# Einstein's clock

1. Feb 10, 2017

### John S

Question: If the hands on a stationary clock will appear to stop when travelling away from it at the speed of light, what would you see if travelling towards it at that speed?

2. Feb 10, 2017

### phinds

Clocks don't travel at the speed of light and neither do you.

3. Feb 10, 2017

### stevendaryl

Staff Emeritus
As @phinds says, nothing can travel at the speed of light. But for traveling near the speed of light, the relevant equation for how things "appear" is the Doppler shift formula:

$f' =f \sqrt{\frac{c-v}{c+v}}$ for travel away from the clock

$f' =f \sqrt{\frac{c+v}{c-v}}$ for travel toward the clock

where $f'$ is the rate that the clock's hands appear to be moving (in degrees per second, or whatever units you like), and $f$ is the rate in the rest frame of the clock, and $v$ is your speed relative to the clock, and $c$ is the speed of light. So when you're moving toward a clock, it seems to be running faster than a clock at rest relative to you, and if you're moving away, it seems to be running slower.

The Doppler shift is not the same thing as time dilation, although the two effects are related (in the sense that the Doppler shift formula takes into account time dilation).