The benefits of black holes

Posted: Wednesday, July 02, 2008 3:48 PM by Alan Boyle

CERN

A simulation shows the pattern of particles that scientists think could be produced by a micro black hole.

What good is a microscopic black hole, and why would you make one on Earth? Can a black hole ever really be safe, even if it's the size of a quark?

Michelangelo Mangano is a theoretical physicist at Europe's CERN particle-physics center, where black holes could conceivably be created as early as next year, and such questions have taken up his time for many months.

In an exclusive Q&A, he provides answers on a cosmic scale.

Mangano is one of the authors of CERN's latest safety report for the Large Hadron Collider, which is destined to become the world's most powerful atom-smasher. The 17-mile-round underground ring on the French-Swiss border is being readied for its official startup next month or so, but the proton-on-proton action isn't likely to reach its peak energy of 14 trillion electron volts, or 14 TeV, until next year.

At that level, there's a chance that the LHC might create microscopic black holes - as well as supersymmetric dark-matter particles, quark-gluon plasma, the elusive Higgs boson (a.k.a. the "God Particle") and other exotic stuff. It was Mangano's job to update past safety reports that concluded particle colliders like the LHC posed no risk of sparking a cosmic catastrophe (for example, creating the planet-gobbling variety of big black holes).

Those previous studies took the view that micro black holes would almost instantly wink out of existence, based on the claim that black holes lost energy through a phenomenon called Hawking radiation. But critics complained that the evidence for Hawking radiation was less than rock-solid - and for that reason, a couple of those critics filed a federal lawsuit seeking the suspension of work on the LHC.

Courtesy of M. Mangano

Michelangelo Mangano is a physicist at CERN.

Answering that criticism, Mangano and a colleague of his from the University of California at Santa Barbara, Steve Giddings, wrote a heavy-duty research paper asserting that the LHC posed no catastrophic risk, even if you assumed that micro black holes remained stable and didn't emit Hawking radiation. The Giddings-Mangano study, or "GM paper," drew high praise from a panel of experts who endorsed CERN's safety report.

The most dedicated critics may not be satisfied, but Mangano hopes that the latest findings should reassure reasonable observers that particle colliders pose no threat. The Italian-born 46-year-old also hopes that he can now get back to some semblance of a normal life, although I have a feeling he's going to be kept busy at least until the LHC reaches its top energy of 14 TeV.

This week, Mangano discussed in detail what led him to conclude that Earth was safe - and explained how microscopic black holes could spark a scientific revolution. Here is an edited transcript:

Q: What new research did you conduct on this question about what the Large Hadron Collider will do, and how does it apply to this controversy over black holes?

A: We took on the suggestion that black holes could be stable, even though the overwhelming majority of experts don’t take this as a serious possibility. It’s quite clear, if you just do some simple estimates, that even in the scenario in which black holes are stable, you should be able to rule out any possible problem. Anything that could destroy the earth cannot just happen on earth, it would have to be able to happen somewhere else. So we just went around and identified what seemed to be the most promising systems we could look at, to establish this connection. In fact, many scenarios can be ruled out directly by the fact that they would have already destroyed the earth or the sun.  But for certain scenarios you need to consider other objects.

Dense objects like white dwarfs and neutron stars seemed to be the best possible candidates, because they’re very dense, and therefore if you produce black holes from cosmic rays, they would be stopped by those objects. Since they are dense, the black hole would eat matter at the highest rate. It would tend to consume such objects much faster than it would consume the earth. So we studied how stable black holes would change lifetimes of these objects.

The crucial point is this: The black holes that could be produced by the LHC would be very, very small objects. Now, the black hole absorbs matter that gets in its way, right? If you assume that the black hole only eats whatever falls into its trajectory, you find out that it would take a nearly infinite amount of time before it could do any damage to earth. It just cannot grow fast enough, because it’s too small.

The only way that you could actually do something macroscopic is by drawing in matter from a much, much larger distance. Since this is the condition under which a black hole can become dangerous, and since it requires the distance at which it affects the matter surrounding it to be large, you only need to understand very basic features of how a black hole works. The starting point for this research is to assume our ignorance about the quantum state of a black hole – whether it’s stable, whether it decays, whether there is Hawking radiation.

We know that in order for the black hole to do anything macroscopic, it has to behave as a big object. It has to have an effect at a large distance. But at a large distance, we don’t worry about the microscopic state of a black hole. It’s all electromagnetism, it’s fluid dynamics, it’s the standard physics that we know very well.

We can arrive at very solid conclusions that do not presume knowledge of physics beyond the Standard Model on a microscopic scale. This is the main contribution, if you wish, of our work – aside from working out the implications of these observations in detail.

Q: But how did you match up your conclusions about the macroscopic effects of a black hole with observations of the universe itself? Did you do a survey of neutron stars?

A: We know what the rate of cosmic rays is, how they permeate the galaxy. Then we look at very specific neutron stars or, better yet, white dwarfs. We’re not doing a statistical analysis. We’re merely looking at a specific object. And we ask ourselves, “How long has that object been there?” For example, we have a very reliable estimate of the age of a white dwarf, based on its temperature and mass – macroscopic parameters that are well-measured and well-understood by astronomers.

And then we ask, “How many cosmic rays with energy beyond the energy of the LHC have hit that star, and how many black holes would have been produced inside that object, if black holes can be produced at the LHC?” We find numbers that are very large – numbers that are in the hundreds, in the thousands.

In parallel, we calculate how long one of these black holes would take to destroy that object. And we find numbers that are on the order of anything between a few years and perhaps a hundred thousand years or 1 million years. It’s basically impossible that this star had not been hit by a cosmic ray that would have produced a black hole. And if it produced a black hole that was capable of consuming the star, the star would be gone. It would not be there.

Q: Maybe we can back up a bit and ask how a microscopic black hole is created in a particle accelerator. Does the impact of the protons lead to the gravitation collapse of a particle to such a degree that it creates a black hole, like a star collapsing? Or is it a different mechanism?

A: It’s slightly different. Two quarks – one quark from one proton, the other from the other proton – come together with the very high energy that they inherit from the protons that contain them. They come very, very close to each other. From Einstein’s theory of general relativity, we know that it’s the energy density that curves spacetime. It’s the mass in E=mc². If you have mass, or you have energy, it’s the same. So if you manage to concentrate enough energy in a very small amount of space, then you curve spacetime, and beyond a given curvature you create a small spatial region that we call a black hole.

This isn’t like the collapse of a star, where you have no radiative pressure after running out of fuel and it falls onto itself under its own gravitational field. Here, it’s shooting two particles very close to each other at very high energy to create this huge energy density.

Now, if we just have the universe as we currently know it, in order to create such a spacetime region, it would require an amount of energy that is 1 million billion times bigger than the LHC. That’s energy on the order of 10¹⁹ GeV [giga electron volts] – much higher than anything that is achievable today.

On the other hand, some speculative theories say that there are more dimensions in addition to the four dimensions of the regular universe we know. If those theories are true, then the force of gravity could become much stronger at very short distances. The gravitational force between these two particles would become much bigger. Therefore, there would be a much higher potential for curving spacetime and producing this black hole even with energies as low as those accessible at the LHC.

So in order for the LHC to produce some of these black holes, we really have to go beyond the normal theory of gravity. We have to assume that there are extra dimensions. By the way, there are many theories that have extra dimensions. Not all of them would give rise to black holes at the LHC. It’s only highly fine-tuned ones that make this possible.

Q: How would these black holes be detected? I assume that you wouldn’t detect them directly, but you’d detect them through their decay products.

A: This is true of pretty much every particle that we produce at this accelerator. Even the top quark is not detected directly, because it decays within 10^-24 seconds. What we see are the decay products. It’d be the same for a black hole. It would decay on a time scale that is about a factor of thousands smaller than that of the top quark. The main feature of a black hole decay is that there would be no bias in the particles coming out of the decay.

The final state would be relatively spherical, with no specific direction. There’d be a uniform distribution, with many highly energetic particles of all different kinds: electrons, muons, quarks, photons. This is something that the typical proton-proton collision would not give rise to. It would be a very distinctive signature.

Q: Is there any expectation of how long it might take to have a confirmed detection of black holes? Does it usually take a couple of years?

A: This would be a very spectacular signature. The number of events would be quite large. So it’s not unreasonable that two or three years could be enough to draw this conclusion. It all depends on the mass, because if the black holes exist, they cannot be arbitrarily light. Otherwise we would have seen them already at the Tevatron.

We haven’t seen them at the Tevatron, so they have to be at least a few TeV. In fact, from other collider limits on extradimensional gravity, we know they must have mass higher than about 5 TeV. If they are close to this, their production rate would be large, and they would be produced abundantly early on. Given the spectacular nature of the final state, I believe there could be a conclusion within a couple of years.

In fact, it could be easier than detecting the Higgs boson. We talk a lot about the Higgs, but the Higgs is not the simplest thing to observe. Supersymmetry could be discovered before the Higgs. Extra dimensions and black holes could be discovered before the Higgs.

Q: And because black holes would imply that extra dimensions exist, that would be a signal achievement for physics. Would that provide the first evidence that all this crazy talk about extra dimensions is true?

A: That would certainly be the most compelling indication that indeed we live in more than four dimensions. Philosophically, that would be at the same level as special relativity or quantum mechanics. That would be a major revolution in our view of the universe – well beyond the Higgs, well beyond supersymmetry and anything else that we have thought of.

Q: I know this is the question that physicists hate, but would there be any implications for daily life? Could extra dimensions lead to new energy sources, for example?

A: I really have no idea what we could get out of extra dimensions. But there is one element of our universe that we don’t understand very well, and that is gravity. Being able to do experiments exposing extra dimensions would for the first time provide us with a new observatory for gravity.

So far, the only place where we can measure gravity is in the macroscopic universe, where we measure the motion of planets and other large objects, or the expansion of the universe itself. But it all follows from the same law of gravity: Newton’s law or, if you wish, Einstein’s theory of general relativity. By looking at how spacetime works at very short distances, we get an entirely different picture. God knows what we will uncover.

All of these discussions about traveling forward and backward in time, or wormholes, you name it – all of these ideas, however bizarre, if they work at all, could be exposed by phenomena in the framework of extra dimensions.

When we’re talking about the proton-proton collisions at the LHC, one manifestation of extra dimensions would be the production of black holes. Maybe there are other manifestations. Maybe you could alter the fabric of spacetime, for example. But again, the risk is something we can rule out, because destroying the fabric of spacetime is not something that would happen only at the LHC. If you look at the much more energetic cosmic-ray collisions taking place out there in the universe, if any of those had created a problem with spacetime, we would know it by now.

So when I say “we don’t know what will happen,” it doesn’t mean we have a new source of uncontrollable risk. I’m simply saying that the picture of the universe that we will see could be very interesting.

You know, 5 billion years from now, the sun is going to blow up. There is nothing we can do about that. This will be worse than global warming. It will be worse than a meteorite hitting Earth. If we want to survive for more than 5 billion years, we have to find a way out of this place – and the way to get out of this place is not just by building more powerful rockets. We have to understand more about spacetime and how to travel in a much more efficient way.

I’m not saying this will come out of the LHC. But it’s quite clear that, if it finds black holes, the LHC will be one of the steps in this direction.

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