Erasure

A hectic weekend that is best described in the words of Big Chris 🙂

My post a couple of days ago about education got me thinking about something interesting today. We have, over the last several centuries, evolved extremely sophisticated pedagogical techniques to help people remember information.

What about the opposite problem: how do you help someone forget?

Besides the obvious emotional applications, I believe forgetting is as critical to learning as remembering is. Often, the biggest stumbling block to learning is a deeply ingrained knowledge of something learnt in the past, that prevents us from approaching a subject from a fresher perspective. Erasure – how can that be achieved? Currently, the only process that we know seems to be ‘starvation’; with the passage of enough time, we forget. Can we do better?

I had a couple of interesting associations regarding erasure. One of them is related to the concept of the ‘coasting computer’, i.e., a computer that does not consume any energy at all. This is a very interesting concept that got a lot of press about 15 years back. The research question being addressed was, what is the minimum energy needed for a computation? And the answer was fascinating – the computational process can be performed with no energy at all; where energy is needed is for the process of ‘reset’. In other words, a computer needs no energy for remembering, but for forgetting. The book I read this in was about quantum computing; I will dig it up for a future blog post. It was one of the best popular science books I’ve ever read. I vaguely remember the author’s name was Julian Brown.

The problem of computational energy is also exemplified by the classic problem of Maxwell’s demon. Maxwell’s demon is the protagonist of a thought experiment that seems to violate the second law of thermodynamics. It sits between two chambers containing molecules, and controls a tiny valve. When it sees a molecule traveling faster than average in one chamber and moving towards the valve, it opens the valve and allows the molecule into the second chamber. Similarly, it lets slower molecules from the second chamber into the first. Thus, it creates a temperature differential in two chambers with apparent zero expenditure of energy, and a decrease in the system entropy. The classical argument refuting this fallacy is the understanding that in order for the demon to sort molecules based on speed, there is a computation being performed, and this computation expends more energy than is gained by this segregation.

Ralf Landauer, one of my heroes of 20th century science, visited the Maxwell’s demon problem, and this is what Wikipedia has to say about his work:

“In 1960, Rolf Landauer raised an exception to this argument. He realized that some measuring processes need not increase thermodynamic entropy as long as they were thermodynamically reversible. He suggested these “reversible” measurements could be used to sort the molecules, violating the Second Law. However, due to the connection between thermodynamic entropy and information entropy, this also meant that the recorded measurement must not be erased. In other words, to determine whether to let a molecule through, the demon must acquire information about the state of the molecule and either discard it or store it. Discarding it leads to immediate increase in entropy but the demon cannot store it indefinitely: In 1982, Bennett showed that, however well prepared, eventually the demon will run out of information storage space and must begin to erase the information it has previously gathered. Erasing information is a thermodynamically irreversible process that increases the entropy of a system.”

In other words, Maxwell’s demon is foiled by the flaw of failing to forget.

Also worth mentioning in connection with forgetting is a science fiction movie that all my friends say is superlatively good, but which I saw in such a state of drunkenness that the only thing I remember is a scene where a pretty girl bounces on a bed. The movie is called “Eternal Sunshine of the Spotless Mind”, and it is about a couple that decide to surgically erase each other from their memories, only to rediscover what they lost.

Another day, I promise to give some more original thought to forgetting. What do you think?

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