This Week’s Science News from Sabine

Scientists have figured out how to make antigravity. Sounds crazy, yet I think it might actually work. But before you go and buy ceiling furniture, let’s have a look at the ifs and buts.

In Einstein’s theory of general relativity gravity is always attractive. The reason is quite simple: in gravity, energy plays the role that electric charges play in electromagnetism. It’s just that since gravity is mediated by a spin-2 field, like charges attract and unlike charges repel. And since there are only positive masses and only positive energies, they all attract each other.

But according to a new paper, quantum mechanics offers a loophole. Under the right conditions, two attractive gravitational pulls can combine into an effective repulsion. That’s a stunning conclusion, and the authors arrived at it by studying an entirely different question, namely how to find out whether gravity itself is quantum.

This is one of the biggest unsolved problems in physics. We have quantum mechanics, which describes atoms and elementary particles through quantum fields. And we have general relativity, which describes gravity as the curvature of space-time. Both theories work extremely well. Unfortunately, they do not fit together. It’s like having two instruction manuals for the universe, one that says “assemble with screws” and another that says “there are no screws”. And then a theorist walks in and says, “Have you tried 11 dimensions?”

For a long time, most people thought that testing quantum gravity would require absurdly high energies, like inside black holes or at the Big Bang. But in recent years, physicists have finally realized that it is possible to test quantum gravity in the laboratory. The currently most-discussed idea is to use what’s called an “entanglement witness”. That is, if you create entanglement using only the gravitational interaction, then you can conclude that gravity must also have had quantum properties.

It’s a difficult test, because for this you have to put massive objects into a quantum superposition. They have to be massive enough so that the gravitational interaction has a measurable effect. The problem is that the quantum properties of massive objects go away very quickly, which is why we don’t normally see people being in two places at once. It’s also why the experiment has not yet been done.

The authors of the new paper suggest a slightly different route. Their proposal is to use one massive object, the source, in a superposition of two places. Nearby is another particle, the probe. The source mass is put into two possible locations at once, and the probe then feels gravity from both possible locations of the source.

If the source were in just one place, the probe would be attracted towards it. If the source were in the other place, the probe would also be attracted towards it, just slightly differently. The twist is that if you have the source in two places at once, you can create a destructive interference that leads to a net repulsion.

Wait, doesn’t this violate some sort of conservation law? No, it’s fine, really. If you average over many runs of the experiment, you get what you expect, namely that the probe particle is attracted to the average position of the source particle. But in single runs, you can get this repulsive effect.

It looks like gravity became repulsive. But really it happens because quantum particles are not just particles, they are also waves. And waves can interfere. If you have a particle in two places at once, then the particle has not one particular momentum but many different ones. And if these add up in a weird way, the result can be a push rather than, as you would expect, a pull.

It’s not a new fundamental force. It is an effective force that only occurs for specially prepared quantum states. But that is exactly why it is interesting. If the gravitational interaction didn’t have quantum properties, it could not produce this effect. If the wrong-way kick is observed and all ordinary forces are ruled out, it would be evidence that gravity has quantum properties.

That said, the experiment is still hard, because the gravitational interaction between small objects is ridiculously weak. The authors make an estimate with caesium atoms as the probe particles and find that the source mass would have to be about 20 µg. That’s about 2 million times larger than what current technology allows. The masses might come down if the decoherence time increases, if force measurements become more accurate, or if shorter distances can be resolved. So no, you cannot yet float to work.

I give this paper 2 out of 10 on the bullshit meter. I have misgivings about this sort of experiment, because even if the effect is observed I think it’ll be extremely difficult to rule out that it was some other interaction. And if it isn’t observed, the result is inconclusive, because that wouldn’t mean gravity has no quantum properties, it would just mean it doesn’t have these particular quantum properties.

Still, I have to say that this is a neat contribution to the literature, and I hope the question of repulsive gravity will receive more attention.