Sean Carroll on Time

SEAN CARROLL ON SPACE AND TIME

Posted November 10, 2011

The words past and future do not appear in the laws of physics. There is no difference between one direction of time and the other in the ultimate elementary laws of physics. What we do is we assign the words past and future to the direction of lower entropy and higher entropy.

Entropy is a way of measuring the disorderliness of any system—whether it is the universe or a cup of coffee or anything like that. So if things are precisely organized, in particular if they're segregated so that your cream is over here and your coffee is over here, that's low entropy, that's a high amount of organization. Then you mix them together and the entropy goes up. And it's a law of nature if you leave things by themselves entropy always goes up. Things go from being orderly, very delicately organized, to being messier, to being more disorganized.

And so we tend therefore to associate time with not just change, but a certain directed kind of change. The direction of time in which the entropy was lower we call the past. The direction of time in which entropy is increasing we call the future. That's what defines the arrow of time. And all of that is part of this underlying dynamic caused by the fact that the universe started very organized and is becoming ever more disorganized as it expands and cools and things happen.

AUDIO

Produced by

David Levin

Original interview by

Randy MacLowery

IMAGE

(Sean Carroll)

© WGBH Educational Foundation 2011

Major funding for "The Fabric of the Cosmos" is provided by the National Science Foundation and the Alfred P. Sloan Foundation.

Additional funding for this program is provided by the Arthur Vining Davis Foundations.