Today we have a special relativity challenge question

for you.

Now, this one is conceptual, not mathematical.

So I expect you all to do your homework.

In a recent episode, we showed you

how the ticking of a clock-- and in particular a photon clock--

slows down if that clock is moving with respect to you.

The photon in the clock has further

to travel from your stationary perspective.

But you still see it traveling at the same old speed of light.

So those ticks take longer.

This simplistic thought experiment

applies to the real world.

And the resulting slowing of clock ticks due to motion

is the time dilation of Einstein's theory

of special relativity.

It's a very real effect.

Time dilation plays a key role in today's challenge question.

Ready for it?

Here we go.

Imagine observing a clock moving toward you

at 50% the speed of light.

As the light from that clock reaches you,

you see the hands of the clock tick.

Eventually the clock reaches you, passes by,

and continues to move away at the same speed.

When the clock was moving toward you,

do you observe its hands to be ticking slower, faster,

or at the same rate compared to the rate of ticks

that you observe when it's moving away from you?

And for you overachievers, here's a bonus question.

When the clock is moving toward you,

do you see its hands ticking slower, faster,

or at the same rate compared to a second clock that is

stationary with respect to you?

Submit your answers to the email on the screen,

using the subject line Photon Clock Challenge Answer.

We filter by subject line, so make sure

you use exactly this phrase.

We'll choose five random entries from the correct submissions

to receive brand-new PBS Space Time T-shirts, which are also

available for anyone to preorder at a link you'll

find in the description.

Submissions are due two weeks from the release date

of this video.

We'll see you next week for a fresh new episode

of Space Time.