The Gravity Particle Should Exist. So Where Is It?

0:02 Physics is this close to understanding the entire universe.

0:06 And what lives in this gap?

0:09 Many physicists think it’s the elusive graviton—the

0:12 quantum particle of gravity—whose discovery will finally allow

0:15 us to stitch together our two great

0:18 theories of nature into a single master theory.

0:22 But what is the graviton, and does it even exist?

1:32 The quantum revolution started when we

1:34 realized that light is made of particles—photons.

1:37 Max Planck guessed it and Albert Einstein proved it.

1:40 Light is a wave in the electromagnetic field,

1:43 and so even from the beginning of modern physics we

1:46 have this idea that a force—the

1:48 electromagnetic force—is communicated by particles.

1:52 As quantum mechanics evolved into quantum field theory,

1:56 we also found the particles for the weak and the strong forces.

2:01 That’s three of the four fundamental forces,

2:04 leaving only gravity lacking a mediating particle.

2:07 So, if we could just figure out the graviton,

2:10 we’d have unified all the forces of nature

2:13 and be on track to a theory of everything.

2:16 We’re now 100 years after the birth of quantum mechanics,

2:20 and much of the past century of work towards

2:23 this master that hinges on the existence of the graviton.

2:27 But does it exist?

2:29 If not, much of that work in unifying

2:31 our theories has been in the wrong direction.

2:34 We can describe nearly the entire universe

2:37 in terms of two theories—quantum mechanics and general relativity.

2:40 Yet they look wildly different to each other.

2:44 taOn the quantum side we have these fields filling all of space,

2:48 whose oscillations give us all known particles

2:51 of matter and three of the four fundamental forces.

2:54 These quantum fields fracture into discrete shards as you zoom

2:59 in, and whose properties and particles can never be pinned down precisely.

3:05 On the relativity side we have space

3:07 and time merging into a unified fabric of spacetime;

3:11 a fabric that can expand and warp.

3:14 It stretches in response to matter and energy,

3:17 leading to what we experience as the force of gravity.

3:20 It’s a pretty effective picture.

3:22 Quantum theory gives us the stuff of the universe,

3:25 relativity gives us the container.

3:27 Matter in spacetime.

3:29 Matter tells spacetime how to curve and spacetime tells matter how to move.

3:33 So, if this picture works so well in describing the universe,

3:37 why do we need to try to force gravity

3:40 to work the same way as the quantum forces.

3:43 Well, partly because the two theories are so different.

3:47 We expect, or at least hope, that all the complexity we observe

3:52 in the universe emerges from unified underlying laws.

3:55 That proved to be true of all matter and most

3:59 of the forces of nature—quantum fields all the way down.

4:02 It would be weird if the most fundamental

4:05 layer of reality was actually two unrelated things.

4:08 And extra weird if the theories describing those things

4:13 are profoundly different—fundamentally discrete and random for the quantum,

4:21 fundamentally continuous and deterministic for relativity.

4:23 But maybe that’s just how it is.

4:26 Not very elegant, but does the universe care what we find pretty?

4:32 There’s another reason besides aesthetics that we

4:35 need quantum theory and relativity theory to connect.

4:38 That’s because at a fundamental level, they actively contradict each other.

4:42 Together they describe most of the universe,

4:45 but in certain places like the center of a black hole or at the big bang,

4:51 they generate contradictions and paradoxes that, it seems,

4:54 can only be solved by finding the master theory

4:57 in which quantum mechanics and general relativity connect seamlessly.

5:01 For various reasons,

5:02 most physicists trying to solve this problem follow a particular path.

5:07 The path of particles.

5:09 They try to quantize gravity, just as they once quantized electromagnetism.

5:15 And the search for this theory of quantum

5:19 gravity implies a particle that mediates gravity.

5:22 That’s the graviton.

5:24 If the graviton exists, then gravity has to be quantum, and vice versa.

5:29 Let’s see why that’s the case.

5:33 To get a photon, we start with the classical

5:36 electromagnetic field—the one described by Maxwell’s equations.

5:39 A field in that sense is just something that extends

5:43 through space and has some value at each point in space.

5:47 That value can be a simple number.

5:49 Like, the density or temperature of the air in the room,

5:52 giving a density or temperature field.

5:55 Or it can be a vector—a number and direction.

5:58 The EM field is like that—a strength of the force

6:02 and a direction at every point in space.

6:05 Fields can support waves—oscillations in field strength that move through space.

6:12 The density field in air supports sound waves.

6:15 The EM field supports EM waves, which we know as light.

6:20 So, when Planck and Einstein showed

6:23 that light is made up of indivisible particles,

6:26 we were set on the path to quantizing the EM field itself.

6:32 This process of “quantization” followed the development

6:36 of quantum mechanics in the mid

6:38 20s and quantum field theory in the following couple of decades.

6:42 Through quantization,

6:43 the EM field is broken down into countless modes of all different frequencies.

6:50 In this new description, even the empty or vacuum-state of the EM field is made

6:59 of those modes—made of infinite electromagnetic

7:02 waves stacked and canceled into nothing.

7:05 And those “virtual” modes are made of discrete packets,

7:08 just as “real” EM waves—so the EM field is made of virtual photons.

7:16 In quantum field theory, we describe the quantum electromagnetic force

7:21 in terms of particles exchanging these virtual photons.

7:24 They take momentum from one particle and give it to another,

7:29 causing the push and pull that we see as a smooth EM force.

7:36 Photons mediate electromagnetism.

7:38 We’ve talked about whether these virtual photons

7:41 can be thought of as “real” before.

7:43 But what IS real is the fact that the EM field

7:47 is extremely well described as being made up of quantized chunks.

7:51 If we use basically the exact same process to quantize

7:55 slightly more complicated fields we get the other quantum

7:59 forces and their mediating particles—the gluons for the strong

8:02 force and the W and Z bosons for the weak.

8:06 It should not be taken lightly that this stuff works so incredibly well.

8:11 We take a classical field,

8:13 make some symmetry arguments to guess what other fields might exist,

8:18 apply quantization rules, and boom, we figure out almost all

8:21 of the particles and forces that make up our universe.

8:24 The standard model of particle physics is

8:26 arguably the most successful theory in physics, and perhaps science.

8:31 It’s not surprising that we’d want to envelop gravity

8:36 and spacetime into the explanatory magnificence of quantum field theory.

8:42 So, how do we proceed?

8:43 Well, we need to identify the field that we’re trying to quantize

8:48 and then apply the quantization rules just like we did with the other forces.

8:55 We do know that gravity behaves like electromagnetism in important ways.

9:00 It has a strength at all points in space.

9:03 It also supports waves—gravitational waves—that also

9:05 travel at the speed of light.

9:07 There are differences, however.

9:09 According to general relativity,

9:11 the gravitational field is not a field on top of spacetime,

9:15 like electromagnetism,

9:15 but rather in a very real sense it’s the fabric of spacetime itself.

9:20 This tangling of the field and its..

9:22 is going to be a problem as we’ll see, but for now let’s proceed.

9:27 Let’s get a little more precise.

9:29 In the Einstein field equations, the field we’re interested in is encapsulated

9:33 in something called the metric tensor—a 10-valued

9:36 object made up of scalars and vectors with a value at every point in space.

9:41 It’s a field, albeit a very complicated one.

9:43 Where the EM field can be thought of as a thing

9:47 lying on top of the background grid of spacetime,

9:50 this tensor field of GR in a real sense IS the grid of spacetime.

9:56 In electromagnetism, the field changes with respect to the grid.

10:00 In general relativity, the grid itself changes.

10:04 This is going to prove complicated, but nonetheless, this grid,

10:10 this spacetime, this tensor field is the thing we need to quantize.

10:14 So let’s do that.

10:16 We’ll start with an approach that hopefully won’t break anything.

10:19 We’re going to treat the gravitational field as a small

10:23 fluctuation– or perturbation– to an imaginary flat and static background.

10:29 We poke flat spacetime very lightly and see what comes out.

10:34 Hopefully a graviton.

10:36 This is a very standard approach in physics.

10:38 It’s called perturbation theory.

10:40 Rather than completely changing our model,

10:42 we just make a tiny adjustment to what we understand and see what happens.

10:48 The result is a good approximation to reality,

10:52 assuming the perturbation is very small.

10:55 Take for example electromagnetism—we can understand its quantum

11:00 behavior by taking account of the exchange of different

11:05 nudges to the electromagnetic field as represented

11:07 by the exchange of virtual photons in different ways.

11:12 For a weak interaction, only the most obvious exchanges matter—like

11:16 the exchange of a single virtual photon.

11:20 Stronger interactions require us to account for more complex exchanges,

11:23 but it all works out beautifully.

11:29 When we do this to the gravitational field everything is… also fine!

11:36 Not only can we do a sort of gentle quantization of a weak gravitational field,

11:41 we get a glimpse of the quantum

11:44 of that field that mediates the gravitational force—the graviton.

11:48 What does it look like?

11:50 Well, it inherits all of its properties in a pretty

11:54 non-negotiable way from the field it comes from.

11:58 It’s a massless, spin-2 boson.

12:00 Massless means it travels at the speed of light.

12:03 Spin 2 means it has a 180 degree rotational

12:08 symmetry that reflects the sort of stretch-squish action on space,

12:13 just like the gravitational waves that should be built from these gravitons.

12:18 And boson means you can stack infinite gravitons together,

12:22 to make gravitational waves.

12:23 And just as virtual photons make the quantum EM field,

12:28 we can build the fabric of spacetime out of virtual gravitons.

12:33 Cool cool.

12:34 In a sense we just succeeded in quantizing gravity.

12:38 This approach of perturbative quantum gravity, and it actually works.

12:43 We can use it to recover classical gravity.

12:47 We can even use it to make predictions,

12:49 like Stephen Hawking’s prediction of Hawking radiation—leaky black holes.

12:53 So why don’t we just stitch this quantum gravity to the rest

12:58 of quantum field theory and say we have a theory of everything?

13:02 Why not add the graviton to the standard

13:04 model library and move on with our lives?

13:07 Not so fast.

13:08 We want our final theory to explain the whole universe,

13:13 but our perturbative quantum gravity only works

13:15 in the case of very weak gravity.

13:17 It doesn’t work for places with extreme spacetime curvature,

13:20 like the centers of black holes.

13:22 We also want to actually verify our quantum gravity theory with experiment,

13:27 and our perturbative theory works only in situations where it’s essentially

13:33 impossible to differentiate it from regular

13:37 Einsteinian gravity with a human-buildable experiment.

13:41 We really need the full theory.

13:43 So, to finish the job of quantizing gravity we have to see

13:48 if we can generalize this to a theory that works everywhere,

13:53 including where gravity is extreme.

13:56 As gravity gets stronger, our perturbations get bigger and the corrections we

14:01 need to make to our classical theory get larger.

14:05 More technically, we need higher-order corrections,

14:08 represented by more complex interactions via virtual gravitons.

14:13 Remember how we do this for the quantum electromagnetic interactions.

14:17 We calculate our quantum EM force by summing over

14:22 all the possible virtual interactions that could have occurred.

14:26 For weak EM fields it’s enough

14:28 to include only the most probable such interactions,

14:31 and that means the simplest ones.

14:33 Like, the exchange of a virtual photon or two.

14:37 For stronger EM fields, more complex interactions become important.

14:41 That includes so-called self-energy terms,

14:44 which result from the feedback between the different fields.

14:47 These higher-order terms involve loops that appear in the many

14:53 Feynman diagrams that we sum together to compute a strong interaction.

14:59 In a previous episode we talked about the connection

15:02 between these loops and the mass of the electron,

15:06 and how they appear to make that mass infinite.

15:10 But these sorts of infinites can’t be real or we wouldn’t have a universe.

15:15 In regular quantum field theory we have

15:17 a way to remove them called renormalization.

15:20 It lets us sort of divide them out by grounding them in measured values,

15:26 for example by measuring the true mass of the electron.

15:30 This is possible with the regular quantum fields because the perturbative

15:36 expansion—the sum of Feynman diagrams—”only”

15:39 generate a finite number of infinities.

15:43 It’s still possible to absorb these into a finite

15:47 number of additional terms in our equations.

15:50 Even if “absorbing infinities” sounds a bit hokey, it works incredibly well.

15:55 By grounding our otherwise-infinite predictions in a few

15:59 finite measurements of the real universe, quantum field theory is able to make

16:04 stunningly precise predictions of many, many things.

16:06 Not a bad trade-off.

16:08 Depicting the EM field as being

16:11 made of virtual photons is incredibly successful,

16:15 which forces us to take seriously this picture of a quantized field.

16:19 At some level we believe that the EM

16:23 and other quantum forces are mediated by particles.

16:27 If we can do for gravity what we just did for electromagnetism

16:31 then we can make the same argument for the reality of the graviton.

16:35 So, we’re ramping up the strength of our quantum gravity,

16:38 increasing the complexity of our perturbative approximation,

16:41 and hoping we can renormalize any infinities to get a sensible theory.

16:46 And this is where everything goes catastrophically wrong.

16:50 The problem is that gravitons interact gravitationally.

16:53 They interact with themselves.

16:55 The photon, on the other hand,

16:58 does not interact electromagnetically—at least not directly.

17:01 It interacts with, say, the electron field, which feeds back on the EM field.

17:08 That extra step limits the complexity of the photon

17:12 self-interaction and limits the number of self-energy loops,

17:17 making renormalization possible.

17:19 But a graviton can directly spawn other virtual

17:23 gravitons which can spawn more virtual gravitons, ad infinitum.

17:27 Instead of a finite number of self-energy loops we get an infinite number.

17:34 An infinite number of terms in our perturbative approximation

17:38 that can’t be renormalized—canceled out—with

17:40 a finite number of real measurements.

17:43 In cases of strong gravity, our quantum gravity theory becomes meaningless.

17:49 We say that perturbative quantum gravity is non-renormalizable.

17:53 We can also think of this in terms of the strength

17:56 of the gravitational coupling—the gravitational constant—getting

17:59 stronger as the field getting stronger,

18:04 ultimately sending the field strength to infinity.

18:07 This straightforward approach at quantizing gravity leads to nonsense.

18:11 So maybe it follows that we can’t build

18:14 the gravitational field out of gravitons after all.

18:16 Does this mean that the graviton doesn’t exist?

18:19 Not really.

18:20 What it means is that this simple picture of quantum gravity that we

18:26 get by applying the same quantization methods as the other forces isn’t right.

18:32 There may be a theory of quantum gravity that just

18:35 can’t be got to by the simple perturbative approach.

18:39 Interestingly, it’s the graviton itself that may lead us to the final answer.

18:48 Even though the standard approach didn’t get us all the way,

18:52 the picture of the graviton we got from it

18:55 is almost certainly right—if the graviton exists at all,

18:59 it has to be a massless, spin-2 boson.

19:03 Nothing else can generate the Einstein field equations in the classical limit.

19:09 And that’s why, when we find a theory that predicts a massless,

19:14 spin-2 boson our ears prick up and we

19:17 wonder if we’ve found a potential for quantum gravity.

19:20 And that’s happened once before, giving us string theory.

19:24 This was originally meant to be a theory for quark interactions,

19:29 but a graviton-like particle popped out of the math in such a profound

19:33 way that it sparked a many-decade quest to find a stringy theory of everything.

19:38 You can’t avoid the graviton in string theory,

19:41 and in string theory you also avoid the rampant infinities because.

19:46 At the smallest scales where gravity can be strongest,

19:50 the graviton field is smeared out over

19:53 the vibrating strings at the theory’s core.

19:56 There are other approaches to quantum gravity,

19:59 like loop quantum gravity and others.

20:01 All of them have a graviton,

20:03 because without the graviton there’s no path to classical gravity.

20:07 It’s again sounding like the graviton is inevitable.

20:10 But again, no.

20:12 It’s only inevitable if gravity is fundamentally quantum in the first place.

20:17 If the final theory looks more like quantum mechanics than general relativity.

20:24 What’s the alternative?

20:26 Well, it’s that the fabric of spacetime

20:29 doesn’t have discrete chunks—it’s not made of gravitons.

20:32 Maybe a smooth, continuous spacetime is fundamental in some way.

20:37 A number theories favour this option.

20:40 Even Roger Penrose is a proponent,

20:43 encouraging us to “gravitize the quantum” rather than quantizing gravity.

20:47 Fit quantum mechanics into general relativity rather than the way around.

20:52 Ultimately, finding evidence of the actual

20:55 existence of the graviton will be critical

20:58 to confirming the quantum nature of gravity

21:02 and even which quantum gravity we have.

21:05 As I hinted, that’s going to be really, really hard to do directly because

21:10 we’d need to build solar-system-sized particle accelerators.

21:13 Fortunately there are brilliant methods to observe the indirect effects

21:18 of the interactions of a quantum graviton that we can do now.

21:23 For example, spotting the mediation of quantum entanglement

21:26 by a gravitational field or a Cavendish experiment,

21:30 but with exceptionally tiny masses.

21:32 One of these methods may pan out very soon,

21:35 or we may have to start astroengineering our mega-collider.

21:39 Nature has set a last gigantic hurdle to understanding her deepest laws.

21:46 We need to understand and then find the graviton,

21:50 and then we’ll hold the building block of spacetime.