Imagine an advanced civilization somewhere in the universe, which developed a particle accelerator that collides electrons at the Planck energy, the scale where gravity must be described quantum mechanically. This energy scale is no small feat for a collider, as it corresponds to ten quintillion (1019) times the rest mass of the proton. To reach this energy with our existing acceleration technology would require a linear collider 10,000 light-years in length.
But various concepts of laser plasma accelerators, on which I worked during my Ph.D., can possibly shorten the required acceleration distance by a factor of 10,000, shrinking the size of such a collider to the size of the Oort cloud that surrounds the solar system. And so, one could optimistically imagine that a highly advanced civilization could generate electron collisions at Planck energies within its home planetary system.
As it turns out, the hypothetical feasibility of such an experiment is a reason for concern to all civilizations in the universe. Let me explain.
Based on the accelerating expansion of the universe, we know that the vacuum is not empty but has some dark energy density. Collisions of particles at the Planck energy can trigger a local tunneling of the vacuum to a lower energy state. The quantum transition between the two states may require high energies to overcome the barrier that separates the two states, as well as to produce a big enough bubble so that the energy gained from the increase in its volume would exceed the energy invested in the tension of its surface area. That surface is reminiscent of a soap bubble skin, here being blown by burning the dark energy “fuel” inside of it.
Triggering vacuum decay in a sufficiently large bubble at the collider location would produce an expanding burning front, a so-called “domain wall,” behind which the vacuum energy density will be converted into heat—just as a detonation wave burns through an explosive material. This spherical burning front will move outward at the speed of light and release an unprecedented amount of energy into space, heating everything along its path. If all the dark energy is converted to heat, it would bring an unlimited volume swept up by the burning front to a temperature of 30 degrees above absolute zero, 10 times hotter and 10,000 times denser in energy than the radiation of the cosmic microwave background, left over from the hot big bang.
Would such a heat wave be a reason for concern? The bad news is that we would not receive any advance warning before this cosmic disaster hit us in the face because no precursor signal can move faster than light to alert us to the risk. But perhaps this is also good news, since it implies that any resulting devastation would occur instantly and be as surprising as the Chicxulub impactor was for the dinosaurs. We would never know what hit us.
One way to avoid a cosmic catastrophe of this type is to establish an interstellar treaty, similar to the Nuclear Test Ban Treaty, signed first in 1963 by the governments of the Soviet Union, the United Kingdom and the United States. The objective of the “Planck Collider Treaty” would be to protect our cosmic environment from artificially produced domain walls. With no such treaty, we could only wish that all civilizations would behave responsibly when they acquired the technological maturity to build a Planck-energy collider. We would have to hope that our neighbors would exhibit cosmic responsibility.
In the long term, the need to sign a treaty is only pressing within our galaxy, the Milky Way, and its nearest neighbor, Andromeda; it does not extend beyond the Local Group of galaxies. Even without a treaty signed or honored on extended 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 an eventual merger between Milky Way and Andromeda, which my colleague T.J. Cox and I named in a 2007 paper) will eventually recede away from us faster than light. As I showed in a 2002 paper, once all other galaxies leave our cosmic event horizon, nothing happening within them could affect us because all causal signals propagate at most at the speed of light. Once the universe ages by another factor of ten, Milkomeda will only be surrounded by dark space.
The accelerated cosmic expansion will carry away from us all the risky Planck colliders within distant galaxies, ultimately protecting us from any mishaps in them. This constitutes yet another example of Mother Nature being kind to us. We are blessed by an inescapable social distancing on a cosmic scale. After all, a cosmic domain wall could be far more dangerous than COVID-19 because fundamental physics offers no escape from its burning sweep at the speed of light.