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  • What would it look like to ride alongside a beam of light?

  • This is a question Einstein had asked over a hundred years ago.

  • Now the trouble was, he couldn't actually do the experiment

  • so he had to use his imagination;

  • he had to do a thought experiment

  • to figure out what a reasonable outcome would be.

  • I could imagine there'd be some element of nausea.

  • I'd imagine everything would get more intense.

  • Maybe it would confuse the brain; I don't really know if you'd be seeing very much.

  • I was on a water slide just a couple of days ago and I found that really difficult.

  • Einstein's intuition told him that the world should feel the same in all frames of reference moving with constant velocity.

  • These are called inertial frames of reference.

  • So it doesn't matter if you're in a train, or on a plane, or in a car, or in a room

  • As long as you're moving with constant velocity

  • [All four] The laws of physics should apply in exactly the same way.

  • And this is what's called the principle of relativity.

  • Is there a speed limit in the universe?

  • –I don't think so.

  • You can go as fast as you want?

  • Probably

  • Could we go as fast as light?

  • Ah, I'm sure eventually, absolutely!

  • Can you compare the speed of sound and the speed of light?

  • Well, it's however many times quicker, isn't it?

  • because you've got the lightning and thunder.

  • Roughly how much quicker is light?

  • Ten times.

  • Roughly ten times?

  • Ten times, I think, is it ten? or a hundred, I don't know.

  • Something like that.

  • The answer for you is—a million.

  • Oh!

  • Light is a million times faster than sound.(Damn)

  • Perhaps thought? or...

  • Thought is faster than light?

  • Yeah, it's immediate! it's there.

  • Let's do a little thought experiment.

  • Let's say I'm on a train travelling the speed of light that way.

  • Who am I kidding, this is city rail! We're lucky to be moving.(I looking at you Chicago and New York)

  • But, let's use our imaginations.

  • So let's say I'm travelling at the speed of light this way.

  • If I hold up a mirror in front of my face,

  • will I be able to see my reflection?

  • Do you see the problem?

  • In order for me to see my reflection,

  • light has to bounce off my face and travel to the mirror.

  • But if the mirror is going forward at the speed of light,

  • how is the light ever going to catch up?

  • So, a logical answer would be that

  • no, you can't see your face in a mirror when you're on a train travelling at the speed of light.

  • But that violates the principle of relativity because

  • it means that I can tell I'm in a frame of reference that's going at the speed of light that way.

  • Aw, crap, not again!

  • So let's consider the other option.

  • Maybe I do see my reflection

  • which means the light must travel from my face, to the mirror,

  • at its regular speedthe speed of light.

  • But now the problem is,

  • someone outside the train would see light travelling at twice the speed of light

  • because I'm already travelling at the speed of light, and the light is travelling at the speed of light relative to me

  • if that makes any sense.(Not really)

  • So, that leads to a really inconsistent answer

  • because how can the speed of light be twice the speed of light in that frame of reference?

  • So Einstein's conclusion was that

  • I must see my reflection; because otherwise, it violates the principle of relativity.

  • But in order for me inside the train to measure the same speed of light as someone outside the train,

  • the train must actually contract in the direction that it's moving.

  • And, my time must tick by slower relative to someone outside.

  • So Einstein changed what we thought were absolutesspace and time

  • and made something that seemed like it should be relativethe speed of lightinto the true universal constant. (Mind Blown yet?)

  • So what would it look like to travel alongside a beam of light?

  • Well, according to einstein, we'll never know,

  • because we can't go that fast.

  • At that speed,

  • our length would become zero;

  • time would stop relative to an outside observer;

  • and perhaps worst of all, our mass would become infinite.(No Mass Effect here guys)

  • but a recent experimental result has suggested that neutrinos can go faster than light.

  • Still, most physicists think that, well, this result probably won't hold up to scrutiny

  • because the principle of relativity is just so well established nowadays.

  • But who knows! Perhaps one day,

  • we'll find a way to break what we thought was the universal speed limitthe speed of light.

  • Subtitles: Thomas Martinez

What would it look like to ride alongside a beam of light?

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A2 初級

光の速さで行けるのか? (Can You Go the Speed of Light?)

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    林宜悉 に公開 2021 年 01 月 14 日
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