Friday, July 20, 2012

Star Trek: Antimatter

Star Trek the Next Generation (1987-1994) (Picture)
Season 1, Episode 22
Skin of Evil

The Enterprise comes in contact with a creature made out of an oily substance, which calls itself Armus. It is the embodiment of evil. In its haste to get to the planet Amus inhabits, the Enterprise must realign the dilithium crystals for its warp core. The warp core supposedly runs on an antimatter reaction. What is antimatter, and is it enough to power a ship like the Enterprise?

What is Antimatter?

You may be wondering whether antimatter is a real thing. I mean dilithium crystals are not real (at least not the way they are presented in Star Trek) so why should antimatter be real? It could very well just be a device the writers used to explain where the ship gets all its energy.

As it turns out, antimatter is real. For every piece of matter there is a corresponding piece of antimatter. For example, we have protons. Therefore, antiprotons also exist. We have electrons and also antielectrons (called positrons). These antiparticles have the same mass but they have opposite charge and quantum spin. We will ignore the quantum spin part - let's just focus on the fact that they are opposites.

Because the positron and electron have the opposite charge, they attract one another. When they come together, they annihilate!


Annihilation is derived from the Latin word nihil, meaning "nothing." When the electron and positron annihilate, all their mass disappears, becoming energy. This applies to all matter-antimatter pairs, including protons and antiprotons,  or neutrons and antineutrons.

A representation of the annihilation process (Source)

The energy is released in the form of two photons, which are electromagnetic radiation (of any and all energy levels). These photons each carry half of the energy released in the reaction.

How much energy is released in the reaction?

Remember Einstein's famous equation E = mc2? Well, this is one of the places it is relevant. The energy released in the reaction is equal to the change in mass times the speed of light squared. This is a lot of energy.

On the Enterprise they use deuterium-antideuterium reactions in their warp core. Deuterium is an isotope of hydrogen. Its nucleus has a neutron and a proton, instead of just the one proton in regular hydrogen. Let's see how much energy is produced from the annihilation of one gram of deuterium with one gram of anti-deuterium:

This means we have two grams of mass and it is being annihilated completely:

E = mc2 = 0.002kg x (3 x 108 m/s)2 = 1.8 x 1014 Joules

For a comparison, the energy yield of Fat Man, the bomb dropped on Hiroshima during WWII, was estimated to be about 8.8 x 1013 Joules.

It is evident that matter-antimatter reactions are incredibly powerful and can supply almost unimaginable amounts of energy. In fact, there is no other process in the universe that can produce energy more efficiently.

And it can go backwards!

It is possible to produce antimatter from energy. In fact, it is all around us, though it does not survive long before annihilating. Here's my favorite example: bananas.

Bananas have potassium, and potassium is good, right? Of course potassium is good - but it is also radioactive! All potassium contains about 0.012% Potassium-40. This is a radioactive isotope of potassium, and it decays by a method called β+ decay. In this process a proton in the nucleus converts to a neutron and emits a positron!

Bananas are a source of antimatter! There you have it. Share the information with your friends or keep the secret to yourself.

Seriously, though, don't be afraid of bananas. They are a good example because they have a lot of potassium, but all potassium sources have the same percentage of potassium-40, so you are not actually doing yourself any good if you avoid bananas and get your potassium elsewhere. Also, you need potassium, so you cannot just avoid it completely.

 A diagram of pair production (Source)

Another example of how the process can go backwards is called pair production: a high-energy photon is traveling at the speed of light (because a photon is light) when it comes upon a nucleus. When it comes close to the nucleus, it can interact, producing an electron-positron pair. The positron will go on to very quickly annihilate with a nearby electron.

The condition for pair production to occur is that the incoming photon must have at least the energy required to produce the mass for an electron and a positron. This corresponds to about 10-13 Joules. This is not much compared to the annihilation process we saw above, but it is a lot of energy for one photon to have. Only high-energy gamma rays are capable of pair-production reactions. 

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