I'm tutoring a Year 12 (high school) physics subject which requires me to understand special relativity, in particular, time dilation and length contraction. I have only studied 1 semester of 1st year uni physics, so bear with me for sounding ignorant. I've tried reading parts of Physics for Scientists and engineers but have more questions than answers.

I get that if you're traveling away from a clock at high speed, the clock will "appear" to slow down with respect to the person. After one tick, you've moved a few more million metres away and so the light has further to travel to reach you, which will take longer. A bit like looking up at the stars and thinking that is it happening now, but it's actually something that happened thousands of years ago.

What happens if you move toward the clock. Does time speed up?

So why is there only time dilation and not contraction? My only understanding I can lean on here is the doppler effect, but I have a suspicion that has nothing to do with it.

PS I have a degree in engineering, but I struggle to get my head around this stuff.

## Best Answer

What you are describing here has nothing to do with time dilation. What you are describing is the classical Doppler shift. (The relativistic Doppler shift is similar but also includes time dilation)

In relativity, the “big three” effects (time dilation, length contraction, and relativity of simultaneity) are what remain after the observer correctly accounts for the delay in the signal reception due to the finite speed of light. They are not optical illusions, they are what remains after eliminating the optical illusions.

If you move toward the clock at relativistic speeds then there is a relativistic Doppler shift. This includes a classical Doppler blue-shift plus time dilation which makes the signal a little less blueshifted than you would expect classically.

The time dilation effect does not depend on the direction, so it even occurs if the clock moves perpendicular to the receiver, thus it is sometimes called the transverse Doppler shift.

It is because of the symmetry. For the first postulate to hold, the principle of relativity, all inertially moving clocks must go slow relative to any inertial frame.

You can get time “contraction”, but only where the symmetry is broken. For example, in a gravitational field, both clocks agree that the clock down lower is slow and the clock up higher is fast. The gravitational field breaks the symmetry since both can tell which clock is the higher and which is the lower.