Last semester I studied Stellar Astronomy, and I started out by saying that it was important because “one day we will need to go there.” In the future the sun will die and we will need to travel to another star. This semester I am continuing my studies in astrophysics, focusing on relativity, and I would like to make a different statement. Relativity is important because it already applies here, because relativity applies everywhere. Einstein's theory of special relativity can be summed up in one sentence. The rules of physics are the same for all reference frames. The laws are the same everywhere. They apply just as much on Earth as they do around black holes.
That sounds simple, but it actually means that Newtonian physics, and many of the common sense rules we’ve learned, are wrong. The implications of SR give us a whole new view of the universe. The second part of special relativity, which most people already know, is that the speed of light is constant. Since c (the speed of light) must remain the same, something else has to give, and that something is space and time. Space and time bend. We don’t notice it on Earth, because the effects are negligible at the slow speeds at which we travel. But near the speed of light, the effects become much more drastic.
Imagine if we had a ship that could move at 0.9c (ninety percent the speed of light). Say it has a mirror on the ceiling and on the floor below. Assuming the mirror reflects perfectly. If somebody shone a light beam straight up, the beam would be caught, bouncing up and down between the two mirrors. To someone standing inside the spaceship as it moved, the path of the light beam would look like figure figure a. But to people watching from Earth (so they were at rest with respect to the ship), the path wouldn’t look vertical. They would see the light beam move with the ship, diagonally in a zig-zag through space, like figure b. To the observer at rest, the light beam would travel a farther distance in the same amount of time. This is the same phenomenon as someone standing inside a train tossing a ball. It makes sense, because the ball is going its speed, plus the speed of the train, so the ball is moving faster with respect to the stationary observer. Except light can't do that; its speed must remain constant. What changes between the frames of reference is the passing of time. The light does not go a farther distance at the same speed in the same amount of time. Instead time runs more slowly for the person in the spaceship. This is called time dilation.
Time dilation doesn’t make sense with our standard idea of time, but in this instance, it is the only explanation, so it must be true. The same phenomenon actually happens with the ball and the train, but we don’t notice because neither is going fast enough for the time dilation to matter. Time dilation is just one effect that occurs when objects move close to the speed of light, but understanding this is a good starting point to start trying to wrap our heads around the fact that reality is not what it appears.
~Sarah P.
Image citation: http://webs.mn.catholic.edu.au/physics/emery/hsc_space_continued.htm
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