The Standard Model is a theory developed by physicists to describe how the seventeen particles listed above interact and behave. The full equation of the Standard Model is mathematically daunting. It cannot be described as a particularly elegant-looking equation, but it is an amazing achievement and provides astonishingly accurate predictions on how particles interact via electromagnetism and the strong and weak nuclear forces. Although complicated, the Standard Model can be broken down into four parts. The first part describes the four forces, while the second captures how the four forces act on each of the fundamental particles in the three generations of matter. The third part explains that the Higgs boson determines the mass of each of the fundamental particles, while the fourth part describes how the Higgs boson goes about doing this. The problem was, up until quite recently, no one had ever detected a Higgs boson. The Standard Model depended upon its existence, but what if it didn’t exist? Physicists would have to go back to the drawing board and derive a new model as their understanding of the universe would be flawed.
Detecting the Higgs boson was never going to be easy. It would require creating conditions similar to those of the early universe, and that requires a huge amount of energy. The boson would need to be detected very rapidly after it was coaxed into appearing before it decayed into other particles. Physicists built the world’s largest machine to search for the Higgs boson, a machine I have been fortunate enough to visit.
My eldest daughter has always been an avid reader and, like me, she went through a spell when she devoured books on physics. Like father, like daughter. The curriculum she was taught had thankfully been significantly reworked since I was at school, and she liked physics so much she went on to study it at university, and then to work for a physics start-up company in Oxford, the town where my wife Sonya and I live. As Sophie’s eighteenth birthday drew near, I asked her what she wanted as a present, and she said she’d like to visit the LHC, the machine that physicists built to detect the Higgs boson.
One of the great joys of working at a university is you get to meet experts in all sorts of subjects, and I happened to know a physicist who splits his time between Oxford and CERN. I asked him when was a good time to visit, and he told me to come when it was shut down for maintenance. I looked confused, and he said that if we came then, he could arrange for us to go underground and see the heart of the machine. Sophie and I duly headed to Geneva, where my friend took us to visit part of the machine called the ATLAS detector. As a field biologist who spends a fair amount of my time in wild parts of the planet, I was used to colleagues in other subjects being a little envious of where I work. I very rarely wish I had chosen a different career path, but I did suffer physics envy when we visited the LHC. It is one of science’s, and humanity’s, greatest achievements.
At the LHC, beams of protons, and occasionally other particles, are accelerated to very close to the speed of light before being collided with one another at one of four locations on the 27-kilometre underground particle speedway that is the core of the machine. At each of these locations there is a huge bit of complicated equipment specifically designed to detect the particles that are created from high-speed proton–proton collisions. The particles produced by these collisions do not last for very long, before they decay into other particles giving off energy, or interact with protons, neutrons or electrons. The ATLAS detector is at one of the eight locations on the LHC, and it was one of two detectors that observed the Higgs boson in 2012. The ATLAS detector measures 25 metres by 45 metres and is housed in an underground cathedral-sized cavern. The detector is designed to measure the masses, trajectories and lifetimes of the particles created when proton–proton collisions occur.
The LHC and its four detectors constitute not only the world’s largest machine but also its most technologically advanced. Particles are accelerated and steered around the loop using super-cooled high-powered magnets before being crashed into one another with remarkable precision. The LHC has significantly advanced our understanding of particle physics, but the discovery of the Higgs boson has been its greatest success.
The Standard Model is almost as remarkable as the LHC. It is a mathematical formulation of how quarks, electrons and neutrinos (and higher generations of particles) interact with one another via electromagnetism and the strong and weak nuclear forces. It accurately describes what happens in the physical world around us, and it predicted the existence of a particle that had not been observed. When physicists looked for it, it was there, just as the Standard Model predicted. Thousands of physicists and engineers collaborated to design and build the world’s biggest machine to detect the particle. It cost £3.8 billion to build, and costs about £1 billion each year to run, but in my opinion the cost was well worth every penny. We now have a good understanding of how the universe works. Except that not all physicists are happy. There is something missing, and this led to some scientists arguing that we need a complete rethink of the Standard Model. Any new version must perform as well as the existing model, but it will be bigger and better.
The reason the Standard Model is incomplete is it does not include gravity. It explains how the seventeen particles it describes behave in the absence of gravity. One of the fundamental forces is missing from the model. A huge challenge in physics today is linking our understanding of gravity with the Standard Model.
Physicists have a theory of gravity that is remarkable, and as impressive in its predictions as the Standard Model. The theory is called the theory of general relativity, and it was developed early in the twentieth century by Albert Einstein. The theory of general relativity is an extension of an earlier theory that Einstein also developed called special relativity, and together the two theories describe how gravity, energy, space, time and mass are linked, and how the speed of light is central to the association between energy and mass. Einstein built on fundamental work on gravity that was begun by Isaac Newton, another colossus of science.
The universe has a maximum speed limit, which is the speed of light in a vacuum. The speed limit, as I explained above, is 299,792,498 metres per second, and this is the speed at which photons travel through empty space. If you were to approach the speed of light, some strange things would happen. First, time would slow down, and second your mass would get greater and greater. It is impossible for something with mass to reach the speed of light as its mass would become infinite, but if you were able to, time would stop. Photons can travel at the speed of light because they have no mass. Each photon is a packet of energy zipping through space.
We must exercise our imagination and forget day-to-day common sense when considering Einstein’s theories of relativity. As an example, if we measure how long it takes light to travel to Earth from the sun, we will record a time of eight minutes and twenty seconds. That is our perception of time. From the perspective of the photons that constitute the beam of light travelling through the vacuum of space, each photon is simultaneously at the sun and on Earth. From the perspective of the photon, there is no time, because time has stopped. Einstein named his theory appropriately, because reality is relative to the speed at which you are travelling.