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Enter the Relativity Challenge
An anonymous reader writes "Any slashdotters wanna pick up a lazy 25,000 Euros? All you have to do is explain Einstein's theory of relativity in a five minute multimedia presentation. The Pirelli Group have laid down this 'Relativity Challenge' to anyone as part of the International Year of Physics. Entries close on 31 March 2005."
"A Man has got to know his limitations."
I found mine in Physics 21 when we hit Relativity. I just flat out don't get it. I can do the math, and get the right answers, but I couldn't truly explain it.
"As God is my witness, I thought turkeys could fly." A. Carlson
Einstein's work was already in very simple laymen's terms. I don't know what the point is in trying to make it into braindead powerpoint.
I've had enough abrasive sigs. Kittens are cute and fuzzy.
You can't just wave it off saying the one that experienced the accelleration will have their clock slow down. If you want to calculate how much less one person aged, you go by how long (time and distance) they were travelling at that speed. For example, if I accellerate to 0.999999c in about a year (I think that's about 1g accelleration) and travel 10 light-years and back, then 24 years will have passed on Earth (20 travel + 4 for accelleration), but I will have aged only about 4. If I undergo the same accelleration but travel 10,000 light-years and back, then 20,004 years will have passed on Earth, but I will still have aged only about 4. The accelleration didn't make time pass more slowly, it was the period while I was tavelling at high speed that made it pass more slowly. Take both examples together and the one would seem to have 4 years pass and the other would seem to have 19,984 years pass, even though they experienced the same accellerations.
This also leads to an absurd result from my point of view. I will have only seen 2 years go by, but I will have travelled 20,000 light-years. From my point of view I would have been travelling 10,000 times the speed of light. How can this be?
I think it has to do with contraction. Lorentz contraction is one thing I haven't understood, how you can measure the length of something that is going nearly the speed of light? Apparently, when you are going nearly the speed of light, everything else contracts in the direction of your travel. For instance, if you were going a certain speed and passed a meter stick, it would appear to be only 1 millimeter long, although a stationary observer by the meter stick would see it as 1 meter long.
Now as for how fast you are going, that is all relative as well. If I take off from earth and accellerate to 0.999999c for about a year and travel 10 light-years, I don't think I'm going 10 light years. Space and the galaxy will seem to contract along the direction of my motion. When I get 10 light-years in space, it will appear to me like I have travelled a much shorter distance.
Here's a more concrete example. Let's say that I pass Earth going at velocity V, which slows down time for me to 1/10th normal. Then I travel to a space buoy that you have measured from earth as 10 light years away. Not only will I reach that buoy in about a year, but I will think I have travelled much less than 1 light-year because space along my direction of motion has contracted. During that time, an earth-based observer thinks 10 years have passed. The reason that his clock doesn't appear to slow down for me is because I don't think he's travelling that fast. To me, he has travelled much less than 1 light-year because space contracted and I think it was in 1 year, so he is travelling much slower than the speed of light and subject only to minor relativistic effects.
First, it does not take a whole book to explain why speed of light is constant. In fact, the postulates of SR is intuitive (once you know enough physics) and even necessary. It's the consequences of those postulates that is so slippery and mind-boggling (I've seen graduate physics students have trouble building up "relativistic intuition," while quantum mechanics, which, IMHO, is much more complicated mathematically, is intuitive to them). So, any good SR book should spend more time on the consequences of the postulate than on the postulate, which is an almost trivial consequence of results in other areas of physics.
The postulates of SR follows naturally from classical electrodynamics--that is, if you choose to accept that Maxwell's four equations of electromagnetism is correct. In fact, without SR (and length contraction and time dilation), classical electrodynamics falls apart, because its laws will not be constant in different inertial frames (now, this is an axiom that you just have to take on faith--if you can't accept this, well, how do you trust that conservation of mass and energy holds true?).
More natural reason given for Einstein's second postulate is, usually, that speed of light is a physical constant (this should follow directly from the fact that, mathematically, when you combine the Maxwell's equations, you have an electromagnetic wave, which consists of time-varying electric field coupled with time-varying magnetic field at right angle to each other). If this physical constant were to change as we change our frame, that would almost be equivalent of changing physical laws themselves--electron charge measurement, for example, in different frames will be different, as, if c changes, that will change permittivity of free space as well.
Now, I suspect that the book does not explain all this (and, no, I didn't go in detail either--I would hate to try to do math in ASCII text, and this (er, electromagnetism?) is a subject that takes a full semester if you want to do it right--and no, this is electrodynamics, not special relativity, so you shouldn't expect this to be in an SR book) because the author doesn't think you would understand, anyway. A mathematician does not start from axiomatic set theory when he tries to explain to an child why binary operations like 2 * 2 makes sense (i.e. that the set of integers is closed under the binary operation).
PS. Now, how is this post insightful? who modded this up?