Scientists Have Figured Out How to Make Antigravity

[00:00:00]Scientists have figured out how to make anti-gravity. 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's always attractive. The reason's quite simple. In gravity, energy plays the role that electric charges play in electromagnetism. It just that since gravity's mediated by 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 the new paper, quantum mechanics offers a loophole.

[00:00:42]Under the right conditions, two attractive gravitational pulls can combine into an effective repulsion.

[00:00:50]That's a startling conclusion they arrived at by studying an entirely different question, namely how to find out whether gravity itself is quantum.

[00:01:00]This 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 don't fit together. It's like having two instruction manuals for the universe.

[00:01:26]One that says assemble with screws, and the other 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 of black holes or at the Big Bang. But in recent years, physicists have finally realized that it's possible to test quantum gravity in the laboratory. The currently most discussed idea is to use what's called an entanglement witness. That's if you create entanglement using only the gravitational interaction. If this is possible, then you can conclude that gravity must also have hard quantum properties. It's a difficult test because for this you have to put massive objects into 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. This 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 now suggest a slightly different route. Their proposal is to use one massive object, the source, in a superposition of two places and nearby is another particle, the probe. The source mass is put in two possible locations at once. The probe then feels gravity from both possible locations of the source. If the source were in just the first place, the probe would be attracted towards that. If the source were in the other place, the probe would also be attracted just towards this different place. 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, what? Doesn't this violate some sort of conservation 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 of the source particle. But in single runs, you can get this repulsive effect. Have you ever taken an online IQ test and instantly gotten a genius-level score?

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[00:05:02]And now, back to the science news. It looks like gravity became repulsive, but really, it happens because quantum particles are not just particles, they're 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 push you away rather than, as you would expect, make a pull. It's not a new fundamental force, it's an effective force that only occurs for specially prepared quantum states.

[00:05:35]But, that's exactly why it's 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 cesium atoms as the probe particles and find that the source mass would have to be about 20 micrograms. That's about 2 million times larger than what current technology allows. Though the masses might go down if either A, the decoherence time increases, B, force measurements become more accurate, or C, shorter distances can be resolved. So, no, you can't yet float to work, but given the current state of public transport, I understand why you ask. I give this paper a two out of 10 on the meter. I have misgivings about this sort of experiment in general because even if the effect is observed, I think it'll be extremely difficult to rule out it was some other interaction.

[00:06:48]And if it isn't observed, the result is inconclusive because that wouldn't mean gravity has no quantum properties. It'd 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. And if this works, I finally have a scientific excuse for why my hair does not obey gravity.

[00:07:18]Thanks for watching. See you tomorrow.