I have said before that the emerging modern consensus is that there are no particles or waves in the classical sense of those terms, although those concepts are still useful to us in visualizing physical processes. (For previous posts in this series, click on the Higgs folder just below the blog post title.)
Relativistic quantum fields, each corresponding to what we refer to as an elementary particle, are all there is and everything is made up from these fields. All these fields have tiny vibrations even in the vacuum where there are no particles. Elementary particles can be considered to consist of larger (i.e., higher energy) vibrations of the fields corresponding to them, beyond the level of vibrations in the vacuum. If these higher energy vibrations become localized in a region of space, that will give the appearance of a particle being present in that region of space.
These fields are present even in the vacuum and vibrate by tiny amounts. This is one necessary consequence of the uncertainty principle. It is the fact that the vacuum always has these tiny vibrations that results in scientists seeing the vacuum as a dynamic entity that can give rise to some interesting effects, rather than being uninteresting and inert as is popularly thought. This is what physicists mean when they speak of the vacuum or ‘nothing’.
One consequence of these ever-present vibrations of fields is that we can have the creation of what are called ‘virtual’ particle-antiparticle pairs out of this quantum vacuum. These virtual particles can exist for a short time before recombining and disappearing, leaving us with nothing again. For example, one can have the sudden appearance of an electron and an anti-electron (called a ‘positron’) pair out of the vacuum that exists for a short time. So the quantum vacuum can be thought of as a kind of foamy sea where pairs of particles are popping in and out of existence all the time.
If matter can suddenly appear out of nothing, doesn’t this violate the law of conservation of energy? Yes it does. But one form of the uncertainty principle says that such violations are possible for a short time, provided that the amount by which energy conservation is violated, multiplied by the duration of the violation, is less than a very small (but non-zero) amount of the size of Planck’s constant, which is a universal constant of nature that was introduced to explain blackbody radiation that heralded the dawn of the era of quantum mechanics back in 1900.
In other words, the larger the violation of energy conservation, the shorter the time the uncertainty principle allows it to last. For an electron-positron virtual pair, the allowed duration of violation is about 10-21seconds, a really tiny time. And this is about as small an energy violation as you are likely to see, since the electron is one of the lightest particles there is. More massive pairs will last even shorter times. This is why we do not see energy conservation violation in everyday life.
The significance of the Higgs particle is that it is a prediction of the Higgs field which, as we will see later is what, via the Higgs mechanism, gives mass to the elementary particles. Finding the Higgs particle confirms the existence of this field and is thus an important prediction of the Standard Model.
So how does the Higgs field give rise to the masses of other particles? It arises from one key difference between the Higgs field and all other fields.
For all fields other than the Higgs, even though they have vibrations even in the vacuum, the average value of those fields in the vacuum (called the ‘vacuum expectation value’) is zero. But the Higgs field has a non-zero average value even in the vacuum. In other words, all of space throughout the universe is permeated by this non-zero Higgs field. This is what makes the Higgs field so special because it is the presence of this non-zero field that gives rise to the mass of particles.
To use a popular analogy, think of this field as a kind of non-physical syrup that permeates all space. In its absence, all particles would travel unhindered at the speed of light, which is equivalent to them having zero mass. But when the syrup is present, the particles are slowed down by it, which is equivalent to them ‘gaining mass’. The amount by which they get slowed down, and hence their mass, depends on the strength of interaction between the particles and the Higgs field.
Moreover, the average value V of the Higgs field in the vacuum is predicted by theory to be given by V=[GF√2]-1/2, where GF is called the Fermi coupling constant that is related to the strength of weak nuclear force, one of the 19 parameters of the Standard Model that were listed in this earlier post and have to be obtained by experiment. Since GF is a known measured quantity (given by GF=1.166×10-5 GeV-2), we can calculate the value of V and it has the value of 246 GeV. Note that this is the average value of the Higgs field in the vacuum. It is NOT the mass of the Higgs particle. It is large local vibrations in this non-zero Higgs field that correspond to the presence of a Higgs particle in a region, just like it is large local vibrations of the electromagnetic field in some region that correspond to the presence of a photon there.
The Higgs particle has zero electric charge and spin and that is one of the reasons why the Higgs particle is so hard to detect. Its properties are such that it just doesn’t stand out clearly from the vacuum background. As someone once said, searching for the Higgs particle is worse than looking for a needle in a haystack, it is more like looking for a particular piece of hay in a haystack.
But it does have a mass. Unfortunately while the average value of the Higgs field is predicted by theory, the mass of the Higgs particle is not and so it could have had any value. Since the mass of a particle is one of the main features we use to design experiments to detect its presence, this ignorance of where to look was one of the reasons that made the search for it so hard.
There is, however, a element of the theory called unitarity that, if it holds (as we think it does), puts an upper limit to the Higgs particle mass of about 1 TeV or about 1000 times the mass of a proton. As we will see, this fact strongly influenced the design of the Large Hadron Collider.
But that still leaves us with a huge range of masses that have to be searched through to find it.
Next: The non-zero Higgs field in the vacuum