Originally posted by LocalX
I'm sure it was on this site but I cant find it. Is there a link or Web Page that explains the theory of relativity in layman's terms? I remember something about a train. My daughter asked me about it yesterday and I wanted to show it to her.
Thanks in advance.
Don't know which link you mean. I found lots.
Here's an explanation that came from a physics course at Purdue. It requires only a basic understanding of the Pyhtagorean theorem. Look at the attached drawings.
Special relativity says that time is a dimension just like space has dimensions. It is at a right angle to the 3 dimensions we know. You can't visualize that, so "collapse" our 3 dimensions into one (because what direction you're moving doesn't matter to realtivity). Now draw a line at a right angle to that line. Call the vertical one space and the horizontal one time. That's the top left drawing.
You move through time just as you move through space. In fact, all you do is move through spacetime. Not space time, but spacetime. It's one thing. And you must always move at a constant rate through spacetime. Let's call it One "C". That's the red line in the second drawing.
When you're sitting still in space, you're still experiencing time, so you're moving in spacetime at the rate of One "C", but only in the time direction. You experience the time at that rate. Let's call this "normal". This is the third drawing; all the red "movement" is along the time direction.
When you start to move through space, your total velocity must remain constant, so you move less through time. In the fourth drawing you'll see that One "C" is drawn between a tiny increment in the space direction, to a point on the time direction slightly less than before. The One "C" remains constant. (Pardon my poor drawing skills and pretend the picture does show this).
If you go fast, the One "C" line has to connect a point well up the space direction with the time direction's line, but does so at a much shorter distance. As you go faster, you experience less time, so that your total remains constant. This is the fifth drawing/
The sixth drawing represents moving at the speed of light, or One "C". You always were, but now you're doing it entirely in the space direction. Since all your velocity is there, you do not move in time.
That's special relativity. There's a precise relationship between space and time. The pcitures describe it, but the equation is a little more difficult. There is also a similar precise relationship betwen mass and energy. The equation for this is a little more straight forward even though it derives from the other. It says that the mass is energy, and the amount of {E}nergy (measured in ergs) represented by a certain mass is equal = to that {M}*** (measured in grams) times 898751347000000000000. Which happens to be One "C" squared.
Now all this moving more slowly in time is as seen from the outside. To you, in your spaceship or whatever (your "frame of reference") everything seems normal inside. It's outside that's getting weird.
The light coming at you from in front is moving at C. You're moving fowards at say 90% of C. The light can't move 1.9C. It must move C total. So, some of what would be greater than One "C" is coverted to energy. Light with more energy has a shorter wavelength -- it's "bluer". The opposite happens to light catching up to you from behind.
There is also strange shortening of space itself in the direction of fast travel, called Lorentz contraction. And an increase in mass as you go faster. This is all because spacetime and massenergy are inextricably linked into one cohesive universe and we only perceive things about it as separate.
Einstien also put forth the equivalence principle, saying you can't tell from inside a box whether you're in a gravity field or accelerating. They're equivalent because bent spacetime due to gravity (the old elastic sheet idea) is an acceleration. "There is no special frame of reference".
General relativity combines special relativity and equivalence and ends up explaining strange things light light "bending" due to gravity as well as more exotic things like the fact thatblack holes can exist.
If that hasn't bent your head enough consider: anything moving at the speed of light experiences n0 time. In comparison it would observe the outside world as moving extremely fast. In fact, at One "C" everything outside happens immediately.
Imagine a photon created in the original big bang, zipping off into the cosmos. It never slows down and never hits anything. It continues at that speed until all matter and ebergy exhuast themselves, some 10(^200) years from now. But to that photon, it will have seemes as though the entire history of the universe from beginning to end happened immediately in a time of zero duration. Stranger yet, it would be every bit as correct as you who sat still and watched the universe unfold over all that period of "normal" time, never moving an inch.