AAS: Scientists have spent billions of pounds building colliders that make a few micrograms of antimatter yet you’ve found antimatter occurring naturally around the Earth. How does it get there?
JB: Antimatter forms when atomic particles travelling near the speed of light collide with one another and convert their energy of motion into matter. If they are travelling fast enough; a process called pair production creates a regular particle and its antiparticle by converting the kinetic energy of motion into mass. Outside of particle colliders, there are very few places on Earth where there is enough energy to create antimatter. The Earth is constantly being bombarded by very high energy cosmic rays which are formed outside of our Solar System. When these galactic cosmic rays strike our atmosphere, their energy of motion can be converted into antimatter. Most of it gets lost in the atmosphere, but a small fraction bounces back into space and gets caught in the magnetic field of the Earth. This creates a steady supply of antiprotons which can populate the radiation belts and coexist with the regular Van Allen radiation belts around the Earth.
AAS: Is there enough antimatter in the antimatter belts found to do anything with?
JB: The amount of antimatter trapped around Earth is comparable to the amount of material in a speck of dust. This may sound like an incredibly small amount by normal standards, but antimatter has unique properties which can make this useful for a number of applications. In particular, when matter and antimatter come into contact, they annihilate and their mass is converted into energy. The process can also trigger other reactions that can be leveraged in a number of ways. Proposed applications include medical treatments, non-destructive material testing, fundamental physics, and of course spacecraft propulsion. It would take hundreds of kilograms to propel a spacecraft to another star if used like a traditional rocket fuel. However, micrograms of antiprotons can be used to catalyze other reactions for missions outside our solar system which couldn’t be reached with traditional propulsion approaches.
AAS: Is there any way we can collect the antimatter to use as a power source for space travel or other activities in space?
JB: The challenge has always been how to collect enough antimatter and then store it for use since it is spread so diffusely in space and it will annihilate when it comes in contact with ordinary matter. As part of my NASA Institute for Advance Concepts (NIAC) program, we looked at how you could use large magnetic fields around spacecraft to funnel and then collect the natural antimatter background in space. The magnetic field can then be used as a bottle to store what is collected until it is ready for use. The spacecraft could basically mine the antimatter from space and then use it to propel itself. Although there isn’t enough antimatter to propel the spacecraft to near light speed, there is enough to fuel some spacecraft concepts that would enable aggressive exploration to the outer solar system and beyond.
AAS: Do you think antimatter can be found around other planets?
JB: By any standards, the amount of antimatter around Earth is still minuscule. However, there is significantly more in other parts of the solar system. During the NIAC study, we evaluated each of the planets and found the Saturn was by far the best place for antimatter to collect in the solar system. This was surprising at first since I had originally assumed that the biggest planet, Jupiter, would have the most. However, it turned out that Jupiter’s magnetic field was just too strong and it reduced the flux of cosmic rays from striking the atmosphere. However, the rings of Saturn have just the right geometry and composition to create antiprotons, and the magnetic field of the planet works to trap it where it can then be collected. Building and then getting a collection system to Saturn would be challenging to say the least, but it is an interesting theoretical problem knowing that such a supply exists in our cosmic neighbourhood.
AAS: Why is antimatter only in very short supply? Why micrograms and why not kilograms of it?
JB: The unique properties of antimatter are also what make it so difficult to create and store. It contains an incredible amount of energy, which also means that it takes an exorbitant amount of energy to create. Even if the production process was 100 percent efficient, it would take years of electrical output from a large nuclear power plant to create the energy contained within a kilogram of antimatter. Once you solve the production issue, you’re still left with the storage problem of how to contain a material that will annihilate when it comes into contact with the walls of its container. When you calculate how inefficient it is to create and store with today’s technologies, it quickly becomes clear that it is impractical, if not impossible, to have large quantities of antimatter around.
AAS: Is antimatter overhyped or can studying it help us to understand new things about the universe?
JB: For the most part, propelling spacecraft to near the speed of light with antimatter lives in the realm of Star Trek. The technical obstacles are non-trivial and probably won’t be solved in the near future, if ever. From this perspective, the potential for antimatter probably has been overhyped. However, the small scale experiments are just the first baby steps that could help us down the long path. More importantly, research and development in this area is part of a broader framework that could help fundamental science and our understanding of the universe. Antimatter plays a central role in some of the Holy Grail problems of physics, such as the nature of dark matter and why matter dominates over antimatter.
Image Credit: NASA