The Universe Itself Might Be Hiding the Gravity Particle From Us

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0:03 To progress to the next level in understanding reality,

0:06 we need to combine quantum mechanics and Einstein’s general relativity.

0:10 And to do that, most physicists believe we need a theory of quantum gravity..

0:15 which means we need gravitons.

0:16 But it also seems like the laws of physics make

0:20 it impossible to ever detect this quantum particle of gravity.

0:23 Almost like the universe is set up to keep

0:26 the final answer forever out of our reach.

0:29 So, can we outsmart the universe, catch a graviton, and finally solve physics?

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1:15 Now onto the episode In 2012,

1:17 the legendary physicist Freeman Dyson gave a talk in which

1:21 he speculated on the possibility of ever detecting gravitons.

1:27 He was not optimistic.

1:29 In his Poincare prize lecture, he laid out how the universe seems to conspire

1:36 to make the detection of the quantum particle of gravity impossible.

1:40 In some cases it seems to be impossible for all

1:45 practical experiments—the experiments are just too outlandish to ever happen.

1:48 But in other cases, graviton detection seems

1:52 to be in-principle and fundamentally impossible—we’re thwarted

1:56 by the appearance of black holes

1:58 or by the quantum vacuum from ever glimpsing a graviton.

2:02 So does the universe really prohibit us from ever

2:05 seeing the building block of the fabric of spacetime?

2:08 As a reminder, the graviton is the quantum particle of gravity,

2:12 just as the photon is the quanta of electromagnetism,

2:15 gluons of the strong force, etc.

2:18 But the graviton is more than that—it’s

2:20 the building block of the fabric of the universe.

2:24 In the same way that an electromagnetic field

2:27 is made of a sea of “virtual” photons, spacetime is made of gravitons.

2:32 At least, that’s true if spacetime has a quantum nature,

2:35 as most physicists seem to believe.

2:37 All theories of quantum gravity require them,

2:40 from string theory to loop quantum gravity.

2:42 So, probably worth trying to spot one if we want to verify these theories.

2:48 Now in the past we’ve talked about indirect

2:50 ways to test the quantum nature of gravity,

2:53 but what about detecting the graviton itself?

2:55 Well, there are two broad ways we can think about graviton detection.

2:59 On the one hand, we can take what we know about detecting classical

3:04 gravitational fields and try to apply

3:07 it to a quantized field of gravitons—basically,

3:10 to detect the gravitational effect of a single graviton.

3:13 On the other hand, we can treat gravitons like every other particle and use

3:19 the same techniques we once used

3:21 to squeeze out photons from the electromagnetic field.

3:24 By tackling both perspectives we can get a reasonably

3:29 comprehensive sense of how impossible graviton detection really is.

3:34 Let’s start by trying to detect the gravitational effect of a single graviton.

3:38 We’ll think about this in terms

3:40 of the most sensitive gravity detectors ever built:

3:43 the Laser Interferometer Gravitational Wave Observatory—LIGO.

3:47 This pair of detectors tracks the subtle differences in phase

3:51 of a pair of laser beams traveling along 4km perpendicular arms.

3:55 When those phases fail to line up, it indicates a relative change in the lengths

4:01 of those arms—a possible signature of a passing gravitational wave.

4:05 We’ve used these observatories to spot

4:07 gravitational waves from hundreds of merging

4:10 black holes and neutron stars since the first detection in 2015.

4:14 So what would it take for a laser

4:17 interferometer along these lines to detect a single graviton?

4:21 We’ll follow Dyson’s argument here.

4:23 LIGO is able to detect gravitational waves with a “strain”

4:27 of 10^-22—that’s the relative change in length it’s sensitive to.

4:33 It corresponds to one-one thousandth the width of a proton

4:39 for the 300 back and forth reflections along LIGO’s 4km arms.

4:45 If gravity is indeed quantum then gravitational waves

4:48 are made of a coherent superposition of gravitons,

4:51 in the same way that our lasers are made of photons.

4:56 A wave at LIGO’s strain limit would contain at least 10^36 gravitons.

5:00 We’re trying to see a single graviton,

5:04 so in principle we’d need a detector that’s 10^36 times more sensitive.

5:09 The critical question we need to ask is how precisely do we need to measure

5:14 the variation in the length of detector arms to pick up a single graviton event?

5:20 It turns out that the answer is independent of the type of gravitational wave:

5:26 for optimal sensitivity, we need to be able to measure

5:30 a length difference of order a single Planck length.

5:33 This should already raise concerns because the Planck length is

5:37 the distance where our current understanding of space breaks down.

5:41 But let’s proceed.

5:43 The limits of measurability of literally everything

5:46 is defined by the Heisenberg uncertainty principle.

5:49 The principal sets an absolute limit on how

5:52 precisely we can know complementary pairs of properties simultaneously,

5:55 for example the position or momentum of something.

5:59 Let’s start simple by imagining one LIGO arm as a pair of free-floating mirrors.

6:05 A gravitational wave causes the distance between them to change,

6:09 and we want to measure that changing distance.

6:12 That’s effectively measured by bouncing a photon between them.

6:15 However that photon also imparts momentum on the mirrors,

6:20 inducing an uncertainty in the mirror position.

6:22 The more precisely we want to measure those positions,

6:26 the higher the frequency of light we need,

6:28 therefore the more momentum we have to impart.

6:31 The position-momentum uncertainty relation can be used to relate

6:35 the limit of the position measurement to the imparted mirror momentum.

6:40 We can then convert this relation to one involving time.

6:45 So in order to accurately measure the separation of the mirrors,

6:49 the measurement has to happen fast—faster than the mirrors

6:52 move out of position due to the act of measuring.

6:56 Long story short, we can maximize the precision

6:59 of our measurement of the mirror separation in two ways:

7:03 by reducing the distance between the mirrors—that reduces measurement time.

7:07 And by increasing the mass of the mirrors,

7:11 heavy mirrors move more slowly when bumped by the measuring photon.

7:16 If we want a 1-Planck-length position precision

7:18 due to the passage of a graviton,

7:21 the mirrors need to be massive enough and close

7:24 enough together that they form a black hole.

7:27 Well, that sucks.

7:28 By definition, the formation of an event

7:32 horizon prevents our distance measurement.

7:34 Dyson shows that we get the same issue via

7:37 a different argument even if we fix the mirrors in place.

7:41 In general, any distance measurement on the scale

7:44 of the Planck length gives us black holes,

7:47 which means that direct measurement of the effect

7:49 of a single graviton by a LIGO-like device is fundamentally impossible.

7:55 Okay, let’s try something else.

7:57 The gravitational wave approach tries to measure

8:00 the actual gravitational effect of a single graviton.

8:04 But maybe treating gravitons gravitationally is using

8:07 a classical hammer to crack a quantum nut.

8:11 Instead, maybe we should look for gravitons by the same

8:15 method as we look for other quantum particles.

8:18 We can summarize that method in very general terms:

8:21 crash particles together and see what happens.

8:24 The physics of colliding particles is inherently

8:27 different to the interactions of classical waves,

8:30 so maybe that’s the right approach for spotting

8:33 a particle that arises from a quantum field.

8:36 The most obvious example of the particle collision method is,

8:40 surprise surprise, a particle collider.

8:42 We discovered the Higgs boson by building a collider so

8:46 large that its collision energies could generate the massive Higgs particle.

8:52 And that’s the 27 km diameter large hadron collider.

8:57 So how large a collider do we need to generate the graviton?

9:02 Gravitons are massless,

9:04 so energy isn’t needed to give them mass like with the Higgs.

9:09 Rather, energy is needed to increase the chance of generating one.

9:13 Gravity is the weakest force by a long, long way.

9:17 It’s 24 orders of magnitude weaker than

9:20 the weakest of the other fundamental forces,

9:22 a fact which is itself a conundrum called the hierarchy problem,

9:25 which obviously we’ve talked about before.

9:27 In quantum terms, that weakness can be expressed as a very

9:32 small coupling constant between the graviton and other particles.

9:36 The probability of generating a graviton

9:38 in a particle collision depends on that coupling constant.

9:42 Despite the word “constant”,

9:44 the coupling factor actually increases with the energy

9:47 of the interaction At around a billion Joules,

9:51 the coupling for gravity reaches the ballpark strength of the other forces.

9:55 So that’s the energy we need to reach

9:58 in our collisions to have a fair chance of producing gravitons.

10:03 How big an accelerator do we need to reach that energy?

10:09 Well the LHC collisions reach a whopping millionth of a Joule.

10:15 For a fixed magnetic field strength,

10:18 collision energy scales directly with collider size.

10:21 To get to the energy to detect a graviton,

10:25 a collider with the same magnets as the 27km LHC would need

10:30 to be around 3 light years in diameter—much bigger than our solar system.

10:37 That sounds insanely difficult, but it’s in-principle possible.

10:42 And once we’ve build a graviton-factory,

10:46 we get to the real challenge— it's not creating gravitons but detecting them.

10:52 Now massless gravitons are stable,

10:53 so we can’t rely on looking for their decay products.

10:56 We’d need to see one via its gravitational effect,

10:59 which we just saw may be impossible,

11:02 or when it’s absorbed by or scatters another particle.

11:06 The OG example of discovering a particle this way is the photoelectric effect,

11:11 in which an electron is ejected from a conducting plate by a single photon.

11:17 Einstein used this effect to demonstrate the existence

11:20 of photons as quanta of the electromagnetic field.

11:22 So can we use a similar test to detect gravitons?

11:27 An equivalent gravito-electric effect?

11:29 Well in principle, yes.

11:31 A sufficiently high-frequency graviton will carry enough energy to kick

11:35 an electron out of an atom or out of a conducting plate.

11:40 We could also imagine a graviton equivalent of the Compton effect,

11:44 in which electrons are ejected from atomic orbitals.

11:47 Either way, we shift the problem

11:50 from detecting gravitons to detecting electrons, which is pretty easy.

11:54 The probability of an electron absorbing a graviton to produce

11:59 one of these phenomena depends on the coupling strength of gravity.

12:04 So just as with graviton creation, it’s super unlikely.

12:10 Another way to think about this interaction

12:13 strength is in terms of the cross-section.

12:16 It’s called a cross-section because it tells us

12:20 how big an electron looks to a graviton—how close

12:24 the graviton needs to get to the electron

12:26 center in order to hit it and get absorbed.

12:29 And for the graviton-electron interaction the cross section

12:33 is proportional to the square of the Planck length.

12:37 So even if we can create a graviton in our interstellar-sized collider,

12:42 let’s call it the SLC—“stupidly large collider”,

12:46 we can probably never detect it.

12:48 The other way we can increase the likelihood of a gravito-electric interaction

12:53 is by increasing the number of gravitons in the right frequency range.

12:58 So maybe we amp up the power of our SLC,

13:02 or we find a natural source of gravitons.

13:05 The theoretical physicist Stephen Weinberg calculated that there

13:09 is potentially a huge quantity of high-frequency

13:13 gravitons being emitted by the Sun.These are

13:17 produced by electron-graviton interactions in the hot, dense core of our star.

13:21 At the energy we need, the sun should produce about 10^24 per second.

13:26 This amounts to about 4 per meter

13:30 squared per second passing through the Earth’s surface.

13:34 You probably don’t notice the several

13:36 that passed through you during this sentence alone.

13:39 The interaction cross-section for this is so small

13:43 that these solar gravitons only interact with a particle

13:47 of matter roughly once every billion years

13:50 or so across the entire volume of the Earth.

13:54 Now we could build a star-sized detector and maybe

13:59 get a detection every 1000 years rather than every billion,

14:04 but that still feels like an unsatisfying yield.

14:07 We probably actually need a stronger source

14:10 of gravitons than a mere entire star.

14:13 The best option might be the collapsed core of a dead star.

14:18 Robert Gould estimates that a hot white dwarf or a neutron star would emit,

14:23 respectively, 100 and 100,000 times more gravitons than the Sun,

14:28 though over a shorter timescale.

14:29 So placing a planet or star-sized detector near such a stellar

14:34 remnant might net us a single graviton on human timescales.

14:40 Of course, the path to building such an experiment

14:44 is far beyond human timescales if it’s possible at all.

14:48 And if we were somehow able to pull this off,

14:51 we’d face an even more basic issue: neutrinos.

14:54 These ghostly particles are notoriously hard to spot.

14:58 Most of the Sun’s neutrinos pass straight through the earth,

15:02 and we’ve built detectors out of the Antarctic

15:05 ice cap to catch the occasional rare interaction.

15:09 But by comparison to the shy graviton, the neutrino is a party animal.

15:14 For any of our graviton sources,

15:16 regardless of whether it’s our Sun, a white dwarf,

15:20 or a neutron star, our detector will

15:24 interact with 10^34 neutrinos per single graviton.

15:28 And distinguishing that graviton

15:30 from the neutrino noise seems practically impossible.

15:34 Let’s try one more method that on the surface looks promising.

15:37 In the 1960s Mikhail Gertsenshtein

15:39 showed that electromagnetic and gravitational waves,

15:43 in a strong magnetic field, can couple together.

15:47 This is more promising than looking

15:49 for interactions between gravitons and, say, electrons.

15:52 EM and gravitational waves, which both travel at the speed of light,

15:57 can experience wave resonance that allows

16:00 energy to transfer between them at much lower coupling strength than is needed

16:05 for the absorption of a graviton by matter.

16:08 This means a photon may be transformed into a graviton and vice versa.

16:13 So, if we shoot a graviton through a magnetic field,

16:17 it will oscillate between graviton states and photon states.

16:21 This is called the Gertsenshtein effect.

16:24 Now to do this, we need a long hollow tube threaded

16:27 by a strong magnetic field passing and pointed towards a source of gravitons.

16:31 The hope is that some of those gravitons

16:34 will turn into photons that we can detect.

16:37 And, as you may have guessed, the universe again conspires against us.

16:41 The magnetic field needs to be quite

16:44 strong for the Gertsenschtein effect to work.

16:47 How strong?

16:48 Strong enough that we have spontaneous

16:50 creation of matter-antimatter pairs inside the tube.

16:53 This vacuum polarization resulting from this limits

16:57 the coherence of EM waves in the tube,

17:01 preventing resonance with gravitational waves

17:04 and shutting down the Gertsenschtein effect.

17:07 So we’re at strike three for detecting gravitons.

17:10 It seems the universe really doesn’t want us to see its deepest structure.

17:14 But is it really impossible, or just nearly impossible?

17:17 In a couple of the cases we looked out the answer is… impossible.

17:23 The barrier to graviton detection seems fundamental.

17:25 The formation of black holes or the breakdown of the vacuum tells

17:30 us that the laws of physics forbid graviton detection by these methods.

17:35 But in other cases it’s just astonishingly difficult—we need star-sized

17:39 graviton sources and detectors and some yet-unknown way to filter neutrinos.

17:45 But next to impossible is still possible.

17:48 I’m saying there’s a chance.

17:50 And maybe the problem is actually just that we

17:53 haven’t come up with a smart enough experiment yet.

17:57 Or maybe we have.

17:59 Since Dyson’s 2012 lecture,

18:01 we’ve detected gravitational waves directly and we’ve

18:05 made major advancements in quantum technology.

18:07 This has inspired a new generation of physicists

18:10 to come up with a new generation of proposals.

18:14 For example, we may be able to combine the two broad approaches I mentioned:

18:18 using a LIGO-like interferometer combined with absorption of gravitons,

18:22 this time by a detector with novel quantum properties.

18:26 How does that work?

18:28 Well that’s for an episode coming soon, when we’ll find out how we really may be

18:34 able to detect the particle at the heart of spacetime.

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