Time dilation question

Question

I remember reading that if you go in a spaceship and travel at the speed of light and come back, you would be younger according to time dilation.

I as yet do not see this as logical.

According to the same theory, it should look towards you and think that you're getting younger, thus you are going more slowly in time... so when it comes back you would be younger than it. So I would think that the easy thing happens and it all cancels out, and no one is older.

Does this make sense?

Answer 1

No, it doesn't make much sense. Go back and read what you wrote and perhaps explain more clearly where your confusion lies.

When you travel at high speed and return, you aren't younger than when you left. You're older than when you left. But you haven't aged as much as someone who remained behind at the starting point and waited for you to return.

It is not the same for you as it is for that person; you are the one who accelerated and switched reference frames, while his motion was unaccelerated the entire time.

EDIT: cd52211 is INCORRECT. His world is a world in which there is an abolute rest frame, and there is no such thing. If you run with a clock, constant speed motion is just as good as at rest. Your clock will run at exactly the same rate as if you were "standing still," whatever that means, as measured by you. Only the observers on the sidelines, who measure you and that clock in motion will observe it to run slow. Conversely, because in your own reference frame you are at rest and it is they who are in motion, then as measured by you, your clock runs normally and theirs run slow. Clocks in motion relative to the observer run slow. Clocks not in motion relative to the observer run normally.

In the spaceship example, the pilot observes the ship's clocks to run normally, and Earth's clocks to run slow. The Earth observer observes Earth's clocks to run normally and the ship's clocks to run slow. But the pilot is the one who is younger when he returns, because he accelerated during the trip, and this effect really has little to do with time dilation and much more to do with relativity of simultaneity, which shifts when he turns around.

Answer 2

I am convinced that time does not dilate. At near light speed an object gains mass and shortens in the direction of motion, the increase in mass and the foreshortening reduces space required for particle motion, motion slows for the particles within the moving object. If you subject a clock to such a speed, the moving parts gain mass and increased inertia, the power supply, however, remains the same, the moving parts will slow and the clock will show time lost, nothing has been lost other the speed of the moving parts of the clock.

Answer 3

Yes it does make sense.

When you sit still, you move at twenty four hours a day. As you move time actually slows down (hold a clock and run with it, it actually ticks slower). If we could reach the speed of light, time would actually stop.

Let's take two identical twins, Bob and Billy. Both of them are seven. Bob has been chosen to go on a thirty year space adventure. He will ride in a spaceship which travels at the speed of light while Billy stays on Earth. The spaceship has a clock which let's assume will not slow down even at the high speed. Since Bob is moving so fast, his body is aging a lot slower, so when he returns to earth after thirty years, Billy will have aged thirty years, but Bob will have only aged one or two years since his metabolism would have slowed down so much.

I know this is a hard concept to comprehend, but keep in mind this concept will undoubtedly become very vital in about thirty to forty years since this theory will be used in transportation to many other aspects of life. So my children will probably understand this better than I do, but for now I understand if you have trouble with this theory.

Answer 4

Considering special theory of relativity, time dilation should be applied in inertial frame of reference. The person on Earth is in inertial frame of reference (not exactly but to a good approximation). In this frame, the person on Earth is older. But what about from the point of view of the person in spaceship? You should not apply time dilation from his point of view because his frame of reference is non-inertial. Why non-inertial? This is because when he turns back, then he experiences huge acceleration. So, his frame is accelerating i.e. non-inertial.

You can look at it from general theory of relativity point of view also. The person in spaceship undergoes huge acceleration as he turns back. Any acceleration is equivalent to gravity and clocks in higher gravity move slower. The person in spaceship is in higher gravity (when he is taking turn) than the person on Earth. Therefore, spaceship's clock runs slower than the clock on Earth. Therefore, the person in spaceship is younger.