Standing on the fire escape of his office in Geneva, Switzerland, assistant professor of physics Steven Nahn, PhD '98, enjoys the evening air as he enumerates some of the universe's greatest mysteries. The question he's working on, he says, is so simple a child could ask it: "Where does mass come from?"

Answering it requires explaining why the fundamental par­ticles that make up all the matter in the universe have mass. So Nahn and about 30 other MIT researchers and students are working on experiments to run on the Large Hadron Collider, the new particle accelerator based at the European Organization for Nuclear Research. The LHC is an exercise in extremes: at temperatures near absolute zero, it will accelerate particles to the highest energies ever achieved experimentally, its thousands of powerful magnets guiding protons along a circular path 27 kilometers in circumference until they collide. The instrument is slated to be fully operational later this year, and Nahn figures he'll spend half of the year working there; his students and other researchers are spending most of their time at the LHC. Each wants to be part of the team investigating one the biggest questions in physics.

"I find it ironic," muses Nahn. "It seems like a very simple question, but it takes an enormous accelerator and thousands of physicists working on it to try to find out the right answer."

Many of the other questions driving MIT physicists are just as basic. Another big one that LHC experiments could resolve concerns the nature of about 23 percent of the universe--the so-called dark matter, whose existence is inferred from gravitational effects on visible objects. Physicists simply don't know what it is. "Where is all the stuff?" Nahn says, half joking. "You'd think we'd know."

To be a physicist is to puzzle over what the rest of us take for granted--that objects have mass, that the universe consists of matter instead of antimatter, that gravity works.

Physicists around the world are now working to expand and revise the parts list of the universe--what's known as the standard model, a compact distillation of about 100 years of research, which attempts to describe the particles and forces that account for all physical phenomena. The standard model includes 12 fundamental particles that constitute matter as we know it, plus their equal but opposite antiparticles. It includes the four fundamental forces governing interactions between particles: gravity, electromagnetism (which is responsible for light, magnetism, and electricity), and the strong and weak forces (which mediate the interactions within atomic nuclei). And it includes particles that carry the four forces--although the one carrying gravity remains hypothetical.

This framework ties together everything that particle physicists know to be true. It tells us that atomic nuclei, once thought to be indivisible, are made up of protons and neutrons; protons and neutrons are further divisible into particles called quarks, which are held together by the strong force, whose carrier is the gluon.