The first antimatter

Reading the post below on the spectrum of anti-hydrogen, it strikes me that I haven’t explained the concept of antimatter very well. AM has always been one of my favourite manifestations of the strange world of quantum physics (hence the blog title), so let’s have a proper post on it…

The idea of antimatter first emerged in 1928. In that year, Paul Dirac derived, from first principles of quantum theory, a wave equation for the electron that included the effects of special relativity. It was a stunning achievement and marked the beginning of modern quantum field theory. However, the Dirac equation had one very strange property – there were dual solutions for the equation, implying that positive and negative energy levels existed for the particle.

What was the physical meaning of a whole extra set of energies of opposite sign for the electron ? It couldn’t be that a repulsive electromagnetic force also existed, as the atom would fly apart. Dirac eventually decided that the only sensible answer was that the equation also described the energy of a particle of opposite sign to the electron.

This was an outlandish prediction of a brand new version of quantum theory and few scientists were convinced. However, in 1932 the experimentalist Carl Anderson discovered the decay track of an intriguing new particle in studies of cosmic rays – a particle that was of the same mass as the electron, but of opposite charge (the anti-electron or positron). It was a spectacular success for Dirac’s equation and marked a watershed in quantum theory. Long years later, other anti-particles were discovered in accelerator experiments, from the anti-proton to the anti-neutrino.

The discovery of the positron (1932): the particle was deflected by a magnetic field in the opposite direction to the electron, but was too light to be a proton

In the 1980s, accelerator physicists managed to create entire anti-atoms of hydrogen, by allowing positrons to be trapped by anti-protons. However, such ‘hot’ anti-atoms are hard to study and the next challenge was to create ‘cold’ anti-atoms so their properties could be studied in detail; this was achieved in the late 1990s.


A fundamental problem

From the first, it was realised that antimatter and matter would annihilate on contact (from relativity), and this raised a new fundamental question in physics: Why do we live in a universe made almost entirely of matter? Why didn’t matter and anti-matter annihilate immediately after the Big Bang? This puzzle hints at a deep asymmetry in the decay of matter and antimatter and is known as the puzzle of baryogenesis, more on this later..


Filed under Particle physics

14 responses to “The first antimatter

  1. cormac

    Yes, can’t wait. I was a little put off by the review in The Sunday Times, but I think it was just a poorly written review..

  2. James

    “From the first, it was realised that antimatter and matter would annihilate on contact (from relativity)”

    Is the “from relativity” bit entirely true? Is the concept of anti-matter even meaningful from a pure-relativity standpoint?

    As far as my memory serves, the creation and annihilation of particles is a result of QFT. Is the fact that the QFT is required to respect special relativity esential for this? (Honest question by the way, I’m out of practice and too lazy to dust of old physics text books and revise…) Can we have “classical” anti-matter? (As a toy toy model at least – what happens with anti-matter after a Wick-rotation?)

  3. James

    Typo: by “classical” I meant non-relativistic, not non-quantum. Perhaps, to use a musical analogy, I should have said “romantic”? ;-)

  4. cormac

    Good question. One learns in class that it is only when SR is taken into account in the wave equation for a particle that the double energy levels (signifying AM) arise. Relativity is invoked a second time in order to explain the conversion of the mass of M/AM into energy.

    However, it could be argued that there are hints of AM in relativity without QT. After all, E2 = m2c4 + p2c2 has double solutions!I too haven’t looked at this in awhile, must look it up..

  5. dvd

    some interesting topics along the peripheries of the webpages

  6. cormac

    Thanks for thre fab links Dave, the first one is an incredible resource. I enjoyed ATOM so much I think I’ll do a post on it tonight…

  7. great links on the on-line documentaries dvd

  8. huh? if at the big bang the universe = +1 (so: universe existed) and
    we can pretty much for sure say that the universe exists now (still at a +1 state) then why all the fuss about matter/anti-matter

    if there was pos/neg symmetry the universe (energy, matter, whatever you
    wanna call it… the ‘fractions’ of the total)… would = 0…

    hence: as soon as there’s existence, surely there’s asymmetry?

  9. Pingback: Antimatter trapped at CERN « Antimatter

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