Want to look at your insides and/or destroy the world? Antimatter may be right for you.

In science fiction and the popular press, antimatter often gets a bad rap. If it’s not breaching a warp core somewhere and destroying a spaceship, it’s being used by a supervillain to power a death ray. In actual fact, though, antimatter can be very useful stuff.

Antimatter, of course, is just normal matter with the electrical charges reversed. Normal protons are positively charged, and normal electrons are negatively charged. Anti-protons are negatively charged, and anti-electrons (or positrons) are positively charged. This is interesting, but the fun (and useful) thing abut antimatter is what happens when it encounters regular matter. Turns out, the bit about the warp core breach blowing up the spaceship is pretty accurate. Matter and antimatter, like pasta and antipasta, do not get along.

To see how this works at the smallest scale, consider what happens when a positron runs into an electron.  This reaction is the basis for positron emission tomography (PET), a medical imaging technique that provides the best method we have for detecting small, distant metastases in cancer patients. When positrons and electrons collide, the mass of both is converted into a neutrino, which we don’t worry too much about, and two high-energy photons (gamma rays) that shoot off in exactly opposite directions. If this reaction occurs inside a cancer patient, and the patient happens to be inside a gamma camera, those photons will strike elements of the camera on opposite sides of the patient’s body more or less simultaneously. This tells us that the positron was somewhere on a straight line between the two detector elements. With enough of these simultaneous detections, we can build up a picture of how the positrons (which are coming from a tracer with an affinity for tumors, like fluorodeoxyglucose) are distributed in the body. This, in turn, tells us where the tumors are.

This is all well and good, but what if you’re interested in doing something more energetic? The problem with positrons is that they have very little mass, so their annihilation produces very little energy. To drive a spaceship, for example, we need to start generating (and destroying) anti-protons. Anti-proton annihilation produces both charged and un-charged pions (sub-atomic particles consisting of a quark and an anti-quark) as well as neutrinos (again, not particularly useful) and a shit-ton of gamma rays.

Depending on what, exactly, you’re trying to do, this reaction can be very helpful from a propulsion standpoint. The charged pions, in particular, are moving at very close to the speed of light, and because they are charged, a magnetic field can be used to funnel them out the back of your ship. One of the main limiting factors in how fast a rocket can go is the speed at which its propellant material shoots out the back. An antimatter rocket provides the greatest specific impulse (a measure of the efficiency of a rocket or jet engine) that is possible using known physics. Because of this, a spacecraft using an antimatter rocket may be able to achieve speeds as great as a bit less than a third of the speed of light. As a point of comparison, this would allow you to travel from Earth to Pluto in about sixteen hours (compare that to the nine and a half years it took the New Horizons probe to get there), and to the nearest star in about fifteen years.

So, why aren’t we building these things right now? Well, the issue is that, like the hydrogen that’s supposed to running all those fuel-cell cars, antimatter isn’t an energy source. It’s an energy storage medium. The energy required to create one anti-proton is just a bit greater than the energy released by its annihilation. So, how much energy would be required to generate the fuel for an Alpha Centauri probe?

Start with an assumption about the mass of the probe. The New Horizons is a hair under 500kg. This one will be exploring an entire new star system, so let’s give it a bit more heft (and also make our math easier) by calling it 1000kg. How much energy will we need to get this guy up to speed?  Well, the kinetic energy of the probe at 0.3c will be about 4.5x10^18 joules. For perspective, that’s roughly the output of 100 large nuclear power plants over a full year. Remember, though, that we’ll need to decelerate the probe at the other end–so, better get another hundred plants on-line.  Remember also that only a fraction of the annihilation energy of our antimatter fuel will actually go toward propulsion. The uncharged pions can’t be funneled, and we don’t really have a good way to make use of the gamma rays either. Probably need to multiply our fuel requirements by another factor of five.

So, in order to generate the fuel to get one smallish robotic probe to Alpha Centauri in about 15 years (meaning we’ll hear back from it in 19 or so) we’ll need to make use of the entire output of 1000 nuclear power stations for a full year. That’s assuming a completely lossless production process, of course, so you might want to have a few hundred extra plants running just in case.

While we’re thinking about how much energy we’re blowing, we might also want to consider that when our little probe is sitting on the launch pad at Cape Canaveral, it will be carrying all that juice with it in highly concentrated form. We’ve been talking about this energy in terms of electrical output.  Here’s another comparison:  In 1961, the Soviets detonated the Tsar Bomba, which is to this day the most powerful nuclear weapon ever devised. It’s explosive yield was 50 megatons. The fireball when it went off was visible from 1000 kilometers away, and the mushroom cloud pierced the stratosphere. If something goes wrong during the launch of our probe–if we suffer a “core breach”–the explosion will be roughly 200 times as powerful. This wouldn’t destroy the world or anything, but I don’t think I’d want to be in Jacksonville when it happened.

One last note on antimatter–there is no reason we know of that the Big Bang shouldn’t have produced it and regular matter in equal amounts. If that had happened, of course, all those protons and anti-protons and electrons and positrons would have annihilated one another, and we’d have been left with a matter-free, life-free universe. For some reason, however, there was a slight excess of our sort of matter, and that excess now makes up all the matter we have left. Chalk this up as another item on the long list of things that had to go exactly right in order for you to exist, and if you can, take just a moment to wonder how many cosmos had to be born and die before all of those things fell perfectly into place.