How can you tell if something is moving?

I mean, yes, you can watch it move compared to the background, okay?

What if that motion is towards or away from you? Right? Let's make it a little bit harder.

What how can you tell if something is moving away from you? Usually we tell that by if it's

getting bigger or smaller. If something's getting bigger, something in our brain triggers,

oh, I think that thing is getting closer. And if it's getting smaller, we think, oh, it's getting further away. But what if that doesn't quite work? What if

something's too far away and you can't really tell if it's getting closer or faster?

If you want to know how fast it's going, not just where it's going, but how fast it's going.

Well, let's say you have a speed gun.

You know, speed guns are these handy little devices where it shoots out a beam of light.

You know, radiation, it's invisible, but it's still light.

It shoots out a beam of light, hits something, that light gets absorbed, and then

gets re-emitted, because that's what light does. And it comes back and there's a little sensor

on the light gun that detects that light. And it compares the light that sent out to the light

that got back and see if there's any differences. And if the object is moving, there will be differences. They'll

be a shifting of the pattern of light. If something is moving towards you, then as it's

emitting the radiation, these waves of electricity and magnetism. You can imagine the waves just like emanating

off the object. And as it's moving towards you, it squishes up those waves. Like it gets in the way

of its own waves. It piles them up. It shifts them to higher energies. It blue shifts them. This is a Doppler shift. The exact

same kind of shift that causes stretching and compressing of sound waves can do it to any wave,

including light waves. And the exact opposite of blue shift is red shift. If something's moving

away from you, it's pulling on those waves. It's tugging at them as it's moving away.

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