A black future

Shortly after the first of the year (if not already), the Large Hadron Collider — the most powerful particle accelerator ever built — will smash protons together at record energies. If the Earth remains intact, doomsayers will once again have been falsified. Every time they forecast the demise of the planet, those prophets of Earthly annihilation prove themselves no more foresightful than mortgage bankers or phony psychics.

This time, the fear of physics focuses on the prospect that the LHC, housed in a tunnel circling beneath the Swiss and French countryside outside Geneva, will condense mass-energy densely enough to create small black holes. Since black holes gobble up any matter that enters them, digesting it in a bottomless gravitational pit, perhaps Geneva, then France and Switzerland, the rest of Europe and the entire planet might all be swallowed and then shredded to subatomic smithereens, a handful of litigious LHC critics have contended.

Do not worry, physicists respond. But not because the LHC is hopelessly incapable of producing anything so dreadful as a baby black hole. To the contrary, many physicists actually hope very much that the LHC will indeed produce black holes — too small to be dangerous, of course, but desirable as a sign of new physical phenomena to study (SN: 9/26/09, p. 22). After all, the LHC may be the only way scientists can learn the deepest secrets of the universe — how it began, what it’s made of, why there’s such a thing as mass.

Others, though, may question whether the LHC is worth any risk at all — whether anything it might find out would really matter to humankind. The answer is yes. In fact, humankind’s future may depend on what the LHC discovers. And it’s a future in which black holes could play a starring role.

In this context, the future is the future envisioned by the visionaries of science fiction, the future in which the human race explores the galaxy. Sober analysis of this plan inevitably concludes that the ability to power long-range, human-occupied space vehicles exceeds earthly energy capabilities. But one proposal on the books (actually, the physics website arXiv.org) identifies a strategy that might make the science fiction future possible. Interstellar spaceflight, in this scenario, would depend on harnessing the power of the tiny black holes that scare the quarks out of people who fear the LHC.

Hawking power

In a paper online at arXiv.org/abs/0908.1803, physicist Louis Crane and collaborator Shawn Westmoreland explore the criteria for building black hole–powered starships.
“We think the possibility should be studied carefully,” they write, “because it would have profound consequences for the distant human future, which no other proposal based on currently known physics could duplicate.”

Currently known physics is, in fact, a source of great pessimism for advocates of long-distance space travel. Living things are acutely vulnerable to cosmic rays and other radiation streaming through space. Any starship capable of protecting its crew would require immense amounts of shielding, on the order of 400 metric tons for a single capsule. That’s a prohibitive amount of mass for a craft so small; for shielding to be a reasonable fraction of the size of a ship, you need to scale up.

“It therefore becomes more economical to think of a larger vessel, weighing many thousands of tons, in which a group of people could live indefinitely,” write Crane and Westmoreland, of Kansas State University. But there seems to be no way to supply the energy required to accelerate a craft that huge to the substantial speeds needed for interstellar exploration.

Star Trek enthusiasts might note that matter-antimatter annihilation seems to work just fine for the Enterprise. And in principle, Crane and Westmoreland agree, antimatter fuel could produce starship power. But the problem lies in making the antimatter to begin with, a process which itself would consume enormous amounts of energy. It’s kind of like the problem with the much-hyped future hydrogen economy. Hydrogen isn’t a source of energy, it’s a means of storage. You still need a lot of energy to get the hydrogen in the first place. Same with antimatter.

But there is another source of cosmic-class power that could perhaps be exploited to drive space vessels: the energetic emissions from black holes. That strategy does not leap immediately to mind, perhaps because black holes have the reputation of swallowing up mass and energy (famously confining even light). But as Stephen Hawking (a Star Trek fan himself) discovered, physics permits (and therefore requires) a black hole to emit “Hawking radiation” — particles and photons that slowly diminish the black hole’s mass as they stream away. Left on its own, with no source of food, a black hole shrinks as surely as a helium balloon with a pinhole leak.

Here’s the best part: The smaller a black hole gets, the faster it shrinks. So a very small black hole spews out very large amounts of energy. A very, very small black hole  — much smaller, say, than even an atom — would emit enough energy to, well, power a starship. Such black holes would release a much higher percentage of the energy used to make them than does either hydrogen or antimatter.

Just possible

Black holes produced at the LHC would not be any good for starship fuel. They would be so small that their decay would take only a tiny fraction of a second, which is also why they pose no danger of breaking the doomsayers’ perfect record of being wrong. But perhaps it is possible to create a black hole a little bigger, one that would produce a sufficient amount of Hawking radiation, for long enough, to drive a massive vessel across space to reach other stars in a reasonable time.

Prospects for success in that quest depend on the ability to compute precisely how much Hawking radiation a black hole releases. It’s not easy math. Most textbooks use rough approximations that may badly underestimate the true black hole energy output. Using more sophisticated approaches, Crane and Westmoreland conclude that a black hole with the proper propellant properties might indeed be consistent with the laws of physics. “It seems that making an artificial black hole and using it to drive a starship is just possible,” Crane and Westmoreland write.

Their calculations suggest that a black hole a few atto­meters or so across (an attometer is a nano-nanometer, smaller than a proton), with a mass on the order of a million metric tons, could provide large amounts of power for decades before shrinking to nothingness and disappearing in a final explosive poof. Making black holes of that magnitude would require an elaborate and massive bank of gamma-ray lasers, powered by a gigantic solar panel orbiting the sun. Converging laser beams would concentrate energy densely enough to create the black hole (somewhere in the vicinity of the sun), which would then be harnessed to a space vessel equipped with a parabolic dish to focus the black hole’s energetic emissions. Steering a black hole is no problem, Crane and Westmoreland say: “It is only necessary to scatter radiation off the black hole to impart momentum to it.” It’s basically pretty simple.

But there’s a catch, of course. It turns out that the amount of power a subatomic-sized black hole produces depends on details of the physics of quantum gravity. And a complete theory of quantum gravity is precisely what today’s scientists don’t have. They need the LHC to provide essential clues.

More specifically, the black hole power output that Crane and Westmoreland calculate may be realistic only if cosmic physics incorporates a mathematical  framework known as supersymmetry. Established laws of particle physics, describing the quarks and leptons that make up matter and the bosons transmitting forces between them, are rooted in mathematical symmetry principles. Symmetry enforces the requirement that the laws of physics apply equally to everybody, no matter where in the universe they live or how fast they are moving or spinning. Super­symmetry takes those laws a step further: In a sense, says Nobel laureate Steven Weinberg of the University of Texas at Austin, supersymmetry acts at “right angles” to the standard symmetries of physics. In practical terms, that means super­symmetry’s math implies that for every known subatomic matter particle the universe should provide a corresponding forcelike particle, and for every force particle there should exist a matterlike partner.

An opportunity for nature

Supersymmetry, Weinberg emphasizes, is the kind of principle that physicists seek to guide them in their search for a deeper understanding of nature.

“The minds of physicists can think of all kinds of possibilities,” Weinberg said in Austin at a recent symposium sponsored by the Council for the Advancement of Science Writing (SN Online: bit.ly/higgs_lhc). “When we speculate aimlessly, the results are likely to be not very interesting. It’s when there are physical principles that narrowly restrict our speculations, so that new ideas can only take one or a very limited number of forms, that we begin to think we’ve discovered something that’s an opportunity that nature probably didn’t pass up. Most of us have this feeling about supersymmetry.”

If supersymmetry particles exist, they have remained un­detected, implying that they are too massive to have been created by experiments on Earth to date. Potentially, though, the LHC has sufficient power to produce such particles. If it succeeds, physicists will have received a major clue in their quest to solve the mystery of quantum gravity, and the prospect of building black hole spaceships (before the world comes to an end) will grow substantially brighter.

But as things stand, nobody knows for sure if super­symmetry is correct. That’s why the LHC is so important.

“There isn’t any one standard supersymmetry theory,” Weinberg points out. “That means either we’re missing something about supersymmetry which will be revealed to us when it’s discovered, or the whole idea is wrong and it won’t be discovered. And we just don’t know. But this is certainly one of the targets for the LHC. And I would say it’s the most important target.”