Transcription
So, the superposition of spacetime itself, of the geometry itself, is what distinguishes a semi-classical or classical theory of gravity being combined with quantum mechanics versus a quantum theory of gravity.
That's an interesting subtlety, actually, that Kiara and I like to discuss, and we did it in the paper that you mentioned.
But, yeah, I think there is a difference between preparing gravity itself in a superposition of different configurations. If you think of a field, you can think of different configurations of the field or superpositions of geometries or whatnot.
The fact that gravity is quantum means you can have some features of quantumness. For example, the ability to create entanglement is one feature, and the ability to be set into a state that is a superposition of classical configurations is yet another feature.
This test that we are discussing checks the former: the fact that you can use gravity as a channel to create entanglement. However, it doesn't really prepare gravity in a quantum state that has the property of being in a superposition of different configurations.
So, it's more about showing that gravity must be described by an object that isn't classical, which in jargon we call a "q number." This means that when you consider it together with the classical features of gravity—such as some properties that you associate with gravity that are classical—then they can be measured simultaneously by the same instrument, by the same measuring device.
But this other q number that mediates the entanglement, which is a non-classical aspect of gravity, doesn't necessarily have the property of being preparable in a quantum state. This is an extra property.
The experiment doesn't show that you can put gravity into a superposition, but it shows that it has, I would say, a minimal set of properties that make it different from a classical object. Therefore, it cannot be described by a classical field or by a classical set of numbers, which are all commuting with themselves in a way and therefore measurable at the same instrument.
I think this is a subtle difference. It is indeed a subtle difference.
To put it this way, as well, to add to what Kiara said: this is a necessary property to talk about superpositions. If you didn't have this, then clearly you wouldn't even have superpositions of different spacetimes.
But to create the superposition of spacetimes, you may need to do more. Then there is the question of whether this is really feasible to do or not.
So, it's interesting that our experiment, if successful, wouldn't necessarily tell you whether you could superpose these things. You would have to do another experiment to really try to prepare a superposition.
The reason why I'm thinking this is that it could simply be that these superpositions decohere. They are simply coupling so quickly to everything else in the universe because gravity basically has that feature that it sees everything.
In fact, you cannot maintain the superposition for too long. These are what you would call superselection rules in physics: there are quantities that, even though they are fundamentally quantum, in this first sense that Kiara was describing, you still cannot utilize fully. You are not allowed to make some superpositions because they would get you into trouble with other things.
So, it's a very interesting thing, but I think it's certainly the thing to check, to test for beyond this proposal.
To make it a bit clearer, if you want to check that something is in a superposition, you need to do an interference experiment directly on this entity. For example, with a photon, you can set it up in a way that performs an interference experiment with some device, like a Mander interferometer or some kind of interferometry, so that you show that it has some wave-like properties.
With gravity, here we are testing a lesser property that the photon has as well, which is this ability to create entanglement and to mediate quantum features between two other quantum objects.
But it's not the full set of quantum properties. If you really want to check that something has the property of being in a superposition, you need to set up an interference experiment on the object itself.
In this case, with gravity, I think that's very hard to do. It's practically impossible to do because the quantum particles of gravity—the gravitons—are not detectable individually.
If you trust Freeman Dyson's conclusion, I think you would say that the corresponding experiment with the graviton would never be done, actually.
With the photon, it's relatively accessible because it's been done many times. Yes, with gravity, this experiment—the interference experiment with gravitons—can't be done, and that's because of the difference in fundamental constants and the way they couple and all of that.
So, that's somehow how this experiment, this test that we proposed, manages to bypass this problem by testing a different property, which still shows that gravity is quantum but in a different way.
It's an interesting fact.
Okay, I think the audience has been teased enough, so we'll get to the experiments in just a moment.
The GIE experiment—first, I want you to spell out specifically what is meant when you both have been using this word "mediate." Gravity mediates entanglement, or supposedly or potentially.
That's something I'd like you to spell out. I'd also like you, when you start to talk about GIE, to distinguish it from Bose's initial experiments or proposals for experiments.
Maybe in your paper you've outlined Paig and Gilker and Cow. If you think those will take us off track, then you don't need to go into Paig and Gilker or Cow, but if you think that’s useful as a stepping stone, then feel free.
No, we can do that. It's, yeah, the floor is yours.
Shall I just?
Yeah, I can say a few things about this mediation generically. Actually, then you can even decide whether this fits in or not in the long story. I think these are all very beautiful topics about the mediation.
So, physics is, like I said, all about fields. Actually, what's behind that is locality. We believe that all interactions in physics take place at the same point. When two entities interact, they must be at the same point at the same time, and then they could affect the neighbors, and then the neighbors affect the neighbors, and then this propagates.
This was a main problem for Newton, by the way. His gravity was action at a distance, and he himself was extremely troubled by this. There are exchanges of letters with other scientists at that time, philosophers, where he acknowledges that no one in their right mind would think that planets can affect one another instantaneously.
The genius idea of Faraday, which by the way Einstein and Maxwell formalized, and then Einstein took on board from electrodynamics and applied to gravity, was basically to introduce a field in between.
So, a planet only disturbs its neighborhood, creates gravity in the neighborhood, and that gravity propagates like a wave at the speed of light. Then it affects other objects when it reaches them, shaking the other objects locally.
That's basically your mediation. That's what we call a mediator: the field mediates the force, if you like, between these entities.
Now, this was very important because you couldn't have conservation principles if you didn't have fields.
Let me give you a very simple example. When a planet is close to the Sun, it moves faster and has a higher kinetic energy. When it's further away, it has lower kinetic energy.
Now, imagine it's very close to the Sun and it starts to move away, which means it loses energy. Where does the energy go?
Now, you say, "Well, it takes 8 minutes for that signal to come from the Earth to the Sun." But that doesn't sound logical to a physicist that for eight minutes energy is not conserved. It goes, God knows where, and then suddenly after 8 minutes, the Sun picks up the momentum and says, "Oh, the Earth just lost some energy 8 minutes ago. I better speed up a bit."
That would be very funny if it operated like that.
With the field, there is absolutely no problem. The energy is stored in the local field, propagates to the Sun, and then the Sun picks that up. Overall, the field, together with the two objects, perfectly conserves energy, momentum, angular momentum, anything else.
That's the true value of this concept of the field.
Now, imagine quantum mechanically—and I think that's part of what Kiara and I, what our argument relies on fundamentally—in quantum mechanics, objects can be in two places at the same time.
So now, in order to conserve anything, the field better understand how to respond simultaneously to the object in place one and the object in place two.
A classical field doesn't understand what that means. That's why, as Kiara was emphasizing, you really need these q numbers to also describe the field.
The field must be able to respond simultaneously and conserve all of these entities in both places at the same time.
Actually, that's key to our experiment that propagates locally to the other particle. In this picture, all of these principles are beautifully upheld.
So, I think a problem for any of these semiclassical theories is even bigger. They literally don't know how to strictly conserve these entities. They may have to do it stochastically, on average, things like that—probabilistic conservation—but they can never do it exactly.
Somehow, to ask this looks like a huge price to pay. That's why it sounds kind of illogical that gravity is not quantum.
But I think you wanted to...
Yeah, no, I wanted to just comment on the fact that, I think more broadly speaking, the mediation requires there to be—if you have two entities that, like in the experiment, interact with each other—there's one way that can happen.
As you were saying, it can happen that they interact directly, so they just talk to each other directly. Or they don't talk to each other directly, but they talk through a third element, a third system.
Now, this third system could be a field with all of the mathematical apparatus that describes a field, but it could also be a more general entity.
So, it doesn't have to be a field necessarily, but as you were saying, it's important to have the idea of the interaction happening not instantaneously directly between these two objects if they are spacelike separated, so they are distant from each other in a sense.
But there has to be a third system, which in this case is gravity, that mediates the interaction indeed.
In a way, this test that we are proposing does assume that the interaction is mediated in this way.
I think it's a good assumption because, as V was saying, this is what most of the rest of the theories of gravity that we've had—those that are meaningful, not just general relativity but other proposals as well—are mediated in this way.
This is a difference from Newton's gravitation theory because, in that case, Newton himself had issues with the fact that there was this instantaneous action at a distance, which bothered him a lot.
That's because the interaction was not mediated.
Indeed, this is a nice point also to link back to what we said. This is yet another set of theories that we couldn't rule out.
So, we can't rule out semi-classical descriptions, but out of the quantum descriptions—even the weird descriptions like non-local signals back from the future and things like that—none of that would actually be invalidated by our experiment.
Yeah, you mean classical descriptions.
Yes, all the classical descriptions that are not mediated are not tested by our experiment.
So, they could still maybe be able to create entanglement, but they would do so in a non-local way.
Yes, and we think that there are lots of strong reasons to just assume in the background knowledge the fact that we have, that you know, that the locality is satisfied and that we have mediated interaction for gravity.
But it's an important assumption.
So, it was nice that you asked the question.
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