Just as sound waves propagate in water, it was once believed that light waves propagate in a medium called aether. This presented an opportunity for experimental physicists to measure the motion of the earth with respect to the aether. In 1887, two scientists named Michelson and Morley conducted an experiment to do just that. But no matter which the direction they pointed their apparatus, they measured the same speed of light through the aether. This forced them to conclude that the aether did not exist and that light would always be seen traveling at the same speed.
앞서 수중 음파의 전달을 다뤘듯이 빛도 에테르(aether)라는 매질을 통해 전달 된다고 믿었다. 1887년 미켈슨(Michelson)과 몰리(Morley)는 에테르를 찾고자 실험을 했지만 실패했다. 에테르는 존재하지 않는 것으로 결론을 내렸다.
This puzzled scientists worldwide, well, except for one, Einstein. Einstein imagined what it would be like to see the universe from the perspective from a beam of light. He asked questions like how would a photon perceive the passage of time? And will distances shrink and stretch depending on the motion of an observer? Of relevance to him was the fact that experiments had proven that light waves were special compared to sound waves or water waves. In that they didn't require a medium through which to propagate.
에테르가 존재하지 않는다는 사실은 많은 과학자들을 혼란케 했지만 아인슈타인은 달랐다. 그가 가진 의문은 시간의 흐름에 광자는 어떻게 변할지, 관측자의 운동에 따라 (광자가 이동한) 길이가 줄거나 늘지 않을까 라는 것이었다. [빛은 매질 없이 전달된다. 매질이 없으므로 빛(광자)의 운동은 물속의 음파 전달의 경우 처럼 매질의 속도에 더해지거나 빼지 않는다. 빛의 속도는 불변이다. 움직이는 물체에서 출발한 광자나 정지한 물체에서 출발한 광자든 속도가 불변이다. 동시에 출발한 광자는 동일한 지점에 도달해야 한다. 이동한 거리가 늘어나거나 시간이 느리게 가야 한다. 빛의 속도로 움직이는 광자의 입장에서 시간과 공간(시공간, spacetime)이 왜곡된다.]
In helping us to understand these new revelations, Einstein had to tackle problems which few could ever even consider. In one of Einstein's most famous papers entitled On the Electrodynamics of Moving Bodies, he introduced two very important ideas. Ideas which are now among the foundation of modern physics.
이런 (빛의 속도 불변으로 제기된) 문제를 풀기위해 아인슈타인은 누구도 하지 못했던 생각을 '이동하는 물체의 전기 동역학에 관하여'라는 논문으로 냈다. [빛은 전자기파다.] 이 논문에는 현대 물리학의 근간이 된 중요한 이론을 세웠다.
They are, 1, the laws of physics are the same in all inertial frames of reference. And 2, light moves at the same speed relative to all observers. That first postulate seems reasonable.
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1. 모든 관성계에서 물리법칙은 동일하다.
물리법칙은 위치에 상관없다. 지구에 있든 달에 가 있든 모두 같은 물리법칙의 지배를 받는다.
An inertial reference frame is either an experiment at rest or one moving with a constant velocity. Inertial frames are not accelerating. For example, someone standing in a high speed train would experience the same laws of physics as someone stationary on the ground, so long as neither are accelerating. The first postulate is intuitive to human beings which makes the second one impossibly hard to believe at first glance.
등속운동하는 곳 어디에 기준 좌표계를 설정해도 물리 법칙은 같다. 관성 좌표계는 가속하지 않는다.
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2. 관측자(등속운동을 하든 정지해 있든)에 상관없이 빛의 속도는 같다.
Einstein's second postulate was that light moves at the same speed relative to all observers. So if we were to measure the speed of light from a fast moving astronaut, we don't add the astronaut's speed to the speed of light. Weird, right? The speed of light always comes out to the same value, no matter how fast the astronaut is traveling. Einstein realized that if the laws of physics are the same for all observers, then all observers must agree on the value of the speed of light.
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상대운동, 서로 다른 속도로 (하지만 등속으로) 움직이는 관측자에 따라 관측되는 속도가 다르다.
You've probably experienced some of the strange effects of changing reference frames before. Have you ever been in a parked car when an adjacent car starts moving? In some cases, your brain tricks you into thinking that you're moving instead of the other car. Since motion is relative, we can always choose a reference frame that is stationary, even if there's relative motion to something else. Let me explain.
Suppose you're riding in a self-driving car. Some fast cars are passing you in the left lane and you are passing some slow cars in the right lane. If all the cars are moving at a constant but different speed, each car is their own inertial reference frame. If you observe the cars from the ground they will all appear to be moving.
If, however, you choose a reference frame of the car in the middle lane, the cars on the left appear to be moving forward. While the cars in the right lane appear to be moving backwards. Without the road in the background, we can't figure out how fast the cars are traveling. The only thing we can tell is how fast they're traveling with respect to one another.
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누구에게나 빛의 속도는 같다는 개념을 도입하니 상대속도를 설명하기 어려워졌다.
However, this becomes problematic when light is introduced. Instead of driving during the day, what if our cars are driving at night? They'll need to turn their headlights on. And If the center car turns on their headlights, they'll see photons leaving at the speed of light, c.
Let's consider the headlights of the cars in the left lane. Does the speed of light coming from the fast car appear faster due to the relative motion? No, even though the red car is moving faster, the photons coming out of the headlights always appear to move at the speed of light, c. The speed of light from the middle car is not c minus 10 kilometers due to the motion of the cars. And the same is true for the slower blue car. The speed of light is always measured as c.
Weird? Einstein thought so, too. Clearly, if Einstein's second postulate, that all observers measure the speed of light as a constant, is to hold true, we need some other kind of transformation group, so that every observer can see the light beams moving relative to themselves, at c. In order to do such a thing, Einstein realised that our intuitions about space and time must be incorrect. And that a new theory is required to describe how all observers, moving at different speeds, can measure the speed of light to be a constant.
모든 관찰자(등속운동을 하는)에게 빛의 (측정한) 속도는 같다. 아인슈타인도 이를 그냥 받아들이기 어려웠다. 그래서 새로운 이론을 세워야만 했다. [솜씨 좋은 목수가 궁한 물건을 만들어 쓰듯이 아인슈타인은 이해가 않되면 이론을 만들어 냈다. 나만 편하면 되는 것이 아니라 우주를 통찰하는 이론이다. 그는 '끕'이 다른 재주꾼 이라고 해야겠다.]
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