A few days before the holidays began, I made a passing mention of an interesting new result from the ALPHA collaboration relating to the measurement of the spectrum of anti-hydrogen. Since that time a few people have inquired about it, and asked for more details.

Let me begin with a little background on the topic.

As most of us recall from high school science class, everything that we see around us is made of atoms. Protons, neutrons and electrons combine in different amounts to form different elements, and comprise all of the regular matter we know about. (Not including the more speculative dark matter, or particles created in a lab that exist for a fraction of a second).

At the start of the twentieth century, physicists began developing the theory of quantum mechanics, and one of its first great successes was in predicting the spectrum of each element. The electrons in an atom can only exist in certain energy levels, depending on the mass and charge of the nucleus of the atom. When these electrons switch between energy levels, they either absorb or emit light at precise frequencies (or colours if it is in the visible spectrum). Since each type of atom has a different set of frequencies, this spectrum is a signature of each element.

This signature is so precise in fact that it is in common use in many areas of science. The elemental composition of distant stars is known because astronomers measured their spectra and related it to the different elements. Chemists can analyze unknown substances by passing light through them and measuring which frequencies get absorbed or re-emitted. Even the ubiquitous neon lights in every store window make use of the signature spectrum of different gasses to produce their colours.

Shortly after the explanation of atomic spectra was provided through the use of quantum mechanics, another interesting result was found. When Paul Dirac was combining the theory of special relativity with quantum mechanics, which would be necessary to study the properties of electrons and protons, the equations predicted the existence of another particle with the same properties as the electron but with an opposite electric charge. For several years he and other theorists tried to explain this new particle or to remove it from the equations, but with no success. Then cosmic-ray experiments detected the positron – a particle that exactly matched Dirac's predictions earning him the Nobel Prize.

Since that time, the existence of anti-matter has been proven in many experiments and is now regularly used in physics experiments ranging from the lowliest undergraduate lab to the most famous high energy particle accelerators.

We now know that every particle in nature has an associated anti-particle, which has the exact same properties but with an opposite electric charge. Particles can only be created with an anti-particle, and they are destroyed when a particle and anti-particle collide and annihilate each other.

The problem though is that for reasons we still do not understand, there is very little anti-matter in the Universe. It might have been pushed out of our region, or perhaps a random bubble of matter allowed our region of the Universe to form, or maybe there is some very small asymmetry in nature that allowed more matter than anti-matter to form. The lack of anti-matter in our Universe is a very important open problem in physics and the moment, and so physicists are quite eager to study its properties in ever greater detail.

The ALPHA collaboration has just completed a very important experiment to improve our understanding of the properties of anti-matter. They have been combining positrons (which are the anti-particle partner of the electron) and anti-protons to form atoms of anti-hydrogen. It is very expensive, and very difficult to do, but they have succeeded in producing a very small quantity of anti-hydrogen gas.

In this latest result they were able to measure the energy levels of anti-hydrogen using the spectrum of light that it absorbs and emits. Our best theories predict that the spectrum will be exactly the same as for regular hydrogen, while the lack of anti-matter in the visible Universe suggests that this symmetry might not be exact.

Their result is that, within the limits of their experiment, there is no difference between the two spectra. Anti-hydrogen and hydrogen have the exact same energy levels.

Of course more work will need to be done, and experimentalists are always pushing for more data and stronger evidence for this symmetry being exact. For now though, matter and anti-matter are equal and the symmetry is preserved.