Imagine an advanced civilization somewhere in the universe that has developed a particle accelerator that collides electrons with Planck energy, the scale on which gravity must be quantum-mechanically described. This energy scale is no small matter for a collider as it equals ten trillion (10)19th) times the remaining mass of the proton. To achieve this energy with our existing acceleration technology, a linear collider 10,000 light years in length would be required.
Various laser plasma accelerator concepts that I worked on during my doctoral thesis can potentially shorten the required acceleration distance by a factor of 10,000 and reduce the size of such a collider to the size of the Oort cloud that surrounds the sun system. So one could optimistically imagine that a highly developed civilization could generate electron collisions with Planck energies in their home planetary system.
As it turns out, the hypothetical feasibility of such an experiment is a matter of concern to all civilizations in the universe. Let me explain.
Due to the accelerated expansion of the universe, we know that the vacuum is not empty, but has a certain dark energy density. Particle collisions at Planck energy can trigger local tunneling of the vacuum into a state of lower energy. The quantum transition between the two states can require high energies to overcome the barrier that separates the two states and to create a bubble large enough so that the energy gained by increasing its volume is converted into voltage of invested energy exceeds its surface. This surface is reminiscent of a soap bubble skin, which is blown here by burning the “fuel” of the dark energy it contains.
Triggering the vacuum breakdown in a sufficiently large bubble at the collider location would create an expanding burning front, a so-called “domain wall”, behind which the vacuum energy density is converted into heat – just like a detonation wave burns through an explosive material. This spherical burning front moves outward at the speed of light, releasing an unprecedented amount of energy into space, heating everything in its path. If all of the dark energy is converted into heat, this would bring an unlimited volume absorbed by the burning front to a temperature of 30 degrees above absolute zero, 10 times hotter and 10,000 times denser than the radiation of the cosmic Microwave background. left over from the hot big bang.
Would such a heat wave be a cause for concern? The bad news is that we wouldn’t get any warning before this cosmic disaster hit us in the face, as no precursor signal can move faster than light to alert us to the risk. But maybe this is also good news as it implies that any devastation resulting from it would be instantaneous and just as surprising as the Chicxulub impactor to the dinosaurs. We’d never know what hit us.
One way to avoid such a cosmic disaster is to enter into an interstellar treaty similar to the Nuclear-Test-Ban Treaty first signed by the governments of the Soviet Union, the United Kingdom, and the United States in 1963. The aim of the “Planck Collider Agreement” would be to protect our cosmic environment from artificially produced domain walls. Without such a contract, we could only wish that all civilizations behave responsibly when they acquire the technological maturity to build a Planck Energy Collider. We should hope that our neighbors show cosmic responsibility.
In the long term, the need to sign a contract is pressing only within our galaxy, the Milky Way, and its closest neighbor, Andromeda; it does not extend beyond the local group of galaxies. Even without a treaty signed or kept on sprawling intergalactic scales, the accelerated expansion of the universe will ultimately save us from the risk of a Planck Collider catastrophe. All galaxies beyond “Milkomeda” (the result of a possible merger between the Milky Way and Andromeda, which my colleague TJ Cox and I mentioned in a 2007 paper) will eventually move away from us faster than light. As I showed in an article from 2002, nothing can affect us when all other galaxies leave our cosmic event horizon, since all causal signals propagate at the speed of light at most. As soon as the universe ages by another factor of ten, Milkomeda will only be surrounded by a dark room.
The accelerated cosmic expansion will carry away all risky Planck colliders in distant galaxies and ultimately protect us from breakdowns in them. This is another example of Mother Nature being kind to us. We are blessed by an inescapable social distancing on a cosmic level. After all, a cosmic domain wall could be far more dangerous than COVID-19, as basic physics at the speed of light offers no way out of its burning momentum.