Need help understanding time dilation

[annms]

I keep reading that an observer will find the clock moving relative to him to be slower than his own local clock. But isn't his own local clock measuring proper time, which should be a shorter time period than the moving clock? How can the slower (moving) clock measure a longer time period? Isn't that a contradiction? I'm so confused please help.

 

[Doc Al]

I keep reading that an observer will find the clock moving relative to him to be slower than his own local clock.

Right.

But isn't his own local clock measuring proper time, which should be a shorter time period than the moving clock?

Proper time between what events?

How can the slower (moving) clock measure a longer time period?

A slower clock measures less time between events, of course.

If the 'events' are the ticks of the moving clock, then it is the moving clock that measures the proper time between them. Not you! (Your clocks measure proper time between events that occur at your location.)

Say you are observing a rocket speeding by. The rocket has a clock that is set to flash a burst of light every time a minute passes. You, using your own Earth clocks, would say that the time between flashes was greater than one minute and thus you would describe the moving clock on the rocket as running slow. Since the flashes take place at the location of the moving clock, that clock measures the proper time between them.

 

[annms]

Thanks. I think I'm getting it a little better.

This was what really confused me: Let's say a spaceship is traveling at a speed near c relative to earth. If one hour has elapsed according to an Earth observer's clock, how much will have elapsed in the spaceship's clock according to the Earth observer? (ie. greater than one hour or less than one hour?) And why?

 

[Doc Al]

If one hour has elapsed according to an Earth observer's clock, how much will have elapsed in the spaceship's clock according to the Earth observer? (ie. greater than one hour or less than one hour?)

The Earth observer would say that during the one hour on earth, less than an hour would have elapsed on the spaceship clock.

And why?

Moving clocks run slowly.

 

[Naty1]

Annms:

What do you think the rapidly moving spaceship observer will say about the elapsed time he observes on earth??

Hint: who is moving rapidly now??