# Why not use this experiment to test gravity's quantum properties?

If a heavy object $$X$$ is in superposition, let's say, at two places "at the same time", to which point is the gravitational pull of that object directed to? This can probably not be answered without having a quantum theory of gravity, but I read (somewhere on this site) that two possible options are:

1. the gravitational field itself goes into superposition, or
2. the gravitational field will be some kind of average of the fields for "the objects being there" and "the objects being here".

I do not want to discuss these options, but I want to know, why we currently cannot design an experiment to just test which is true. It is always said that the problem is getting a heavy enough object into superposition, but what about the following:

1. Design a quantum experiment which yields a 0/1-answer based on quantum randomness (e.g. did the atom decay or not?).
2. Based on the result of this, push a large mass (100 tons) to the left or to the right.
3. Detect where the gravitational pull of that mass is directed to (to the left/right? some average?).

This will probably not work, and I am here to understand why? Isn't the large mass in superposition now (induced by the random decay)? Maybe not relative to us who conduct the experiment. What if the random choice and the subsequent push of the mass happens automatically in some isolated container from within the mass can interact only gravitationally? Shouldn't it now be in a superposition (Schrödinger's cat style) and we can check in which direction the pull goes?

• Superposition is not a real and physical thing and it's only used to satisfy the math. You can't really have something in two places, nor do you need to go to that extreme to explain things. Commented Jan 16, 2020 at 20:50
• @Bill I don't understand this. What's the difference between "satisfying the math" and being a real/physical thing? It's as real as necessary to explain reality. Commented Jan 16, 2020 at 20:56
• When you said experiment I thought you meant a real experiment. Commented Jan 16, 2020 at 21:22
• What you're asking about is called the Page-Geilker experiment. We've had several questions about this in the last few days. Commentary at backreaction.blogspot.com/2012/01/… . Basically we can't do a good version of the experiment without getting killed by decoherence.
– user4552
Commented Jan 17, 2020 at 1:43
• @BenCrowell Great! So the experiment was done and its result was that the gravitational pull is in the direction where the mass actually is. Commented Jan 17, 2020 at 9:58

This is a perfectly good idea, and there are definitely experiments that test this. The issue, however, is that there are really three possibilities. Suppose that we have a mass in superposition, $$|\psi \rangle \sim \frac{|\text{here} \rangle + |\text{there} \rangle}{\sqrt{2}}.$$ Let the gravitational fields correspond to the two states be $$|g_{\text{here}} \rangle$$ and $$|g_{\text{there}} \rangle$$. The possibilities are:

1. The gravitational field behaves like any other quantum field; it enters the superposition, so that the joint state of the field and mass are $$|\Psi\rangle \sim \frac{|\text{here}, g_{\text{here}} \rangle + |\text{there}, g_{\text{there}} \rangle}{\sqrt{2}}.$$
2. The gravitational field is inherently classical, so it has a definite value given by averaging over the possible positions of the mass, $$g = (g_{\text{here}} + g_{\text{there}})/2$$.
3. The gravitational field is inherently classical, and it causes widely separated superpositions to collapse, through a mechanism which is beyond the standard rules of quantum mechanics. In other words, you end up with either $$|\text{here} \rangle$$ and $$g_{\text{here}}$$, or $$|\text{there} \rangle$$ and $$g_{\text{there}}$$, with 50/50 probability, not a superposition.

Using your idea, it would be very straightforward to test (2). The issue is, just about nobody believes that (2) is true! Forget about moving masses around in the lab: things have been going into quantum superpositions since the beginning of the universe, in a way that impacted cosmological structure formation. The very fact that the Earth orbits the Sun, instead of some very widely smeared out mass distribution, rules out (2).

It is possible to test (3), but it's trickier. The first issue is that the particular mechanism of collapse, and its rate, depends on the (speculative) way you extend quantum mechanics. (You recover (2) in the limit of zero collapse rate.) There is a wide range of possibilities, some of which are much harder to test than others. I don't think Penrose, who is the most famous proponent of (3), has a concrete model in mind either.

Putting that aside, you could test (2) by performing a sensitive interferometry experiment, and looking for gravitationally induced decoherence, i.e. a reduction of the interference fringe visibility. This has indeed been investigated, e.g. see Quantum Gravitational Decoherence of Matter Waves (2006) and Gravitational Decoherence of Atomic Interferometers (2002). Upcoming experiments which seek to produce macroscopic superpositions, such as MAQRO, would also automatically test this. However, it's all hampered by the lack of a concrete model to target.

The reason you don't hear about these experiments too often is that almost everyone believes (1). In general, it doesn't seem to make sense to couple totally classical and totally quantum objects, without modifying the rules of quantum mechanics. We've been through this with the electromagnetic field, where a treatment like (2) runs into paradoxes$${}^1$$.

As a result, the default assumption is that the gravitational field should be described at low energies just like the electromagnetic field, namely with the rules of quantum field theory. Just about all approaches to quantum gravity, such as string theory, loop quantum gravity, and asymptotic safety, try to reproduce this. It is, however, still important and interesting to check it. So it's good that a number of experiments are doing just that!

$${}^1$$ Sometimes (2) is used in semiclassical gravity calculations, but I think it's well-accepted that it doesn't make physical sense in general. In the cases where it is used, you don't get matter superpositions that produce radically different gravitational fields.

• That's a great answer. Thank you. The only thing is that it is not so absolutely clear to me that (2) can be rules out so quickly. How precisely can we measure the point that the earth actually orbits? And if "very precisely", should we really expect that the suns mass is "smeared out" that much today? After all, all these "copied masses" attract each other too and keep themselves together. Also, its not like we are searching for a lot of invisible mass right now, isn't it? Are there any sources you can point me to were this option is debated and ruled out beyond any doubt? Commented Jan 16, 2020 at 20:53
• @M.Winter Well, (2) is really in trouble if you believe in the standard rules of QM "all the way up". For example, it is thought that all structure in the universe is seeded from quantum fluctuations -- which means not only the position of the Sun, but even the existence of our galaxy, is in principle in a superposition. But I don't know of a source where this argument is made carefully and explicitly, hopefully somebody else can leave one. Commented Jan 16, 2020 at 20:59
• These two scenarios are very different; e.g. if the source mass is in an equal superposition of being in just two locations, then in the first scenario, the $\vec{g}$ field will be very weak near the midpoint of the two locations, but in the second scenario it will be very strong. I think that these scenarios are both vaguely plausible to consider, so there should really be four options to consider, not three. Commented Dec 23, 2022 at 15:30
• @tparker Hmm, I think I mean your first scenario, which should correspond to what people call semiclassical gravity ($G_{\mu\nu} \propto \langle T_{\mu\nu} \rangle$). I do agree they're both interesting possibilities though! Commented Jan 6, 2023 at 4:13
• @Andrew You're right, thanks; I edited! Commented Jan 6, 2023 at 4:29