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The experiment that could save physics
🕑 Added 2023-12-21 16:00:06 +0000 UTC[This is a transcript with links to references.]
Physicists at the University of Warwick in the UK are starting to build an experiment that could just save physics. Let’s have a look.
Research in the foundations of physics has been stuck for about 40 years, ever since the development of the standard model was completed. The big questions that were open then are still open: What is dark matter, how does a measurement work, and how does gravity work together with quantum physics? Physicists have tried very hard to answer these questions, but nothing has come out of it.
This new experiment is taking on the latter question, how does Einstein’s theory of General Relativity work together with quantum physics. The problem is that, mathematically the two don’t want to cooperate. We just talked about a possible solution to this problem last week. This new solution has it that gravity doesn’t become a quantum theory, but instead has random fluctuations.
But most physicists don’t think this is how it works. Most physicists think that gravity, too, is a quantum theory. And since Einstein taught us that gravity is really caused by the curvature of space-time, this means that space-time itself must have quantum properties.
So we have these two possibilities, space-time gets quantum properties and with that, gravity does, too. Or it doesn’t.
How could you find out? For a long time physicists thought it’s just impossible to test this because the effects of quantum gravity are just too small to be measurable. I’ve spent 10 years trying to argue against that but eventually gave up because I couldn’t get funding for it, but that’s another story. Be that as it may, I’m very excited to see that finally some experiments are underway, and little Albert agrees.
This news experiment in Warwick rests on a very simple idea. Entanglement is a type of correlation between two objects that is specific to quantum mechanics. To create entanglement between two quantum objects, the mediator that creates the engagement must also be quantum.
So the idea of the experiment is to take two small diamonds, make sure that gravity is the only force between them, and then measure whether they became entangled. If they did become entangled, gravity must be a quantum force.
The idea for this experiment has been around for some time. It’s a good idea in principle but in practice I think it’ll basically be impossible to eliminate the possibility that something else created the entanglement. You see, most people think of entanglement as something very special, but rather the opposite is the case. Everything gets constantly entangled with everything else around you. That’s because all those atoms bounce off each other and so on.
Indeed, the press release contains this very nice quote from David Moore,“We need to eliminate all interactions between the nanoparticles other than gravity, which is incredibly challenging since gravity is so weak.” Yes indeed.
The problem is then this following. The most likely thing this experiment will find is that they two diamonds do get entangled. Then they’ll have a hard time to argue it must have been gravity and nothing else. If that the diamonds don’t get entangled even though gravity acts between them, then that could be used to rule out the possibility that gravity was quantum. But this seems to me extremely unlikely, both because I don’t think that it’s possible to measure and because I don’t think that gravity is non-quantum.
I am therefore more enthusiastic about another experiment that’s logically kind of the opposite. This is an experiment which the group of Marcus Aspelmeyer in Vienna is working on. They want to bring two small masses into a superposition of two places and then measure where the gravitational pull goes. Depending on whether its quantum or not, it should go to the middle or fluctuate between the sides.
Now the thing is that in this kind of experiment, if you manage to keep the noise down, it’ll show you that gravity is quantum. With the other experiment it’s the opposite. So this is why I am more in favour of the Aspelmeyer experiment, though I think both should be done.
If you didn’t really understand that, don’t worry. I’ve been saying this for 10 years and no one understands it. I’m just recording this so that when they do their experiment in Warwick and the result is inconclusive, I can put on a smug grin and say “See, I told you so.”
Comments
I'm no physicist, but I am a computer scientist. If my area were quantum computing (it's not), I would have some niggling doubt in that back of my mind that should gravity turn out to be a strictly classical effect, this might place some limit on the scale of computer computers, a point in size at which we can't maintain the quantum behavior.
Rad, I agree wholeheartedly. The ArXiv paper of the Vienna group is here: https://arxiv.org/pdf/2203.05587.pdf "How to avoid the appearance of a classical world in gravity experiments" Markus Aspelmeyer [the single author] The text is less enthusiastic about proving that gravitation is non-quantum, than about hoping to prove its opposite. Also it seems to indicate that the table-top experiment they are beginning to design may take years to be realized: Excerpt: "It is, however, also fair to say that optimistic claims about the technical feasibility of these experiments are -at least from today’s view- highly exaggerated. It is certainly worthwhile to also investigate other phenomena involving coherent source mass distributions that provide us with observations that are inconsistent with what we expect from a classical (fixed) space-time metric. Most likely, the outcome of such experiments will be consistent with the predictions of an effective quantum field theory. Beyond the possibility that this is not the case, the actual benefit of this undertaking lies in ruling out semi-classical models of gravity and, probably more importantly, provide empirical evidence that gravity in fact requires a quantum description. An even more thrilling perspective is to gain access to the field degrees of freedom of gravity, either directly via optical clocks or indirectly by observing decay of entanglement between the masses through additional entanglement with the field. This represents an even more demanding experimental challenge, but who knows what the next 60 years will bring."
Rad Antonov
I don’t understand why the Warwick group thinks gravity will entangle the diamonds. They are starting out by entangling the spins of nitrogen vacancy centers. Nothing new there. An inhomogeneous magnetic field is then somehow going to kick them into a spatial superposition?! 🤔 The second approach makes more sense. Physicists have gotten really good at making large BECs, so maybe a matter wave interferometer could work, like the 700 atom one in this paper: https://www.nature.com/articles/s41586-022-05197-9 No matter, table top physics could still teach us a lot!
When we think and ponder about gravity, why don't we try the approach that Einstein started in 1911. There he considered the fact that the speed of light varies in a gravitational field. Unfortunately, he used a wrong equation for this dependence; perhaps he realized this and so returned to spacetime. - But now his original direction: We know that light is deflected by a large body like the sun, and this deflection can in fact be calculated as classical refraction. The result is exactly what Einstein achieved with his spacetime (even analytically!). On the other hand, we know that there is a permanent oscillation in elem. particles, which Schrödinger called "zitterbewegung" - for the electron; but valid for all particles. This oscillation is also subject to this refraction, and the resulting acceleration is exactly that which Einstein obtained through curved space-time. Recommendation to all: please continue along this path and you will find a theory of gravitation that avoids all the problems mentioned here (as dark matter, dark energy, quantum gravity).
