The Standard Model (below) is a highly successful theory of physics. It describes the most fundamental particles we know and their interactions, helping us to understand the deep inner workings of nature all the way back to fractions of a second after the Big Bang. While it is now regarded as one of the major successes of particle physics, it was the result of much hard work. Back in the 1950’s particle physics was struggling to explain what its experiments were finding.
The problem that physicists had was they did not have a theory to explain two of the fundamental forces, the strong and the weak interactions, and they were confronted with a myriad of sub-atomic particles, nicknamed the particle zoo, that they again could not explain.
The explanation for the particle zoo came in the form of the quark model proposed by Murray Gell-Mann and George Zweig. The idea is that all of the sub-atomic particles that had been found were not in fact fundamental particles but were made up of particles called quarks.
The original proposal only contained the up, down and strange quarks, the charm (Sheldon Glashow, John Iliopoulos and Luciano Maiani, 1970), the top and the bottom (Makoto Kobayashi and Toshihide Maskawa, 1973) were added later. The problem of the weak and the strong interactions proved more difficult to solve. The issue was that the methods of performing calculations used to formulate the theory of Quantum Electrodynamics (QED) that describes the electromagnetic interaction (and describes it incredibly well), a) produced nonsense results for the weak interaction that could not be fixed in the same manner as for QED and b) did not even work for the strong interaction.
From left to right, Tom Kibble, Gerald Guralnik, Carl Hagen, François Englert and Robert Brout
(Credit: Wikipedia Commons/Tim Roettger)
By the early 1960’s physicists were tantalizingly close to a theory for the weak interaction, one that would unify the weak interaction with the electromagnetic interaction, similar to the unification of electricity and magnetism by James Clerk Maxwell in 1873, a game changing idea that we are all now taught at school it is so fundamental.
Components of the Yang-Mills theory and spontaneous symmetry breaking mechanism could explain the weak interaction but required massless bosons. Physicists knew that the bosons for the weak interaction couldn’t be massless or they would have already seen evidence of their existence so it was back to the drawing board, or so it seemed.
In 1964 three papers were authored by two Belgian physicists Robert Brout and François Englert working at the Université Libre de Bruxelles in Brussels, the British physicist Tom Kibble along with his two American colleagues Gerald Guralnik and Carl Hagen who were all working at the department of physics at Imperial Collage London and most famously Peter Higgs of the University of Edinburgh.
The theory proposed by these three papers gave an explanation for the masses of the bosons of the weak interaction and would allow physicists to explain the weak interaction and unify it with the electromagnetic interaction. However, the theory proposed by these scientists was originally not accepted by the scientific community.
Peter Higgs at the CMS experiment at CERN, one of the two experiments at CERN which discovered the Higgs Boson
In fact Higgs’ original paper outlining his theoretical model was rejected by the scientific journal Physics Letters (edited at CERN) as they judged it to be “of no obvious relevance to physics”. Higgs modified slightly his paper and submitted it to a different journal Physical Review Letters who published it along with the papers from the other two groups.
Eventually the community realised the importance of the theory and now Higgs’ name has gone on to become synonymous with the theory, and key elements such as the Higgs boson, the Higgs field and the Higgs mechanism now bear his name. However, it is believed by some that the names should credit all those involved in the theory and names such as the BEHGHK (pronounced ‘berk’) particle, or the ABEGHHK’tH mechanism (proposed by Higgs himself to credit Anderson, Brout, Englert, Guralnik, Hagen, Higgs, Kibble and ‘t Hooft) have been suggested but for some reason have failed to stick.
For the sake of simplicity we will continue to refer to the boson, mechanism and field associated with the theory as the Higgs boson, the Higgs mechanism and the Higgs field. Looking at the history of the Standard Model the reason that Higgs’ theory is regarded as so important is that it is the last piece of the jigsaw that enabled physicists explain the mass of particles. You might ask how his theory works, which is a good question, the best way to get an answer to that question is to take a university course in particle physics, but for those of you looking for a quick fix your best bet is an analogy.
The mathematics behind Higgs’ theory, the Higgs Lagrangian with the (in)famous Mexican hat potential
(Credit: Wikipedia Commons, modified by Daniel Potter/STFC)
Higgs proposed a field, now called the Higgs field, which endows particles with a mass via what became known as the Higgs mechanism. The basic idea of a field is that at every point in space there is a number which tells you something about that point. So if you drew a grid for every point in the universe that would be a field.
Fields exist for the four fundamental forces (gravitational fields are still theoretical) but Higgs’ field is a little different. When you have a field you have associated bosons: the photon for the electromagnetic field, the W and Z bosons for the weak field, gluons for the strong field and the Higgs boson for the Higgs field. So the discovery of the Higgs boson infers the existence of the Higgs field which therefore infers that it is indeed the Higgs mechanism that endows particles with mass.
So these are the key players in the theory now for the analogies of how they work (remember they are only analogies). These analogies were proposed by David Miller and are probably the most famous analogies but have been slightly modified in light of the discovery of the Higgs Boson.
Event recorded with the CMS detector in 2012 showing characteristics expected from the decay of the Higgs boson into a pair of photons (dashed yellow lines and green “towers”)
On the day of the announcement of the discovery of the Higgs boson the rumour got out that a big announcement was set to be made and the venue where the announcement was set to be made was packed full of physicists, including Professor Higgs. So imagine the scene the physicists are evenly distributed across the hall.
After the announcement of the discovery imagine as the physicists near Professor Higgs clamber to congratulate him and shake his hand. As Professor Higgs makes his way across the hall to leave, the physicists he comes close to move in to congratulate him and the ones he leaves behind return to their even distribution.
The well-wishers clustered around Professor Higgs making it hard for him to move through the hall. In this analogy the hall is space, the physicists are the field with each one representing a point, their clustering is the Higgs mechanism and Professor Higgs is a particle. The mass of a particle is the resistance it offers against a force that acts to change its velocity. If Professor Higgs was moving through an empty hall then he would encounter no resistance to his movement and would therefore be a massless particle.
News of the discovery travelled quickly and caused great excitement the world over. If we imagine physicists gathered for a conference, it’s a popular conference and the hall is packed. Imagine once again that the physicists are evenly distributed throughout the hall. One of the physicists receives a text message informing them of the discovery at CERN.
The excited physicist informs those nearby and a cluster of physicists form to read the CERN press release. The physicists near the cluster want to know what’s happening and those in the cluster turn to tell them and a wave of clustering passes through the room. The rumour may spread to all the corners of the hall or it may form a compact bunch which carries the news along a line of physicists from the one who received the text message to the chair of the conference at the other side of the room.
An illustration of the above analogy showing a rumour moving through a cocktail party.
Since the information is carried by clusters of physicists, and since it was clustering that gave mass to Professor Higgs, then the rumour-carrying clusters also have mass. In this version of the analogy the rumour carrying clusters of physicists (or clusters in the field) represent Higgs Bosons (no physicists were harmed in the making of these analogies).