Later this month, Astronomer Royal Professor Martin Rees is releasing a new book about the risks and challenges humanity faces over the next century. The book argues that humanity's prospects depend on changing our approach to planning for the future. We should abandon such things as short-term thinking, polarizing debates, and alarmist rhetoric. That said, the book is currently in the news because Rees casually mentions that Earth could theoretically be shrunk down to just over 300 feet (100 meters) across thanks to experiments by particles accelerators.
“Maybe a black hole could form, and then suck in everything around it,” Rees wrote, reports the Telegraph. “The second scary possibility is that the quarks would reassemble themselves into compressed objects called strangelets."
“That in itself would be harmless," he explained. "However, under some hypotheses a strangelet could, by contagion, convert anything else it encounters into a new form of matter, transforming the entire Earth in a hyperdense sphere about one hundred metres across.”
This is not the first time that Rees has publically mentioned this strangelet hypothesis. It has been around in popular media long enough that it often pops up in arguments against particle accelerators. It’s either black holes or strangelets. Two law professors even used it in their argument to commission a look at the risk of the Brookhaven National Laboratory’s Relativistic Heavy Ion Collider destroying the Earth. (The risk is vastly exaggerated.)
So, is the risk of Earth being shrunk real?
Firstly, let’s talk about the nature of strangelets. Strangelets are hypothetical particles that have a surprisingly large number of quarks. In particular, they have the three lightest quarks – up and down quarks (which make up every nucleus), and a strange quark that has the same charge as the down quark but is 19 times heavier.
Strangelets could possibly be created at extremely high energy, such as in particle accelerators, but also by many natural events. They could also make up neutron stars, the extremely high-density remains of certain supernovae.
So why are they "scary"? It depends on a particular series of hypotheses.
Strangelets might be even more peculiar than thought. While most things in the universe like to be in the simplest state, with the lowest possible amount of energy, strangelets may be more stable the bigger they are. They reach stability by turning regular matter into strange matter, so, hypothetically, producing strangelets could create a chain reaction that would over time turn every atom on Earth into a strangelet and eventually condense our entire planet into a single very dense sphere.
“Innovation is often hazardous, but if we don’t forgo risks we may forgo benefits,” Rees wrote. “Nevertheless, physicists should be circumspect about carrying out experiments that generate conditions with no precedent, even in the cosmos.”
Fair enough, but so far no experiment has generated conditions that have never existed in the cosmos. Cosmic rays can hit the atmosphere with energies similar to what is produced in particle accelerators, and the much rarer ultra-high-energy cosmic rays could produce strangelets directly hitting our atmosphere. OK, they have not been discovered yet but if they can be produced at these energies, they still won’t lead to a shrunken Earth. After all, cosmic rays have been hitting our planet for billions of years and it is still here.