The Arrow of Time

The arrow of time is something of great simplicity that we experience constantly. It also presents some of the most difficult philosophical and physical questions concerning the nature of time and our perception of it.

What is the Arrow of Time?

The arrow of time is a very simple concept: time goes one way and one way only. This is so natural, so obvious that we rarely think about it. People are born, age and die. Fires burn wood to ash. We remember the past not the future.

That's just the way the universe is - it fits with our rational, common sense view of the world. Unfortunately "common sense" is often one of the first casualties of theoretical physics.

It's a complex issue. I'm not a trained physicist, just an interested layman - on this page I'll try to introduce some of the basic points in layman's terms.

Reversibility: T-symmetry

For physicists the arrow of time is anything but obvious. On the one hand it fits perfectly with the second law of thermodynamics (*). Entropy has a "preferred" direction of time - but the other laws of the universe do not.

The problem is that the fundamental mathematical equations at the core of modern physics are reversible. Technically speaking they are "invariant" if time is considered to be running the other way (under the time-reversal transformation T: t -> -t).

According to the mathematics of physics, time could equally well run backwards. The arrow of time could point either way or even change direction.

So why does time always run the same way? Why can we not remember the future? According to the basic laws of physics the bull might just as easily tidy up the china shop.

Entropy Wins?

The most popular answer to this contradiction between entropy and every other law of physics is effectively to state that entropy "trumps" the rest. If only one of the laws of the universe "cares" about the direction of the arrow of time then the universe will go in that direction.

A more scientific justification of this position comes from looking at the difference between the very small microscopic world and the larger everyday one. It's true that the laws of physics are reversible at a microscpopic level - but we don't live at that level. The world we live in and experience is made up of huge numbers of these tiny microscopic particles and processes.

Austrian physicist Ludwig Boltzmann is credited with proposing the statistical explanation of time's arrow. Put simply, eggs break because they are more likely to break than reassamble themselves. If you think of all the millions of molecules in an egg then there are far more "broken" arrangements than "unbroken" ones. If a bull runs around in a china shop there are far more ways that it could smash the place than tidy it up.

According to this theory, the arrow of time as we perceive it could move backwards - it's just very unlikely to do so.

Maxwell's Demon

The problem with the statistical explanation for the arrow of time is that it doesn't account for human perception. Our memory consists of information, not random collections of molecules. Why should we remember the past any better than the future?

A possible explanation for this is linked to a famous thought experiment: Maxwell's Demon. James Clerk Maxwell proposed imagining a demon that sat between two containers of gas. The demon controls an opening between the two containers, it allows "fast" molecules through one way and "slow" ones through the other. In doing so it is constantly heating up one container at the expense of the other, cooler one. It is breaking the second law of thermodynamics.

One possible explanation for this is that the demon itself must be considered part of the system. When the information which the demon possesses is taken into account, the precious Second Law remains intact.

This introduces the concept of "information entropy" which is thought by some to provide a possible explanation for the psychological arrow of time.

External Links:
T-symmetry

(*) The Second Law of thermodynamics was summed up memorably by the wonderful Flanders and Swann as:

"Heat won't pass from a cooler to a hotter
You can try it if you like but you far better notter"

	All About Clocks, Watches and Time
The Arrow of Time The arrow of time is something of great simplicity that we experience constantly. It also presents some of the most difficult philosophical and physical questions concerning the nature of time and our perception of it. What is the Arrow of Time? The arrow of time is a very simple concept: time goes one way and one way only. This is so natural, so obvious that we rarely think about it. People are born, age and die. Fires burn wood to ash. We remember the past not the future. That's just the way the universe is - it fits with our rational, common sense view of the world. Unfortunately "common sense" is often one of the first casualties of theoretical physics. It's a complex issue. I'm not a trained physicist, just an interested layman - on this page I'll try to introduce some of the basic points in layman's terms. Reversibility: T-symmetry For physicists the arrow of time is anything but obvious. On the one hand it fits perfectly with the second law of thermodynamics (). Entropy has a "preferred" direction of time - but the other laws of the universe do not. The problem is that the fundamental mathematical equations at the core of modern physics are reversible. Technically speaking they are "invariant" if time is considered to be running the other way (under the time-reversal transformation T: t -> -t). According to the mathematics of physics, time could equally well run backwards. The arrow of time could point either way or even change direction. So why does time always run the same way? Why can we not remember the future? According to the basic laws of physics the bull might just as easily tidy up the china shop. Entropy Wins? The most popular answer to this contradiction between entropy and every other law of physics is effectively to state that entropy "trumps" the rest. If only one of the laws of the universe "cares" about the direction of the arrow of time then the universe will go in that direction. A more scientific justification of this position comes from looking at the difference between the very small microscopic world and the larger everyday one. It's true that the laws of physics are reversible at a microscpopic level - but we don't live at that level. The world we live in and experience is made up of huge numbers of these tiny microscopic particles and processes. Austrian physicist Ludwig Boltzmann is credited with proposing the statistical explanation of time's arrow. Put simply, eggs break because they are more likely to break than reassamble themselves. If you think of all the millions of molecules in an egg then there are far more "broken" arrangements than "unbroken" ones. If a bull runs around in a china shop there are far more ways that it could smash the place than tidy it up. According to this theory, the arrow of time as we perceive it could* move backwards - it's just very unlikely to do so. Maxwell's Demon The problem with the statistical explanation for the arrow of time is that it doesn't account for human perception. Our memory consists of information, not random collections of molecules. Why should we remember the past any better than the future? A possible explanation for this is linked to a famous thought experiment: Maxwell's Demon. James Clerk Maxwell proposed imagining a demon that sat between two containers of gas. The demon controls an opening between the two containers, it allows "fast" molecules through one way and "slow" ones through the other. In doing so it is constantly heating up one container at the expense of the other, cooler one. It is breaking the second law of thermodynamics. One possible explanation for this is that the demon itself must be considered part of the system. When the information which the demon possesses is taken into account, the precious Second Law remains intact. This introduces the concept of "information entropy" which is thought by some to provide a possible explanation for the psychological arrow of time. External Links: T-symmetry (*) The Second Law of thermodynamics was summed up memorably by the wonderful Flanders and Swann as: "Heat won't pass from a cooler to a hotter You can try it if you like but you far better notter" Clocks-Watches-Time / Time / Arrow / All original material copyright © Trevor Mendham 2005-2008. All trademarks are the property of their respective owners Please read the site terms of use		Clocks Watches Time 60 24 Hours 7 Days Arrow Atomic Bells Biological Calendars Daylight Saving Leap Seconds Planck Quotes Solar Standard Travel Zones Time & Motion More...