# Question about light and relativity

1. Dec 9, 2005

### thunderchicken

Another question about light and relativity:

The way I “understand” the theory of relativity is that it is based on the fact that light always moves at a constant speed in the direction which you perceive it to be traveling. This is how (I think?) Einstein’s light-clock and spaceship analogy works.

My question is this: If relativity is based on the observer’s perception of light’s direction, does optical perspective (reality) have an effect?

As the light clock gets farther and farther away from the observer, the distance the light is traveling appears to decrease dramatically until it eventually disappears. If the perceived vertical distance the light has to travel becomes shorter, and light continued to travel at the same speed as always, would the clock not tick faster and faster the farther away it gets?

It seems to me that the thought experiment only works if the two clocks can be perceived as being exactly equal in height – something that is simply not possible. In reality, shouldn’t the person the spaceship age way faster??

Thanks for your help.

2. Dec 9, 2005

### Staff: Mentor

Optical perspective is an illusion, not a reality.

3. Dec 9, 2005

### thunderchicken

So, what would the observer see then as the clock moves away?

4. Dec 10, 2005

### bernhard.rothenstein

in special relativity to "see" means to detect at the same time light signals that have left a luminous object at different times. special relativity is involved with many clocks. The light clock you have singled out measures a proper time interval whereas the stationary clocks it meets during its trip measure by the difference between theirs readings a non-
proper time interval.
I think you should rephrase your question. Please have a look at a paper of mine "Illustrating Einstein's special relativity" arxiv phyhsics education 2005.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook