# One more (possibly dumb) question.

1. Oct 4, 2011

### AH_AHSAN

I want to know that if we are standing on the earth's equator facing west and throw a beam of light at some distance, say 100 km towards west. then would the eastward rotation of earth around its axis would decrease the actual distance for light to travel.?
because as the photons emitted from the light source started moving towards the target, in the mean time earth rotates towards east along its axis. So does the photons emitted from the source have to move less distance when they reaches the target which is 100 km away ?.

i think as the photons have 0 mass so gravity have no effect on them and they will actually have to cover less distance than the actual distance.

2. Oct 4, 2011

### ghwellsjr

If one distance is 100 km to the west and the other one is 100 km to the east, then the two distances are equal. I think you are really asking if the light can get to one target quicker than the other one, correct?

3. Oct 4, 2011

### AH_AHSAN

YES this is what i actually means.

4. Oct 4, 2011

### ghwellsjr

Well, it's not possible to know if the light gets to one target quicker than the other.

But suppose instead of a pair of targets, we placed a pair of mirrors 100 km away, one to the east and one to the west. Then we launch a beam of light to the west and another one to the east at the same time and we wait until the light traverses the distance to the mirrors and comes all the way back to us and we will discover that it arrives at exactly the same time from both directions. It will take both beams of light about 2/3 of a millisecond to make the round trip.

Einstein says that we define the time it takes for the beams of light to make it one-way to the mirror to be exactly one half of the round trip time so that it will be 1/3 of a millisecond for both directions.

And so Special Relativity says that until and unless you define the times it takes for the beams to make the trip, you cannot talk about how long it takes for each of them.