Another interesting result from the Canadian led team at the ALPHA collaboration, with a paper publish today in Nature in which they present the hyperfine spectrum of anti-hydrogen.

Anyone who has the least interest in either physics or astronomy is aware of atomic spectra. Over a century ago scientists discovered that each chemical element emits a signature series of wavelengths of light, which is unique to that element. From the colour of emitted light we can identify each element that is present in some system or substance. It was this discovery that allows scientists to study the chemical composition of distant stars, using just the light that our telescopes receive from them.

Another important discovery in the history of science came nearly a century ago when Paul Dirac was studying the properties of the newly developed quantum mechanics in relativistic systems. According to his eponymous equation, every particle should be accompanied by an anti-particle, an exact copy of it that has all of the same properties except that it contains the opposite electric charge. When a particle and antiparticle collide, they both are annihilated. 

However there is a problem with our understanding of antiparticles. Our best theories predict that matter and antimatter have the same properties, and that they are always created and destroyed in pairs. Therefore we would expect that the Universe would have an even mixture of each, and we should be able to see antimatter stars and planets. However in spite of spending centuries cataloging matter in the form of planets,stars, and galaxies, so far astronomers have been unable to find any significant quantities of antimatter!

And that is why it is critical for particle physicists to study antimatter in the lab and try to determine some unknown property of it that allowed matter to dominate during the Big Bang. The more antimatter that we can create, the more we can understand why the predicted matter/antimatter symmetry is not being preserved in nature.

That is why this new result is potentially so interesting. Scientists at the ALHPA collaboration have been producing small numbers of anti-hydrogen atoms, which contain an anti-proton and an anti-electron (also known as a positron), and studying their properties. In this latest result, the team were carefully studying the spectrum of light that is emitted by the anti-hydrogen, and recording each wavelength as precisely as possible. If matter/antimatter symmetry is preserved in nature, then the wavelengths will be identical to those found in hydrogen atoms.  If there is some symmetry breaking between matter and anti-matter, it could show up as a very slight difference in the hyperfine spectrum.

Unfortunately for the particle theorists and astrophysicists, their results do not show any differences. The experiment made very accurate measurements, but could find no discrepancies with the well known hydrogen spectrum. It would have been perhaps more interesting if they had found a difference between the hydrogen and antihydrogen spectrum, and probably aided in our understanding of the early evolution of the Universe, but they have instead strengthened the evidence that the two atomic spectra are identical and provided further evidence that nature displays a matter/antimatter symmetry.

It is a most interesting result, and I am proud to say that it is under the leadership of Canadian scientists!