SCIENCE
12/31/2012 08:03 am ET

What Is Time? Theoretical Physicist Sean Carroll Explains Time's Arrow (VIDEO)

Have you ever stopped to ask yourself, "what is time?" We all understand its passage intuitively, experientially. But does time actually exist? Is it a force of nature? A tangible entity? Or is time a human construct--an attempt to comprehend the incomprehensible?

I spoke with theoretical physicist and perhaps the world's premier expert on the science of time, Sean Carroll, to learn more.

Watch the video above and/or click on the link below for a full transcript. And don't forget to sound off in the comments section at the bottom of the page. Come on, talk nerdy to me!

SEAN CARROLL: We remember things in the past and you don’t remember the future. But we can make choices toward the future. We can choose what to have for dinner tomorrow night, we cannot, sitting right here, make a choice about what to have had for dinner last night. We think of the past as in the books, it’s settled. The future is still up for grabs.

CARA SANTA MARIA: Hi everyone. Cara Santa Maria here. And that's Sean Carroll. He's a theoretical physicist at CalTech and author of a new book about the Higgs Boson called “The Particle at the End of the Universe.” He also wrote “From Eternity to Here: The Arrow of Time and the Origin of the Universe.”

SC: A lot of people from ancient greek philosophers to modern physicists have wondered whether or not maybe time doesn’t exist. Maybe it’s just an illusion. But when someone says to you ‘be at the restaurant at 7 p.m.’ you don’t get seized by existential anxiety and go ‘what could that mean? 7 p.m.? I have no idea.’ We all know what time is, how it works, it plays a very deep role in our fundamental physical theories. I personally think that rather than get rid of it, we should try to understand it better.

CSM: And one way to better understand time is to think of it in terms of directionality--a so-called arrow of time.

SC: What we call the arrow of time is just the fact that the past is different from the future. It’s a very, very basic fact. But there's many ways in which one direction away from us (toward the past) is different from the other one.

CSM: And we measure this arrow of time using clocks. A clock is just something that keeps pace of time--it repeats itself, predictably, over and over. Calendars are clocks. So are wristwatches. We have circadian clocks that keep biological time.

SC: When we think about the Earth going around the sun, it goes around the sun one time and rotates around its axis, 365 and a quarter times, those two things are in synchrony with each other and that’s what makes either one of them a clock.

CSM: That's where we get days and years. Seconds come from our own personal heartbeat clocks. And in physics, entropy itself can be viewed as a type of clock, since it's directional. It follows the arrow of time.

SC: The arrow of time is built into physics under the name of the second law of thermodynamics. This is this famous law of physics that says entropy increases over time. And entropy is just a measure of how messy things are, how disorganized or how disorderly they are. So you know that if you stack papers up very neatly on your desk, they will naturally over time become scattered all over the place. Whereas if you scatter papers all over the place they’re not going to naturally over time neaten themselves up, you have to do that. There’s a natural tendency in the universe for things to move from orderly to disorderly all by themselves.

CSM: And just as you cannot unscramble an egg, neither can you halt entropy. It's a fundamental principle in physics, running through every system in the universe, at least at the macroscopic level.

SC: If it’s true, and we think it is, that entropy is increasing in the whole universe, then in the past the entropy was lower. The mystery is why it ever was low in the first place. And both the data and our thoughts converge on saying that ‘indeed 13.7 billion years ago at the Big Bang the entropy of the universe was extremely low.’ That is an absolute puzzle for cosmology right now. We don't know why that is true.

CSM: Another puzzle is just how and why the arrow of time emerges. Sean says time doesn't really work the same when we probe way, way down to the microscopic level in particle physics. In fact, there doesn't seem to be an arrow of time at all when matter's reduced to its basic building blocks. When we look at molecules and atoms, entropy doesn't make a whole lot of sense, since the fundamental laws of nature are thought to be time-reversal invariant.

SC: If you do believe that the fundamental laws of physics are reversible then what you believe really is that information is conserved. So if you knew everything about the universe at one precise moment in time, in principle, you could turn a little crank and predict what the future would be like and reconstruct what the past would be like.

CSM: Wait a minute. That sounds like an argument against free will.

SC: Yes if you knew everything about you and the universe then you would not have free will. Then the future would be clear to you. However, you don’t and you never will. It is absolutely impossible in principle for your brain to obtain enough information to accurately predict what the future’s going to hold.

CSM: And because of that, we are moved along by the arrow of time, bound to remember the past and dream of the future. Alright everyone. It's that time again. Drop me a line on Twitter, Facebook, or leave your comments on The Huffington Post. Come on, talk nerdy to me!

MITCH HEDBERG: One time a guy handed me a picture and said "here's a picture of me when I was younger." Every picture is of you when you were younger.

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