The Higgs Was the Missing Puzzle Piece. The Real Mystery Is Why It Exists at All

Fridman opens by asking whether the Higgs deserves its "god particle" nickname, which Cliff notes was coined by someone trying to sell popular science books. To answer the question about its unifying power, Cliff goes back to the electroweak theory of the mid-1970s, which joined electromagnetism and the weak force. That theory predicted four new particles: the W+, the W-, the Z and the Higgs boson.

The W and Z particles were found at CERN's Super Proton Synchrotron in 1983 and 1984. Three of the four pieces were therefore in hand, and from the 1980s onward physicists were confident the Higgs had to exist, because the theory only works if it does. The tunnel that now houses the LHC was dug for an earlier machine, the Large Electron-Positron Collider, which studied the W and Z with great precision until it shut down in 1999.

Cliff describes the Higgs discovery as a huge experimental and theoretical achievement that was also an anticlimax. It is like a jigsaw puzzle with one gap in an otherwise beautiful picture, where everyone already knows what the missing piece looks like. Completing the Standard Model, a layout known for forty years or more, was not the interesting part.

What makes the Higgs interesting, Cliff says, is that it is unlike any other particle in the Standard Model and theoretically troublesome. Physicists do not really understand how such an object can exist in its observed form. That has led to speculation that it comes with a family of other particles, or that it is not fundamental but made of smaller things, which he stresses is an idea and not a demonstrated fact.

The key difference is that the Higgs field has a value everywhere. Shield a box of empty space and measure the electromagnetic field, and you get almost nothing apart from quantum fluctuations. The Higgs field does not vanish. It is as if all of space holds stored energy, like a raised background temperature, and the interaction with that energy is why electrons and quarks have mass.

The precise value of that energy has to be carefully tuned for an interesting universe. Naive calculations suggest two likely configurations. In one the field is zero everywhere, so particles are massless, cannot form atoms and fly by at light speed. In the other it explodes to the Planck scale, where even an electron would be massive enough to collapse into a black hole.

Cliff says that reaching the Goldilocks value requires fiddling with the other Standard Model fields and their properties, and physicists dislike this. Fridman puts two explanations on the table: a designer who set things up perfectly, or an infinite number of universes in which we happen to live in the one that allows complexity. He also asks whether some mechanism might push the universe toward complexity.

Cliff grants that a cosmic creator is a possibility but not a scientifically testable one. He mentions an idea in which universes are born inside black holes and evolve in a Darwinian way, so that universes able to form black holes produce more offspring. As an experimentalist he finds such ideas interesting but untestable, so he tends not to spend much time on them.

The most popular solution without multiverses or tuning was supersymmetry. It is one of the last possible symmetries not yet seen in nature, a symmetry between matter particles (fermions) and force particles (bosons). Every particle in the Standard Model would have a superpartner, and these extra fields would cancel the unwanted effects on the Higgs field and stabilise it at a sensible value.

Supersymmetry had other attractions too. It predicts a dark matter particle and suggests that the strong force and the electroweak force unify at high energy. If it stabilised the Higgs, the superpartners should have masses near the energies the LHC probes, so before the machine switched on many expected to find them alongside the Higgs. Cliff explains that colliding hard enough should make these fields vibrate and send superparticles flying out.

That has not happened. After a decade of LHC data taking, no supersymmetric particles have been found. Cliff adds that no new particles beyond the Standard Model have shown up at all. Other theories, involving extra dimensions of space or a Higgs made of smaller things, are still on the table.

Asked what the Higgs could be made of, Cliff describes the oldest idea, Technicolor. In it the Higgs is a bound state of strongly interacting, quark-like particles held together by a new, very strong force. He says Technicolor has fared badly, particularly since the LHC found the Higgs, but related ideas survive. One is partial compositeness, developed by some of his Cambridge colleagues, in which the Higgs and heavier particles like the top quark are partly mixtures of composite objects.

Such a theory could address more than the Higgs's tuned value. The Standard Model has six quarks and six leptons that can be arranged in a table with repeating patterns, much like the periodic table, which suggests something deeper is going on. The first generation, with the electron, the electron neutrino and the up and down quarks, is what we are made of. Heavier copies such as muons and top quarks appear in the second and third generations, and no one knows why.

Fridman asks whether the dream is something simple, beautiful and unifying. Cliff says it still is for many people, and recommends Steven Weinberg's book "Dreams of a Final Theory", written around the end of the 1980s and early 1990s. Reading it, you sense the feeling that such a theory was imminent, at the time of the superstring revolution. In string theory, tiny vibrating loops can be plucked like a string, with each vibration mode corresponding to a different particle.

The trouble is that the mathematics is hard. People have spent three decades or more on it, and Cliff says most string theorists would freely admit that no one really knows what string theory is. The bigger obstacle is that it mostly makes predictions about physics at energies far beyond anything a laboratory will probe.

The LHC collides protons at 14 teraelectronvolts, enough to reach the Higgs and the weak particles. The Planck scale, where strings would become manifest, is roughly fourteen orders of magnitude higher. With today's technology, Cliff says, you would need a particle accelerator circling the Milky Way. Barring an almost unimaginable breakthrough, the testable predictions of unified quantum gravity theories lie out of reach, though he says you never say never. Fridman jokes about Elon Musk building a collider of that size.