NOBEL PRIZE IN PHYSICS 2013
The Nobel Prize in Physics 2013 was awarded jointly to François Englert
and Peter W. Higgs "for the
theoretical discovery of a mechanism that contributes to our understanding of
the origin of mass of subatomic particles, and which recently was confirmed
through the discovery of the predicted fundamental particle, by the ATLAS and
CMS experiments at CERN's Large Hadron Collider"
François
Englert and Peter W. Higgs are jointly awarded the Nobel Prize in Physics 2013
for the theory of how particles acquire
mass. In 1964, they proposed the theory independently of each other (Englert together with his now deceased
colleague Robert Brout). In 2012, their ideas were confirmed by the discovery
of a so called Higgs particle at the CERN laboratory outside Geneva in
Switzerland.
The awarded mechanism
is a central part of the Standard Model of particle physics that describes how
the world is constructed. According to the Standard Model, everything, from
flowers and people to stars and planets, consists of just a few building
blocks: matter particles. These particles are governed by forces mediated
by force particles that make sure everything works as it should.
The entire Standard
Model also rests on the existence of a special kind of particle: the Higgs
particle. It is connected to an invisible field that fills up all space. Even
when our universe seems empty, this field is there. Had it not been there,
electrons and quarks would be massless just like photons, the light particles.
And like photons they would, just as Einstein’s theory predicts, rush through
space at the speed of light, without any possibility to get caught in atoms or
molecules. Nothing of what we know, not even we, would exist.
Both François Englert
and Peter Higgs were young scientists when they, in 1964, independently of each
other put forward a theory that rescued the Standard Model from collapse.
Almost half a century later, on Wednesday 4 July 2012, they were both in the
audience at the European Laboratory for Particle Physics, CERN, outside Geneva,
when the discovery of a Higgs particle that finally confirmed the theory was
announced to the world.
The model that created order
The idea that the world can be explained in terms of just a few building
blocks is old. Already in 400 BC, the philosopher Democritus postulated that
everything consists of atoms — átomos is Greek for indivisible. Today we know
that atoms are not indivisible. They consist of electrons that orbit an atomic
nucleus made up of neutrons and protons. And neutrons and protons, in turn,
consist of smaller particles called quarks. Actually, only electrons and quarks
are indivisible according to the Standard Model.
The atomic nucleus
consists of two kinds of quarks, up quarks and down quarks. So in fact, three
elementary particles are needed for all matter to exist: electrons, up quarks
and down quarks. But during the 1950s and 1960s, new particles were
unexpectedly observed in both cosmic radiation and at newly constructed
accelerators, so the Standard Model had to include these new siblings of
electrons and quarks.
Besides matter
particles, there are also force particles for each of nature’s four forces —
gravitation, electromagnetism, the weak force and the strong force. Gravitation
and electromagnetism are the most well-known, they attract or repel, and we can
see their effects with our own eyes. The strong force acts upon quarks and
holds protons and neutrons together in the nucleus, whereas the weak force is
responsible for radioactive decay, which is necessary, for instance, for
nuclear processes inside the Sun. The Standard Model of particle physics unites
the fundamental building blocks of nature and three of the four forces known to
us (the fourth, gravitation, remains outside the model). For long, it was an
enigma how these forces actually work. For instance, how does the piece of
metal that is attracted to the magnet know that the magnet is lying there, a
bit further away? And how does the Moon feel the gravity of Earth?
Invisible fields fill space
The
explanation offered by physics is that space is filled with many invisible
fields. The gravitational field, the electromagnetic field, the quark field and
all the other fields fill space, or rather, the four dimensional space-time, an
abstract space where the theory plays out. The Standard Model is a quantum
field theory in which fields and particles are the essential building blocks of
the universe. In quantum physics, everything is seen as a collection of
vibrations in quantum fields. These vibrations are carried through the field in
small packages, quanta, which appear to us as particles. Two kinds of fields
exist: matter fields with matter particles, and force fields with force
particles — the mediators of forces. The Higgs particle, too, is a vibration of
its field — often referred to as the Higgs field. Without this field the
Standard Model would collapse like a house of cards, because quantum field
theory brings infinities that have to be reined in and symmetries that cannot
be seen. It was not until François Englert with Robert Brout, and Peter Higgs,
and later on several others, showed that the Higgs field can break the symmetry
of the Standard Model without destroying the theory that the model got
accepted.
This is because the
Standard Model would only work if particles did not have mass. As for the
electromagnetic force, with its massless photons as mediators, there was no
problem. The weak force, however, is mediated by three massive particles; two
electrically charged W particles and one Z particle. They did not sit well with
the light-footed photon. How could the electroweak force, which unifies
electromagnetic and weak forces, come about? The Standard Model was threatened.
This is where Englert, Brout and Higgs entered the stage with the ingenious
mechanism for particles to acquire mass that managed to rescue the Standard
Model.
The ghost-like Higgs field
The Higgs
field is not like other fields in physics. All other fields vary in strength
and become zero at their lowest energy level. Not the Higgs field. Even if
space were to be emptied completely, it would still be filled by a ghost-like
field that refuses to shut down: the Higgs field. We do not notice it; the
Higgs field is like air to us, like water to fish. But without it we would not
exist, because particles acquire mass only in contact with the Higgs field.
Particles that do not pay attention to the Higgs field do not acquire mass,
those that interact weakly become light, and those that interact intensely
become heavy. For example, electrons, which acquire mass from the field, play a
crucial role in the creation and holding together of atoms and molecules. If
the Higgs field suddenly disappeared, all matter would collapse as the suddenly
massless electrons dispersed at the speed of light.
So what makes the Higgs
field so special? It breaks the intrinsic symmetry of the world. In nature,
symmetry abounds; faces are regularly shaped, flowers and snowflakes exhibit
various kinds of geometric symmetries. Physics unveils other kinds of
symmetries that describe our world, albeit on a deeper level. One such,
relatively simple, symmetry stipulates that it does not matter for the results
if a laboratory experiment is carried out in Stockholm or in Paris. Neither
does it matter at what time the experiment is carried out. Einstein’s special
theory of relativity deals with symmetries in space and time, and has become a
model for many other theories, such as the Standard Model of particle physics.
The equations of the Standard Model are symmetric; in the same way that a ball
looks the same from whatever angle you look at it, the equations of the
Standard Model remain unchanged even if the perspective that defines them is
changed. The principles of symmetry also yield other, somewhat unexpected,
results. Already in 1918, the German mathematician Emmy Noether could show that
the conservation laws of physics, such as the laws of conservation of energy
and conservation of electrical charge, also originate in symmetry. Symmetry,
however, dictates certain requirements to be fulfilled. A ball has to be
perfectly round; the tiniest hump will break the symmetry. For equations other
criteria apply. And one of the symmetries of the Standard Model prohibits
particles from having mass. Now, this is apparently not the case in our world,
so the particles must have acquired their mass from somewhere. This is where
the now-awarded mechanism provided a way for symmetry to both exist and
simultaneously be hidden from view.
The symmetry is hidden but is still there
Our
universe was probably born symmetrical. At the time of the Big Bang, all
particles were massless and all forces were united in a single primordial
force. This original order does not exist anymore — its symmetry has been hidden
from us. Something happened just 10–11
seconds after the Big Bang. The Higgs field lost its original
equilibrium. How did that happen? It all began symmetrically. This state can be
described as the position of a ball in the middle of a round bowl, in its
lowest energy state. With a push the ball starts rolling, but after a while it
returns down to the lowest point.
However, if a hump
arises at the centre of the bowl, which now looks more like a Mexican hat, the
position at the middle will still be symmetrical but has also become unstable.
The ball rolls downhill in any direction. The hat is still symmetrical, but
once the ball has rolled down, its position away from the centre hides the
symmetry. In a similar manner the Higgs field broke its symmetry and found a
stable energy level in vacuum away from the symmetrical zero position. This
spontaneous symmetry breaking is also referred to as the Higgs field’s phase
transition; it is like when water freezes to ice.
In order for the phase
transition to occur, four particles were required but only one, the Higgs
particle, survived. The other three were consumed by the weak force mediators,
two electrically charged W particles and one Z particle, which thereby got
their mass. In that way the symmetry of the electroweak force in the Standard
Model was saved — the symmetry between the three heavy particles of the weak
force and the massless photon of the electromagnetic force remains, only hidden
from view.
Extreme machines for extreme physics
The Nobel Laureates
probably did not imagine that they would get to see the theory confirmed in
their lifetime. It took an enormous effort by physicists from all over the
world. For a long time two laboratories, Fermilab outside Chicago, USA, and
CERN on the Franco-Swiss border, competed in trying to discover the Higgs
particle. But when Fermilab’s Tevatron accelerator was closed down a couple of
years ago, CERN became the only place in the world where the hunt for the Higgs
particle would continue.
CERN was established in 1954, in an attempt to
reconstruct European research, as well as relations between European countries,
after the Second World War. Its membership currently comprises twenty states,
and about a hundred nations from all over the world collaborate on the
projects.
CERN’s grandest achievement, the particle collider LHC (Large Hadron
Collider) is probably the largest and the most complex machine ever
constructed by humans. Two research groups of some 3,000 scientists chase
particles with huge detectors — ATLAS and CMS. The detectors are located 100
metres below ground and can observe 40 million particle collisions per second.
This is how often the particles can collide when injected in opposite
directions into the circular LHC tunnel, 27 kilometres long.
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Protons are
injected into the LHC every ten hours, one ray in each direction. A hundred
thousand billion protons are lumped together and compressed into an ultra-thin
ray — not entirely an easy endeavour since protons with their positive
electrical charge rather aim to repel one another. They move at 99.99999 per
cent of the speed of light and collide with an energy of approximately 4 TeV
each and 8 TeV combined (one teraelectronvolt = a thousand billion
electronvolts). One TeV may not be that much energy, it more or less equals
that of a flying mosquito, but when the energy is packed into a single proton,
and you get 500 trillion such protons rushing around the accelerator, the
energy of the ray equals that of a train at full speed. In 2015 the energy will
be almost the double in the LHC.
A puzzle inside the puzzle
Particle
experiments are sometimes compared to the act of smashing two Swiss watches
together in order to examine how they are constructed. But it is actually much
more difficult than so, because the particles scientists look for are entirely
new — they are created from the energy released in the collision.
According to
Einstein’s well-known formula E = mc2, mass is a kind of energy. And it is the magic of this equation
that makes it possible, even for massless particles, to create something new
when they collide; like when two photons collide and create an electron and its
antiparticle, the positron, or when a Higgs particle is created in the
collision of two gluons, if the energy is high enough.
The protons are like
small bags filled with particles — quarks, antiquarks and gluons. The majority
of them pass one another without much ado; on average, each time two particle
swarms collide only twenty full frontal collisions occur. Less than one collision
in a billion might be worth following through. This may not sound much, but
each such collision results in a sparkling explosion of about a thousand
particles. At 125 GeV, the Higgs particle turned out to be over a hundred times
heavier than a proton and this is one of the reasons why it was so difficult to
produce.
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However, the experiment is far from finished. The scientists at CERN
hope to bring further ground-breaking discoveries in the years to come. Even
though it is a great achievement to have found the Higgs particle — the missing
piece in the Standard Model puzzle — the Standard Model is not the final piece
in the cosmic puzzle.
One of the reasons for this is that the Standard Model
treats certain particles, neutrinos, as being virtually massless, whereas
recent studies show that they actually do have mass. Another reason is that the
model only describes visible matter, which only accounts for one fifth of all
matter in the universe. The rest is dark matter of an unknown kind. It is not
immediately apparent to us, but can be observed by its gravitational pull that
keeps galaxies together and prevents them from being torn apart.
In all other
respects, dark matter avoids getting involved with visible matter. Mind you,
the Higgs particle is special; maybe it could manage to establish contact with
the enigmatic darkness. Scientists hope to be able to catch, if only a glimpse,
of dark matter, as they continue the chase of unknown particles in the LHC in
the coming decades.
François Englert
Born: 1932
Affiliation at the time of the award: Université Libre de Bruxelles, Brussels, Belgium
Prize motivation: "for the theoretical discovery of a mechanism that
contributes to our understanding of the origin of mass of subatomic particles,
and which recently was confirmed through the discovery of the predicted
fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron
Collider"
Peter W. Higgs
Born: 1929
Affiliation at the time of the award: University of Edinburgh, Edinburgh, United Kingdom
Prize motivation: "for the theoretical discovery of a mechanism that
contributes to our understanding of the origin of mass of subatomic particles,
and which recently was confirmed through the discovery of the predicted fundamental
particle, by the ATLAS and CMS experiments at CERN's Large Hadron
Collider"
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