A Simple Introduction to Special Relativity

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Think Einstein’s theory of Special Relativity is beyond you? Think again.

It sounds intimidating. But what most people don’t realize is that it only requires a knowledge of the Pythagorean theorem, the definition of velocity (or really just speed, in the sense of distance divided by time), some basic algebra, and a willingness to embrace an unintuitive new understanding of time (and distance).

Here’s a simple introduction that I wrote for my 9th and 10th grade students at Village High School in May of 2010:

Relativity

Feel free to share it, copy it, distribute it, shred it, or burn it.

Okay, I admit that there’s actually a lot more to Special Relativity than what’s discussed in this little paper. The first three inescapable conclusions that emerge from the theory are that (1) time slows down in a moving reference frame (“time dilation”); (2) moving objects are shortened (“length contraction”); and (3) events that are simultaneous in one reference frame occur at different times in other reference frames. What’s hard to wrap your mind around is that these effects are not just matters of perception. Rather, the times and lengths actually change.

Those three effects are only the beginning, though. From them can be derived all sorts of other fascinating phenomena. Velocities, energies, momenta, and forces also change from one reference frame to another. The most amazing thing about Special Relativity, in my opinion, is the fact that the magnetic force is actually just a consequence of these time and length transformations.* One could say that the magnetic force isn’t even a real force. It’s an effect that arises as a consequence of relativity whenever an electric charge moves. (That’s why the “electromagnetic force” is considered to be just one force.) In fact, you can set up a situation in which, from the point of view of one person who’s sitting still, there’s a magnetic field; but from the point of view of another person who’s moving in a certain way, there is no magnetic field. It all depends on your point of view.

It’s tempting to go one step further and draw parallels between the physical theory of relativity and various philosophical ideas — relativism in ethics, culture, and religion, for example. It seems like there’s a scientific basis for saying that ideas that are right from one point of view might be wrong in another, and vice versa. But in fact, such thinking is contrary to the very heart of special relativity. The physical theory is built on the following two axioms: (1) The speed of light is always exactly the same in any reference frame; and (2) The laws of physics are always exactly the same in any reference frame. Thus, special relativity is actually a theory of absolutes. In fact, Einstein himself wanted to call it “Invariance Theory.”** It was other people who gave it the name “Relativity.”

* Haskell, Richard. “Special Relativity and Maxwell’s Equations.”

** Isaacson, Walter. Einstein: His Life and Universe.

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