6.3 Collisions on a small-scale
In case you think it’s a long time since you personally were involved in a collision, you should be aware that even the air that you breathe has its properties regulated by the innumerable collisions that occur every second between the molecules in the air. The air pressure that helps to keep your lungs inflated and enables you to breathe is a result of the rate at which momentum is transferred between the molecules in the air and lung tissue.
Collisions continue to be of importance in nuclear physics, but they are even more significant in subnuclear physics. Collision experiments, usually at very high energies, are almost synonymous with the experimental investigation of elementary particles such as protons, and their supposedly fundamental constituents, the quarks and gluons. These investigations are carried out with the aid of purpose-built particle accelerators, such as the ones at The European Centre for Particle Physics (CERN) (Video 2) or Brookhaven National Laboratory in the USA. Sophisticated detectors allow the energies and momenta of the emerging particles to be measured, aiding the identification of the particles and the analysis of their behaviour. The results give an indication of the underlying structure of the colliding particles, and have revealed the existence of forms of matter that would still be unknown and possibly even unsuspected were it not for collision experiments. The most recent of these results is the observation of a particle consistent with the Higgs boson announced by CERN in July 2012.
Transcript: Video 2 A look inside CERN.
BRIAN COX (VOICEOVER): Every civilisation has its own creation story. The ancient Chinese, Indian mystics and Christian theologians all place a divine creator at the heart of their creation stories. Science, too, has an elaborate story that describes the Universe’s genesis. It tells us how the fundamental constituents of the cosmos took on their form.
The difference with this story is that we can test it. We can find out if it’s true by tearing matter apart and looking at the pieces. All you need is a machine powerful enough to restage the first moments after creation.
In the beginning, there was nothing. No space, no time, just endless nothing. Then, 13.7 billion years ago, from nothing... came everything. The Universe exploded into existence. From that fireball of energy emerged the simplest building blocks of matter. Finding experimental evidence of these fundamental entities has become the holy grail of physics.
PROFESSOR ALVARO DE RUJULA: Well, the Universe is an object that is not stable. It is expanding and cooling. It’s doing things. And it was therefore different in the past and it will be different in the future. It has a history. It has a life. It has an evolution.
BRIAN COX (VOICEOVER): As the early Universe grew, its mysterious primeval constituents transformed themselves into atoms, then molecules and, eventually, stars and planets. Now, billions of years on from the big bang, the Universe is so complex that all traces of the enigmatic building blocks are lost.
PROFESSOR ALVARO DE RUJULA: Understanding the evolution of the Universe requires understanding what it is made of. As it turns out, most of that of which the Universe is made are things that we do not understand at all.
BRIAN COX (VOICEOVER): But we hope that the LHC is about to bridge this profound gap in our knowledge, by peering further back in time than ever before. The LHC is truly colossal. Its accelerator ring is 27 kilometres long - big enough to encircle a small city. And around it, we’ve built four enormous experiments that will investigate the big bang in exquisite new detail.
BRIAN COX: This is my experiment, the experiment that I work on - ATLAS. And what you can see is just the surface buildings. The experiment is actually a hundred metres below the ground, which is where the LHC is. And, basically, this is just a building that covers cranes, where we winch everything down.
And this is pretty much the last time that not only TV crews but me and the people that built it will be able to go down. Because, once it starts operating, the whole area becomes a radiation area. It becomes mildly radioactive.
You’ve always got to be worried when you see those things. One of the most expensive bits, if not the most expensive, bit of ATLAS actually, was digging the cavern. We even have iris scanners. So, a little bit of science fiction.
BRIAN COX (VOICEOVER): It’s down here, in caverns brimming with the latest technology, that the big bangs will be made.
BRIAN COX: We just take little bits of matter, little bits of this stuff, and accelerate them to as close to the speed of light as we can get, and then smash them together, right in the middle of that detector, to re-create the conditions that were present back at the beginning of time.
BRIAN COX (VOICEOVER): The bits of matter we’re going to fire around the LHC are called protons. They come from a family of particles that give the collider its name - the hadrons.
BRIAN COX: Protons are going to fly around here, so close to the speed of light that they go round this 27-kilometre tunnel 11 000 times a second.
BRIAN COX (VOICEOVER): The ring has two barrels that will shoot beams of protons around in opposite directions. When they collide, they’ll have the energy equivalent to an aircraft carrier steaming at 30 knots. All this energy will be focused into a space just a fraction of the width of a human hair. The resulting explosion will be so intense that no one’s quite sure what will happen.
BRIAN COX: This machine really is a leap into the unknown. I mean, it’s often said with scientific experiments, but I think, in this case, it’s absolutely right. We’re a step - something like a factor of ten in energy. So it’s a huge jump up in energy. It’s a huge jump up in the number of times we can smash particles together per second. It collides protons together so often that your chances of seeing something incredibly interesting and profound are increased way beyond anything that we’ve found before. And I can think of no better place to be, actually, at the moment. This is exciting.
BRIAN COX (VOICEOVER): The dream of understanding the building blocks from which the Universe is constructed has inspired the greatest minds for over two millennia.