Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: chiralSPO on 18/06/2015 20:13:00
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Imagine a simple chiral amino acid such as (S)-alanine (the natural isomer; https://en.wikipedia.org/wiki/Alanine). If we somehow were able to synthesize a sample of anti-(S)-alanine from antiprotons, antineutrons and antielectrons, would it rotate light the same way as (S)-alanine or the opposite way?
I can say with certainty that the magnitude of the effect would have to be identical. And I would guess that it would have the opposite effect because each of the bonds would have polarity of equal magnitude and opposite direction (effectively performing a inversion through the center of the molecule--or an improper rotation of 180°)
On the other hand, the interaction between a photon and alanine should be indistinguishable from that of an antiphoton with antialanine. If we consider a photon is its own antiparticle, then a photon should not be able to distinguish matter from antimatter. Is this correct?
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......If we consider a photon is its own antiparticle, then a photon should not be able to distinguish matter from antimatter. Is this correct?
Don't have an answer, but can we start with basic particles, for example I assume photon interaction with electron or positron is same result? What about other fundamental particles?
There must be info on the net about these perhaps we should have a look, or maybe there are experts out there......
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I talked with some of my colleagues, and I think I have the answer:
The normal matter and antimatter versions of (S)-alanine will both rotate light the same direction.
On the other hand, the interaction between a photon and alanine should be indistinguishable from that of an antiphoton with antialanine. If we consider a photon is its own antiparticle, then a photon should not be able to distinguish matter from antimatter.
↑ is correct.
I would guess that it would have the opposite effect because each of the bonds would have polarity of equal magnitude and opposite direction
It is true that the polarity is exactly opposite, but light isn't just an oscillating electric field. The magnetic component also interacts, and the antiparticles will have the opposite magnetic influence as well as opposite charges, so the combination results in identical interaction with light. Nifty!
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to what extent is a photon its own antiparticle?
Does the "anti" version have the same spin?
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to what extent is a photon its own antiparticle?
Does the "anti" version have the same spin?
As I understand it, the photon is it's own antiparticle so there is no 'anti version'.
Ordinary photons have 2 spin versions anyway, but that doesn't make them anti.
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If converting a photon to its antiparticle reverses its spin (andI don't know if it does or not) then you would get round this
"On the other hand, the interaction between a photon and alanine should be indistinguishable from that of an antiphoton with antialanine. If we consider a photon is its own antiparticle, then a photon should not be able to distinguish matter from antimatter.
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If converting a photon to its antiparticle reverses its spin (andI don't know if it does or not) then you would get round this
No, there is no conversion to an antiparticle because there is no separate antiparticle.
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It is true that the polarity is exactly opposite, but light isn't just an oscillating electric field. The magnetic component also interacts, and the antiparticles will have the opposite magnetic influence as well as opposite charges, so the combination results in identical interaction with light. Nifty!
Does it react differently to circular polarised light RH vs LH? These 2 have opposite spin.
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Chiral samples rotate linearly polarized light because the two different circularly polarized lights (that the linearly polarized light can be decomposed into) will experience different refractive indices, different absorptivities etc. through interaction with the chiral sample.
https://en.wikipedia.org/wiki/Circular_dichroism
https://en.wikipedia.org/wiki/Optical_rotation
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Chiral samples rotate linearly polarized light because the two different circularly polarized lights (that the linearly polarized light can be decomposed into) will experience different refractive indices, different absorptivities etc. through interaction with the chiral sample.
So similar cause as Faraday rotation in a dielectric + mag field, each circ polarisation has different spin, each effected differently, but come out as linear.
Interesting.
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Can you get "rotation of the plane of polarised light" with a single photon?
That rotation is a relative phase shift between left and right handed spins.
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Can you get "rotation of the plane of polarised light" with a single photon?
That rotation is a relative phase shift between left and right handed spins.
I don't think so. I suppose I have been mixing the logic of single interactions with that of interacting populations, which may have helped confuse myself and others (oops!). On further consideration, I would have to say that plane polarized light is necessarily a "racemic mixture" of photons (1 to 1 mixture of left handed and right handed). Rotation of the plane can only be realized by the interactions of many photons (of both handednesses--what a terrible word...) with many molecules. On an interaction-by-interaction basis there are only a few different ways the photon and molecule can interact, each leading to a slight difference in the properties of the photon after the interaction (including whether or not the photon was absorbed without being re-emitted).
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Can you get "rotation of the plane of polarised light" with a single photon?
That rotation is a relative phase shift between left and right handed spins.
I don't think so. I suppose I have been mixing the logic of single interactions with that of interacting populations, which may have helped confuse myself and others (oops!).
No, I don't think you've been confusing yourself or others, I think your mind like ours automatically assumes group functions for some effects and single photon for others.
The problem with phase shifting a single photon is how do you know you did it? You can't detect it before and afterwards except as a group phenomenon.
However, of much greater importance, where did you get the phrase 'racemic mixture' from? I'm dying to work that into a conversation!
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a "racemic mixture" is a chemist's term for talking about a precisely 50/50 mixture of right-handed and left-handed versions of a compound (and I extended it to discuss photons). I believe the term comes from raceme, which describes a cluster of flowers, stalks or grapes (I guess a group of high symmetry?)
For instance a synthesis of alanine from two achiral precursors, ammonia and pyruvic acid, would be expected to form both S-alanine and R-alanine in precisely equal quantities (the law of large numbers is especially powerful when those numbers are >1020...). The product would be described as a "racemic mixture" or "racemate." If a chiral catalyst were used such that one enantiomer was favored over the other (even by 1%), the mixture would be referred to as "scalemic," or "enantio-enriched." If your catalyst were good enough (like our own enzymes) to produce only one enantiomer, or if you were able to separate the enantiomers from a racemic or scalemic mixture, the samples would be "enantiopure."
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a "racemic mixture" is a chemist's term for talking about a precisely 50/50 mixture of right-handed and left-handed versions of a compound (and I extended it to discuss photons).
Thanks.
Very appropriately extended I think.
Ever since I learnt that photons come in a 50:50 mix I've been trying to find a definitive source that describes the mechanism. There is no suggestion they are generated in pairs, so is it just a heads/tails mechanism?
If you find something let me know.
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I don't think that a pair-wise mechanism applies in general, though there might be specific cases in which it does. My guess is heads/tails mechanism (equal probability of forming either), and given the large number of photons usually considered, 50-50 mix is practically guaranteed.
I think I read somewhere that stars that spin very quickly emit circularly polarized light (polarized one way from one hemisphere, and the other way from the other hemisphere), this would be especially obvious for pulsars...
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My guess is heads/tails mechanism (equal probability of forming either), and given the large number of photons usually considered, 50-50 mix is practically guaranteed.
Yes, I was thinking the same, in such large quantities.
Star spin effect is intriguing! Must look that up.
Thanks
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I am reviving this old thread (despite the warnings!) because I came across an article describing the detection of polarized light from a spinning star (Regulus).
The paper I had come across before 2015 must have just been a theoretical paper, since this one appears to have been the first report, in 2017...
summary: https://phys.org/news/2017-09-secrets-bright-rapidly-star-revealed.html
actual paper (paywall): https://www.nature.com/articles/s41550-017-0238-6
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I don't think that a pair-wise mechanism applies in general, though there might be specific cases in which it does
One example of pairwise photon generation is in particle-antiparticle annihilation.
Another is generation of entangled photons.
See: https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion
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I am reviving this old thread (despite the warnings!) because I came across an article describing the detection of polarized light from a spinning star (Regulus).
Thanks for raising the dead!
Interesting that the predicted effect was detected in this way.