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  • The speed of light is meant to be the ultimate speed limit in the universe. According to

  • Einstein’s special theory of relativity, nothing should move through space faster than

  • light. But that doesn’t stop people from trying. Every day I get a lot of messages

  • proposing ways to go faster than the speed of light.

  • There is the classic method where you shine a laser at the moon. If you can flick that

  • beam across the moon’s surface in less than a hundredth of a second, which is not hard

  • to do, then that laser spot will actually move across the surface of the moon faster

  • than the speed of light. Imagine what that would look like if you were standing on the

  • moon. If you were quick enough to perceive it, you would see this laser spot move faster

  • than the light coming out of your own laser. How is that even possible?

  • Well, in truth, nothing here is really travelling faster than the speed of light. The individual

  • particles of light, the photons coming out of my laser are still traveling to the moon

  • at the speed of light. It is just that they are landing side by side in such quick succession

  • that they form a spot which moves faster than the speed of light. But really it is an illusion,

  • nothing is actually going faster than the speed of light. So you couldn’t transmit

  • any information this way. Dan asked: What if instead of a laser we used

  • a long rigid stick instead? Now surely if you flick your wrist, the tip of this stick

  • must move across the surface of the moon, faster than the speed of light.

  • Well, unfortunately this won’t work either. As we learned in the slinky drop experiment,

  • the fastest a force can propagate through an object is the speed of sound, that is because

  • each atom needs to bump into the one next to it to transmit that force. And this is

  • a lossy process. So you would be lucky if you any of the energy you have put in at the

  • start actually made it to the tip. You would be lucky if the tip moved at all.

  • Now this is a really sophisticated idea. Gerard writes: A very special space age engine would

  • need to be designed that is capable of doing 10,000 plus rpm in outer space with very high

  • torque. Consult Elon Musk for this. As the engine is spinning it slowly deploys two very

  • long tethers made from carbon nanotubes on opposing sides. Eventually each carbon nanotube

  • tether reaches an amazing length of 285 kilometers. At this point, the end of the tether will

  • be traveling at the speed of light. Can you point out some reasons as to why it would

  • not work? Yes, Gerard, yes, I can.

  • First, any object going in a circle requires a force pulling it in towards the middle of

  • that circle. That is called centripetal force. And you can feel it when you whirl a ball

  • above your head. Now that force is dependent on the speed of the object squared. So if

  • that gets to be too great the tether breaks. Now if you had a single gram rotating at 99

  • percent of the speed of light, the amount of force required to pull it towards the center

  • would be 300 meganewtons. That is the weight of 6000 fully African elephants. But, you

  • are right, carbon nanotubes are tremendously strong. If you had a fiber just eight centimeters

  • wide, you could support all of that force.

  • But now he problem is if you have less than a centimeter of that fiber, it adds another

  • gram to the tip of you tether. And so now you need a thicker fiber in order to support

  • that additional force. And that would happen all the way to the base, so the fiber would

  • need to get thicker and thicker and thicker all the way back to the motor. And if you

  • do the calculation you find that basically 30 meters from the tip the fiber already has

  • to be as wide as the observable universe in order to support all of that force. It is

  • nuts. But it gets worse. As an object moves faster

  • its inertia actually increases. That means it requires more force to accelerate it. In

  • fact, that one gram mass going 99 percent the speed of light would require seven times

  • the amount of force we calculated before. And so the tether would have to be even thicker.

  • But things get even more problematic if you think about speeding up the tip of the tether

  • that extra one percent to the speed of light. I mean, since the inertia keeps getting greater

  • and greater, it requires more and more force to accelerate it. And, in fact, to speed it

  • up that extra little bit to go the speed of light would require an infinite amount of

  • energy. Ok, well putting the infinite energy aside,

  • let’s say we could create an incredible motor and we could find a material much stronger

  • and lighter than carbon nanotube. Is it at least in principle possible that the tip could

  • go faster than light? No. There is one final problem which is insurmountable which is that

  • a tether, like anything, is held together by the electromagnetic interaction. That is,

  • the attractions between all the tiny little charges that makeup the material. Now the

  • problem is, electromagnetism is a force carried by photons. I mean, the way that something

  • knows that another thing is there to attract it, is by the exchange of photons, these force

  • carrying particles. And the problem is the photons themselves move at the speed of light.

  • So even if you could create this incredible apparatus with ridiculously strong materials

  • and spin it up with infinite energy, it still wouldn’t go the speed of light, because

  • the force carrying particles that hold the whole thing together only go the speed of

  • light.

  • The speed of light really is the ultimate speed limit in the universe.

  • Hey, did you see that I made a video about the problem with Facebook over on my second

  • channel? It really seems to have struck a cord, so you should check it out if you haven’t

  • already. Now I want to thank Audible for supporting

  • this episode of Veritasium. They are a leading provider or audio books with over 150,000

  • titles in all areas of literature from fiction to non fiction and periodicals. Now this week

  • I wanted to recommend the book by Bill Bryson called A Brief History of Nearly Everything.

  • When this book first came out I really wanted to dislike it, because I felt like it was

  • just piggy backing on Stephen Hawking’s Brief History of Time, but what Bill Bryson

  • has done is something truly different and extraordinary. I really think it is a great

  • summary and a great sort of investigation of what happens in science. It is a brilliant

  • thing to listen to.

  • Also, if you go to Audible.com/Veritasium, you can download this book for free, or another

  • of your choosing. Now they actually have this book in an abridged form read by Bill Bryson

  • himself. It is really interesting to hear the author’s voice. To me he sounds a little

  • bit like C. G. P. Gray, but with a hint of a British accent. So you should really check

  • that out. Just go to Audible.com/Veritasium.

  • All right. Thanks for watching and thanks to Audible for supporting me.

  • But there are some things which are going faster than the speed of light, relative to

  • us. There are some distant galaxies which are receding at a velocity than the light,

  • so we will never be able to see the light that they emit.

  • But this doesn’t violate Einstein’s theory of relativity, because they are not moving

  • through space faster than light, it is just that the space between us and them is expanding

  • so quickly that their effective velocity is greater than the speed of light.

The speed of light is meant to be the ultimate speed limit in the universe. According to

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B1 中級

これは光よりも速く進むのか? (Will This Go Faster Than Light?)

  • 57 5
    鄧北宸 に公開 2021 年 01 月 14 日
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