Framing the debate: relativity

15 Mar

Note: Albert Einstein’s birthday was yesterday, March 14th. I didn’t realize that until after I had decided to write a few facts about relativity. Sometimes you just get lucky.

I’ve seen a couple of links to a Popular Science article, “Warp Factor,” about a NASA engineer’s effort to develop an engine that could travel faster than the speed of light. I read it last night and I was rather skeptical, purely from a theoretical standpoint. Still, it did get me thinking about relativity and some of the funky things that happen at high speeds. Yet in order to talk about the weird stuff, you have to know what relativity means. Relative to what?

The most common way of explaining this, used by Daniel Frost Comstock in 1910 and Albert Einstein in 1917, is more or less this; imagine there is a man on a train moving past a man on the side of the tracks. Then, at the exact moment that the passenger and the bystander pass one another, a flash of light happens at the center of the train car. What does each man observe?

735px-Traincar_Relativity1.svgThe observer on the train experiences precisely what we’d expect him to see. He is traveling the same speed as the train, so relative to him the train is at rest. The flash of light happened at the center of the train, so the light reaches both ends of the train at the same time.

800px-Traincar_Relativity2.svg

The observer on the ground has a very different experience. The flash of light happened at the center of the train but relative to him the train was moving to the right. The left side of the train moved towards the light while the right side of the train moved away from the light. As a result, the light reaches the left side before it reaches the right side.

This raises a problem. One event happened, but there were two entirely different experiences. Which one is valid? The surprising answer is both! The key words in each of the explanations was the term relative to him and his frame of reference (hence the field of relativity).¬† Either of these can happen, depending on the observer’s frame of reference.

And that’s just the beginning of the weird stuff that happens in relativity (Still if you have any questions, now is a good time to ask). Stay tuned for the next episode where I’ll tell you all about time dilation.

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