The discovery of the Higgs boson particle only takes us half way to replicating the extreme energy conditions of the Big Bang
July 4th witnessed not just the celebration of American independence from the Brits, but the discovery of the Higgs boson, the herald of how beauty and order emerged from the chaotic debris of the Big Bang. Why is the universe full of huge, interesting structures of galaxies, stars and planets, with microscopic fabrics of atoms and molecules, rather than some disordered hotchpotch?
Our best theories had posited that the debris from the Big Bang was like goo: massless particles flitting around hither and thither at the speed of light. But this model of mathematical perfection is not the universe that you and I know; it is the plaything of theorists. In the real world of particles which have mass, these theories worked only in restricted cases. The moment they were applied beyond the simplest approximation, nonsense erupted.
Something had to be changed, or added. But if you try changing even a few symbols in Einstein’s theory of relativity or quantum theory, the edifice collapses. How can you fix the problem without destroying the foundations of modern physics?
Nearly half a century ago, Peter Higgs and five other theorists, independently in the space of a few months, discovered the key. The price was that they had to assume that the universe is filled with a field of influence—today known as the Higgs field. The problem was that no one had ever seen any evidence for this all-pervading essence.
Alone among the sextet, Higgs pointed out a consequence of the theory: a particle with mass—the Higgs boson—should exist. This ephemeral subatomic lump had to be produced in an experiment and its properties measured. The challenges were immense: how to produce the boson, how to “beware of imitations” in the hordes of data, and how to respond to any surprises it revealed.
All atomic particles are either bosons or fermions. Fermions are the basic seeds of matter, such as electrons and quarks, which in quantum mechanics act like cuckoos, where two in the same nest are forbidden. Bosons, by contrast, are like penguins, where large numbers cooperate as a colony. Bosons can collect together into the lowest possible energy state—an effect known as Bose-Einstein condensation, after the two scientists whose work explains this phenomenon. It is manifested in weird phenomena, such as the “superfluid” ability of liquid helium to flow through narrow openings without friction; in superconductivity; and, we now know, in forming a pervasive essence throughout the cosmos—the Higgs field. This field acts on particles, giving them mass, enabling atoms to form, stars to shine, and ultimately life to occur.
But contrary to many media reports, the Higgs field is not the source of all mass, only that of the most basic particles. It is the atomic nuclei in your body that give about 99.5 per cent of your weight. This has nothing to do with the Higgs field. What the field does is give structure by acting on the fundamental particles, such as the electron found in the outer reaches of atoms, and the quarks, the ultimate seeds of the atomic nucleus.
It is because quarks have mass that atomic nuclei are compact; the massive electron gives atoms their size. So your weight has little to do with Higgs, but your size does.
We are familiar with electromagnetic radiation, and photons, which are manifestations of electric and magnetic fields. The Higgs boson is analogously a manifestation of the Higgs field. However, while it is easy to make radio waves, it is hard to make Higgs bosons. They were common in the heat of the Big Bang and we have to recreate those conditions in the laboratory before a Higgs boson has even a chance of bubbling into view.
Looking for the Higgs boson in the Large Hadron Collider (LHC) experiments at Cern is like watching a demon rolling a pair of dice inside a closed box. If sixes happen every time, you can be certain that the dice are special—in our analogy, the Higgs boson exists. However, if sixes occur on average once every 36 throws, the results are no more than chance and there is no Higgs boson.
If only it were that simple. In practice what happens is that the sixes turn up slightly more often than mere chance. You have to decide whether this is significant—evidence for the Higgs boson—or the vagaries of luck. You need to do more tests, until either the excess of sixes becomes convincing, or instead gradually dies away towards randomness. In the jargon, one is trying to decide between signal and noise.
At the LHC the beams circulate 11,000 times a second and two independent teams of physicists are able to peek into the box. The longer the LHC collects data, the more confident they can become in discriminating signal from noise.
That’s the good news. The catch is that when you open the box, almost invariably you find that the demon hasn’t thrown any dice at all; to extend our analogy, it is found to be playing roulette, tossing coins or indulging in some different game of chance, which are themselves interesting to other physicists, but your precious dice, the ones relevant to the Higgs search, are very rarely on view. This is why it has been easier for the LHC to discover phenomena other than the Higgs boson and why it has taken two years to be certain enough to go public.
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Following the discovery of the Higgs boson, we crossed from history to a future built on certainty. Where next? Powerful though the LHC is, it takes us less than half way, in energy terms, to the extreme conditions of the Big Bang, known as the Planck energy, when the fabric of space and time fragment into foam. The theory relevant to that extreme is unknown. It requires a marriage of quantum theory and Einstein’s theory of gravity—general relativity.
Some believe that string theory is the way forward, but we are not yet certain. In practice we can ignore this limitation. The Planck scale is so far away that its effects are indiscernible by the most sensitive experiments. So there is no practical implication in our lack of knowing the so-called “theory of everything.” That is the irony of research. The realm of quantum gravity is so far away that we can ignore it, but the very lack of any observable effects also leaves us clueless on how to proceed in constructing it.
We have hints of novel phenomena that should be within reach of the LHC. They may give clues on finding the theory of everything—time will tell. But discoveries are anticipated with confidence. As I said, in terms of energy, the LHC is only half way towards the Planck extreme. Yet in that half we have life, molecules and atoms, the atomic nucleus, quarks and now the Higgs boson. So many riches. Is there nothing but desert from here to the Planck limit?
It seems unlikely, not least because if it were like that, the Higgs boson should not be here. The boson, weighing in at about 130 times the mass of a hydrogen atom, is like a snowball in hell. The conditions required to make it in the LHC are hot to our senses but are nothing compared to Planck energy. Why has the Higgs not metaphorically melted?
The backstory is that Higgs bosons can interact with one another, and with other particles; when they do, according to our theories, the projected answers are sometimes nonsense. However, all is well if there is a whole new family of “supersymmetric” particles, whose fermions match our bosons, and vice-versa. The presence of these new particles can leave sensible results. Whether they are within reach of the LHC is the question.
Hints of them may already be with us. The Higgs is the final piece in the cast of characters needed to describe our world, but over 90 per cent of the universe consists of “dark stuff,” which does not shine but gives itself away by its gravitational pull on the galaxies of stars. There are no candidate particles known with the required dark properties, but supersymmetry theory contains such possibilities. The search is already on at the LHC, but there is no direct sighting of them yet.
However, particles that are too heavy to have revealed themselves directly can still make their presence felt courtesy of quantum theory. Like seeing dawn before the sun actually rises above the horizon, such particles can illuminate the Higgs boson in its brief moment of existence, subtly altering the options for its behaviour. The slight differences between what the simplest theory expected for the properties of the boson, and the fine details of what may be revealed are what experimentalists are now seeking. There is speculation that the fine details of the boson’s behaviour suggest something unusual. It is far too soon to get excited, and as data accumulates in the coming months this may turn out to be nothing—or a breakthrough. If the latter, we will have the hints of the discovery of new varieties of matter, yet to be included in our computations. These might be examples of supersymmetry particles, or something utterly novel.
These are unknowns that seem likely to become known. There are also the unknowns that, barring some unexpected breakthrough, look likely to remain unknown. If, as seems likely, we now know how the fundamental particles gain mass, this leaves open the question of why they have the particular masses that they do. If the electron were slightly heavier, some crucial forms of radioactivity would not occur, elements would not form, and we would not exist. Were it much lighter, these processes would change in other ways, once again unfavourable for life. Exactly what determines the strength of the Higgs’s affinity for one particle or another is what experiment might reveal, but to do so will require some quirk in the data, some clue to guide us.
The July 4th results imply that the Higgs gives mass to the carriers of the weak nuclear force—the “W and Z bosons”—and possibly the quarks too, but there is not yet evidence that it gives mass to the electron and its siblings. Proving that the Higgs boson is responsible for the mass of the electron, and hence for the origins of chemistry, will be harder to establish. But it should be settled one way or the other in a year or two.
