Rosio Pavoris

Special relativity is the theory introduced by Einstein in 1905 which overthrew our understanding of classical physics. It’s a special case of general relativity, which he would come up with later on.
The central ideas of special relativity are so easy to explain that I’m not sure why it isn’t taught in high schools. Well, not in ours, anyway.
There are two important ones:

1. All uniform motion is relative

If you aren’t accelerating, you can’t tell if you’re moving, except if you take some other object as a reference point, in which case all you can say is that you’re moving in reference to it. If you’re floating in space and you see someone else float by, it’s meaningless to ask if you’re stationary and they’re moving past you, or if they’re stationary and you’re moving past them, or if you’re both moving past each other.

If you’re in a space shuttle with no windows, there’s no experiment you can do to tell if you’re moving or not; there’s no absolute frame of reference, so the question is meaningless.

This isn’t true for accelerated motion or rotation, but special relativity doesn’t deal with that. That’s general relativity’s area.

2. Light appears to move at the same speed to all observers

If you’re moving at 30 kilometers per hour (from the point of view of a second observer), and you throw a ball ahead of you at 60 kilometers per hour (from your point of view), it will be moving at 90 kilometers per hour (according to that observer), yes?

So, if you’re moving at 30 kilometers per hour, and you know the speed of light is 299,792,458 meters per second, and you shine a flashlight ahead of you, how fast will that second observer say the light is going?
Exactly 299,792,458 meters per second, turns out. And that’s also the speed you’ll measure from your own frame of reference.

To make this easier to think about, let’s consider a very simple clock.
Take two mirrors, and bounce a single photon back and forth between them. Every time it hits a mirror, there’s a soft tick. If the mirrors are slightly less than thirty centimeters apart, a billion ticks will equal one second.

Now, imagine that clock is moving past you at a uniform velocity, on a tiny train, perhaps. How will that photon appear to move?
(Leaving aside the issue of how you’d see a single photon bouncing between mirrors.)

The photon appears to be travelling a greater distance than the thirty centimeters it appears to be travelling to the person on the train with the clock.
But light being what it is, it will appear to still be moving at 299,792,458 meters per second to both of you, which means that a billion ticks will actually take longer from the point of view of the person watching it go past (you) than to the person in the train with the clock, so time is actually moving more slowly on the train!

Even more interestingly, imagine what would happen if you had a second clock, which you kept with you as you were watching the train go by.
In exactly the same way, your photon would appear to be travelling more than the thirty centimeters as well to the person on the train, so to him, you would be moving more slowly too. Both of you perceive the other as moving more slowly!
How can this be?

Suppose you were both wearing wristwatches (or whatever; something to tell how much time has passed, rather than how quickly time is passing, which is all the photon clock is capable of). If after a bit, you and the guy on the train came together to compare watches, you should be able to tell which one of you was really moving more slowly, right?
Well, no. Bringing the watches together requires motion as well, and for the other guy to change his direction (or for you to start moving) so you two can meet requires acceleration (even if only briefly), which actually doesn’t work according to special relativity.
The acceleration breaks the reference frames and makes the guy doing the accelerating experience time lag in a way that neatly cancels out the apparent paradox and will make him the slow one.
Bringing the watches together might seem like a simple matter of logistics, but it’s not actually exempt from the laws of physics.

This is rather more complicated to explain properly, though, so I’m not going to bother.
An interesting consequence of this, though, is that for any two events, you can never say in an absolute sense which occured first, which is quite interesting.