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Has the Problem of Time’s Arrow Been Solved? August 19, 2009

Posted by Dwight Furrow in Dwight Furrow's Posts, Science.

It is commonplace in the philosophy of physics to note that time has a direction.

If you spill milk,  you can’t unspill it. We always grow older, never younger. It looks like all physical processes move in one direction—they are time-irreversible. That is the “arrow of time” pointing toward the future.

But most of the basic laws of the universe can go in either direction. That is, the theoretical statements that describe the universe remain true even if time is reversed.

There is, one exception to the time symmetric laws of the universe—entropy. Entropy is the tendency for physical systems to go from a state of higher organization to a state of lowest organization.

Entropy always increases (i.e. toward less order)  in a closed system so time running backwards doesn’t get you back to where you started, despite the fact that there are no physical laws that explain this. We observe that entropy always increases but we don’t know why.

This leads to the problem of time’s arrow – if the laws of nature permit all processes to be run backwards in time, why don’t we observe them doing so?

A new paper by physicist Lorenzo Maccone has a solution. It is helpfully explained by Chris Lee at Ars Technica:

Imagine I do something that increases entropy slightly, and my wife observes the results of my actions and records the consequent increase in entropy—we will leave the fight over who should tidy up the mess out of the story.

Now, I can choose a set of operations that can return the entropy to its previously low value. However, doing so involves not just reversing my actions, but also reversing all correlated systems. In other words, I have to wipe my wife’s memory of the event and her subsequent recording of it. If she wrote it on a piece of paper, I have to wipe the paper clean etc, etc. But at the end of it, there would be no record of the event ever having occurred.

The upshot is that entropy-decreasing events can occur, but can never be observed from within the system. You can extend this to the universe, which may well be a closed system: we are within it and, even though events that reduce the entire entropy of the universe are possible, we can never observe such things.

How does this resolve the arrow of time problem? Well, put simply, running time in one direction allows records to be kept and events to be observed. In the other direction, observation becomes impossible. Therefore, although time could be running in either direction (or, who knows, both directions simultaneously), it is only possible for any observer (not necessarily a human one) to experience time in the forward direction.

So for all we know, we are getting younger, we just can’t know it.

That is comforting.

As someone in the comments section at AT quips:

So does this mean that if a tree falls in the forest, and nobody hears it, the tree can brush itself off and get back up again?

book-section-book-cover2 Dwight Furrow is author of

Reviving the Left: The Need to Restore Liberal Values in America

For political commentary by Dwight Furrow visit: www.revivingliberalism.com

philosophy of physics, time’s arrow, Lorenzo Maccone



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