Antimatter

Many of you have probably heard of antimatter, a strange form of matter that seems to be opposite of what we call "normal" matter. If you've seen Star Trek, the Enterprise and all the rest of the Starfleet fleet are run by matter-antimatter collisions. We will see why this is a very efficient way to power starships, cities, almost anything.


First, we need to know what antimatter really is. Antimatter is a form of matter that has the same physical properties as normal matter, but with one huge difference. For a charged particle, the corresponding antimatter particle has an opposite charge. So for an electron, its corresponding antimatter particle is an antielectron, or more commonly, a positron. A positron has the same mass and spin, but is positively charged. In the same way, the antimatter equivalent of a proton is the antiproton with a negative charge. A neutron has the corresponding antineutron. Wait a minute, neutrons have no charge, so how does an antineutron differ from a neutron?


To look at an antineutron, we should know a couple of things about protons and neutrons. They are a part of set of particles known as baryons, which are particles that can be subdivided into smaller particles called quarks. A proton is made up of three quarks: two up quarks and one down quark. A neutron is composed of two down quarks and one up quark. Quarks come in six flavors, have a electric spin of +1/2 and they have unique charges.

• up quark (u) - charge of +2/3e, mass of 2.3 MeV/c²
• down quark (d) - charge of -1/3e, mass of 4.8MeV/c²
• charm quark (c) - charge of +2/3e, mass of 1.275 GeV/c²
• strange quark (s) - charge of -1/3e, mass of 95 MeV/c²
• top (or truth) quark (t) - charge of +2/3e, mass of 173.07 GeV/c²
• bottom (or beauty) quark (b) - charge of -1/3e, mass of 4.18 GeV/c²

Where e is the elementary charge, 1.602e-19 coulombs. An electron has a charge of -1.602e-19 coulombs and a proton has a charge of +1.602e-19 coulombs. An eV is an electron-volt, a unit of energy that is approximately equal to 1.602e-19 Joules (surprise, surprise) and eV/c² is the unit of mass for elementary particles (equal to 1.783e-36 kg) with M the prefix for Mega (10e6) and G the prefix for Giga (10e9). The mass of an electron is 0.511 MeV/c². So obviously, these are very tiny masses.


Quarks are particles known as fermions, while electrons belong to a family of particles called leptons (which also include neutrinos, muons, and tau particles as well as all their corresponding antiparticles). But I digress.


The antiquarks have opposite charges: anti-up quark (u) has a -2/3e charge and an anti-down quark (d) has a +1/3e charge. (The line above the letter distinguishing the particle as an antiparticle). This means that an antiproton is made up of two antiups and one antidown while a neutron is made of one antiup and two antidowns.

A cool thing about matter and antimatter (or weird, depending on how you look at it) is that when matter and antimatter interact, they completely annihilate each other. In other words, 100% of the mass is converted to energy via Einstein's energy-mass relation, E=mc². If an electron and a positron collided, the energy produced would be 1.022 MeV (1.67e-13 Joules) which is really not a lot. But when there are trillions upon trillions of interactions like this, the energy can add up. Compare this to the proton-proton chain, which is the process in the core of the Sun where four hydrogen nuclei fuse into one helium nuclei where only 0.07% of the total mass of the four protons is converted to energy (one proton has a mass of 938.3 MeV/c² or 1.673e-27 kg). Four protons have a mass of 3753.2 MeV/c² which only 2.63 MeV/c² is converted to energy, or 2.63 MeV (4.21e-13 Joules). One proton and one antiproton would create 1.877 GeV of energy (3.01e-10 Joules) - almost 1000 times as much energy. By the way, one kg of matter reacting with one kg of antimatter would equal 1.8e17 Joules (43.02 millions of tons of TNT - 4 times the energy released by one hydrogen bomb).


So why don't we use matter-antimatter reactors to power the world? For one thing, antimatter is rare. There is not a repository of antimatter near to us in the universe. However, antimatter can be created in the lab, but only in small quantities. But once it is created, you have to find a way to store it without allowing it to interact with normal matter. That means placing it in a vacuum in a magnetic field so it does not touch the container it is in. Air itself would interact with antimatter.

Anything we need to know, is why do we have more matter than antimatter? Nothing in the laws of physics says that their should be an imbalance between the two, but we know that there is. Early in the universe, just a difference of one quark or one electron in a billion quark-antiquark or electron-positron pairs would lead to the imbalance we see. But we don't know why there was an imbalance in the first place.


One theory is the idea of virtual particles. Virtual particles are particle/antiparticle pairs that are created when a photon with the correct energy spontaneously is turned into a particle and an antiparticle (as long as their masses equal the energy of the photon). In most cases, virtual particles don't last very long because they immediately react with each other and annihilate. However, if a photon spontaneously decayed into a particle/antiparticle pair near a black hole, it is possible that the antiparticle can fall into the black hole. However, this still can't explain why there are more particles than antiparticles as random matter/antimatter creation should not choose one over the other.


Another theory is that matter and antimatter are subject to anti-isotropy, meaning that our region of the universe is dominated by matter while there could be regions that are dominated by antimatter. The only way we could be able to confirm this is to travel to that region of the universe since observation alone won't tell us that antimatter is the dominate matter. However, since we are made of "normal" matter, travelling to a region of space dominated by antimatter would not be healthy.