Quantum Smell

By Sean Carroll | March 25, 2011 10:37 am

Over on the Facebooks, Matt Strassler points to a BBC story about the role of quantum mechanics in explaining our sense of smell. There aren’t any equations in the article, and I haven’t read the research papers, but the idea seems to be that electrons move from one part of a protein to another part via quantum tunneling. The potential that allows this to happen is only set up if you have the right chemical involved, which is how the protein purportedly “smells” the existence of this chemical. The resulting mechanism is just absurdly sensitive — apparently fruit flies can smell the difference between hydrogen and deuterium (chemically identical, but tiny differences in atomic energy levels from having an extra neutron in the nucleus).

It’s still a controversial theory, but apparently not crackpotty. The question of how important quantum mechanics (as opposed to just its classical limit) is for biological processes was brought up in our earlier post on quantum photosynthesis. Which reminds me in turn of this worthwhile talk by Seth Lloyd, on the basic topic of “quantum life” and photosynthesis in particular. In between learning about how quantum phenomena might remain relevant in the hot, warm environment of a plant, you can enjoy Lloyd’s principled stance not to use PowerPoint under any circumstances.

  • Kerub

    Are we sure this isn’t gonna smell fishy in less than a week?

  • Sili

    How do they tell their mechanism apart from the hypothesis that what we’re smelling is in reality molecular vibrations? They’d be similar affected by isotopic substitution.

  • CoffeeCupContrails

    Sean, I think you spend too much time on Reddit. Like the rest of us.


  • Leblebi

    He is cool, but honestly, his fake laugh is so annoying.

  • http://sapping.tumblr.com Sapping

    IEEE spectrum carried a discussion on Dr Turin’s recent paper at http://spectrum.ieee.org/biomedical/diagnostics/nose-works-like-a-scanning-tunneling-microscope back in February if anyone wants a bit more info.

  • http://tgd.wippiespace.com/public_html/ Matti Pitkanen

    An alternative interpretation is that IR light -rather than electron tunneling- induces the sensation of smell: in both cases transitions between electron’s energy levels would be in question. Callahan has long ago collected evidence supporting this interpretation.

    The killer test would be a check whether a beam of IR coherent light at preferred frequency can induce a predictable odor percept. If so, a breakthrough in entertainment industry is to be expected;-).

    For TGD based model of olfaction as analog of vision in infra red see http://matpitka.blogspot.com/2011/02/isotope-effect-of-olfaction.html .

  • Low Math, Meekly Interacting

    Interesting stuff. One of a very short list of phenomena in which aspects of “quantum weirdness” affecting electronic configuration plausibly play a biologically relevant role. The only other ones I find very convincing are the aforementioned antenna superposition phenomenon and contributions to nucleobase tautomerization. I’m sure more subtle examples will emerge.

  • Evin

    It always amuses me when someone who thinks he’s smart (and it’s almost always a “he”) says something like, I don’t use powerpoint because it’s so limiting. Oh but your overhead projector combined with your mouth is so enlightening! The reason you don’t like tools like ppt is because you don’t know how to use them. That’s like saying the piano sucks for relaying musical ideas because it only has 88 keys of defined frequency. He seems to be entertaining these high school kids, though.

  • Karl

    @ 4. Leblebi,

    You’re kidding right? Either that or your not very adept at reading social signals — clearly that was a nervous and awkward laugh — something not unusual around MIT I suspect. Annoying – perhaps. Fake — hardly.

    @ 8. Evin,

    Wow, such outrage. I agree that making not using powerpoint a matter of principle is a little much, but then, I find being outraged at someone not using powerpoint as a matter of principle also little much. :)

    Anyway, I hardly think he arrived at this conclusion because he couldn’t figure out how to use it.

  • http://quantumtantra.blogspot.com nick herbert

    Chandler Burr’s popular account of Luca Turin’s frustrating attempt to introduce his new quantum theory of smell to a hostile scientific community is a fascinating window both into how the perfume industry works and how some scientific fields erect powerful barriers to maverick ideas. “Emperor of Scent”–a marvelous read.

  • Greg

    Very cool. Enjoyed it a lot.

  • Pingback: Kvantbiologi | Sentio Ergo Sum()

  • HI

    I’m very skeptical. Olfactory receptors belong to G-protein coupled receptors. If this happens with olfactory receptors, one would expect that it also happens with other G-protein coupled receptors. But is there any evidence for that? Rhodopsin is also a G-protein coupled receptor. The way rhodopsin works is certainly quantum mechanical in so far as it involves absorption of photon. But that is presumably because absorption of photon changes the shape of retinal.

    Of course, everything is quantum mechanical at some level. But I’m not convinced with this particular hypothesis by Luca Turin. And the evidence they have is very indirect. They should use more direct biophysical method to test it.

  • Stolen Dormouse

    Karl @9, it may not be that Lloyd can’t figure out how to use PowerPoint to communicate what he wants. Edward Tufte, the author of “The Visual Display of Quantitative Information,” also dislike PowerPoint greatly. He wrote and self-published an essay “The Cognitive Style of PowerPoint: Pitching Out Corrupts Within” (http://www.edwardtufte.com/tufte/powerpoint), concerning PowerPoint’s effect to limit our ability to communicate ideas. His cover visually suggests that the presentation software is as limiting as Stalin’s dictims were to thought and discussion in the Soviet Union.

  • Nancy Mills

    Very interesting & enjoyable talk.
    By the way — neither fake nor annoying.

  • stephen s

    Thanks for posting this. I loved this lecture!

  • Pingback: Perfumes, smells and quantum wells | Empirical Zeal()

  • Neal J. King

    Sean, you’re original posting seems to display a misunderstanding:
    “The resulting mechanism is just absurdly sensitive — apparently fruit flies can smell the difference between hydrogen and deuterium (chemically identical, but tiny differences in atomic energy levels from having an extra neutron in the nucleus).”

    According to Turin’s theory (as discussed in the referenced article), we would be talking about differences in the VIBRATIONAL, not ATOMIC, energy levels. The vibrational levels should be significantly more sensitive to a doubling in a nuclear mass than the atomic levels, since they depend on the reduced mass of the atom with respect to the rest of the odorant molecule; whereas the atomic levels depend on the reduced mass of the electron with respect to the nucleus.

    As a rough estimate, doubling the nuclear mass will shift the Bohr levels by about 1 part in a thousand; but the vibrational levels by about a factor of 2.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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