A reader emailed me to ask if I had any opinion on the Large Hadron Collider (LHC). I didn't, so he sent me a link to a forum that discusses concerns about the project's safety. I spent a little time over there, and while I am far, far from being an expert on these issues, my layman's impression is that the pro-LHC people know what they're talking about, and the anti-LHC people seem to be rather confused.
The biggest fear cited by the anti-LHC crowd is that a micro black hole will be created by the collider, and this MBH will stayed lodged in the Earth until it eventually devours the planet. The usual comeback is that cosmic rays are interacting all the time in the Earth's atmosphere, and no planet-killing MBHs have been created so far. Naysayers respond that a MBH created outside the Earth's gravitational field would speed harmlessly away, while one created within the Earth's gravitational field would stay with us and gobble us up.
There are at least two problems with this argument. The first is it doesn't matter where the particle collisions take place. As one poster – a physicist working on the LHC, with the screen name "North of the North Pole" - wrote,
[T]here is absolutely nothing different between hadron collisions in the center-of-mass frame that you happen to be sitting in, and the center-of-mass frame that is moving with respect to you. (Don't believe me? Listen to Albert Einstein who showed it was true over 100 years ago).
I'd be happy to view over some of [the critics'] more eloquent theories as to why special relativity happens to be violated in this particular case, whereas a hadron-hadron collision produced at rest with respect to the third rock from our average yellow sun [is] somehow different than hadron-hadron collisions produced with some velocity with respect to us.
The other problem is that any MBHs produced by cosmic-ray collisions would almost certainly disappear in a split second. If they did not – if they instead went careering through the universe – they would eventually be attracted to the powerful gravitational field of a neutron star, and would swallow up that star. Under even the most conservative projections, there would be enough wandering MBHs to snap up any given neutron star after (at most) 100 million years. Thus, 100 million years would be the upper limit of a neutron star's lifespan. In fact, however, neutron stars that are a billion years old have been identified.
In short, even if MBHs are produced by cosmic radiation – or by the kinds of collisions taking place in the LHC - they will wink out almost instantly. Moreover, it should be pointed out that there is no certainty that MBHs even exist, much less that they can be produced at the LHC's energy levels.
Another concern is that the collider will produce strangelets, which will convert our planet to "strange matter." The problem with this view is that there is another collider already in operation, the Relativistic Heavy Ion Collider or RHIC, which would be more likely to produce strangelets than the LHC. The RHIC has been running since 2000, with no ill effects.
The RHIC is less powerful than the LHC, so why would it be more likely to create strangelets? A poster explains:
[T]he production of strangelets is not directly correlated to the power of the accelerator. It is actually inversely proportional to it….
The theory behind strangelets determines that, if they are actually real, they will be produced at greater rates if there is a greater baryon density…. So, the higher the baryon density, the higher the probability of these strangelets forming because you will actually have the particles you need lying around. Another thing essential for the formation of strangelets is that they actually need to form; meaning that the dissociation energy of the strangelet (the energy needed to break up the strangelet) has to be greater than the kinetic energy in the collisions. Otherwise they wouldn't be stable, and any particle colliding into it would provide enough energy for the system to simply dissolve. Now, with the LHC, the energies are greater …, and this means that the kinetic energy will be greater than that at the RHIC. Thus, because the RHIC has not produced any strangelets, the LHC will be even less likely to. Going back to our earlier point; the baryon density will also be lower. Even assuming an equal amount of baryons, the volume of the LHC is much bigger than the RHIC. So, you have the same amount of baryons over a greater volume, giving less baryon density and thus a smaller probability of producing strangelets.
Yet another concern is voiced by those who fear that the LHC will be "re-creating the Big Bang," as some media outlets have claimed. Re-creating the Big Bang certainly sounds dangerous, but the reality is that the LHC will be re-creating only certain conditions believed to be associated with the Big Bang. It does not have anything near the power of the actual Big Bang, of course.
Some of the critics seem to be quite ignorant of even high school physics. One befuddled person, when informed that cosmic ray collisions are going on all the time, for instance in the sun and moon, responded:
But there isn't gravity on the moon and sun…
Oops! Yeah, there is. Any object with mass exerts gravitational pull.
Trying to rescue his credibility, the poster follows up:
I meant there was far less than there is on earth, haha…
Ha ha, yourself, but you're still wrong. The sun has far more mass than the Earth, and thus a stronger gravitational field, which is why the Earth orbits the sun and not vice versa.
Another poster cites a supposed "expert" who said:
[C]osmic rays have a limit of 10^12 GeV, while the collider will produce 1.4 х 10^13, i.e. more than one order higher.
Prompting this response:
10^12 GeV = 10^21 eV
The LHC operates at 14 TeV = 1.4 X 10^14 eV.
Someone's units are way off.
In other words, the so-called expert's argument is like saying that a book weighing 32 ounces is heavier than a boulder weighing 16 tons. After all, 32 is bigger than 16! The energy level of the LHC is 14 tera electron volts, which is far lower than the energy level found in cosmic radiation. (10^14 is much less than 10^21.)
Then there are the people who fret because scientists refuse to affirm that the LHC is 100% safe – that there is absolutely no possibility of something going wrong. What these folks don't realize is that scientists are trained to think in terms of probabilities, not absolutes. There is a nonzero possibility of almost anything happening, but most of these nonzero possibilities are so remote as to be effectively impossible. For instance, there is a nonzero possibility that a spaceship will land on the White House lawn and Donald Duck will waddle out. But the actual chance of such a thing happening is so small that it's hardly a possibility at all except in a theoretical sense.
Overall, it seems to me (again, purely from a layman's perspective) that the Large Hadron Collider poses no real risk, and that media alarmism and Internet scare stories have created a phony issue.
A good summary of the LHC experiments, emphasizing safety aspects, is found here.