by Alok Jha
This idea is largely correct. Cosmic black holes form in the dying moments of supermassive stars in a process whereby all of what a star once was is crushed by gravity into a dimensionless point of infinite density called a singularity. This implosion is so violent that it tears apart space itself, and within a certain radius of the singularity, nothing can escape. That includes light, hence why black holes are black. We know nothing about what happens inside this radius, the event horizon, because no information can escape the grip of the gravity there.
The size of a black hole depends on its mass—you would have to compress the matter in the Sun to about three kilometers (two miles) across, four millionths of its present size, to make it collapse into a black hole; the Earth would need to be squeezed into a radius of nine millimeters, about a billionth of its present size. Neither of these celestial bodies could turn into black holes naturally, however—there just isn’t enough gravity present to crush the masses down.
But cosmic monsters are not the only type of black hole that physicists think is possible in nature. In the 1970s, astrophysicists Stephen Hawking and Bernard Carr looked at whether there might have been black holes in the early universe, when the energies were fierce and the density of matter was immense enough to collapse some regions into tiny singularities. The laws of physics allow for matter to have a density of up to the so-called Planck value of 1097 kg per cubic meter, at which point gravity becomes so strong that the material will collapse into a black hole 10-35 meters across, with a mass of 10-8 kg. Hawking and Carr called these theoretical singularities “primordial black holes.”
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This implosion is so violent that it tears apart space itself.
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Creating these tiny black holes in the early universe, with its unimaginable density and temperature, is one thing. Producing anything similar on Earth would require an attempt to re-create the conditions of the early universe, in the moments after the Big Bang.
Black holes on Earth
Re-creating the moments after the Big Bang is exactly what the LHC is designed to do. By accelerating protons toward each other at nearly the speed of light, scientists hope to find evidence of new particles in the debris of the collisions and to push back the boundaries of what they know about nature.
When a proton is accelerated in the LHC, it will reach an energy of around seven tera-electron-volts. According to Albert Einstein’s equation E=mc2, a proton with this much energy is equivalent to a mass of 10-23 kg, around 7,000 times more massive than a proton at rest. “When two such particles collide at close range, their energy is concentrated into a tiny region of space. So one might guess that, once in a while, the colliding particles will get close enough to form a black hole,” says Carr.
At first glance, there is a big problem with this argument. Two protons with masses of 10-23 kg constitutes far too small an amount of material to create Hawking and Carr’s primordial black holes, which are a minimum of 10-8 kg. Two protons could be made to form a primordial black hole if they were accelerated to much greater velocities, but to satisfy Hawking and Carr’s criteria, the particle accelerator needed would have to be the size of our galaxy.
So why is anyone worried about black holes on Earth? The reason is that Hawking and Carr’s theory relies on standard general relativity, the mathematical description of gravity formulated by Einstein in the early 20th century. More recent ideas of how gravity might work have proposed that the required density to form a tiny black hole might be much lower than the two scientists had originally speculated.
“String theory, one of the leading contenders for a quantum theory of gravity, predicts that space has dimensions beyond the usual three,” says Carr. “Gravity, unlike other forces, should propagate into these dimensions and, as a result, grow unexpectedly stronger at short distances. In three dimensions, the force of gravity quadruples as you halve the distance between two objects. But in nine dimensions, gravity would get 256 times as strong. This effect can be quite important if the extra dimensions of space are sufficiently large, and it has been widely investigated in the past few years. There are also other configurations of extra dimensions, known as warped compactifications, that have the same gravity-magnifying effect and may be even more likely to occur if string theory is correct; these have been extensively studied in recent years.”
The innards of the CMS experiment at the Large Hadron Collider, located at Cern, the European particle physics lab. The collider sits in a 27 km underground ring in Geneva and smashes together protons at nearly the speed of light.
If gravity really does extend into other dimensions in this way, it would mean that the threshold to form a black hole is much lower than the Planck density. And this in turn means that the density required to make tiny black holes on Earth would lie within the range of the LHC. In 2001, scientists calculated that the lowest boundaries for the Planck density meant that the LHC would produce a black hole every second it carried out proton collisions. They concluded that the particle accelerator at Cern would be a factory for making black holes.
Do we need to worry?
Given the violent nature of the black holes in space, it might be less than comforting to think that similar objects will be popping into existence thousands of times every day at Cern. Could these objects burrow their way out of the collider and begin to eat away at our planet?
This worry is not purely hypothetical, by the way. People become anxious every time a big particle accelerator goes online; before the LHC was due to be switched on, two residents of Hawaii filed a federal lawsuit in an attempt to prevent scientists from going ahead with their work until the potentially catastrophic effects of the particle collisions had been reassessed.
But calm your fears, Frank Wilczek, a physicist at the Massachusetts Institute of Technology, points out that a black hole in space is a very different thing to anything we could create on Earth. He compares the problem to having a single word for all the animals in the world and having elephants in mind when originally defining that word. Amoebas, however, are animals too, he reminds us.
The first thing to remember about any man-made black holes in the LHC is that they would be so tiny that their gravitational fields would not extend very far. A black hole capable of consuming the Earth would have to have a mass of several tons—anything generated by the LHC would not even reach an appreciable fraction of a gram.
In any case, the black hole would evaporate and disappear far too quickly to cause any damage. This conclusion comes from the predictions of Stephen Hawking in the 1970s. The idea that black holes could be small led the physicist to wonder whether quantum mechanics, the physics used to describe the smallest constituents of matter, would have any important effects on their behavior. “In 1974 he came to his famous conclusion that black holes do not just swallow particles but also spit them out,” says Carr. “Hawking predicted that a hole radiates thermally like a hot coal, with a temperature inversely proportional to its mass. For a solar-mass hole, the temperature is around a millionth of a kelvin, which is completely negligible in today’s universe. But for a black hole of 1012 kilograms, which is about the mass of a mountain, it is 1012 kelvins—hot enough to emit both massless particles, such as photons, and massive ones, such as electrons and positrons.”
This emission carries off energy, which means that the mass of the black hole would steadily decrease over time. As it shrank, it would get hotter, emitting more particles and shrinking even faster. “When the hole shrivels to a mass of about 106 kilograms, the game is up: within a second, it explodes with the energy of a million-megaton nuclear bomb,” says Carr. “The total time for a black hole to evaporate away is proportional to the cube of its initial mass. For a solar-mass hole, the lifetime is an unobservably long 1064 years. For a 1012-kilogram one, it is 1010 years—about the present age of the universe. Hence, any primordial black holes of this mass would be completing their evaporation and exploding right now. Any smaller ones would have evapo
rated during an earlier cosmological epoch.”
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BLACK HOLES CREATED EACH YEAR
In Earth’s atmosphere: 100
In the universe: 10 trillion
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If a proton–proton collision at the LHC was to create a black hole, it would disappear in a flash of Hawking radiation in less than 10-26 seconds.
What are the chances?
A study carried out by scientists for Cern pointed out that, although the LHC would achieve energies that no other particle accelerators had previously reached, nature routinely collides particles at much higher energies. Whatever the LHC will do, said officials in a statement reviewing several safety studies of the accelerator, nature has already done many times over during the lifetime of the Earth and other astronomical bodies.
“Cosmic rays are particles produced in outer space, some of which are accelerated to energies far exceeding those of the LHC,” said Cern. “The energy and the rate at which they reach the Earth’s atmosphere have been measured in experiments for some 70 years. Over the past billions of years, Nature has already generated on Earth as many collisions as about a million LHC experiments—and the planet still exists. Astronomers observe an enormous number of larger astronomical bodies throughout the Universe, all of which are also struck by cosmic rays. The Universe as a whole conducts more than 10 trillon LHC-like experiments per second. The possibility of any dangerous consequences contradicts what astronomers see—stars and galaxies still exist.”
The same calculations that show that the LHC will become a factory for mini black holes also predict that around 100 black holes are created every year in the Earth’s atmosphere because of cosmic-ray collisions. Over and above everything else, perhaps our own continuing existence is the best evidence there is to reassure us that the LHC is unlikely to create any monstrous black holes that could eat the Earth.
Hostile Extraterrestrials
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What would happen if we ever made contact with aliens, and they decided to come and visit? They might be benevolent, but there is no guarantee. They might just see Earth, with its rich array of elements and resources, as a filling station to be plundered on their way to more interesting places.
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When the aliens in science-fiction stories are bad, they tend to be very, very bad. In Independence Day, they lay waste to all the world’s cities. In War of the Worlds, they kill everyone they can find. And it is hard to forget the mischievous aliens in Mars Attacks!—blowing up buildings and people with uncontained glee.
No doubt any real-life visitors would be from an advanced civilization, in order to have mastered interstellar travel, and they would probably have the technology and power to do whatever they pleased with us and our planet. If we were lucky, they might not perceive us as a threat, and would ignore us during their visit. Then again, they might leave behind toxic waste or inadvertently wipe us out by bringing a virus or other pest to our planet.
The eminent physicist Stephen Hawking is one of those who is worried. “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans,” he said in a 2010 documentary made for the Discovery Channel. He argued that instead of trying to find and communicate with life in the cosmos, humans might be best off doing everything they can to avoid contact. When someone as smart as Hawking makes a statement like that, it is worth listening, isn’t it?
Is it a bad idea to look for ET?
Astrobiologists and astronomers have been on the hunt for extraterrestrial life for more than half a century, part of a human desire to know whether we are alone in the universe, and to learn from and meet civilizations that are more advanced than our own.
There are billions and billions of stars in our galaxy, and, it is reasonable to expect, an even greater number of planets orbiting them. Some of these, surely, will exist in the “Goldilocks zone” of perfect temperature and distance from their star (in the same way the Earth exists with the Sun), which would make life possible? And going on sheer numbers alone, it is not unreasonable to expect that some of this life is intelligent and capable of interstellar communication.
The Earth’s biggest and most active hunt for alien life started in 1960, when the astronomer Frank Drake pointed the Green Bank radio telescope in West Virginia toward the star Tau Ceti. He was looking for anomalous radio signals that could be signals sent by intelligent life. Eventually his idea turned into the Search for Extra Terrestrial Intelligence (SETI), which used the downtime on radar telescopes around the world to scour the sky for telltale alien signals. For the past 50 years, SETI has continued its quest, but in all that time, the sky has remained silent.
There are lots of practical problems involved in hunting for aliens. Chief among them is distance. Our galaxy is big—it would take a beam of light 100,000 years to cross from one end to the other. If our nearest neighbors were life forms on the forest moon of Endor, 1,000 light years away, it would take a millennium for us to receive any message that they might send. A response would take the same amount of time to reach the aliens. It is not a timescale that allows for quick banter.
This picture was taken by a US Coast Guard photographer in Salem, Massachusetts. It was one of several reports of unidentified aerial phenomena sighted across the country.
And they might not be communicating in our direction anyway. If the Endorians were watching us, the light reaching them at this very moment from Earth would show them our planet as it was 1,000 years ago. In Europe, that means lots of fighting between knights around castles, and in North America, small bands of natives living on the great plains. If our nearest aliens are tens of thousands of light years away, they would see only the ancestors of modern humans living among much greater beasts. You could forgive them for not bothering to get in touch.
How dangerous could aliens be?
Answering this question in any definitive way is impossible, at least until an extraterrestrial species lands on Earth and makes its intentions clear.
According to Jack Cohen and Ian Stewart of the Mathematics Institute at the University of Warwick, aliens would not look like the canonical “little green men.” In a commentary in Nature, they wrote how extraterrestrial life forms “might look exactly like people. Or cats. Or houseflies. Or they are invisible, or lurking just outside our space–time continuum along a fifth dimension.”
The lack of a signal from ET has not prevented astronomers and biologists from coming up with a whole range of ideas about what aliens might look like. In the early days of SETI, astronomers focused on the search for planets just like ours. Their idea was that, since the only biology we know about is our own, we might as well assume that aliens are going to be something like us. But there’s no reason why that should be the case.
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In all that time, the sky has remained silent.
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Humans evolved on a planet rich in oxygen and water, where a carbon-based molecule called DNA became the copying mechanism for all life. From our point of view, we seem to exist in a world with just the right parameters of temperature, water and nutrients.
Aliens, of course, are not restricted to our point of view. You don’t need to step off the Earth to find life that is radically different from our common experience of it. Extremophiles are species that can survive in places that would quickly kill humans and other “normal” life forms. These single-celled creatures have been found in boiling-hot vents of water thrusting through the ocean floor, or at temperatures that are well below the freezing point of water. The front ends of some creatures that live near deep-sea vents are 200°C warmer than their back ends.
“In our naïve and parochial way, we have named these things extremophiles, which shows prejudice—we’re normal, everything else is extreme,” says Stewart. “From the point of view of a creature that lives in boiling water, we’re extreme because we live in much milder temperatures. We’re at least as extr
eme compared to them as they are compared to us. Similarly for the ones in very cold water.”
He calls this anthropocentric view of life—that whatever is right for us sets the agenda for everything else—the “Goldilocks mistake.” The problem with the original fairy story, he says, is that even though Goldilocks found baby bear’s porridge to her liking, daddy bear was perfectly happy with his hot porridge and mummy bear was happy with her cold porridge. Not to mention the whole forest outside the house that probably didn’t even like porridge.
On Earth, life exists in water and on land, but on a giant gas planet, it might exist high in the atmosphere, trapping nutrients from the air swirling around it. “Most aliens would not wish to visit Earth at all, any more than we would care for a ramble across the surface of a neutron star, or to live, as do some extremophiles, in boiling water,” wrote Stewart and Cohen in Nature.
Faced with such diversity, it is still possible to make an educated guess at what aliens might be like. For a start, says Stewart, you have to carve up biological features into things that might be universal across all life forms in the galaxy, and those which are parochial to the Earth.
Parochial features include anything unique to a single species—such as the five fingers on a human hand. There is no reason, other than a quirk of evolution, why we don’t have four or even six digits. The eye, for example, has evolved more than 40 times in completely unrelated creatures.