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I've heard descriptions, particularly from folks like Penrose, that the brain may be influenced by quantum events...and that cellular organelles may act as a receiver or amplifier for such noise to such a point that it may influence will.
My question is, how do know with such a size difference between the most mundane cellular structure and the planck scale what influences what? It's like wondering what tug the earth gives on the sun and how much that tilts it in our direction. I know it does have an influence, just like the drop beneath a certain temperature changes millions of molecules of water to ice, but how do we go on getting new information?
- Devon (age 24)
Devon- That's an important question.There are a few points to clarify for those who haven't followed those arguments. The brain, like any chemical system, is "quantum" in the sense that all the chemistry is governed by quantum mechanics. The question Penrose raises is whether the brain is "quantum" in the sense that large-scale structures in it maintain quantum interference effects. The Planck scale is not directly relevant for this issue.
It does not seem possible for Penrose to be right. The rate at which quantum interference effects are lost ("decoherence") is frequently measured for a variety of physical systems. In part, that's because people are now trying to build quantum computers which require keeping that decoherence rate low. One of the keys to reducing deoherence is to reduce the temperature. Another is to design physical variables which are only weakly coupled to their environment. the absolute temperature of the brain is more than 100 times that of typical quantum computational ingredients ("qbits"). No parts of the brain, including the microtubules discussed by Penrose, appear to be isolated from their warm aqueous environment.
Thus the brain is very far removed from the conditions required for any coherent quantum computation.
(published on 02/05/2011)
Follow-Up #1: biological quantum coherence?
I see...Do you think it's possible that it's the great undiscovered country of biophysics? Sort of like the superconductor temperature slowly rising up?
- Devon (age 24)
That's an interesting thought, that evolution could have slowly increased some quantum coherence times to the point where brains can do some quantum computing. I'm convinced it didn't happen, because it seems impossible under the physical conditions of the brain. If there is some mechanism, it's deeply hidden. If it were somehow possible, it would be much harder to do it stealthily, leaving no obvious microscopic mechanism, and there's no selective benefit to taking that stealthy route.
It would certainly be exciting, however, to be wrong about something this big.
(published on 02/08/11)
Follow-Up #2: quantum coherence in photosynthesis
As a follow up on your comments:
There is some evidence evolution lead to biology taking advantage of quantum coherence for some processes:
However, research strongly suggests the brain does not use quantum coherence. Similations of neurons work well with only classical mechanics, and also researchers have been able to successfully replace parts of a living rat's hippocampus with circuitry.
- Kevin M. (age 30)
Urbana, IL, USA
Kevin- Good point. The particular quantum coherence you mention is the delocalization of the initial photosynthetic electron excitation over a collection over a collection of antenna chlorophyll molecules, as opposed to classical hopping among them. I remember when I joined a research group working on photosynthesis 40 year ago, we wondered which way that would come out. It's nice to see that the tough experiments are one.
The spatial scale of these quantum coherent processes is small (a few big molecules) and the time scale is very short. Check the label on the graph in the picture in that link- the coherence extends for a picosecond or so. This short duration suggests why it's so hard to get coherence on a time scale useful for quantum computation in biology.
(published on 02/11/11)
Follow-up on this answer.