by Tim James
It is really the Higgs field and the Goldstone bosons that do interesting stuff, but they are slippery rascals so we have to create a pointless particle in the field to see if it is there. The Higgs boson does not give particles mass but it proves our theory of how they get mass is correct, and that is what the Large Hadron Collider was all about.
You take a bunch of hadrons (particles made from quarks) and send them into a giant tube, which uses electromagnetism to accelerate them around a loop, like an epic centrifuge. Cooled to a few degrees below the coldness of space, moving at 99.9 per cent of the speed of light, these hadrons are then smashed together at various points around the ring and the energy released ends up distributed into all the fields.
You sometimes hear it phrased as if quarks are smashed together and particles inside come falling out, but that is not right. Quarks do not have particles inside but when we slam them together, the energy released is so enormous it gets transferred to every other field and the collision creates electrons, muons, tauons, neutrinos, antimatter, gluons, photons, Ws, Zs and, if we are very lucky, it will shake the Higgs field and we will see a tiny blip on our readout.
The momentous announcement was finally made to a packed lecture hall in France on Independence Day 2012, using a PowerPoint presentation written in comic sans, no less. A particle with the expected properties of a Higgs boson had at last been found.
Peter Higgs, who was in the audience, began weeping as applause detonated around him. His forty-eight-year search was over and his hypothesis was vindicated.
THE HOUSE THAT EMMY BUILT
The complete list of particles in quantum field theory is huge. Take the number of quarks, for instance. We start with the six main fields: up, down, charm, strange, top and bottom, but each one can also be found in three possible colours (red, green and blue), giving us eighteen particles/fields. Then we have to include all the antimatter quarks, giving us thirty-six. And if you want to include the left and right chiral versions, that becomes seventy-two.
Most of the variations are nearly identical, however, so rather than putting every possible version of a particle into a periodic-table of particles we can draw up a simplified list such as this:
FERMIONS (MATTER)
BOSONS (FORCES)
Up
Quark
Charm
Quark
Top
Quark
Photon
Gluon × 8
W+
W−
Z
Higgs
Down
Quark
Strange
Quark
Bottom
Quark
Electron
Muon
Tauon
Electron
Neutrino
Muon
Neutrino
Tauon
Neutrino
What you are looking at here represents a century of scientific experiment tied together with the symmetric bow of Noether’s theorem.
The foundations she laid were built upon by Dirac, Feynman, Gell-Mann, Weinberg, Salam, Glashow, Higgs and many others to create a structure of ingenuity and beautiful complexity.
It is called the standard model of particle physics because it models every particle and every interaction between them and is the crowning achievement of physics.
To some, the Large Hadron Collider might seem like a waste of effort but it is worth remembering this was a machine built to test the grandest theory we have about the universe. Quantum field theory and Noether’s theorem present us with the biggest question, so it makes sense that our response should be to build the biggest answer.
CHAPTER FIFTEEN The Trouble with G
EVERYTHING EVER ALMOST
The reality we live in owes almost all its beauty and complexity to the three forces of quantum field theory: the strong, the electromagnetic and the weak.
Without the strong force, nuclei at the centres of atoms would not be stable and protons would simply fly apart before anything could form. The gluons and quarks that bind everything together are the reason we are treated to a universe of more than just hydrogen. The carbon, nitrogen, oxygen, phosphorus, sulfur, sodium, chlorine, calcium and iron in your body, in fact the entire periodic table of elements, owe their existence to the laws of quantum chromodynamics.
Those elements, when combined, can form the intricate molecules of chemistry with electrons exchanging energy through photon transfers, making every reaction possible. Not only that, the ground you stand on which repels against you is only doing so because electrons in your feet and electrons in the Earth are able to exert a force on each other due to the interplay of electrons and photons.
When you push or pull against something, when you feel friction or drag, when you experience buoyancy, upthrust or any other phenomenon described by Newtonian mechanics, you are experiencing electric repulsion from the photon field, not to mention the fact you can see any of this in the first place due to the existence of light. All of chemistry, all of classical physics and all of optics owe their behaviour to the laws of quantum electrodynamics.
And then there is the magnificence of biology. The most complicated of the sciences whose simplest structures are made from molecular chains billions of atoms long. As these DNA strands in your cells replicate, mutations in the code lead to new features which get passed along, allowing a species to diverge and evolve over time. These mutations are sometimes caused by transcription errors in the copying process, but that only takes things so far. The real gift to life on Earth are the radioactive particles that filter into our atmosphere and disturb the coding process: remnants of solar winds and astronomical phenomena caused by particle decays out in the coldness of space.
Without W or Z bosons, such radioactive decays would not occur and life on Earth would be confined to a few dozen bacteria swirling about in rock pools. And without the Ws or Zs, quarks in the core of the Sun would not be able to turn protons into neutrons, preventing nuclear fusion from taking place and thus preventing the Sun from shining at all. The overwhelming splendour of biodiversity and the sustainability of life itself is only possible due to the laws of electroweak theory.
The quarks and leptons that make up your body, the photons and gluons, the bosons that tell them to interact, the Higgs that give you mass and the neutrinos that keep everything balanced, are all accounted for in the standard model of particle physics and the underlying quantum field theories.
We do not know everything, and there are still wonderfully juicy questions to be answered but quantum field theory points us in the right direction. We are taking baby steps in this new realm, it is true, but we are no longer fumbling blindly. Every event in the history of history is the result of particles interacting through fields and we now have a framework to explain them all. Except for one problem. The thing quantum field theories cannot handle. Gravity.
WEAKER THAN WEAK
The story of the apple striking Newton’s head is slightly apocryphal. He was actually watching it fall from a tree near his home in Lincolnshire, when it struck him as odd.1
Most of the forces on a falling apple can be explained using simple mechanics. The snapping of the branch is the result of atoms in the stem rearranging. When it moves through the air it ‘bumps’ into air particles, slowing its acceleration, and once it hits the ground we can use more simple laws to explain why it lands, rolls and splits in a certain way.
All of these phenomena are the results of particle–particle interactions, mostly handled by QED and electron-photon theories. But what makes the apple fall in the first place? That is the real question.
Newton did not invent gravity, sadly; things were not floating about in the air prior to 1687. What he realised was that gravity is a force of its own. Apples do not move downward out of preference as had been supposed by Aristotle; they get faster during the fall, which means something is actively pulling on them.
A simple way of proving this is t
o show that an apple hitting the ground from a tree hits with more of an impact than one being dropped from a few centimetres up. Clearly the longer you are falling for, the faster you get, and if something is making the apple get faster, something must be exerting a force.
Newton’s realisation was that objects with mass (later modified by Einstein to include energy) are in communication with each other through some invisible attracting medium we now call the gravitational field. Along with the weak, electromagnetic and strong fields, gravity is a fundamental force of nature.
Your gut instinct might be that gravity is the strongest of the four, but in fact it is weaker than the weak force by a factor of about a trillion. The reason gravity is still a force to be reckoned with (you’re welcome) is because it has infinite range and acts on absolutely everything. All the quarks, leptons, gluons, photons, Ws, Zs and the Higgs are drawn together by its ruthless greed.
As the strong, weak and electromagnetic forces are jumping merrily from particle to particle, barely noticing gravity, it is lurking in the shadows, subtly and silently binding things together without remorse, and nothing escapes it. Not even light. Not even time itself.
Right now, as you sit reading this, the book in your hand is gravitationally attracted to your face and vice versa. You do not notice this amount of gravity because you need a planet’s worth of the stuff for it to become important, but everything around you is collapsing towards everything else very, very slowly.
You can defeat gravity temporarily and on a small scale – a magnet can lift a paper clip off the ground with ease – but when you look at the big picture, gravity is holding everything to the Earth’s surface and will not let go without a wrenching effort.
The other three forces can be explained by introducing virtual particles moving between matter and so an analogous particle can be introduced to account for gravity: the graviton.
Discovering the graviton is going to be tricky though, because it is such a weak force that in order to work up enough energy to agitate the field, we would need a particle collider roughly the size of the galaxy. And even if we did that, the energy of such a collision would be so tremendous it would make a black hole at the point of impact, which would suck the gravitons back inward and we would never see them. Unless we can figure out another way of detecting particles and fields, gravitons are going to remain hidden.
ODD ONE OUT
Gravity is different to the other three forces in many ways. It is not just a little bit asymmetric from them (the way electromagnetic and weak are a bit asymmetric from each other), it is the undisputed Quasimodo of physics. Here is some of the nastiness we run into when we talk about gravity.
1. Gravity is weaker than the other forces by an alarming amount. If we represent the strengths of the other three forces like pins stuck on a line, we could place them all within a centimetre of each other as they are all comparable in strength. Gravity’s pin would be somewhere out towards the Andromeda galaxy.
2. Using quantum field theory we can predict how much energy there should be in the vacuum of empty space (the result of adding all those virtual particles together). The total amount of energy in the vacuum should be around 10105 Joules per cubic centimetre but when we actually measure the energy for real, which we do by observing the impact of gravity on galaxies, we get 10−15 Joules per cubic centimetre. The value quantum field theory predicts and the value gravity measures are out by several million quadrillion. Quantum field theory on its own boasts the most accurate prediction in all of science, as we have seen, but including gravity in the equation gives us the worst prediction in science.
3. One of the features of fermions (matter particles such as quarks, electrons and neutrinos) is that they occupy their own space. It is a feature called the Pauli exclusion principle, which states that particle identities including their energies and locations are exclusive to them. Fermions stay separated at all costs, but when we get enough gravity together at the heart of a black hole, particles get crunched together. In quantum field theories, the Pauli exclusion principle is inviolable. In gravity theories, it can be broken quite easily.
4. The theory that describes how gravity works is Einstein’s general theory of relativity, which relates energy, mass, time, light and empty space. The official record states that Einstein came up with it in 1916 although what is less widely known is that he originally discovered it in 1912 but threw the equation away and told nobody,2 presumably because he thought it was wrong. General relativity is a theory which describes empty space bending around objects to create distortions, which we perceive as gravity. The theory matches experiment perfectly and rests on a key assumption: space is smooth at every point with a clearly defined value. In quantum mechanics, however, the Heisenberg uncertainty principle says that can never happen. All fields and particles jiggle, which means no theory describing things as smooth can possibly be correct. Gravitons themselves would have to obey Heisenberg uncertainty laws while gravity itself would apparently not.
5. The other three force fields are sitting against the backdrop of empty space. Particles can be bumped out of them but the shape of the fields obeys sensible geometry. In general relativity, however, empty space can curve. All the particles used to moving in straight lines suddenly find the space around them changing shape and we have no way of explaining how this affects them. We can use gravity to explain what happens to a whole crowd of particles, but the force of gravity on a single particle cannot easily be calculated.
Gravity is not just the loner kid at the house party. It is the bully who goes around kicking everyone in the back, knocking over lampshades and urinating on the television. Quantum field theory works neatly until we factor in gravity, at which point it all breaks down. And this is the most thrilling thing that could have happened.
THE TREE OF KNOWLEDGE
Every object near the surface of the Earth is pulled towards it through gravity. Newton realised that this same force was working on all the suns, moons and planets in the cosmos because gravitation was a universal law, applying to the heavens and the Earth alike. Thanks to him, two apparently different domains of reality were connected by one simple explanation.
A few centuries later, Einstein discovered that the laws of energy were part of the same framework and incorporated them into relativity. Once again showing that two separate branches of physics were bound together by a previously hidden connection.
Meanwhile, Michael Faraday discovered that electric and magnetic fields, thought to be separate, were different facets of the same field, giving us a unified explanation that encompassed electricity, magnetism and light.
Then quantum physicists took these electromagnetic laws and joined them with particle physics to birth QED, before combining QED with radioactivity and the weak force to give us electroweak theory.
The same thing happens again and again. We start at different places in our knowledge, places that appear unrelated, but as we follow them to their logical conclusions we discover links between unconnected theories like twigs joining up to form the branches of a tree. The more we study the universe, the more those branches connect.
At the moment, our tree of knowledge is a network of theories that sprout from three main boughs. One is general relativity, which explains astronomy, cosmology and gravity. One is the electroweak theory, which explains mass, light, radioactivity, classical forces and chemistry. The third is quantum chromodynamics, which explains the nucleus. And we are pretty close to uniting those last two.
Right now we are finding ingenious ways to combine the electroweak theory with quantum chromodynamics to give us a ‘grand unified theory’ or GUT that will account for the entire standard model in one go.
If we can devise a successful GUT, we would have everything in physics explained by two boughs: general relativity for gravity and the completed quantum field theory for everything else. Could we eventually bring them together to form a trunk? Is there a single theory of everything that will expla
in away the contradictions between gravity and the quantum forces? We do not know but we are sure as hell going to try.
THE BEGINNING
A century ago we thought we had the answers. Quantum physics has taught us some humility since then. Rather than science drawing to a denouement, it appears that things are just getting started and that is a good reason to get excited. We have a daunting task ahead of us but the fact that we have got so far in such a short time gives me tremendous hope.
Humans are born with not only a thirst for knowledge but a brain capable of acquiring it, We do not like the phrase ‘nobody knows’, and we are determined to find out where we fit in the grandest scheme. That is why we never give up answering questions and questioning answers. The universe may be unimaginably complicated, but if we can make sense of quantum mechanics, who knows what else we can do?
For that reason, and for many others, I truly believe that science will save our species.
Timeline of Quantum and Particle Physics
Science rarely happens in straight lines. Sometimes the theory we invent makes a prediction we accidentally verified years before, without realising what we were doing. From our current vantage point we can put the puzzle pieces together in a narrative, but we sometimes lose historical accuracy in the process.
Throughout this book I have focused on storytelling in order to make this complicated topic easier, but I have occasionally sacrificed chronology to do so. Here, in the interests of authenticity, is a more precise timeline of how things went down.