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  • [♪ INTRO]

  • In 2008, there was a lot of excitement when the Large Hadron Collider

  • (the LHC) was first turned on in Switzerland.

  • The particle accelerator was the largest ever made,

  • representing decades of work by thousands of people, and it promised

  • to unravel some of the deepest mysteries of the universe.

  • But some people weren't so excited.

  • In fact, they were afraid, because they worried the LHC

  • would make a black hole and destroy the Earth.

  • I mean, that wasn't true.

  • The fears were the result of some bad science reporting,

  • and the LHC was never going to do thatbut there was actually

  • a kernel of truth to this idea.

  • After all, there are some theoretical physicists who believed

  • the LHC could make a black hole.

  • It would just be a sub-microscopic one that would

  • fizzle out instantly.

  • But while the accelerator hasn't found any of these micro black holes yet,

  • it's still thought that maybe the next generation

  • of particle accelerators will be able to.

  • And if they can, that would provide evidence for some

  • fascinating ideas in theoretical physics.

  • We're talking about stuff like extra spatial dimensions.

  • The foundation of this idea is that black holes aren't just things

  • out in space; they're a natural consequence of Einstein's theory

  • of general relativity.

  • The theory tells us that matter warps the fabric of spacetime,

  • and that the more matter there is in a region of space,

  • the more it warps its surroundings and draws nearby objects closer.

  • If there's a lot of matter stuffed into a very small volume,

  • then space becomes so warped that it becomes impossible

  • to move away from the matter if you get too close.

  • That is a black hole. And while they usually form

  • when huge stars collapse, they can technically form

  • any time there's enough matter in a small area

  • so it's possible to get some really tiny ones.

  • Weirdly enough, though, making a tiny black hole

  • doesn't involve something really heavy, but something really light.

  • And that's where the Large Hadron Collider comes in.

  • In many of its experiments, the LHC smashes extremely light particles,

  • protons, together really fast.

  • Like, “just a hair short of the speed of lightfast.

  • That allows us to learn all kinds of things about what happens

  • when particles collide.

  • Like, we've seen some brand-new particles emerge from these collisions.

  • But the important thing to know here is that those super-fast protons

  • have a lot of energy for their size.

  • And that's a big deal when it comes to black holes.

  • See, we almost always talk about black holes forming

  • because of some amount of mass.

  • But in the high-energy collisions of the LHC, mass and energy

  • actually become interchangeable.

  • That's the point of Einstein's famous “E equals m c-squared.”

  • It says that mass is proportional to energy.

  • So, theoretically, if you get some super-fast-moving particles

  • really close together, then the energy of all that motion

  • in one place can act like a lot of mass and be enough

  • to form a black hole.

  • Just, y'know, a very tiny one.

  • The problem iswith our current understanding of physics,

  • that cannot happen at the LHC.

  • To make the lightest possible black hole, the particles

  • would each need about 10 quintillion electron volts of energy.

  • That's a one with 18 zeros after it.

  • And while the LHC is goodit can get particles up to

  • 14 trillion electron voltsit's not that good.

  • But! There's a reason some physicists are still thinking

  • about all of this.

  • And it's because there's a catch here:

  • These calculations all assume that there are no problems

  • with general relativity.

  • And, well, we already know the theory has problems.

  • I mean, it is very good and it makes lots of good predictions,

  • but technically, it also predicts it should be possible

  • for places to have infinite density.

  • And we know that's just not a thing.

  • If infinities start popping up in your physics results,

  • you know you probably pushed your theory past its limits.

  • So, what is likely happening is that general relativity

  • is only approximately true, and there's a more correct theory

  • we don't know about.

  • That theory would produce different results when you have

  • a lot of energy in a really small space, which would solve

  • thisinfinite densityproblem.

  • Right now, scientists think this new theory will probably be

  • some sort of quantum gravity — a combination of quantum mechanics

  • and relativity. And there are already a few candidates.

  • But because there is no direct evidence for them yet,

  • scientists spend a lot of time probing the mathematics

  • of these theories, looking for predictions they make

  • that can be tested.

  • And one prediction some of these theories make is that

  • the space we live in actually has more than three spatial dimensions.

  • That would mean that in addition to being able to go forwards, up,

  • and to the side, it'd be possible for things to go in a direction

  • that's at right angles to all of those directions at once.

  • If you're having a hard time visualizing thatwell, yes.

  • Because to our eyes, that's clearly not the world we live in.

  • There is no direction that we can see that's at right angles

  • to all three at once!

  • So if extra dimensions do exist, they have to be extremely thin.

  • This is similar to how a piece of paper is a 3D object,

  • but if you don't look closely, it's basically 2D.

  • Only when you zoom in can you see that very thin extra dimension.

  • So, in some quantum gravity theories, the extra dimensions

  • can be up to about a millimeter in sizebut in those cases,

  • only gravity can interact with them.

  • That means that if you measured anything except gravity

  • at those scales, like, say, the strength of a magnet,

  • it would behave as it does in a 3D world.

  • But if you measured the strength of gravity at those scales,

  • it would behave as it does in a higher-dimensional world,

  • and that behavior would be different.

  • Why thisgravity onlyrule?

  • Well, if anything else interacted with this extra dimension,

  • we would already know about it!

  • On that kind of tiny scale, we've seen every other force of nature

  • work as-predicted, but we have not been able to test gravity

  • that's just because it's so weak compared to the other forces,

  • not because of anything to do with relativity.

  • So we're not sure what gravity is like in those situations

  • and it could very well act differently.

  • Now, the nice thing is, if gravity were interacting

  • with other dimensions like this, it'd be pretty easy to tell.

  • See, in the 3D world we live in, gravity follows

  • what's called the inverse square law: If you decrease the distance

  • between two objects by half, the gravitational force

  • between them increases by a factor of four.

  • But in a world with more dimensions, it's no longer

  • the inverse square law.

  • In, say, a 9D world, cutting the distance in half

  • might mean increasing the force by a factor of 256.

  • So if there are extra dimensions smaller than one millimeter,

  • then below that scale, the gravity between colliding objects

  • gets a lot stronger a lot more quickly.

  • And that means good things for black holes in the LHC.

  • It can smash particles together with really high energy,

  • which means they get pushed really close together.

  • And if the pull of gravity were stronger on tiny scales,

  • it would be easier to push lots of energy really close together,

  • which is what you need to make a black hole.

  • In fact, depending on which theory you use, it's thought that

  • if enough extra dimensions exist, you mayonlyneed

  • to smash protons together at around 10 trillion electron volts

  • to get a black hole.

  • And that's an energy that's reachable with the LHC!

  • So far, though, the search hasn't been promising.

  • One paper from 2016 reviewed LHC data with collisions at up to

  • 13 trillion electron volts, and found no evidence

  • that black holes were being made that way.

  • Then, building on that, a 2018 paper found that, at those levels,

  • gravity wasn't behaving differently due to

  • extra spatial dimensions, either.

  • But there is a lot we don't know about these theories.

  • It could be that the energy we need is just a bit higher than what we

  • have now, meaning the LHC or a future replacement could reach it.

  • That could give us the first experimental evidence

  • for quantum gravity!

  • And, as a very nice bonus, it could also teach us something

  • about black holes themselves.

  • See, in the 1970s, Stephen Hawking predicted that all black holes

  • from big ones to tiny ones you'd see in the LHC

  • should release energy in what's called Hawking radiation.

  • And while most physicists are completely confident that this is true,

  • no one has been able to observe it.

  • That's because more massive black holes, like the ones you'd study

  • in space, should emit radiation that's way too faint

  • to see with telescopes.

  • But according to Hawking, less massive black holes

  • should release hotter radiation that's easier to detect.

  • So, if you had an itty-bitty black hole pop up

  • in your particle acceleratoryou would be able to measure it.

  • If the LHC made a black hole, it would be so small

  • that it would radiate away all its energy in about

  • an octillionth of a nanosecond.

  • But there would still be enough radiation to detect

  • with our instrumentsand thus, provide the first

  • definitive proof that Hawking radiation is real.

  • So, even if the evidence isn't promising so far,

  • there's a reason people are looking into this.

  • Because if there's any chance that the LHC could make a black hole,

  • we would probably want to try to make it.

  • Unfortunately, though, the evidence keeps piling up

  • that it might not be possibleeven with

  • those extra spatial dimensions.

  • And some of that evidence doesn't come from our big,

  • fancy particle accelerator: It comes from nature.

  • All the time, high-energy particles from deep space,

  • called cosmic rays, are hitting our atmosphere.

  • And lots of them have energies even higher than

  • the particles in the LHC.

  • So if these black holes can be made, they should be forming

  • in the upper atmosphere.

  • But we don't see them there.

  • Or from collisions anywhere else in space.

  • Based on these observations and some modeling assumptions,

  • one 2019 paper predicted that extra dimensions can be ruled out

  • up to the exa-electronvolt range.

  • That's almost a million times higher than the energies in the LHC,

  • which means the prospects of seeing these micro black holes

  • do not look so good.

  • But other people have used different assumptions to say that

  • a near-future LHC replacement could maybe see micro black holes.

  • So while things aren't looking great, that doesn't mean

  • the case is closed.

  • There are a ton of quantum gravity models out there, so there may be

  • a way to test our hypotheses and theories down the road.

  • And even if it doesn't work out in the endWell, black holes or not,

  • when the LHC is turned back on in 2021 after some maintenance

  • and upgrades, people are hoping that it will discover

  • all sorts of new things, so there are still plenty of reasons

  • to get excited.

  • Thanks for watching this episode of SciShow!

  • And thanks to our patrons on Patreon for helping us make it happen.

  • If you want to help us make more free science content online

  • for everyone and learn more about our Patreon community,

  • you can head on over to patreon.com/scishow.

  • [♪ OUTRO]

[♪ INTRO]

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如果大型強子對撞機制造了一個黑洞呢? (What If the Large Hadron Collider Made a Black Hole?)

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    林宜悉 發佈於 2021 年 01 月 14 日
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