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This might be more on the philosophical side.

We already know a lot about light. How it is generated and how it interacts with a material. Phenomenon such as Interference, Diffraction, the Photoelectric effect is very well known. We also know how our eyes will perceive the light of different wavelengths. We have developed spectroscopy to analyze things using light. This clearly indicates that we understand a lot about it.

However, I have never come across something which discusses how smell is generated? What exactly gives rise to a different kind of smell and how different kinds of smells interact with each other to give something different? The only thing I know of is that different molecules smell differently, but where does that property come from? Is it propagated in the form of waves? If so, the phenomenon such as Interference occurs in this case too?

I wanna know how far have we achieved on this topic?

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    $\begingroup$ Jay: smell is something you experience depending on what molecules hits the inside of your nose and activates receptors that are on the cell membranes. If you believe there are special "smell-fields" you are not dealing with mainstream physics. (I don't know of any way to remotely activate the receptors at least) $\endgroup$
    – Emil
    Sep 13, 2018 at 9:17
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    $\begingroup$ "I wanna know how far have we achieved on this topic?" A Nobel prize was awarded for work on it, in 2004. That suggests to me that we are still a long way from "fully understanding" it! en.wikipedia.org/wiki/… $\endgroup$
    – alephzero
    Sep 13, 2018 at 9:36
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    $\begingroup$ I'm voting to close this question because it's not really about physics $\endgroup$
    – Kyle Kanos
    Sep 13, 2018 at 12:33
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    $\begingroup$ I'm voting to close this question as off-topic because it looks like it's a chemistry/biology question. $\endgroup$
    – user191954
    Sep 13, 2018 at 12:57
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    $\begingroup$ @Jay In that case you're talking about why certain compounds in the air are detected as odorous, which is chemistry. But as far as the physics goes, it's just that there are certain special compounds in the air. $\endgroup$
    – user191954
    Sep 13, 2018 at 16:20

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This isn't a physics question and should be at Biology SE. But: smell is caused by so-called olfactory molecules. If your body detects them, then you feel the appropriate smell. An example is hydrogen sulfide ($\rm H_2S$). This gas smells of rotten eggs. If you're in a room with hydrogen sulfide, then you'll smell the "rotten eggs" even if there are no rotten eggs in the room.

Detection occurs because the olfactory molecule binds to receptors in the nasal system. The smell response is to the molecule's structure. The receptors that detect hydrogen sulfide for example have evolved to detect exactly hydrogen sulfide (c.f. lock and key model of enzymes). It is desirable for the body to be able to detect hydrogen sulfide, because the gas is poisonous. You don't want to be in a room with it! So the body has evolved to produce strongly-negative responses if it detects hydrogen sulfide. A different biological organism can be attracted to hydrogen sulfide (because they need it for food, etc) - these organisms "smell" hydrogen sulfide differently than we do.

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    $\begingroup$ The question of what physical property of hydrogen sulfide makes it smell different from, say, water vapour is a physics question. $\endgroup$ Sep 13, 2018 at 7:18
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    $\begingroup$ @MarkMitchison I think "what is the shape of a molecule (and why is it that shape)" or "why does molecule X bind to molecule Y, while molecule Z does not" are usually considered as a chemistry questions rather than a physics questions - though of course you could classify the whole of "chemistry" as "applied physics" if you wanted to. $\endgroup$
    – alephzero
    Sep 13, 2018 at 9:39
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    $\begingroup$ @alephzero You are making an implicit "physics" assumption that olfactory receptors work based on shape and chemical binding, which is not explicitly made anywhere in the question nor this answer. (You could add an answer about that). As far as I am aware, the actual mechanism underlying olfaction is not completely understood. The idea that physical mechanisms such as vibronic resonances in electron transfer has been widely promoted by, e.g. Luca Turin. I believe so far these theories have "disputed" status, i.e. there is evidence both for and against. I'm no expert, though. $\endgroup$ Sep 13, 2018 at 10:44
  • $\begingroup$ @MarkMitchison Things smell different based on how the chemicals in our nose interact with chemicals in the air. If that's not chemistry, I don't know what is. $\endgroup$
    – Chris
    Sep 14, 2018 at 10:53
  • $\begingroup$ @Chris It sounds like biology to me ;) These discussions about what field something should belong to are always futile. My point was only that some physicists are actually interested in this question, for what they consider to be “physicsy” reasons. $\endgroup$ Sep 14, 2018 at 13:19

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