At What Point Does Spacetime Become Quantum?

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0:03 the quantum nature of gravity and of spaceime itself,

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0:12 Or we could just physics smarter and build one on a lab bench.

0:20 Here's how.

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0:54 The quantum world is certainly strange.

0:56 Objects in multiple places and states at once.

0:59 Random transitions between states and places.

1:02 weird instantaneous communication over large distances.

1:05 All that good quantum weirdness.

1:07 But perhaps the strangest thing about quantum mechanics is that its

1:12 rules seem so different from the classical large scale world.

1:16 And yet the latter comes from the former.

1:20 But how?

1:21 And at what size does that happen?

1:24 A related mystery is the connection between gravity and the quantum.

1:28 Gravity seems more in line with the classical world.

1:32 It is crisply defined and very non-random.

1:35 But gravity is the fabric of spacetime which is

1:39 knit together on scales far smaller than the quantum.

1:43 So at what point does spaceime itself become quantum?

1:47 The question of the quantum classical transition and the related

1:51 quantum gravity connection are notoriously difficult to probe

1:54 directly due to the challenge of accessing the minuscule

1:58 quantum world and the minuscular quantum gravity world.

2:02 But maybe the answer is not to build tiny experiments or giant colliders,

2:08 but to make the quantum world big enough

2:11 to meet gravity and the classical world halfway,

2:15 perhaps inside a regular laboratory.

2:17 The known laws of physics can be pretty clearly broken up into these two realms,

2:23 those describing the large scale classical world and the tiny quantum world.

2:27 Classical physics works great above about a micrometer

2:30 where objects are made of many many atoms.

2:33 Everything larger is as we say macroscopic.

2:38 Quantum mechanics rules below the nanometer scale.

2:42 At single molecules and atoms and smaller but the in between scale,

2:48 the meos scale feels both of these worlds.

2:52 It is, as we say, semiclassical, behaving mostly classically in some ways,

2:57 but exhibiting quantum effects in certain conditions and when carefully coaxed.

3:01 Today, we're going to look at approaches

3:05 and some example experiments for probing fundamental questions

3:09 by looking at this intermediate space where

3:12 both classical and quantum mechanics are at play.

3:15 Maybe we don't have to wait for that solar

3:18 system size particle accelerator to do this cool stuff.

3:20 We'll look at two paths.

3:22 One is to look for a breakdown

3:24 in our classical understanding of gravity at the mesos scale.

3:27 The second will be to look for true

3:31 quantum effects in as large a system as possible, but also in the mesos scale.

3:36 So, we're going to try to make gravity small and then make quantum big.

3:40 If we can get them to meet somewhere in the middle,

3:44 maybe we can even make gravity quantum.

3:47 Let's start with an experiment so elegant

3:49 and simple that it's been our go-to for measuring

3:52 the strength of gravity since we well

3:54 first tried to measure the strength of gravity.

3:57 When Newton first wrote down his law of universal gravitation,

4:01 he reasoned his way to the proportionalities based

4:03 on a falling apple and the orbit of the moon.

4:07 The force of gravity had to be proportional to the product

4:11 of those masses divided by the square of their separation.

4:15 But he had no idea of the value of the constant of proportionality,

4:21 the gravitational constant.

4:22 For that, he would have needed to know the Earth's mass.

4:25 But how do you weigh the Earth?

4:27 The answer is that you first measure G with known

4:30 masses and then work your way back to the planet.

4:33 But that requires us to measure the minuscule gravitational

4:37 attraction between masses that we can make and then weigh.

4:42 And it wasn't for another century after

4:44 Newton's law that this became remotely possible.

4:47 The guy who did it should have been John Mitchell,

4:51 the most underrated of the geniuses of physics.

4:54 This is the same guy who first conceived

4:56 of the Newtonian version of the black hole, the dark star.

4:59 Mitchell's idea was to measure the minuscule gravitational pull

5:03 between a pair of metal spheres using a remarkable device, the torsion pendulum.

5:09 This cutting device suspends a rod on a thin wire.

5:13 Rotating the rod twists the wire leading

5:15 to a restoring force that pulls it back.

5:18 This can produce pendulum-like oscillations.

5:19 But it can also be used to measure

5:22 the strength of the force doing the initial displacement.

5:25 The restoring force increases with twist.

5:28 So more twist means more force.

5:31 And the torsion pendulum is almost completely free from friction.

5:36 With a thin wire, it can be sensitive to extremely tiny forces.

5:41 Perfect for the minuscule gravitational

5:43 force exerted between non-planet-sized objects.

5:46 Mitchell actually built his torched pendulum,

5:49 but he died before he could complete the experiment.

5:53 His close friend Henry Cavendish inherited the device,

5:57 refined the design, and saw the experiment through.

6:00 It looked something like this.

6:01 Two lead balls of around a kilo uh were

6:05 fixed to the arms of the pendulum while a pair

6:09 of 160 kg balls were fixed near these two

6:12 with the intention of them gravitationally attracting the smaller balls.

6:17 The amount of twist in the pendulum would measure

6:20 that force and because all the masses and distances were known,

6:23 the gravitational constant should fall out as the last unknown in the equation.

6:28 The experimental design and the care taken were extraordinary.

6:32 So much so that in 1798, Cavendish measured the gravitational constant to within

6:39 1% of the best modern value.

6:42 Fun side fact, Caendish described his experiment

6:45 as weighing the Earth because with G in hand,

6:49 we could now calculate the Earth's actual mass

6:52 just from the gravitational acceleration at the surface.

6:55 In the 225 plus years since Cavendish's measurement,

6:59 this experiment has been greatly refined,

7:03 but we've improved on that first measurement only by about one

7:07 more decimal place because Cavendish and Mitchell did such a good job.

7:12 However, we're also now able to do this experiment with much smaller masses,

7:17 perhaps even small enough to see if

7:20 the rules change as we approach the quantum realm.

7:24 And there are some reasons to think they might.

7:27 In a previous episode,

7:28 we talked about how gravity might deviate from the Newtonian inverse square law

7:33 at very short distances if there are extra coiled up dimensions on those scales.

7:41 Essentially, gravity leaks into those dimensions very close to the source,

7:45 causing it to weaken faster.

7:47 But at longer ranges, it settles into the regular inverse square fading.

7:51 But there are other reasons to think that gravity might behave this way.

7:55 In string theory, there are ways for gravity to behave like

7:59 the quantum forces whose coupling strength

8:02 changes with distance due to self-shielding.

8:05 There's also the hypothetical chameleon field,

8:08 which is a dark energy candidate that has

8:11 a distance dependent variable mass that could do something similar.

8:15 And give a theorist enough chalk and a sbatical,

8:18 I'm sure that they'll come up with several more options.

8:22 But how to test?

8:23 The problem is performing a Cavendish type experiment

8:27 with quantum or even meoscopic masses brings with it

8:31 a new slew of noise and possible interfering effects

8:35 that not even Cavendish's dedication to precision could overcome.

8:40 For starters, we know the gravitational force is absurdly weak between

8:43 two small masses when compared to the other forces of nature.

8:47 For example, the gravitational attraction between two electrons is about 42

8:50 orders of magnitude weaker than

8:52 the repulsive electromagnetic force pushing them apart,

8:55 making it essentially impossible to measure gravity.

8:58 In that case, we can use neutral masses,

9:01 but even those tend to have internal electric charge distributions

9:05 that can lead to dipole interactions when objects are close by.

9:09 And when surfaces get very close,

9:11 the Casmir force and the Vanderwal's force come into play,

9:15 typically with much greater strength than gravity.

9:18 Even if we could compensate or outright avoid these effects,

9:21 there are many other potential sources of noise.

9:24 natural vibrations in the experimental setup, seismic noise,

9:28 and even gravitational noise from nearby massive objects.

9:31 A recent example of a successful teensy tiny Cavendish

9:35 experiment came from a group of physicists in Vienna.

9:39 They built a Cavendish setup with tiny gold spheres a thousand

9:43 times lighter than the original lead source mass at under 100 mg.

9:49 These 2 mm gold beads kept a pendulum rod suspended on a wispy silica thread.

9:56 This experiment was all about minimizing external noise and interference.

10:01 As with typical modern Cavendish experiments,

10:03 this one was conducted in a high vacuum

10:07 and the masses were carefully discharged here using ionized nitrogen.

10:11 A conductive Faraday shield between test and source

10:15 ensured no electromagnetic interactions between the balls.

10:19 The tiny gravitational field they were trying to measure between the balls

10:23 which were separated by about 2 and 12 mm was no

10:27 stronger than that of a grad student standing 2 and 1/2

10:30 m away or a vianese tram rumbling by on the street outside.

10:36 To help quieten the gravitational noise,

10:39 the experiments were done mostly between midnight and 5:00 a.m.

10:43 during the quiet Christmas season.

10:46 And I guess grad students were

10:48 positioned strategically and asked to remain motionless

10:51 on top of asking them to stay up all night and to miss the holiday.

10:54 Even with these precautions, more cleverness was needed to make the measurement.

10:59 Now Henry Cavendish directly measured his displacement of the torsion

11:03 pendulum arms by seeing how far they moved.

11:07 But in this and other modern Cavendish experiments,

11:10 researchers have a way to massively amplify that signal.

11:14 They oscillate the position of the source mass.

11:17 So the test mass experiences a varying gravitational field.

11:21 The pendulum also oscillates and the varying

11:24 gravitational field will very slightly perturb that oscillation.

11:28 This allows a direct measurement of the gravitational

11:31 acceleration caused by the source mass.

11:33 The benefit of this method is that the researchers can watch

11:37 for long periods of time as the tiny pertabbations build up.

11:41 By integrating the signal for say half a day,

11:44 they can detect a gravitational acceleration of 10

11:47 the power of minus 10 meters per second squared.

11:51 That's a 100 billion times smaller than what we feel at Earth's surface.

11:55 The gravitational constant measured in this experiment

11:58 turned out to be consistent with the known gravitational constant

12:01 as measured by Cavendish and later experiments.

12:04 There was a difference of around 9%

12:06 but that's within the experimental uncertainty of this setup.

12:09 So, the laws of gravity may not be

12:12 too different for these sub hundred milligram masses,

12:15 but since the gold balls used here are more

12:19 comparable to a baseball in size than to an atom,

12:22 that may not be too surprising.

12:24 It's impossible to know exactly how small we need to go

12:28 to see if the strength of gravity changes, if it even does.

12:32 But this team feels that it should at least be possible to get down to the plank

12:37 mass nearly 10,000 times smaller than the current experiment

12:41 with several significant but plausible refinements of the methods.

12:45 If we wanted to get these experiments down to truly quantum scales,

12:49 then we need to go down another 9 to 12 orders of magnitude.

12:52 That may not be possible with the Casemir setup,

12:55 but other approaches like levitating nano particles

12:58 or cryogenic suspension may make it possible.

13:00 So, we've been talking about measuring gravity down to the near quantum scale.

13:04 The other way we can go is try to observe

13:07 quantum effects on as large a scale as possible.

13:10 One of the most defining of quantum

13:12 phenomena is quantum entanglement in which a pair

13:15 of quantum objects can become correlated with each

13:18 other in a way that defies classical explanation.

13:20 We typically think of entanglement

13:22 as existing between truly quantum scale entities.

13:25 For example, a pair of electrons may have entangled spins.

13:29 Each spin direction undefined when taken separately,

13:32 but those entangled spins are defined relative to each other.

13:36 For example, as having opposite directions,

13:38 entanglement should exist between large systems.

13:41 But the larger the system, the harder it is to observe.

13:46 The correlations between individual particles can be smeared out

13:49 and lost to the surrounding environment in a process called decoherence.

13:54 Decoherence plays a huge role,

13:56 perhaps the entire role in making the quantum classical.

14:01 And we could understand that process a lot better if we

14:05 could somehow observe entanglement in a macroscopic

14:07 property of a macroscopic system.

14:09 One of the most promising approaches is in the field of optochanics.

14:14 Imagine a laser light bouncing between two mirrors.

14:17 We call this an optochanical cavity.

14:20 The mirrors in this case are suspended so they can oscillate forward and back.

14:24 A photon from the laser hitting the mirrors

14:27 will transfer momentum that depends on the photon frequency.

14:30 That sets up an oscillation in the mirrors which in turn changes the size

14:34 of the cavity which in turn changes the frequency

14:36 of the modes of light in the cavity.

14:39 This leads to a feedback cycle in which the oscillation

14:42 of the mirrors is correlated with the frequency of the photons.

14:45 And done carefully enough, this correlation can be a true quantum entanglement.

14:50 This has been achieved with very very tiny mirrors.

14:54 The first was in 2010 with an entanglement demonstrated between a light

14:59 field and just a single membrane of silica nitride as the mirror.

15:04 But in 2011, a pair of mirrors was put into entanglement,

15:08 not just with the bouncing photons, but also with each other.

15:11 That means two macroscopic systems in a state of entanglement with each other.

15:16 So here we are really blurring the line between the quantum and the classical.

15:21 But these mirrors were still tiny, really messcopic.

15:24 It would be nice if we could

15:26 achieve the same result for something truly macroscopic.

15:29 Of course, it would be incredibly expensive to build

15:34 a much larger optochanical cavity with all the precision engineering,

15:38 the noise mitigation tech, etc.

15:40 to even attempt a macroscopic optochanical entanglement measurement.

15:43 Good thing we already built one.

15:46 The laser interroter gravitational wave observatory,

15:48 LIGO, was designed to detect space-time ripples,

15:52 but in principle could be used to detect correlations in the oscillations

15:56 of the mirrors that point to true

15:59 entanglement between these genuinely macroscopic objects.

16:02 Many of the challenges for doing this with LIGO have already been solved.

16:07 In order to detect actual gravitational waves,

16:10 incredible work was done to minimize random noise,

16:13 to flag and remove seismic vibrations,

16:15 and even remove spirious gravitational signals.

16:18 The major outstanding challenge is dealing with a special

16:22 type of noise that can confuse the entanglement signal.

16:26 This is non-marovian noise which unlike

16:29 regular noise develops correlations over time sort

16:32 of like a memory that can be confused for the entanglement source correlations.

16:37 Motivated by the goal of teasing out any signs of entanglement in LIGO,

16:42 in 2024, a group of researchers went back through the LIGO's data,

16:46 now equipped with new and improved models to account for the non-makovia noise,

16:53 and they haven't found anything yet.

16:56 But with better noise modeling and more integration time,

17:00 we may spot the quantum whisper of entanglement between

17:04 LIGO's 40 kg mirrors which span its 4 km arms.

17:09 Detecting entanglement between classical or semic-class objects can help

17:12 us understand the boundary between the quantum and the classical.

17:16 The Cavendish experiment we started with was

17:19 about exploring the quantum nature of gravity.

17:21 Now the holy grail would be to bring these goals together to detect

17:26 entanglement between systems in which

17:28 the entanglement is actually mediated by gravity.

17:31 If gravity can do this, it means

17:34 that gravity itself must have quantum properties.

17:36 Now we did look at one approach a while

17:40 back with a stern-gerlach type experiment in which

17:43 nano diamonds in a super position of positions nudge

17:47 at each other gravitationally and so become quantum entangled.

17:51 But there are other proposals that don't

17:54 require the super position of positions.

17:57 For example, putting test masses in a superp position of spin states whose

18:01 energy difference then leads to a difference

18:04 in the gravitational fields that they generate.

18:07 or even a real Cavendish-like experiment in which

18:11 a pair of oscillating pendula develop non-class correlations

18:14 over long periods of time that can only

18:17 have arisen from a gravitationally mediated quantum connection.

18:21 All of these ideas are still in early

18:24 phases from thought experiment to active planning.

18:27 The technological hurdles are major but the crazy thing is that the hurdles

18:32 are just technological and there are clear paths to solving them.

18:35 This is what's so cool.

18:37 We will soon have a lab bench experiment that can start to map

18:43 the quantum classical divide and even observe the quantum behavior of gravity.

18:48 Physics is cool.

18:50 Just a couple hundred years from lead balls

18:53 suspended from wires to the quantum nature of spacetime.

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