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The Arrow of Time

May 22, 2008 on 5:01 pm | In Uncategorized | 21 Comments

The laws of physics are the same forwards in time as they are backwards, right?

At least, that’s what some physicists will tell you. They have (some) good reasons for saying this. After all, if you watch the planets orbit the Sun, they look like they obey the same laws of physics whether you watch them orbit clockwise (forwards in time) or counterclockwise (backwards in time):

But is everything time symmetrical? Well, no, of course not. There are certain things that are easy to do moving forwards in time, like to take a raw egg or two (left) and fry them (right). Well, that’s pretty easy.

But what about going backwards; what about taking a fried egg and turning it back into a raw one? Good luck with that. It doesn’t happen. Why not? Because some things involve decision making; they contain more information. Think about the raw egg, for example. There are many, many possibilities for what that egg could become.

(This will teach me to blog when I’m hungry.) But once you’ve gone and cooked it, you can’t go back! That’s basically what the second law of thermodynamics tells you: once you do something to increase the amount of information in a system, you can’t take it out without expending a tremendous amount of energy!

This is known, in layman’s terms, as the arrow of time. Things get messier as you go forward in time. There are some things you can see that make perfect sense going both directions in time (like the planets orbiting the sun), and some things that only make sense going in one direction. Here’s a short video clip with some examples of water; again, they only make sense going forwards in time:

If we want to think about this more deeply, think of it on a quantum level. We have a world, a plethora, of possibilities for the outcome of any event. Time passes, we make “an observation,” and we see an event. You can’t go backwards from there; the amount of information was just increased. My cat might be both dead and alive until I open the box:

But once I open it and find Shanie’s corpse, there’s nothing I can do to return the cat to a mixed dead/alive state. (Sorry, kitty!) And that’s why the laws of physics aren’t completely the same forwards as they are backwards.

Now, can someone tell me why physicists have trouble with this?


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