The photons are the propagators for QED, and we rely on photons to see the world around us.
The gluon is the propagator in QCD. Why have our eyes not evolved to see gluons (either on top of being able to "see" photons, or instead of)?
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7$\begingroup$ I'm voting to close this question as off-topic because it is about why nature took one evolutionary path instead of another and is almost virtually devoid of actual physics. $\endgroup$– Kyle KanosCommented Apr 14, 2016 at 23:00
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63$\begingroup$ Closing this question would be a disaster. The issue doesn't really seem to be that it lacks physics content (it's about gluons for pity's sake!) but rather that it also contains biological content. It's irritating enough to keep closing questions based on philosophical hair-splitting about which things count as "physics" and which don't, but if we also start closing questions based on containing applications of physics to other fields then we're just completely lost as a community. $\endgroup$– N. VirgoCommented Apr 15, 2016 at 1:18
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10$\begingroup$ This kind of cross-site question illustrates the limitations of Stack Exchange sites. There are plenty of valid questions like these which belong on multi-sites. Stack Exchange should not be making artificial barriers to inquiry. $\endgroup$– ThomasWCommented Apr 15, 2016 at 9:59
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18$\begingroup$ My wife is actually an evolutionary biologist (which is probably why I took an interest in this question as framed). She had zero clue as to how to answer this question, because she didn't know the physics. I think we can agree that the behavior of gluons is a physics matter, and I would argue it is central to this question and its answer. We're all intelligent people here; we can deal with a little elementary evolution theory while discussing physics. $\endgroup$– MikeCommented Apr 15, 2016 at 13:52
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5$\begingroup$ I'm voting to close this question as off-topic because it's not about physics $\endgroup$– John RennieCommented Apr 20, 2016 at 14:54
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4 Answers
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In short, the answer is: because gluons behave in a way that makes them useless for this purpose. To understand why, let's back up a little and look at how photons are useful, and then see how gluons behave differently.
We (animals pretty broadly) evolved to see photons because they allow us to move around in and respond to our environment more efficiently. This, in turn, is because our environment is pretty well supplied with photons from the sun (and other sources, in some cases). It so happens, as ulidtko rightly points out in the comments, that we only use a select range of photons for vision. In fact, we (humans) can only see photons from a fairly narrow range right around the peak emission of the sun, which incidentally corresponds to a range over which the atmosphere is fairly transparent. They interact with electrons, which are everywhere, so they bounce off of things in our environment (or are produced by things, in some cases). Yet they travel in fairly straight lines through air, so they can transmit very precise information to us that we can use to adapt to that environment. Photons can tell us about distant threats to avoided, nearby obstacles to be negotiated, food, water, potential mates, etc.
Now, the main reason we would not see gluons is because there aren't many — or any — gluons bouncing around in our environment. This is primarily because of a phenomenon called confinement. Gluons don't typically travel freely away from quarks, and quarks aren't exactly flying around as readily as photons. In fact quarks are also subject to confinement, so you won't see one outside of a hadron (proton or neutron, typically). But those are generally charged or short-lived, and stuck in a nucleus, which is stuck in an atom, which is stuck in some sort of molecule in our environment. So you'd only get any benefit from "seeing" these things if molecules and nuclei were routinely broken down and sent flying all over the place with great momentum. And even then, it would probably be easier to "see" these flying hadrons with something other than the gluons. In any case, that wouldn't be the healthiest place to be, and the photons would typically have told you to get out of that situation much earlier — thus preserving your molecules, which is a distinct evolutionary advantage.
It is possible that things called "glueballs" exist, which are just what they sound like: particles that are just balls of gluons stuck to each other. They could travel away from quarks, and would move in pretty straight lines. But they have not yet been observed; they are rare, difficult to produce, and hard to unambiguously identify. Their theoretical mass (unlike the massless gluon itself) is in the neighborhood of 1GeV — heavier than most of the elementary particles — which means they would only be produced in very energetic processes (e.g., nuclear reactions, rather than chemical reactions). So they certainly wouldn't be common enough to transmit much information about that saber-toothed tiger that's coming to eat us.
So to recap, photons are plentiful in our environment, and they travel long distances in more-or-less straight lines through the atmosphere, so they transmit information efficiently. Gluons are hard to produce in a form that travels long distances (with or without atmosphere), and so cannot transmit information usefully. Basically, gravitons are too weak to be useful, and gluons are too strong — but photons are juuust right.
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1$\begingroup$ Are you sure the term
moleculeis right in here? shouldn't it be replaced byatomic nucleus? Because molecules are routinely broken down and sent flying everywhere, that is what chemistry and gases are about. But those (hadrons) are generally charged (sic) and stuck in some sort of ______ in our environment. - hadrons are in the nucleus. $\endgroup$ Commented Apr 14, 2016 at 20:55 -
1$\begingroup$ Well, it may have been a larger jump than I should have taken, but no I meant to say molecules, which do indeed contain hadrons. I would argue that molecules are not frequently broken apart and sent flying everywhere; most of your molecular constituents stay within your body for a while. I was trying to draw the contrast with photons scattering off predator or prey at some distance. True, scents and other emissions do emanate, but not with the rate and momentum required to be analogous to photons. That's why our nasal angular resolution is poor compared to the visual. $\endgroup$– MikeCommented Apr 14, 2016 at 21:09
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7$\begingroup$ Photons: the Goldilocks particle. $\endgroup$– corsiKaCommented Apr 14, 2016 at 22:34
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4$\begingroup$ Well, photons of a narrow energy range (the visible spectrum) are "juuust right". Our eyes don't detect pretty much the whole diversity of radio photons, microwave photons, infrared photons, gamma & X-ray photons. Each of those kinds has much wider spectrum than the visible band. So: even among photons, by far not all of them are suitable for vision capabilities. It's also easy to deduce (evolutionarily) why. $\endgroup$– ulidtkoCommented Apr 15, 2016 at 9:46
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8$\begingroup$ "preserving your molecules, which is a distinct evolutionary advantage" +1 :) $\endgroup$ Commented Apr 15, 2016 at 10:41
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Short answer: because the Sun emits photons, not gluons.
Having a long range sense is vital for finding food and recognizing predators. Seeing light and forming an image of our surroundings is one of the three long range senses we have (the others are hearing and smelling).
Gluons are extremely short range; they don't even exist as naked particles. How could a gluon eye even detect them? It couldn't. Probably the same reason why we don't have neutrino eyes to gaze at far-out supernovae. :-)
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To put it simply, there is an evolutionary advantage to be able to see the objects around you using photons, but there would be no particular survival advantage to be able to see gluons, even if they weren't essentially confined to the nucleus.
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Why have our eyes not evolved to see “gluons”?
Because gluons are virtual particles rather than real particles. See the Wikipedia article:
"Although in the normal phase of QCD single gluons may not travel freely, it is predicted that there exist hadrons that are formed entirely of gluons — called glueballs. There are also conjectures about other exotic hadrons in which real gluons (as opposed to virtual ones found in ordinary hadrons) would be primary constituents."
Bit of a random question...photons are the propagators for QED, and we rely on photons to see the world around us.
Yes, but virtual photons aren't real particles. See anna's answer to this question and note this: "virtual particles exist only in the mathematics of the model". Virtual photons aren't short-lived real photons popping in and out of existence like magic. That's a lies-to-children popscience myth. Instead they're "field quanta". It's like you divvy up an electromagnetic field into chunks and say each is a virtual photon. Then when the electron and the proton attract one another, they exchange field, such that the resultant hydrogen atom doesn't have much of an electromagnetic field left. Hence you can see the logic of the exchange idea. But the electron and the proton aren't actually throwing photons at one another. Hydrogen atoms don't twinkle, and magnets don't shine.
The gluon is the propagator in QCD. Why have our eyes not evolved to see gluons (either on top of being able to "see" photons, or instead of)?
Because whilst there are photons flying around, there aren't any gluons flying around. Nobody has ever seen a gluon. Not for forty years. And since gluons are virtual as opposed to real particles, that's not going to change any time soon.
answered Apr 21, 2016 at 7:24