Einstein’s first postulate tells us that neither the passenger on the train nor the person on
the platform is wrong. It’s just as correct to say that the train is still and the Earth is
moving as it is to say that the Earth is still and the train is moving. Any inertial reference
frame is as good as any other.
Second Postulate
The speed of light in a vacuum is a constant— m/s—in every reference frame,
regardless of the motion of the observer or the source of the light.
This postulate goes against everything we’ve learned about vector addition. According to
the principles of vector addition, if I am in a car moving at 20 m/s and collide with a wall,
the wall will be moving at 20 m/s relative to me. If I am in a car moving at 20 m/s and
collide with a car coming at me at 30 m/s, the other car will be moving at 50 m/s relative
to me.
By contrast, the second postulate says that, if I’m standing still, I will measure light to be
moving at m/s, or c, relative to me, and if I’m moving toward the source of light
at one half of the speed of light, I will still observe the light to be moving at c relative to
me.
By following out the consequences of this postulate—a postulate supported by the
Michelson-Morley experiment—we can derive all the peculiar results of special relativity.
Time Dilation
One of the most famous consequences of relativity is time dilation: time slows down at
high speeds. However, it’s important to understand exactly what this means. One of the
consequences of the first postulate of special relativity is that there is no such thing as
absolute speed: a person on a train is just as correct in saying that the platform is moving
backward as a person on the platform is in saying that the train is moving forward.
Further, both the person on the train and the person on the platform are in inertial
reference frames, meaning that all the laws of physics are totally normal. Two people on a
moving train can play table tennis without having to account for the motion of the train.
The point of time dilation is that, if you are moving relative to me in a very highspeed
train at one-half the speed of light, it will appear to me that time is moving slower on
board the train. On board the train, you will feel like time is moving at its normal speed.
Further, because you will observe me moving at one-half the speed of light relative to
you, you will think time is going more slowly for me.