Prof S. M. Deen [
] University of Keele, UK
Note My New Email: email@example.com
Recently there has been great excitement about Higgs bosons, so much so that its sighting at the CERN LHC (Large Hadron Collider) made front-page news, an unheard of public interest in science. More recently, Higgs boson was celebrated at the opening ceremony of the 2012 Paralympic Games. Some people even revere it, calling it the God particle – a term detested by scientists, and hence is not used here. In this article I shall try to explain in simple terms with non-scientists in mind, what is a Higgs field, what are Higgs bosons and how they are thought to create the mass of fundamental particles. However, to understand Higgs, we first need to understand what is meant by mass in physics. So I shall start with mass, and then offer a simple view of the Higgs field and bosons, before delving into slightly more elaboration.
Mass is the amount of matter (or substance) in an object. The mass is meant to be an intrinsic property of an object, which does not change, unless the object is moving (as given by an Einstein’s equation), but here we shall generally assume all objects to be stationary (i.e. at rest), unless otherwise implied.
The mass of an object is most commonly defined as its weight at sea-level on earth, but this definition confuses mass with weight. Weight is a force that is given as Mg (where M is the mass of the object, and g is the gravity). Therefore in the mass definition, the earth’s gravity is taken to be 1 (i.e. g = 1). Since the gravity does not change much on the earth’s surface, mass and weight have roughly the same numerical value, unless we leave the earth. If you go to the sun where gravity is 28 times that of the earth, the weight of a 1Kg object would be 28Kg, and likewise due to the weaker gravity of the moon, the same 1Kg object from the earth would weigh 1/6 of a Kg, but mass in both cases will remain unchanged at 1Kg. So let us summarise: the difference between mass and weight on the surface of the earth is mainly conceptual, but on other places where gravity is very different, the weight will change, but the mass by definition will remain the same.
There are many alternative ways of defining mass, though they all yield equivalent mass contents. One convenient alternative definition for our purpose here is: the mass of a (stationary) object is measured by the resistance it offers in moving it from its stationary position [more strictly, by its resistance to acceleration]. Imagine two solid cubes of identical dimensions, one of aluminium and the other of gold, sitting (i.e. stationary) on a friction-free surface. The gold cube will be the heavier of the two, but neither of these two cubes will move unless they are pushed (i.e. force is applied to them); a stronger push (and hence a greater force) will be needed to move the gold cube. But why?
You can truthfully say that this is so because gold is heavier, or you can equally truthfully say that this is because gold has more mass. But a physicist, aware of the definition of mass given above, would look at this slightly differently and say that this is so because these objects encounter resistance to movement, the gold cube encountering greater resistance than the aluminium cube. Now from the resistance encountered by each (i.e. the push required to move each) you can calculate their respective masses. Remember that this definition will yield the equivalent mass values that you would get by weighing them on the earth’s surface.
Aside: the unit used in measuring the mass of a fundamental particle is not gram (gram is too large for it, given that the mass of an electron in grams is about 10 –27). The units used are: million electron volt (MeV), billion electron volt (GeV, not called BeV for some reason), etc. These units are the energy equivalent of mass in grams obtained by applying Einstein‘s Mass Energy Equivalence. An electron has a mass of ½ MeV, while a proton has roughly 1 GeV (2000 times of an electron). Strictly speaking the proton mass is 938.27 MeVs, but people often use the rough value of 1 GeV.
The Basic Idea: Higgs Field and Higgs Boson
I trust you are comfortable with the idea that the mass of an object is nothing but the effect of the resistance it encounters in moving from its position of rest. If it is not clear, then please do email me for further clarification.
Now I assume we agree that it is the resistance that causes mass. A photon (the light particle) encounters no resistance, and hence it has zero mass. If you are a clever theoretical physicist like Peter Higgs, you would now be thinking: “Hmm. Can I contemplate a field (force) in the early universe that creates resistance?” There you will have the starting idea. But first I shall offer two analogies:
(1). If you are walking in water, say 3 feet (1 metre) deep, you will find it hard, due to water resistance to your movement. So you will have to apply more force than you would do while walking on the ground at the same speed. In this case water is the equivalent of the Higgs field and the water molecules are the Higgs bosons, which are interacting with your body creating resistance to your movements.
(2). Imagine a large bowl (the early universe), in which many balls of different types are moving freely at the speed of light, in this otherwise empty bowl. Now fill the bowl with sand completely, thereby submerging all the balls. Sand, we assume, will react differently with different types of balls, creating different amounts of resistance to different balls as they try to move though that sand. We imagine some balls will not react with sand at all – they are the mass-less balls that will still be moving at the speed of light, the photon being one such ball. Of the other balls, each will have a different amount of resistance (and hence different mass) in this sand-filled bowl. The sand that filled the bowl is equivalent to the Higgs field, and the grains of sand reacting with those balls (dragging them if you prefer) and causing resistance are the Higgs bosons.
Now some caveats to the analogies given above. First you can get out of the water or the balls can be taken out of the sand. But you cannot get out of the Higgs field, it permeates the whole universe, it is everywhere, inside and outside everything including of course ourselves. Thus everything in the universe is submerged in a Higgs ocean, if you like. Secondly while water and sand are visible, the Higgs ocean is not, like the gravitational and electromagnetic fields, which are not visible.
Physicists have defined over the years the properties of fundamental particles, such as electron, quarks, W and Z bosons, photon, etc. However, one very important property they were significantly unable to explain was why a particle has mass. Peter Higgs came out with the idea of a new field with a characteristic force particle in 1964 – an idea that subsequently induced several other physicists to join in. One might speculate that these theoretical physicists realised that the only way to create resistance was to have a new field in which its force particles would react with the fundamental particles, obstructing their movements. I imagine they also came to the conclusion that since the mass of a fundamental particle is constant everywhere, the resistance it encounters also has to be constant everywhere, and hence the number of force particles has to be sufficiently large and uniformly distributed to maintain that constant resistance everywhere, unlike the electromagnetic field (or the gravitational field), where the average number of its force particles is zero everywhere. This is indeed the case with that new force and its particle as explained below. Soon the field became known as the Higgs field, and the particle the Higgs boson.
First why it is called a boson? There are two classes of particles in physics, one class is called Fermions following the work of Enrico Fermi of Italy with Paul Dirac of England, Fermi later emigrating to the USA. Electron, neutron, proton, etc. are fermions. The other class of particles is called Bosons, following the work of Satyendra Nath Bose of Dhaka University (Bangladesh), with Einstein in 1927. Bose emigrated to India in 1947, some years before my entry to the Dhaka University as a Physics student. Photon, W and Z Bosons and the Higgs particle belong to the boson class. Some sub-nuclear particles, such as neutrons and protons are made up of other particles (such as quarks), but the fundamental (equivalently, elementary) particles, such as electrons and quarks, are not made up of other particles.
In the mid-1970s came what is called the Standard Model of Particle Physics, which explains the creation of the early universe and its contents, including of course the formation of fundamental particles. Prof Abdus Salam of Imperial College with two other physicists from the USA shared the 1979 Physics Nobel Prize for it. Salam (from Pakistan) was the first Muslim to receive a Nobel Prize, and as it happened he was my teacher at Imperial College during my Particle-Physics PhD study (apologies for bragging). The Standard Model uses the idea of Higgs boson to create mass. Many aspects of the Standard Model have been subsequently verified by experiments, including the existence of the Higgs boson, it would seem, if the results of the CERN LHC (Large Hadron Collider) are finally reconfirmed.
Universe at the beginning and the evolution of the Higgs Field
Before we proceed, note that the restriction that ‘nothing can go faster than the speed of light’ never applies to the expansion of the universe itself (how privileged can you get?). Now, when the universe was born, it was very, very hot. At time 10-43sec [this is: 1 divided by (1043), i.e., 1 divided by (1 followed by 43 zeros) sec], its temperature was about 1032 degrees [this is 1 followed by 32 zeros]. The universe then started expanding very fast at a huge rate, possibly 1060 [i.e. 1 followed by 60 zeros, which is a trillion trillion trillion trillion trillion] times faster than the speed of light, or perhaps even faster. This enormous expansion took place between 10-36 sec and 10-34 sec after the birth of the universe [Note 10-36 sec is one trillionth of a trillionth of a trillionth of one sec, while 10-34 sec is 100 times larger than 10-36 sec, which is thus 100 trillionths of a trillionth of a trillionth of one sec]. By 10-34 sec, the universe grew by a colossal factor of at least 10100 [that is 1 followed by 100 zeros] in volume, possibly to an infinite size.
In other words, in 100 trillionths of a trillionth of a trillionth of one sec, the universe attained a huge size (possibly an infinite size), cooling itself down as it expanded. This is called the inflationary expansion phase of the early universe, after which the universe still continued (and still continues) to grow, though not at that fast rate. Before I proceed further, let us consider an analogy of water transformation: hot steam cools down, first to condensation, then to water and finally to ice, as the temperature gradually goes below zero. All these various forms are really the different phases of water displayed as it cools. Similarly the universe has many phases that emerge as it cools down.
As the universe cooled down further, some of the forces such as the gravitational and the electromagnetic forces evolved. Some fundamental particles were also born, but they all had zero mass. As the universe cooled further, the Higgs field (with its Higgs bosons) came into existence, but with fluctuating field strengths, which settled to its final value at about 10-18sec [one billionth of a billionth of one sec] after the birth of the universe when the temperature cooled down to about 1015 degrees [1000 trillion degrees]. The universe had now reached the second billionth of a billionth of the first sec of its life, and many other amazing things would happen before it became one sec old. If you are wondering about the stars and galaxies, they were formed some 380,000 years later – beyond our scope here. I should mention here that these times and temperatures are rough and speculative. Another point is that the famous BigBang idea of the creation of the universe is not favoured by many physicists these days, and hence I do not use the term here.
The Higgs field reacts with itself, and a Higgs boson has a mass of about 125 GeVs (i.e. about 120 proton masses). Note that the Higgs field and the mass of fundamental particles would disappear if we could raise the temperature of the universe above 1015 degrees, which of course we cannot do. The current average temperature of the universe is about -270C (about 2.73 degrees above the Absolute zero).
As mentioned earlier the average value of the other fields (such as electromagnetic or gravitational) is zero everywhere in the universe, the average number of their respective characteristic force particles being zero. In contrast (again as hinted earlier) this is not so with the Higgs field, which has a large average value (i.e. strength) everywhere in the universe, implying a large number of Higgs bosons everywhere, which react with the fundamental particles endowing them with their mass. If you add up all the masses thus created and the values of the Higgs bosons (field) in the universe, they would cancel each other out making the total mass content of the universe zero. This would happen if you could raise the universal temperature above 1015 degrees as mentioned earlier. So everything in this universe of ours is magic – nothing really exists.
The Higgs process discussed above is one of several possible ways in which Higgs field/bosons might work, the correct one is expected to be determined by further experiments at CERN and perhaps elsewhere. In fact there could be several types of Higgs field and several types of Higgs boson. Also the large strength of the Higgs field I mentioned above could be negative. Worse still, the strength of the Higgs field could be different in different regions of the universe, creating chaos – that is, the masses of fundamental particles (and hence the physical laws) could be different in different parts of the universe. In that case we do not need to invoke the theory of multiverses, even this one universe of ours could have different physical laws in its different regions, where there may not be any atoms and molecules, let alone stars and galaxies, as we know them.
Finally, do not forget that the mass content I have discussed here constitutes only 5 percents of the universe – we know nothing much about the other 95 percents, made up of Dark Matter and Dark Energy. The little that we do know fills us with so much wonder and amazement, that even Einstein exclaimed (and we concur): Allahu Akbar.
© S. M. Deen, 2012.
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