Particle Pair Production in Deep Space

Many of you know that a matter-antimatter reaction results in a pair of gamma rays. Fewer of you will know that you can take a couple of gamma rays, run them into each other, and get a pair of matter-antimatter particles. This has been done experimentally, and there’s a bit of data about it under “Two Photon Physics” in Wikipedia. Generally, if a subatomic reaction can occur, then it’s reversible. Maybe not statistically probable, but still reversible. This is a concept I used in a story I recently sold to Analog SF. In an area of space with high-density, high energy gamma rays, you’ll get a lot of positrons and anti-protons produced (more positrons, since they are 1/2000th the mass, of course), but there will also be some small production of antihydrogen if the antimatter doesn’t recombine right away with normal matter. And the antihydrogen may be neutral enough to survive and drift in deep space for a while, maybe long enough to be used as a resource.

Some reactions result in the release of more than two photons. A particle and antiparticle meet, three photons are emitted. The photons are lower energy, but the reverse reaction, 3 photons meeting, is a much, much lower probability than 2 photons (gamma rays) meeting. Still, on rare occasions, it might happen.

In fact, it’s my belief that if you have enough photons, even low-energy photons, passing through the same bit of space at the same time, you can also have pair-production, spitting out particles and antiparticles. One calculation for photons from the cosmic microwave background radiation (CMB) estimates 400 photons per cubic centimeter, average, plus whatever higher-energy visible light and gamma rays pass through from billions of stars. And there are a lot of cubic centimeters in a light-year (about 4.9 x 1050). Even if the probability of pair production is very, very low, I still imagine that it would happen on occasion.

As a side-note, the probability of a positron and electron meeting in deep space is very high, since they attract one another, while the probability of two gamma rays meeting at just the right time in just the right way is fairly low. The reaction looks symmetric, but the probability of it happening in a certain direction is much higher one way than the other. Ditto for any two-particle reaction that creates three particles. This contributes to the increased entropy of the universe and the “arrow of time”; there’s a preferred direction for these subatomic reactions to occur.


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