Read The Big Questions: Physics Online
Authors: Michael Brooks
The Big Questions: Physics (12 page)
The theories of these two forces seemed uncannily similar in many aspects. The theory of electromagnetic forces known as quantum electrodynamics, and the theory of the ‘weak’ force that creates some forms of radioactivity and powers the sun’s nuclear fusion, looked something like two sides of the same coin (see
Which is Nature’s Strongest Force?
). Weinberg and Salaam showed that this was indeed the case, and unified them into the ‘electroweak’ theory. There was a problem, though. That theory required that a couple of as yet-unseen particles, dubbed the W and Z bosons (a boson is a particle that creates a force), be added to the so-called particle zoo.
Rather embarrassingly, these two particles had mass. That seemed wrong, because the most famous boson is the photon, which creates the electromagnetic force, and the photon has no
mass. If the photon and the W and Z bosons do the same kind of job in a unified theory, there ought to be a kind of ‘symmetry’ between them. The fact that there isn’t, because of the masses of the W and Z bosons, leads physicists to suspect that something is breaking that symmetry, in the same way that adding a weight to a carefully balanced kitchen scales will upset its delicate balance. But what was that weight? This is the question to which Peter Higgs offered his field as an answer.
By 1967, Weinberg and Salaam had incorporated the Higgs field into their electroweak theory. In 1983, at CERN, the W and Z bosons were seen, exactly as Weinberg and Salaam had predicted. It was a triumph, the closing refrain of the particle physics odyssey. Except for one tiny detail. No one knew whether the Higgs field was really there.
You can envisage the Higgs field in various ways, but one is to rub your finger along the groove in a sheet of corrugated metal. It feels smooth and your finger runs without resistance. Now pull your finger across the grooves. It is much rougher. In the standard model of physics, this is how things are for the W and Z bosons. While the photon always moves along the Higgs field’s grooves, the other two move across them, encountering a resistance that translates into mass.
It’s an elegant idea, but it needs proof. And the only way to find that proof that the Higgs field really does provide a directional ‘grain’ to the universe, felt by W and Z bosons but not by the photon, is to find the particle that the field produces. Every field has its own particle. The electromagnetic field has the photon, the gravitational field has the graviton (though no one has
ever seen one), and the strong interaction comes through the gluon. The Higgs field, according to received wisdom, endows things with mass because of the Higgs boson. The question is, is received wisdom to be trusted?
Physicists do not have unlimited confidence in their theory of particle physics. In some ways it is hugely successful. We have predicted the existence of particles that (with the exception of the Higgs particle – so far, at least) have always been found – and in many cases the theory even told us exactly where to look. Physicists measure particle energy in electronvolts or eV; an electron would gain 9eV of kinetic energy when pulled by the voltage across the terminals of a 9 volt battery, for example. Salaam and Weinberg told CERN researchers that if they smashed particles together at 80 and 90 gigaelectronvolts, or GeV, they would find the w and z bosons. And that’s exactly what happened.
However, the standard model does not predict everything. The fact that 26 of its fundamental constants have to be found in experiments then written into the equations, for instance, is a little frustrating. Some particles had to be found by trial and error too. There was a 20-year gap between the prediction that the fundamental particle the ‘top quark’ must exist and the moment we finally found it. That’s partly because the theory gave us no idea where to look (it turned up at 170 GeV). Unfortunately, we are in the same boat with the Higgs boson. It should be there, but no one knows where ‘there’ is. And so we keep building bigger and bigger atom-smashers in the hope that we’ll eventually get to the right energy.
All this is not as desperate or random as it might appear. Smashing atoms together has a solid history as an experimental tool. That is, after all, how Ernest Rutherford discovered the atomic nucleus. In 1909, he decided to test the ‘plum pudding’ model of the atom, which suggested its positive and negative charges were mixed together. Rutherford fired a beam of alpha radiation – essentially the nucleus of a helium atom – at a thin sheet of gold foil. Most of
the alpha particles were unaffected, but some were deflected wildly. From his results, Rutherford deduced the existence of a tiny region of concentrated positive charge at the centre of the atom which caused the occasional wild deflections. Nuclear physics was born.
Since Rutherford, we have built up a roster of ever-larger particle accelerators in order to probe the complexities of the nucleus, culminating in the current state of the art: the Large Hadron Collider at CERN. Though it might seem like it from the media coverage, the LHC is not the first particle accelerator to be raised up as a detector for the Higgs boson. Because we have no way of telling at what energies the Higgs boson might be found – though the standard model suggests 96 GeV as a likely target – we have been hoping to stumble across it for many years. But particle accelerator after particle accelerator has been hailed as the bright new hope and we are still not there.
The first one with a serious chance was the Large Electron Positron (LEP) collider at CERN (see
Why is there Something Rather than Nothing?
). Housed in a circular tunnel 27 kilometres (16.5 miles) in circumference, LEP accelerated electrons and positrons to close to the speed of light. A ring of 4,600 magnets guided these particles in a circle that extends out across the Swiss border into France, to the foothills of the Jura Mountains, with the electrons going in one direction and the positrons in the other. The magnets could be tweaked to guide these beams into each other, releasing a cascade of particles from any collisions. Four vast detectors, each the size of a small house, would pick up the tracks of these particles. The experiments would run for hours, with potential collisions every 22 millionths of a second. Then the scientists had to examine the output of the detectors, and try to work out what had happened as electron and positron smashed into one another.
LEP, which came into operation in 1989, accelerated particles to 45 GeV, enough to produce the Z boson. Later upgrades enabled
it to produce the W boson too. By the time it was scheduled for shutdown, LEP was operating at 209 GeV. But just before that, in September 2000, it produced a tantalizing glimpse of something that looked like the Higgs boson.
High energy particle accelerators
The observation was made at collisions involving energies of a whisker under 115 GeV, which made sense in the standard model’s view. Unfortunately, there were simply not enough of the observations to make it a statistically significant result. The only conclusion was that, using Einstein’s
E = mc
2
energy mass equivalence, the Higgs boson was heavier than 114 GeV.
The mass of the Higgs boson is tightly linked to the mass of the top quark and the mass of the W boson. As scientists pin down these masses ever more precisely, the range of energy scales over which the Higgs boson could appear gets narrower. The latest constraint on the mass of the W boson has brought the most likely Higgs in at 153 GeV, and the race is now on to find it. In 2009, scientists at Fermilab announced they have a 50:50 chance of spotting the Higgs boson before the end of 2010. LHC researchers still might get there first. This new collider, the world’s most powerful machine, occupies the tunnel vacated by the LEP in 2000. It will accelerate protons and antiprotons to a stunning 99.9999991 per cent of the speed of light. The particles will crash
together at 14 TeV (teraelectronvolts). With all that energy concentrated into beams only a thousandth of a millimetre across, it’s a prospect that has some people worried that the collider could produce unexpected and catastrophic effects (see box:
Will We Find the Higgs Boson or Destroy the Earth
).
Technical issues caused massive delays to its start-up, but the LHC remains the best hope for detecting the Higgs particle. It may take years before the LHC produces any meaningful data, though. Its detectors are enormously complex, and will need an unprecedented amount of calibration. Once that’s done, the experiments will start. So what happens if the Higgs doesn’t appear in the LHC? It may sound shocking when you consider the £2.6 billion cost of the collider, but physicists are sanguine about that eventuality.
The accepted view is that, if there is no Higgs boson, the standard model of physics will crumble and fall. To explain the asymmetry between the massless photon and the massive W and Z bosons requires a Higgs boson, or something like it. But even here, there is wiggle room. Some physicists claim, for instance, that we have oversimplified our understanding of what the Higgs signature will look like. If things are more complicated, it’s because of a theory that goes beyond the descriptions offered by the standard model. This theory is called ‘supersymmetry’.
In supersymmetry, every particle has a ‘superpartner’ that is a heavier version of itself. The electron is partnered by a selectron. The quarks have squarks. And so on. This creates a much more complex particle zoo than we might perhaps like, but the idea has teeth. Most importantly it solves numerous difficulties with the idea of ‘unifying’ all the forces of nature. The link between the electromagnetic and the weak forces, for instance, hints that all the forces evolved from one superforce just after the Big Bang. As things cooled down to lower energies, the superforce split into the forces we now recognize. Supersymmetry suggests that there are perhaps as many as five
particles to be associated with the Higgs boson. So what does this do to the search? Well, predictably, it makes it more complicated than we would like.
WILL WE FIND THE HIGGS BOSON OR DESTROY THE EARTH?
The Large Hadron Collider is such a powerful machine that it could bend and tear the fabric of the universe – so could it destroy the Earth? The question has become the subject of intense debate, and even legal proceedings, but the answer is almost certainly not. The issue is the high concentration of energy in the LHC’s colliding particles. The energy is not enormous – it is something around the kinetic energy of a small insect. But it is concentrated into a very small region of space. We know from Einstein’s theory of relativity that energy warps space. In some models of the universe, where there are more dimensions of space than the three we experience, such a concentration of energy can produce tiny black holes, where the extreme bending of space creates what is effectively a rip in space and time.
In this scenario, the black holes generally disappear in a fraction of a second and pose no threat. There is a tiny, but finite possibility, though, that they can grow to a significant size and become a real danger. Or that’s the scaremongers’ theory. The reality is far more prosaic. The scare stories have prompted researchers to go through the theory with a fine toothcomb. They unanimously conclude that the probability of disaster is infinitesimally small.
Perhaps more important, though, is that whatever the theory says is possible (but highly improbable), we actually have some experimental results that also bear on the discussion. In our upper atmosphere, charged particles from outer space are causing higher energy collisions than those that will take place at the LHC. They are happening at the rate of 10,000 billion LHC collisions per second. As the LHC’s safety report puts it, ‘over the history of the Universe, Nature has carried out the equivalent of 10
31
LHC projects’. That’s 10,000 billion billion billion LHCs, with no sign of a black hole opening up and eating the Earth. On that basis alone, there is no reason to think that the LHC poses any danger to the future of humanity.