What is Space Time? | StarTalk

What is space time? You already know. You have never met someone at a place unless it was also at a time. You have never met someone at a time unless it was-- OK, I get it. I get it. So we-- Whoa, well, wait a minute.

What happens to a photon from 13 billion 0.800 million years that comes this way and enters my eye so I can see it? Where's space involved in that? It entered your eye at a time and at a place right here. That's all that matters here. Is that all we're saying?

Well, once you have formalized space and time and know that they're conjoined, then you can make all kinds of fascinating calculations with relativity. Well, what is all that? The train's going and I'm walking down there. - Exactly!

I'm walking down the train. I'm walking on-- And the time change. And the time is going to change. What is all that? That's all the consequences of thinking about space and time as conjoined. But it's confusing. So?

And not only is it-- The universe is under no obligation to make sense to William Shatner. OK? No, but William Shatner's under the obligation to make sense of the universe, as you are doing. And why do I slow down as I approach the speed of light?

It doesn't apply to a photon 13 billion-- No. You want to freak out? I don't want your head to explode. You ready? - Yeah. - OK. - No, you're not ready. Are you ready? - No, I'm ready. I'm ready. - OK, OK.

The faster you go, the slower time ticks. Say that again. The faster you move, the slower time ticks for you as seen by others. Right. As you approach the speed of light, time continues to slow down.

WILLIAM SHATNER: Yes. At the speed of light, time stops. Which means for a photon moving at the speed of light, when it is absorbed in your retina, it is the same instant it was emitted at the Big Bang 14 billion years ago.

That's what I thought. The photon gets emitted. Bam! As far as it's concerned, it is in your eye in that same instant. Can we measure that photon and observe-- Yes!

WILLIAM SHATNER: --the Big Bang? I know that that came from the Big Bang, and I'm watching it. And it's taken 13.8 billion years to reach you. But if you are that photon, it does not experience that time delay.

What a great science fiction story. That-- Instantaneous. [applause]