# Speed of Light

### Particles and Interactions

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빛의 속도가 일정하다는 것은 단순한 진술처럼 들립니다.,,en,그러나 그것에 더 많은 것이 있습니다,,en,꽤 많이,,en,이 불변성이 실제로 무엇을 의미하는지 살펴 보겠습니다.,,en,어딘가에 서 있다면,,en,그리고 오른쪽에서 왼쪽으로가는 광선이 있습니다.,,en,속도가있다,,en,씨,,en,그리고 왼쪽에서 오른쪽으로가는 또 다른 광선도 속도가 있습니다.,,en,지금까지,,en,너무 좋아,,en,이제 당신이 로켓 선에 있다고 가정 해 봅시다.,,en,오른쪽에서 왼쪽으로 이동,,en,특수 상대성 이론,,en,아인슈타인과 특수 상대성 이론을들을 때,,en,또는 특수 상대성 이론,,en,실명 사용,,en,우리는 유명한 것을 생각한다,,en,E = mc ^ 2,,en,방정식,,en,그리고 쌍둥이 역설 같은 이상한 것들,,en,그 모든 것이 사실이고 중요하지만,,en,SR이 해결하려는 문제는 완전히 다른 문제입니다.,,en. But there is more to it, quite a bit more. Let’s look at what this constancy really means. At first glance, it says that if you are standing somewhere, and there is a ray of light going from your right to left, it has a speed c. And another ray of light going from left to right also has a speed c. So far, so good. Now let’s say you are in a rocket ship, as shown in the figure below, moving from right to left.

The yellow arrow is the rocket ship moving at a constant speed of v, and you are the scientist standing in there measuring the speed of the light beam as it passes you. Even that simple statement is not without its complications. When I say your ship is moving, I have to say with respect to what. Let’s say the motion is with respect to the younger gentleman standing outside.

Let’s also say you are trying to measure the speed of the light coming from the source S1, which is outside your spaceship. You would naively expect to measure c-v, because you are moving away from it to your left. But no, you will get c. The same holds true if you measure the speed of the light coming from source S2, which is inside the spaceship, moving with you.

If you think these results are peculiar, you haven’t seen anything yet. Let’s say the younger fellow measures the speed of the same light beams. He would also get c! In order for this to happen, the space in the rocket is contracted (shrunk), and the time dilated.

The special theory of relativity is highly counter-intuitive (which is physics speech for nonsensical) because it is not as though the rocket is really moving. Remember, only relative motion has physical significance, which is the principle of relativity. For you, the scientist in the rocket, you are stationary, and it is the younger fellow outside who is moving in the opposite direction at a constant speed of v. So it is his space and time that are contracted and dilated. This is all very strange, but true. This is exactly the basis of the thought experiment they call the twin paradox.