Time Dilation

Makes me think of pupils.

Anyway, learn that a light clock is a device which uses a beam of light reflected between two mirrors to measure time. This is what you see below:



Now lets tackle Time Dilation.

So Jack is at rest.

Set Jill to move at 0.3 the speed of light c and click "play".

Notice that for Jack, the photon travels up and down in a straight line...nothing complicated.

Jill is moving away from Jack from Jack's perspective (we would say from Jack's frame of reference). Notice that Jill's photon travels a further distance as seen by the traingle shape path.

The description on the top right hand side is wrong...this is the correct interpretation

From Jack's frame of reference: Jack measures 10.0 seconds on the clock he is holding. When he looks at Jill's clock (so he takes out some binoculars to see Jill's clock which is very far away), he sees the clock read 10.4 seconds. This is time dilation.

From Jill's frame of reference: the ship, the floor, the walls and everything else around her are stationary while Jack appears to be moving away. From Jill's frame of reference, she will see 10 seconds on her clock and when peering through binoculars to Jack's clock, will see 10.4 seconds.

This is like the man and boat example. For the man on the boat, a ball dropped from the mast of the ship will follow a straight path downwards. For the external observer, the ball appears to follow a parabolic path as the boat is moving. Identical situation but different conclusions from different frames of reference. (This is Simultaniety...the same 

Time stretches and contracts because it is the speed of light which ios constant.