# Time measuring in relativity

1. Sep 20, 2009

### rajeshmarndi

In relativity example , a time measuring device is shown in which a ball moves up and down.

This device is then placed in a moving frame and the motion of the ball is observed from the ground.

To an observer on the ground, the path followed by the ball will be a slant line. And therefore when it strike the top, it has to travel a greater distance and therefore it takes more time and the vertical velocity of the ball also decreases to the observer on the ground.
Which is the reason why time run slower in a moving frame.

My question why the vertical velocity of the ball decreases and the velocity of the ball on the path as observed by the observer on the ground doesn't change?

Plz correct where i'm wrong!

2. Sep 20, 2009

### Staff: Mentor

Generally such a clock uses pulses of light instead of balls and is called a "light clock". (Analyzing the motion of balls would be more complicated.)

With light pulses instead of balls we can apply one of the fundamental assumptions of special relativity: that the speed of light is the same for all observers. Given that the speed of light is the same for all observers, and that the observer watching the moving "light clock" sees the light travel a longer slanted path, that observer must measure a longer time for that pulse to travel back and forth.

3. Sep 20, 2009

### rajeshmarndi

How do we know that the assumption is correct.

4. Sep 20, 2009

### Staff: Mentor

One important way is to compare the predictions of that assumption--the various relativistic effects, such as time dilation--with the results of experiment. So far, so good! (See the FAQ at the top of this forum.)

5. Sep 20, 2009

### rajeshmarndi

what was the earlier thought on the observation of this moving frame and the path of the light pulse or its movement.
Just eager to know its earlier thoughts. Thanks

6. Sep 21, 2009

### Staff: Mentor

I believe that Einstein was the first to think about a "light clock", so there were no earlier thoughts. At least none recorded that I know of.

7. Sep 22, 2009

### rajeshmarndi

before the assumption that light speed is same for all observer.

when velocity of the moving frame and the velocity of the light were given
was it possible to calculate the time and velocity of the light on the slant path in the moving frame mathematically and not by experimenting.