Wednesday, June 11, 2008

Sensitivity to a Single Photon

Patrick Suppes and Jose Acacio de Barros wrote a paper called Quantum Mechanics and the Brain (HT: Clark’s sideblog). Obviously the title was irresistable to me. However, it turns out that the title is a bit misleading. The paper has a few paragraphs discussing previous proposals regarding the role QM might play in the brain and briefly gives the authors’ views on these -- frankly without saying anything new. The paper then shifts to what is its main focus, which is to highlight an interesting instance where there is a good indication that biological systems do exploit quantum level phenomena. This is the animal eye’s demonstrated ability to react to a very faint light signal: specifically the ability to detect and respond to a handful of photons, and probably a single photon.

The paper describes previous experimental results with insects and animals. Sensitivity to single photons was inferred using statistical methods in some of these tests (some of the older references in the paper were also discussed in this Usenet posting from 1996). The authors then outline a proposal for future experiments to confirm the results by utilizing the technological ability we now have to shoot single photons.

This result would demonstrate that organisms are sensitive to the quantum realm. While we remain far from understanding the role quantum effects might play in conscious experience, this is another step toward exploring the subject.


Mike Wiest said...

Hi Steve,

Did you ever review the "Quantum mechanics in the brain" essay in Nature by Koch & Hepp (cited by the present paper)? I think their thought experiment is strictly irrelevant to the question of whether the brain exploits quantum phenomena. Also they say no quantum computational algorithms have been developed of similar power to Shor's has been found. But what about "quantum associative memory" reported by Ventura and Martinez that has exponential memory capacity as a function of the number of neural units, as compared to classical neural net memory capacity that is linear in the number of units? Was it bogus for some reason? It was based on the well-established Grover's algorithm (which in itself is more practical than Shor's but unmentioned by Koch).

I think I've also mentioned before that Tegmark's "proof" that the brain decoheres too fast is predicated on the false assumption that the brain is at thermal equilibrium, whereas there is a concrete physical model (the Frohlich model) that shows how pumping energy through a system of electric dipoles (metabolism, biomolecular dipoles) condenses the system into a coherent state--at high temperatures.

They also don't mention the binding motivation for quantum processing in the brain. Or that sub-neural quantum states could influence neural firings by modulating synaptic transmission.


ps. Just read Dennett's "Sweet Dreams." I need somebody to gag me with a spoon.

Steve said...

I agree the authors were wrong to cite and then imply the Koch/Hepp essay and the Tegmark paper were somehow definitive statements on the computation and decoherence topics. They just should have skipped the first sections of their paper and gone straight to the discussion of the eye.

Steve said...

Hey Mike. Here's something that just caught my eye on this topic (and the related discussion we having on freedom vs. randomness). Following on some of those Bertrand Russell-related posts I reread 1927's The Analysis of Matter. Check out this quote (keeping in mind the full formulation of QM by Heisenberg had just happened in the prior couple of years.):

"We have seen that, on the basis of physics itself, there may be limits to physical determinism. We know of no laws as to when a quantum transaction will take place or a radio-active atom will break down. We know fairly well what will happen if anything happens, and we know statistical averages, which suffice to determine macroscopic phenomena. But if mind and brain are causally interconnected, very small cerebral differences must be correlated with noticeable mental differences. Thus we are perhaps forced to descend into the region of quantum transactions, and to desert the macroscopic level where statistical averages obtain. Perhaps the electron jumps when it likes; perhaps the minute phenomena in the brain which make all the difference to mental phenomena belong to the region where physical laws no longer detemine definitely what must happen. This, of course, is merely a speculative possibility; but it interposes a veto upon materialistic dogmatism. It may be that the progress of physics will decide the matter one way or other; for the present, as in many other matters, the philosopher must be content to await the progress of science."

Blue Devil Knight said...

I like their evidence-based approach, even if the conclusions are somewhat uninteresting to those less interested in biology.

I bet we won't thoroughly understand how certain ion channels work without QM. Also, the electron transport chain shuffles electrons along a path to produce ATP. Does this require QM to understand? It seems whenever there is an interaction between a single (small enough) particle like a photon or electron and biological tissue we'd expect QM to play a role in explaining the mechanism.

In other words, their claims seem somewhat unsurprising given what we already know in biology, and certainly less provocative than the QM-consciousness folks ambitious view of things.

I like the quote from Heisenberg, especially the last sentence (though I would add biology too to the science the philosopher must wait for).

Mike Wiest said...

That is a sweet quote. But it's Russell, right?

BDK--aka Mr. Conservative:

It goes without saying that ion channels and ATP interactions require quantum mechanics to understand, as does all chemistry. Similarly, if there's an individual photon involved a quantum description is mandatory.

But Russell is clearly allowing for the possibility that MIND might play a role in determining the body's behavior, as against "dogmatic materialism," which seems exactly as or more ambitious than the "QM-consciousness folks view of things." It's the audacity of hope man!

Si se puede!

ps oops, I just realized BDK was comparing the target article to "QM folk," not Russell/Heisenberg...

Blue Devil Knight said...

Ah right I misread that quote as attributed to to Heisencain, not Russellbama.

Yes, I get Russell's original point, I just like it when philosophers get the beat down, even if from their own kind (originally I thought it was Heisenmaster that said it).

If the Nernst equation were quantum, we'd have a nice example of strong quantum effects in neuroscience. A vanilla example, but clearly important nonetheless.

I don't mind radical theories, especially with predictions. After all, the view that it's all neurons is 'The Astonishing Hypothesis.' If it's called that, it must be astonishing and radical.