# The Naked Scientists Forum

### Author Topic: Time dillation problems  (Read 1598 times)

#### jopie64

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##### Time dillation problems
« on: 20/12/2012 20:19:31 »
The first story about relativity, which made alot of impression on me, was the one with the twins and the rocket. It is as follows:
There're 2 twins. One takes a rocket and flies for a while at nearly lightspeed. When he comes back, his brother has aged more then himself.

This is explained with the light-clock. A light signal bounces between 2 mirrors. The twin in the rocket, which I shall call John from now on, takes a light-clock with him. Seen from earth, his light-clock ticks slower, because light has to travel more between the 2 mirrors. Seen from John of cource, his clock ticks regulary because light has to travel the same distance between the mirrors.
With this you can explain John ages less then the twin on earth called Bob.

Now what happens when Bob also has a light-clock?

Seen from John, Bobs light-clock runs slower then his own, because of the same reason as Johns clock runs slower seen from Bob.

So... what did I miss?

#### flr

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##### Re: Time dillation problems
« Reply #1 on: 20/12/2012 21:08:43 »
Quote
Seen from John, Bobs light-clock runs slower then his own, because of the same reason as Johns clock runs slower seen from Bob.
So... what did I miss?

While moving at constant speed, Bob will see John aging slower and John will see Bond aging slower. Both Bob and John are correct from their own frame of reference.  That means for time to be relative. Note that, while moving at constant speed there could be inertial frames of reference from which John is seen aging faster but also there could be frames of references from which Bob is seen aging faster. All observations are correct from their own frame.

On the other hand, taking into account that John is the one accelerating / decelerating then  an observer could tell John is the one older than Bob when returned back from the trip.

#### jopie64

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##### Re: Time dillation problems
« Reply #2 on: 20/12/2012 21:45:01 »
On the other hand, taking into account that John is the one accelerating / decelerating then  an observer could tell John is the one older than Bob when returned back from the trip.

Euh... seen from Bob, John is aging slower, so Bob will be older then John in this case right?

And this accelerating / decelerating thing... I never understood that explanation because, how can you explain this with the light-clock?
It seems to me that the light-clock explaination is not an explaination of Bob getting older then John, because that depends on which frame you take as reference. (According to John, Bob ages slower.)

Now the following:
John takes of in his rocket. Flies around earth for about a year seen from Bob. According to Johns clock, he's only flying for a day. Bob starts missing him, so he also takes a rocket and matches Johns speed and docks to his rocket.
They both accelerated.

According to light-clock theory Bob is almost a year older then John now. But from Johns point of view, Bobs clock ran slower al this time.
Now I think about it, Bobs clock ran slower seen from John, but this just lasted for a day. So Bob is just a little younger (not more then a day) seen from John.

I'm confused.

#### flr

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##### Re: Time dillation problems
« Reply #3 on: 21/12/2012 01:07:09 »

#### yor_on

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##### Re: Time dillation problems
« Reply #4 on: 22/12/2012 06:56:49 »
Now, that one was truly sweet flr :) I'll keep that link.

And it's true that it is the acceleration that breaks the symmetry, but it is also true that different uniform motions measured between different frames of reference in relative motion will present us with a 'time dilation'. But it is also so that all 'time dilations' are perfectly solvable just by moving yourself into that frame you recently measured, proving to yourself that yours, and that frames 'time' now is perfectly correlated :)

#### The Naked Scientists Forum

##### Re: Time dillation problems
« Reply #4 on: 22/12/2012 06:56:49 »