What’s the Matter with Antimatter?

Illustration by John Henry


We see symmetry in nature all the time. At our scale, almost every species on earth has some form of external body-plan symmetry; notable exceptions exist, such as flounder. But the symmetry of objects extends into natural processes and even fundamental physics principles. One such symmetry arises with the property of matter known as charge; a rather abstract notion that we can understand as a driving principle behind circuitry and chemistry. But, questions as to why the most basic components of nature have the charges they do and what the significance of charge is are the questions that are on the minds of physicists. An experiment conducted by The Alpha Collaboration published in Nature earlier this year sought to expound on and explore these questions. First, however, some background.

In 1928, a young physicist, Paul Dirac, developed an equation that married Einstein’s Special Relativity – the theory of the very fast with Schrodinger’s quantum wave mechanics – a description of the fundamental nature of matter. His goal was to produce an equation that accurately describes the quantum behavior of matter at high speeds. But Dirac had outdone himself; not only did the equation seamlessly blend special relativity with quantum mechanics, but it made unexpected predictions. In the same way that the square root of 4 is both positive 2 and negative 2, when solving his own equation for the electron, Dirac found that there were two energies for the resulting particle – one positive and one negative. Instead of discarding the negative solution, Dirac supposed that it may represent a different particle, one with characteristics identical to the electron in every way with the exception of one – its charge. Thus the solutions to the equation are a negatively charged electron and a positively charged positron. The notion of antimatter was born. Following this, Dirac’s idea was treated as being an artifact of the mathematics. That is, until antimatter was discovered four years later in 1932.

Since then, the last 85 years have seen an incredible amount of knowledge and application as a result of the discovery of antimatter. One such example is our current understanding of radioactive decay, in particular beta-radiation, in which either a proton or neutron transforms into the other, producing an antimatter particle. This property of the decay is actually exploited in a modern medical technology – positron emission tomography, or P.E.T. scanning for short.

Whilst we’re at it though, an aside. Beta radiation, when you hear about it, can seem as if its reasoning is pulled out of thin air. It turns out that its cause, is actually due to a fundamental physical force known as the weak interaction. The weak interaction is one of the four fundamental mechanisms for change in the universe, and can cause an interchange of matter with antimatter as well as altering the constituents of protons and neutrons. There’s some modern physics for you.

Oh, and in case you’re curious, when normal matter and antimatter engage with one another, they undergo a friendly process known as annihilation and are converted into pure energy in a quantum explosion. In fact, on the same trail of logic: if matter and antimatter create pure energy, can the reverse be said? Astonishingly, the answer is yes! This is actually the main method of antimatter production; we pump enough energy into a vacuum or fire it at an object and the result is a plethora of matter and antimatter particles. When this happens with an electron and positron spontaneously in a vacuum, it is known as pair production.

We’ve already established that the only difference between particles and their respective antiparticles is their charges. We also know that atoms are composed of positive protons and negative electrons (with most having neutral neutrons for stability), making them neutral overall. So it stands to reason if we just flip the charge of every constituent – protons to anti-protons etc., then the overall properties shouldn’t change –because the atoms don’t have total charge anyway. So here are the real questions: is the logic right, does anything change?

Now we’re equipped to talk about the experiment. One of the most well-understood systems in all of physics is that of the hydrogen atom; composed of a proton and an electron. But a classic way of probing hydrogen is to give the electron some energy and see how it responds. The goal of the experiment was to determine whether or not anti-hydrogen responds the same as normal hydrogen. Due to the obvious issues in handling antimatter, the anti-hydrogen must be created, contained and experimented on in very creative ways. First, the basic ingredients must be made in a particle accelerator. Then, the anti-hydrogen was cooked up by combining its pre-prepared ingredients: an antiproton and positron. It was then trapped and contained in a magnetic field so that it wouldn’t interact with anything made of matter. In order to determine its response to light, the antihydrogen was stimulated with a laser and its response recorded.

The results were rewarding and found an identical response to light from anti-hydrogen as they found from hydrogen. As far as interactions with matter and antimatter go, the experiment found light doesn’t seem to care which is which. This is a big step experimentally into understanding the fundamental symmetries of the universe.

But here’s the rub: why is there more matter than antimatter in the universe, and why are we made of matter? We look out at the cosmos and see galaxies and stars only composed of one. We know this is true because otherwise there would be annihilation constantly and we could see its effects. Perhaps it’s all antimatter and we’re the only matter. But that raises more questions of its own and answers less of ours. In any case, this is a big problem in modern astrophysics and cosmology. Had the experiment shown a different response to light from anti-hydrogen we might have an inkling; but no cigar. Every day we understand antimatter and its properties better both experimentally and theoretically. Its origins and seeming lack of ubiquity on the other hand? Right now it’s a matter of speculation.


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