Black holes at the CERN collider - will physicists bring the earth to a premature end?

In December, I did a short post referring readers to this website: www.risk-evaluation-forum.org/index.htm. I found the site intriguing, as the author did appear to have some knowledge of physics and made arguments which, at least on the face of it, appeared plausible and concerning. It's time I did a full post about this.

Background - how black holes may be coming to your neighbourhood

As James Blodgett (the author of the above site) explains, there is a large particle accelerator being built in Europe by CERN called the Large Hadron Collider (LHC) which is expected to come "on line" in 2007. This is a huge project, which was the subject of a recent BBC background report here.

For a relatively straightforward explanation of how it is that black holes might be created by the LHC, see this article from Analog. My explanation (subject always to correction, as I am no scientist) is this:

Since at least 2001, which was presumably well after the LHC project was planned, physicists have realised that if there are large "extra dimensions" to the universe, the energies that might be required to create a microscopic black hole may be within the reach of the LHC. For papers describing this in full technical detail, try this one by Webber, and Blodgett cites others others in different parts of his website. Physicists are excited by the possibility, as it would really be a very new field of inquiry for particle physics.

The idea of there perhaps being large extra dimensions has become a popular theory relatively recently, but there has been a lot of work about it the last, I think, decade or so. (Also, see this search at the arxiv.org site to see the large amount of attention possible black hole production at the LHC is getting.) From what I understand, the large extra dimensions idea is different from, but not inconsistent with, string theory, which proposes that there are very tiny hidden extra dimensions bound up so tightly that no one has any idea yet how they can be verified experimentally.

Black holes are created by compressing stuff so tightly that its gravity prevents anything escaping, even light. (Although there may be a type of radiation from them anyway, as I will note below.) If other matter is nearby, they can suck it in and grow, as indeed is believed to be happening at many astronomically sized black holes throughout the universe. Sounds dangerous, and physicists have thought about the safety aspects of creating a micro black hole.

Back in 1999 there was a bit of media attention given to the RHIC accelerator in the USA, and whether it would cause the end of the earth, or even the entire universe, through production of a "strangelet" or other ways. A risk assessment paper was done on this, and it also considered whether black holes could be created at that accelerator. That paper is here.

The paper quickly concluded that there was no where near enough energy in that accelerator's collisions to create a black hole. They spent much more time on "strangelets", but concluded that they are a very small risk. The argument that clinches this is that cosmic rays (which are tiny, naturally produced, subatomic particles traveling at enormous speed) have been crashing into the planets and our moon for billions of years at much higher energies than what can be achieved in particle accelerators, and they have not caused the destruction of any solar system body yet. Work at lower energies is therefore also presumed safe.

There is a similar "risk assessment" style paper dated June 2002 on the LHC, which is a substantially more powerful collider.

This paper acknowledges that micro black holes might be created at the LHC, but also assumes that they will not be any danger to the earth because they are expected to evaporate (my term, not there's) almost instantaneously with their creation.

(Famous physicist Stephen Hawking predicted early in his career that black holes will, in effect, evaporate due to quantum effects at their edge. It's called Hawking Radiation, or HR in the rest of this post. The smaller the black hole, the faster the HR process, and the theory goes that a tiny black hole created in the LHC would instantaneously evaporate in a spray of subatomic particles. It would be this process by which the detectors would in fact know that a black hole had [very temporarily] been created. Black holes which absorb surrounding matter at a rate faster than they lose weight through HR will grow in size, but once there is nothing around them to "accrete", they will start losing weight again.)

The 2002 paper does not mention the 'cosmic ray' argument except in the context of strangelets. However, at the CERN website there is a page where this argument is referred to in the context of black holes too. See here, where it is said:

"It should be stated, in conclusion, that these black holes are not dangerous and do not threaten to swallow up our already much-abused planet. The theoretical arguments and the obvious harmlessness of any black holes that, according to these models, would have to be formed from the interaction of cosmic rays with celestial bodies, mean that we can regard them with perfect equanimity."

So - what's the problem?

Well, as Blodgett notes, and a search of the arxiv site confirms, that there are credible physicists who doubt that HR actually exists. Although astronomers believe they have strong evidence of massive black holes in the centre of galaxies, HR is too faint to be observed that way.

So, the fundamental problem comes down to this: the CERN risk assessment paper is based on HR definitely happening. They do not consider in any detail what may happen if a micro black hole does not disappear quickly.

What about the cosmic ray argument?

Blodgett notes that if cosmic rays create black holes, they would nearly always be doing it by a very fast particle (a cosmic ray) smashing into a relatively stationary one (a bit of the moon, say.) The micro black hole created that way should therefore have high velocity. This is quite different from the LHC process, which would create its high energy interactions by head-on collisions of 2 streams of particles traveling at similar speeds in opposite directions.

In the case of the LHC, the momentum of the particles would often cancel each other out resulting in potential particles (such as micro black holes) that are moving below the earth's escape velocity.

This is important, because all micro black holes would be so small that no one expects them to be highly interactive with ordinary matter, and a speeding one might zip through a planet in much the same way a bullet might pass harmlessly through a room full of balloons. Ones created in the LHC, on the other hand, have time to settle into the core of the earth, and lots of time to interact with matter there. It is also possible that the LHC will create hundreds of such black holes during its experiments. (Blodgett has obtained definite confirmation on this point from a physicist wrote a paper and who did correspond with him for a time.)

How fast could a stable micro black hole absorb other particles?

Blodgett readily acknowledges that micro black holes may not be capable of absorbing anything at a rate which represents a real problem anyway. His point is, however, that physicists do not seem to have done detailed work on 'worst possible case' scenarios because they assume that HR means this is just not going to be a problem in the first place.

Blodgett points out that at least one paper suggests that a black hole based on the 'extra dimensions' theory could have a larger radius than a 'normal' black hole in a universe without extra dimensions. He also raises the issue of conditions in the interior of the earth and how that would affect a micro black hole's accretion rate.

He worries, although without providing any real detail on his site, that there are some scenarios in which a dangerous accretion rate (with expotential growth of a micro black hole) is possible. Just how fast this may mean that it could eat the earth is not clear. I take it that Blodgett does not think it likely that the earth would disappear in such a hole in a day or a year. However, even if it may mean that the earth could end up as a tiny black hole within, say, 10,000 or a 100,000 years, wouldn't people be a little concerned about that?

What do I think of all this?

During the weeks since I found Blodgett's site, I have had email correspondence with him about his background and motives. He is not a physicist, but does have some qualifications in statistics and other topics. He certainly seems to have better maths than me.

He appears genuine; and not a "nutter".

What I like about his site is that he is open about being willing to be proved wrong.

He told me that he has tried to obtain some publicity for his site, with very limited success. He has tried contacting quite a few physicists, most of whom have been immediately dismissive.

I has some experience of this myself. I raised his site at a 'group blog' run by a bunch of particle physicists called Cosmic Varience. This particular thread was about what the LHC may or may not find. (Some people fear it won't turn up anything very new at all, at great expense.) My first comment was at comment no. 45. It is well worth reading the comments thread from that point on. Note the initial snide reaction of particle physicist Mark (who is one of the group bloggers).

I emailed James Blodgett and told him about the thread. He made a couple of subsequent posts; I think it is fair to say that Mark did not take well to being questioned by a non physicist. I ended with a polite request for further response to articles I found myself indicating that they is a lot of uncertainly about HR as a process, but did not get an answer.

Blodgett says this is fairly typical of the reaction he has received in contacting other physicists.

I have emailed a couple of other physicists who appear to know a lot about black hole theory about the issues (very briefly), and while I did receive an answer from one, it seemed clear that he had not read Blodgett's site in any detail.

There are threads on other physics forums about the issue, but none that I have read seem to have dealt with Blodgett's arguments in adequate detail.

My conclusions:

1. It is clear that the current published paper by the CERN safety committee is inadequate in that it bases its arguments regarding micro black holes solely on the assumption that HR does exist. For this point alone, I think Blodgett deserves praise.

2. I see no published evidence that particle physicists have taken seriously Blodgett's suggestion that the "cosmic ray" argument is a flawed analogy in the case of micro black holes from the LHC.

3. It is possible that, even without HR, accretion rates for micro black holes within the earth's interior might be so slow that the worst possible case is not worth worrying about. But again, I see no clear evidence that they have done the work of looking at "worst case scenarios".

4. It is possible that some physicists have done some calculations on the scenarios that Blodgett suggests and have formed the opinion that there really is no problem. If so, they are doing a terrible job at explaining to anyone asking the question whether Blodgett's concerns are misplaced. It certainly seems that some particle physicists simply don't like to be questioned about this. The fact that there is a huge investment at stake may help explain some of the animosity, although I am not suggesting that a physicist who realised there was a danger would try to hide it. I do wonder, though, whether it makes them not want to look into it in too much detail.

5. One other point I have not yet mentioned: if micro black holes can be created by cosmic rays and do evaporate via HR, then it should already be happening above our heads in the earth's atmosphere. New Scientist ran a story about that here. There is already work underway to see if the decay of such micro black holes can be detected. If it is, it would be a confirmation of HR really working, and the LHC could go ahead confident in that knowledge. It seems to me that a strong case could presently be made for not starting up the LHC until the search for atmospheric decay of 'cosmic ray' black holes has been given a good chance of success.

Also, a very recent article suggest that there might be another way of testing if "extra dimensions" exist. If they are confirmed, it would presumably suggest that the LHC will definitely create micro black holes, although it may not add much to the issue of whether HR works.

An appeal

This issue has not exactly caused me to lose sleep, but I have spent a fair bit of time reading on the internet about the issues, and thinking about it. It does worry me that a published risk assessment paper from CERN about the possible risk of their destroying the earth does seem to be clearly inadequate in detail when addressing micro black holes. Although I think Blodgett is not as transparent on his web site as he could be, his arguments make some intuitive sense to me.

If they re-wrote it to address the issues Blodgett makes, I would be much happier. But I get the impression that this has not yet happened because:

a. relatively few physicists think there is much risk of HR not existing (even though there seem to be quite a few credible papers on exactly this point)

b. the possibility of micro black holes being created at LHC was only realised a few years ago, and papers are still being produced at a great rate with new ideas about their possible character and fate. (Some papers suggest that a "remnant" will be left from the HR process. I guess that these are not thought to be potentially dangerous either, but the exact nature of such a particle has not been clearly explained in anything I have read);

c. Blodgett's questions are being asked by non physicists.

If any reader of this post has any contacts with physicists who are interested in addressing the issues Blodgett raises, please ask them to have a look at it for us. It's only the issue of the future of the earth at stake.

Update: in the interests of showing independence, I did not provide a copy of this to James Blodgett prior to posting. I have advised him of its existence, and invited him to comment if he thinks I have got any point wrong.

Also: only today I found this old thread on a physics forum site which contains posts by Blodgett and some responses he receives. They are well worth reading, especially the last one by him here.

Blodgett's point about the difficulty of trying to work out possible accretion scenarios when quantum gravity is not understood seems very valid. It is the apparent lack of response to these issues that worries me.

UPDATE SEPT 2008: I see that this post is still getting quite a bit of attention from people Googling for information now that the LHC is nearing operation.

Readers should be aware that I made many subsequent posts relevant to the topic after this one. As I don't tag my posts, the best way to find them is to go to the current page for my blog and use the search blog facility for "black holes".

The short story is that I had been somewhat relieved by the work published mid 2008 by Mangano that gave reasons why micro black holes could not be a danger. Then this month, I found that there was potentially a new reason to worry. The status of the Plaga suggestion of how Hawking Radiation itself might be a danger is unclear to me: Mangano says he makes a fundamental mistake, but I find it hard to follow.