Wednesday 20 January 2016

Many worlds of Surface Hopping

What happens during a surface hop in Surface Hopping dynamics? Does the wavefunction collapse or do you create a parallel universe?

Imagine the process of vision. We start with retinal in its excited state after it has absorbed a photon. In the next picosecond retinal has several distinct possibilities to hop to the ground state. Once such a hop occurrs, the molecule can isomerize, which can in turn change the configuration of the rhodopsin protein and ultimately trigger vision. In our simulations we need a random number generator to decide when the hop occurs. Why do we need a random number generator to simulate reality?

There are two equally disturbing answers to this question. Answer 1: Random wavefunction collapses happen, or as Einstein put it - God plays dice. Answer 2: The wavefunction never collapses. Everything we know of is just one big wavefunction. That means that for every possible quantum transition both branches are equally real (weighted by the transition probability). Effectively, we spawn a parallel universe with every quantum transition. In one branch of reality the hop occurs at 100 fs and triggers the isomerization, in another branch it happens at 200 fs. In yet another branch the isomerization does not occur and no nerve signal of vision is transmitted. All these branches exist in parallel. But if decoherence occured fast enough they do not know about each other and it seems like the wavefunction collapsed.

If you want to know more about parallel universes, read Max Tegmark's book "Our Mathemtatial Universe".

Looking back, I wrote a similar post before. The topic still captivates me ...

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