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In a sense, antimatter beams are commonplace. The Tevatron machine at Fermilab in Chicago had the best beam of antiprotons, and used it to find the top quark. The LEP collider, which between 1990 and 2000 sat in the tunnel at CERN now occupied by the Large Hadron Collider, had a beam of positrons - the antiparticle of the electron. However, proper matter, everyday matter, is made of atoms. That is, electrons bound to an atomic nucleus. Slowing positrons and antiprotons down and making them stick together into anti-atoms of antihydrogen is difficult.
If you can do it, the rewards are substantial though. With a big enough sample of anti-hydrogen, one can make detailed studies of the energy levels that the positron can occupy in its journey around the antiproton. These energy levels have been measured very precisely for hydrogen, and the expectation is that they should be identical in antihydrogen. But we won’t know until we look.
The ALPHA experiment at CERN made a start on this almost exactly two years ago, with a rough first measurement. They will be more precise in the future. But one of the problems with precision in these experiments is that you need big magnetic fields to traps the anti-atoms, stop them meeting atoms, and annihilating. Unfortunately these magnetic fields also distort the energy levels, and will likely in the end limit the precision of any comparison to hydrogen.
The ASACUSA experiment plans to get around that by making a beam of antimatter, and measuring the energy levels as the beam travels in a vacuum, away from the magnetic fields and away from any nasty annihilating matter. They published a paper on it in January and there is a nice article in the CERN courier this week giving some more details of the technique and how they plan to develop it.
The symmetry principle which these experiments are designed to test is whether physics, and therefore the whole universe, would look the same if we simultaneously swapped all matter for antimatter, left for right, and backwards in time for forwards in time. This is called a CPT (Charge/Parity/Time) inversion. The Standard Model of physics, and almost all variants on it, require that indeed the universe would be identical after such an inversion. However, in string theory for example it is possible to violate this principle (see “CPT and the Standard Model Extension”). The ASACUSA people do not plan to test this by crossing their antimatter beams (thank you and RIP Harold Ramis) but by measuring those antihydrogen energy levels very precisely. Any difference would mean a violation of CPT inversion symmetry.
This kind of idea, of looking at or imagining the world from a standpoint different to the everyday, is an important technique for building models of how the universe works. Knowing whether or not the laws of physics change under a CPT inversion would tell us an enormous amount about the structure of reality. Thought experiments, such as those Einstein used in developing relativity, often consist of imagining what physics would look like to different observers, with different points of view, and requiring that in the end it should look the same.
As well as reading about ASACUSA, this week I attended the first in a series of seminars on the “Wonderments of the Cosmos”. The series, organised by Martin Holbraad and Lucy Calder, is part of a cross-disciplinary activity at UCL involving physical scientists, anthropologists, historians, artists and more, where we discuss various ideas of cosmology and our place in the Universe. Ofer Lahav, a colleague from Physics & Astronomy, kicked it of with a description of the best, evidence-based but always provisional astrophysical “standard model”, involving as it does copious unknown dark matter and energy.
Something that struck me in the discussion was the regret expressed by Ofer, and echoed by the anthropologists, that these days we communicate continuously and too much. Developing independent points of view on cosmology, or indeed other matters, is therefore very difficult. Having been surrounded by a culture in which communication is seen as generally a good thing, this came as a surprise to me, but it is a very good point. We gain confidence in the correctness of ideas if they are arrived at independently from different points of view. A good example is the independent, almost simultaneous development of quantum electrodynamics by Richard Feynman, Julian Schwinger and Sin-Itiro Tomonaga. They all three had very different approaches, and Tomonaga in particular was working in wartime Japan, completely cut off from the others. Yet Freeman Dyson was able to prove that the theories each had provided for the quantum behaviour of electrons and photons were not only all equally good at describing nature, but were all mathematically equivalent - that is, the same physics, seen from different points of view.
Part of the power of the scientific approach which has a direct bearing on our culture (and emanates from that culture, I guess) is this idea of standpoint-independence, the absence of “privileged observers”. Or at least the idea that even if privileged observers do exist in some senses, there is a reality that remains independent of them.
Why would reality perceived directly with our own senses be more real than reality revealed via instrumentation? There is nothing special about wavelengths around 500nm apart from the fact that our eyes have evolved to be sensitive to them. We can extend our vision, and I don’t believe that the way our minds interpret signals from an infrared telescope or from the LHC is any more or less theory-laden that the way we interpret optical light - it’s just not as automatic. Does the world suddenly become less real if you wear glasses?
Whether we are using thought experiments, antimatter beams, sophisticated instrumentation, or sending spaceships to the outer solar system as in the photograph above, the ability to loosen the constraints of our own point of view is hugely important. It is also, I think, closely related to the ability to put ourselves into the place of other people in society and to perceive ourselves as seen by them - to check our privilege, if you like. Imperfect and difficult, but a leap away from a childish self-centredness and into adulthood.
Jon Butterworth’s book, Smashing Physics, is out on 22 May. Order it now!
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