WHAT are the fundamental building blocks of the Universe? Once we were told
they were atoms. Then it turned out that these were not fundamental at all, but
made of protons, neutrons and electrons. Protons and neutrons are in turn made
of quarks. Deeper still, we now learn, come tiny vibrating strings and membranes
living in a space of 10 or 11 dimensions. But we all expect that one day
physicists will finally discover the deepest structures of nature. Won鈥檛
they?
Not necessarily. Maybe it鈥檚 impossible to discover these deepest structures.
What鈥檚 more, maybe it doesn鈥檛 matter what they are. That鈥檚 the startling claim
of Robert Laughlin, a Nobel laureate at Stanford University. According to
Laughlin, it may be that what we call reality is a spontaneous phenomenon,
emerging like a wave out of some forever unknowable cosmic medium.
In some ways, Laughlin鈥檚 ultimate aim is not so different from that of other
theoretical physicists. Their common goal is to find a single theory that unites
quantum mechanics鈥攖he theory that describes the behaviour of matter on
tiny scales鈥攚ith Einstein鈥檚 general theory of relativity, which describes
space, time and gravity. Such a 鈥渢heory of everything鈥� would unite all the
forces of nature and explain why time and space exist, as well as answering such
trifles as how the Universe began and what happens at the centre of a black
hole. Ambitious stuff.
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The well-trodden route towards this ultimate theory is to try to find the
right building blocks of reality and then see whether they can account for the
natural phenomena we observe. Laughlin is treading a very different path,
however, because he believes you can鈥檛 build a theory of everything from the
bottom up. The laws that govern large-scale phenomena will not be deduced from
the laws that govern tiny particles, he says. 鈥淚t鈥檚 in the same way that
flocking behaviour can be characterised without understanding everything about
birds, or superconductivity without understanding atomic theory.鈥�
This idea is called emergence. It鈥檚 a familiar phenomenon in the theory of
condensed matter, which is Laughlin鈥檚 background. Solids and liquids sometimes
play host to strange entities that bear little resemblance to the atoms making
up the substance.
For example, in some materials there are things called spin waves. Every atom
acts a bit like a small magnet, with a north and a south pole aligned along its
spin axis, and spin waves are oscillations in the alignment of these spins.
鈥淪omewhat like what would occur if one took a supple picket fence and rapidly
twisted one end back and forth,鈥� says Laughlin. Because this is the quantum
world, waves can be considered as particles, and vice versa, so spin waves
behave like a kind of emergent particle.
Many other kinds of emergent creature live inside matter, including
vibrational waves called phonons, electrical excitations called excitons in
semiconductors, and waves of charge called plasmons. These are called variously
鈥渃ollective excitations鈥� and 鈥渜uasiparticles鈥�. From inside the material, these
bizarre objects would seem as real as any other particle.
But if quasiparticles are indistinguishable from real particles, could it be
that things we think of as real鈥攅lectrons and so on鈥攁re themselves
quasiparticles, emerging out of some ubiquitous but undetectable cosmic
stuff?
It鈥檚 a controversial idea. Sure, even in string theory and other bottom-up
theories, matter particles arise from the behaviour of smaller building blocks.
But for Laughlin there鈥檚 a crucial difference鈥攚e can never determine what
that basic 鈥渟tuff鈥� is.
In a solid, for example, quasiparticles can鈥檛 be derived or predicted from
the behaviour of the individual particles they are made of. In general it has
proved impossible to solve the quantum equations of motion for each interacting
atom to predict the existence of spin waves or phonons: there are just too many
equations to handle. This means knowing about the quasiparticles may tell you
nothing about what they are made of.
This isn鈥檛 a problem for ordinary materials, because by studying these at
higher temperatures, we already know what they are made of. But if the Universe
works like this, then maybe the underlying nature of reality is hidden from us.
Everything is emergent, but we鈥檒l never know what from. It would explain why
physicists have so far had such trouble finding the right fundamental particles
to unify the whole of physics in a theory of everything. 鈥淚f what you see is
model-independent then you can鈥檛 learn anything about the underlying equations
by observing it,鈥� says Laughlin. 鈥淵ou could call this the dark side of
别尘别谤驳别苍肠别.鈥�
At first sight, this is a depressing conclusion. Does it mean physicists
should just give up their quest? Thankfully not. Laughlin thinks we just have to
look elsewhere for the fundamental nature of reality鈥攊n the process of
emergence itself.
The range of emergent particles found in most condensed-matter systems is far
too limited to be a blueprint for a theory of everything. Such a theory would
need to account for a plethora of particles, including their charge, spin and
mass, and also spawn the whole of space and time. 鈥淜nown cases of emergence are
too primitive to serve as a model for real space-time,鈥� Laughlin says. 鈥淲e need
to find better ones.鈥�
Whatever that model will eventually be, Laughlin鈥檚 betting it will exploit a
phenomenon called quantum criticality. This is a kind of behaviour seen in some
materials near absolute zero, when they are poised between one state and
another. For example, in some magnetic solids individual spins become so highly
correlated that the behaviour of one affects them all, and the collective
wavefunction of the material lacks any sense of scale.
And to Laughlin, this is a highly desirable property, because scale
invariance is also a fundamental property of space-time. In the standard model
of particle physics, particles are thought of as collective oscillations of the
vacuum of space. In this model, a small chunk of space oscillates exactly as
much as a larger chunk of space. This is just like when you zoom in on a stretch
of coastline while looking at a map. You see as much variation in the coast no
matter what scale you鈥檙e looking at, because as the scale of the map gets
smaller, you lose sight of larger variations but become sensitive to smaller
ones.
So modelling the Universe with quantum criticality gives you scale invariance
for free. But it also means that any sense of the material being made up of
building blocks is lost. In a superconducting material, for example, nothing
about what the material is made of follows from the behaviour of spin waves. 鈥淚f
all we could observe was the quasiparticles, we wouldn鈥檛 be able to tell,鈥� says
Matthew Fisher, who works on the theory of quantum criticality at the University
of California, Santa Barbara.
Likewise, if the very fabric of the Universe is in a quantum-critical state,
then the 鈥渟tuff鈥� that underlies reality is totally irrelevant鈥攊t could be
anything, says Laughlin. Even if the string theorists show that strings can give
rise to the matter and natural laws we know, they won鈥檛 have proved that strings
are the answer鈥攎erely one of the infinite number of possible answers. It
could as well be pool balls or Lego bricks or drunk sergeant majors.
Just a minute, though. If you can warm up a quantum-critical solid so that
the bits become visible again, why not heat up a piece of the Universe鈥攁
little matter, say鈥攖o do the same? This is effectively what experimental
particle physicists have been doing for decades with particle accelerators. The
trouble is, to see the underlying medium of reality you have to reach beyond the
maximum energy that a quasiparticle can carry. The quasiparticle might then
start to show signs of its component parts. And according to quantum mechanics,
this would be at a temperature of 1032 kelvin鈥攖en million billion times
hotter than anything we鈥檝e achieved so far, and probably out of reach of any
conceivable civilisation. 鈥淲hat we emerge from is unknowable,鈥� says Laughlin.
鈥淭he underlying equations of the Universe cannot be determined from what we
办苍辞飞.鈥�
This isn鈥檛 enough to convince most theorists. 鈥淎ll these hopes are fine and
grand,鈥� says David Gross, a string theorist at the University of California,
Santa Barbara. 鈥淚鈥檇 like to see concrete examples of how you can leap to quantum
gravity from this idea.鈥�
So Laughlin needs to build a testable theory from this idea, and derive the
things we see from the ideas of quantum criticality. 鈥淭he hope is to find the
right equations that get it all to come rolling out,鈥� he says.
His first problem is a big one: explain how space and time emerge. Gross
thinks it鈥檚 impossible. 鈥淪pace and time are coordinate systems that sit there,
they鈥檙e not something that can emerge.鈥� But Laughlin says Gross is voicing a
mere article of faith.
Lee Smolin, a theoretical physicist at Perimeter Institute for Theoretical
Physics in Waterloo, Canada, is more sympathetic. Smolin has worked on a theory
called loop quantum gravity, in which space and time emerge from abstract
creations called spin networks. They produce a quantum version of space-time
called spin foam, which is grainy on very small scales鈥攋ust what is needed
for a quantum version of gravity.
Smolin has had to give his spin networks one basic property, a fundamental
symmetry of the Universe called general covariance. This states that however you
are moving, all the laws of physics (for example, the observed speed of light)
look the same. It is the principle behind general relativity, which is still our
best description of gravity, space and time. But it鈥檚 at odds with all quantum
theories of how subatomic particles behave. If you could derive a model of the
Universe that includes generally covariant particles, then you鈥檇 finally have a
theory of quantum gravity.
But Smolin says it鈥檚 hard to see how general covariance could come out of
quantum criticality. And Laughlin can鈥檛 just assume, as Smolin did, that his
building blocks have this property, because there are no building blocks in the
first place. If he can鈥檛 assume any property at all, is there any hope of
getting general covariance to emerge?
Laughlin cites the work of Subir Sachdev, who is modelling quantum-critical
systems together with Yale colleagues Matthias Vojta and Ying Zhang. In 2000,
the Yale group explained some mysterious fluctuations in the electrical current
in a superconductor. They worked out that it was in a quantum-critical state
with a new kind of quasiparticle. Astonishingly, these quasiparticles obey their
own version of special relativity. For example, there鈥檚 an absolute speed that
they can鈥檛 surpass (although it鈥檚 not the speed of light). This implies that the
particles observe a spatial symmetry called Lorenz invariance鈥攖hat is,
frames moving with constant velocity all see the same physical laws. This is a
weaker symmetry than general covariance, but if the one can emerge, says
Laughlin, why not the other?
He is now working on a three-dimensional quantum-critical system in which he
hopes objects similar to black holes will emerge. It builds on the work that won
him a Nobel prize, in which he predicted the existence of quasiparticles with
fractional electric charge.
So far, he has one very tentative prediction. He and George Chapline of Los
Alamos National Laboratory speculated that black hole event horizons might be a
kind of phase transition in the vacuum. By analogy with a condensed matter phase
transition, they concluded that relativity might fail at the horizon, altering
the spectrum of the light that comes off the black hole. So this theory could in
principle be tested.
And another kind of evidence could back Laughlin鈥檚 ideas up. A real material.
So far, no material is known to have the right kind of quantum critical
behaviour to include generally covariant quasiparticles, but if we did find such
a material, Laughlin thinks it would put emergence ahead of string theory or
loop quantum theory as the leading approach to quantum gravity.
With this in mind he鈥檚 looking at all sorts of low-temperature materials in
the hope of spotting some overlooked phenomena that might be models of reality.
The nearest so far comes from Grisha Volovik at the Helsinki University of
Technology, who claimed in 2000 that there are quasiparticles in liquid helium-3
that respect special relativity.
This is still a long way from modelling the whole of particle physics and
general relativity. But perhaps, in some chemist鈥檚 lab somewhere, or even buried
in the rock under your feet, is a small crystal that will do the trick鈥攁
material that鈥檚 a microcosm of the Universe.