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Is the designation as matter vs. antimatter arbitrary?

  1. Nov 26, 2006 #1
    Is the designation as "matter" vs. "antimatter" arbitrary?

    I think this should be a very elementary question, but I haven't seen it addressed anywhere before. (but pardon me if it has been, and I just missed it.)

    I know that it is arbitrary to designate the electron as a matter particle and the positron as antimatter, but once this designation has been made, is there any connection that forces us to designate the up and down quarks as matter as well? For that matter (pun unintended, honestly!), does the designation of the up quark as "matter" force us to designate the down quark the same way?

    I think I'm asking if there is any significance to the designation as "matter" other than between particles that have otherwise identical quantum numbers?
  2. jcsd
  3. Nov 26, 2006 #2
    The symmetry between matter and antimatter is not exact. Charge-Parity conservation violation is observed in the disparity of our matter-dominated universe, and in quantum (quark) chromodynamics, both concerning Kaon decay.
  4. Dec 5, 2006 #3
    It's the weak interactions (flavordynamics) not the strong (chromodynamics) that violate CP, thereby allowing us to distinguish anti-matter from matter. Griffiths chapter on CP violation (4.8) gives a convention-free definition of positive charge. It is the charge carried by the lepton preferentially produced in the decay of the long-lived neutral K meson.

    Once we have a set definition of positive, we can choose how we want to define anti-matter and matter (and how we define right-handed and left-handed). We choose that the +2/3e charged quark is the up quark and the negative one is the antiquark. Once we make that choice, the other quarks fall into their place. Baryons are comprised of three quarks or of three anti-quarks. Mesons have one quark and one anti-quark. You can't have an up and a down quark in a meson, only the up and the anti-down (pion +) or the anti-up and the down (pion -).

    I'm not sure if defining the quarks that way absolutely forces us to define the leptons as we do. But the universe certainly gives us a hint. Everything around us is pretty much made up of protons and neutrons (with quarks we defined as matter) and electrons. Defining the electrons as matter makes sense. Then the other leptons (including neutrinos) have to fall in on the electron into their proper matter/anti-matter formation.
    Last edited: Dec 5, 2006
  5. Dec 5, 2006 #4


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    Of course you know the trivial answer is that we call matter both to the proton and the electron, and this naming extends to the valence quarks of the proton, thus the up with +2/3 and the down with -1/3. This is very trivial so I guess you are asking a deeper question.

    The secondary one is easy: yes, quarks are connected by the exchange of a W boson, so the +2/3 and the -1/3 are in the same multiplet, we must to designate them in the same way.

    This answer hints about what the "deeper question" can be: Is there any GUT multiplet containing the +1, 0, +2/3 and -1/3 particles?

  6. Dec 5, 2006 #5
    Is there any GUT multiplet containing the +1, 0, +2/3 and -1/3 particles?

    So now that you've tantalised us with the "deeper question", is there a "deeper answer"?
  7. Dec 5, 2006 #6


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    Agg, I do not remember. In fact the SU(5) multiplets, see
    are a funny mess of matter and antimatter.
  8. Dec 5, 2006 #7
    Thanks, arivero and BB1974, for your responses - they pretty much clear up my original simple-minded question. If I understand it, there is nothing that forces us (in the Std. Model, at least), to name the electron the same way as we do the up and down quark (right?), but what I was really wondering was if we had to name the up and down the same way, and it seems that the answer is "yes", due to their coupling via the W.

    I started wondering about this because of the matter/anitimatter asymmetry problem, and I wondered if the ratio was really an absolute thing or if it was in any way a by-product of convention, i.e. if we could decide electrons are actually antimatter, and then the asymmetry would be somewhat reduced, but obviously nowhere near enough to resolve the problem.
  9. Dec 6, 2006 #8


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    Well, particle and antriparticle are not completely a convention because they happen to live as solutions of the same equation, Dirac's, at least for elementary fermions.

    But 20 years ago you could had been right on the money. Remember why we built all these "neutrino detectors"? Not to measure oscillations, but to look for the disintegration of proton. Had we found it, it had been a proof of mixing between matter and antimatter in GUT theories, getting the proton to disintegrate all the way down to produce a positron (?!). Even in such case the question was not solved; it transmutes to "why we only find atoms with negative leptons?"
  10. Dec 6, 2006 #9
    Since charge is always conserved, isn't the excess of negative leptons an inevitible result of the excess of protons (the stable baryon) vs. anti-protons (the stable anti-baryon). All matter baryons will ultimately break down into protons, electrons, and electron anti-neutrinos. So we end up where we started: why the excess of matter vs. anti-matter?

    I know BABAR and BELLE were supposed to be probing for a more satisfying explanation to this question through observing CP-violating events. I don't know that they've found anything groundbreaking.
  11. Dec 6, 2006 #10


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    Yes, it's quite arbitrary. What we commonly see in our chunk of spacetime is called "matter", rare stuff we call "antimatter". And there are things that are on the line, such as the pions. These are composites, and they are made up of partly what we call quarks and partly what we call anti quarks. And the pi+ and pi- are equally common, so we can't decide whether to call them matter or antimatter. On the other hand, protons are very common in our neck of the woods, antiprotons are not, so that's what we call them.

    In Feynman's interpretation, antiparticles are particles traveling backwards in time. If we could distinguish between particles that travel forwards in time from particles that travel backwards, we could distinguish between particles and antiparticles.

    One of the mysteries of the Dirac equation was that it is natural to put together eigenstates of velocity. This was cause for much debate back when it was a new idea. Most of the debate has died away.

    However, the fact that the Dirac equation solutions can be put into eigenstates of velocity provides a clue for how to distinguish between particles and antiparticles in a non arbitrary (well, less arbitrary) way.

    If you can expand the Dirac equation so that it contains solutions for more particles, you can look at the velocity eigenstates to divide the elementary particles into two sets, and then arbitrarily call one set particles, and the other set anti particles. You then have a 50% chance of being right, which is better than if you assign particle / anti particle labels to all the elementary particles at random.

    The velocity eigenstates of the Dirac equation are known as the zitterbewegung theory of the electron. So you might try searching for: "zitterbewegung" and "velocity eigenstate"
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