Today I want to talk about a topic that most physicists get wrong: How to test quantum

gravity. Most physicists believe it is just is not possible. But it is possible.

Einstein’s theory of general relativity tells us that gravity is due to the curvature

of space and time. But this theory is strictly speaking wrong. It is wrong because according

to general relativity, gravity does not have quantum properties. I told you all about this

in my earlier videos. This lacking quantum behavior of gravity gives rise to mathematical

inconsistencies that make no physical sense. To really make sense of gravity, we need a

theory of quantum gravity. But we do not have such a theory yet. In this video, we will

look at the experimental possibilities that we have to find the missing theory.

But before I do that, I want to tell you why so many physicists think that it is not possible

to test quantum gravity. The reason is that gravity is a very weak force and its quantum

effects are even weaker. Gravity does not seem weak in everyday life. But that is because

gravity, unlike all the other fundamental forces, does not neutralize. So, on long distances,

it is the only remaining force and that’s why we notice it so prominently. But if you

look at, for example, the gravitational force between an electron and a proton and the electromagnetic

force between them, then the electromagnetic force is a factor 10^40 stronger. One way

to see what this means is to look at a fridge magnet. The magnetic force of that tiny thing

is stronger than the gravitational pull of the whole planet.

Now, in most approaches to quantum gravity, the gravitational force is mediated by a particle.

This particle is called the graviton, and it belongs to the gravitational force the

same way that the photon belongs to the electromagnetic force. But since gravity is so much weaker

than the electromagnetic force, you need ridiculously high energies to produce a measureable amount

of gravitons. With the currently available technology, it would take a particle accelerator

about the size of the Milky Way to reach sufficiently high energies.

And this is why most physicists think that one cannot test quantum gravity. It is testable

in principle, all right, but not in practice, because one needs these ridiculously large

accelerators or detectors. However, this argument is wrong. It is wrong

because one does not need to produce a quantum of a field to demonstrate that the field must

be quantized. Take electromagnetism as an example. We have evidence that it must be

quantized right here. Because if it was not quantized, then atoms would not be stable.

Somewhat more quantitatively, the discrete energy levels of atomic spectral lines demonstrate

that electromagnetism is quantized. And you do not need to detect individual photons for

that.

With the quantization of gravity, it’s somewhat more difficult, but not impossible. A big

advantage of gravity is that the gravitational force becomes stronger for larger systems

because, recall, gravity, unlike the other forces, does not neutralize and therefore

add up. So, we can make quantum gravitational effects stronger by just taking larger masses and

bringing them into quantum states, for example into a state in which the masses are in two

places at once. One should then be able to tell whether the gravitational field is also

in two places at once. And if one can do that, one can demonstrate that gravity has quantum

behavior.

But the trouble is that quantum effects for large objects quickly fade away, or “decohere”

as the physicists say. So the challenge to measuring quantum gravity comes down to producing

and maintaining quantum states of heavy objects. “Heavy” here means something like a milli-gram.

That doesn’t sound heavy, but it is very heavy compared to the masses of elementary particles.

The objects you need for such an experiment have to be heavy enough so that one can actually

measure the gravitational field. There are a few experiments attempting to measure this.

Check the information below the video for references. But presently the masses that

one can bring into quantum states are not quite high enough. However, it is something

that will reasonably become possible in the coming decades.

Another good chance to observe quantum gravitational effects is to use the universe as laboratory.

Quantum gravitational effects should be strong right after the big bang and inside of black

holes. Evidence from what happened in the early universe could still be around today,

for example in the cosmic microwave background. Indeed, several groups are trying to find

out whether the cosmic microwave background can be analyzed to show that gravity must

have been quantized. But at least for now the signal is well below measurement precision.

With black holes, it’s more complicated, because the region where quantum gravity is

strong is hidden behind the event horizon. But some computer simulations seem to show

that stars can collapse without forming a horizon. In this case we could look right

at the quantum gravitational effects. The challenge with this idea is to find out just

how the observations would differ between a “normal” black hole and a singularity

without horizon but with quantum gravitational effects. Again, that’s subject of current

research.

And there are other options. For example, the theory of quantum gravity may violate

symmetries that are respected by general relativity. Symmetry violations can show up in high-precision

measurements at low energies, even if they are very small. This is something that one

can look for with particle decays or particle interactions and indeed something that various

experimental groups are looking for.

There are several other ways to test quantum gravity, but these are more speculative in

that they look for properties that a theory of quantum gravity may not have. For example,

the way in which gravitational waves are emitted in a black hole merger is different if the

black hole horizon has quantum effects. However, this may just not be the case. The same goes

for ideas that space itself may have the properties of a medium give rise to dispersion, which

means that light of different colors travels at different speed, or may have viscosity.

Again, this is something that one can look for, and that physicists are looking for.

It’s not our best shot though, because quantum gravity may not give rise to these effects.

In any case, as you can see, clearly it is possible to test quantum gravity. Indeed I

think it is possible that we will see experimental evidence for quantum gravity in the next 20

years, most likely by the type of test that I talked about first, with the massive quantum

objects.

Thanks for watching. See you next week.