Why physics needs such enormous machines to study the smallest things

Seeing smaller things takes more energy, and delivering that energy to a single proton takes a very large machine. That is why the Large Hadron Collider is a ring 27 kilometres around. Three physicists explain the chain of reasons, and all three say each further step gets harder.

Lisa Randall, a theoretical physicist at Harvard, gives the physics in four words: "Higher energy means shorter distances." Energy also becomes mass, so a heavy particle can never be seen unless the collision carries enough energy to make it. Harry Cliff, a particle physicist at Cambridge, calls the collider a gigantic microscope that looks at the structure of the vacuum. In his account the real building blocks are fields that fill all of space, and you have to hit a field hard to make it ripple. The Higgs boson was found that way.

Cliff explains the size. A ring lets the protons come round again and get another push on every lap. The hard part is steering. Protons moving at nearly the speed of light need magnets to hold them on the curve, and the collider used the strongest magnets that could be built at the time. A bigger tunnel can use weaker magnets and a smaller tunnel needs stronger ones, so the ring had to be that wide.

Don Lincoln, who works at Fermilab, explains the rest. Bunches of protons pass through each other like swarms of bees and mostly miss, so the machine makes about 40 million crossings and roughly a billion collisions every second. Most collisions are boring. The detectors that watch them weigh thousands of tons, and fast electronics called triggers throw away nearly everything, keeping about 1,000 events a second. The numbers pay off: his team found the top quark in 1995 with perhaps 19 real events, and the newer collider may make one every second.

The cost is counted in decades and countries. Cliff puts the collider at around 10 to 12 billion euros after about 25 years of campaigning. A proposed 100-kilometre ring would cost about 30 billion euros over roughly half a century. Lincoln says collider energy has grown about sevenfold every twenty years, while the energies that would test a final theory are about a quadrillion times beyond today's machines.

They differ on emphasis. Randall calls the Higgs discovery a major victory, Lincoln calls it the last piece of the existing theory, and Cliff calls it a triumph and a puzzle. Randall regrets the cancelled American collider, which would have been a more robust test, and draws a lesson from the failure of supersymmetry to appear: "you don't want to assume things before they've been actually found." She and Cliff also point to a second route, higher precision instead of higher energy, which is where the collider's most interesting hints now come from.

What to do with it: when you read that a theory will be tested, ask what energy the test needs and whether a real machine can reach it. Ask about the collision rate as well as the energy. Watch the precision experiments. Be cautious about predictions of what a machine will find. Spread a headline cost over its decades and countries before judging it. Count ruling out a favoured idea as a result.