## Saturday, October 20, 2007

### Special Relativity Part 1: The problem

So I'm an undergrad physics major, as my brief profile mentions on the left. But I don't know that there's a whole lot to write about. I actually love all my classes, and manage my time very well, so there isn't exactly a whole lot of drama going on. I'd love to explain physics concepts, but I have to stick to what I know, which is certainly less than, say, the physicists over at Cosmic Variance.

But there's still a whole lot, and it includes Special Relativity (abbreviated SR). Lay people seem to think that you need to be an Einstein to understand it. There's also the idea that not even physicists can get a true understanding of the theory; they only know how to do the math and output the numbers. These are all wrong, at least when it comes to SR, which is a relatively simple theory. I think SR is something that any interested reader can come to fully understand.

Oh, and don't confuse Special Relativity with General Relativity, which by contrast is a much more difficult theory to understand. If you're imagining the warping of space--that's General Relativity.

The first thing to understand is why SR is even needed. The problem arose from many experiments involving the speed of light, and most famously the Michelson-Morley Experiment. These experiments showed that light moves at the same speed in all directions, regardless of the motion of the source of the light, or the observer. That means that if I shoot a beam of light at you, you will measure the speed of light to be going at the exact same speed regardless of whether you are moving towards me or away from me.

If that seems only vaguely paradoxical at first, consider this thought experiment: you are on a longboard, and I am standing still. Right when you pass by me, I shoot two beams of light, one forwards and one backwards. Because both beams of light move at the same speed, I will always be right at the midpoint of the two beams of light. However, the same is true for you, even though you are moving on past me. So at any instant, each of us is at the midpoint of the two beams of light, even though we are at two different locations. How can that be?

The solution to this, as many people know, involves radically changing our view of time, space, and whatnot. I'll leave that explanation for future posts.