[00:00] Scientists have figured out how to&nbsp; make anti-gravity. Sounds crazy,&nbsp;&nbsp; yet I think it might actually work. But before&nbsp; you go and buy ceiling furniture, let's have a&nbsp;&nbsp; look at the "ifs and buts." In Einstein's theory of&nbsp; general relativity, gravity is always attractive.&nbsp;&nbsp;
[00:18] The reason is quite simple. In gravity, energy&nbsp; plays the role that electric charges play in&nbsp;&nbsp; electromagnetism. It's just that since gravity is&nbsp; mediated by a spin-2 field, like charges attract&nbsp;&nbsp;
[00:30] and unlike charges repel. And since there are only&nbsp; positive masses and only positive energies, they&nbsp;&nbsp; all attract each other. But according to the new&nbsp; paper, quantum mechanics offers a loophole. Under&nbsp;&nbsp;
[00:42] the right conditions, two attractive gravitational&nbsp; poles can combine into an effective repulsion.&nbsp;&nbsp; That's a stunning conclusion they arrived at by&nbsp; studying an entirely different question, namely&nbsp;&nbsp;
[00:56] how to find out whether gravity itself is quantum.&nbsp; This is one of the biggest unsolved problems in&nbsp;&nbsp; physics. We have quantum mechanics which describes&nbsp; atoms and elementary particles through quantum&nbsp;&nbsp;
[01:11] fields and we have general relativity which&nbsp; describes gravity as the curvature of spaceime.&nbsp;&nbsp; Both theories work extremely well. Unfortunately,&nbsp; they don't fit together. It's like having two&nbsp;&nbsp;
[01:23] instruction manuals for the universe. One that&nbsp; says, "Assemble with screws," and the other says,&nbsp;&nbsp; "There are no screws." And then a theorist walks&nbsp; in and says, "Have you tried 11 dimensions?" For&nbsp;&nbsp;
[01:36] a long time, most people thought that testing&nbsp; quantum gravity require absurdly high energies,&nbsp;&nbsp; like inside of black holes or at the big bang. But&nbsp; in recent years, physicists have finally realized&nbsp;&nbsp;
[01:48] that it's possible to test quantum gravity in the&nbsp; laboratory. The currently most discussed ideas&nbsp;&nbsp; to use what's called an entanglement witness.&nbsp; That's if you create entanglement using only the&nbsp;&nbsp;
[02:00] gravitational interaction. If this is possible,&nbsp; then you can conclude that gravity must also&nbsp;&nbsp; have had quantum properties. It's a difficult&nbsp; test because for this you have to put massive&nbsp;&nbsp;
[02:12] objects into a Quantum Superposition. They have&nbsp; to be massive enough so that the gravitational&nbsp;&nbsp; interaction has a measurable effect. The problem&nbsp; is that the quantum properties of massive objects&nbsp;&nbsp;
[02:24] go away very quickly. This is why we don't&nbsp; normally see people being in two places at once.&nbsp;&nbsp; It's also why the experiment has not yet been&nbsp; done. The authors of the new paper now suggest&nbsp;&nbsp;
[02:37] a slightly different route. Their proposal is to&nbsp; use one massive object, the source in a superposition of two places and nearby is another&nbsp; particle, the probe. The source mass is put&nbsp;&nbsp;
[02:51] into two possible locations at once. The probe&nbsp; then feels gravity from both possible locations&nbsp;&nbsp; of the source. If the source were in just the&nbsp; first place, the probe would be attracted towards&nbsp;&nbsp;
[03:04] that. If the source were in the other place, the&nbsp; probe would also be attracted just towards this&nbsp;&nbsp; different place. The twist is that if you have&nbsp; the source in two places at once, you can create&nbsp;&nbsp;
[03:17] a destructive interference that leads to a net&nbsp; repulsion. Wait, what? Doesn't this violate some&nbsp;&nbsp; sort of conservation law? No, it's fine, really,&nbsp; if you average over many runs of the experiment,&nbsp;&nbsp;
[03:30] you get what you expect, namely that the probe&nbsp; particle is attracted to the average of the source&nbsp;&nbsp; particle. But in single runs, you can get this&nbsp; repulsive effect. Have you ever taken an online&nbsp;&nbsp;
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[04:52] assessment today. Head over to cognitivetesting.org to&nbsp; take a legitimate IQ test. That's cognitivetesting.org.&nbsp;&nbsp; And now back to the science news. It looks like&nbsp; gravity became repulsive, but really it happens&nbsp;&nbsp;
[05:09] because quantum particles are not just particles.&nbsp; They're also waves, and waves can interfere. If&nbsp;&nbsp; you have a particle in two places at once, then&nbsp; the particle has not one particular momentum,&nbsp;&nbsp; but many different ones. And if these add up in&nbsp; a weird way, the result can push you away rather&nbsp;&nbsp;
[05:25] than, as you would expect, make a pole. It's&nbsp; not a new fundamental force. It's an effective&nbsp;&nbsp; force that only occurs for specially prepared&nbsp; quantum states. But that's exactly why it's&nbsp;&nbsp;
[05:37] interesting. If the gravitational interaction&nbsp; didn't have quantum properties, it could not&nbsp;&nbsp; produce this effect. If the wrong-way kick is&nbsp; observed and all ordinary forces are ruled out,&nbsp;&nbsp; it would be evidence that gravity has quantum&nbsp; properties. That said, the experiment is still&nbsp;&nbsp;
[05:53] hard because the gravitational interaction between&nbsp; small objects is ridiculously weak. The authors&nbsp;&nbsp; make an estimate with Cesium atoms as the probe&nbsp; particles and find that the source mass would have&nbsp;&nbsp;
[06:06] to be about 20 micrograms. That's about 2 million&nbsp; times larger than what current technology allows.&nbsp;&nbsp; Though the masses might go down if either a) the&nbsp; decoherence time increases, b) force measurements&nbsp;&nbsp;
[06:19] become more accurate or c) shorter distances can&nbsp; be resolved. So no, you can't yet float to work.&nbsp;&nbsp; But given the current state of public transport, I&nbsp; understand why you asked. I give this paper a
[06:33] 2 out of 10 on the [&nbsp;__&nbsp;] meter. I have misgivings&nbsp; about this sort of experiment in general because&nbsp;&nbsp; even if the effect is observed, I think it'll be&nbsp; extremely difficult to rule out it was some other&nbsp;&nbsp;
[06:47] interaction. And if it isn't observed, the result&nbsp; is inconclusive because that wouldn't mean gravity&nbsp;&nbsp; has no quantum properties. It'd just mean it doesn't&nbsp; have these particular quantum properties. Still,&nbsp;&nbsp;
[07:00] I have to say that this is a neat contribution&nbsp; to the literature and I hope the question of&nbsp;&nbsp; repulsive gravity will receive more attention. And&nbsp; if this works, I finally have a scientific excuse&nbsp;&nbsp;
[07:14] for why my hair does not obey gravity.&nbsp; Thanks for watching. See you tomorrow.