Knocking on Heaven's Door (28 page)

Even this isn’t a serious loophole however. Few people, if any, worried about these objects. Only black holes that can grow to be big can possibly be dangerous. Small black holes can’t accrete enough matter to pose any problem. The only potential risk is that the tiny objects could grow to a dangerous size before evaporating. Yet even without knowing exactly what these objects are, we can estimate how long they should last. These estimates yield lifetimes that are so significantly less than would be required for a black hole to be dangerous that even the very unlikely events on the tails of distributions would still be extremely safe. Small black holes wouldn’t behave very differently from familiar unstable heavy particles. Like these short-lived particles, small black holes would very rapidly decay.

Some did, however, still worry that Hawking’s derivation, although consistent with all known laws of physics, could be wrong and that black holes might be completely stable. After all, Hawking radiation has never been tested by observations since the radiation from known black holes is too weak to see. Physicists are rightfully skeptical of these objections since they would then have to throw away not only Hawking radiation, but also many other independent and well-tested aspects of our physical theories. Furthermore, the logic underlying Hawking radiation directly predicts other phenomena that have been observed, giving us further confidence in its validity.

Nonetheless, Hawking radiation has never been seen. So to be super-safe, physicists asked the question: If Hawking radiation was somehow not correct and the black holes the LHC might create were stable and never decayed, would they be dangerous then?

Fortunately, even stronger proof exists that black holes pose no danger. The argument makes no assumptions about black hole decay and is not theoretical but is based instead solely on observations of the cosmos. In June 2008, two physicists, Steve Giddings and Michelangelo Mangano,
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and soon afterward, the LHC Safety Assessment Group,
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wrote explicit empirically based papers that convincingly ruled out any black hole disaster scenario. Giddings and Mangano calculated the rate at which black holes could form and what their impact would already have been in the universe if they were indeed stable and didn’t decay. They observed that even though we haven’t yet produced the energies required to create black holes—even higher-dimensional black holes—at accelerators here on Earth, the requisite energies are reached quite frequently in the cosmos. Cosmic rays—highly energetic particles—travel through space all the time, and they often collide with other objects. Although we have no way to study their consequences in detail as we can with experiments on Earth, these collisions frequently have energies at least as high as that which the LHC will achieve.

So if extra-dimensional theories are correct, black holes might then form in astrophysical objects—even the Earth or the Sun. Giddings and Mangano calculated that for some models (the rate depends on the number of additional dimensions), black holes simply grow too slowly to be dangerous: even over the course of billions of years, most black holes would remain extremely small. In other cases, black holes could indeed accrete enough matter to grow big—but they often carried charge. If these had indeed been dangerous, they would have been trapped in the Earth and in the Sun, and both of the objects would have disappeared long ago. Since the Earth and Sun seem to have remained intact, the charged black holes—even those that rapidly accrete matter—can’t have dangerous consequences.

So the only possibly dangerous scenario that remains is that black holes don’t carry charge but could grow big sufficiently quickly to be a threat. In that case, the Earth’s gravitational pull—the only force that could slow them down—wouldn’t be sufficiently strong to stop them. Such black holes would pass right through the Earth so we couldn’t use the Earth’s existence to draw any conclusions about their potential danger.

However, Giddings and Mangano ruled out even that case too, since the gravitational attraction of much denser astrophysical objects—namely, neutron stars and white dwarfs—is sufficiently strong to stop black holes before they could escape. Ultra-high-energy cosmic rays hitting dense stars with strong gravitational interactions would have already produced exactly the sorts of black holes that are potentially possible at the LHC. Neutron stars and white dwarfs are much denser than the Earth—so very dense that their gravity alone would suffice to stop black holes in their interior. If the black holes had been produced and had been dangerous, they would have already destroyed these objects that we know have lasted billions of years. The number of them in the sky tells us that even if black holes exist, they certainly are not dangerous. Even if black holes were formed, they must have disappeared almost immediately—or at worst left tiny innocuous stable remnants. They wouldn’t have had sufficient time to do any damage.

On top of that, in the process of accreting matter and destroying such objects, black holes would have released large amounts of visible light, which no one has ever seen. The existence of the universe as we know it and the absence of any signal of white dwarf destruction is very convincing proof that any black holes the LHC could possibly make cannot be dangerous. Given the state of the universe, we can conclude that the Earth is in no danger from LHC black holes.

I’ll now give you a moment to breathe a sigh of relief. But I’ll nonetheless briefly continue with the black hole story—this time from my perspective as someone who works on related topics such as the extra dimensions of space necessary for low-energy black holes to be created.

Before the black hole controversy blew up in the news, I’d already become interested in the topic. I have a colleague and friend in France who used to work at CERN but now works on an experiment called Auger, which studies cosmic rays as they descend through our atmosphere toward Earth. He complained to me that the LHC takes away resources that can be used to study the same energy scales in his cosmic rays. Since his experiment is far less precise, the only type of events it might find would be those with dramatic signatures such as decaying black holes.

So along with a postdoctoral fellow at Harvard at the time, Patrick Meade, I set out to calculate the number of such events they might observe. With a more careful calculation, we found that the number was much less than physicists had originally optimistically predicted. I say “optimistic” since we are always excited about the idea of evidence for new physics. We weren’t concerned about disasters on the Earth—or in the cosmos, which I hope you now agree were not a real threat.

After recognizing that Auger wouldn’t discover tiny black holes, even if higher-dimensional explanations of particle physics phenomena were correct, our calculations made us curious about the claims other physicists had made that black holes could be produced in abundance at the LHC. We found that those rates were overestimates as well. Although the rough ballpark estimates had indicated that in these scenarios, the LHC would copiously produce black holes, our more detailed calculations demonstrated that this was not the case.

Patrick and I had not been concerned about dangerous black holes. We had wanted to know whether small, harmless, rapidly decaying higher-dimensional black holes could be produced and thereby signal the presence of higher-dimensional gravity. We calculated this could rarely happen, if at all. Of course, if possible, the production of small black holes could have been a fantastic verification of the theory Raman and I had proposed. But as a scientist, I’m obliged to pay attention to calculations. Given our results, we couldn’t entertain false expectations. Patrick and I (and most other physicists) don’t expect even small black holes to appear.

That’s how science works. People have ideas, work them out roughly, and then they or others go back and check the details. The fact that the initial idea had to be modified after further scrutiny is not a mark of ineptitude—it’s just a sign that science is difficult and progress is often incremental. Intermediate stages involve forward and backward adjustments until we settle theoretically and experimentally on the best ideas. Sadly, Patrick and I didn’t finish our calculations in time to prevent the black hole controversy from permeating the newspapers and leading to a lawsuit.

We did realize, however, that whether or not black holes could ultimately be produced, other interesting signatures of strongly interacting particles at the LHC might provide important clues about the underlying nature of forces and gravity. And we would see these other signals of higher dimensions at lower energies. Until we see these other exotic signals, we know there is no chance for making black holes. But these other signals themselves might eventually illuminate some aspects of gravity.

This work exemplifies another important aspect of science. Even though paradigms might shift dramatically at different ranges of scales, we rarely suddenly encounter such abrupt shifts in the data itself. Data that was already available sometimes precipitated changes in paradigms, such as when quantum mechanics ultimately explained known spectral lines. But often small deviations from predictions in active experiments are preludes to more dramatic evidence to come. Even dangerous applications of science take time to develop. Scientists might be held accountable in some respects for the nuclear weapons era, but none of them suddenly discovered a bomb by surprise. Understanding the equivalence of mass and energy wasn’t enough. Physicists had to work very hard to configure matter into its dangerous explosive form.

Black holes could even possibly be worthy of worry if they could grow to be large, which calculations and observations demonstrated won’t happen. But even if they could, small ones—or at least the gravitational effects on particle interactions just discussed—would nonetheless signal the presence of a shift in gravity first.

In the end, black holes don’t pose any danger. But just in case, I’ll promise to take full responsibility if the LHC creates a black hole that gobbles up the planet. Meanwhile, you can do what my freshman seminar students suggested and check out http://hasthelargehadroncollider destroyedtheworldyet.com.

RISKY BUSINESS

Nate Silver, the creator of the blog FiveThirtyEight—the most successful predictor of the results of the 2008 presidential election—came to interview me in the fall of 2009 for a book he was writing about forecasting. At that time we faced an economic crisis, an apparently unwinnable war in Afghanistan, escalating health-care costs, potentially irreversible climate change, and other looming threats. I agreed to meet—a bit in the spirit of tit for tat—since I was interested to learn Nate’s views on probability and when and why predictions work.

I was nonetheless somewhat puzzled at being chosen for the interview since my expertise was predicting the results of particle collisions, which I doubt that people in Vegas, never mind the government, were betting on. I thought perhaps Nate would ask about black holes at the LHC. But despite the by then defunct lawsuit that suggested possible dangers, I really doubted Nate would be asking about that scenario, given the far more genuine threats listed above.

Nate in fact wasn’t interested in this topic. He asked far more measured questions about how particle physicists make speculations and predictions for the LHC and other experiments. He is interested in forecasting, and scientists are in the business of making predictions. He wanted to learn more about how we choose our questions and the methods we use to speculate about what might happen—questions we will soon address more fully.

Nonetheless, before considering LHC experiments and speculations for what we might find, this chapter continues our discussion of risk. The strange attitudes about risks today and the confusions about when and how to anticipate them certainly merit some consideration. The news reports the myriad bad consequences of unanticipated or unmitigated problems on a daily basis. Perhaps thinking about particle physics and separation by scale can shed some light on this complicated subject. The LHC black hole lawsuit was certainly misguided, but both this and the truly pressing issues of the day can’t help but alert us to the importance of addressing the subject of risk.

Making particle physics predictions is very different from evaluating risk in the world, and we can only skim the surface of the realities pertinent to risk evaluation and mitigation in a single chapter. Furthermore, the black hole example won’t readily generalize since the risk is essentially nonexistent. Nonetheless, it does help guide us in identifying some of the relevant issues when considering how to evaluate and account for risks. We’ll see that although black holes at the LHC were never a menace, misguided applications of forecasting often are.

RISK IN THE WORLD

When physicists considered predictions for black holes at the LHC, we extrapolated existing scientific theories to as yet unexplored energy scales. We had precise theoretical considerations and clear experimental evidence that allowed us to conclude that nothing disastrous could happen, even if we didn’t yet know what would appear. After careful investigations, all scientists agreed that the risk of danger from black holes was negligible—with no chance that they could be a problem, even over the lifetime of the universe.

This is quite different from how other potential risks are addressed. I’m still a bit mystified how economists and financiers a few years back could fail to anticipate the looming financial crisis—or even after the crisis had been averted possibly set the stage for a new one. Economists and financiers did not share a uniform consensus in their prognoses of smooth sailing, yet no one intervened until the economy teetered on collapse.

In the fall of 2008, I participated in a panel at an interdisciplinary conference. Not for the first or last time, I was asked about the danger of black holes. The vice-chairman of Goldman Sachs International, who was seated to my right, joked to me that the real black hole risk everyone was facing was the economy. And the analogy was remarkably apt.