Scientists Have Figured Out How to Make Antigravity

[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 is always attractive.  

[00:18] 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  

[00:30] 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. Under  

[00:42] the right conditions, two attractive gravitational  poles can combine into an effective repulsion.   That's a stunning conclusion they arrived at by  studying an entirely different question, namely  

[00:56] 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  

[01:11] fields and we have general relativity which  describes gravity as the curvature of spaceime.   Both theories work extremely well. Unfortunately,  they don't fit together. It's like having two  

[01:23] instruction manuals for the universe. 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  

[01:36] a long time, most people thought that testing  quantum gravity require absurdly high energies,   like inside of black holes or at the big bang. But  in recent years, physicists have finally realized  

[01:48] that it's possible to test quantum gravity in the  laboratory. The currently most discussed ideas   to use what's called an entanglement witness.  That's if you create entanglement using only the  

[02:00] gravitational interaction. If this is possible,  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  

[02:12] 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  

[02:24] 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  

[02:37] 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  

[02:51] into 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  

[03:04] 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  

[03:17] a destructive interference that leads to a net  repulsion. Wait, what? Doesn't this violate some   sort of conservation law? No, it's fine, really,  if you average over many runs of the experiment,  

[03:30] 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  

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[04:52] assessment today. Head over to cognitivetesting.org to  take a legitimate IQ test. That's cognitivetesting.org.   And now back to the science news. It looks like  gravity became repulsive, but really it happens  

[05:09] 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  

[05:25] than, as you would expect, make a pole. It's  not a new fundamental force. It's an effective   force that only occurs for specially prepared  quantum states. But that's exactly why it's  

[05:37] 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  

[05:53] 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  

[06:06] 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  

[06:19] 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 asked. I give this paper a

[06:33] 2 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  

[06:47] interaction. 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,  

[07:00] 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  

[07:14] for why my hair does not obey gravity.  Thanks for watching. See you tomorrow.