# Vacuum Stability

In the distant past, deep within the very first second after the start of the big bang, the vacuum state of the universe has had a higher energy than it has now. During the phase transitions that have occurred since then, this extra energy was converted into thermal radiation. It is conceivable that our current vacuum is still not the lowest vacuum possible. If that is the case, a transition to a lower vacuum state will be catastrophic: not only will it liberate enormous amounts of energy, it will also alter the very properties of the particles that make up all known forms of matter. Given this dramatic, if remote, possibility, it is worthwhile to ask under which conditions such a vacuum phase transition could be triggered.

### Limits from Highest Energy Cosmic Rays

On Earth, the most energetic reactions between elementary particles take place in collisions between the highest energy cosmic rays and nucleons in the upper atmosphere. In the center of mass frame, such collisions can have energies up to 1 Pev. At present, these are far more energetic than the collisions that are produced in laboratory accelerators, where collisions barely exceed 1 TeV. However, in future generations of particle accelerators, energies beyond 1 PeV could be attained, later in this century. If so, such experiments would create conditions that have never before been achieved naturally on Earth. Would it be possible, in principle at least, that such experiments would threatened the vacuum as we know it?

Our conclusion was that there was no need to worry: plenty of collisions between two of the highest energy cosmic ray particles have taken place in our past light cone, with energies up to 100 Eev (100,000 Pev).

### Limits from Heaviest Cosmic Rays

Although our analysis made a convincing case against the possibility that elementary particles in accelerators could trigger a vacuum transition, it did not directly address what would happen in laboratories accelerating much more complex objects, such as heavy ions. I extended our analysis to this case, in the paper:

• Is It Safe to Disturb the Vacuum? by Hut, P., 1984, Invited Talk at Quark Matter '83, the Third International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, Brookhaven, NY, September 1983, ed. T.W. Ludlam and H. Wegner (Amsterdam: North Holland Publ.); in Nuclear Physics A418, 301c-311c.

My conclusion was that there was still no reason to worry, but that it would be interesting to probe the composition of the highest energy cosmic rays, in order to sharpen the limits even further.

Many years later, our results were quoted in a report commissioned by John Marburger, then Director of Brookhaven National Laboratories, to address various disaster scenarios' that had been put forward, ours being one of them. See Review of Speculative `Disaster Scenarios'' at RHIC, by Jaffe, R.L., Busza, W., Sandweiss, J., and Wilczek, F, 2000, Rev. Mod. Phys. 72, 1125-1140.

On a lighter note, around the same time physicist Gregory Benson published a science fiction story, Cosm (1999, Avon Eos), about the creation of a whole new universe through a vacuum phase transition triggered in a particle accelerator.