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Time Going Backwards?…….Forget About It!

We recently received a question from a listener asking me to help figure out the following article:

Is quantum mechanics messing with your memory?

This article refers to a recent attempt by physicist Lorenzo Maccone of MIT to address what’s been called “The Arrow of Time Mystery”

In it Maccone tries to address one of the many long-standing apparent differences between the microscopic world and the macroscopic one, namely this arrow of time mystery.

Processes at the atomic scale are time-symmetric or slightly more technically known as “time-reversal invariant”. This means that the laws governing physics at this scale make perfect sense even if time is reversed. Imagine atom A colliding into atom B sending it careening away. Reversing this so B hits A  makes perfect sense and is permitted by all the rules governing such behavior.

When you scale this up however, it doesn’t work nearly as often. Macroscopic events that we deal with every day are often time-asymmetric like glass breaking or my coffee cooling as I type this post. We never see a shattered piece of glass coming back together to form a whole object; or do we?


These macro events are described by the statistical second law of thermodynamics which tells is that the entropy or amount of disorder in a closed system never decreases. Your bedroom just keeps getting dirtier and messier and more disorganized as time goes on. You can clean and organize it but that uses up energy as well, ultimately further increasing entropy leaving less usable energy available in the future.

Geeky Aside:
Extrapolating this far into the future reveals one of the likely ultimate fates of the universe called Heat Death. At this point, entropy has reached maximum in the universe meaning that all available energy has dissipated evenly and is close to absolute zero. With no energy gradients anywhere, life cannot exist and things get pretty dull. Fortunately this won’t happen for more than 10 to the 100 years…wait…that’s a Googol years isn’t it?…Cool

The problem with entropy and the arrow of time though is that even the macroscopic processes described by entropy are ultimately composed of atomic scale particles which in turn are time-reversal invariant. So why isn’t this scaling up?

Maccone’s solution to this arrow of time mystery is that there is no mystery. He posits mathematically that reverse entropy (reverse time) events do happen and we do observe them but all the information these events leave behind are erased quantum mechanically leaving behind the one arrow of time that we observe.

He describes how this happens as a quantum entanglement between your memory and the entropy decreasing or time reversal event such as glass unbreaking.

Maccone says that when the entanglement eventually dissipates:

“the disentangling operation will erase this entanglement, namely the observer’s memory”


“all phenomena which leave a trail of information behind (and hence can be studied by physics) are those where entropy necessarily increases or remains constant. All phenomena where the entropy decreases must not leave any information of their having happened. This situation is completely indistinguishable from their not having happened at all.”

There you go, problem solved.

As you might imagine, some physicists have a problem with this.

Huw Price, head of the Centre for Time at the University of Sydney, characterizes Maccone’s solution as trading one mystery for another.

“The proposal to explain the thermodynamic arrow in terms of the [quantum] effects of observers has an obvious flaw, It doesn’t explain why all observers have the same orientation in time … Why don’t some observers remember what we call the future, and accumulate information towards what we call the past?”

My take on this situation is based on the statistical nature of entropy. Entropy increases simply because there are so many more disordered states for a system than ordered ones. There are multiple ways that your room can be organized and neat; but the number of ways it can be messy and disordered is so high that there is essentially no comparison.

Another common example involves the entropy of the randomly moving particles of a gas. There are few low entropy configurations and innumerable high entropy configurations. As time marches on, it is simply overwhelmingly likely that the next state will be less organized. These particles can spontaneously all organize themselves into a low entropy state but time does not need to reverse itself for this to happen. The problem is that you’d probably have to wait the age of the universe to see it.

I can only hope that if you wait so long for such an event that you’d at least remember it.

2 comments to Time Going Backwards?…….Forget About It!

  • dcardani

    Technically, that should be spelled “googol” (I think). Google is a trademark, but googol is a number.

  • Thanks dcardani,

    I did a quick check before I posted and I thought both spelling were acceptable. A little more research just proved me wrong.

    I made the change to the post.

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