Last week, blogged about a rather interesting experiment, designed to test the quantum properties of gravity. Normally, quantum gravity is essentially unobservable: quantum effects are typically only relevant for very small systems, where gravity is extremely weak. However, there has been a lot of progress in putting larger and larger systems into interesting quantum states, and a team of experimentalists has recently proposed a setup. The experiment wouldn’t have enough detail to, for example, distinguish between rival models of quantum gravity, but it would provide evidence as to whether or not gravity is quantum at all.
Lubos Motl, meanwhile, argues that such an experiment is utterly pointless, because there is no possible way that gravity could not be quantum. I won’t blame you if you don’t read his argument since it’s written in his trademark…aggressive…style, but the gist is that it’s really hard to make sense of the idea that there are non-quantum things in an otherwise quantum world. It causes all sorts of issues with pretty much every interpretation of quantum mechanics, and throws the differences between those interpretations into particularly harsh and obvious light. From this perspective, checking to see if gravity might not actually be quantum (an idea called semi-classical gravity) is a bit like checking for a monster under the bed.
You might find semi-classical gravity!
In general, I share Motl’s reservations about semi-classical gravity. As I mentioned back when journalists were touting the BICEP2 results as evidence of quantum gravity, the idea that gravity could not be quantum doesn’t really make much sense. (Incidentally, Hossenfelder makes a similar point in her post.)
All that said, sometimes in science it’s absolutely worth looking under the bed.
Take another unlikely possibility, that of cell phone radiation causing cancer. Things that cause cancer do it by messing with the molecular bonds in DNA. In order to mess with molecular bonds, you need high-frequency light. That’s how UV light from the sun can cause skin cancer. Cell phones emit microwaves, which are very low-frequency light. It’s what allows them to be useful inside of buildings, where normal light wouldn’t reach. It also means it’s impossible for them to cause cancer.
Nevertheless, if nobody had ever studied whether cell phones cause cancer, it would probably be worth at least one study. If that study came back positive, it would say something interesting, either about the study’s design or about other possible causes of cancer. If negative, the topic could be put to bed more convincingly. As it happens, those studies have been done, and overall confirm the expectations we have from basic science.
Another important point here is that experimentalists and theorists have different priorities, due to their different specializations. Theorists are interested in confirmation for particular theories: they want not just an unknown particle, but a gluino, and not just a gluino, but the gluino predicted by their particular model of supersymmetry. By contrast, experimentalists typically aren’t very interested in proving or disproving one theory or another. Rather, they look for general signals that indicate broad classes of new physics. For example, experimentalists might use the LHC to look for a leptoquark, a particle that allows quarks and leptons to interact, without caring what theory might produce them. Experimentalists are also very interested in improving their techniques. Much like theorists, a lot of interesting work in the field involves pushing the current state-of-the-art as far as it will go.
So, when should we look under the bed?
Well, if nobody has ever looked under this particular bed before, and if seeing something strange under this bed would at least be informative, and if looking under the bed serves as a proving ground for the latest in bed-spelunking technology, then yes, we should absolutely look under this bed.
Just don’t expect to see any monsters.
4 gravitons 
Exciting! Certainly evidence of QG would let me stop hoping GR gets it right! (I do realize what a faint hope it is.) XD
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I can picture doing quantum mechanics with Newtonian Gravity, just as we treat Coulomb interactions in non-relativistic QM. We use the classical expression for the Coulomb interaction. We don’t consider the intricacies of the EM field and bare vs ‘dressed’ charges.
I’m not familiar with Lubus Motl’s work, but I’ll but take his word that we can’t incorporate classical GR directly into quantum field theory without quantizing the gravitational field.
I am reminded of some work that happened with neutrons:
http://physicsworld.com/cws/article/news/2002/jan/17/neutrons-reveal-quantum-effects-of-gravity
my understanding of this is that they saw the neutrons behaving like they should if they were in a linear potential , according to non-relativistic QM. This is not the same as showing the gravitational field is quantized, but it at least shows that particles move in the gravitational potential in a quantum mechanical way.
apparently a similar experiment was just done recently:
http://www.nature.com/news/bouncing-neutrons-probe-dark-energy-on-a-table-top-1.15062
Personally, I’m wondering if these experiments call tell us anything about the validity of entropic theories of the gravitational force. I have sort of a minimal understanding of entropic theories, but it seems one of the ideas is that gravity may be a collective phenomena, therefore it doesn’t effect individual particles the same way as large objects. Am I right here?
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So there’s a difference here between treating gravity as classical at a first approximation, and thinking of it as classical “all the way down”. Doing the former is perfectly normal, just like it is for E&M. The latter, though, opens one up to all kinds of awkward “Schrodinger’s cat”-esque situations, which is what Lubos fills most of his post ranting about.
I had not heard of either of those experiments before, interesting. At a glance, I think the difference with this experiment is that it is set up to detect not only energy levels under a gravitational field, but energy level splitting due to a gravitational field. But it’s not immediately obvious to me that that’s an especially important distinction, now that I think about it.
I don’t know much about entropic theories of gravity, but my guess is that even they would view gravity as a quantum phenomenon. If they lead to differences in experimental predictions, it’s probably at much higher energies.
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Yeah, I read most of the blog post you linked to, and it wasn’t clear to me exactly what distinguishes the proposed experiment from the ones with neutrons. It sounds like the proposed experiment probes deeper. I sorta understood his analogy about measuring the gravitational effect of a particle going through a double slit, but it wasn’t clear how that was linked to the proposed experiment. I have 0% understanding of the “Schrodinger-Newton equation”. It sounds interesting, but I think I’ll focus on my research at hand rather than trying to understand it right now. It sounds a bit sketchy, anyway =).
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it might be that since they measure the effects of gravitational ‘self interaction’, they measure the perturbation of the energy levels of the (quantum) system due to its own gravitation. That may be an important distinction from measuring the perturbation of the energy levels due the Earth, which, since it is a macroscopic object, doesn’t have any quantum character.
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That seems plausible, yeah.
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The reason it matters to rule out that gravity is classical – even though we all seem to agree it’s hard to see how it could be theoretically inconsistent – is that theoretical arguments don’t count as evidence for most people. I’m not sure why that is and one can cry over it, but it’s true. If you don’t know what I am referring to, check out some quotes by Freeman Dyson, who has been going about for decades claiming that “no inconsistency can ever arise” from using quantum theory and classical gravity because it’s presumably impossible to ever make measurements in a regime where both are non-negligible.
The moment that experimental precision becomes sensitive to measuring quantum properties of gravitational fields, or their absence, is the moment when quantum gravity becomes a “real science” in the eyes of many people.
Besides this, there is another reason this is an exciting development. Once you have one experiment that is sensitive to the general scales involved in quantum gravity, you can probably conceive of other tests, tests that might be more conclusive. See, the guys who wrote this paper have been on the SN equation for a while and that’s what they do, fine. But I would expect that by orders of magnitude there should come up other possibilities in this parameter range. I am pretty sure we’ll hear more about this in the future.
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Thanks for the comment!
I completely agree that you have to make some things experimental in order to convince the naysayers, that’s what the cell phone analogy was about. And yeah, it’s extremely encouraging that it’s possible to test this sort of thing now. A large part of the value of this experiment is in pushing technology to the point where they can ask something more meaningful than just “quantum gravity, Y/N?” (I would hope even Lubos would appreciate that 😉 )
By the way, you wouldn’t happen to have an answer to MoreIsDifferent’s query above? Off-hand it wasn’t obvious to me why the experiments he links didn’t already in some sense rule out the monster under the bed.
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Dear Tetragraviton, thanks for your remarks, ideas, and nice blog. I tried to make some readers of mine aware of your blog. It seems obvious that we would agree about all the things.
Right now, I have spent 5 minutes looking for the Quantum Fields and Strings postdocs at the Perimeter who might be doing the “amplitudes”. Correct me if I am wrong but your initials are rather similar to the maximally helicity violating amplitudes, right? 😉
Hi Sabine, don’t you think that the essence of the theories, experiments, and their claimed relationships are a different issues than “what most people want” or whether some people are called “naysayers”? Could you please separate these things? When something is being said by most people, it doesn’t mean that they’re right, and if someone is a naysayer, it doesn’t mean that he has reasons and it doesn’t mean that he will change a mind according to some logical argument.
Now, if you could address the essence. I think that that the experiments testing “whether the gravity is quantum” are pointless because no currently conceivable result of any experiment could be correctly interpreted as evidence that “gravity is not quantum”. If you disagree, could you please tell us what the experiment is, what is the desired outcome of the experiment, and what is the logical argument that implies that this outcome says that “gravity is not quantum”?
There is a simple reason why I am sure that all such “arguments” proving that “gravity is not quantum” are invalid. The reason is that basic experiments are enough to show that at least microscopic systems obey the probabilistic rules of quantum mechanics; large systems may be made correlated to the properties of microscopic systems; one may measure gravitational fields that also depends on outcomes of microscopic experiments. Because the gravitational fields have been measured to be correlated with masses and positions of large objects and because it’s easy to design experiments where those are tightly correlated with properties of microscopic objects and because the microscopic objects have been verified to obey the laws of quantum mechanics and not classical physics, it follows that the large objects and their gravitational fields also have to follow the rules of quantum mechanics and in particular, they can’t be deterministic.
So every experimenter or would-be theorist claiming that his experiment may prove or will have proven that gravity is not quantum is simply making a mistake. Whether he is a naysayer or whether he can find 7 billion people whom he can fool is totally irrelevant.
LM
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Thanks for the comment. Indeed, while I maintain a thin veneer of anonymity on this blog, it’s relatively simple to figure out who I am. 😉
I think you might appreciate the following: while testing whether or not gravity is quantum doesn’t make much sense (and in this case is likely to be largely experimentalist bluster), using the fact of quantum gravity to test the precision of state-of-the-art large quantum states seems like a much more worthwhile endeavor. We know that gravity is quantum, but we don’t know whether the experimental community has the tools to observe that, and that seems like a scientific question worth asking.
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Dear Tetragraviton, don’t get me wrong, I do think that an experiment seeing energy levels of a slowly spinning mesoscopic osmium disk – energy eigenvalues that require the self-gravity of the disk to be taken into account – would be a fun experiment. I would love to see it, too. It would surely be great as a lab problem for undergraduates or something like that.
But such an experiment isn’t testing quantum gravity and “gravity is not quantum” just isn’t a possible valid conclusion from this experiment. Concerning the first point, extremely low-energy mundane lab experiments like that just can’t test “quantum gravity”. “Quantum gravity” only starts when one becomes sensitive to features like the high-dimension operators and effects that dominate the Planck scale physics. This just isn’t one. It’s still quantum mechanics of a mesoscopic object in a smooth external gravitational field. If one decomposes what’s going on, it’s still testing “whether the gravitational field is centered around the masses and drops like the inverse square of the distance etc.” and “whether the matter moves in the external potential energy fields as we thought” i.e. according to the quantum mechanical equation.
Concerning the second, if there were deviations of the results of this experiment from the predictions, there would be no possible interpretation of “gravity has been shown to be non-quantum” simply because there doesn’t exist any viable theory of this kind that could compete with theories that actually do exist – and all these viable theories are variations of the theory we have within the QM framework. If the levels had different spacings or something like that, people would start to include some higher terms in the gravity and non-gravity part of the dynamics, but everything 100% exactly within the quantum framework. And the deviations from the predicted spectrum could also very well be due to some non-gravitational extra terms so one would have to study the rigidity of the disk and other things, essentially condensed matter physics first. Even the idea that the deviation has something to do with gravity would be far-fetched.
The very opinion that “gravity has been shown to be non-quantum” would be one of the first conclusions from a result of this experiment unmasks the incompetency of the speaker in theoretical physics. It is at most a widespread popular-physics myth. Serious physics just wasn’t testing whether “the world follows the laws of quantum mechanics” for very many decades. These questions were sensible in the 1920s and perhaps the 1930s but not afterwards. Every competent physicist has known that the world has been demonstrated to be non-classical from those decades. Much finer questions were the subject of the later state-of-the-art physics.
Claims that a slowly rotating disk of osmium contributes to the research of quantum gravity are ludicrously arrogant self-promoting hype.
LM
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Kudos on making a blog post that secures comments from both bloggers discussed in it.
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