Time (and existence)

[richhhhhard]

Originally Posted by insanefreddy926

I think you guys need to understand special relativity a bit more to understand what Reach has been saying. So here's a crash course :

One of the most important things we've discovered about the universe is that light always travels at the same speed (through a given medium). This may sound obvious, but if you think about it, it's really weird. Consider standing on a street, and throwing a ball. You are stationary, and you throw the ball and it goes 10 meters per second. Now let's say you hop on your skateboard with the ball. You ride down the street at 5 m/s and throw the ball while on your board with the exact same force as before. Now the ball will travel at 15 m/s, since it's already moving at 5 m/s and you throw it at 10 m/s. Now let's say you repeat both steps, but instead of throwing a ball, you shine a flashlight. The speed of light is about 3X10^8 m/s. So you shine it while standing still and it goes at 3X10^8 m/s. You would think that when you shine the light while moving on your skateboard, it would go 3X10^8 m/s plus the 5 m/s you're already moving, just like the ball. But it doesn't. It still only travels at 3X10^8 m/s. This type of experiment has been done (albeit not exactly in the same way) and proven.

So knowing this, we can generate a "thought experiment" which makes use of this curious property of light. Now the actual experiment is impossible to do (hence "thought experiment") but you can figure out a lot from it: Say you are in a spaceship, traveling at a constant speed. The ship has a large window. You have a large transparent cylinder which has mirrors at each end of it. There is a light beam which is continuously bouncing back and forth between these mirrors.

There is also an observer standing on the Earth, stationary. We say that you and the person on the earth are in two different "reference frames" because you are moving while the observer is stationary.

So you're going in your spaceship, and you fly right by the observer while the light is bouncing in the cylinder. You and the observer both see that light travel from the top of the cylinder to the bottom of it. You and the observer both measure how far you saw the light travel. To you, the light went in a vertical line, from top to bottom. So the distance you measure is the length of the cylinder. To the observer, however, the light traveled in a diagonal line, because while it was moving up and down in the cylinder, the ship was also moving sideways past the observer.

Now we can form a right triangle. One side is the distance you saw the light travel. The hypotenuse is the diagonal distance the observer saw the light travel. The other side is the distance the ship traveled while the light went from top to bottom. We know that distance/time = speed. So distance = speed*time. We know that light always travels at the same speed.. We know that the observer saw the light travel a longer distance than you did, because the diagonal side of a right triangle is longer than the other sides. This means that the light traveled two different distances while going at the same speed. Since distance = speed*time, this must mean that you and the observer both experienced two different times. Since the light traveled a shorter distance for you, "your time" was passing slower, relative to the observer.

That's pretty much the basis of it. If you were to use the Pythagorean Theorem on that triangle and fiddle around with it, you would end up with a form of that formula which Reach referenced. But this really shows that the act of traveling through space alters the time which you experience. In fact it slows it down.

Now if you were to use that formula you would see that the faster you travel the slower time passes. When you travel at the speed of light (which isn't really possible) time passes infinitely slowly; or it stops.

edit:



Actually the classic example of a black hole is an object so dense that it bends space so much that every direction you can travel inside the event horizon leads to the center of it. So when light crosses the event horizon, it travels in "straight" lines which lead to the center of the black hole.

Thanks for the information. However, this does not change anything in my opinion. What you are saying is that the way we *perceive* time changes, but what is *really* happening with the light? Regardless of how the two people perceive it, it is doing the same thing. Because we are limited by our perception does not mean that altering how you perceive something changes the time that passed. To refrence your example, "To the observer, however, the light traveled in a diagonal line, because while it was moving up and down in the cylinder, the ship was also moving sideways past the observer." Is time limited by our perception? Outside of the perception of the two people the light is not doing anything differently. The only thing that is different is that one of them is moving really fast so they perceive a change. The equation d=s*t is irrelevant because that is *supposed* to be measuring only the light, not your perception of it. You are taking a scientific equation and using it to try to rationalize differences in perception.

Edit.
What you said about black holes means the same thing as what I said.