Is the Brain a Quantum Device?
Most recent answer: 02/05/2011
- Devon (age 24)
Lansing
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 decoherence 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.
Mike W.
(published on 02/05/2011)
Follow-Up #1: biological quantum coherence?
- Devon (age 24)
Lansing
It would certainly be exciting, however, to be wrong about something this big.
Mike W.
(published on 02/07/2011)
Follow-Up #2: quantum coherence in photosynthesis
- 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 done.
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.
Thanks,
Mike W.
(published on 02/08/2011)