In Microcosm, CERN's science centre

LHC and black holes?

Hugely interesting? Scary? Whatever your personal approach to the unexplored is, two things are 100% sure:

  • the creation of black holes at the LHC is very unlikely

  • if black holes will be created, they will be 'quantum' black holes, that can NOT harm the Earth

Discover why collisions at the LHC are safe, through questions asked by the public and answered by experts who have accurately studied the situation

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Nobody really knows what black holes are and how they behave. They can be dangerous!

This is incorrect as a general statement. Standard black holes, namely what remains after a big star collapses at the end of its evolution, are quite well understood theoretically, and there are several examples of objects in the sky that match the expected properties very well. In the case of microscopic black holes, the correct statement is that we don't even know whether they exist!

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Is there an intention to create black holes with the LHC? How will that be done?

The LHC has not been built to create black holes. The LHC is being operated to study the behaviour of elementary particles and forces at energies higher than previously studied. One object that the LHC is expected to produce is the Higgs boson, a very innocuous particle that we believe is responsible for the masses of the other particles we already know (the electron, the quarks, etc).

The LHC cannot create the type of black holes we see in the sky as a result of the collapse of stars that run out of fuel. This is because the energy required to do that is 10 million billion times (1016) larger than the energy available at the LHC. The black holes that are under discussion are objects that could only appear if some hypothetical theory is realised in nature. This theory suggests that, beyond the three spatial dimensions we experience (plus time, which can be regarded as an additional dimension),  our Universe has some extra dimensions. These could have a "size" of about 10–16cm (0.000....001 cm, with 16 zeros in between). Since the LHC can explore such distances in virtue of its energy, if this theory is correct, then we could create them. Many theories are based on the idea that there are extra dimensions, but typically these are much much smaller, well below the reach of the LHC. Only a small subset of theories has this peculiar feature. There is no current indication in our understanding of the world that these specific theories are right, they are just speculative exercises. And, when these scenarios are put to test in experimental situations other than the LHC, one can already today conclude that it is rather unlikely that indeed sizes accessible to the LHC are allowed. Ths is because, if they were true, then other phenomena would have appeared, which have not been seen in a series of other measurements performed in the very recent years. But if black holes exist, they will be created, and if so, we'll want to study them, together with other possible new particles that other theories predict.

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What hopes lie in the creation of a black hole with the LHC?

If this happens, then it will confirm that our universe is not 4 dimensional (3 space plus 1 time dimensions), but indeed hosts other dimensions. It would be quite a spectacular philosophical outcome! In the same way that the theory of relativity or of quantum mechanics revolutionized our way of thinking, discovering the existence of extra dimensions will be a major new milestone in our understanding of the universe. But aside from this, there is no obvious application. Many people will start speculating about using these extra dimensions for space and time travel, or as a source of clean energy, and who knows what else. But we really have no solid clue to any application. It is rather like the Phoenicians, who discovered magnetism and knew how to use a compass for their navigations, but could not foresee that out of electricity and magnetism we would have built our modern civilization!

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Some scientists say that black holes are not dangerous because Hawking says they would quickly decay via the Hawking's radiation. But there is no proof of its existence. What if it does not exist?

This is not what physicists believe in general. Those few who pointed out issues with Hawking's radiation were simply trying to get a more rigorous proof of it. But no-one ever claimed that his proof of the decay is wrong, and that therefore they should be stable.However, even if they were stable, they would pose no threat. Please read the next Q&A.

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Why do you believe it is not dangerous to create them?

Several physicists in Europe as well as in the US have been studying this issue in great detail recently, as a result of the popular concern about this topic. The first reason to believe they are not dangerous is that, as said before, we believe that they will indeed decay (therefore, disappear!) very quickly, well before they get into any contact with matter. (The inner part of the LHC experiments, where the collisions take place and black holes could be created, is in a total vacuum).

However, even if they were stable, they would pose no threat. The main lines of the argument go as follows: 
The size of their potential macroscopic effects is defined by the rate at which they can accrete matter. If they accrete very slowly, then they have no time to absorb significant parts of the Earth during the 5 billion years that we have left before the Sun explodes anyway, and life on Earth will be impossible (and this is not something we can do anything about!). On the other hand, if they accrete faster, then we can check whether they could have done damage elsewhere. The universe is filled with cosmic rays, high energy particles that are emitted during the catastrophic explosions of stars. We know this, since we observe them! It turns out that many of these cosmic rays have energies much larger than the energies that we'll be producing at the LHC. So IF the LHC can produce black holes, THEN these cosmic rays can do it as well. One can show, in particular, that several old neutron stars we see in the universe have been bombarded by a great number of such cosmic rays, and that therefore they should contain such black holes. If these black holes accrete matter fast enough to damage the Earth, they would do even more damage in a neutron star. This is because a neutron star is 10 thousand billion times (1 followed by 13 zeros) denser than the Earth, and black holes would accrete their mass much faster. This would lead to a collapse of a neutron star within an amount of time much shorter than the age we measure. We know neutron stars that are as old as a billion years, and they simply could not be there if stable black holes existed!  A detailed account of the technical argument is available here. The conclusion is that there is a contradiction between the assumption that black holes are stable and that they can do harm. The contradiction is based on empirical evidence, so this is not just a theoretical speculation.

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What would happen if a black hole created at the LHC turns out to be stable?

Most of the times it would simply pass through the Earth and disappear in the cosmos. This is because black holes from the LHC collisions typically travel very fast, and because early on they interact very weakly with matter. They behave like neutrinos, which can travel through several billion kilometres of iron before they hit something. A few will be slower, and will be trapped by the Earth's gravitational field. In this case, they will start accreting matter, but at a very slow rate. Depending on the number of extra dimensions present in the theory that predicts them, after 5 billion years their mass will typically be of few tons only, sitting in the dense Earth’s core. This would have no impact on any macroscopic aspect of the Earth, be it the radiation levels, the surface temperature, the geological stability, or the magnetic field strength and direction. It would be just too small to be noticeable. The possibility that the accretion rate be faster is ruled out, as mentioned above, by the long-term stability of old neutron stars. 

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Black holes, strangelets, god knows what else. Despite the fantastic reassurance from CERN I just can't get a worst-case scenario thought out of my head. What are the odds of that, do you know?

The point here is that there is NO worst-case scenario. What should we expect to happen? It is not possible to set a probability on something that is not expected to take place in first place! Let me give you one practical example:
Let’s assume you have just bought a new electric shaver. I have never seen you shaving and therefore I fear that by shaving yourself in the morning, you could actually trigger a chain reaction that blows up the universe. This is not something our theories expect to happen, but how can you prove to me that it CANNOT (zero probability) happen? What would you do to convince me of the opposite? I think you would start by saying that many people have shaved themselves in the past, and nothing ever happened. OK, so let's look at what limits we can extract from this observation. Let's say that each day 1 billion people shave themselves, and that they have being doing this for the past 10 thousand years with all sorts of shavers. This means 104 years, times 350 days per year, times 1 billions shavings each day, giving a total of 3·1015 shaves. Since nothing has happened, empirically you can state that the probability of something wrong happening has to be less than 3·10-16. If you are willing to believe that we don't know everything about the universe, and only known facts count, you are left with this estimate of an UPPER limit to the probability that shaving can do damage to the Earth. So one more shaving has “only” less than one chance in 3·1015 of destroying the Earth. Are you still willing to shave tomorrow morning on the basis of these odds? The numbers we obtain on similar "worst case scenarios" (e.g. looking at the probability that a strangelet particle is produced) are even smaller. If you look at the previous reports, they will give you analyses (based e.g. on the study of cosmic ray productions of strangelets) leading to UPPER limits of the probability of a disaster in the range of 1 in 1022, so over 1 million times smaller than the probability of causing a mess by shaving. These are not probabilities that something wrong will happen, but UPPER limits on these probabilities, obtained by analysis based on things that have actually happened (such as cosmic ray interactions during the lifetime of the universe). Using available data, this is the best we can do, in the same way that using available data, the best limit we can set on the danger of shaving is 3·1015. If you ask however what you really expect to happen, the answer is NOTHING, as in the case of shaving. There is no theoretical expectation that a strangelet should be produced, or that it should be dangerous, if produced. 

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I remain skeptical. Is there anything else that could reassure me?

Yes. The most important thing perhaps is that, like you, we CERN’s scientists also have families, parents, children, and friends. There is no way that we'd put their lives at risk. And we are not talking about accepting "small chances", like 1 in 50 million or whatever. We are talking about wanting to be absolutely certain that absolutely nothing can happen.  

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