331 | Solo: Fine-Tuning, God, and the Multiverse

Interactive Transcript

spk_0 Hello everyone, welcome to the Mindscape Podcast. I'm your host, Sean Carroll.

spk_0 Those of you who've been around here for a while know that here at Johns Hopkins, I'm teaching this semester two different courses.

spk_0 Both are a lot of fun in very different ways. One is quantum mechanics, the standard quantum mechanics course for all physics undergraduates.

spk_0 And the other is one called philosophy of cosmology. That's an upper level lecture course in the philosophy department, obviously.

spk_0 And it's for a general audience. So there's some philosophy majors there, but there's a whole bunch of different people. So a wide variety of levels of expertise are there.

spk_0 The quantum mechanics course is fairly standard, like I said, I'm actually doing threads on blue sky where I try. I think I've been successful so far.

spk_0 Every day after the lecture, I give a couple of little sentences about what was in that lecture. And if you follow the thread on the quantum mechanics course, you'll get a feeling for how very, very different quantum mechanics is for the working physicist, then it is for the popular discussions of quantum mechanics.

spk_0 I love the popular discussions of quantum mechanics. I'm part of them myself, but they rarely involve how to deal with operators that have degenerate eigenvalues. And therefore their eigenvectors do not form a unique basis. Oh my goodness. What do you do with that?

spk_0 No one ever talks about that in the popular level books. So you learned a little bit about that in the thread where I cover everything I'm doing.

spk_0 The other course, the philosophy of cosmology course, though, that's almost me talking for myself in some sense. I think that the topics we're covering, we're focusing on three big questions. One is the multiverse and the anthropic principle. One is entropy in the arrow of time. And the other one is the foundations of quantum mechanics, especially the many worlds approach to it.

spk_0 These can be interesting to just about anyone, you know, these big topics, but they're also questions with very big unanswered issues floating around issues that I'm interested in myself from a research level.

spk_0 And therefore, you know, I get to think through how I think about these things often with questions that I don't have strong opinions about what the answers are. I'm trying to learn, sharing that lack of complete confidence with the class. Maybe they could help me out.

spk_0 Maybe I can learn something from them. That's always possible. And in particular, there's one issue that it occurred to me would make for a good solo podcast. Actually, there's a whole bunch. I could, you know, do a series of solo podcasts, basically covering the whole philosophy of cosmology class. But, you know, I like to hear other people also. So I thought I would talk about this one. It's an old chestnut. It is by no means as a topic.

spk_0 New and fascinating. Nor have there been any recent breakthroughs in it. But in many of these philosophy of cosmology questions, I'm just not that impressed with the level of conventional discourse about these hard questions. The questions are hard.

spk_0 Quantum mechanics, I think, is the one area where people really, really thought hard about it. I disagree with a lot of things that people say.

spk_0 But it's certainly a high level of discussion. Entering the arrow time, there are levels of discussion that are very high, but it's not absolutely universal. But the multiverse anthropic thing is a place where there's a whole bunch of things both on the philosophy side and on the physics side, the cosmology side, if you like, where I think we could just do better. It's not just that I disagree. I'm like, come on, you got to think harder about this stuff.

spk_0 And in particular, this issue of fine tuning, that is what I want to talk about today. Fine tuning is roughly speaking when you look at the world and you characterize the world in the form of some numbers, right? Some physical parameters for constants of nature, but also some numbers that specify the initial conditions of the universe.

spk_0 And you notice, if something is fine-tuned, one way or the other, it's surprising. It's not what you would have expected. And therefore, you wonder whether or not there's some deeper reason why this number, this constant of nature, this feature of the initial conditions is what it is.

spk_0 Now just by putting it that way, you can tell, this is a little fuzzy, right? This is a little vague. Like, what do you mean? Unexpected surprising. Who gave you what the expectations were? And that's what things we're going to have to talk about.

spk_0 But it's a substantive conversation, and for better or for worse, two of the leading ways of thinking about fine-tuning are either, like we said, the multiverse and the anthropic principle, or the existence of God, the argument from design, the idea that the principle is fine.

spk_0 And the parameters are what they are because some higher being made them that way. And, you know, I actually don't have strong opinions about what is the right explanation for fine-tuning.

spk_0 I do have a strong opinion that it's not the existence of God, but what we can talk about that. But there are serious discussions of it. I think that the idea that God is the answer for the fine-tuning we observe in the universe is 100% a respectable idea to think about.

spk_0 And we should think about it. I personally come to the conclusion that it's not a very good answer, but we have to establish that. We shouldn't dismiss it out of hand.

spk_0 My point, though, is that on both the side of the multiverse and the side of fiasom or the existence of God, there's many people who kind of get emotional. They kind of lose their ability to sort of think dispassionately about these very, very difficult issues. And they get insulted very quickly if you don't agree with them, or they disparage the intelligence or the existence of God.

spk_0 And they're the good faith of the people who are disagreeing with them, which is a trap I don't want to fall into. So I want to think about these things, and I'm not going to be giving you the final answers.

spk_0 I think it's important to think through all of these things carefully, and, you know, include the possible existence of God as a scientific hypothesis that we can think about.

spk_0 To include the multiverse as a scientific hypothesis we can think about to include other possible explanations again dispassionately fairly thinking them all through.

spk_0 That's what we're going to try to do in the in this little kind of easy and fun solo podcast. I'll say just before we start in that you are always encouraged to support the mindscape podcast on Patreon.

spk_0 You can go to patreon.com slash Sean M. Carroll and toss in a dollar or whatever for every episode of mindscape. We have plenty of Patreon supporters.

spk_0 So it's very, very much appreciated that we're getting support that we are. I will say though the whole podcast landscape is changing kind of wildly now with AI and with people trying to sell their series and be sponsored and be created by big companies rather than individuals.

spk_0 So it's good that there is Patreon. Let's put it that way. I'm very, very happy that I get the support from Patreon. It is very important to the continued success of mindscape.

spk_0 But I just also it gives me a warm feeling to think that so many people appreciate it that much. We have a nice little community and I try to get back to the supporters some little goodies in return. And with that thought, let's go.

spk_0 We might as well start right there at the beginning with the question of what is fine tuning supposed to be we already alluded in the intro to the fact that it's kind of a fuzzy notion. And in fact, if you get on the internet and Google around and whatever and look for different definitions of it, the definitions will not be the same.

spk_0 And some of that is just because people are trying to be careful and they be they're careful in different ways. Some of it is because they truly do mean different things by fine tuning. And there's many things they mean let's focus on two of them.

spk_0 One is the idea that the parameters, the numbers, the physical quantities that go into describing our universe.

spk_0 So the typically the laws of physics numbers, right, the strength of a given force of nature or the mass of a certain particle or when you get into the weeds, something like the expectation value of the Higgs field in empty space, right. These are all numbers.

spk_0 And there's some math you have to do, but it's not very hard math and you realize that you could easily have imagined these numbers being very different.

spk_0 In other words, in the space of all possible worlds that are somehow physically reasonable, whatever that means, the numbers that you're seeing are located within a very, very narrow band typically near zero.

spk_0 In other words, the simplest version of fine tuning is just that some parameter of physics is unnaturally small. It could have been in some units that you'll have to specify somehow. It could have been one and it's 10 to the minus 20 or 10 to the minus 100 or something like that.

spk_0 And that's what fine tuning is a lot to a lot of physicists in particular.

spk_0 Sometimes that's phrased as saying that fine tuning is a matter of sensitivity. In other words, if you changed the parameters substantially, the results would be very different. What do you mean by the results?

spk_0 You mean, well, the higher level emergent phenomena that you would get, you know, if you change the mass of the electron, chemistry would be very different, things like that.

spk_0 But I don't think it's really just sensitivity because it's sort of sensitivity plus unnaturalness, like not only, I think that the usual emphasis of fine tuning is not only that the numbers that you're looking at are ones whose values have important effects on higher level emergent phenomena.

spk_0 It's that end they actually in the real world are in this narrow window that is special somehow that allows something to happen. It's the unnaturalness of the value plus the sensitivity of different sort of large scale phenomena to the value that makes something fine tuned.

spk_0 But unnaturalness itself raises some questions like after all, it's the universe. It's it is nature nature by definition can't be unnatural.

spk_0 But we say that a parameter of nature appears unnatural to us and I'm going to give some examples soon enough to put some meat on these bones. But when we say that a parameter appears unnatural, that's a statement at least as much about us as it is about the universe, right? It appears unnatural to us.

spk_0 And that's okay, that's perfectly okay. And we're going to discuss the extent to which this is a problem at all, like one attitude towards fine tuning is this is a you problem, not the universe's problem.

spk_0 This is because you have the wrong conception of what is natural and unnatural for the universe to be in, but that doesn't mean you should go inventing wildly new theories of the universe, maybe you just need to accept that the numbers are what they are. That's possible.

spk_0 But there's another sense of the phrase fine tuning that is sometimes what people mean and it's very similar to just the simple unnaturalness idea.

spk_0 So I wanted to highlight it just because some people come into the conversation with this in mind, which is the idea that not only are parameters unnatural in some way, but they're unnatural in just the right way to allow life to exist.

spk_0 In other words, there's parameters of nature where if the numbers were very different, then chemistry would be impossible, biology would be impossible, life would be impossible.

spk_0 Clearly in our universe, those things are possible. So this aspect of fine tuning is not just that the numbers are small in some unknown probability distribution.

spk_0 It's there exactly what they need to be for a certain reason, which is very special, namely the existence of life. Clearly the ones who want to point in the direction of either an anthropic explanation or a theistic explanation are going to emphasize these aspects, because these are going to be as well discuss those kinds of mechanisms, either an anthropic principle explanation or a theistic explanation, point at making the parameters what they need to be to allow for the existence of the question.

spk_0 So I'm just letting you know that physicists don't always even care about that. As long as some number is way smaller than we might have naturally expected it to be, whether or not it has anything to do with the existence of life, we might still call it fine tuned.

spk_0 Okay. Anyway, that sounds maybe a little vague. So I do want to put some meat on the bones by giving you examples and the examples are intentionally a wildly diverse lot. Okay. There's some examples of fine tuning out there in physics and cosmology that are kind of crowd favorites.

spk_0 There are some words that are always fraught out and you might have heard them before. There's others examples that are very true puzzles, but are less likely that you've heard them. There are some examples where we know the answer. So we don't usually count it as a fine tuning, but by the criteria of oh, there was a small number in the dimensionless quantities of physics, then yeah, it really should count.

spk_0 And I emphasize the word dimensionless here. What does that mean? When we have quantities in physics, like the mass of the electron, we measure it in some units, right? We measure mass in units of kilograms or something like that.

spk_0 Typically, particle physicists have this habit where we use what we call natural units. That's where we set plonks constant h bar equal to one and also the speed of light c equal to one in general relativity land. Sometimes people said Newton's constant equal to one, but particle physicists don't do that. And I'm not going to do that either.

spk_0 So if you set h bar and c equal to one, well, just setting c equal to one all by itself means that energy and mass have the same units, right? Because e equals mc squared. So in a world where c equals one, the speed of light is set in units to be equal to one, e equals m, e and m are the same kind of thing. They use the same units. And therefore we can use energy units and mass units interchangeably. You just have to multiply by the right factor of c squared or divide by the right factor of c squared to go back and forth.

spk_0 I say all this, which is completely uninteresting to you, because the units in which physicists usually think about the masses of elementary particles are something called electron volts, which are actually units of energy.

spk_0 And their units of energy, which are defined in a completely goofy way, like the energy it takes to move an electron across one volt of electrostatic potential that has absolutely no intuitive feeling for anybody.

spk_0 So here's a way to think about it. A proton is roughly a billion electron volts. Okay, that's what's going on. And electron is roughly half a million electron volts. Okay.

spk_0 So we say one g EV, which is one giga electron volt for the proton and half an M EV capital M EV is a million electron volts for the electron.

spk_0 You're not going to remember any of these. They're not on the test. This is the other thing that teaching these courses puts you in the mode of making sure your listeners know what's going to be on the test and what's not going to be on the test.

spk_0 And nothing about the masses of particles or units or anything like that is going to be on the test. So I'm just letting you know this is how physicists think. Okay.

spk_0 So the mass of the electron can't be fine tuned in a very real sense, because it is a dimensionful quantity. Like you can't say the mass of the electron is a big number or a small number compared to one.

spk_0 That dramatically depends on what units you use, right? So physicists have this idea that the things that could possibly be fine tuned. The things that you should worry about being numbers much, much less than one being numbers very, very different than one. And therefore somehow unnatural are dimensionless numbers are numbers that don't have any units attached to them at all.

spk_0 So you might say, well, what are the interesting dimensionless numbers? Sometimes you just are handed an interesting dimensionless number like the fine structure constant.

spk_0 The famous constant alpha of electromagnetism has a dimensionless value of approximately one over 137. And that is two physicists that's close enough to one. It's actually less than 1% right?

spk_0 But in this world that we're talking about one over 137 is small, but it's not crazy small, right? I mean there might be four pies and things like that that you're neglecting and what you compare to what else.

spk_0 So if you want to fine tuning, you want something that is really less than 10 to the minus 10 at the very least, right? Then you can say like, okay, yeah, that's real. If you have something that's 10 to the minus two, that doesn't really bother you.

spk_0 We're happy that the fine structure constant is small because it makes it easy to do calculations in quantum electrodynamics, but we're not worried about it being too small.

spk_0 So that is a non fine tuned dimensionless constant of nature. But even if you have dimension full quantities, that is to say quantities that are measured in some units like mass or time or length or whatever, you can always just divide it by a different quantity that has mass or time or length or whatever.

spk_0 And very often since we're doing fundamental physics, the natural thing to divide by our plank units. You've all heard about plank units by invented by Mox Plunk, the famous father of quantum mechanics.

spk_0 And it was interesting as soon as he noticed, Punk back in 1900, as soon as he noticed that he had to invent a new unit, a new constant of nature in order to describe this phenomenon of quantum mechanics.

spk_0 One of the first things he did was that with my new constant of nature, I can now define universally agreed upon length, mass, time, energy, temperature, all those things.

spk_0 And he literally said like, now we can talk to aliens because now we can tell the aliens how tall we are and how long we live because we can just use plunk units. I don't think he called them plunk units.

spk_0 But those are units which are more or less given by the constants of nature, which is always very nice. However, those units are very, very far away from everyday life.

spk_0 The plunk length, the length that we get by combining the units of H bar for quantum mechanics, G, Newton's constant for gravity, and C, the speed of light, we combine them in the right form to get a length just by canceling the units appropriately.

spk_0 And we get about 10 to the minus 33 centimeters. That is a very, very short length for the plunk length. Likewise, the plunk mass is something like 10 to the minus 5 grams.

spk_0 That actually is pretty macroscopic, like it's small. It's certainly smaller than anything you would come across in your everyday life, but it's imaginable.

spk_0 Like you could imagine holding a little dust grain that was exactly the plunk mass. The thing that makes it actually very, very big by human standards is that when we think about quantum gravity or particle physics and stuff like that, we want to put all of that mass into a single particle or a single collision between two particles or something like that.

spk_0 And that's an enormous amount of density of energy, like the mass of the proton, which is a billion electron volts, is roughly 10 to the minus 15.

spk_0 No, 10 to the minus 17 times the plunk mass, right? That's very, very far away. So that's something which is the plunk mass in energy and particle physics units is huge compared to our everyday way of thinking about things.

spk_0 Okay, all that is to say, I'm trying to like lay the groundwork to get to the fun stuff here. Don't worry about it. All that is to say, when we talk about numbers in physics being a naturally fine tuned, we have to be sure to talk about dimensionless numbers.

spk_0 When the thing we have in mind naturally has units of something, we want to divide it by something else with units of that thing. Sometimes very, very often, in fact, it is natural to use the plank units as the measuring stick.

spk_0 We're going to measure things with respect to plank units. In fact, there is a whole justification for doing this that I'm not going to get into.

spk_0 If you have read Quanta and Fields, my most recent book in the biggest ideas series, I go on a lot about effective field theory. And it's really within the context of effective field theory, where you're summing up a whole bunch of influences that are happening at short wave lengths and high energies into their effects on lower energy, longer wave length phenomena.

spk_0 Then you really get this idea that certain parameters of nature, especially masses of particles, should naturally be big, should naturally be driven up because of all of these hidden effects at high energies to very high values themselves.

spk_0 And so there is some justification for using this very high scale of the plank scale as our measuring stick. If you don't want to use it, that's okay.

spk_0 You can pick some other scales, and if you're anywhere close to being reasonable, you'll still get fine-tuning problems coming out of it. But this is something that, you know, because we do have these problems, we don't always know what the solution to them is.

spk_0 You're welcome to worry about these problems in a very realistic way. Just understand, I mean, you're welcome to worry about it, but you should understand why the physicists pick the things they do.

spk_0 So, dimensionless numbers would be things like the mass of the electron divided by the plank mass, or the mass of the proton divided by the plank mass. In other words, the masses of these particles in plank units, those are things that we can ask, are they fine-tuned or not?

spk_0 Okay, so going through some famous and less famous examples, one is the spatial curvature of the overall universe.

spk_0 Remember, I said we're not talking just about numbers built into the laws of nature, we're also talking about the initial conditions of the universe.

spk_0 Casually and informally, because physicists don't know the final theory of everything, in the usual way of thinking about physics, the entire history of a system depends on two things.

spk_0 Number one, the dynamical laws, that say how the system evolves from time to time, and number two, the initial conditions.

spk_0 So, for the universe as a whole, we imagine that there were some initial conditions set at or near the big bang.

spk_0 We have to be a little bit fuzzy about that, because the big bang probably didn't occur as an actual moment of time.

spk_0 There's probably some quantum gravity substitute for it, but not long after, whatever happened happened, there is a more or less sensible classical universe that we can talk about.

spk_0 And that's what we mean by conditions near the big bang.

spk_0 And the spatial curvature of the universe today is related to the initial conditions.

spk_0 You just understand what the initial conditions are, you understand the stuff that is in the universe that helps explain how it expands and evolves over time, and you can predict the spatial curvature today.

spk_0 What do we mean by spatial curvature? Well, in cosmology, you know that the universe is pretty uniform, over large scales.

spk_0 There's plenty of conditions that you can imagine like if you're next to a black hole, conditions are not uniform.

spk_0 The black hole is over there, and empty space is on the other side, right?

spk_0 But in the very, very specific context of the universe as a whole, on very, very large scales, the matter distribution as far as we can observe looks pretty uniform to us.

spk_0 So in that very, very specific context, you've heard that in general relativity, space time is curved, right?

spk_0 So in the context of cosmology where the distribution of matter is uniform, the curvature of space time takes on a very natural decomposition.

spk_0 It's the sum of two different contributions. One is the curvature of just space.

spk_0 So you have three dimensional space, evolving with time, expanding as a matter of fact in the universe that we live in, and that three dimensional space has curvature.

spk_0 And the curvature could in principle be wildly different from place to place, and in fact it is.

spk_0 But the fact that things are smooth and uniform on very large scales means that there is a single number, which is the same throughout the observable universe, which is the overall curvature of space itself.

spk_0 And that's different than the sort of all the different components of the remont tensor of space time curvature that you could have in general.

spk_0 We're not talking about in general, we're talking about the very, very specific example of a uniform space in an expanding universe.

spk_0 So one contribution to the curvature of space time is the curvature of space, and that's just a single number.

spk_0 The other contribution is the expansion rate, the rate at which space is changing over time.

spk_0 And the famous Friedman equation of cosmology relates the expansion rate to the energy density, the amount of stuff in the universe, and to the spatial curvature.

spk_0 So there is something called the flatness problem in cosmology, which I don't know exactly who invented it first, but was certainly highlighted by Robert Dickie and Jim Peoples back in the 1970s.

spk_0 And it was one of the motivations for Alan Gooth in the late 70s when he invented the theory of cosmological inflation, as we will see.

spk_0 And the point of the curvature problem, the flatness problem, the flatness problem is that the curvature is very small.

spk_0 Small curvature means flat. The universe is flatter than it should be, okay? That's one way of putting it.

spk_0 Now, what do you mean by what it should be? How do you know what it should be?

spk_0 The usual way that the flatness problem is presented is to say the following.

spk_0 Without inflation, if you didn't believe in inflationary cosmology, and I'll talk a little bit about what inflation says in a bit, but in conventional cosmology, where the universe is full of matter and radiation, basically.

spk_0 And space could or might not be curved. That's an initial condition that you put into your model.

spk_0 In that theory, if you have some amount of curvature, some amount of matter and some amount of radiation, the effective energy density and matter and radiation dilute away faster than the effective spatial curvature.

spk_0 Spatial curvature lingers around longer, and if they were approximately equal at early times, then at later times, the curvature is relatively huge, and the matter and radiation are relatively negligible.

spk_0 That is not at all what we see in the actual universe. In fact, we see negligible curvature, and we see a decent amount of matter and a certain amount of radiation.

spk_0 So in order for that to be true, if you thought about it in terms of initial conditions, the early contribution of spatial curvature to the universe had to be less than the contribution from matter and radiation by something like a factor of 10 to the minus 25.

spk_0 That's a very, very rough number because it depends on exactly when you start your calculation of what's going on in the early universe.

spk_0 But the beauty of this whole discourse is that nobody cares what the exact number is. The point is, if you are randomly picking universes from a hat, and you got a number that was 10 to the minus 25 that could have been 1, or it could have been minus 1, you're weirded out a little bit. You think like, oh, I don't know.

spk_0 That seems like it would not have been what I would have guessed would have come out, and that is the flatness problem. Why was the universe so spatially flat at early times?

spk_0 That's fine tuning problem number one is the flatness problem.

spk_0 Fine tuning problem number two is the cosmological constant problem.

spk_0 You must have heard that our universe is accelerating. We attribute that acceleration to the presence of dark energy in empty space.

spk_0 And the simplest candidate for dark energy is Einstein's cosmological constant, which he started thinking about back in 1917, which is equivalent precisely equivalent precisely the same as energy density of the vacuum.

spk_0 Energy density in empty space itself, it turns out that in the context of general relativity, where you have an unambiguous relationship between energy sources and the curvature of space time, space time itself can be an energy source, empty space can contain energy.

spk_0 And this is basically what Einstein came up with in 1917. He came up with it for bad reasons, but his conclusion is still true.

spk_0 Empty space could have energy, and you go out and measure it. Maybe it's zero, maybe you were guessing it was zero all along, but in fact, in modern particle physics, a better guess would be the Planck scale.

spk_0 Or since it's an energy density, energy density is energy per volume, that is to say energy per length cubed.

spk_0 So you might say, I'm going to guess the cosmological constant should be one Planck energy per Planck volume.

spk_0 Now you're welcome to disagree with that guess. Go ahead and disagree with that, but it's a natural guess. Had it been right, no one would have complained.

spk_0 It is not right. In fact, the observed value of the dark energy, if we attribute it to the cosmological constant, divided by what we just said was the natural guess for the vacuum energy, is a famous number of about 10 to the minus 120.

spk_0 That's an incredibly absurdly tiny number, famously the worst prediction in all of physics. Not really a prediction, of course. It was just a guess, just a natural value, but so very, very far off,

spk_0 that we think like there's got to be a reason for that, right? Like it can't just be, it happened that way. We couldn't have just got lucky, right? That's a fine tuning, a fine tuning between the natural value for the vacuum energy, which doesn't after all involve both quantum mechanics and gravity.

spk_0 And the Planck scale, which you might have thought, sorry, the Planck scale is the natural value versus the observed value that we get from astronomical data. So that was fine tuning number two.

spk_0 Fine tuning number three is what we call the hierarchy problem of particle physics. So the hierarchy problem, we just called the hierarchy problem, but there's lots of different hierarchy problems.

spk_0 The big one, the famous one, is a difference of two energy scales. One scale is the energy scale characterizing the weak interactions of particle physics.

spk_0 You can think about this in different ways. You can think about it as the mass of the Higgs boson. The Higgs boson gets a non-zero expectation value in empty space, and this is what makes the weak interaction special, what makes the W and Z bosons heavy and therefore the weak force weak, but it also gives masses to all the other particles.

spk_0 So once the Higgs gets a value in empty space, that value of the Higgs is a number expressible in terms of its units are actually energy units, the same as the units of the mass of the Higgs boson.

spk_0 So it is true and natural that the mass of the Higgs and the expectation value of the Higgs should be close to each other, that turns out to be correct, that is not a fine tuning.

spk_0 But they're close to each other, they're very far away from the Planck scale. The ratio of the expectation value of the Higgs boson to the Planck mass is something like 10 to the minus 16.

spk_0 Another big number, as long as we're 10 to the minus 10 or smaller, we're going to count that, when I say big number, I mean big discrepancy, we're going to count that as a big discrepancy, 10 to the minus 16.

spk_0 And the hierarchy problem is an interesting one, because with the cosmological constant, you know, it does involve quantum mechanics and gravity and all these things, and you figure like maybe there's just something I'm missing.

spk_0 The hierarchy problem is pretty down to earth. It's kind of within the wheelhouse of what we expect to be able to understand, it's just particle physics really.

spk_0 We're comparing the Higgs scale to the gravity scale, because it's a convenient benchmark, but really just in a more general sense, the weak interaction scale is very tiny compared to anything we might have expected it to be.

spk_0 If you just ignored gravity and imagined grand unification or something like that, the Higgs scale is very different than that, and it shouldn't be, it should be nearly the same one, and we have good effective field theory reasons to expect that.

spk_0 So that's the hierarchy problem. That's number three. So we've gone through the spatial curvature, the cosmological constant and the Higgs expectation value.

spk_0 Number four is the proton mass. Now this one's going to get me in trouble, because particle physicists don't consider the proton mass to be fine tuned.

spk_0 But if you just plug in the numbers and go by the definition we gave you, the mass of the proton divided by the Planck scale is about 10 to the minus 17, 10 to the minus 18, depending on where you put the two pies in there.

spk_0 Again, it doesn't matter. It's much less than 10 to the minus 10, okay? So the mass of the proton much lighter than the Planck scale. That is an example of a fine tuning. The reason why I'm going to give away the answer here.

spk_0 The reason why we don't list it among the many fine tunings that we know about is because we have an answer to this one. This one makes perfect sense.

spk_0 We're not surprised by this one exposed back though, but if you're just looking for small numbers, it would absolutely count. So let's put it on the list.

spk_0 Number five, and this one is a little bit different than the other ones. The ratio of the neutron mass to the proton mass.

spk_0 So, or I could just say like the neutron mass. But the neutron mass being small in Planck units is not surprising once you've already told me the proton mass is small in Planck units.

spk_0 They're basically the same thing. Protons and neutrons are very, very similar to particle physicists. And so it's not surprising at all that the neutron and the proton have roughly equal masses.

spk_0 But the difference here is that they are roughly equal. The mass of the neutron is about 1.0014 times the mass of the proton.

spk_0 So that's not a small number compared to one. That's very, very close to one. The small number, if you wanted to force yourself to make a small number, would be the mass of the neutron minus the mass of the proton divided by the mass of the proton. That would be 0.0014.

spk_0 What's weird about that one? I mean, it's given that the neutron and proton are kind of similar creatures. Maybe we shouldn't be surprised they have similar masses.

spk_0 Here is where the question of life comes in to the game. The question of life already came in as we'll talk about in a second. All these other parameters do in principle affect the possibility of life existing.

spk_0 But here's one that you might not have thought of as fine tuned until you started thinking about how desperately important this number is for life as we know it, the mass of the neutron divided by the mass of the proton.

spk_0 The neutron is a little bit heavier than the proton. That's the motto that we get from measuring their values.

spk_0 What that means is heavier particles tend to decay into lighter particles. One way of thinking about that is a collection of light particles has higher entropy than a single heavier particle. So they like to decay.

spk_0 And so the neutron decay is into the proton. And indeed it does. If you let a neutron out there all by itself, it will at some point decay into a proton and electron and in what we call an anti-nutrino.

spk_0 But you go through the calculations. You can actually calculate exactly how this happens. You get the right answer. This goes back to Enrico Fermi and his theory of Fermi's theory of beta decay as it's known, the decay of the neutron.

spk_0 So we understand this very, very well. And what happens is the neutron is going to decay because it can, all of its conserved quantities are still conserved in this decay and all of the masses of the particles it decays into add up to less than the mass of a single neutron.

spk_0 So in the neutron decay process, neutron converts into proton, electron, and neutrino. The extra energy that got disappeared turns into kinetic energy. The mass of the electron plus proton plus neutrino is less than the mass of the neutron.

spk_0 And the change in mass goes into energy equals mc squared. It's converting the kinetic energies of the other particles. But because the difference in mass of the neutron and the proton is so small, there's not a lot of room for that to happen.

spk_0 The way quantum mechanics works is basically every single way a process can happen adds up to the total is a contribution and you just add up all the contributions. In this case, the different contributions are basically just different ways to share the energy, to share the kinetic energy between the proton, electron, and neutrino.

spk_0 But because there's so little energy to share, there's not that many ways to do it. And that means that the decay is small. The mass of the lifetime of the neutron is something like 10 minutes, which by particle physics standards is hilariously long.

spk_0 The lifetime of the Higgs boson, which is only 100 times more massive than the neutron, is 10 to the minus 21 seconds. That's a decent particle physics time scale. The neutron lives a very, very long time.

spk_0 But it still does decay. And both those facts matter. If the neutron were much heavier, and by much heavier, I don't mean like a billion times heavier. I mean if the neutron were twice as massive as the proton, it would decay right away.

spk_0 It would decay very, very quickly. And furthermore, you notice that there are neutrons in the world, right? There are neutrons out there. There's a bunch of neutrons in your body right now, probably most of the mass of your body is actually neutrons. I actually don't know that for sure, so don't quote me on that.

spk_0 But of order the same mass in your body comes from neutrons and protons. And how is that possible if neutrons decay away? The answer is that even though a free neutron by itself will decay away,

spk_0 neutrons can be bound into atomic nuclei. Neutrons bound with protons have, there's a negative energy in that binding. And you can have a situation where let's say in a single helium nucleus where you have two neutrons and two protons.

spk_0 Even though the mass of the neutron is bigger than the mass of the proton, because of the negative binding energy in the helium nucleus, the mass of a helium nucleus is less than the mass of four protons.

spk_0 So the helium nucleus is perfectly stable. It's not going to go anywhere. The neutron can be stable as long as it is in a stable nucleus. If the mass of the neutron were twice the mass of the proton, it would be so unstable that you would not be able to make nuclei. You would not be able to make stable nuclei.

spk_0 You would only have protons as stable nuclei, which means that the only element in the universe would be hydrogen. It is thought that you cannot make living creatures out of nothing but hydrogen.

spk_0 The only molecules you can make are basically H2. There's no organic chemistry if you only have H2 in the world, double hydrogen atoms. So of course we don't really know the precise set of conditions under which living beings can be constructed.

spk_0 But certainly by the methods that we know, you can't make life if the neutron is twice as heavy as the proton. What if the neutron were a little bit lighter than the proton that we're close to it but a little bit below?

spk_0 Also, no chance of getting life. In that case, it's the proton that is now unstable. If a proton is heavier than the neutron, the proton will decay into the neutron. And then you just get a world full of neutrons.

spk_0 There's no electrons hanging around. They've all been converted electrons plus protons have been converted into neutrons plus neutrinos. And once again, there's no chemistry.

spk_0 Now rather than having a universe with nothing but hydrogen, you have a universe with nothing but neutrons. You don't even have atoms, much less chemistry.

spk_0 So once again, it seems like it's very difficult to get life out of the laws of physics if the neutron has a lower mass than the proton.

spk_0 So the neutron proton mass difference is not a case where a number is an almost small, less than 10 to the minus 10. It's a case where a number is special in the sense that it's just right to the allow for the existence of life, apparently, as far as we know.

spk_0 So that counts as a fine tuning that we might want to keep our eyes on when we're trying to explain why fine tuning's occur in the universe.

spk_0 The next example is actually my favorite. And you've heard me talk about it before, which is the entropy of the early universe.

spk_0 This is once again a matter not of the constants of nature, but rather the initial conditions of the universe, the configuration the universe was in at early times.

spk_0 And I've talked about this multiple times on other occasions, but the upshot is that there is a scale by which we can judge the entropy of the universe, which is sort of what its maximum entropy could be.

spk_0 Roger Penrose figured this out back in the 70s, just by asking the simple question, after Stephen Hawking figured out the entropy of a black hole, Penrose asked himself, well, okay, if we took all of the known matter and energy in the universe,

spk_0 put it into a single black hole, what would its entropy be? And the answer is enormously bigger than what the entropy is.

spk_0 And the early universe, where we started right after the big bang, has even a much lower entropy than that.

spk_0 The entropy of the early universe divided by what its entropy could have been, its maximum entropy, is something like 10 to the minus 122.

spk_0 It depends on the details once again, a factor of 10 or 4 pi among friends isn't going to bother us.

spk_0 10 to the minus 122 is a very, very tiny number.

spk_0 This is reflecting the fact that the conditions in the early universe were very special.

spk_0 Which is, if you don't believe they're special, notice that the early universe doesn't stay like the early universe.

spk_0 The early universe rapidly expands and cools, and turns into the universe like we have now, and if you give it a few more quadrillion years, it will empty out.

spk_0 It will empty out, stars will burn out. That's the natural state for the universe to be in.

spk_0 Flung all the matter and radiation flung to the four corners of space time, and everything looks almost empty.

spk_0 The early universe looks the opposite of that. It's a very, very unusual configuration, where it's super duper dense.

spk_0 And not only is it dense, it's super duper smooth. That's very, very strange to us. If the universe were to collapse into a big crunch, there's no known reason why it would smooth itself out along the way.

spk_0 We would expect it to get lumpier and lumpier, indeed, as it collapsed. That would be the natural, high entropy thing to happen.

spk_0 But that's not what we see. In the early universe, it's smooth and dense at the same time. That's very weird. That's a reflection of the fact that it's entropy is very, very tiny.

spk_0 So that's a fine tuning, not of a constant, but rather of the initial conditions of the universe.

spk_0 I know this is a lot of fine tunings, but that's the kind of the whole point. We have two more to go, and we should be struck by the fact that there's a whole bunch of ways in which the universe really isn't what you might guess in a natural way.

spk_0 Okay, the seventh example of fine tuning is something called the strong CP problem, which those of you who are really fans of particle physics or cosmology will have heard about, but many others might not have.

spk_0 CP here stands for charge parity, which are two transformations you can do in particle physics.

spk_0 The charge transformation basically says, let's convert all particles into antiparticles. The parity transformation basically says, let's switch right-handed with left-handed.

spk_0 Let's take a mirror image of whatever configuration of physical stuff we have.

spk_0 And it was an interesting and no-bell prize-worthy set of events back in the 50s, 60s, where we realized that these transformations were not actually symmetries of the universe.

spk_0 Parity was violated, and then we realized that even the combination of charge and parity is violated, and that was very surprising.

spk_0 But then once we did realize that, we realized that there's no reason for these transformations to be symmetries. It could matter what the orientation of your axes were, for example, in a particle physics experiment.

spk_0 Then we started asking, well, okay, should we violate them? Can we write down terms, again, from an effective field theory perspective, for example?

spk_0 Can we write down things in the standard model of particle physics that would violate CP?

spk_0 The answer is yes, and there are terms that do violate CP, and they're small in the weak interactions. There's also a term you can write down in the strong interactions that violates CP, and it's not there.

spk_0 That is to say, if this term were there with a natural value, it would be very, very noticeable. People have calculated how we would detect it.

spk_0 If you want to know the details, it's the electric dipole moment of the neutron that is proportional to this parameter called theta QCD.

spk_0 QCD is quantum chromodynamics, the strong interactions. So there's this parameter we know had to write down that we could put into the dynamics of the standard model.

spk_0 If we did, it would show up. It would be detectable, and we've looked for it, and it's not there.

spk_0 In fact, you can put a limit on how big it is. It's guess what? About 10 to the minus 10, or less, we haven't detected it. It could be 10 to the minus 100 for all we know.

spk_0 10 to the minus 10 of its natural value. Okay, so that's the strong CP problem. And again, we're still not quite sure what the answer to that one is.

spk_0 I'll give you one final one. This is the eighth example of a fine tuning. Although it's a slightly different example,

spk_0 because we don't actually know if it exists or not. This is the mass of the dark energy field. Okay, so what is we talking about here?

spk_0 I just said that we know that there's dark energy, or there's something, anyway, making the universe accelerate, and we call it dark energy. Maybe it's the cosmological constant.

spk_0 But maybe it's not. I had mentioned on the podcast, there's been little hints in recent years that maybe the dark energy is not simply the cosmological constant.

spk_0 Maybe it is changing over time, and changing very gradually, otherwise you would have discovered a long time ago. Okay, and this is something that I thought about a lot back in my days as an enthusiastic young cosmologist.

spk_0 And it's hard to make the idea work. It's a natural idea. A lot of the motivation for thinking about whether or not dark energy can be variable, can be dynamical rather than just constant, is kind of an expression of humility.

spk_0 You know, back when I was your age, when we were thinking about the vacuum energy, but hadn't yet discovered it, right, the cosmological constant.

spk_0 We knew the cosmological constant was too small compared to its natural value. We knew that it was 10 to the minus 120 or less times its natural value.

spk_0 And there was this school of thought that said, look, in the space of all possible theories that we haven't thought of yet, I don't know what explains why the cosmological constant is small, but it's easier for me to imagine a theory that sets the cosmological constant to exactly zero, even though I don't have that theory yet.

spk_0 Then it is to imagine a theory that sets the cosmological constant to 10 to the minus 120. It's natural value. I don't know how to do that. Exactly the value that we haven't yet observed yet.

spk_0 But that's what it is. That's what it turned out to be. We were wrong. Okay, so we were wrong. And when I say we, I include me literally not just figuratively, it's not the royal we. I was there and I was wrong about this.

spk_0 The cosmological constant is or apparently looks like 10 to the minus 120, it's natural value. So maybe it's not the cosmological constant that we're observing. Maybe the cosmological constant really is zero. Maybe we were right in some weird way that there is some mechanism that we haven't yet invented that sets the vacuum energy equal to zero.

spk_0 And what we're seeing in the acceleration of the universe is some field that has an almost constant energy, but not quite. Okay.

spk_0 Here's the problem with that idea. We would call this dynamical dark energy or quintessence. Sometimes it's called the problem is the same as the problem with the hierarchy problem in the week interactions.

spk_0 The Higgs boson field is a field that pervades all of space. And its mass is small compared to the plank mass, right? Its mass is 10 to the minus 16 something like that, the plank mass.

spk_0 And we say that's a small number. Masses in particle physics under the rules of effective quantum field theory tend to be big, tend to be driven up to very large numbers. Okay.

spk_0 And so the hierarchy problem is just that there's a factor of 10 to the minus 16 between the Higgs mass and the plank mass. If you have a dark energy field that is making the universe accelerate, it needs to be changing very slowly.

spk_0 Otherwise, we would have noticed a long time ago. If we were changing rapidly on cosmological scales, that would have been easy to detect. It wouldn't even have acted like dark energy.

spk_0 The whole thing that dark energy needs to do is to change slowly so that its energy density is approximately constant and it can make the universe accelerate.

spk_0 So if it's an ordinary, not weirdly, a typical field, filling all of space that has some kind of mass, we talk about the mass of the field, what we really mean when we say that is the mass of the particle you would make by jiggling that field.

spk_0 That's what we mean when we talk about the mass of the Higgs field or something like that is the mass of the particle you would make.

spk_0 In the case of dark energy, you're not actually making any particles, you're not jiggling it and doing any particle physics experiments.

spk_0 But we can still mathematically talk about the mass parameter that characterizes the field that we can generally quite easily relate to its rate of change.

spk_0 The mass is basically how much push the field is getting from its potential energy, which in turn is related to its rate of change.

spk_0 Anyway, at the end of the day, the mass of the dark energy is about needs to be in order to fit the data, something like 10 to the minus 33 electron volts, which is lower than the plank mass by a factor of 10 to the minus 60, not 16.

spk_0 So the Higgs boson, we already said, was anomalously low mass, its mass is 10 to the minus 16, the plank mass.

spk_0 The dark energy needs to have a mass of 10 to the minus 60 times the plank mass in order to fit the data, oh my goodness.

spk_0 And of course, I have invested interest in this. One of my better papers that I've written as a physicist was pointing this out, pointing out that this was very unnatural, pointing out there's a whole other set of unnaturalness involved with dynamical dark energy.

spk_0 Because that dynamical dark energy can also interact with photons and with electrons and with neutrons and things like that. And it apparently doesn't.

spk_0 So there's a whole bunch of new parameters you have to invent and then fine tune all of them. So I wrote a little paper explaining that that's a problem and also suggesting a way out and also suggesting an experimental test of the way out.

spk_0 And that test would potentially lead us to cosmological by-refringence, rotation of the polarization angle of light from the cosmic microwave background or other distant sources.

spk_0 And that's something we're looking for and there's even little hints we might have found it. So my fingers are crossed that we do find it and then forget about this cosmological constant stuff. I'm going to be a dynamical dark energy guy.

spk_0 But we haven't found it yet. It's still a little bit up in the air. So cautious optimism, but you have to wait for more data to come in.

spk_0 Okay. Anyway, that took a longer than I expected. That's usually what happens because I get into it. I get enthusiastic. I warm to my subject.

spk_0 We have eight different fine tunings on the table that I specifically and intentionally chose to be very different from each other in character.

spk_0 The spatial curvature is much smaller than it might have been the cosmological constant is much smaller than it might have been the mass of the Higgs boson or equally well the scale of electric weak symmetry breaking is much smaller than it might have been the proton mass is much smaller than it might have been the neutron proton mass ratio is close to one but a little bit above but not too much above or too much below, which seems a little fine tuned.

spk_0 The entropy of the early universe is very tiny. The violation of CP in the strong interactions is very tiny and the mass of the dark energy if it's there would be very tiny.

spk_0 So why am I giving you all these very, very different examples of purported fine tunings just because I think that the discourse about fine tuning tends to kind of wander all over the place sometimes and part of that is the different fine tunings come in different forms.

spk_0 So as I mentioned as we were going through that most of these fine tunings involve small numbers but not all of them do the neutron to proton mass ratio is a number that is very close to one just like it's supposed to be.

spk_0 It's the specific close to one value that is fine tuned but not the fact that it's a very tiny number compared to one.

spk_0 So there's one way of being fine tuned which is just to be small and there's another way that says it's just a weird value that seems to be purposeful intentional special whatever you want to call it.

spk_0 Some of these fine tunings either we know the answer to them we know why they're like that or we have a really, really good theory for why they might be like that.

spk_0 Remember I told you that the mass of the proton is never listed as something that's finally tuned that's because we know where it comes from we know that the mass of the proton depends on the symmetry symmetry breaking is not the right way to put it even though it would count the scale at which the strong interactions become strong.

spk_0 There's an energy scale at which below which the strong interactions are strong but the strong interactions have this interesting property called asymptotic freedom as you go to higher and higher energies interestingly the strong interactions become weaker and weaker.

spk_0 They don't become that strong anymore and there's a crossover threshold point where the strong interactions go from effectively being strong to effectively being weak.

spk_0 That interaction point that the energy which that happens is roughly the mass of the proton and that makes a hundred percent sense that's what it should be it's roughly the mass of the neutron roughly the mass of most strongly interacting particles.

spk_0 So the question is this energy scale which is called the QCD scale again QCD is quantum chromodynamics why is it so different from the Planck scale but we know that one it wasn't actually that different from the Planck scale and dimensional this unit.

spk_0 But then it turns out that there's a relationship between the high energy scale where we imagine our minds maybe correctly maybe not we imagine that you know God sets the parameters of the universe at high energies and then they flow down to low energies using what is called

spk_0 renormalization but for something like QCD they flow very very slowly. So if you start with a number that is not much different than one at very very high energies and move it down to lower energies to ask where does it cross over from being less than one to being greater than one so the strong interactions go from being weak to being strong.

spk_0 You very very naturally get a big big difference a big big hierarchy between the high energy scale and the QCD scale. So I know I'm skipping over a lot of deep and and esoteric physics here all I want to let you know is the reason why people don't worry about the mass of the proton being fine tuned even though it's ten to the minus eighteen to the Planck mass is we know exactly a good theory that explains that one.

spk_0 Okay, compare that to the case of the spatial curvature right the flatness problem the flatness problem only became famous once we learned the solution to it or at least a very good solution to it which is inflationary cosmology.

spk_0 The flatness problem was popularized in Alan Gooth's paper about inflation where he said I have solved this problem that you didn't know was a problem and indeed no one quoted as a problem until he solved it.

spk_0 And the solution of inflation is imagine there is some field that we don't have detected directly in a laboratory maybe never will but it's a field that has the property that it's kind of like we imagine the dark energy field to be today except it wasn't dominating the mass of the universe recently was dominating the mass of the energy of the universe near the big bang.

spk_0 So basically what you're imagining with inflation is there's a period of super high density quasi dark energy super rapid expansion inflation pushes the universe apart and incredibly high rate and in doing so it smooths out the universe it also flattens it it decreases the ratio of the spatial curvature to the energy density of the universe.

spk_0 It's the opposite of what happens if you live in the universe with just matter and ordinary radiation with matter and radiation the relative effects of spatial curvature grow with time with this quasi inflationary dark energy the relative effects of curvature shrink over time.

spk_0 And then the thing is that the end of inflation all this energy that was trapped in this quasi dark energy field gets converted by ordinary particle physics process in a process known as reheating into matter and radiation but it does not get converted into spatial curvature there's no way for that to happen.

spk_0 So roughly speaking the inflaton field as we call it which is again purely hypothetical at the moment it dominates the energy density of the universe it flattens the universe out and then it converts into matter and radiation in a universe which is very very close to spatially flat just as we observe it to be right now.

spk_0 So inflation provides a dynamical explanation for why the curvature of the universe is so small.

spk_0 Now a footnote there I wrote a paper with Haywood Tam years ago a decade ago or more which pointed out or tried to make the argument semi successfully I mean we were right but people haven't listened to us yet that there the flatness of the universe was never fine tuned to begin with the flatness of the universe is a is a perfect case of where you really actually do need to think more carefully about what you should have expected the number to be.

spk_0 You know that I gave you the flatness problem is saying the curvature spatial curvature of the universe had to be incredibly tiny at early times for it to be sufficiently tiny today not to be noticeable that's 100% true but then the other half of the argument in the flatness in the flatness problem is and it had no right to be so tiny at early times.

spk_0 I'm not going to go into details about this because it's not the main focus about what we're talking about today but I argue and Haywood and I argue that if you do the math correctly almost all universes are exactly spatially flat it's very very difficult to randomly pick a universe out of the set of all universes and get one with substantial spatial curvature there isn't really a flatness problem in my mind it's very natural for almost smooth universes to be essentially spatially flat.

spk_0 That's just an example that I'm giving you as a possible set of ways of thinking about these fine tuning problems maybe they're just not there if you think better about what the probability distribution for these parameters is.

spk_0 Okay anyway do we have other good theories yeah you know there's also this Higgs boson thing the weak interaction thing right for the cosmological constant we essentially have zero good dynamical theories that explain why it is smooth.

spk_0 I say that as someone who has written papers putting forward theories I just don't think they're very good sometimes right you have good theories sometimes you have less good theories for the Higgs boson it's a tough one you know the weak interaction strength the electro weak scale the thing that sets what we call the hierarchy problem of particle physics it is very possible to come up with good dynamical theories that naturally explain the existence of a hierarchy between the Planck scale and the

spk_0 electro weak scale indeed an enormous amount of effort over the past 30 or 40 years has gone into doing exactly that indeed the famous hopes for super symmetry being discovered at large had run collider were based on the idea that super symmetry could help explain the hierarchy

spk_0 between the electro weak scale and the Planck scale and if it did it would predict a whole bunch of new particles to be discovered near the Higgs boson in mass there were other possibilities involving extra dimensions of space and things like that that also could plausibly explain the hierarchy problem and also predicted new particles near that energy scale.

spk_0 We turned on the LHC the large had run collider at CERN in Geneva we spent $10 billion building it part of the motivation for building the LHC was to search for the Higgs boson itself but a big part of the motivation was to solve the hierarchy problem was to solve the fine tuning problem given to us by the difference in energy scales between the electro weak theory and the Planck scale because almost all of the favorite solutions predicted new particles there to be discovered.

spk_0 We have discovered none of them now look maybe we'll discover one tomorrow that is possible maybe just haven't found it yet that's conceivable but you know we absolutely could have found it the day after turning on the LHC back in like 2008 or whatever was as soon as we turned on the LHC the first time it blew up and we took a while to fix it again but by 2008 was really running so that's another cautionary tale as far as physics and fine tuning is concerned we had a fine tuning where

spk_0 there were a handful of different theories that could have reasonably explained it they all made certain kinds of predictions and none of them apparently is right as far as we can tell that again that might go away we the LHC doesn't have infinite reach only reaches a little bit beyond where the Higgs boson is so I would say we have a dynamical theory with issues in the case of the electro weak scale

spk_0 and then you know there's just a different sort of sense of some of these problems because some of them are relevant to the existence of life like the neutron and proton masses the cosmotrol constant is relevant to the existence of life if the cosmotrol constant the vacuum energy were really really really big it would explode the universe apart that's such an enormous rate that you couldn't make atoms much less stars or galaxies or living beings or anything like that if the cosmotrol constant would be a real big point of view of the world and that would be a real big point of view of the world.

spk_0 So if we were huge in magnitude and negative in value it would cause the universe to recalapse really really rapidly and that would not give you enough time for life to develop so something like the cosmotrol constant the vacuum energy the energy inherent in space itself turns out to be sneakily relevant to the existence of life whereas other things like the strong CP problem the value of the parameter that violates the CP transformation symmetry in the strong interactions

spk_0 that has nothing to do with life it wouldn't matter at all if it were that's natural value so there are dynamical theories that help explain it the axion particle is a particle you might have heard of it's a very popular candidate to be the dark matter

spk_0 but the reason why the axion was initially proposed was as a dynamical explanation for why there is no CP violation in the strong interactions a dynamical explanation for that particular apparent fine tuning

spk_0 so if that turns out to be right we don't know whether axions exist or not one of the reasons they're so popular they're popular as a particle physics idea because they help explain that strong CP problem

spk_0 and then later it was realized they could also be the dark matter and then later it was realized that the other popular dark matter candidate the weak interacting massive particles could have already been discovered and haven't been yet so they become less popular so the axion is getting a resurgence in popularity as a possible explanation for the dark matter

spk_0 if that turns out to be true then what looks right now to us like a fine tuning will be explained dynamically the axion is essentially a field that rolls in a potential you know the usual picture you have of a ball rolling in a hill

spk_0 and you can explain very very naturally why the bottom of the hill is a place where the effective violation of CP is minimized in the strong interactions it's very very close to zero basically the axion takes whatever violation of CP you thought you had and it cancels it out almost exactly

spk_0 and so that will be very very natural and good for all sorts of reasons we just haven't quite detected it yet so you see it's a wide variety of things going on right I mean it's not at all crazy or misguided to talk about fine tunings in particle physics and cosmology they are there when I say fine tunings are there I don't mean God did it I don't mean they're necessarily there because life exists I just mean there are a lot of things that are not going to be there

spk_0 but more than one numbers in the fundamental constants of nature that have values that seem very wrong to us and that is something that should be addressed by fundamental physics by thinking about it in some way and again the ways for the answers for the different fine tunings might come from different directions that will be perfectly okay

spk_0 or there might be one giant theory that explains them all I think it's actually much more likely they they come from different directions because these problems are so different from each other then there's one giant theory but you know we keep an open mind

spk_0 so now finally I get to talk about what I wanted to talk about a little bit which is and it won't take too long to worry what the answers could possibly be okay what are the possible attitudes we can have towards these different fine tunings

spk_0 and I can I think that there's basically four different attitudes one can have I don't want to call them solutions because people can debate whether some of these attitudes actually even count as solutions or explanations but their attitudes people can and do have some of the attitudes are literally theoretical constructions so they wouldn't normally be called attitudes but okay we have to call them something scenarios if you want to call them that

spk_0 one scenario is for each of these fine tunings there is a dynamical theory that explains why this apparently small number has the value it does let's get into that in a little bit more detail in a second but let me go through the different scenario so one scenario is the existence of a dynamical theory like inflation for example

spk_0 the second scenario is there's a multiverse this is not the quantum mechanical many worlds that we've talked about lots of times here on the podcast in quantum mechanical many worlds if I measure the spin of an electron that spin up and spin down then there's one world in which it's spin up and one world in which it's spin down

spk_0 but the fundamental parameters of physics are the same in those two worlds this is not moving me in the direction of explaining any fine tunings anywhere okay in the cosmological multiverse is a very different story people noticed soon after inflation was proposed that in most models of inflation and I'm not quite sure what the word most means there but in lots of models of inflation

spk_0 inflation keeps going forever it ends in some places and keeps going in other places and where it keeps going it can sort of end differently in different places this is called eternal inflation and you marry this to something called the string theory landscape the string theory landscape

spk_0 does end when it turns into matter and radiation the local laws of physics in the region where it ends can be very different in one region of the universe than somewhere else by the local laws of physics I mean the parameters exactly what we're talking about the vacuum energy the mass of the quarks and the leptons the strength of the constants of nature like the fine structure constant

spk_0 even things like what forces of nature there are you know we have the famous su3 cross su2 cross u1 standard model of particle physics but maybe in another region of space very very far away the forces of nature are things that are completely different than those or at least a little bit different even the number of dimensions of space in these models you know we have a world where there are three dimensions of space all around us and string theorists believe there's at least six dimensions that are in the form of space

spk_0 hidden from us curled up in very tiny balls maybe somewhere else there are six dimensions of space that everyone sees and three dimensions are curled up into little tiny balls so this cosmological multiverse two aspects of it that are really important one is conditions are very different literally in different regions of space it's really not a multiverse at all okay it's just things are so far away much further away than we could ever get to moving slower in the speed of light that for all

spk_0 intents and purposes we're labeling different regions of space with different local laws of physics as different universes different pocket universes Alan goof calls them okay and the other is it's not a theory in the sense that you don't start by saying well what if there's a multiverse okay the theory is eternal inflation plus string theory or some substitute for string theory that gives you some landscape of different possible local laws of

spk_0 physics which people can invent and have invented that the multiverse is a prediction of such models it is not a theory by itself the theory is inflation plus the landscape and both of those theories inflation plus the landscape were invented for other reasons they were not invented to get you a multiverse in that sense it is very very analogous to the many worlds of quantum mechanics and quantum mechanics we didn't get many worlds out because we wanted to you ever didn't

spk_0 say can I sit down and come up with a theory that invents a multiverse he noticed that his multiverse was predicted by the Schrodinger equation likewise the cosmological multiverse is predicted by eternal inflation plus a landscape it's relevance to fine tuning happens if the fine tunings are relevant for the existence of life if you have a cosmological

spk_0 multiverse you can say well some places in the multiverse life couldn't exist and they have certain constants of nature some other places life can exist they have different constants of course we're going to find ourselves in the regions that can have life

spk_0 and I'll talk about that in a little bit more detail in a second that's the anthropic principle at work the third scenario is theism god did it the argument from design

spk_0 and again I take this argument seriously I'm not convinced by it for reasons that will become clear but it's it plays by the rules this argument I think it's completely wrong to think that this argument is just non scientific

spk_0 I think that the people who are going to say that is non scientific do so because they don't like the conclusion that it's trying to reach I don't agree with the conclusion it's trying to reach but I disagree for scientific reasons and I can try to explain what those are

spk_0 but the argument from design as a explanation for fine tuning should be pretty straightforwardly obvious to the extent that some of these apparently fine tuned parameters of physics and cosmology are indeed necessary for our existence here

spk_0 it's not completely crazy to suggest they were set that way by somebody by an agent by an intelligence with intentionality and with purpose and with the goal of creating a universe in which creatures could exist right

spk_0 that's just the argument from design and I think that it's not dismissible even though I am ultimately not persuaded by it

spk_0 and then the fourth scenario is we just got lucky there is no explanation I think that even though some of these numbers are kind of wild 10 to 10 and 120 and things like that

spk_0 it's absolutely allowed to just say there's no explanation for these things none of these is a disagreement between theory and experiment because we don't have a well tested theory or even a very respectable theory that predicts the numbers

spk_0 we're talking about all it is is a disagreement between observation and our guesses and so one very plausible attitude is your guesses were wrong deal with it our job is to measure these things not to worry that they weren't the values that we would have guessed had we measured them

spk_0 and I'm not making this up there are people who absolutely have these attitudes and I think that it is a plausible attitude just like the argument from design I'm not persuaded by this attitude but I don't think it's disreputable to think that so we should give it its due

spk_0 so with those four scenarios on our plates the existence of a dynamical theory explain the fine tuning a multiverse plus anthropic explanation a theistic argument from design explanation or we just got lucky shut up and measure rather than shut up and calculate

spk_0 let's think about these different possibilities in comparison to each other let's think about them a little bit more carefully so let's think about the possibility that there are dynamical theories that explain some of these fine tunings

spk_0 you know I think this is the one that everyone agrees is very respectable right like when you see a number and it's very very small if you can come up with a theory that provides an explanatory framework that says you know what

spk_0 you shouldn't have been surprised by that in the first place everyone's happy this is why people like inflation so much right we don't have direct evidence that inflation is right we have indirect evidence that it's right it's pretty compelling in some ways there's some sort of theoretical shortcomings of it that I've talked about before

spk_0 but it really makes things nice and easy you know what again when I was a grad student in the 80s and 90s astronomers didn't like inflation why didn't they like inflation because as far as they could tell inflation made one prediction

spk_0 namely the universe was spatially flat you can convert that prediction the universe is spatially flat into a prediction for the energy density of the universe

spk_0 and they measured the energy and see the universe they had gone out and counted the stars and measure the dark matter and all that and they didn't get the right answer they only got like about 30% of the way there

spk_0 so as far as astronomers were concerned you made one prediction it was wrong I'm not going to give you a lot of credit for that but of course by the end of the 90s in the early 2000s we realized that the other 70% of the universe was dark energy

spk_0 and indeed that one prediction inflation had made comes spot on and so that's actually pretty good and now astronomers all believe that inflation is right that's kind of the gold standard even though I personally am still a little bit less willing to declare inflation completely successful

spk_0 this is what you're looking for you're trying to invent some dynamical theory that makes these things explained in a very natural way

spk_0 the problem is they're hard to find right inflation works pretty well like I said the theory that we have for the mass of the proton and QCD and a running of the coupling constants that works perfectly well that's wonderful

spk_0 but what about like the entropy of the early universe right I mentioned this because I care about it I've tried very hard to explain the entropy of the early universe

spk_0 and as I try to say very honestly I think that my explanation the one that Jennifer Chen and I put forward 20 years ago is on the one hand still the best explanation on the market on the other hand still not great as an explanation

spk_0 I mean it might be the right explanation I absolutely believe it as a chance of being correct but there's an enormous amount of ill understood physics that goes into it

spk_0 and so it's not like it's so good that people should think that it's the right answer yet I mean maybe we'll get there someday

spk_0 the point is just that even though you try very hard to do it coming up with explanations for these things is difficult

spk_0 as I said before no one has come up with a good dynamical theory that explains the value of the cosmological constant

spk_0 even though it's been a super interesting problem for 50 years now

spk_0 and so we should look for those dynamical theories but we can't be guaranteed to find them so we should also be open to other possibilities

spk_0 so let's contemplate the anthropic explanation the multiverse explanation

spk_0 the one example that should always be contemplated when you're thinking about the multiverse and the anthropic principle is Stephen Weinberg's prediction of the cosmological constant

spk_0 so to set the stage it is 1987 so it's more than a decade before we actually gathered evidence in favor of the cosmological constant

spk_0 but we already knew about the cosmological constant problem okay so we knew that given the current constraints that existed at the time

spk_0 the cosmological constant couldn't be bigger than 10 to the minus 120 its natural value that's the cosmological constant problem

spk_0 and as I mentioned almost every one thought even though we don't know what the answer is the eventually will find the answer and it'll be zero

spk_0 there'll be some symmetry some dynamical mechanism that we don't know about yet that will set it equal to zero

spk_0 there's another reason which I could have included among the fine tunings but it's a little messier so I didn't

spk_0 there was another reason why people thought that the cosmological constant was probably zero not just this idea of the space of all possible theories

spk_0 the given the limits the observational limits we had in the 1980s for the cosmological constant

spk_0 if it were noticeable if we were able to detect it there would be what is called the coincidence problem

spk_0 it would be that the cosmological the vacuum energy the energy density in vacuum the cosmological constant is of the same order of magnitude in size as the matter density of the universe

spk_0 and the weird thing about that coincidence between the matter density and the vacuum energy density is that one of them the matter density changes rapidly as the universe expands

spk_0 while the other one the vacuum energy density remains constant so the statement the vacuum energy and the matter density

spk_0 or of the same order of magnitude in size is a temporary statement in fact in a in a real sense it only describes a relatively short period in the history of the universe

spk_0 and in order for the cosmological constant to be detectably large it's true that we live in that period that's the coincidence why would we be so lucky to live in the period where the vacuum energy is exactly that size

spk_0 okay so that's that was the sort of mind state that people were thinking about in the late 1980s and Steven Weinberg had you know he knew about inflationary cosmology and the possibility of a multiverse

spk_0 and he thought about the cosmological constant himself for other reasons so he decided to be a little bit more systematic than average in thinking about what you should predict the cosmological constant to be if there is a multiverse

spk_0 he didn't have any specific model of the multiverse or the string theory landscape or anything like that he just imagined that there were different regions of space and in different regions of space or different

spk_0 some things different universes one way or the other the cosmological constant was different okay that's all he imagined he did need to be a little bit more specific in that he needed to give a probability distribution or sort of a frequency a relative number of universes where the cosmological constant was one value versus another

spk_0 but he made the following clever argument he said look the allowed range that we know the cosmological constant is in is incredibly tiny compared to its natural range right it's plus or minus 10 to the 120 or 10 to the minus 120 times its natural range

spk_0 so if you think about the some true distribution of universes with the true distribution of cosmological constants there's some curve right some probability density of being in one universe versus another and you're sampling that curve over an incredibly tiny window

spk_0 10 to the minus 120 of its distance from left to right and so he knew that if you sample a curve that is relatively smooth over a very very very tiny interval that curve will look like a constant it will just look flat unless it's like really wildly varying at every point which seemed unlikely seem unmotivated anyway

spk_0 so Weinberg said look within the allowed window I am perfectly allowed justified in assuming there is a constant probability distribution a flat uniform probability distribution a measure if you want to call it that on the set of possible cosmological constants

spk_0 and then he knew enough astronomy to say the following thing I can calculate I mean I can't strong Carol but I Stephen Weinberg can calculate the number of densities of galaxies that will form in a universe with a given value of the cosmological constant he could do it back of the envelope pencil and paper he and some collaborators actually did better computer simulations afterward

spk_0 but even just using pencil and paper he could say if the cosmological constant gets bigger and bigger it gets harder and harder to make a galaxy the galaxy is trying to pull themselves together the cosmological constant trying to push them apart so there's a competition and when you get a larger and larger cosmological constant you get fewer and fewer galaxies okay

spk_0 you see where this is going you get the largest number of galaxies when the cosmological constant is smallest and so they said I can use these data to make a prediction with one extra assumption the extra assumption which deserves a whole podcast on its own I'm not going to go into it right now is that you and I are randomly chosen from the set of all of these possible observers

spk_0 so there's some observers in universes with zero cosmological constant there's some observers in the universe where the cosmological constant is 10 to the minus 121 times its current value there's universes where it is what we think it is now 10 to the less 120 there are fewer observers in universes where it's 10 to the minus 119 that's a slightly bigger value of the cosmological constant which pushes galaxies apart makes it harder to form stars and therefore form observers

spk_0 so basically he used the number of galaxies as a proxy for the number of observers he assumed we were typical in that distribution and he made a prediction what is the likely value of the cosmological constant the answer is zero for the likely value overall

spk_0 but then you can ask ask okay what's the typical deviation from zero right a small positive number is allowed a small negative number is allowed and what he found given all the many many huge uncertainties and what he's doing is at the ratio of the cosmological constant to the matter density in a typical observer's universe should be between zero and 10

spk_0 the cosmological constant should on the one hand not be very big compared to the matter density but on the other hand there's no anthropic reason for it to be very small compared to the matter density either and this is 1987 he's saying this so he makes a really provocative point using this anthropic reasoning in 1987 he says if this is the reason why the cosmological constant is small we should probably be able to observe it someday

spk_0 that was a very radical thing to say in 1987 and guess what he was exactly right I mean the observed ratio of the cosmological constant to matter is about three if you pick a random number between minus 10 and 10 and you get three you are not surprised by that that's not a fine tuning that's a perfectly legitimate thing for it to be now this is on the one hand amazing that he got it right and it's also provocative it makes you think it's certainly not a reason to declare victory

spk_0 okay you can't just say well this is the right answer we know it now there's a lot of assumptions that went into Weinberg's argument that I think are questionable what is that probability distribution are we typical observers and so forth are there other things that you should also be scanning over as people say like what if you just had bigger density fluctuations in the early universe then you could make more galaxies do you are you saying your theory predicts the largest possible density fluctuations because that doesn't seem to be true

spk_0 so there's a lot of work to be done a lot of provocation out there still 10 years before the observation he made the prediction he was right in science you always get points for that so it's at least one example we're in a temp at reasoning anthropically in other words reasoning by saying there's a lot of different values in different parts of the universe we are typical observers what should we predict that kind of reasoning worked

spk_0 and I want to emphasize that the anthropic principle sometimes gets a bad rap for being kind of total logical like people think of it as saying life exists only when life can exist I mean that would be total logical right but this is more than that clearly I hope I hope that it's clear that this is more than that this is not just saying we exist where we can exist this is saying we should reason as if we are typical observers in some ensemble and that allows us to make quantitative predictions

spk_0 for things that have different values within that ensemble that's not total logical at all that's actually quite powerful it only makes sense it only works if the multiverse exists okay and that has its own problems for all sorts of reasons also just to say you know there's people who don't like the multiverse idea because they don't believe you can calculate anything like Weinberg did calculate something

spk_0 but there's kind of an infinity divided by infinity problem that he lost over a little bit if you truly have an infinitely big universe then the number of people who observe a cosmological constant like us is infinite

spk_0 and the number of people who observe a cosmological constant ten times bigger is also infinite or a hundred times bigger is also infinite is it really legit to say that it's more probable that we observe what we observe

spk_0 you're taking infinity by infinity and you might think well maybe I can be careful and regularize and take a limit guess what people try to do that doesn't really work it's what is called the cosmological measure problem is one of the things we talked about in the class in my philosophy of cosmology class

spk_0 it turns out to be really difficult to wrangle these infinities and their ratio down to a finite number not that it can't be done but there are people out there who despair or ever being able to do it

spk_0 and they therefore think that the multiverse is just not predictive at all maybe they're right or maybe we're just not being clever about it quite yet enough

spk_0 okay let's switch here to the very different idea of theism or the argument from design you know this goes back to William Paley back I don't know 1800s I think just before Darwin

spk_0 he was making this idea he was a theologian who said look if you're walking down the beach and you stumble across a rock you go ah there's a rock and you go on your way it's not like a big deal you see rocks in the beach all the time

spk_0 but if you stumbled across a wristwatch lying there on the beach you wouldn't think to yourself that the random motions of the surf and the sand and the wind and the earth's natural ways of being had randomly put together a wristwatch

spk_0 you would think rather that someone had built it and then someone had lost it there on the beach why because the wristwatch is an exquisitely designed mechanism every piece inside the wristwatch fits together in a certain way for a certain purpose

spk_0 and that is the sign in Paley's view that it was the it outcome of a designed process not a random process and of course what he was having in mind was biology

spk_0 what he was thinking was biological organisms are also exquisitely organized machines clearly this is evidence for the existence of a creator who designed them

spk_0 and Charles Darwin more or less came up with a better explanation not too long afterwards so that lost a little bit of power but maybe you can use the same logic for the universe as a whole

spk_0 people like Robin Collins who is a contemporary theologian have done things like this so Collins made the case look what if we eventually do get on a rocket and travel to Mars

spk_0 and when we go to Mars we find not just what we already know is there but also a habitat like an artificial looking habitat like you know a dome that has air in it and there's a source of water and there's food and things like that

spk_0 and that is exactly right for human beings to step into would you think that that was just something that randomly arose due to the Martian version of geology and plate tectonics and things like that?

spk_0 no you would think that someone had put it there either human beings had gone secretly before that or aliens did it for us or some supernatural force did it but you wouldn't think that it had just happened

spk_0 and in Collins's view again the universe is like that the universe is a hospitable habitat for humanity that needed to be designed in order for life to be possible

spk_0 so the idea of all of these arguments you know they sort of change with time as our scientific understanding improves but the idea one way of thinking about it is to be a good Bayesian right?

spk_0 you know what it means to be a good Bayesian you have a different set of propositions and you assign prior probabilities or prior credences to these propositions and then you update your priors on the basis of new data

spk_0 so in this case the propositions are theism that is to say that there exists a godlike supernatural being that cares about us and shapes things for our benefit or naturalism

spk_0 the idea that there's just a natural world obeying laws of physics nothing more

spk_0 and the point is of the fine-tuning argument for the existence of god that these observed fine-tunings in nature in physics and cosmology have the feature that if they weren't there some of them anyway life would be impossible

spk_0 if the cosmological constant were its natural value life would be impossible if the neutron were much much heavier in the proton life would be impossible

spk_0 if the entropy of the early universe was maximal that is to say if the universe was just in thermal equilibrium all the time life would be impossible

spk_0 so what they claim is that these fine-tunings are evidence that naturalism is getting the wrong answer

spk_0 if your prior for naturalism was a similar order of magnitude to your prior for theism so the argument goes you should update it on the fact that life exists

spk_0 and the probability that life exists under theism is of order one our notion of what god is includes the fact that god wanted us to be here

spk_0 the fine-tunings are evidence that the likelihood of life existing under naturalism in the set of all possible worlds where all these physical parameters could have taken on different values is infinitesimally small

spk_0 so when you update your prior is using basis theorem if the priors are similar the likelihood that life exists under theism is so much bigger than under naturalism that you end up concluding that theism is true

spk_0 that's the argument from design

spk_0 okay I'm obviously skimming over some details just because you've heard things like it before the details matter to the experts for sure

spk_0 but I don't believe it so and it's my podcast so I'm gonna tell you why I don't believe it I think there's a lot of issues here

spk_0 some of the issues that are more obvious issues are not actually the most important ones

spk_0 the most obvious issue is do you really think you know what the probability of life existing is under naturalism

spk_0 that is to say how well do we really know that if the constants of nature were different life would be impossible

spk_0 I on the one hand I try to be humble about our knowledge I do think that life could be possible under a wide variety of circumstances that we don't actually know about

spk_0 let's put it this way if someone just handed you the standard model of particle physics without any knowledge of the macroscopic world and said is life possible under these laws of physics

spk_0 you would have a difficult time showing the life would be possible so it's possible that under very different values of the parameters life is still possible

spk_0 but nevertheless I take the gist of the argument the strength of it the thrust of it because some of these fine tunings are both very big and very blunt

spk_0 you know the cosmological constant if it were 10 to the 10 times its current value I cannot imagine a way that life would be able to exist

spk_0 much less 10 to the 120 times its current value so I think it's okay I'm gonna give him that I'm gonna grant the theologians and theists out there

spk_0 the probability of life given theism in this perspective is the greater the probability of life giving given naturalism

spk_0 the reason I don't accept that conclusion all the way is of course the multiverse is of course the anthropic argument

spk_0 if you think about the probability of life given naturalism as the probability of life given a single universe cosmology

spk_0 times your prior for a single universe cosmology plus the probability of life given a multiverse cosmology

spk_0 times your prior for a multiverse as long as the multiverse has some substantial prior probability to it

spk_0 the probability of life given that we're in a multiverse is very very large

spk_0 right somewhere in the multiverse you're bound to have parameters of nature that allow for the existence of life

spk_0 so that's why people who are using the fine tuning argument to argue for the existence of God are so dead set against the multiverse

spk_0 they can only sort of get their argument off the ground if you think the multiverse doesn't work for one reason or another

spk_0 just to give you one example of someone who doesn't think it works who wants to get the theistic answer

spk_0 former mindscape guest Philip Goff has been writing about the fine tuning argument for the existence of God

spk_0 and for the anthropic multiverse argument he he makes an interesting and I think ultimately not very convincing argument

spk_0 but I'll give you a brief idea of what it is he says that the anthropic argument is an example of the inverse gamblers fallacy

spk_0 what is that so the gamblers fallacy is when you have some random process going on and there's some result you're hoping to get

spk_0 and you've gotten it even less than you should have by sheer random chance

spk_0 so let's say you're rolling two dice and you want double sixes and you should get that one out of 36 times

spk_0 but you've rolled a hundred times in a row and you've gotten it not at all

spk_0 okay so that's less likely than you would have predicted

spk_0 the gamblers fallacy is I'm due because it hasn't happened yet

spk_0 it's probably more likely than average to happen next time that's a fallacy if you actually have a good random number generator

spk_0 or random stochastic process it's still a one and 36 chance every time even if you haven't had it that's the gamblers fallacy

spk_0 the inverse gamblers fallacy is this weird thing it's kind of unconvincing I don't know why anyone would fall for it

spk_0 but the idea would be if someone walks into a room I'm not exactly sure what the set up so is BC

spk_0 you walk into a room you see someone rolling dice and they roll double sixes the first time

spk_0 okay the inverse gamblers fallacy says well that was really unlikely that you would have rolled double sixes in just one roll

spk_0 therefore I conclude you've been rolling for a long time there were a lot of rolls prior to this

spk_0 because that's the only way to make it likely there would have been double sixes on the roll that I saw

spk_0 this is clearly a fallacy I mean I think I agree like this is such a bad fallacy I can't imagine anyone getting it

spk_0 just because you see an unlikely number doesn't mean there it you happen to see the last example of a large series of trials of that process

spk_0 okay one way or the other you only saw one of them now you kind of see the analogy with the anthropic principle

spk_0 Philip is saying look we see one trial we see the universe we're in it seems unlikely to us

spk_0 and therefore the multiverse people are saying therefore there must be a multiverse but that's just the inverse gamblers fallacy

spk_0 I think it's just not a good analogy I think that that's not what the multiverse people are saying

spk_0 because it's different seeing someone roll double sixes than seeing life existing the question you're asking

spk_0 in the cosmological example is different than the question you're asking in the dice example

spk_0 in the dice example you've seen one roll and it happened to be double sixes and you're saying what does that tell me about other rolls

spk_0 the answer is nothing in the cosmology example you're seeing that life exists and you don't know how many

spk_0 universes there are out there but you know that somewhere in the ensemble of universes life exists

spk_0 so the question you're asking now is not how likely is it or what can I conclude from the fact that in my universe life exists

spk_0 you're saying that as grand cosmological scenarios go if I'm comparing a cosmological scenario with a multiverse

spk_0 in which in different parts of the universe different cosmological parameters take different values everywhere

spk_0 versus a cosmological scenario with just one universe and one randomly selected set of constants of nature

spk_0 which one is it likely for life to exist anywhere

spk_0 that's the difference and why it's not the gamblers fallacy because you are by the nature of being a living being

spk_0 post selected to be only in the part of the universe you're looking where life exists

spk_0 that's not the case in the dice example there was no principle of physics

spk_0 it said when you walked into the room you were definitely going to see the example where it's double sixes

spk_0 in the universe you're definitely going to find yourself in the case in the part where life can exist

spk_0 so if your universe is amenable to the existence of life that theory fits the data better

spk_0 so goes the anthropic reasoning

spk_0 so I don't think that that argument really works the inverse gamblers fallacy argument

spk_0 but you kind of see the philosophical to and fro that is necessary to think about this

spk_0 I think that it's actually it's funny because Philip will say and other people will say

spk_0 that the fine-tuning argument does give you our evidence for the existence of God

spk_0 he will you will buy into the general design argument

spk_0 but that would be if the inverse gamblers fallacy argument worked at all

spk_0 just as bad for the theistic argument I mean one way or the other

spk_0 you can't reason from this one particular thing that we've observed

spk_0 I do think that the that that Phillips argument is right next to an important argument

spk_0 which I actually don't know how to think about which is the issue of old evidence in Bayesian reasoning

spk_0 this is an issue that was brought up by philosopher Clark Glimor all the way back in the 1980s

spk_0 he said the following look what if you you're trying to be a good Bayesian

spk_0 which means you have some priors collect some new data update your priors on the basis of the likelihood functions

spk_0 that's how it's supposed to work what if you had the data all along

spk_0 what if you always knew the data the example he had in mind was actually Albert Einstein inventing relativity

spk_0 and knowing the procession of mercury

spk_0 Clark Glimor claimed that Einstein couldn't really count the procession of mercury as a successful prediction of general relativity

spk_0 because he already knew it was true and people have gone back and forth about whether or not that counts

spk_0 I think it doesn't count I think that people correctly say that Einstein might have known that the procession of mercury was off

spk_0 but he didn't know that his new theory predicted the right one

spk_0 that calculation that he did counts his new information that is perfectly fair for you to update your priors with

spk_0 but the basic issue of old evidence is still there and it is especially

spk_0 important for anthropic reasoning there's no evidence older than the fact that we exist

spk_0 you know we wouldn't have be having this conversation if we didn't exist and I think this is sort of spiritually

spk_0 if you'll forgive the word what Philip golf is getting at

spk_0 that it is not quite fair maybe to use our existence as evidence of anything at all

spk_0 because we literally couldn't be having this conversation without that

spk_0 it's a precondition not a surprising experimental result

spk_0 at least maybe like that's an argument that I could at least take seriously

spk_0 I don't think it's true I don't think that's what I just said is correct the right way of thinking about it

spk_0 I think there's better ways of thinking about it but it's on the table there as something to think about

spk_0 let me I don't have that much time left sorry I've scheduled myself badly here

spk_0 so I actually had to wind up a little bit so let me get to what I think is actually the more important worry about the argument for design

spk_0 from yeah argument from design the argument from design is stated as an argument about the probability of life existing

spk_0 under theism versus naturalism but the fine tunings that are purportedly necessary for life to exist

spk_0 aren't necessary for life to exist under theism that is to say these fine tunings with the cosmological constant the mass of the neutron or whatever

spk_0 what do they really allow to exist what they allow to exist are physical configurations of matter

spk_0 that come into the form of complex information gathering adaptive systems right you can make atoms and chemistry and molecules and cells

spk_0 and biological organisms all without violating laws of physics that's what the fine tunings allow for but guess what

spk_0 you only need to have physical configurations that are complex adaptive information gathering systems under naturalism

spk_0 you don't need that under theism the fine tunings are not evidence that souls are possible because the value of the cosmological constant is so small

spk_0 god could have made life no matter what the constants of nature were if you any of you saw you know we had Daniels as podcast guests they were the writer directors of everything

spk_0 everywhere all at once there's a funny scene where there's two people talking to each other and both people are portrayed as boulders on a beach wearing googly eyes

spk_0 in the world where god exists that's totally plausible he's god he can do anything he's not bound by the laws of physics he could attach individual souls or essences or minds to any configuration of stuff he wanted

spk_0 doesn't have to be complicated get able to gather information adaptor any of those things so secretly the fact that the constants of nature are so amenable

spk_0 to precisely the kind of complex systems that would be life like without any supernatural help is actually evidence for naturalism not for theism

spk_0 especially when you take into consideration all of the other ways I think that if you're going to play this game at all you have to be good Bayesian about it

spk_0 okay and that means not only updating your priors but updating your priors with all of the evidence that you collect and here gets is where it gets very very tricky you're trying to say what kind of universe would you expect given theism given the existence of god

spk_0 and you're absolutely welcome to say well I don't know I don't know gods intentions that's fine if that's your attitude hundred percent fine

spk_0 but don't tell me that the argument from design works because the argument from design relies on knowing gods intentions and deriving the probability of life existing under theism is large okay

spk_0 but if you do know gods intentions then why is god making all these galaxies why is god doing things so extravagantly why is so much of the universe inhospitable to life right

spk_0 and you might say well no no god would have done it that way okay but guess what you had a chance to say that before we discovered that there were other galaxies and other planets and things like that and nobody did

spk_0 before the existence of modern astronomy the conventional way of thinking about cosmology was the earth was all there was right that was the apparent prediction that theism was giving you not the universe that we see

spk_0 so I think if you're a good Bayesian and you take into account all the data we have about the universe it doesn't look like one god would have created

spk_0 so because of that and because of the fact that god could have done life even without all these fine tunings I don't think the argument from design really works so the last one then is we just got lucky

spk_0 right and I know people I some of the my favorite smartest physicist and cosmologists have a very explicit attitude that our job is not to predict these numbers our job is to measure these numbers

spk_0 and we have no right to have any expectation for these numbers like the cosmological const to be big small blue yellow green or anything like that okay and that's it there's nothing to be learned we move on with our lives

spk_0 I think that's wrong also I think that's a wrong headed attitude but I think I get why they do that because sometimes these fine tunings are portrayed like I said as disagreements between theory and experiment

spk_0 and we don't have a theory that predicts them so it's not quite right to think of them in that way what they are is surprising and I think that the surprisingness to us human beings isn't nothing it's not to be discounted and the reason why it's not to be discounted is we don't know the final true theory of the universe

spk_0 right we are not done yet with science and physics and cosmology we don't have our theories of everything we're still trying to work our way toward them and the importance of fine tunings in my mind is that they might be clues to finding the correct

spk_0 further future theories of everything the theories we don't have yet but let's put it this way forget about big numbers and small numbers what if we had measured the mass of the mu on which is exactly like an electron but heavier right it's the heavier cousin of the electron it's about 200 times heavier than an electron

spk_0 what if we had measured the mass of both the electron and the mu on and in this different possible world we're imagining the ratio of the mass of the mu on to the mass of the electron had been exactly pi it was 3.14159 I don't know to 10 decimal places okay for no good reason we just measured it we measured the mass of mu on measure the mass electron divided them by each other the answer was pi to 10 decimal places

spk_0 what are you going to say are you going to say oh we just got lucky that's nice makes it easier to have the particle data book table where we list the mass of the mu on it's just pi times the mass of the electron maybe or maybe you would say to yourself you know I bet there's an explanation for that I bet that there is a formula in a theory that we don't have yet that predicts mass of the mu on is pi times mass of the electron now that's not what we have we have these tiny numbers

spk_0 not these fun geometrical quantities but the point is that these apparent fine tunings to me are special places in the set of all possible future theories the set of all possible worlds if you like the set of all possible physical realities there's a subset of all the possible ones that have these explanatory relations in them that say oh here's why spatial curvature is small

spk_0 here's why the cosmological constant is small et cetera and if you find those and they're really there that teaches you something incredibly valuable that might be important for other reasons you know inflation was motivated first actually by they were certain grand unified theories that over predicted the abundance of magnetic monopoles in the universe and then

spk_0 gooth was extra motivated when you learn about the horizon and flatness problems and that was great motivation trying to explain these fine tunings but eventually it was realized that inflation actually offered something in addition it offered a predictive theory of the perturbations the density fluctuations that come out of the early universe and that we see today

spk_0 in the cosmic microwave background and in large scale structure so by taking a fine tuning seriously by looking for a theory that provide an explanatory reason for that fine tuning and by pushing that forward into regions you hadn't thought about before you learned something new about the universe that's why I think that fine tunings are important to pay attention to I'm not saying it can't be possible that we just got lucky

spk_0 it might be that for some of these values it's just how it is and you know they in the set of all possible worlds they could have been different life wouldn't exist in those worlds and too bad for them whether or not there is a multiverse maybe we're just lucky that the reality of the cosmos allows the existence of life even without a variety of different environments in different places

spk_0 but we don't know that and we might be missing a big clue about what the correct theory of the world is if we don't try to explain these fine tunings the explanation might be a new dynamical theory it might be the anthropic principle and the multiverse it might even be the existence of a supernatural designer or for that matter a college level computer programmer in a higher reality who's running a simulation and we live in that simulation

spk_0 you know the design argument works just as well for simulation as it does for the existence of God so I think all of these are worth thinking about carefully as I said I didn't try to pick on too many people I picked on Philip a little bit sorry Philip no hard feelings but I didn't try to pick on too many specific people

spk_0 but I do think that this whole field of fine tuning and anthropic reasoning and old evidence and basing updating I think there's a lot of work to be done to turn our philosophical musings into something more rigorous and useful

spk_0 and it might even help us predict things in the future for the next generation of experiments that's the best we can hope to do thanks for listening I'll talk to you next time bye bye

331 | Solo: Fine-Tuning, God, and the Multiverse

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