A decade ago, and even right after the Higgs discovery, lots of us were giving numerous popular talks about the collider. One of the laymen's questions that kept on reappearing was: Will the LHC destroy Earth? Will it eat all of us?
It may sound ludicrous but the energy of the collisions is extreme enough and physicists have been obliged to think seriously about this question. And many physicists did. Well, they have thought about it already a decade before the LHC.
In particular, in 1999, the not yet Nobel prize winner Frank Wilczek and three co-authors published the preprint
Just to calm you down, all the complete enough arguments end up with the conclusion that the Earth has to survive such experimentation – and it has surely survived 10 years of the LHC collisions, including several years at 13 TeV (the total energy in the center-of-mass frame).
The arguments that the experiments are OK may be ordered along the "fineness of the analysis". The finest arguments deal with the precise theories describing the hypothetical dangerous objects produced at the accelerators and their fate. For example, one may discuss the properties of strangelets (macroscopically large nuclei – matter about as dense as the neutron star – with strange quarks inside) or black holes (those evaporate by the Hawking evaporation quickly, or have a small chance to eat something before they fly away, and there are other guarantees).
The advantage of these explanations is that you see "what is exactly happening". On the other hand, such arguments in favor of the safety of the LHC depend on some theories that you could consider "untested". The Hawking radiation is nice but we can't really be sure that it exists, can we? Well, I would say that I am sure but other people could fail to be persuaded because they don't understand arguments rooted in profound theoretical physics well enough. So if the LHC produces black holes but those don't evaporate?
If you allow such speculative combinations of assumptions, it's possible to find a loophole in any argument and determine that "the gadget could be dangerous, after all". However, you still have the practical or historical, less fine arguments. In 1999, Wilczek et al. have noticed that the Moon still exists. This makes it almost certain that even after billions of years of high-energy collisions involving cosmic rays, the strangelets inside celestial bodies, even if they exist, don't become as large as the Moon. So even if they exist, they are well smaller than the Earth, and therefore safe.
I don't want to repeat all such arguments. There are lots of subtleties, loopholes, counter-loopholes, and so on. There have been lawsuits and some anti-LHC activists were actually hoping that a judge would prevent the LHC from operations. They were not jobs, right? Some sort of quasi-chimps who couldn't have possibly completed their high school studies.
However, as Gordon Wilson has pointed out that famous astronomer Martin Rees is releasing a new book:
On the Future: Prospects for Humanity (Princeton University Press)And The Telegraph promotes the book in the article
Earth could shrink to 330ft across if particle accelerator experiments fail, top astronomer warns (The Telegraph)Yes, Rees is telling us that the particle colliders may compress our Earth to a round strangelet of diameter 100 meters. Like Gordon, I don't know what to say, it sounds crazy. It is not clear to me whether he really believes it or whether he has converged to a great idea to earn some money from a new book that contains this sort of fearmongering.
Science Alert: a press release
In both cases, it's pretty bad. In some sense, I think that Rees' fearmongering is even worse or more stupid than the fearmongering by the professional LHC alarmists a decade ago because those alarmists at least reacted quickly. Rees has needed 10 more years to think about the question – and he's ended up with a book that promotes these almost "rigorously disproven" scenarios. I don't even see any added value or original ideas contributed by Dr Rees.