What Is Antimatter?

 

Antimatter may sound like science fiction, but it actually surrounds us every day. Antimatter is used commonly in modern medicine and even gets produced by the bananas sitting in your kitchen. (More on that in a minute.)

Antimatter also poses one of the biggest mysteries in physics. When an antimatter particle and its opposite, regular matter particle encounter one another, they annihilate and produce a burst of energy. During the Big Bang, regular matter (the stuff you and I are made of) and antimatter should have been created in equal amounts. Thus we would have expected all of those matter and antimatter pairs to have crashed into one another leaving behind a universe full of energy in the wake of their destruction. And yet, we are here, in a universe full of regular matter stuff.

So what is antimatter? And how can we study it to figure out why it hasn’t destroyed us all?

What is Antimatter?

Matter is made up of atoms which we have mapped out as a nucleus of subatomic particles called protons and neutrons with a number of electrons swirling around it. The kind of atom you have—whether it be hydrogen, carbon, oxygen, etc.—is defined by the number of protons in the nucleus. (Hydrogen has one proton and carbon has six, while oxygen has eight.) Those protons have positive charge, so when an atom is in its normal state, it has an equal number of negatively-charged electrons to balance out those protons, resulting in a neutral atom. Neutrons, as their name suggests, are already neutral.

So subatomic particles—those proton, neutron, and electron pieces that build together to make an atom—can have charge, and they can also have spin, a quantity that represents the particle’s angular momentum which relates to its speed and direction of rotation.

Every matter particle is believed to have an associated antimatter particle which is identical except for having the opposite charge and spin. For electrons, that antimatter particle is a positron which has the same mass but positive charge and a counter-rotating spin. Physicist Paul Dirac first predicted the existence of antimatter in 1928 and positrons were soon discovered by Carl Anderson in 1932. Both men received Nobel Prizes in Physics for their work.

Although our predictions suggest that antimatter was created in large amounts during the Big Bang, it is rare now, at least compared to normal matter. Antimatter electrons, or positrons, are used in medical imaging to get high resolution views of our bodies. If you’ve ever had a PET scan, that “P” in “PET” stands for Positron (Emission Tomography).

The bananas sitting in your kitchen also produce positrons. You have probably heard that bananas are rich in potassium, an element that always has 19 protons in its nucleus. Different forms of potassium, called isotopes, however, have different numbers of neutrons and differing levels of stability. Bananas tend to have a lot of the isotope potassium-40 which has 21 neutrons and will occasionally decay into argon, a process that spits out a positron every 75 minutes. That may sound like a lot, but keep in mind that those antimatter positrons are quickly annihilated. Our bodies also contain similar isotopes that result in tiny amounts of antimatter production.

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How Do We Study Antimatter?

In order to study antimatter to determine its uses and ultimately to understand how our universe ended up with the surplus of matter that allows our existence, particle physicists can generate antimatter in particle accelerators like the Large Hadron Collider buried ~100 meters underground at the France-Switzerland border.

The antimatter particles are first generated through very high speed collisions but then must be slowed down for physicists to get a good look at them using something called an Antiproton Deccelerator.

There are a variety of ongoing antimatter experiments at CERN, the European Council for Nuclear Research, and the institution that runs the Large Hadron Collider. Particle physicists are investigating the biological effects of antiprotons, whether we can use antimatter to target cancer tumors, how antihydrogen atoms behave relative to their normal hydrogen atom counterparts, and even whether gravity affects antimatter the same way it does matter. CERN provides an interesting historic timeline of antimatter research on their site.

Could We Build an Antimatter Spaceship?

Fans of Star Trek may recall that the starship Enterprise used the energy created by matter-antimatter annihilation to propel itself faster than the speed of light. Certain groups, like the Tau Zero Foundation, founded by a former NASA physicist, are investigating whether such propulsion could become a reality and thus make interstellar travel possible.

A huge inhibiting factor is cost. NASA estimated a price tag of $62.5 trillion per gram of antihydrogen back in 1999. CERN has quoted a cost of a few hundred million Swiss francs to produce the ~1 billionth of a gram created in the experiments there.

As for why we exist at all and haven’t been annihilated in a burst of light thanks to matter-antimatter collisions? The best theory so far is that there was one measly extra matter particle for every billion matter-antimatter pairs created in the early universe and that was enough to leave behind the matter-filled universe as we know it today. The reason for that initial asymmetry, however, remains a mystery.

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

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