Your Brain on Entropy

26 06 2008

Recently on a physics forum someone posted a question about entropy: how does the concept of
entropy apply to the human brain?

A number of posters responded with textbook definitions of entropy copied from their
statistical mechanics courses, with the dulling statement that the entropy of the human brain is defined by the logarithm of the number of energetically accessible quantum states, just as in any other system.

True, perhaps, but illuminating? Not very. The human brain is certainly a tour de force in complexity that makes our supercomputers look like ganglia. Given that such an object could spontaneously arise out of the primordial soup in a universe bathed in high-energy ionizing radiation, in which everything decays, crumbles, and otherwise increases its entropy, we ought not be surprised by the popularity “intelligent design.” The brain itself, if it did arise out of evolution as we understand, would seem to be an “intelligent accident”.

If it is an accident – if it didn’t arise out of the manipulations of an omnipotent divine force – but by the physical processes we know and understand, what’s the chance for it to arise anyway? If every thermodynamic process increases entropy, how could any irreversible process have caused life to arise?

We’ve all heard about the spark chamber experiments, where simple chemicals abundant in early Earth’s (and other planets’ and moons’) atmospheres combine to form the building blocks of the building blocks of the building blocks that living organisms use to build the building blocks of life. In those things you can make complex molecules out of simple ones by raising the to many thousands of degrees and then letting the gas cool off. In the discharge, everything is ionized and there is a huge number of possible molecules that can be formed out of this mess. Most of them would not last long in solution, but a few of them, including amino acids, miraculously survive for a long time at room temperature in a water solution.

So far, so good: we’ve got a more complex molecule now, formed from simple molecules, in a process that took in a lot energy from the discharge and gave back out a little bit less; the missing energy was used in forming the chemical bonds and putting the system in a slightly more ordered state (there are fewer degrees of freedom with one big molecule than with a lot of little molecules.) The process must have increased the total entropy of the universe; in this case, the entropy increase simply from releasing the electrons from their capacitors certainly overwhelemed the tiny entropy decrease in forming the complex molecules.

Still, this is no recipe for life. The complex molecules formed this way may hang around for a while, but not forever. And another discharge may come along and dissociate them as fast as they were created. We know how to get the building blocks of life, but how does life get to really get going – how does it carry on organizing the world’s puddles into complex molecules on its own?

The trick must be to provide lots and lots of molecules with lots of internal energy, and some molecule that catalyzes a reaction that creates more of that molecule. Every time the catalyst collides with the two components and the energy molecule, bang! it fuses the components into a bigger molecule. Where does the entropy go? Into the shattering of the “energy” molecule.

This is happing all the time, over and over, ad infinitum, in every cell of every living creature, and it’s going on in your brain all the time, too. Thanks to this process, your brain has tremendously less entropy than a soup made of the elements it’s composed of. Your brain maintains its low-entropy existence by using your bloodstream to suck in big molecules and constantly flush out little molecules. As long as the disorder in the waste is more than the order in the fuel, your brain can continue to become more orderly.

I know, your brain doesn’t seem all that orderly. In fact mine’s becoming quickly more disordered with every passing sentence, since I’m about to go on vacation, in – let’s see – 10 minutes.

Better wrap it up. Stay tuned for more, the saga of your brain on entropy continues!

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2 responses

24 01 2011
natty nish

I like

10 04 2011
Arun

i think we are familiar with the processes in which the disorder increases and the processes which are highly ordered are few and complex, our brain lies among the few cases.

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