Black Holes and Particle Accelerators

A general summary of the recent discussion of the existential threat from colliders, such as the LHC.

- The Large Hadron Collider is the latest shiny new particle accelerator in Europe, scheduled to come online in mid-2008. It will be able to collide protons at an energy of around 14 TeV, and heavy nuclei at much higher energies (over one thousand TeV).

- The high-energy collisions in the LHC might produce exotic new particles, such as microscopic black holes or stable strangelets. These particles could interact with regular matter and start a chain reaction, which wouldn’t stop until the Earth was destroyed.

- Particle physics is a multi-billion dollar field of research, employing thousands of people worldwide. If there is a possibility of existential risk from doing high-energy physics, it will be very difficult to mitigate this risk without clear evidence and a lot of lobbying power.

-  Cosmic rays have been observed with energies in the 3*10^8 TeV range, which is five orders of magnitude larger than anything the LHC might produce. These cosmic rays have been bombarding us since the planet was formed four billion years ago, and so we know that they are not dangerous.

- Standard general relativity predicts that a black hole must have a minimum mass on the order of the Planck energy, or 2.4*10^15 TeV. Such a black hole would decay almost instantly, and could not even be directly measured (it would decay before other particles could interact with it, much like the top quark). A black hole with a lifetime of one nanosecond, under general relativity, requires a total energy of 1.3*10^26 TeV, or around five thousand megatons of TNT.

- We know that general relativity breaks down at the quantum scale; however, we don’t have a good theory of quantum gravity that can predict where it breaks down. Some theories predict that general relativity breaks down very quickly at the quantum scale, and black holes with a mass in the TeV range should last around 10^-26 seconds, rather than the 10^-90 seconds predicted by standard theory. If this holds, the LHC could produce and detect miniature black holes, although they will be far too short-lived to represent a threat.

-  Although cosmic rays are similar to LHC collisions, there are some differences; a cosmic ray has net momentum relative to the Earth, while a proton collision does not. Cosmic rays also collide with nitrogen or oxygen atoms in the upper atmosphere, while LHC protons collide only with each other.

- If black holes are generated by cosmic ray collisions, they must be incapable of eating an entire planet or star. If a star was eaten by a black hole, the matter of the star would heat up, creating a huge radiation source with a unique spectral signature. No such sources have been observed, in this galaxy or any other one.

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