Antimatter Drives
"Any sufficiently advanced technology is indistinguishable from magic," - Arthur C. Clarke

     Antimatter is one of the most promising fuels for future space travel. Mainly because it is the most efficient fuel possible.  The following discussion covers how antimatter was predicted before it was even found, how it is created, and how it can be used for Starship propulsion.  If you would like a "less-technical" explanation of what antimatter is. Please read my essay, entitled What is Antimatter?  Otherwise, continue reading below:
     The Schrödinger equation of Quantum Mechanics is a highly useful equation. It only works, however, for slow moving particles (in the same same sense that Newton's laws of motion only work for slow moving objects).  As it turns out, Einstein's theory of special relativity can be applied to Quantum Mechanics as well. A Cambridge physicist named Paul Dirac combined special relativity and quantum mechanics to create a relativistic wave equation for an electron.  This joining solved one mystery: previously, electrons were observed to be able to have two different energies while in what appeared to be the same quantum state. In fact, however, the electrons were not in the same state. Dirac's equation showed clearly that electrons (actually all particles) have an additional intrinsic property.  This property is known as "spin" because it adds angular momentum to the particle (however, it is considered unlikely that the electron is actually spinning in the conventional sense of the word). Dirac showed that this spin is actually a relativistic effect, which is why it was not predicted by the non-relativistic Schrödinger equation.  Furthermore, this "spin" could take on only two values for the electron: -1/2 (called "down") and +1/2 (called "up").
     Dirac's equation, though, does not simply predict the existence of spin up and spin down particles, it actually predicts four different kinds of electrons: spin up with positive energy, spin down with positive energy, spin up with negative energy, and spin down with negative energy.  A negative energy electron would speed up as it lost energy, until it was traveling at the speed of light with an energy of negative infinity. Since particles can jump to lower energy states by emitting photons, an electron in the lowest positive energy state could emit a photon with energy equal to 2mc^2, and jump to the highest negative energy state!  The electron should, then, continue on its merry way, emitting photons and dropping ever farther into negative energy, speeding up as it did so. Dirac arrived at the startling conclusion that the world would end in 10 billionths of a second!
     Obviously, a tremendous amount of experimental data indicated that the world did, indeed, exist. Therefore, Dirac theorized that the universe was already filled with densely packed electrons (called the "Dirac Sea"). These electrons were all negative energy electrons, but because the universe was uniformly filled with them, they could not directly be observed.  Each of these negative energy electrons had the following properties: negative mass, negative energy, negative charge. Because electrons are a kind of particle known as a fermion, they obey the Pauli exclusion principle that states that no two fermions can exist in the same place at the same time in the same quantum state. Because of the Dirac sea, the normal, positive energy electrons could not turn into negative energy electrons, because that quantum state was already filled.
     So, what if an extremely high energy photon (such as a gamma ray), with energy equal to 2mc^2, promoted a negative energy electron into a positive energy electron (effectively popping it out of the Dirac Sea). Well, that electron would now exist as a normal, positive energy electron. However, since there was a "hole" in the Dirac sea, that hole would behave as particle as well.  Since the hole is the absence of negative energy electron, the hole would have exactly the opposite properties of a negative energy electron. It would have positive mass, positive energy, and positive charge! These particles would be anti-electrons (or positrons). Furthermore, if a normal electron ever encountered a positron (which was actually a hole in the Dirac sea), the electron would emit 2mc^2 energy, and fall into place in the Dirac sea.
     A graphical depiction of the Dirac sea is shown in figure 5, the hole in the negative energy states represents the existance of a positron, while the blue ball in the positive energy states represents the existence of an electron:

Figure 5: Dirac sea, showing both an electron and a positron.

     In 1932 the existence of antimatter (as predicted by Dirac's theory) was verified when Carl Anderson at CalTech discovered some positrons being produced in cosmic ray induced events.  Since then, not only has the existence of antimatter been verified again and again, but small amounts of it are actually produced on a daily basis. In fact, the existance of anti-bosons has been discovered (bozons are dissimilar from fermions, in that they do not obey the Pauli exclusion principle). Since bozons do not obey the Pauli exclusion principle, Dirac's sea doesn't work for them.  Independently, Richard Feynman and Ernst Carl Stückelberg discovered a way to solve the negative-energy problem, without using a Dirac sea. Instead, the Stückelberg-Feynman (S-F) theory utilizes the time symmetry of the Dirac equation. S-F states that negative energy electrons always travel backwards in time. To us humans, who only perceive time in one direction, a negative energy electron traveling backwards in time looks exactly as if it were a positive energy positron traveling forward in time.  Again, antimatter-matter annihilation is possible (just as it was in the Dirac sea theory). A positive energy electron may be traveling merrily along, forward through time, then suddenly emit a super energetic photon (energy equal to 2mc^2), and turn into a negative energy electron traveling backward in time. To us, it would appear as if an electron and a positron came together and disappeared, emitting a gamma ray while doing so.  To many, this theory seems a bit far fetched. However, for fermions it yields all the same results as the Dirac sea, plus it has the added benefit of working for bozons as well as fermions. Since anti-bozons do exist, one must conclude that the S-F theory is somehow closer to the truth than the Dirac Sea.

     No matter which theory of antimatter one accepts, the existence of antimatter is undeniable. Currently, both FNAL (in the U.S.) and CERN (in Switzerland) produce small amounts of antimatter every day (for a grand total of 1 to 10 nanograms a day). At FNAL (Also known as Fermilab), normal protons are accelerated to extremely high velocities by a particle accelerator. The protons are then allowed to strike a target, made of nickel, tungsten, or copper. Since the protons have so much energy (from the acceleration process), a plethora of various particles are produced by the collision.  Some of these particles are anti-protons. At FNAL, the antiprotons are almost immediately used again in scientific experiments, but at CERN some of them are actually stored. Figure 6 shows the general method currently used to produce antimatter (not just antiprotons are produced by this process, but they are the only ones that are captured):

Figure 6: Antimatter production

     The normal protons (blue) are accelerated, then smashed into a metal target. From the spew of particles, a few of the antiprotons that just happen to have the right speed and ejection angle are captured. These antiprotons are then decelerated and stored in a storage unit.
     Storing antimatter is a particularly difficult problem because it can't be allowed to touch normal matter (for obvious reasons). Although antiparticles can interact normally with normal particles of a different type (for example, collisions with electrons can be used to slow down fast moving antiprotons), this is not really useful in the trapping problem. The reason being that you can not build a container out of pure electrons, or pure protons, etc... Therefore, magnetic and electric fields must be used to trap the antimatter.

     The collision of one antiproton with one proton produces their entire combined mass in energy, as given by the equation E=mc^2. That means that one gram of antimatter is capable of producing the total energy equivalent to twenty-three Space Shuttle External Fuel Tanks. Of course, this is useless for powering Earth-based systems, because the amount of energy required to make the antimatter in the first place is enormously higher. However, it offers a perfect fuel for long range spacecraft.  The reason for this is that, traditionally, if a spacecraft needs to go a very long distance, it needs to go very fast. Otherwise, its crew would die of boredom long before it reached its destination. In order to go very fast, the spacecraft must carry a lot of fuel. The fuel, in turn, adds weight, which means the spacecraft will need an even greater increase in fuel to go faster! Obviously, before long, the fuel requirements are quite preventative to space travel. Antimatter, however, has such a high energy-density (energy yield per mass of fuel), that it provides a way for a spacecraft to actually reach a significant fraction of the speed of light.  Current ideas for "Beamed Core" pure antimatter/matter reaction drives would allow speeds of up to 120 million meters per second. That's 2/5ths of the speed of light! The antimatter requirements for this drive are way beyond our current production rate, but someday we will most likely travel to nearby stars using just such an antimatter drive.
     There are a large number of engineering problems associated with using antimatter as a fuel for starships. However, the problems are just that, engineering... there is no theoretical reason to disallow this huge energy source from being used to propel starships. The obstacles that must still be overcome are mainly in the production and storage of large (on the microgram scale) quantities of antimatter. However, two spacecraft designs are currently in the works that would utilize a hybrid antimatter/nuclear drive to allow manned exploration of our own solar system. Certainly, we shouldn't complain about not being able to reach other star systems until we've fully explored our own!
     The current drive/spacecraft concepts, and their relative ranges and fuel requirements are listed below:

Drive Concept: Spacecraft: Max Antimatter Required: Max Range Recommended:
Antimatter catalyzed
micro fission/fusion
ICAN-II 1 microgram Pluto (intrasystem)
Antimatter initiated
micro fusion
AIM Star
10 milligrams Oort cloud (10,000 AU)
slow (50 years)
Plasma Core,
pure antimatter/matter
Theoretical 10 kilograms Oort cloud (10,000 AU)
not as slow as AIM
Beamed Core,
pure antimatter/matter
Theoretical 1,000 megagrams Interstellar

     Obviously the extreamly high antimatter requirements for the "pure" drives are quite prohibitive, which is why there are no current designs for such drives. However the hybrid drives seem likely possibilities for near-term use.

Previous Topic: Quantum Mechanics
Next Topic: Antimatter: Fission/Fusion Drive