The model that created order. Invisible fields fill space. The ghost-like Higgs field

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 — á tomos 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 Franç ois 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.