Why Matter Really Matters

Firing neutrons under Japan may explain why the universe exists

Why isn’t tomorrow like yesterday? Having grown up in a world of growth and decay – of directed change – most people might consider this question a little odd. After all, a movie taken of just about any large-scale natural process has a clear direction of time – if the movie is played backwards, seedlings dwindle into seeds and water jumps up a waterfall. However, this macroscopic “arrow of time” is due to the randomising effects of the very large number of atoms involved. The underlying laws of classical physics (CP) have no arrow of time. Run a movie of the frictionless pulleys and massless springs of our A-level physics books backwards and they look fine.

The macroscopic arrow of time arises because a situation is spectacularly unlikely to arise where all the molecules in a river happen to be flowing with just the right directions and speeds to collide and bounce up just as a waterfall appears. Hence a river won’t flow backwards.

We can therefore conclude that tomorrow isn’t like yesterday because of this macroscopic arrow of time. However, we also need the fundamental laws of physics to have an arrow of time. The reason is related to one of the great mysteries of current science – why is the Universe made of matter rather than anti-matter? Anti-matter is extremely rare in our universe, in fact so rare that when Paul Dirac formulated his theory of the electron in 1928 and found solutions that looked like positively-charged electrons, he didn’t know what they might be (as normal electrons always have a negative charge). He was still groping for an answer in 1932, when Carl Anderson found positively-charged electrons in cosmic rays, and the penny dropped that the Dirac Equation implies that for every particle there is an equal and opposite anti-particle. Of course (as viewers of Star Trek are aware), if the two should meet, they will annihilate into radiation (with unfortunate consequences for anyone standing nearby).

What Star Trek never told you, however, is that the same thing works the other way – radiation can turn into matter and anti-matter (that is, the same reaction, but with time running the other way). In fact, the matter in the universe today was born in just this way, congealed out of the blazing radiation of the Big Bang. But if the laws of physics are the same for anti-matter and matter, the exact same amounts of matter and anti-matter would have been created, and we would not be here.

So are the known laws of physics exactly the same for matter and anti-matter? The answer is no. In the 1960s, physicists first noticed that the decays of obscure particles called neutral kaons were very slightly different for the matter and anti-matter versions. Makoto Kobayashi and Toshihide Maskawa realised that they could explain this based on the “mixing” of different kinds of quarks by different interactions, but only if there existed two new kinds of quarks. These two new kinds of quarks were subsequently discovered, and decades of experiments on the decays of mesons have shown that the mixing of quarks is just as Kobayashi and Maskawa predicted (hence their award of the 2008 Nobel Prize for Physics). But the surprise is that the differences between matter and anti-matter explained by Kobayashi and Maskawa would produce something like one-trillionth of the amount of matter we see around us, leading to an exciting conclusion – since the known laws of physics did not produce the matter we see, there must be entirely new laws of physics which are different for matter and anti-matter (or for time running forwards or back).

So what are these new fundamental laws of physics? Unfortunately, the evidence that they exist is compelling but completely non-specific, so there is very little theoretical guidance on where to search. The new Large Hadron Collider at Cern, Europe’s particle physics laboratory, is one potentially fruitful hunting ground. One of the big experiments on the LHC, which is called LHCb, will continue the study of meson decays searching for deviations from predictions based on the work of Kobayashi and Maskawa that could signal new CP-violating physics. The big general purpose detectors, Atlas and CMS, will search for any other new physics, most versions of which could produce CP violation.

I have put my money on two other horses. The first has to do with the properties of the enigmatic neutrinos, the ghosts of particle physics. Neutrinos are actually quite common. They are produced in huge numbers by the thermonuclear reactions that power the sun (there are 67 billion of them travelling through every square centimetre of your body every second), but they interact so weakly with normal matter that the neutrinos from the sun pass effortlessly through the Earth. Despite their ghostly nature, these solar neutrinos have now been detected many times, and so have neutrinos from reactors, accelerators, radioactive sources, cosmic-ray interactions in the atmosphere and even a supernova. Experiments have now shown that the same sort of mixing seen in quarks also takes place in neutrinos, where we call it neutrino oscillations. Neutrino oscillations could also violate CP, but to confirm that will require far more precise measurements than we have been able to make up to now. I am therefore part of an ambitious new experiment called T2K, which is building a beam of neutrinos using a new accelerator at Tokai on the east coast of Japan. The beam will be measured there, and then fired 295 kilometres through the ground to the vast Super Kamiokande neutrino detector under a mountain near the west coast of Japan. By looking at changes in the neutrino beam as it oscillates we can make the first tentative steps towards measuring CP violation in neutrinos, which could be the signpost of the physics that led to the excess of matter in the universe.

Just in case neutrinos are not the source of CP violation, I am also involved in another experiment at the ILL reactor in Grenoble, France. My British colleagues – assisted by other physicists from around the world – have been working there for many years building experiments to try to measure the static electric dipole moment of the neutron (nEDM). An nEDM would violate time-reversal invariance, but is almost unaffected by the mixing of Kobayashi and Maskawa, so just observing one would be a sign of currently unknown physics. The measurement sounds simple. You make an atomic clock out of neutrons, then you put it in a whopping electric field and look for tiny shifts in the frequency of the clock (of one part in 1011, or one second in 3,000 years) as you switch the sign of the electric field. So far no shift has been seen, producing a limit that the nEDM cannot be bigger than 6.3×10-26 e.cm. To appreciate just how tiny this number is, consider a neutron made of one unit of negative charge (that is, the charge on one electron) and one unit of positive charge. Then expand it to be the size of the Earth. To produce a nEDM equal to our limit you would only need to offset the centres of the two charges in the Earth-sized neutron by about ten microns.

We can do even better, however, and we are currently building an entirely new experiment, which should be about a factor of a hundred more sensitive than the existing one, based on storing the neutrons in liquid helium cooled to a fraction of a degree above absolute zero. We hope to see a signal this time, but whether we do or don’t, we will have learned something important about the structure of the laws of physics. One of these quests to find CP violation must get an answer, because if the universe exists, those laws of physics which produced the matter are out there somewhere.

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