| Could physicists
accidentally make killer black holes or lethal strange matter that
would swallow the Earth? At least there'd be no one left to say
sorry to, says Robert Matthews
|
UH-OH, the mad scientists
are at it again. In their determination to extract nature's secrets,
physicists in America have built a machine so powerful it has raised fears
that it might cause The End of The World As We Know It.
"Big Bang
machine could destroy Earth" ran the headline over a story in The
Sunday Times last month. It claimed that our planet was in peril from
a vast new American particle accelerator on Long Island, the Relativistic
Heavy Ion Collider (RHIC), which will collide pairs of gold nuclei at high
energies. According to the article, RHIC could trigger a catastrophic
event: the creation of a black hole or a ravenous "strangelet" that could
swallow up our entire planet.
Within 24 hours, the laboratory
issued a rebuttal: the risk of such a catastrophe was essentially zero.
The Brookhaven National Laboratory that runs the collider had set up an
international committee of experts to check out this terrifying
possibility. But BNL director John Marburger, insisted that the risks had
already been worked out. He formed the committee simply to say why they
are so confident the Earth is safe, and put their arguments on the Web to
be read by a relieved public.
Even so, many people will be stunned
to learn that physicists felt worried enough even to mull over the
possibility that a new machine might destroy us all.
In fact,
they've been fretting about it for over 50 years. The first physicist to
get the collywobbles was Edward Teller, the father of the hydrogen bomb.
In July 1942, he was one of a small group of theorists invited to a secret
meeting at the University of California, Berkeley, to sketch out the
design of a practical atomic bomb. Teller, who was studying the reactions
that take place in a nuclear explosion, stunned his colleagues by
suggesting that the colossal temperatures generated might ignite the
Earth's atmosphere.
While some of his colleagues immediately
dismissed the threat as nonsense, J. Robert Oppenheimer, director of the
Manhattan Project, set up to build the atom bomb, took it seriously enough
to demand a study. The report, codenamed LA-602, was made public only in
February 1973. It concentrated on the only plausible reaction for
destroying the Earth, fusion between nuclei of nitrogen-14. The report
confirmed what the sceptics had insisted all along: the nuclear fireball
cools down too far quickly to trigger a self-sustaining fire in the
atmosphere.
Yet in November 1975, The Bulletin of the Atomic
Scientists claimed that Arthur Compton, a leading member of the
Manhattan Project, had said that there really was a risk of igniting the
atmosphere. It turned out to be a case of Chinese whispers: Compton had
mentioned the calculation during an interview with the American writer
Pearl Buck, who had got the wrong end of the stick.
Even so, the
Los Alamos study is a watershed in the history of science, for it marks
the first time scientists took seriously the risk that they might
accidentally blow us all up. The issue keeps raising its ugly head.
In recent years the main focus of fear has been the giant machines
used by particle physicists. Could the violent collisions inside such a
machine create something nasty? "Every time a new machine has been built
at CERN," says physicist Alvaro de Rujula, "the question has been posed
and faced."
One of the most nightmarish scenarios is destruction
by black hole. Black holes are bottomless pits with an insatiable appetite
for anything and everything. If a tiny black hole popped into existence in
RHIC, the story goes, it would burrow down from Long Island to the centre
of the Earth and eat our planet--or blow it apart with all the energy
released. So why are physicists convinced that there's no chance of this
happening?
Well, the smallest possible black hole is around
10-35 metres across (the so-called Planck Length). Anything
smaller just gets wiped out by the quantum fluctuations in space-time
around it. But even such a tiny black hole would weigh around 10
micrograms--about the same as a speck of dust. To create objects with so
much mass by collisions in a particle accelerator demands energies of
1019 giga-electronvolts, so the most powerful existing collider
is ten million billion times too feeble to make a black hole. Scaling up
today's technology, we would need an accelerator as big as the Galaxy to
do it.
And even then, the resulting black hole wouldn't be big
enough to swallow the Earth. Such a tiny black hole would evaporate in
10-42 seconds in a blast of Hawking radiation, a process
discovered by Stephen Hawking in the 1970s. To last long enough even to
begin sucking in matter rather than going off pop, a black hole would have
to be many orders of magnitude bigger. According to Cliff Pickover, author
of Black Holes: A Traveler's Guide, "Even a black hole with the
mass of Mount Everest would have a radius of only about 10-15
metres, roughly the size of an atomic nucleus. Current thinking is that it
would be hard for such a black hole to swallow anything at all--even
consuming a proton or neutron would be difficult."
So we needn't
lose sleep about creating an Earth-eating black hole in an accelerator.
But according to John Wheeler of Princeton University, there is another
way: detonating a big hydrogen bomb. He showed that the pressures
generated by a suitable explosion could crush matter to the densities
needed (around 1017 kilograms per cubic metre) to stand a
chance of creating a black hole. However, Wheeler estimated that a
"suitable" H-bomb would require all the heavy water in the oceans, and
weigh many billions of tonnes. Some bomb.
The more discerning mad
scientist might instead opt to pick a black hole "off the shelf". One left
over from the Big Bang or an exploding star, for example. The temptation
is certainly there, for as the Oxford mathematician Roger Penrose showed
30 years ago, black holes make wonderfully clean sources of energy. Just
throw a skipful of junk at a black hole in the right way, Penrose
discovered, and it will eat up all the junk and then hurl the empty skip
back out again with more energy than it had before.
Fortunately,
there's not much chance of bringing a black hole to Earth any time soon.
After all, they would be rather unwieldy and the nearest one is likely to
be many light years away.
It was while dismissing the black-hole
threat in last month's Scientific American that theorist Frank Wilczek of
the Institute for Advanced Study in Princeton mentioned an altogether more
exotic form of killer blob: "strangelets".
Strangelets are chunks
of matter made from "strange" quarks as well as the usual "up" and "down"
types of ordinary matter. It might be possible to make them in particle
accelerators like RHIC. The risk is that a strangelet might consume nuclei
of ordinary matter and convert them into more strange matter, transmuting
the entire Earth into a strange-matter planet. But having raised this
appalling prospect, Wilczek quickly dismissed it.
And quite
rightly, says the world's leading expert on strangelets, Robert Jaffe of
the Massachusetts Institute of Technology. "Strangelets are almost
certainly not stable, and if they are, they almost certainly cannot be
produced at RHIC," he says. "And even if they were produced at RHIC, they
almost certainly have positive charge and would be screened from further
interactions by a surrounding cloud of electrons." Every one of these
steps in the argument would have to be flawed for strangelets to be a
risk.
Blown to
smithereens
But don't heave a sigh of
relief just yet. The Brookhaven scientists have also considered an even
more alarming possibility than the destruction of the Earth. Could their
mighty machine trigger the collapse of the quantum vacuum?
Quantum
theory predicts that the Universe is filled with a seething melee of
so-called vacuum energy. That might seem an unlikely threat to
civilisation. After all, it's simply the average energy of the mess of
particles that flit in and out of existence all around us. As the Universe
expanded and cooled, that vacuum energy dropped down to the lowest
possible level.
Or did it? What if the Universe is still "hung up"
in an unstable state? Then a jolt of the right amount of energy in a small
space might trigger the collapse of the quantum vacuum state. A wave of
destruction would travel outwards at the speed of light, altering the
Universe in bizarre ways. It would be rather bad news for us, at least:
ordinary matter would cease to exist.
In 1995, Paul Dixon, a
psychologist at the University of Hawaii, picketed Fermilab in Illinois
because he feared that its Tevatron collider might trigger a quantum
vacuum collapse. Then again in 1998, on a late night talk radio show, he
warned that the collider could "blow the Universe to smithereens".
But particle physicists have this covered. In 1983, Martin Rees of
Cambridge University and Piet Hut of the Institute of Advanced Study,
Princeton, pointed out that cosmic rays (high-energy charged particles
such as protons) have been smashing into things in our cosmos for aeons.
Many of these collisions release energies hundreds of millions of times
higher than anything RHIC can muster--and yet no disastrous vacuum
collapse has occurred. The Universe is still here.
This argument
also squashes any fears about black holes or strange matter. If it were
possible for an accelerator to create such a doomsday object, a cosmic ray
would have done so long ago. "We are very grateful for cosmic rays," says
Jaffe.
But RIHC is special, goes the counter-argument, because it
collides gold nuclei together. What if some subtle unforeseen physical
effect makes collisions between heavy nuclei particularly dangerous?
Fortunately, there are some heavy nuclei among the multitude of cosmic
rays that fly through the Solar System. "We believe there are relevant
cosmic ray "experiments" for every known threat," says Jaffe. "Even if one
insists on gold-gold collisions, there have been enough such collisions on
the surface of the Moon since its formation 5 billion years ago to assure
us that RHIC experiments are safe."
So until we can build atom
smashers so powerful that they can exceed the energy of the punchiest
cosmic rays, we needn't lose any sleep over them. Paranoiacs should look
elsewhere, and a good place to start would be in the pages of journals
like Physical Review Letters, which have carried schemes for
extracting energy from the quantum vacuum. The worry here is that no-one
knows how much energy might be unleashed: calculations give answers
anywhere between zero and infinity. Arthur C. Clarke once raised the
possibility that some of those vast explosions we see in the cosmos may be
smart-alec alien scientists getting their comeuppance for tinkering with
the quantum vacuum: "they might be industrial accidents" he said.
Those of a nervous disposition should stop reading now. For some
top physicists are toying with the idea of recreating the birth of the
Universe right here on Earth (see "Cosmos-making
for amateurs"). One of the big names backing this idea is cosmologist
Andrei Linde of Stanford University. He admits that he has no idea how to
trigger a little big bang, yet insists that the experiment would not be
catastrophic.
But then, as the Russian theorist Lev Landau once
said: "Cosmologists are often wrong, but never in doubt." Perhaps Linde's
reassurance will turn out to be the very last Famous Last Words.
Robert Matthews is science
correspondent of the Sunday Telegraph
|
Cosmos-making for
amateurs
SCIENTISTS ARE OFTEN
ACCUSED of trying to play God. But
obviously they can't really mimic the feats of the putative Creator
of the Universe, and make a universe in the laboratory. Or can they?
Before you snort in disbelief, you should know that some serious
cosmologists have considered the idea. Indeed, one of them has
already had a shot at creating a universe--albeit inside a computer.
The idea dates back to the late 1970s, when Andrei Linde, now at
Stanford University, and Alan Guth of the Massachusetts Institute of
Technology separately came up with the concept of "inflation".
According to this idea, an incredibly short, violent burst of
expansion occurred around 10-32 seconds after the birth
of the Universe. Propelled by concentrated vacuum energy, inflation
boosted the size of the Universe from one billionth the width of a
proton to the size of a grapefruit. That's what the theorists claim,
but showing that inflation really did take place like this is
hard... unless, of course, someone can recreate the right conditions
in the lab and watch what happens. Linde and his colleagues have
already done a dry run on a computer. "Setting up the simulations
was hard work, and only on the seventh day did we finish the first
series," he reported in Scientific American in 1994, adding
in Strangelovian terms: "We looked at the shining screen, and we
were happy--we saw that the universe was good!" This isn't enough
for Linde: he wants to do it for real. But theory suggests that
matter has to be squeezed to densities similar to those in the
primordial Universe before such fields appear. No-one has the
faintest clue how to create such densities, yet. Linde is sanguine
about the dangers involved, if it ever becomes possible. "You can
think of our Universe as being like a smooth surface, with one part
of it inflating like a balloon. The new universe will be connected
to ours by just a tiny passage--what we call a wormhole--the size of
a subatomic particle." Quite how we'd know we'd succeeded isn't
obvious, but at least there seems little danger of someone tumbling
into the new universe by mistake, or anything nasty getting
out. |
Further reading:
From New
Scientist, 28 August 1999
Subscribe
to New Scientist
|
