Something like… if Einstein had an identical twin brother (lets call him Frank), who got into a spacecraft when they were both twenty, and traveled round the galaxy (or maybe just some of it?) at the speed of light and then came back, Albert would have aged to be an old man but his brother would still be twenty, because the closer you get to the speed of light, the slower time goes, and what Albert perceives as fifty years is mere days to Frank… right?
The Straight Dope Message Board-Even our dumb posters are smarter than you
Has a ring to it, don’tcha think?
I am constantly joyfully surprised at the knowledge Dopers can pull out at the drop of a hat. Even if I do not always participate in threads like these, I constantly learn from them.
E=MC squared. Energy equals mass times the speed of light squared. It’s been a long time since high school physics for me, but I remember that Einstein came up with it. Nothing can go faster than the speed of light (warp speed is science fiction) but as you approach the speed of light things look flat. I was recently listening to the Queen song '39 about a guy who volunteers to go on a space mission and he’s going almost as fast as the speed of light and when he comes back more time has passed on earth than it had in the spaceship. That effect is part of the Theory of Relativity too. Everything looks red because they move closer to the infared end of the spectrum. Somehow they use that to find out the distance of stars. Things look flat at almost the speed of light. I think Carl Sagan covered this on one of his shows, but it’s been a long time since I’ve seen it.
Alrighty then! I’ll change my to a 
in that case. Or maybe a ;), but certainly not a :dubious:.
There’s no such thing as absolute movement in this universe; it only makes sense to talk about relative movement between bodies.
Something going ‘fast’ with respect to you experiences time slower. Time is a difficult concept to formalize, though, so physicists define time as ‘that which is measured by a clock.’
The speed of light is constant. Weirdly so. Even if you run away from it, it still reaches you in the same amount of time. If you bounce light off a mirror on the moon, it returns to you in a measurably non-zero amount of time, but as though the earth hasn’t moved in the interim.
An easy thought experiment demonstrates 2, and depends on 3. You have a clock that is just two parallel mirrors bouncing a photon back and forth. The number of times the photon cycles between the mirrors is the measure of time, because the speed of that photon is constant. So you move the photon clock relative to you, and see that the photon is now taking a non-vertical path as you view it - it’s slanted (were you within the clock, you’d see the photon traveling straight up and down between the mirrors, as you move along with it. Outside the clock the photon is clearly moving horizontally as well as vertically). It is traveling further than when it was standing still, but the speed of light is constant. Therefore it is taking longer to make each round trip - time has slowed down for it.
There are real-world implications of this. Something about microwaves that I don’t understand, for one. Another that I do understand has to do with GPS devices. A GPS satellite has to have very, very accurate clocks. Your device is trying to figure out how far away it is from the satellite based on how long a signal travelling the speed of light took to get there.* The first devices worked well for a few years, then got worse and worse. They finally realized that the satellites, travelling 15,000 miles an hour relative to the earth’s surface, had experienced less time than the devices. So they had to take the slowing of time into account in using the satellite information.
Einstein knew a body can detect acceleration even without another body for comparison. So his paper on velocity that ignored acceleration was about *special *relativity (hence the name). His later paper that used centripetal force as the acceleration under consideration made it general relativity.
*The clocks in the device don’t have to be amazingly accurate, interestingly. They assume they’re right at a given satellite, then take the errors they get from the various calculations, and figure out what time it must be on the device. But that’s not this discussion.
I think I got the time dilation and length contraction, but I was wrong about the speed of light. The universe is expanding and the farthest parts are moving away from us faster than the speed of light. Forget the infrared stuff, it’s from something else.
The speed of light in a vacuum is a constant. Time and distance aren’t.
There was a young woman named Bright,
Whose speed was much faster than light.
She set out one day
In a relative way
And returned on the previous night.
My own brain, she is filled with limericks.
Is that the one where the stars and planets make dents in a big rubbery blanket-looking thing with squares on it and smaller stuff spirals into them unless they go fast enough?
No, wait. There’s two, right? General and Special? One of them produced E=MC2.
Time slows down the closer you get to the speed of light, but stays the same on your planet of origin so you run the risk of getting a Planet of the Apes effect if you go flitting about the solar system in your almost-at-the-speed-of-light rocket ship. Things will look funny too, when you get going that fast.
The concepts of space-time and curved space and wormholes and Black Holes (or was that Hubble?).
Ignorance revealed!