# B Question on the Special Theory of Relativity

#### Visair1

I have a doubt ,the thought experiment performed by einstein he thought that the clock was behind him and he is moving away from clock with speed of light what if he is approaching a clock with speed of light won't time run at very high speed

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#### phinds

Gold Member
I have a doubt ,the thought experiment performed by einstein he thought that the clock was behind him and he is moving away from clock with speed of light what if he is approaching a clock with speed of light won't time run at very high speed
Einstein NEVER said he was moving at the speed of light, since that would be impossible. I think you have misunderstood what you read.

At any rate, no matter how fast you move relative to something else, your clock always runs at one second per second. It does not run faster or slower because of your motion. It may look to some OTHER observer as though it is running faster or slower but for you it is not.

#### Nugatory

Mentor
I have a doubt ,the thought experiment performed by einstein he thought that the clock was behind him and he is moving away from clock with speed of light what if he is approaching a clock with speed of light won't time run at very high speed
(Phinds has already pointed out that you can’t be approaching at the speed of light; but of course some very high speed that is still less than lightspeed is OK)

If you and a clock are moving towards one another (it’s the same thing whether you say it is moving towards you or you are moving towards it) you will SEE the clock running fast. That is, the light that left the clock at 12:00:01 will reach your eyes less than one second after the light that left the clock at 12:00:00. That’s called the Doppler effect and it has nothing to do with relativity or time dilation; all that’s going on here is that the second light signal doesn’t have to travel as far because you and the clock have moved closer.

Likewise, if you and a clock are moving away from one another you will SEE the clock running slow because of the same Doppler effect. The light that left the clock at 12:00:01 will reach your eyes more than one second after the light that left the clock at 12:00:00, because the second light signal had to travel further to get to you.

However, you can calculate when the light signals were actually emitted by subtracting out the light travel time: light that leaves the clock when it is a distance D lightseconds away will reach your eyes $D/c$ seconds later, so you just subtract that from the time it reaches you to know when it left.

When you allow for that, you will conclude that the clock is running slow compared with your own. That’s the relativistic time dilation effect.

#### Visair1

Yes thats doplers effect

#### Ibix

Yes thats doplers effect
However, the point @Nugatory was making is that once you correct for the Doppler effect (or if you construct an experiment that ignores it) you will find that the moving clock is ticking slowly compared to your stationary clocks.

#### Ibix

Also, note that this kind of experiment has been done. Ives-Stilwell is the original test, and more are linked from that article and listed in the "Tests of time dilation and transverse Doppler effect" section of the Experimental Evidence for SR FAQ.

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