On 4th July 2012, Rolf Heuer,
director of CERN, announced that the Higgs boson had been discovered. The
packed auditorium of scientists and reporters erupted in applause, cameras
flashed and history was made.
A few hours later, Googling “Higgs boson
found” gave 262 million results. Replacing “Higgs boson” by “God particle” gave
many million more.
So, what is the Higgs boson, what does it
mean for physics, and where does God come in?
The
Layman’s Picture
To start the story, let’s roll back 80
years.
In 1932, the discovery of the neutron
completed what I call the Layman’s Picture of the universe familiar nowadays to
anyone with a high school education (if they haven’t slept through all their
science classes, that is…).
In this picture, all the “stuff” in the
universe, aka “matter”, is made up of atoms, which are themselves composed of
protons, neutrons and electrons.
All matter is subject to the force of
gravity which shapes the stars and galaxies.
If, in addition, the matter has a charge, it also feels the forces of
electricity and magnetism.
Nearly 70 years before our story begins,
James Clerk Maxwell had shown that electricity and magnetism were two aspects
of the same force, henceforth called electromagnetism.
This was the first of the so called “unifications of physics” and as a stunning
consequence was that light was revealed to be an electromagnetic wave – a disturbance of the electromagnetic field
that propagates through space like waves through water.
The three decades, 1900 to 1930, had
witnessed possibly the most spectacular advances ever in theoretical physics.
On the one hand we had Einstein’s
theories of Relativity. Special Relativity reconciled Maxwell’s theory with
mechanics in a revolutionary way – among the implications was the equivalence
of mass and energy (yes, E = mc2). General Relativity subsumed
special relativity and revealed gravity an effect of the curvature of space and
time in the presence of matter and energy.
On the other hand, the theory of Quantum
Mechanics, developed by a group of physicists in the 1920’s had incredible
success explaining the world of atoms and molecules. This theory came with its
own baggage of bizarreness – wave-particle duality, the Uncertainty Principle
and more.
Many intellectuals believed that our
picture of the world was essentially complete. All that remained was to unify
gravity with electromagnetism and Einstein was already working on it.
The truth would turn out to be far
stranger…
The
Standard Model
Fast forward to the late 1940’s.
The terrifying potential of nuclear power
had engendered interest in peering even
deeper into the fundamental constituents of matter – after all who knew what
mightier sources of energy were waiting to be tapped ?
This was the age of the great particle
accelerators – huge machines that accelerate atoms to extremely high velocities
and smash them together.
The hope was to reveal even simpler
ingredients underlying protons and electrons.
What ensued instead was complete chaos.
Instead of a few simple components, the accelerators turned out a profusion of
literally hundreds of new particles at an astonishing rate!!
Physicists despaired of ever making sense
of the mess. Some speculated that rather than getting simpler, maybe matter
gets ever more complex as one goes further down…
Then as the 50’s rolled into the 60’s, a
picture began to emerge, and here it is:
Welcome to the Standard Model. This is
the best picture we have so far of the fundamental constituents of our
universe.
To the left of the picture are the basic
building blocks of matter.
You will notice that protons and neutrons
are nowhere in the picture. That’s because they themselves are made of even
more basic constituents – the quarks.
Quarks come in 6 varieties – up, down, charmed, strange, top and bottom.
Electrons, however, are in the picture.
The “e” in the upper left corner of the
group called “leptons” is the electron.
Along with them in the same row are the muon and the tau particle.
The bottom row on the left shows the 3
types of neutrino – strange,
ghostlike particles than can pass through entire planets without leaving a
trace.
Why these twelve? Why no more or fewer?
Nobody knows.
The right side shows the “force
particles”. Come again?
Well, the theoretical framework
underlying the forces of nature is Quantum Field Theory (QFT).
In QFT, each force is represented by a “quantum
field” pervading all of space. Associated with each such quantum field is a
characteristic particle.
In the parlance of QFT, a matter particle
“feels” a force if it is able to “interact with” or “couple to” the particle of
the corresponding quantum field.
Just to make things fun, the force
particles can even interact with each other, or even themselves. (This comes up
later)
Clear? So now, the “γ” at bottom right is
the photon, which is the particle
corresponding to the electromagnetic force.
As for the rest – two new forces were
discovered via all the particle smashing.
The strong
nuclear force binds quarks together into protons and neutrons. Its particle
is the gluon, which is the “g” on top
right.
The weak
nuclear force is responsible for radioactive decay and its particles are
the W and Z in middle right.
[A diversion: You will notice that the “force particles”
are called bosons. This is because all force particles share some commonalities
that were elucidated in the early 1920’s by Bengali physicist Satyendranath
Bose. So, all Bengalis reading this – enjoy your 2 seconds of reflected glory.]
But talking of forces, where’s gravity?
Welcome to the biggest gap in the current foundations of theoretical physics.
Despite decades of effort, nobody has come up with a successful QFT for
gravity. Anybody who succeeds will usher in a new era of physics.
But we digress.
Another force particle is missing in this
picture – missing because until very recently, nobody was sure it even existed.
Enter the Higgs boson.
The
Higgs Field(s)
Remember Maxwell joining electricity and
magnetism together?
The quest to unite the forces of nature
under a single description has been a prime motivator for theoretical physics
over the last century.
Starting with Paul Dirac in the 1920’s,
physicists managed to merge Maxwell’s theory with quantum mechanics to develop
the spectacularly successful theory of Quantum Electrodynamics – the QFT for
the electromagnetic field over the next several decades.
(Heard of Richard Feynman? This was his
biggest work.)
Why stop with electromagnetism? From the
1960’s onwards an ongoing effort was made to incorporate both the strong and
weak nuclear force into the same framework.
But there was a problem.
The photon and gluon have zero mass while
the W and Z particles of the weak force have mass.
Why is that a problem?
The reasons are extremely technical
(here’s where a PhD in physics comes in handy).
Suffice it to say that the theoretical
framework constrains the particles of all three forces to have zero mass. So,
how to reconcile theory with reality?
But what if there’s another “force” – another quantum field pervading the world? Maybe
the reason why the W and Z particles seem to have mass is because they feel
this strange new force.
To make a very crude analogy, suppose you
are doing experiments with marbles on a very smooth floor. You flick all the
marbles with the same force. Some of them shoot off, while others roll away
slowly and then come to a stop. How to explain this?
One possibility – they have different
weights and respond differently to your flicking.
But here’s another – they all weight the same, but
some of the marbles are made of iron, and there’s a powerful magnet behind you.
The iron marbles feel the magnetic field
and the others don’t, hence the different reactions.
The explanation proposed for the masses
of the W and Z is similar.
The mysterious field, analogous to the
magnetic field of our example, is the Higgs
field – first proposed by Peter Higgs in 1964.
The idea of a Higgs field was used to
unify electromagnetism with the weak force by Sheldon Glashow, Abdus Salam and
Steven Weinberg – a feat which won them a Nobel prize in 1979.
But now the bonus:
If interaction with the Higgs field can
“give mass” to the W and Z particles, could
it be the case that all particles
derive their mass from the same mechanism?!
If true, this would mean that mass is not
a fundamental property of matter – just a consequence of some particles
“feeling the Higgs field”.
So, does this really happen? Yes. If you believe the Standard Model
Why do different particles have different
masses?
Nobody knows. The model says that a
particle’s mass is proportional to how strongly it interacts with the Higgs,
but that just pushes the question one step back.
Is there just one Higgs field or many?
Nobody knows.
In
fact, until a week ago, nobody was sure that there was even a single
Higgs field.
A number of alternative theories had been
proposed to explain the particle masses without any need for a Higgs – although
all these theories have a bunch of side-effects.
So, how would one know if the Higgs field
was just a figment of the imagination?
Answer: As mentioned above, every quantum
field has an associated particle. The particle associated with the Higgs field
is the Higgs boson. (Finally, we get
to the title of the post!)
Detect the Higgs boson and you know the
field exists.
And that’s
what we did on July 4, 2012!
(Note: Strictly speaking I should say we
detected a Higgs boson, and so there
is at least one Higgs field. For all
we know, there could be dozens of them.)
Stagnation
Okay, so if you’ve followed so far, we
just verified a theory proposed nearly 50 years ago.
What’s the big deal? And how did we do
it?
Answering the first question requires a
bit of scientific history.
The late 70’s were a heady time for
physicists.
The past six decades had been a period of
unprecedented progress, leading us deeper than ever into the secrets of Nature.
The constant stream of insights and
breakthroughs, it was felt, could only have one end – the Theory of Everything,
unifying space, time, matter, energy and forces into one stupendously grand
overarching framework.
Many – including the very outspoken
Stephen Hawking – believed that this would happen by the turn of the
millennium, bringing the Century of Physics to a supremely triumphant end.
What followed instead was three decades
of massive stagnation.
String Theory, hailed as the most
promising candidate for unification, degenerated into a thicket of wild
speculation and unverifiable hypotheses. Currently, supporters claim that they
have made great “conceptual progress”, while detractors argue that the theory
is “not even wrong”, i.e. it can be tweaked arbitrarily to fit any observation.
On the experimental side, things ground
to a halt with Congress refusing funding for more powerful particle accelerators.
Only the astronomers maintained an iota
of progress, indicating that not all was well. Evidence steadily mounted,
especially in the last decade, that the matter described by the Standard Model
only constitutes about 15% of all the matter there is. Meanwhile, cosmology
yielded a huge surprise – the expansion of the universe is accelerating and
nobody knows what’s causing it.
But alas, no help was forthcoming from
the theorists to explain any of this, lost as they were in the wild goose chase
of strings…
For those not enamored of string-world,
the easiest path beyond the Standard Model lay in the investigation of the
Higgs field(s). This was the only feature of the theory which remained somewhat
speculative, but concrete experimental data was needed to make headway.
Hence, all hope focused on the Large
Hadron Collider, scheduled to start operating in early 2009 at CERN in
Switzerland.
Discovery
The Large Hadron Collider (LHC) is the
most powerful particle accelerator ever built.
Magnets of tremendous power accelerate
beams of protons are along a circular tunnel 27 km long and smash them into
each other with savage force. Hundreds of detectors track the debris that
erupts and record its characterestics.
How does this help?
Well, quantum theory predicts that
collisions of particles don’t just give you constituents of the things
colliding. Completely new particles can arise, born out of the conversion of
energy into mass. The higher the energy of collision, the higher the mass of
the particle that may appear. (Remember, E = mc2).
[This, to my mind is one of the weirdest
aspects of the quantum world. Imagine a situation where you smash two stones
hard enough and a butterfly pops out and flies off. That is pretty much what
goes on in these collisions!]
The hope was that the collision energy of
the LHC would be sufficient to generate a Higgs boson.
But that would just be the beginning,
because the Higgs could not be detected directly.
Instead, theory predicts that it would
quickly decay into other types of particles.
So, the hope was to spot the relics of the Higgs rather than the
particle itself.
To make matters even worse, the
collisions at the LHC would generate zillions of other particles.
Thus, a Higgs that forms and quickly
decays would only manifest as a slight
excess of particles spotted at certain energies.
And finally, the Standard Model did not
predict what the mass/energy of the Higgs boson would be. So, the experimenters
would have to comb through a vast range of energy bands, looking for tiny
excesses in the number of particles of certain types.
See the problem? Seeking a needle in a
haystack is trivial by comparison.
But this is what was achieved!
In Dec 2011, after the LHC has run for
about 2 years, researchers reported a slight excess of particles in the 120 to
130 GeV range.
(Gev stands for “Giga-Electron-Volts” and
is a unit of energy. The mass of a proton is about 1 GeV, which is about 1.8×10−27 kg)
But statistical techniques indicated this
was only a “two sigma” result – there is a 2% chance of seeing something like
this purely by chance. More work needed to be done.
Over the next 6 months, a massive effort
was undertaken to scrutinize the mountains of data that had accumulated.
Towards the last weeks of June, excitement rose in the scientific blogosphere.
Rumors circulated that this was indeed the real thing. And so it was.
On July 4th, after months of
speculation, the announcement was finally made.
Two separate sets of experiments had
verified the existence of a particle with a mass of about 125 GeV.
In each case, the probability of a chance
occurrence was “five sigma”– less than one in a million.
The Higgs boson had been found.
Dreams
and Nightmares
So what does this mean for the future of
physics?
Very different things, depending on whom
you ask.
Optimists like cosmologist Sean Carroll
believe that the Higgs only heralds the beginning of a series of discoveries by
the LHC, ushering in a new dawn for physics.
But not everyone agrees.
Detractors argue that all the properties
of the Higgs discovered are completely consistent with the Standard Model which
is nearly 40 years old. There is no hint yet of anything inexplicable, no trace
of any new particles or phenomena, nothing to suggest the next step forward.
Maybe the only new physics occurs at much
higher energies, way beyond what the LHC can probe and way beyond our
technological capabilities.
This would be a physicist’s nightmare.
So, what shall it be? Dream or nightmare?
Progress at last or stagnation without end?
Only time – and more data – will tell.
------x------
Afterword
on the “God particle”:
This nickname for the Higgs boson was
used by physicist Leon Lederman in a popular book on the subject. It remains,
to date, the most egregious example of “God-mongering” to sell a popular
science book, closely followed by Stephen Hawking’s “Brief History of Time”.
So just to be clear:
No, the Higgs boson has no religious
significance or divine powers, and its discovery does not prove the existence
of God.
