杏吧原创

IN THE beginning, space was filled with a liquid hovering below its normal
freezing point. Supercooled liquids like this are on a hair-trigger: the merest
nudge is enough to set off a runaway frenzy of freezing. That nudge might be
provided by a dust-like impurity in the liquid or perhaps by a small region
which by chance is a little colder than the rest. Whatever it was, something
triggered the cosmic liquid, seeding a crystal that grew explosively, racing
outwards.

Does this scenario ring any bells? According to Michael Grady from the
University of New York College at Fredonia, it should. He is convinced that the
seeding of the crystal is nothing other than the big bang, which spawned our
Universe.

As Grady sees it, all sorts of mysterious observations fall into place if our
Universe was brought into being by such a phase nucleation event, triggering a
transition from liquid to solid in a pre-existing fluid.

If this extraordinary idea hasn鈥檛 fazed you yet, hold onto your hat. For the
liquid Grady has in mind is unlike any liquid you have ever imagined. Instead of
the familiar three space dimensions and one time dimension of the world we see,
Grady鈥檚 liquid would have filled four dimensions of space and one of
time鈥攁 total of five dimensions. 鈥淔or want of a better word I call it
`protospace鈥�,鈥� says Grady.

So why do we see only three space dimensions? Grady鈥檚 answer is that we are
stuck on the expanding solid surface. Imagine an ordinary crystal growing in a
familiar three-dimensional liquid. The boundary between the crystal and the
liquid is two-dimensional, like the surface of an expanding soap bubble. But in
the strange liquid envisaged by Grady鈥攐ne with four space
dimensions鈥攖he phase boundary is a three-dimensional surface, something
that is impossible to visualise. 鈥淭hat is our Universe,鈥� says Grady. 鈥淲e think
we鈥檙e in a three-dimensional Universe but we鈥檙e actually riding the surface of
four-dimensional bubble.鈥�

So what about time? In Grady鈥檚 version of the Universe, time comes in two
distinct varieties. First there is the 鈥渦niversal time鈥� which ticks away in the
bulk liquid. This time is completely hidden from us, because our Universe exists
only on the surface. The second kind is the time we experience. This, Grady
believes, arises as we are carried along the fourth space dimension,
perpendicular to the phase boundary. 鈥淲hat we perceive as time is actually the
extra space dimension,鈥� says Grady. 鈥淚t is different from the other space
dimensions because it extends out of the phase boundary and so is
颈苍补肠肠别蝉蝉颈产濒别.鈥�

Grady says he has not had any direct reaction to his idea from any other
physicists. No matter. According to Martin Rees of the University of Cambridge,
such ideas are not without value. 鈥淚t鈥檚 good to float alternatives to
conventional cosmology because it tests the limits of plausibility,鈥� he says.
鈥淏ut if you think hard about any of them, you can always find an
颈苍肠辞苍蝉颈蝉迟别苍肠测.鈥�

The idea that the Universe is an expanding phase boundary first occurred to
Grady in the mid-1980s. It was the recent claim that the expansion of the
Universe is actually accelerating, or at least not slowing down, that spurred
Grady to develop his ideas and submit a paper to the journal Physics Letters
A. 鈥淭he phase boundary would in the early Universe have undergone
accelerated expansion,鈥� says Grady. 鈥淓ventually, if the fluid dissipates energy,
the expansion would settle down to a constant rate.鈥�

Grady says he has gone to such lengths to concoct an alternative picture of
cosmology because it explains many puzzling things about our Universe in a very
natural way. Take the so-called horizon problem鈥攖he fact that regions of
the Universe that are today on opposite sides of the sky have the same
temperature. According to Einstein鈥檚 famous speed limit, different parts of the
Universe cannot behave in synchrony unless light has had time to travel between
them鈥攚hich means that regions on opposite parts of the sky should have
been out of touch with each other when their temperature was set in the very
early Universe.

Cosmologists have had to come up with the bizarre idea of inflation鈥攁
mind-bogglingly rapid expansion early in the Universe鈥攖o account for this.
Grady鈥檚 explanation is more straightforward. 鈥淭he seed that formed the Universe
was born with a uniform temperature for the simple reason that the fluid that
existed before had had time to reach a uniform temperature.鈥�

Another cosmological puzzle that Grady鈥檚 model explains is the 鈥渇latness
problem鈥濃�攖he fact that the Universe today is balanced on the knife-edge
between one that will expand forever and one that will eventually re-collapse.
It鈥檚 a puzzle, because this requires the expansion rate to have been
ridiculously fine-tuned in the early Universe. But in Grady鈥檚 model 鈥渢he
expansion of the boundary is naturally fine-tuned because a phase transition
occurs only at a critical temperature when the energy of the two phases is
别辩耻补濒鈥�.

Despite the apparent successes, however, not everything in Grady鈥檚 garden is
rosy. He cannot, for instance, explain the uniformity of the cosmic microwave
background radiation, the microwave afterglow of the big bang which still
permeates all of space. 鈥淭his is my most serious problem,鈥� he admits. 鈥淏ut my
idea explains so many other things that I鈥檓 hopeful things will eventually fall
into place.鈥�

Having an idea is one thing, proving it quite another. But Grady points out
that his theory makes some predictions which could be used to put it to the
test. For instance, he says, banging heavy ions together at close to the speed
of light might concentrate enough energy in a local region of space to
temporarily re-melt the solid phase. 鈥淚t would change the rate at which the
phase boundary advanced鈥攊n other words, the rate at which time passes
locally,鈥� says Grady. 鈥淎nd this might be noticeable in particle events triggered
by the ion-ion collision.鈥�

Another of Grady鈥檚 predictions emerges from the possibility that the
phase-nucleating event that was the big bang might not have been alone. 鈥淚f the
seed was some kind of dust-like impurity, or eddy current, we might expect other
seeds鈥攑erhaps concentrated in a small region of the fluid,鈥� he says. 鈥淚t
raises the possibility of collisions between universes, rather like collisions
between soap bubbles.鈥�

What would such a collision look like in our Universe? Grady says the area of
contact between colliding crystals would first appear as a point. Then it would
become an expanding spherical surface, rather like the shell of a supernova
explosion. Associated with this expanding spherical shell would be dramatic
dislocations of the phase boundary. These would create large amounts of matter
and antimatter, and copious radiation which would be further increased when some
of the matter and antimatter annihilated (see 鈥淭urn of the screw鈥�).

According to Grady, the most dramatic effect of a collision between our
Universe and another would be that everything from our Universe in the interior
of the expanding shell would be destroyed and replaced by matter from the other
universe. 鈥淚t鈥檚 just like one soap bubble colliding with another,鈥� he says. 鈥淭he
portion of the surface where one bubble touches the other eventually pops and is
replaced by the other bubble.鈥�

In fact, says Grady, there is some tentative evidence that this could have
happened in the past. If matter in the form of galaxies was created in this way,
it should be distributed as if on the surface of giant bubbles, the remains of
the joining boundaries of the now merged universes. 鈥淭his is exactly what
astronomers observe,鈥� says Grady. 鈥淎nd there is no real explanation within the
conventional big bang theory.鈥�

So is there any other evidence of universe-crunching collisions? 鈥淚t鈥檚
conceivable that prodigiously luminous quasars are where small universes are
colliding,鈥� says Grady. 鈥淭he fact quasars existed only in the early Universe
implies our Universe formed from a tight group of initial crystals which very
quickly merged.鈥�

It鈥檚 amazing that an idea as simple as an expanding crystal could explain so
many puzzles. But cosmologists are unlikely to abandon their current theories in
favour of Grady鈥檚 unless he can turn up some overwhelming evidence in its
favour. Grady points out that there is one dramatic way that his theory could
make itself felt: there could be another crystal growing right next to us in the
fluid that fills protospace. 鈥淚f it collided with us, there would be no
warning,鈥� he says. 鈥淚t would be like a mini big bang going off in our
neighbourhood.鈥� There is only one problem. If it did happen, it鈥檚 unlikely that
any of us would survive to witness his triumph.

Michael Grady鈥檚 idea that the Universe is a giant crystal growing in a
five-dimensional liquid scores successes in the subatomic realm as well as on
the cosmic scale. Take quantum fluctuations, the random seethings of the sea of
the vacuum. According to Grady, these fluctuations are simply the random
sloshing back and forth of heat in the bulk of the fluid. 鈥淭hese heat
fluctuations continually buffet the phase boundary in which we live,鈥� he
says.

Grady also thinks he can make sense of the behaviour of the subatomic
particles that form the building blocks of our world. These particles come in
two types: 鈥渇ermions鈥� such as electrons, which obey the Pauli exclusion
principle forbidding two particles from occupying the same quantum state; and
鈥渂osons鈥� such as photons, which observe no such restrictions. Grady believes
bosons are 鈥減honons鈥�, or vibrations of the crystal lattice, while fermions are
defects of the lattice known as 鈥渟crew dislocations鈥�. 鈥淭hink of the planes of a
crystal as the stacked floors of a multistorey parking lot,鈥� says Grady. 鈥淎
screw dislocation is like the spiral ramp connecting the floors.鈥�

Two identical screw dislocations obey the Pauli exclusion principle because
they repel each other when forced together. And a mirror-image pair of screw
dislocations, differing only in the sense in which they spiral, behave just like
a particle and its antiparticle. When they meet, they cancel each other and are
annihilated in a burst of energy. The opposite of this process is 鈥減air
production鈥�, in which two screw dislocations of opposite sense pop into
existence if vibrational energy is supplied to the lattice.

According to Grady, screw dislocations even obey Einstein鈥檚 special theory of
relativity, with the speed of sound in the solid acting like the speed of light
in our Universe. The link between the speed of sound and the rate at which screw
dislocations can travel was first shown by the Russian physicists J. Frenkel and
T. Kontorowa in 1938. According to their picture, as a screw dislocation
approaches this limiting speed it compresses in the direction of motion by
exactly the amount predicted by Einstein. At the same time, the stress energy of
the screw dislocation rises, again in accord with Einstein. 鈥淎t the speed of
sound, the energy, and hence the effective mass, of the dislocation becomes
infinite,鈥� says Grady. 鈥淔ermionic matter is therefore prevented from travelling
faster than light.鈥�