Theory of everything: do we really need one?

Interactive Transcript

spk_0 Great Mysteries of Physics is a series supported by FQXI, the foundational questions institute.

spk_0 A think tank and funding agency that explores the foundations and boundaries of science.

spk_0 Find out more at fqxi.org.

spk_0 Welcome back to Great Mysteries of Physics from the conversation.

spk_0 I'm Maryam Frankel and I'm your host.

spk_0 In this series we've explored five different but equally great enigmas of physics.

spk_0 But why is physics so full of mysteries? Is it an indication that it is in fact broken?

spk_0 That's what we'll discuss this time.

spk_0 Our two best theories of nature are quantum mechanics and general relativity,

spk_0 describing the smallest and biggest scales of the universe respectively.

spk_0 Each is tremendously successful and each has been tested experimentally over and over.

spk_0 The trouble is however that they clash.

spk_0 Quantum mechanics is riddled with randomness and tanglements and fundamental uncertainty

spk_0 that we don't see in general relativity.

spk_0 And while time in quantum mechanics is absolute, it is relative in general relativity.

spk_0 So physicists have long been trying to come up with frameworks for unifying the two into a theory of everything.

spk_0 Popular approaches include string theory or loop quantum gravity.

spk_0 But these theories apply on scales that are difficult to test experimentally requiring much more energy

spk_0 than we can currently produce in the lab.

spk_0 That said, physicists have managed to unite quantum theory with Einstein's other big theory

spk_0 that of special relativity. Together they form something called quantum field theory,

spk_0 which is the basis of the standard model of particle physics, which is our best theory to describe

spk_0 the most basic building blocks of the universe.

spk_0 And while the standard model seems to be able to describe a lot of the experimental results

spk_0 that we can actually produce, there are some gaps.

spk_0 And particle accelerators have failed to discover the very particles that would close those gaps.

spk_0 At the same time, recent results from particle physics experiments hint that there may be forces

spk_0 and particles still to be discovered, potentially even mandating new physics.

spk_0 So what's going on? Will physicists ever develop a theory of everything?

spk_0 What would happen if they didn't? And if they did, could we ever test it?

spk_0 Flatka Vydral is a professor of physics at the University of Oxford in the UK.

spk_0 He says that trouble with uniting quantum mechanics and general relativity

spk_0 is partly down to their different mathematics they use.

spk_0 I think even at a simpler level, possibly, before we even start discussing things like physical

spk_0 notions of space and time, you could actually argue that the two theories are based on different

spk_0 kind of mathematics, interestingly enough. So just if you look at it, not as a physicist necessarily,

spk_0 as a mathematician, you would say that general relativity is all about geometry.

spk_0 It's how space is curved and how space time, ultimately, this unified entity that contains

spk_0 three dimensions of space and one dimension of time is itself also curved.

spk_0 And in fact gravity is just a manifestation of this curvature of space time, all about geometry.

spk_0 Whereas quantum physics is actually all about algebra. It's what we call linear algebra. So they're

spk_0 even two different branches of mathematics, which is interesting. Even at that level, the question

spk_0 is how do we put this together now? Yeah, yeah. I think is why I always preferred quantum mechanics

spk_0 because I like algebra and I do not like geometry. By the way, it may explain why people like

spk_0 Roger Paners, who is extremely intuitive in terms of geometry. He's all about visual things.

spk_0 Why actually he tends to prefer general relativity and he thinks quantum mechanics will collapse

spk_0 ultimately. Interesting how it can come down to those kind of preferences that we intuitively

spk_0 feel closer to one of the two theories. That's a really interesting point. What about you?

spk_0 I am probably among people who weirdly enough may not think that there is a problem,

spk_0 at least in the foreseeable future. But you're saying you don't think there's a problem,

spk_0 but if there was a problem, what's your hunch, which one do you think is more likely to have to

spk_0 be modified? My intuition will be both. And it's simply based on a historical observation. I don't

spk_0 think I can base it directly on general relativity or quantum mechanics, but if you looked at how

spk_0 theories in the past were modified, even when we had a tiny discrepancy in our theories, even when

spk_0 something small could not be explained, then what it usually requires is a radical modification.

spk_0 And this we see both with relativity and quantum mechanics. They are not just small departures.

spk_0 So we have to change classical physics in two very different ways to arrive at relativity and

spk_0 quantum mechanics. And my feeling now is that the next revolution, if there is such a thing,

spk_0 I hope that there is such a thing. I'm almost betting on that. That revolution will somehow

spk_0 unify both into a completely different theory. And then you will take a special limit and

spk_0 derive general relativity in that limit. You will take another limit and derive quantum mechanics.

spk_0 But when you put them together, they will lead to some new entities. And then we will end up

spk_0 discussing the philosophical meaning of these new structures and what is the nature of reality and

spk_0 all of these questions would happen in the new theory. So, Flatko is a necessarily expecting physics

spk_0 to persevere with just small tweaks to our best theories. Just as general relativity and quantum

spk_0 mechanics in different ways ended the common sense physics before them. A new theory of everything

spk_0 may be a radical departure from the physics we have today. But before we get to that,

spk_0 let's just consider the standard model of particle physics.

spk_0 The starter model, I think most people would agree, is very heuristic in many ways. So

spk_0 it's the best description we have that unifies certainly special relativity, at least with

spk_0 quantum mechanics. What we call quantum field theory. But it's not a theory like quantum mechanics

spk_0 or general relativity. You're right, not really. It's a model or how would you?

spk_0 It's a model that actually contains probably far too many fundamental constants. That already

spk_0 shows you that somehow we don't really understand it very well because it really should boil down

spk_0 to possibly, you know, Newton's gravitational constant, the speed of light planks constant as we

spk_0 understand nature at present. But going beyond it, it seems unnecessary somehow. Which usually

spk_0 signals that it's not the most compressed theory that we could come up with. There must be something

spk_0 going beyond this. Fundamental constants are quantities that we have to measure from an

spk_0 electrons charged to the mass of a quark. There's simply no theory explaining what values they

spk_0 should have. So ideally, we need a deeper theory of everything to tell us that.

spk_0 That's it. So it says that we have these fundamental particles which constitute matter. And then

spk_0 we have particles which constitute what we would call forces. They're exchanged between these

spk_0 material elements, quarks and electrons. So we exchange photons for instance, which are the

spk_0 particles of light. And that's the electromagnetic force. Or you exchange gluons for instance,

spk_0 which would glue, as the name suggests. They would glue quarks together into protons and neutrons

spk_0 and then glue these together into atomic nuclei. So there are these handful of fundamental particles

spk_0 and the standard model contains them. But the interactions between them and their various properties,

spk_0 like I said, are not really explained. They're taken as given as constants if you like. But they

spk_0 look remarkably arbitrary if you think about it. It almost begs a question to go beyond it and to

spk_0 understand them. Why they are the way they are. Generable ativities sits outside the standard

spk_0 model of particle physics with quantum field theory, failing to describe the force of gravity.

spk_0 Here's Chanda Prescott Weinstein, an assistant professor in physics and astronomy and core faculty

spk_0 in women's and gender studies at the University of New Hampshire. She's also author of the

spk_0 disordered cosmos, a journey into dark matter, spacetime and dreams deferred.

spk_0 It doesn't come in at all. In fact, the astute science reading general public has probably heard

spk_0 many times over the last decade, particularly since we observed the higs at the large hydrant

spk_0 collider that the standard model is finished. It's complete. I could point to a couple of places where

spk_0 that's just like, paint and lean not true. Like what I think is turning out to be extraordinarily

spk_0 bad PR. I understand why people use that line, but it's track firing a lot. It is not true. But one of

spk_0 the things that it's missing is that it explains three of the known forces, but not the fourth one,

spk_0 which is gravity. So the three ones being electromagnetism and the two nuclear forces, the weak

spk_0 nuclear force and the strong nuclear force. Okay. So we have four known forces and so the standard

spk_0 model and quantum mechanics explain and work with three of them and then gravity in the fourth.

spk_0 General relativity and gravity. I find all of these nomenclature questions about do we call it

spk_0 relativity to be really interesting, but there are a lot of ways in which gravity is really weird.

spk_0 And one thing that distinguishes it as a mathematical picture and even as a physical picture is that

spk_0 general relativity is geometric by nature, right? So the way that people have maybe heard references

spk_0 to this is through the idea that space time is curved or that space time can curve and that really

spk_0 one of the lessons of Einstein's general relativity is that when there's a massive object in

spk_0 space time, it causes the space time to curve and the curvature of that space time tells the object

spk_0 how to move, which I'm very badly paraphrasing. I think John Wheeler, but there's a dynamical

spk_0 relationship there where they are creating movement in each other and it's a kind of dance,

spk_0 a partnership between space time and massive objects. The standard model is not geometric in that way.

spk_0 And so again, really, I think the thing that fascinates me about these questions is how do you

spk_0 bring those two into conversation with each other when they're really living in different mathematical

spk_0 worlds? So given that gravity is the odd one out, does that mean it is perhaps not a fundamental force

spk_0 unlike the other three? Or are quantum field theory and the standard model simply wrong? While the

spk_0 standard model has been enormously successful at explaining experimental results, it does contain

spk_0 a few gaps. And to bridge those, an extension called supersymmetry, suggesting that particles

spk_0 are connected through a deep relationship, has been suggested. According to supersymmetry,

spk_0 each particle has a super partner with the same mass but opposite spin. For example, the electron

spk_0 would have a super partner called the selectoron. Interestingly, supersymmetry is also an important

spk_0 feature of string theory. But so far, particle accelerators such as the large Hadron colliderate

spk_0 CERN in Switzerland have not found any such partners, despite being explicitly designed to do so,

spk_0 threatening both the idea of supersymmetry and string theory.

spk_0 I do think that we have to redevelop an appreciation for incremental learning and an appreciation.

spk_0 And this was actually something that I picked up from a conversation with a particle experimentalist

spk_0 at University of Texas, Peter Unisey, that not finding something is science. That's a piece of

spk_0 information. We now know that the particle does not have the particular properties we were looking

spk_0 for. That's information. So then what we got out of the LHC was the Higgs. And the Higgs boson was

spk_0 a huge accomplishment. That was really kind of like the cherry on the top in terms of the basic

spk_0 pieces of the standard model that we were looking for. But no supersymmetry observations emerged

spk_0 from any of the experiments that happened afterwards. So there are some people in the community who

spk_0 have this fact to argue that supersymmetry as a theory is dead. It's a hypothetical.

spk_0 And thereby also string theory? Yeah, it's possible there's a string theory out there that doesn't

spk_0 require supersymmetry, but not that I know of. So I think that that would really be kind of the

spk_0 nail in the coffin for string theory. I am actually not in the community of people who think that

spk_0 Susie, as people sometimes call supersymmetry, is dead and shouldn't be studied anymore.

spk_0 There's no cosmic rule saying that the energy scale of supersymmetry would be at the energy

spk_0 scale that the LHC was built for. So there is one model of supersymmetry, which was the simplest

spk_0 and lowest energy one that's basically been ruled out. Okay, but there are other models with different

spk_0 energy that just we haven't been able to probe. Yes, and I am a firm believer in pursuing what's

spk_0 possible until we know for sure that it's not possible.

spk_0 Even though the LHC hasn't found any super partners yet, it has come across a strange and

spk_0 normally lately. In fact, both the LHC and the Muon G2 experiment that's Fermilab in the US

spk_0 have discovered hints of new and surprising physics. Dithel studies from the LHCB experiment

spk_0 found that a particle known as the beauty quark, so quarks are particles which make up

spk_0 neutrons and protons in the atomic nucleus, decays into an electron much more often than it decays

spk_0 into an electron's heavier cousin called a Muon. And according to the standard model,

spk_0 that shouldn't happen, hinting that new particles, or even forces of nature, may be influencing

spk_0 this process. The uncertainty of this result is over 3 sigma, meaning that there's a 1 in a

spk_0 thousand chance that the result is a random fluke. And conventionally, particle physicists call

spk_0 anything over 3 sigma evidence, while 5 sigma would be needed for a confirmed discovery. And that's

spk_0 a 1 in a million chance that the findings are just random. The Muon G2 experiment, meanwhile,

spk_0 has recently investigated how Muon's wobble when magnetic fields interact with their spin.

spk_0 It found a small but significant deviation from some theoretical predictions,

spk_0 again suggesting that unknown forces or particles may be at work. And the chances of this

spk_0 discrepancy being a fluke is about 1 in 40 thousand, so also below the threshold of what can be

spk_0 considered a discovery. Thermalab has also made a surprising measurement of the mass of a particle

spk_0 called the W boson. And that suggests that the particle is significantly heavier than theory predicts.

spk_0 And this result is, in fact, impressive deviating by an amount that would not happen by

spk_0 chance in more than a million, million experiments. However, a reanalysis of old data from the large

spk_0 Hadron Collider's Atlas experiment just recently contradicted this by indicating that the particle's

spk_0 mass is in line with a standard model. So at the moment, we simply don't know, and the debate is

spk_0 likely to go on. The findings could be explained by an alternative theory to super symmetries

spk_0 suggesting there's a fifth force of nature. And that would mean that the Higgs particle may not be a

spk_0 fundamental particle, but instead made up of other fundamental particles bound together by this

spk_0 unknown force. But how seriously should we take this? Is it actually evidence?

spk_0 I think that would be amazing. Again, it would challenge this thing that now existed for well

spk_0 over a half a century that there are four fundamental forces, like I said. We're still uncertain

spk_0 about gravity. There are various views, but I think coming up with another force would definitely

spk_0 radically change this. It would make matters more complicated. I haven't taken this as seriously

spk_0 myself simply because I think already this question of gravity is very big. And I think with gravity,

spk_0 we literally have no experiments telling us either way. And it seems to me this really is a very

spk_0 pressing question. You mean how gravity affects particles that on the scale? Quantum mechanics,

spk_0 that's right. How gravity and quantum mechanics couple together? Can we explain gravity? Can we

spk_0 quantize it? Quantum mechanics describes light and matter which exists as tiny discrete chunks.

spk_0 So quantizing gravity essentially means cutting up space time into similar minute bits and making

spk_0 them obey the laws of quantum mechanics. But still though, I mean there's been a lot of debate

spk_0 about particle physics and these large expensive experiments not producing more particles,

spk_0 not finding evidence of super symmetry and stuff like that. But there's also been these

spk_0 interesting hints that there might be a fifth force of nature. So I mean, should we be more excited

spk_0 about that? Why is the discussion sort of about how particle physics has failed? Because it might

spk_0 be on the verge of finding something really revolutionary. You're right. Here is very difficult to

spk_0 make that judgment simply because these experiments are extremely complex. So even to come up with

spk_0 the next level of energy. So the whole idea of course is to probe smaller and smaller distances

spk_0 and shorter and shorter times. And because energy is inversely proportional to these things,

spk_0 this means that you require higher and higher energy. And of course, that becomes extremely

spk_0 complicated. Already these experiments that we have currently caused a huge amount of money,

spk_0 really. And it's not even clear whether we can scale them up to the next level, which is I think

spk_0 why people like me are looking for tabletop experiments that could tell us something. Maybe there

spk_0 are regimes that are simpler to access, but they could still tell us something. But even having said

spk_0 this, I think if we can, I think it's worth pursuing high energy physics experiments. It is

spk_0 possible that they will tell us something new at new energies. And I think something of the kind

spk_0 that you are describing of another force that we have not been aware of would be a huge momentous

spk_0 discovery. But you know, the decision and whether we go in that direction is at least of all based

spk_0 on science, isn't it? Because it's really to do with us as a society. Can we afford these things?

spk_0 And so if there were evidence of a fifth force of nature, would that say anything about quantum

spk_0 mechanics or general relativity or the various approaches to develop a theory of everything or

spk_0 a theory of quantum gravity? Would that given hint about where we're going to be? Oh, absolutely.

spk_0 Immediately. In fact, the first instinct would suggest that we should immediately think about

spk_0 this force in terms of the other forces as we understand them. So the first question there is

spk_0 can we think about a mediator of this force? Is there a kind of particle whose exchange

spk_0 leads quantum mechanically to this new force? So in other words, can we explain it in the same quantum

spk_0 mechanical way that we explain the electromagnetic interaction? That's always the case. Even the

spk_0 stronger and the weak forces are basically understood in exactly the same way as the electromagnetic force.

spk_0 If the answer is no, then this becomes very interesting actually because it would be a different

spk_0 paradigm. That would, to me then, suggest that possibly we have to modify the way we understand

spk_0 quantum mechanics. But the first instance, I wouldn't go in the direction of modifying quantum

spk_0 mechanics. I would simply ask could this new fifth force also be quantized in the same way that the

spk_0 other forces are quantized? If a discovery of a new force of nature was announced and this could be

spk_0 described by quantum mechanics, that would suggest that quantum mechanics is indeed fundamental and

spk_0 that any new forces or even gravity could potentially have quantum effects. But Chanda is still

spk_0 skeptical about the new experimental results. I need a five sigma result before I take anything

spk_0 seriously. It's funny because like I'm all for let's explore the thing until we are certain

spk_0 that we know it doesn't exist or until we discover it. And I'm fairly conservative when it comes to

spk_0 how much information I need before I agree that we have seen a thing that we have discovered a thing.

spk_0 So I'm all for tantalizing hints, but as far as I'm concerned, those are just exciting possibilities

spk_0 until I see like a five sigma. So where we at now three sigma? One of those things is three

spk_0 sigma. And I think what if it might have even been like 2.5 sigma? And fine, these are hints and they

spk_0 have not been proven or anything like that. We don't know what these anomalies are. But if

spk_0 you had five sigma showing these results, what would that mean and what could it be?

spk_0 The question as to whether something would be interpreted as say a fifth force or another particle.

spk_0 I think it gets really interesting and just to pick maybe a simpler example, there are changes

spk_0 that we can make to our models of gravity to relativity that depending on how you write them down,

spk_0 they look like you've created a new particle like phenomenon or you can actually rewrite them to

spk_0 look like changes to the rules that govern how space time curves. And so I think that there would

spk_0 actually be like some interesting questions there about like what's the proper interpretation of

spk_0 these results? In that scenario, I think it would be a very exciting opportunity for theoretical

spk_0 physicists because like the best time for theoretical physicists is the time when we have no idea what's

spk_0 going on. When we have like a new result that's not consistent with our old results.

spk_0 Chanda says there may also be ways to experimentally rule out any such theories rather quickly.

spk_0 This would involve monitoring the life cycle and evolution of stars.

spk_0 I will also just start by saying this is not my area of expertise, but I will tell you that as an

spk_0 outsider, the first thing that comes to mind is actually that where I would have questions is about

spk_0 stellar astrophysics. And the reason that stellar astrophysics comes to mind is because stars are

spk_0 where everything happens kind of all at once at the same time. You have really strong electromagnetic

spk_0 interactions. You have weak interactions. You have strong nuclear interactions. And gravity also

spk_0 plays a really big role. And you don't get what happens in a star without all four of those things

spk_0 working together simultaneously. And in a way that we are relatively good at modeling. So we know

spk_0 about how many neutrinos should be produced in certain reactions. And we know that fusion is

spk_0 happening and that that fusion happens in a particular sequence. And this is actually something

spk_0 we understand so well. And so I actually think one of the challenges faced by any kind of new

spk_0 discovery at this scale is whether it messes up any of our stellar astrophysics so sufficiently that

spk_0 maybe there will be a conflict between this experimental discovery and what we know about how

spk_0 stellar astrophysics works. Ultimately something needs to shift if we want to more fundamental

spk_0 understanding of nature. Something which could explain everything we see around us regardless

spk_0 of scale and including the particles and the standard model. But what exactly is a theory of

spk_0 everything? So what we really think about theory of everything is unifying all the four fundamental

spk_0 forces which is why frequently people talk about quantum gravity because gravity is the only

spk_0 outstanding force that we are not able to unify with the other three. So what does that really mean?

spk_0 It means writing down a quantity that goes under various different names in physics. It's a

spk_0 mathematical quantity called a Lagrangian or Hamiltonian or whatever you want to call it but it's a

spk_0 quantity that would actually contain all of these forces and which you could simply use to calculate

spk_0 any experiment in principle. Whatever experiment you want to perform whatever forces this experiment

spk_0 may contain and depend on this entity in this grand unified theory should be able to actually

spk_0 calculate ultimately. So that's kind of the whole thing of physics. It's of course a big question

spk_0 whether this is possible. So I think physics is clearly aware that this may well be just an

spk_0 intuition and a dream but it doesn't mean that nature works this way. You know there is nothing

spk_0 out there that really necessitates this kind of description.

spk_0 You're listening to great mysteries of physics from the conversations. But the clash between quantum

spk_0 mechanics and general relativity isn't the only mystery of physics. Chanda works on dark matter

spk_0 for example which is an unknown substance which makes up most of the matter in the universe.

spk_0 Similarly there is dark energy and unknown force causing the universe to expand at an accelerated

spk_0 rate which makes up most of the energy in the universe. So shouldn't a theory of everything

spk_0 explain these things too? I mean is it a theory of everything if it's not about most things?

spk_0 So dark matter and dark energy are most of the matter energy content in the universe. So it's not

spk_0 really a theory of everything if it's not accounting for most of the matter energy content in the

spk_0 universe. But it goes back to what we're saying about is it just to theoretically bring together a

spk_0 quantum and general relativity that might not necessarily explain what those things are.

spk_0 This is why I'm glad we don't actually use theory of everything in our work. But I think if we

spk_0 were to go out and declare to the public that something was a theory of everything

spk_0 that probably should do those things. That's one of the requirements if that's what we

spk_0 were going to be telling people it is. Otherwise I think you can call it like a fundamental theory

spk_0 of quantum gravity and that doesn't necessarily have to explain dark matter. I might argue that

spk_0 I feel differently about the source of the cosmic acceleration. So it's commonly referred to as

spk_0 dark energy and that actually can cause a lot of confusion because people might think that

spk_0 they're very similar phenomena. They are similar phenomena socially in that they're both things

spk_0 that we don't understand what they are. And that's literally when the cosmic acceleration problem

spk_0 and the question of why space time is not only expanding but the speed of that expansion is

spk_0 increasing with time. When that came along people were like well it's just called this dark

spk_0 energy because we called the last thing we were confused about dark matter. So really the thing

spk_0 they have in common is our confusion which is not necessarily a physical commonality except that

spk_0 they're both matter energy content that we are unsure about. The reason that I wonder about dark

spk_0 energy in a distinct way from dark matter with respect to a theory of quantum gravity because I

spk_0 do think that the presence of cosmic acceleration is maybe a hint about the nature of quantum gravity

spk_0 because one way to think about that problem is that the nature of the vacuum as you can see of it

spk_0 in general relativity and the nature of the vacuum as you can see of it in quantum field theory.

spk_0 When you try and put those two notions of the vacuum together they don't agree.

spk_0 And that's really there's a mismatch there so then we go out and make measurements and the

spk_0 measurement is giving us a value that's not predicted by quantum field theory and it's not

spk_0 predicted by general relativity. You can plug that value into general relativity and just say it's

spk_0 something we measure it's not something the theory tells us our priori but in my ideal theory of

spk_0 quantum gravity that's something that gets told to us our priori that our theory of quantum gravity

spk_0 says ah well it should have this value and this is why it has the value that it does. In the case of

spk_0 dark energy and really the cosmological constant which is this thing that you can add to Einstein's

spk_0 equation and say this is causing the acceleration and it's a form of vacuum energy. There are a

spk_0 couple of like theoretical quantities that come up so one is why is it so so small because it's

spk_0 almost zero but it's just big enough to be observably impactful. So that particular problem is really

spk_0 annoying when you try and then calculate from quantum field theory which the value B and the quantum

spk_0 field theory answer is off by 120 orders of magnitude and if you assume that super symmetry is real

spk_0 you can shave 60 orders of magnitude off but then you're still 60 orders of magnitude away. Exactly

spk_0 yeah so you're saying that you know we want a theory of everything whatever quantum gravity theory

spk_0 to explain what values things like the cosmological constants should have and other fundamental

spk_0 constants that we have no idea why they have the values they have. Right so what is the value of the

spk_0 vacuum energy? At the very least quantum gravity should be able to answer what the energy

spk_0 level of empty spaces and it should match with our observations and it should match with our

spk_0 observations. So from my point of view the fact of cosmic acceleration and the observation of an

spk_0 apparent cosmological constant is actually our first data point about quantum gravity.

spk_0 Flatko agrees that a theory of everything really should explain everything. That's actually this

spk_0 big question whether these things require any new concepts. So if dark matter and energy are

spk_0 really of different kind something that is not already part of our standard model let's say

spk_0 then I think the theory of everything must also explain that. Do you think that people sometimes

spk_0 forget about those when you talk about a theory of everything? I think so and I think we forget about

spk_0 them possibly consciously because there is a much bigger uncertainty about these things than about

spk_0 other things that we discussed like the high energy experiments of course laboratory based

spk_0 experiments which are even better confirmed. It seems to me that the degree of uncertainty there

spk_0 is much smaller than when we talk about dark energy and matter. So I think until that's kind of

spk_0 stood better and we have even more experimental evidence it seems to me that most people would not

spk_0 consider that yet as part of this grand unified theory but ultimately of course it must be explained.

spk_0 It will have to be. But perhaps it makes sense to start with a theory that unites gravity and quantum

spk_0 mechanics and such proposals already exist. One is string theory which suggests that the universe

spk_0 is ultimately made up of tiny vibrating strings. And different vibrations can give rise to familiar

spk_0 particles including a hypothesized but as yet undiscovered particle called the graviton which is

spk_0 related to gravity. But string theory makes one vital assumption that instead of the universe having

spk_0 three spatial dimensions so width, depth and height plus one for time it has 10, 11 or even more.

spk_0 And these different dimensions are compacted so tightly together that we don't really notice them

spk_0 at all they're hidden. And each compactification describes a different possible universe with its own

spk_0 physical laws. Another approach is called loop quantum gravity. While string theory incorporates

spk_0 gravity as well as quantum mechanics it sort of just assumes that Einstein's space time exists

spk_0 in the background. Loop quantum gravity however puts space time at the center and then try to show

spk_0 how it can arise from quantum effects. Essentially the theory is trying to divide up space time into

spk_0 tiny chunks and show that it does behave quantum mechanically. And one of the strengths that people

spk_0 were pointing to was string theory is that string theory built on quantum field theory which is the

spk_0 framework that we use to explain the standard model and to the standard model was built into it.

spk_0 So this is essentially a picture where in order to bring them together you have to move into higher

spk_0 dimensions. I think that that's probably the most user-friendly way of talking about it. And

spk_0 I think it does have a genuine strength which is that it brings the whole standard model with it

spk_0 which loop quantum gravity doesn't do not in the same way. So loop quantum gravity takes the

spk_0 perspective that the goal should be to maintain the lessons of general relativity while bringing

spk_0 it into conversation with the framework of quantum mechanics. And so possibly thinking about

spk_0 space time being quantized at the smallest scales. That's the broad brush strokes picture of how

spk_0 loop quantum gravity sees things. As Chanda started her career in loop quantum gravity,

spk_0 how does she feel about it now? Has she changed her mind about the approach or does she still think

spk_0 it's the best theory? Oh man I'm probably going to get in trouble but the good news is that I don't

spk_0 work on quantum gravity anymore. So I'm just speaking from the peanut gallery at this point. So like I

spk_0 did my PhD ostensibly on cosmology and loop quantum gravity and loop quantum gravity is one particular

spk_0 approach to quantum gravity. And I would say that one of the critiques that's been

spk_0 lobbed at loop quantum gravity is that it's insufficiently ambitious because what loop quantum

spk_0 gravity is trying to do is explain in a coherent mathematical picture the quantization of space time

spk_0 and really how you think of like a quantum general relativity. That might be one way of just saying

spk_0 a quantum general relativity. And I don't know maybe this is an ambition and I'm being shortsighted

spk_0 here but the way that it was framed to me as a student was the goal was not necessarily to explain

spk_0 neutrinos and neutrino masses. That you can think of the standard model as something that

spk_0 connects to that picture but the standard model is not going to be explained by it. And this is

spk_0 really a perspective difference from say string theory which Lee Smolin was one of my PhD advisors

spk_0 and I came of age right as he put out the trouble with physics which a lot of people saw as kind of

spk_0 like anti-string theory warfare. So I'm very much shaped by that particular moment in science where

spk_0 people were talking a lot about loop quantum gravity versus string theory. I think as a student I

spk_0 thought I was just evaluating the scientific picture but I do think that there was a social

spk_0 evaluation there and it's true that I got an opportunity in loop quantum gravity and was welcomed

spk_0 in in a way that I wasn't in string theory. String theory is in some ways far more fantastical.

spk_0 It builds on the framework of quantum field theory which is something that we know we've tested

spk_0 and works but it also requires extra dimensions that we've never seen right and so I think that's

spk_0 very easy to capture the public's attention with it because it has all of these really fantastical

spk_0 features and you have all of these folks who are really excited about that and it depends on which

spk_0 theory some of them I think they're 11 space-time dimensions and some there are 26. Not my area of

spk_0 expertise obviously because I chose the other side. I guess to go back to your question maybe agnostic

spk_0 is the word I think my job as a scientist is to be creative and interested and also to be willing

spk_0 to be told know by the universe. Vladko who is a quantum physicist is quick to point out that

spk_0 if loop quantum gravity turns out to be correct it would suggest that quantum mechanics is more

spk_0 fundamental than general relativity. So it's what people would call a canonical just

spk_0 is another name for standard it's a standard way of quantizing something so what that means is you

spk_0 take certain elements of general relativity. So general relativity for instance we talk about

spk_0 volumes of space or we talk about areas or distances or intervals of time and then you would

spk_0 think what they would mean quantum mechanically what does it mean to quantize a volume what does

spk_0 it mean to have a classical volume but actually to behave like a quantum mechanical object so I think

spk_0 loop quantum gravity is a standard way of imposing if you like quantum mechanics on general relativity.

spk_0 So I think people in loop quantum gravity would certainly bet on the fact that quantum mechanics

spk_0 wins if you see what I mean over general relativity and general relativity will have to conform

spk_0 to quantum mechanics and it seems to me that both string theory and loop quantum gravity as well

spk_0 as more or less any what I call canonical standard quantization approach that all of them would agree

spk_0 that gravity is quantum at that level so I don't think there would be a disagreement there.

spk_0 So what that means is that in order to really see how this theory is differ you would have to ramp up

spk_0 the gravitational strength and what that means is that it's very hard for us to test it because

spk_0 now you have to take a larger and larger object which means it gravitates more and more and then

spk_0 be able to put it in a quantum superposition of being in two or more states at the same time

spk_0 and this is exceedingly difficult actually. So testing a theory of everything won't be easy

spk_0 but is it impossible? I don't think it's impossible we probably have to think harder about it because

spk_0 frequently even with ordinary quantum mechanics we are talking about effects that are tiny right

spk_0 because we usually talk about this plants constant as being your quantum of action if you like and

spk_0 unless you are close to these regimes where plants constant matters if you like it's going to be

spk_0 very hard to see genuine quantum effects but we know the quantum effects can be amplified to the

spk_0 microscopic scales that they actually do matter at microscopic level you know things like for

spk_0 instance superconductivity it's a genuine quantum effect it really is an effect that can be seen

spk_0 at objects that are visible you know these supercurrents that are generated in superconductors

spk_0 are actually microscopic currents and yet they exist in superposition of different classical states

spk_0 if you like. So you know I'm always optimistic that even when some theories claim that some of

spk_0 these effects are tiny for instance the scales at which space is discretized people say oh this

spk_0 are tiny dimensions this is something like 10 to the power of minus 35 meters planks distance

spk_0 and some people say oh we're never going to be able to do experiments to test these kind of

spk_0 distances but what's not clear to me is whether these kind of effects could actually be amplified

spk_0 to lead to some things that are significant even at our scales so it doesn't mean that we have to

spk_0 observe them directly we could observe them indirectly through some manifestations providing

spk_0 that of course we understand what these manifestations are after all think about cosmology you know

spk_0 cosmology suggests that all the objects in the universe we observe now all the huge astronomical

spk_0 objects such as stars clusters of stars galaxies clusters of galaxies in fact can be traced back to

spk_0 quantum fluctuations in the early universe and so it's amazing you could actually argue that the

spk_0 whole classical structure of the present universe owes its existence to quantum fluctuations of geometry

spk_0 in the early stage it's a very speculative idea but it's a possible idea so that's why I'm

spk_0 somehow always optimistic that even if you have a theory where the effects seem very hard to reach

spk_0 it's possible that actually they have consequences which are really microscopic and could be used as

spk_0 weaknesses of these effects. Vlatko has an idea for an experiment to test quantum gravity developed

spk_0 with Chiara Marletto which you can hear about in episode four of the series but is also optimistic

spk_0 about experiments in space in labs on the ground physicists have already created exotic quantum

spk_0 states called both Einstein condensates for instance and I've also shown that it is possible to

spk_0 transmit or teleport information about a quantum state from one location to another

spk_0 but could these be replicated in satellites? People are thinking about you know creating both

spk_0 condensates in space making quantum superpositions on satellites and so on and this has many advantages

spk_0 in the sense that you could actually amplify certain gravitational effects you could suppress other

spk_0 effects and these satellite experiments would be different to earth-based experiments you could

spk_0 actually test different components of gravity on these satellites than what we are able to

spk_0 test on our planet so that's certainly already moving away a little bit from earth-based experiments

spk_0 people are suggesting some very exciting variations there. Like what can you give an example?

spk_0 There are these very cute nano satellites I mean they are tiny in the sense that they have

spk_0 dimensions 10 centimeters by 10 centimeters by 10 centimeters so they are really small as satellites go

spk_0 but in fact you could compress many of our earth-based quantum experiments into this kind of volume

spk_0 and that's remarkable as well the state of engineering is simply mind-blowing at present that you

spk_0 could take a whole laboratory that's huge you know we are talking about large rooms basically and

spk_0 you could compress all of that atom optics into this kind of nano satellite cube and I think

spk_0 what would be interesting already to test is whether the same quantum principles are obeyed in

spk_0 these kind of experiments could you really make a superposition of different massive objects on

spk_0 a satellite as well could these objects interfere quantum mechanically could you get them entangled

spk_0 could you teleport on these satellites but isn't it like they would be in sort of free fall

spk_0 so microgravity indeed wouldn't it be easier than to see those effects or some of it because I think

spk_0 if you're talking about the effects within the objects in these superpositions exactly then that

spk_0 would be easier to see because they would effectively be in free fall as you say you would eliminate

spk_0 all other gravitational fields which is why some people are advocating that that's the way to go

spk_0 but bear in mind that we've never done any quantum experiment there so I think even confirming that

spk_0 that some of these basic experiments work the way we think they ought to work is also an open question

spk_0 what would that say about quantum mechanics that it is more fundamental than gravity or I think

spk_0 it would be yet another confirmation of quantum effects at that level yes to me that would also

spk_0 signal that this works even in this different setting whether we will ever have experimental

spk_0 evidence for any approach to uniting quantum mechanics and general relativity is hard to say

spk_0 but chanda thinks we need to be patient I think when challenge that we're facing right now is that

spk_0 for most of the last century physicists and the general public have gotten a little bit spoiled

spk_0 it was a time of extraordinary learning at a rapid pace about particle physics so about the smallest

spk_0 fundamental constituents of matter and as far as I know there is no cosmic rule saying that physics

spk_0 has to be like that all the time I think that we now have a social expectation that doesn't

spk_0 necessarily align with how the universe works like the universe is not designed to be understandable

spk_0 on the timescale of a human lifetime there's no cosmic rule that says that dark matter which is

spk_0 the problem that I work on has to get resolved before I die it could be like I die and like the next

spk_0 day is the day that like a detector goes off like that could be it right or maybe it's like age

spk_0 away when there's like some sort of post human AI species right I mean it kind of doesn't matter

spk_0 what I think right the universe is just going to calculate regardless of what I think about how

spk_0 it should be calculating and I've spent most of my career as a professor and as a post doc working on

spk_0 a hypothetical dark matter candidate the axion the axion might be forever hypothetical it may not in

spk_0 fact be the dark matter I have to be ready for that I can't be the kind of person who refuses to

spk_0 accept data because it doesn't line up with my world view from a lack of new particles being

spk_0 discovered to fundamental clashes between different theories you may wonder whether physics is

spk_0 ultimately broken but as we've seen in this episode there are lots of theoretical physicists

spk_0 working on various proposals for creating a theory of everything from string theory to loop quantum

spk_0 gravity yes each suffers from its own set of challenges but perhaps the theorist will soon be

spk_0 guided by experiments perhaps they'll discover that quantum mechanics and general relativity

spk_0 aren't as incompatible as we previously thought and perhaps they'll glean insights into which one

spk_0 is most fundamental but let's not forget that physicists are people too as flatgo pointed out

spk_0 they may be drawn to certain theories because the maths is more closely aligned with their own thinking

spk_0 and as Chanda pointed out perhaps some bright young minds out there are put off from pursuing

spk_0 certain ideas because they don't feel they fit in in the community a theory of everything sounds

spk_0 like something that transcends human experience but proposals are being created within the messy

spk_0 realm of human beings full of beliefs, hunches, experiences and prejudices but despite that humanity

spk_0 has got pretty far in understanding the cosmos and that might be because we all have different

spk_0 perspectives and different ideas we also have tremendous levels of curiosity and creativity which

spk_0 when coupled with a rigorous scientific method can achieve the seemingly impossible

spk_0 so we thought we'd end the series with some different thoughts and perspectives on whether physics

spk_0 actually is broken from the brilliant minds who are tackling these mysteries every single day

spk_0 it's more like we are starting to uncover different parts of this story and just to make

spk_0 sense of the whole thing is very hard no in just little bits here and there but that's why we need

spk_0 more experiments to try and push these theories to the limit and try and see what comes next and which

spk_0 pieces are we missing? the closest to a real problem that we're facing is that our theories are

spk_0 too good so that makes life hard I feel like we are using the best tools that we have available to

spk_0 us to answer really difficult and really interesting questions and it would be so incredibly

spk_0 arrogant of us to think that we would answer those questions quickly I think physics is always

spk_0 evolving what happens is that certain areas of physics become more active and other as a physics

spk_0 become more difficult to move forward in so I think that there's always a changing of what's

spk_0 the next big thing would be terrible if we ran out of mysteries what worries me more is that we

spk_0 don't seem to make any progress on solving those mysteries so what do you expect to happen is

spk_0 that we solve one mystery and then a new one pops up but what's actually been going on at least

spk_0 in the foundations of physics is that we're still discussing the same questions that we have been

spk_0 discussing for a hundred years most of the running sofa has been in the very small and the very

spk_0 large particle physics and cosmology but now increasingly physics is tackling the very complex

spk_0 up to the third grade frontier and this is where physics and biology intersect and I think that

spk_0 there's still huge opportunity for physics to move into those fields and to maybe develop new laws

spk_0 and principles to describe them I think that it's been extraordinarily successful I mean look at

spk_0 what we know we have an extremely successful model of the entire history of the universe from

spk_0 the tiniest fraction of a second to now we can observe the cosmic micro background but light

spk_0 from the big bang itself we can observe the expansion of the universe we can see galaxies that

spk_0 existed in the first couple of hundred million years after the beginning of the cosmos we have

spk_0 general relativity of quantum mechanics we're creating quantum computing I think that physics is going

spk_0 great I think it's a sign that there's a lot of exciting questions to be answered and you know what would

spk_0 you rather have would you rather have a physics where everybody agrees on the correct direction of

spk_0 travel and we just you know tick things off and basically dot the eyes and cross the T's I mean

spk_0 that sounds utterly boring to me and I wouldn't be a physicist if that was the situation we're in

spk_0 physics is wide open and there's a lot of genuine disagreements about what we should be doing next

spk_0 but for me that's actually just part of the excitement physics isn't broken I think the more

spk_0 problems there are in it the better it is for physics because you know we don't run out of jobs

spk_0 and I think it's very fruitful that there are problems that can be solved and even if we haven't

spk_0 been able to solve them so far this actually just means we haven't looked at it from the right

spk_0 angles I don't think it's broken I just think it's adolescent I mean it was only invented by our

spk_0 species about like 300 years ago so I think that our theories of physics are very early and there's

spk_0 just a lot of work to be done and you know the most interesting places are the places where our

spk_0 theories are breaking because it's telling us that we're missing things about how reality works

spk_0 I feel very lucky to be alive at this point in time where I think we are about to see another

spk_0 revolution in physics all my bets are that we are going to sooner or later be forced to come up

spk_0 with a new theory and I think it will supersede both quantum mechanics and general relativity I

spk_0 think physics is the only way to understand the universe

spk_0 that was Natalia Arras Sean Carroll, Chanda Prescott, Weinstein, Fred Adams, Sabine Hussenthalder,

spk_0 Paul Davis, Katie Mack, Andrew Ponson, Kyora Marletto, Sarah Walker and Flattega Vadral and

spk_0 thanks to all our contributors throughout the series

spk_0 Although everyone we've interviewed across this series has a different perspective none of them

spk_0 believe that physics is broken there was a time when we thought that there was nothing new to

spk_0 discovering physics and just because that is no longer true doesn't necessarily mean we're on

spk_0 practically and experimentally that it will take time to get there

spk_0 ultimately we need patience and long term thinking something humans aren't that great at

spk_0 but it is becoming increasingly clear that it is a skill we must nurture

spk_0 this podcast was created and presented by me Miriam Frank and produced by Hannah Fisher

spk_0 the executive producers are Joe Editunji and Gemma Ware and the advisory editor is Zia Morally

spk_0 the sound design is by Eloise Stevens and music is by Mita Sarle great mysteries of physics is a

spk_0 podcast from the conversation UK with funding from fqxi

spk_0 this was the sixth and final episode of great mysteries of physics from the conversation

spk_0 I hope you've enjoyed it as much as we have thanks so much for listening

spk_0 I'm Gemma Ware host of the Conversation Weekly podcast each week I speak to an academic expert

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Theory of everything: do we really need one?

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