Why does the Large Hadron Collider (LHC) have to be so physically large, when it is only designed to detect particles that are extremely tiny?

The LHC is a synchrotron, a circular accelerator that uses carefully synchronised electromagnetic fields to accelerate particles to very high speeds. When this involves charged particles on a curved path they release synchrotron radiation, which wastes energy. This is not desirable because most of the particles that physicists are looking for, such as the recently discovered Higgs boson, have large masses and can only be created in high-energy collisions.

The large radius of the LHC’s track is big enough to limit the radial acceleration given to the particles, thus minimising the loss of energy the particles suffer as synchrotron radiation. The superconducting magnets used to control the flow and direction of the particles can accelerate them up to speeds in excess of 99.99 (but less than 100) per cent of the speed of light.

The magnets lie central in answering the question. The size of the LHC is actually a trade-off between three things: the magnets that are available; the energy (or velocity) it is necessary to give the particles; and, the feasible dimensions of the structure.

The faster the particles are moving, the more likely you are to see something interesting happen in a collision. So, it’s important to accelerate the particles, mainly protons, as much as possible.

The protons need to follow a circular path so they can be continuously accelerated by an electric field, and this is done using magnets positioned around the tunnel. The faster the protons travel, the stronger the magnetic fields need to be to keep them on track.

To increase energy there are two possible choices: make the magnets stronger or the accelerator ring larger, so that the particles’ path does not need to be bent so much. At some point there is either a technological or financial limit on the strength of the magnets, leaving the ring size as the only remaining variable.

However, to keep the costs of the project manageable, the LHC was built in an existing tunnel that housed a previous experiment, called the Large Electron-Positron Collider. So the energy to which protons can be accelerated was actually predetermined by limits of technology and funding.