Transcript:

Hi! Today we look at the arrow of time. You know, time only goes one way. You can't go backwards in time. Unlike space or distance, you can go forwards or backwards or left or right. In time, you can go only forwards. So, this idea of the arrow of time comes from Professor Carlo Rovelli in his book called "The Order of Time." And he makes a very interesting point.

I'm going to show you two videos, and then we'll talk about it. Here's the first one. This video is a video of a bouncing ball. So you see, as the video moves from left to right, the ball bounces lesser and lesser. And if I move the video backwards—if I move the video backwards—you'll see that there is something fishy, that the ball starts from rest and then starts bouncing on its own. There is something fishy.

But here's the other video. Here's the video of motion of planets. You see this video of motion of planets. Now this video, if I move backward—if I reverse the direction of this video, the motion of planets—you won't see that there's something funny. It would be looking just as normal.

So, the difference between the bouncing ball and the planetary motion is heat. Each time the ball hits the ground, there is some heat released, which is why the next bounce is less high compared to the first bounce, and then subsequently, the ball stops. So when the video moves backwards, it is a little bit bizarre because the ball suddenly starts bouncing. And this bizarreness of the backwards video for the bouncing ball versus the non-bizarreness of the backwards video for the planetary motion—the essential difference is heat.

Whenever there is heat involved in interaction, it sets the arrow of time. Time only goes one way. And the reason is probabilistic. We'll come to that later. Boltzmann figured that out many, many years ago. But for now, this is the key insight. Whenever there is friction involved, there will be an arrow of time.

That's it for now. Bye!