CAUGHT in a high-tech trap, a lone atom is blinking. From the spectrum of its
light it looks like a cadmium atom. But to the scientists who have trapped it,
there鈥檚 something not quite right: the frequency is shifted by a puzzling 0.002
per cent. Their unease gives way to astonishment as they discover that their
captive is a billion billion times heavier than an ordinary atom. Staring them
in the face is a freak of nature鈥攆or in place of a nucleus, this atom has
a gaping hole. A black hole.
According to Victor Flambaum and Julian Berengut of the University of New
South Wales in Sydney, black hole atoms such as this are a real possibility. In
a paper submitted to Physical Review A, the two physicists have calculated the
bizarre properties of these objects. Black hole atoms could be part of the dark
matter that binds the Galaxy, they might accelerate nuclear reactions in stars,
and they could tell us about conditions in the first fraction of a second of the
life of the Universe. Strangest of all, there could be black holes inside you,
and you wouldn鈥檛 even notice.
In an ordinary atom, negatively charged electrons orbit positively charged
protons and uncharged neutrons, which are clumped into a tiny nucleus 1/100
000th the diameter of the atom. There is no reason why this central positive
charge could not be provided by something else, as long as it is small enough.
But how could black holes fit the bill?
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Astronomers have detected several black holes in our Galaxy, the relics of
massive stars which gravity has crushed out of existence
(see 鈥淭he Great Annihilators鈥�).
These holes weigh several times as much as the Sun, and are a few kilometres
across鈥攈ardly a handy size to slip inside an atom. Even less suitable are
the colossal black holes with billions of times the mass of the Sun that power quasars
(see 鈥淢asters of the Universe鈥�).
But theorists think that microscopic black
holes might have been squeezed into existence in the first split second after
the big bang. It is these that could form the kernels of black hole atoms.
To make an atom, a primordial black hole would have to carry a positive
electrical charge, just as a nucleus does. Flambaum says that black holes are
more likely to be charged than not. 鈥淭hey get their charge simply by swallowing
charged particles,鈥� he says. 鈥淚t could be negative or positive, depending on
what they swallow.鈥�
But to make a substitute nucleus, a black hole must also be
stable鈥攚hich microscopic black holes aren鈥檛 supposed to be. In 1971, the
Cambridge University physicist Stephen Hawking showed that black holes radiate
subatomic particles, causing them to slowly evaporate away. As the hole becomes
smaller, the sleet of particles becomes a ferocious blizzard, until the last
shred of the black hole vanishes in a blast of radiation. On the face of it,
this spells death to any black hole atom. However, Flambaum and Berengut reckon
that Hawking鈥檚 evaporation could eventually stop, leaving a tiny, long-lived
black hole behind.
Jacob Beckenstein discovered in 1995 that the area of a black hole鈥檚 event
horizon鈥攖he boundary which marks the point of no return for in-falling
matter鈥攊s quantised. In other words, it can only have certain discrete
values. As a hole evaporates, its area, and therefore its mass, falls step by
step. By the time it has only a few quanta of horizon area left, Hawking鈥檚
calculations break down. No one can work out exactly what should happen, but
many physicists think the black hole might stop evaporating altogether, or at
least hang around for many billions of years before evaporating. To conserve
such physical quantities as charge and angular momentum, the residual black hole
would have to turn into a collection of particles with very particular
properties鈥攕omething that quantum mechanics makes highly improbable.
If so, the Universe could be awash with charged primordial black holes of
about a Planck mass, or 0.00001 grams. They would have an incredibly tiny radius
of 10-35 metres. And in the aeons they have been drifting through the gas of
interstellar space, they would have gathered about them a cloud of electrons to
neutralise their positive charge. 鈥淭he electrons would orbit just like in a
normal atom,鈥� says Flambaum.
Super-atoms
If such a beast were to run into the Earth, it could get trapped on the
surface and become lodged in a rock, in a laboratory bench, or even in your
head. What would this super-atom be like?
At first glance, there appears to be no limit to the number of orbiting
electrons a black hole atom can have鈥攊t should depend merely on the charge
of the black hole. It is easy to imagine atoms with a thousand electrons, or a
million. But it turns out that there are limits: a fundamental one, and a
practical one set by the age of the Universe.
The fundamental limit was discovered by the British physicist Paul Dirac. He
knew that in a normal atom, an electron is prevented from spiralling into the
nucleus by Heisenberg鈥檚 uncertainty principle. Electrons are attracted by the
positive charge on the nucleus, but any electron trying to orbit too close to
the nucleus would be stuck in a very confined space. According to the
uncertainty principle that means it would have to move very fast, too fast to be
trapped there by the electrical attraction of the nucleus.
But if the nucleus has a very large charge, the increased pull would be
enough to overwhelm the uncertainty principle. 鈥淒irac found that for an atom
with more than 137 electrons there are no stable electron orbits,鈥� says
Flambaum. 鈥淎ll electrons spiral down to nuclear oblivion.鈥�
So any charged black holes that formed in the big bang could not have
collected more than 137 electrons. Any surplus would have been swallowed by the
hole almost instantly, cancelling out part of its positive electrical
charge.
But that鈥檚 not the only constraint. Any hole that started out with a charge
as high as 137 would be less charged by now, because it turns out that electrons
don鈥檛 have to spiral in to be gobbled up. An electron isn鈥檛 a discrete little
ball: its existence is actually smeared out. This means that a small part of the
existence is always within the black hole鈥檚 horizon, which translates into a
small probability that the electron will be swallowed. This probability
increases with the cube of the charge on the hole. So highly charged black hole
atoms quickly polish off their inner electrons. According to Flambaum, the most
complex black hole atoms that we could expect to see in today鈥檚 Universe would
have 70 electrons, corresponding to the element ytterbium.
If experimenters were to trap a black hole atom in their lab, what would it
look like? The wavelength of light emitted by a normal atom depends on the
energy levels that its electrons can occupy. These are affected slightly by the
motion of the nucleus: a nucleus moves a little as a result of the pull of an
electron orbiting it, just as the Sun wobbles slightly under the influence of
the Earth鈥檚 gravity. The effect in an ordinary atom is to shift the wavelength
by a tiny amount from what it would be if the nucleus were immovable鈥攖hat
is, if its mass were infinite. 鈥淏ut a Planck mass black hole is effectively an
infinite mass,鈥� says Flambaum. 鈥淪o you wouldn鈥檛 get the shift you get for a
normal atom. Such a difference would be easy to see with modern
颈苍蝉迟谤耻尘别苍迟蝉.鈥�
A more dramatic effect would occur if an inner electron was gobbled. Then the
black hole atom would do a quick change. A cadmium black hole atom with 48
electrons would spontaneously turn into a silver black hole atom with 47
electrons. And as the hole鈥檚 mass depends on its charge, such a transformation
would produce a huge burst of radiation.
So what are the odds of detecting a black hole atom, either in space or on
Earth? It really depends on how many of them are kicking around the Universe. We
can get a rough idea of the maximum possible number by postulating that the
Universe鈥檚 invisible 鈥渄ark matter鈥� is made entirely of black hole atoms. 鈥淎t
best we could hope for one black hole atom for every 1000 trillion trillion
hydrogen atoms,鈥� says Flambaum. 鈥淭hat would make them about 30 times rarer than
uranium atoms.鈥�
So there is no doubt that black hole atoms would be rare creatures, but
Flambaum鈥檚 point is that they might not be impossibly rare. He suggests that the
best bet could be to look for their anomalous spectral lines in gas around the
Sun. The real problem in detecting one would be in spotting its light amid the
blaze of light from countless normal atoms. Flambaum admits that this would be
quite a challenge.
Exotic entities
Atoms with a positively-charged black hole in their hearts may seem weird.
But Flambaum and Berengut have imagined some even more exotic entities, formed
when a minimum-mass black hole has a negative charge. 鈥淚nstead of gathering
electrons about it as it travelled through space, it could gather protons or
even nuclei,鈥� says Flambaum. Such a 鈥減rotonic鈥� atom would be about 2000 times
smaller than a normal atom鈥攕o small that short-range nuclear forces would
come into play. Two protons in orbit around a hole would react to form a
deuterium nucleus. Such a reaction, which is the first step in the process of
making sunlight, takes about 10 billion years to happen in the Sun. However, it
would happen much faster around a black hole, where the two protons are close
enough for nuclear forces to snap them together.
The arrival of a third proton would trigger the nuclear reaction to make
helium-3, and the process carry on to build ever heavier nuclei around the black
hole. Flambaum and Berengut point out that the nuclei would be very different
from normal nuclei, which have to contain large numbers of neutrons to glue them
together because of the tremendous electrical repulsion between protons. But
around a black hole, the protons would be squeezed together by the hole鈥檚
gravity. 鈥淐onsequently, you would expect nuclei far richer in protons than
normal nuclei,鈥� says Flambaum. This type of black hole atom could act as a
catalyst for building new elements in stars. The energy released in the reaction
could make a nucleus fly off, allowing new protons to be captured.
Another, weirder possibility, arises because a proton is a very large object
compared with a point-like black hole. From the point of view of the hole, a
proton is not a single object but an assembly of three quarks. 鈥淭his means that
there is the possibility of the proton actually losing one of its quarks to the
black hole,鈥� says Flambaum. The black hole would then possess colour charge, a
property thought to be the preserve of quarks alone. In effect it would become a
superheavy quark, and from a distance the whole system of two quarks orbiting a
quark black hole would appear to be a superheavy proton. If we could capture
these weird particles they would make a great test bed for theories of how
quarks interact with each other. The snag is that they鈥檇 be very hard to catch.
They are so small that they would fall between the ordinary atoms of a solid and
so could never be trapped on the Earth鈥檚 surface.
But most significantly, black hole atoms might provide a window onto the very
early Universe, all the way back to the first instant, within 10-43 seconds of
the beginning. This is far earlier than any other relics, such as photons or
neutrinos. According to Bernard Carr of Queen Mary and Westfield College in
London, primordial black holes would tell us how lumpy the early Universe was.
They might even preserve a memory of a time when the fundamental constants of
nature were different from their values today. For example, in some cosmological
models the strength of gravity varies in the very early Universe. Relic black
holes might record this variation in the values of their mass.
So although Flambaum admits that black hole atoms of all types would be very
difficult to detect, the rewards could be great. 鈥淟ooking for them has got to be
worth a try,鈥� he says. Just don鈥檛 expect to see black hole paperweights in the
shops any time soon.