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Our future, our universe, and other weighty topics

Wednesday, October 14, 2015

Can Quantum Biology Explain the Origin of Life?

The interesting book Life on the Edge: The Coming of Age of Quantum Biology by Johnjoe McFadden and Jim Al-Khalili discusses some ways in which scientists are trying to discover a relevance between quantum mechanics and biology. Although the book is largely speculative, the general idea of looking for a dependence of life on quantum mechanics is probably a sound one.

One of the under-noticed trends of the past 150 years is the gradual discovery by science of how greatly life depends on physics. While someone might have thought 150 years ago that life might arise in any old universe, we now know of a long list of physics requirements for the existence of life. For example, scientists have talked about how life would not exist if there were not a strong nuclear force that binds nucleons together in the atom; it would not exist unless such a force were balanced so as to produce large abundances of carbon and oxygen; and life would not exist unless we had a programmatic set of rules resembling the current laws of electromagnetism, with an exact balancing of the proton charge and the electron charge (as discussed here). So why should we not speculate that advanced life may has some dependence on some of the more exotic features of quantum mechanics – things such as quantum entanglement and quantum tunneling? But the case for this type of quantum dependence has not yet gelled very much.

In discussing the origin of life, McFadden and Al-Khalili refer to a startling analysis made by chemist Graham Cairns-Smith in his book Seven Clues to the Origin of Life. This book can be found online here. On page 60 Cairns-Smith tries to estimate the odds of a self-replicating molecule arising by chance on the early Earth:

It would be a safe oversimplification, I think, to say that on average the 14 hurdles that I referred to in the making of primed nucleotides would each take 10 unit operations-that at least 140 little events would have to be appropriately sequenced. (If you doubt this, go and watch an organic chemist at work; look at all the things he actually does in bringing about what he would describe as 'one step' in an organic synthesis.) And it is surely on the optimistic side to suppose that, unguided, the appropriate thing happened
at each point on one occasion in six. But if we take this as the kind of chance
that we are talking about, then we can say that the odds against a successful
unguided synthesis of a batch of primed nucleotide on the primitive Earth
are similar to the odds against a six coming up every time with 140 throws
of a dice. Is that sort of thing too much of a coincidence or not?

Cairns-Smith then goes on to point out the odds that he has just mentioned are equivalent to a probability of 1 chance in 10 to the 109th power (1 chance in 10109), where 10109 is 1 followed by 109 zeros. He points out that if you had one chemical reaction per second throughout all of the Earth's history, that would be only 1015 trials, so you would need 1094 such trials per second before there would be a likelihood of success. And that would be totally impossible, since the whole observable universe is estimated to have less than 1082 electrons. 

We can do a similar calculation by using the estimated number of atoms in the ocean, which is about 1047. If we assume that every such atom was trying its hardest to achieve this result with a probability of 1 in 10109, for each of 1015 seconds of the Earth's history, that would still give you only 1062 trials (or somewhat less, since the origin of life would require multiple atoms). So given the probability mentioned by Cairns-Smith, the chance of success would still be only about 1 in 1047, or about 1 in a hundred billion trillion trillion trillion.

McFadden and Al-Khalili suggest a possible way to overcome such a difficulty. On page 285 of their book they vaguely speculate that some primordial soup might have acted like a “quantum computer” to somehow create a self-replicating molecule needed to get life started:

Our proto-self-replicator could, if it survived long enough, act as a 64-qubit quantum computer; and we have already discovered how powerful such a device would be. Perhaps it can use its huge computational resources to compute an answer to the question: What is the correct molecular configuration for a self-replicator?

But this is very woolly speculation that seems rather on the cheesy side. Even if we grant that there could somehow be some kind of effect by which a primordial soup might act  like a quantum computer – a huge leap – one is still left with the issue of how such a thing (acting like a computer) got programmed in the first place. Remember that computers never accomplish useful things unless software has been loaded into them.
McFadden and Al-Khalili concede on page 288 of their book that “any scenario involving quantum mechanics in the origin of life three billion years ago remains highly speculative.”

McFadden and Al-Khalili try their best to suggest that quantum mechanics can assist in explaining some of the origin problems puzzling scientists, such as the origin of life and the origin of consciousness. But does quantum mechanics make such mysteries easier to understand or harder to understand? We should not forget the “vacuum catastrophe” issue raised by quantum mechanics. This is the fact that quantum field theory predicts that the vacuum should be super-dense because of an army of virtual particles constantly popping into existence and out of existence.

A fundamental part of quantum mechanics is the idea of virtual particles – that short-lived particles should be constantly arising because of what are called quantum fluctuations. Even though such particles are short-lived, according to quantum field theory, there should be so many of them that they should combine to make every cubic meter of the vacuum far denser than steel. Imagine if there was some weird rule that every second a trillion insects were materializing in your living room, each existing for only a tiny fraction of a second before disappearing. Even though the lifetime of each of those those insects would be extremely short, at each instant there would be so many of them that your living room floor would collapse from the weight of all of them. Quantum mechanics predicts a similar situation, except that instead of insects it says virtual particles should be appearing in such great numbers everywhere that the universe should be totally uninhabitable, because empty space should be super-heavy everywhere. This is one of the greatest anomalies of modern physics, known as the vacuum catastrophe or the cosmological constant problem.

So rather than making our existence seem less of a miracle, quantum mechanics would seem to make our existence seem like more of a miracle. But when we have finally finished discovering all of the countless physics dependencies of our existence (something we are probably only halfway finished doing), we will probably find that quite a few of them are quantum mechanical in nature. I can think of one such quantum dependency McFadden and Al-Khalili fail to highlight (and seem to overlook completely when they state that most of the time we don't need quantum mechanics). Were it not for quantum mechanics limiting the possible orbits of an electron, electrons would be dragged into the nucleus of an atom (because of the electromagnetic attraction of protons and electrons), and there would be no elements other than hydrogen. So we do know one exotic feature of quantum feature on which life depends – the exotic feature known as quantum jumps, which require electrons to instantly change from one orbit to another in the atom. Our very existence depends on this “crazy” feature of nature. Far from not needing quantum mechanics “most of the time,” we would not exist a minute without it. I suspect we will find that other “crazy” aspects of quantum mechanics are also necessary for our existence.