Monday, May 5, 2008

Ping: Relative Dog Motion

Great article by Chad over at Uncertain Principles about explaining Special Relativity to his dog.

I really like the way he uses "the house is where I keep my stuff" to illustrate the huge draw that some places have as a preferred frame of reference. This is as important when teaching classical physics (helping students recognize why they prefer to look at fictitious forces rather than real ones) as it is when teaching physics at "relativistic" velocities. We think Newton's First Law is valid everywhere we might be, everywhere we keep our stuff, even if we are turning a corner in a car.

That might have been a better point of digression for Chad, since the relativity fallacies that are not due to frame jumping are usually due to hidden acceleration. He needs to explain that to his dog before Emmy falls prey to a door-to-door paradox salesman.

My only complaint is that he did not answer Emmy's main question very well.

"Why do they call relativity 'relativity?' Why not something cooler, like Superfast Timeslowing Squirrelcatching Dynamics?"

Actually, it is called "Special Relativity", and that name was added to distinguish it from the General theory that came along a few years later. The original paper actually was about Squirrelcatching Dynamics. It was about the Electrodynamics of Moving Bodies (squirrels, for example, and how they are influenced by electric forces).

... picking up from where I had to leave this ...

Further, there was nothing at all radical in Einstein's notion of relativity. His paper starts with the same postulate that Newton has: that all physical laws are the same in every inertial frame (that is, every coordinate system where the First Law is exactly true). [No cheating here: If you allow a tiny but non-zero acceleration that violates the first law, you can generate as many bogus paradoxes as you want. Just like you discover when shooting a long-range cannon while ignoring the Coriolis effect.]

What was radical was that he changed which laws should be covered by the first postulate. He picked Maxwell's equations over the Galilean Newtonian ones, thus predicting (correctly, we now know, based on bazillions of high energy experiments) that classical equations of motion are only an approximation. His paper was written around the experimental data for light, an empirical starting point that would not surprise Karl Popper, but today it might make more sense to start with Maxwell and the very obvious result that those equations are obviously false in a frame moving at c. [You have a stationary wave, so the spatial derivative of a B or E field is not zero but the time derivative of the field is zero. QED.]

Actually, even that was not a radical notion, it's just that no one had thought of it (except one guy who did not see its significance). Instead, they were busily rejecting relativity in favor of crazy ideas like "time changes when you are moving" so that all time had to be compared to where Emmy (Chad's dog) lives even if any other reference point (like Greenwich or the Williams Rugby field) works just as well. Einstein's solution was obviously more sensible than that used by Lorentz, but he did not get the Nobel Prize for it because the experimental proof of it was not clear for another 40 years or so.

He also failed to tell Emmy that her "time" is not affected by motion. It is affected by acceleration (which also resulted in a speeding ticket for Chad, if we believe the entire story) but not by speed alone. We also know this from experiment, most notably the clocks in GPS satellites.


And if you like that article by Chad about his schizophrenic conversations with his dog, there are also the all-time classics

that are indicative of great things to come when his book makes it through the publication process.

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