If we discover how to connect quantum  mechanics with general relativity   we’ll pretty much win physics.

There are  multiple theories that claim to do this,   but it’s notoriously difficult to test them.

They seem to require absurd experiments   like a particle collider the size of a  galaxy.

Or we could try to physics smarter,   instead of physicsing harder.

Let’s talk about  some ideas for quantum gravity experiments that   can be done on a non-galaxy-sized lab bench,  and in some cases already have been done.

It’s been almost right on 100 years since quantum  mechanics was discovered, and 110 since general   relativity.

Together these theories explain almost  everything—quantum gives us the behaviour of the   subatomic world and so the building blocks of  all matter and their interactions via the quantum   forces, and GR gives us the the background in  which that matter lives—space and time along with   the force of gravity.

Despite their incredible  successes in their own domains, the two seem   to contradict each other in deep ways.

There  must be some master theory that encapsulates   both and resolves these contradictions, but  in the century following the discoveries of   general relativity and quantum mechanics, our  efforts to find that theory have come up short.

The common approach to reconciling general  relativity with quantum mechanics has been   to quantize gravity—to make the gravitational  field and so the geometry of spacetime obey the   strange rules of quantum mechanics.

And why not?

It’s worked for everything else; for example we   made the electromagnetic field “quantum”,  showing it was mediated by force-carrying   photon—and that was a stunning success.

But  quantizing gravity proved extraordinarily   difficult.

The hypothetical force-carrier for  gravity—the graviton—has an energy so tiny that   it’s hard to even conceive of an experiment that  could detect it directly.

And then there’s the   fact that the field of gravity is the fabric  of spacetime itself.

We quantized the other   forces of nature with respect to a continuous  background, but so far it’s been exceptionally   challenging to quantize that background itself.

The most prominent approaches, like string theory   and loop quantum gravity, may have made progress,  but they struggle to make testable predictions.

So what if we’ve been looking at it the wrong way  around.

What if, instead of quantizing gravity,   we should be trying to “gravitize the quantum”.

That is, instead of trying to give quantum   properties to gravity, we should accept that  gravity is fundamentally classical and try   to understand how truly quantum matter  can result in truly classical gravity.

Now we talked about approaches in which  gravity stays classical in the last episode,   when we explored the new Postquantum gravity  hypothesis by Jonathan Oppenheim, in which random   fluctuations in the gravitational field act on  the quantum wavefunction, causing it to collapse.

We also talked about the Diose-Penrose model  a while ago, in which a superposition of   different spatial distributions mass and  energy is ultimately forced to choose one   such distribution consistent with  a singular gravitational field.

There are compelling reasons on  both sides of the discussion,   but which is it?

Is gravity quantized or is the  quantum gravitized?

Unlike many questions about   quantum gravity, this one may be possible  to answer with actual doable experiments.

Let’s consider the two scenarios and see what   predictions they make that  we might be able to test.

One: Gravity is quantum - this means gravity  and by extension the fabric of spacetime exhibit   the various weird properties of quantum world,  like superposition and entanglement.

If we can   experimentally demonstrate one of these  things then we know gravity is quantum.

Two: The quantum is gravitized - this can be  tested in the context of specific scenarios   like the Diosi-Penrose theory or Oppenheim’s  Postquantum gravity.

I’m going to talk about   this first because it’s shorter, then I’ll  get back to tests of actual quantum gravity.

In both the Diosi-Penrose and Oppenheim  approaches, the gravitational field is responsible   for collapsing the quantum wavefunction.

In these  ideas, there’s only one gravitational field,   only one spacetime, and quantum systems do  quantum stuff within that classical background.

One of the things quantum systems can do is  be in superpositions of states—they can exist   simultaneously in multiple states at once—like  Schrodinger’s alive-and-dead cat.

The job of the   quantum wavefunction is to describe this range  of possible states.

Measurement or observation   “collapses the wavefunction” - it forces the  system to choose one state from the possibilities.

If gravity is classical, then the superposition  of its contents shouldn’t cause a superposition   of the gravitational field.

A superposition  might involve different distributions of   mass—which normally would mean different spacetime  curvatures—perhaps a superposition of spacetimes.

But if gravity is classical then there’s only one  spacetime regardless of its contents.

This could   lead to a tension between the gravitational  field and the matter producing that field.

The Diosi-Penrose approach says that  when this tension gets too large,   the wavefunction collapses.

Another way to  think about it is that the fuzziness in the   matter distribution is within a certain range  of matching a single spacetime curvature And   in that case it can stay in superposition.

But  if the distribution of matter strays outside   that range then the superposition collapses  into a well-defined matter distribution.

Oppenheim’s postquantum  gravity is slightly different:   it states that the gravitational field  contains random fluctuations—called   gravitational diffusion—that mess up quantum  superpositions, causing them to collapse.

Both the Oppenheim and Diosi and  Penrose make a similar prediction:   that there should be a stochastic quality  to the gravitational force or its effect.

This gravitational diffusion might be the most  straightforward thing to test if it exists.

It   would imply that there’s a fundamental limit  to the precision with which any mass can be   measured.

So if we can build a set of scales  with a certain very high precision, and if the   weights it measures appear to fluctuate outside  that precision, it may suggest an intrinsic   uncertainty in the gravitational field.

The most  accurate experiment measuring a mass conducted   so far has led researchers to rule out versions  of the theory with rapid spacetime diffusion,   implying that any gravity-induced collapse from  this mechanism is very weak.

Oppenheim and team   propose other tabletop experiments that  could constrain this better in the future.

Rather than look for the funky  behavior of the gravitational field we can also approach this from the other  side—by trying to see when the collapse of   the wavefunction happens.

If we can see where  quantum superposition gives way to classical matter we may be able to figure out  if it's gravity causing that collapse.

Both the Oppenheim and Diosi-Penrose models,  as well as any of the family of models known   as objective collapse theories predict that  wavefunction collapse becomes increasingly   likely as an object grows in size or mass.

If  any of these ideas are correct, there should   be a limit to how large an object we can put into  superposition for any appreciable amount of time.

So far we haven’t found such a hard limit,  but we’ve been able to put some pretty big   things in superposition—for example, getting  interference patterns from large molecules in   double-slit experiment.

Based on the largest  systems that have been put in superposition,   we can rule theories where the  gravity-induced collapse is very strong.

So we have some constraints on these ideas  where a classical gravity is influencing   the quantum wavefunction of matter, and  more advanced tests are being planned   that may turn up something.

Of course, it  may be that gravity really isn’t classical   in the way these theories propose.

It may  be that gravity really is quite quantum,   just like matter.

If that’s the case then, we’ll need different experiments.

Two of the strangest properties of quantum  objects are superposition—which we just   talked a lot about—and entanglement.

Quantum entanglement is when you have   two quantum systems that each may  be in a superposition of states,   but the systems are also correlated so that the  state of one depends on the state of the other.

For example, if a “quantum coin” was in  a superposition of both heads and tails,   while a pair of quantum coins could be both in  superposition, but also be entangled so be in   the opposite state to each other.

Even though  each of the coins is fundamentally undefined in   its state.

If the first coin is measured so that  it’s wavefunction collapses to either heads or   tails, its entangled partner’s wavefunction  will immediately collapse to the opposite.

Quantum entanglement sounds like an exotic  phenomenon, but it’s not; quantum particles are   constantly getting entangled with each other.

In fact, every interaction between particles   generates entanglement, even if it doesn’t last  very long.

We’ve observed these entanglements   when the interaction happens via the quantum  forces—for example, when particles interact by   exchanging the photons of the electromagnetic  force.

So what if we could cause particles to   become entangled by a gravitational interaction?

As I’ll try to convince you, this would mean   that the gravitational field itself has to be  part of the entanglement of those objects and   spacetime has to be in a superposition of states.

Which would mean spacetime has to be quantum.

One particular proposal for this type of  experiment has recently been put forward,   known as a Quantum Gravity-induced Entanglement  of Masses, QGEM, experiment.

I’ll describe the   original version proposed by Sougato  Bose and collaborators in 2017.

It uses a Stern-Gerlach interferometer.

This is the device that first demonstrated   the existence of quantum spin back in the  20s.

The original version used silver atoms,   which have a single unpaired electron in their  outer shell, so the atom takes on the quantum   spin of that lone electron.

That gives the atom  a tiny dipole magnetic field.

This causes the   atom to be deflected when it passes through the  external magnetic field of the Stern-Gerlach   device.

It’s deflected upwards if  its spin  axis is point up and downwards if its spin is   down.

Because quantum spin is quantum, all  atoms will be measured to have either spin   fully up or spin fully down (reflected all  the way up or reflected all the way down)— there are no in-betweens for  quantum properties like this.

If the atom starts in a superposition state of  being both spin-up and spin-down then it’ll end   up in a superposition state of being physically  up or down just before it hits the detector   screen.

So we've turned a spin superposition  into a spatial superposition.

At the screen,   the superposition collapses  into one or the other location.

But we can also remove the screen and  use more magnets to bring the spatial   superpositions back together, in which  case we turn the spatial superposition   back into a spin superposition.

Now  if we measure that spin we should find   it collapses completely randomly  into 50-50 spin up or spin down.

If we were to run a second Stern-Gerlach  interferometer next to the first, the spins   we’d measure at the end of the second device  would have no relationship to the spins measured   at the end of the first.

Atoms in the different  devices are uncorrelated—they are not entangled.

But now let’s move the devices really, really  close together.

The spin-up path of interferometer   one is right alongside the spin-down path  of interferometer two.

Let’s pretend for   a moment that the silver atoms can interact  gravitationally.

The spin-up-one and spin-down-two   atoms tug on each other ever so slightly, while  the other paths are too far for the atoms to   interact.

Remember that the spin measurements at  the output of interferometer one versus two used   to be random—uncorrelated.

Up-up, down-down,  up-down, down-up should all be equally likely.

Any quantum interaction should generate  entanglement, so now, if gravity is quantum,   the outputs of the two interferometers  are entangled and correlated.

For example,   depending on how this is set up you may  find that that the interferometers produce   opposite spin results to each other very very  slightly more often than same spin results.

Now single atoms are way too light  to produce a measurable entanglement.

But what if we could do this with many  atoms?

Specifically, with a nanodiamond.

These are tiny diamonds with widths measured in  nanometers.

A little defect inside the diamond   crystal structure can be created by removing one  carbon atom and replacing it with a neighbouring   nitrogen atom.

This leaves an unpaired electron  which gives the nanodiamond a net spin.

The   same technology is actually a candidate  for qubit storage in quantum computers.

So we create nanodiamonds and put their  unpaired electrons in a spin superposition,   then shoot them through the pair of Stern-Gerlach  devices.

In principle we should get a spatial   superposition of these relatively massive  crystals, and if gravity is quantum, we should   get entanglement between the devices that will be  reflected in correlations in their final spins.

What would all of this mean if it proved to work?

Well from the point of view of particle  physics, the masses get entangled when   gravitons are exchanged between the two masses.

This is just like in electromagnetism, where   photon exchange leads to the electromagnetic  force, and sometimes entanglement.

We’ve   made a video before on how hard it is to  detect single gravitons even in principle,   so this entanglement between the two masses in  the QGEM experiment might be considered indirect   evidence for the existence of gravitons—the  hypothetical quantum carrier of gravity.

From the point of view of general  relativity, the superposition of   gravitational forces could be interpreted as  a superposition of spacetime geometries.

So,   spacetime itself would be exhibiting one of  the most quantum of all properties.

In general,   a positive QGEM result would be strong  evidence that gravity is indeed quantum.

These are just some examples of experiments that  have been proposed—or actually conducted—to test   some of these ideas about the link between  quantum mechanics and gravity.

Whether to find   quantum effects in the gravitational field,  or to show that gravity doesn’t even need   to be truly quantum.

There are lots of other  ideas and efforts in progress beyond these,   and there’s a good chance that some  will bear fruit—perhaps finally   unlocking that century-old mystery  of the quantum nature of spacetime.