Reality could be made of anything from
blancmange to billiard balls and we'd never know.
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
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't they?
In some ways, Laughlin's 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-the
theory that describes the behaviour of matter on tiny scales-with Einstein's
general theory of relativity, which describes space, time and gravity. Such
a "theory of everything" 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. 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't 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. "It's
in the same way that flocking behaviour can be characterised without
understanding everything about birds, or superconductivity without understanding
Maybe the nature of reality is hidden from us.Everything is emergent but we'll never know what from.
This idea is called emergence. It's
a familiar phenomenon in the theory of condensed matter, which is Laughlin's
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. "Somewhat
like what would occur if one took a supple picket fence and rapidly twisted
one end back and forth," 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.
But if quasiparticles are indistinguishable from real particles,
could it be that things we think of as real-electrons and so on-are themselves
quasiparticles, emerging out of some ubiquitous but undetectable cosmic
This isn't 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'll 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. "If what you see is model-independent then you can't learn anything
about the underlying equations by observing it," says Laughlin. "You could
call this the dark side of emergence."
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-in the
process of emergence itself.
Whatever that model will eventually be, Laughlin's 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're 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. "If all we could observe was the quasiparticles,
we wouldn't be able to tell," 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 "stuff" that underlies reality is totally
irrelevant-it 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't have proved that strings are the
answer-merely 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-a little matter, say-to 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
quasi-particle might then start to show signs of its component parts. And
according to quantum mechanics, this would be at a temperature of
1032 kelvin-ten million billion times hotter than anything we've
achieved so far, and probably out of reach of any conceivable civilisation.
"What we emerge from is unknowable," says Laughlin. "The underlying equations
of the Universe cannot be determined from what we know."
This isn't enough to convince most theorists. "All these hopes
are fine and grand," says David Gross, a string theorist at the University
of California, Santa Barbara. "I'd like to see concrete examples of how you
can leap to quantum gravity from this idea."
His first problem is a big one: explain how space and time
emerge. Gross thinks it's impossible. "Space and time are coordinate systems
that sit there, they're not something that can emerge." But Laughlin says
Gross is voicing a mere article of faith.
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's 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'd finally
have a theory of quantum gravity.
But Smolin says it's hard to see how general covariance could
come out of quantum criticality. And Laughlin can't 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't assume any property at all, is there
any hope of getting general covariance to emerge?
And another kind of evidence could back Laughlin's 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
With this in mind he's 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's lab somewhere,
or even buried in the rock under your feet, is a small crystal that will
do the trick-a material that's a microcosm of the Universe.
Entangling particles is easy when you know how
THE dream of teleporting atoms and
molecules and maybe even larger object has become a real possibility for
the first time. The advance is thanks to physicists who have suggested a
method that in theory could be used to "entangle" absolutely any kind of
particle. Quantum entanglement
is the bizarre property that allows two particles to behave as one, no
matter how far apart they are. If you measure the state of one particle,
you instantly determine the state of the other. This could one day allow
us to teleport objects by transferring their properties instantly from one
place to another. Until now, physicists have only been able to entangle photons,
electrons and atoms, using different methods in each case. For instance,
atoms are entangled by forcing them to interact inside an optical trap, while
photons are made to interact with a crystal.
"These schemes are very specific," says Sougato Bose of the
University of Oxford. But Bose and Dipankar Home, of the Bose Institute in
Calcutta, have now demonstrated a single mechanism that could be used to
entangle any particles, even atoms or large molecules.
To see how it works, consider the angular momentum or "spin" of an electron.
To entangle the spins of two electrons, you first need to make sure they're
identical in all respects but their spin. Then you shoot the electrons
simultaneously into a beam splitter. This device "splits" each electron into
a quantum state called a
superposition, which gives it
an equal probability of travelling down either of two
paths. Only when you try to detect the electron do you know which path
it took. If you split two electrons simultaneously, both paths could have
one electron each (which will happen half of the time) or either path could
Bose and Home show mathematically that whenever one electron
is detected in each path, they will be entangled. While a similar effect
has been demonstrated before for photons, the photons used were already entangled
in another way, even before they reached the beam splitter. "One of the advances
we have made is that these two particles could be from completely independent
sources," says Bose.
The technique should work for any objects-atoms, molecules and
who knows what else as long as you can split the beam into a quantum
superposition. Anton Zeilinger, a quantum physicist
at the University of Vienna in Austria, has already shown that this quantum
state is possible with buckyballs-football-shaped
molecules of C60.
Although entangling such large objects is beyond our technical abilities
at the moment, this is the first technique that might one day make it possible.
"The theory of everything" by Robert Laughlin and David Pines, proceedings of the National Academy of Sciences (vol 97, p 28) "Quantum phase transitions and the breakdown of classical general relativity" by George Chapline and others (www.arxiv.org/abs/gr-qc/0012094)