Science Sundays with John Duffield: Missing Antimatter

Google for a heartbeat and you can find a whole stack of articles on ”the mystery of the missing antimatter”. A CERN webpage called The Search for Antimatter is fairly typical. As is Don Lincoln’s little lecture on Missing Antimatter. The idea is that the Big Bang happened, and for a while the universe was a seething cauldron. Energy was being turned into matter and antimatter via pair production, and matter and antimatter was being turned back into energy via annihilation.

No problem there. When you know a bit of physics, the Big Bang is pretty reasonable. And since pair production and annihilation can be done in a lab, what’s not to like?

What’s not to like is people peddling mystery. The problem comes when people start saying “Woo! Where’s all the antimatter gone?” Especially when they spin this into a yarn to attract media attention. Because when you know a bit of physics, you know that they’re abusing ”the mystery of the missing antimatter” to curry favour with a gullible public. The aim is to make people think that theoretical physicists are really useful engines, so that people won’t want to cut their funding. No problem there either, but people are people. Huff puff, and before you know it they’re on a slippery slope wherein simplification has turned into deception. Political parties do it, trade unions do it, and theoretical physicists do it too. Fortunately for physics, once you know a bit of physics, you can spot it fairly easily. See this picture from the Australia Telescope Outreach website?

Image credit: CSIRO

Image credit: CSIRO

It’s a depiction of proton-antiproton annihilation alongside electron-positron annihilation. Pair production is just the opposite process, check out gamma-gamma physics. Keeping things simple, the early universe was a mess of photons turning into electrons and positrons and protons and antiprotons, and vice-versa. On top of that the only other rule in the game  is that particles can be individually “melted” into the background plasma as per this physicsworld article. It’s a simple game, and photons aside, we’re only thinking about four types of particle. So draw a 2 x 2 table and fill in the electron and the positron, like this:

antimatter table

Now, on particle properties alone, fill in the proton and the antiproton. Start with the proton. It isn’t much like the electron or the positron, but since it’s positively charged like the positron, it’s more like the positron than it’s like the electron. So put it under the positron. That leaves only one cell free, so we have to put the antiproton under the electron.

OK, now take a look at Jim Mitroy’s article on positronium. Positronium is an “exotic” atom. It’s short-lived, but whilst it exists, it consists of an electron and a positron orbiting one another. The electron is matter, and the positron is antimatter. Positronium however is neither matter nor antimatter. It’s both. Note the bit in Jim’s article that says “it can be regarded as a sort of light hydrogen atom”. Also note that another short-lived exotic atom is protonium, which consists of an antiproton and a proton. Again it’s neither matter not antimatter. It’s both. And it’s a bit like positronium. And a bit like hydrogen too. Catch my drift? Time to look at our little 2 x 2 table again:

antimatter table2

Positronium is essentially row 1, and protonium is essentially row 2. Remember we put the proton under the positron? I’ve taken the liberty of filling in the column headings. And guess what? Hydrogen, which consists of an electron and a proton, isn’t matter or antimatter either. It’s both. And antihydrogen, which consists of an antiproton and a positron, isn’t matter or antimatter either. It’s both too.

The ”mystery of the missing antimatter” is now starting to look a little shabby. Especially if you know that baryon asymmetry is matched by lepton asymmetry. Especially if you’ve ever played tennis. Especially if you’ve ever played mixed doubles tennis. Because then you know that it doesn’t matter how evenly matched the players are, one side is going to win. And in the cauldron that was our early universe, the winning side was the blue team. The electron and proton. The losing side was the positron and antiproton. Only if they had won, we’d be calling them the electron and proton. And all it took for one side to gain the advantage was some chance event or “melting” that meant they started to gain an advantage over the other side. Such that the chances of survival for members of the losing team started to diminish, which tips the odds further. Before you know it you’ve got a stability tip. Then it’s game set and match. Then it’s game over.

The thing is, electrons and positrons attract one another. Then they then annihilate, as do antiprotons and protons. That’s because they have the opposite chirality or “handedness”, and they fit like a lock and key. But whilst electrons and protons attract another, they don’t annihilate because they also have different topologies. Ditto for positrons and antiprotons. It’s like the electron and positron are mirror-image trivial knots of wave energy, and the antiproton and proton are mirror-image trefoil knots of wave energy. They’re opposite, but different too. It’s like men are from Mars, women are from Venus, and they don’t annihilate but everybody’s happy, if you catch my drift.

Yes, this lock and key thing is there in biology too, with things like glucose and enzymes. And we know that glucose is “right handed” in nature. We call it dextrose, or D-glucose. The left-handed version of glucose is called L-glucose. It isn’t found in nature, not on Earth anyway. But biomedical scientists don’t feel the need to peddle ”the mystery of the missing L-glucose”. Or the mystery of the missing anti-men. That’s because they’ve been pretty useful in recent decades. Unlike some.

I’m afraid to say that some like to peddle mystery to look useful rather than solve the mystery and be useful. And I’m afraid to say that some would rather keep on peddling mystery than ever admit that actually, there’s no mystery at all.

And never ever was.


12 comments on “Science Sundays with John Duffield: Missing Antimatter

  1. dr
    November 17, 2013 at 11:47 am #

    I am not sure that your argument explains why, when we look out into the universe, we observe fewer positrons than electrons. Likewise, I am not sure that your argument explains why, when we look out into the universe, we observe fewer anti-protons than protons.
    This is despite the fact that you have provided an explanation for missing anti-matter.

    • duffieldjohn
      November 17, 2013 at 1:33 pm #

      It doesn’t explain why we observe fewer positrons than electrons just as it doesn’t explain why we see less D-glucose than L-glucose. But it would be even harder to explain a universe comprised of equal numbers of all four particles, because that wouldn’t be a stable habitable environment. It would instead be a fireball of annihilation that would have to evolve according to some “rules of the game” to leave a stable situation.

      The article does however point out that we can classify particles as matter as antimatter, that baryon asymmetry is balanced by lepton asymmetry, and we should examine the way we classify atoms composed of two particles. Positronium is neither matter nor antimatter, and if hydrogen is like it, you have to say that hydrogen is neither matter nor antimatter too. Instead it’s both. The same applies to oxygen, carbon, et cetera. Which means that weight for weight, rather than antimatter being missing, then weight-for-weight, you are more than 99.95% made of it.

  2. James Eadon
    November 18, 2013 at 8:36 am #

    Hi John,
    What about the violation of Charge Parity conservation then?

  3. duffieldjohn
    November 18, 2013 at 8:42 am #

    James: have a look at this:

    See this bit:

    “A University of Warwick physicist has produced a galaxy sized solution which explains one of the outstanding puzzles of particle physics.”

    Now I don’t know he’s right, but it does make sense. Only you’ve never heard of Mark Hadley. But you hear plenty about “mysteries”.

  4. shorelark
    November 18, 2013 at 12:21 pm #

    Hi DJ

    I fear you have missed the point of this discussion. The question is why there was anything left over from the big bang apart from light. I don’t see why there should have been any neutrinos either, as electron-positron annihilation doesn’t go that way.

    Seeing as there was some matter left over, either it must have been there in the beginning or its anti-matter counterpart is hiding somewhere else. Call it “missing” if you want, but it knows where it is and it’s wondering where we are. .


  5. duffieldjohn
    November 19, 2013 at 1:42 pm #

    Hi shorelark. If all you’ve got is light, you’ll have gamma-gamma pair production going on, so you will have matter. You’ll have annihilation going on too, but don’t forget that “melting”. You start with one electron, one positron, one antiproton, and one proton. Two out of four get melted and you’re left with one electron and one proton. Repeat ad infinitum, and as per mixed doubles, one side will win.

    Best set neutrinos aside for now. They’re classed as fermions, but actually when you look at their brute properties, they’re more like photons than they’re like electrons. They move so close to the speed of light we can’t tell the difference, and they have little or no mass/charge.

    • shorelark
      November 20, 2013 at 4:34 pm #

      Hi DJ

      “Hydrogen, which consists of an electron and a proton, isn’t matter or antimatter either. It’s both. And antihydrogen, which consists of an antiproton and a positron, isn’t matter or antimatter either. It’s both too.” You can’t be serious.

      As for ” melting”, this is on a par with the homeopaths’ notion that water has a memory. In both cases, the proposition beggars belief.

      Let’s see some validation of these concepts in the laboratory before they are trotted out as explanations.

      Even the notion that proton anti-proton annihilation goes to two gamma rays is news to me. The predominant decays involve meson, and then muon decays. These process can scarcely be described as reversible. Again, I hadn’t heard that these particles played any part in the big bang.


  6. duffieldjohn
    November 22, 2013 at 1:20 pm #

    I’m serious, shorelark. The words matter and antimatter are used for particles AND for combinations of particles. Hydrogen is called matter purely by convention, and yet positronium is like light hydrogen. The “melting” is legit, read the physicsworld article. It’s been done in a lab, as has pair production and annihilation and positron chemistry.

    There’s something like a 1% chance of direct proton/antiproton annihilation to gamma photons. Even if that doesn’t happen, see wiki: “The newly created mesons are unstable, and will decay in a series of reactions that ultimately produce nothing but gamma rays, electrons, positrons, and neutrinos”. You can of course annihilate electrons and positrons to gamma photons.

    NB: I don’t think these particles played a part in the big bang per se. The “seething cauldron” came after that.

  7. duncanpt
    November 22, 2013 at 3:47 pm #

    DJ – the rows of your table are not so much “both” as leptons (top), baryons (bottom).

    Using positronium and protonium to justify a matter/antimatter imbalance is sleight of hand. You say yourself that they are “short-lived” – which is basically because they are bound only by electromagnetism and will radiate away the orbital energy so the particles “reach” each other and annihilate. In a sense, it’s only a special case of their approach trajectory to annihilation.

    You also imply in your Nov 19 answer to shorelark that the positron in your set of four particles “melts” into the plasma. Having read the article you referenced, it is referring to a quark-gluon plasma and leaving aside the special circumstances involved in creating the plasma, may I remind you that a positron is a lepton and so has no quarks or internal structure so cannot become part of such a plasma. It might be attracted to it if the plasma has a net positive charge but I think the most it can do is orbit it or pass through it: this plasma is an effect of the strong interaction which by definition doesn’t affect electrons/positrons. So you cannot magic away the positron’s identity into the plasma, which means you are still left with an equal quantity of matter and anti-matter leptons. You might find the anti-proton merging with the plasma – but all that means is that the constituent quarks and anti-quarks are no lurking in the plasma: they haven’t gone away. What one needs to remember is that although conveniently seen as a particle, when you get down to it, the proton is really made up of three quarks; the anti-proton of three anti-quarks and those are where the matter-antimatter balance needs to be examined.

    As shorelark says, we see a lot of matter in our bit of the universe but not a lot of antimatter. If the two kinds were produced equally from gamma-gamma interactions, the questions of how did they get separated so completely in space, or how did we end up with more of one than the other (if the pan-universal sum is not zero) remains unanswered by anything you put forward.

    Lastly, because its a bit of a tangent, neutrinos aren’t really more like photons than electrons. Neutrinos are definitely fermions (no two can ever be in exactly the same quantum state, Fermi statistics vs Bose-Einstein statistics) like electrons; they interact through the weak force, like electrons. OK, so they don’t have an electric charge – in which case view them as a neutral electron is your wish. They are massless (at least not that we have detected or deduced) so must move at light speed as a consequence of that (I think that comes out of either special relativity or general relativity, but it’s a long time ago). Photons are not fermions – they are unrestricted as to quantum state and they don’t interact through the weak interaction. Big difference. If anything, photons are more like gluons – they are mediators of one of the four forces or interactions: in the case of photons, the electromagnetic interaction; in the case of gluons, the strong interaction. I’m not going to get into the W and Z, because then it will get really of the point of your article.

    • shorelark
      November 23, 2013 at 3:20 pm #

      Hi duncanpt: I couldn’t have put it better myself, though the evidence that the proton is a quark composite seems a bit flaky; left to its own devices the proton never decays.

    • duffieldjohn
      November 30, 2013 at 4:20 pm #

      Sorry duncan, I didn’t see your comment. It’s a bit late to reply but I’ll do it anyway:

      There’s no sleight-of-hand re positronium & protonium. The important point is that electrons can’t annihilate with protons because they have different topologies. It’s like the lock and key don’t fit. Instead the sleight-of-hand is making a mystery out of baryon asymmetry without mentioning lepton asymmetry.

      Yes, the positron will “melt” into the plasma. Think of pea soup. There are no peas in it. In similar vein there are no quarks or gluons in a quark-gluon plasma. Gluons in hadrons are virtual anyway. Don’t think the strong interaction has nothing to do with electrons. In low-energy proton-antiproton annihilation you’re typically left with electrons, neutrinos, and light. Where did the strong force go? Where did the quarks go?

      What one needs to remember is that we’ve never seen a free quark.

      You’ve missed the point of the article, which is that we label protons as matter merely by convention. And yet they are more like positrons than electrons.

      Yes, neutrinos really are more like photons than electrons. They travel at c or thereabouts, and they have virtually no mass or charge. They aren’t much like electrons at all. And photons aren’t much like gluons because gluons are virtual!

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