# Special Relativity for Special People :) Posted on 2016-11-21

Category: other

For the New Higher Physics course.

Special relativity is based on two fundamental principles from the original mad yin Alberto Einstein:

1. All motion is relative. Basically, the idea is that everything moves relative to something else. So, for example, if car A drives by my house, they're moving at 30 mph relative to me, but if there's a second car B going by at 25 mph, car A is only moving at 5 mph relative to car B.
2. The speed of light is constant no matter what. So, if you went by my house at 90% of the speed of light in a car, we would both agree that the light coming from your headlights is travelling at c, which is the speed of light. This what makes Special RelativitySpecial.

All of the other mental stuff that special relativity proved to be true follows from these two basic assumptions (and a lot of maths, but that's for the plebs amongst you). Since light moves at c no matter how fast you're going relative to anything else, some weird stuff goes down in funky town.

One of the most well known of said "weird stuff" is time dilation. If you're moving really fast relative to me, you're going to experience time differently. Why? Those two rules up there!

Let's imagine that you've taken more ectos than you've got teeth, you're moving close to the speed of light, and I'm standing on the side of the road. You have a light bulb that shoots pulses of light up at a mirror cos you're a mad rocket, and you measure how long it takes for the light to get back to the light bulb. What you would see is:

I, however, am standing on the platform, and I want to measure the same thing. While you see the light travelling up and down vertically, I see something like this (below) because in the time it took for the light to travel up to the mirror, then back down to the source, the mirror moved.

We have to agree that the same thing happened (we were both observing the same event, after all), but my watch has gained time or yours has lost time. Why? Well, the only way to explain this is to accept that time itself moves along at a different rate when you're moving close to the speed of light. If we agree that we both measured different amounts of time for the same event, but that your idea of a second is different from mine because of your speed, things work out.

Another mental thing that happens because of special relativity is length contraction - things get smaller the faster you are travelling. Of course, to you, things don't get smaller, someone observing sees that. To you, in your frame of reference, everything is hunky dory. Suppose you're riding a bottle of bucky straight into the sun at the speed of light like the true bucky bandit you are. If I'm sitting on the moon and see you flying past I will perceive the length of the bottle of bucky to be shorter than you perceive it to be. This can, again, be explained by the two rules. Since the only thing that is constant is the speed of light and time slows down the closer you travel to c, length must get smaller to make maths not break.

Alberto didn't actually figure all of this out himself, he just added all the bits of work that folk had done before him into a single unifying theory. So we shouldn't thank him we should thank the whole young team:

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