Can this be put in layman's terms?

[Lestat]

I'm not sure about philosophy, but this definitely has to do with science and math. I know there are several smart people here who might be able to answer this.

I have never been able to grasp the concept of why time slows down as velocity increases. I read Einstein came up with the theory as he watched a passing train and determined that time was moving slower for the people on board than the people standing beside the train. But I can't get it to make sense to me... Is this math? I'm terrible at math, but can understand most abstract things.

If someone on that train were to bounce a ball at the exact same time someone standing alongide the train, wouldn't they both catch the ball at the same time? Granted the person on the train would've covered a greater distance, but the time elaplsed from both balls bouncing should be the same.

Now I try to extend this to the speed of light. Someone on a space ship traveling just under the speed of light bounces a ball at the same time you do. Granted, he'd be well past the moon before he caught the ball, but if someone had a stopwatch, would you both catch the ball at the same time?

This is killing me. Is there a physics for dummies book that might explain this for me? Thanks.

[gumpzilla]

In a nutshell, this is what goes on.

The underlying structure of the theory of relativity all comes back to this basic idea: people moving at constant speed should always agree on what the laws of physics are. In particular, for the purposes of this argument they should agree on Maxwell's equations. Now Maxwell's equations predict a certain velocity for light in vacuum. Thus observers moving at different speeds should agree on the speed of light. Experiments done in two different inertial frames should be the same, even if they are moving at different speeds.

How does lead to time dilation? The easiest way (in my opinion) to see this is through the light clock. Imagine that I have a clock that works as follows. I have two mirrors parallel to each other and bounce a pulse of light between them. I can use the regular "ticks" of the pulse hitting the top mirror as a timing mechanism. Now let's imagine that somebody watches one of these light clocks whiz by at high speed. The path that the light takes is no longer a straight line up and down, but two diagonal lines; after hitting the top mirror, the light travels horizontally as well to hit the bottom mirror. These lines are longer, and so it takes more time per tick from the perspective of the outside observer because the light is still going the same speed (this is crucial). You can work out that the amount of time extra taken is just what Lorentz transformations would tell you.

But a person travelling with the light clock won't notice this. He'll still think that the clock is going at its normal speed. This, in essence, is time dilation. The person perceives each tick of the clock as taking less time than an outside observer. And this has to be more than just a quirky feature of light clocks, otherwise we could come up with an experiment to determine the absolute motion of the moving clock, which doesn't jibe with the axiom that physics should look the same in all frames moving at constant velocity. So the conclusion is that time itself moves slower for a moving person.

Try Googling "light clock;" I bet you'll find some diagrams that will demonstrate geometrically some of the things I'm talking about.

[Lestat]

Wow... Thanks! Yeah, pictures would help and I'll try googling it. One last silly question...

Does time have to exist? Or can it merely be a man-made instrument we use to measure a dimension we don't understand and aren't sure exists?

[JoshuaD]

Gumpzilla: That's a really good way of envisioning the effects. I like that alot.

[PairTheBoard]

[ QUOTE ]
In a nutshell, this is what goes on.

The underlying structure of the theory of relativity all comes back to this basic idea: people moving at constant speed should always agree on what the laws of physics are. In particular, for the purposes of this argument they should agree on Maxwell's equations. Now Maxwell's equations predict a certain velocity for light in vacuum. Thus observers moving at different speeds should agree on the speed of light. Experiments done in two different inertial frames should be the same, even if they are moving at different speeds.

How does lead to time dilation? The easiest way (in my opinion) to see this is through the light clock. Imagine that I have a clock that works as follows. I have two mirrors parallel to each other and bounce a pulse of light between them. I can use the regular "ticks" of the pulse hitting the top mirror as a timing mechanism. Now let's imagine that somebody watches one of these light clocks whiz by at high speed. The path that the light takes is no longer a straight line up and down, but two diagonal lines; after hitting the top mirror, the light travels horizontally as well to hit the bottom mirror. These lines are longer, and so it takes more time per tick from the perspective of the outside observer because the light is still going the same speed (this is crucial). You can work out that the amount of time extra taken is just what Lorentz transformations would tell you.

But a person travelling with the light clock won't notice this. He'll still think that the clock is going at its normal speed. This, in essence, is time dilation. The person perceives each tick of the clock as taking less time than an outside observer. And this has to be more than just a quirky feature of light clocks, otherwise we could come up with an experiment to determine the absolute motion of the moving clock, which doesn't jibe with the axiom that physics should look the same in all frames moving at constant velocity. So the conclusion is that time itself moves slower for a moving person.

Try Googling "light clock;" I bet you'll find some diagrams that will demonstrate geometrically some of the things I'm talking about.

[/ QUOTE ]

Nice. But why doesn't the person traveling conclude the same thing about the stationary light clock? From the traveler' perspective doesn't the stationary clock appear to have it's light bouncing diagonally?

PairTheBoard

[gumpzilla]

[ QUOTE ]

Nice. But why doesn't the person traveling conclude the same thing about the stationary light clock? From the traveler' perspective doesn't the stationary clock appear to have it's light bouncing diagonally?

[/ QUOTE ]

If you have light clocks in two different moving frames, this is exactly what happens. Each person perceives the clock in the other frame as moving slower. This is the origin of the famous twin paradox, which I think somebody else mentioned as its own thread somewhere else around here.

The real thrust of the twin paradox is sometimes missed by people who aren't familiar with relativity. If you take two twins, and put one in a spaceship and send them off for 10 years from the perspective of the twin on Earth at .877 * lightspeed, the twin in the spaceship should come back only 5 years older. A lot of people have a hard time believing this, but if special relativity is true (which all evidence that I'm aware of suggests) then it must be so. But this itself isn't the twin paradox. The twin paradox involves a further step: from the perspective of the person in the spaceship, time on Earth should seem to be going twice as slow as time in the spaceship, yet when he comes back he finds that just the opposite has occurred. Why is it that the person on the spaceship hasn't aged excessively and the Earth twin stayed young?

Perhaps I'll explain more later, but for now I'll just say that the flaw in the argument leading to the twin paradox is assuming a symmetry between the two situations that's not entirely there.

[xniNja]

http://casa.colorado.edu/~ajsh/sr/time.html

The above site has good diagrams for light clocks & a solution of the paradox. However, I've always thought the light clock explanation isn't particularly intuitive without an understanding of special relativity. On top of that, once you have a basic understanding of special relativity, it still isn't clear, logically, (at least not to me, I'm no physicist but have studied physics) what the solution in space-time means or all of the implications it could have.

[ QUOTE ]

Does time have to exist? Or can it merely be a man-made instrument we use to measure a dimension we don't understand and aren't sure exists?

[/ QUOTE ]

I'm pretty sure "time" as a concept would exist even without a measurement of it. The only way to record an event is to give its location in space and the time at which the event took place.

For some reason I seem to recall that the solution to the twin paradox lies in the acceleration stages required for the spaceship to achieve a large velocity when compared to the speed of light and then return to rest. A simple lorentz transformation doesn't apply to those stages.

[daryn]

not sure if anyone said this.. i am too lazy to read the responses.

imagine you were sitting on a rocket sled going the speed of light. if you turn around to look where you've been, you'll see that time is standing still, since no new information can reach your eye.