A query for any lurking physicists


in Geek stuff, Science

I was having a conversation this afternoon about the Tunguska event: a huge explosion that occurred in Russia in 1908. I had always heard that it was caused by a meteor impact, though apparently some other explanations have also been considered. One that I just heard about is the possibility that it was caused by a collision between the earth and some antimatter.

Wikipedia suggests that this explanation isn’t credible, but it does leave me wondering: in the event of a collision between a particle of matter and a particle of antimatter, where the two particles are traveling at different velocities in opposite directions, how does the net momentum of the two translate in their annihilation? If antimatter did hit the earth, it would start to strike particles of matter in the upper atmosphere, the particle pairs would annihilate one another, and energy would be released. Would all that happen before the antimatter hit the ground? Presumably, it would strike its mass in antimatter before then. If so, what would the effect on the surface of the planet be?

Conservation of energy dictates that the kinetic energy in the faster particle would need to go somewhere. Presumably, it would manifest in the production of more energy during the annihilation event. As such, I suspect the antimatter clump would get blasted apart in the upper atmosphere and produce some kind of horrible shower of radiation, though nothing in the way of direct physical debris.

{ 2 comments… read them below or add one }

. January 10, 2008 at 10:48 am

Electron-positron annihilation occurs when an electron and a positron (the electron’s anti-particle) collide. The result of the collision is the conversion of the electron and positron and the creation of gamma ray photons or, less often, other particles. The process must satisfy a number of conservation laws, including:

* Conservation of charge. The net charge before and after is zero.
* Conservation of linear momentum and total energy. This forbids the creation of a single gamma ray.
* Conservation of angular momentum.

As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by elastic scattering.

. January 10, 2008 at 10:49 am

If the electron and/or positron has appreciable kinetic energies, other heavier particles can also be produced (e.g. D mesons), since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. It is still possible to produce photons and other light particles, but they will emerge with higher energies.

At energies near and beyond the mass of the carriers of the weak force, the W and Z bosons, the strength of the weak force becomes comparable with electromagnetism. This means that it becomes much easier to produce particles such as neutrinos that interact only weakly.

The heaviest particle pairs yet produced by electron-positron annihilation in particle accelerators are W+/W- pairs. The heaviest single particle is the Z boson. The driving motivation for constructing the International Linear Collider is to produce Higgs bosons in this way.

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