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Aidan Randle-Conde | Université Libre de Bruxelles | Belgium

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Advent Calendar 2012 December 8th

8 is for gluons! They’re strong, they’re coloured, and you can never catch one all by itself!

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8 Responses to “Advent Calendar 2012 December 8th”

  1. Juliet Wakelam says:

    Do gluons have mass? I have read contradictory answers to this.

    • Hi Juliet, good question! In the Standard Model gluons are massless, however it’s very difficult to verify this experimentally, since we can’t isolate gluons. I don’t think there’s any process we can use to measure the mass of the gluon, so it’s possible we’ll never know. The only thing we do know is that if gluons have mass it must be a small mass.

  2. Xezlec says:

    1. Each gluon has a color and an anticolor.
    2. There are three possible colors and three possible anticolors.
    3. There is no color-anticolor combination that is not represented by a gluon.
    4. There are 8 types of gluons.

    The above 4 statements logically contradict one another as stated. Yes, yes, I know, complicated explanations and hand-waving, but because logic is logic, all those complicated explanations must ultimately amount to falsifying (or modifying) at least one of those statements.

    This is what frustrates me about explanations of physics: each concept is *always* explained the same way, using the same words and the same metaphors, and these words are often clearly nonsensical as stated. In the rare cases when a physics discussion online is about a subject I actually understand, a lot of the time I can step in and clear everything up right away by explaining in clear and unambiguous (even if a little more verbose) language, and that is often easy enough that it kind of boggles my mind that no one else will do this.

    Based on the wikipedia article on gluons, it appears to my very uneducated eyes that the problem here is that several different concepts are being named with the same words. The 3 colors and 3 anticolors combine to form 9 gluon “colorings” (for lack of a better word), but every gluon is a superposition (combination) of several of these 9 colorings, and the rules for these superpositions are such that only 8 superpositions are possible. We unfortunately refer to the “colorings” and also their superpositions as “colors”, so we’re left with the contradictory statement that “there are three colors, so there are nine colors, but there are only 8 colors”, where each occurrence of the word “color” is being (ab)used to mean a different thing.

    • Hi Xezlec, thanks for the comment! Yes, it is complicated, and a fuller treatment requires first discussing concepts such as superpositions and representation, which is hard to do in a video, so if I over-simplified this concept then I apologize. It’s hard to gauge these things in advance, so your feedback is appreciated! In the video I hinted at the superposition by saying that we could swap colors about without actually changing anything physically, so that means we would not expect every gluon to be in a pure color eigenstate. The subtlety that I skipped over altogether is that if we try to form the ninth gluon in a similar kind of representation as the others (ie if we move from an SU(3) to a U(3) representation and get our ninth gluon back) then this gluon is in a state known as a “color singlet”. Every isolatable and observable object in the universe is in a color singlet state, and this means that even if we had the ninth gluon it would just escape the rest of the quarks and gluons and it would not interact. This means that we can count up the gluons in the following way: we have six color-anticolor states where the two colors are distinct (eg red-antiblue) and three states where the colors are the same (eg red-antired). These three states have no net color, so they mix to give us the remaining gluons. They give us two gluons which are not color singlets, and one color singlet state. The color singlet state does not interact, so not only can it not be seen, it also cannot be produced by the strong force! (A slight over-simplification here. The real reason we exclude the ninth gluon is because of the symmetries imposed by the SU(3) group, but it’s neat to have this second way of looking at things.) As a sort of epilog to all this, some people have tried to interpret the ninth gluon as the photon, or some component of it, which would lead to some very interesting unification as they have the same quantum numbers. Unfortunately the couplings come out all wrong, but I’m still hoping someone can make it work in the future!

    • Stephen Brooks says:

      A similar thing happens with pions: in theory you have every combination of u,d and anti-u, anti-d. But as I understand it, in reality you only get 3 pions (u anti-d=pi+, d anti-u=pi-, {u anti-u,d anti-d}=pi0).

    • Yep, that’s true! You can then add the s quark to the mix to recover the “fourth pion”, but it gets a bit tricky. It comes about as the eta_3 and the eta_8, and then these mix to give the eta and eta’, where the eta is the “fourth pion”. It’s neat that SU(2) is a subset of SU(3).

  3. Stephen Brooks says:

    –[It’s hard to gauge these things in advance]–

    :D Physics pun

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