THREE hundred feet below the outskirts of Geneva lies part of a 17-mile-long tubular track, circling its way across the French border and back again, whose interior is so pristine and whose nearly 10,000 surrounding magnets so frigid, that it’s one of the emptiest and coldest regions of space in the solar system.

The track is part of the Large Hadron Collider, a technological marvel built by physicists and engineers, and described alternatively as heralding the next revolution in our understanding of the universe or, less felicitously, as a doomsday machine that may destroy the planet.

After more than a decade of development and construction, involving thousands of scientists from dozens of countries at a cost of some $8 billion, the “on” switch for the collider was thrown this week. So what can we expect?

The collider’s workings are straightforward: at full power, trillions of protons will be injected into the otherwise empty track and set racing in opposite directions at speeds exceeding 99.999999 percent of the speed of light — fast enough so that every second the protons will cycle the entire track more than 11,000 times and engage in more than half a billion head-on collisions.

The raison d’être for creating this microscopic maelstrom derives from Einstein’s famous formula, E = mc2, which declares that much like euros and dollars, energy (“E”) and matter or mass (“m”) are convertible currencies (with “c” — the speed of light — specifying the fixed conversion rate). By accelerating the protons to fantastically high speeds, their collisions provide a momentary reservoir of tremendous energy, which can then quickly convert to a broad spectrum of other particles.

It is through such energy-matter conversion that physicists hope to create particles that would have been commonplace just after the big bang, but which for the most part have long since disintegrated.

Here’s a brief roundup of the sort of long-lost particles the collisions might produce and the mysteries they may help unravel.

Higgs Particles

One of the mysteries that continues to stump physicists is the origin of mass. We can measure with fantastic accuracy the mass of an electron, a quark and most every other particle, but where does mass itself come from?

More than 40 years ago, a number of researchers, including Peter Higgs, an English physicist, suggested an answer: perhaps space is pervaded by a field, much like the electromagnetic fields generated by cellphones and radio broadcasts, that acts like invisible molasses.

When we push something in the effort to make it move faster, the Higgs molasses would exert a drag force — and it’s this resistance, as the Higgs theory goes, that we commonly call the object’s mass. Scientists have incorporated this idea as a centerpiece of the so-called standard model — a refined mathematical edifice, viewed by many as the crowning achievement of particle physics, that since the 1970s has described the behavior of nature’s basic constituents with unprecedented accuracy.

The one component of the standard model that remains stubbornly unconfirmed is the very notion of the Higgs’ “molasses” field. However, collisions at the Large Hadron Collider should be able to chip off little chunks of the ubiquitous Higgs field (if it exists), creating what are known as Higgs bosons or Higgs particles. If these particles are found, the standard model, more than a quarter-century after its articulation, will finally be complete.

Supersymmetric Particles

In the early 1970s, mathematical studies of string theory revealed a striking step toward Einstein’s unfulfilled dream of a unified theory — a single theory embracing all forces and all matter. Supersymmetry, as the insight is called, is mathematically complex but has a physical implication of central relevance to the Large Hadron Collider.

For every known species of particle (electrons, quarks, neutrinos, etc.), supersymmetry implies the existence of a partner species (called, with physicists’ inimitable linguistic flair, selectrons, squarks, sneutrinos, etc.) that to date has never been observed.

Physicists believe these “sparticles” have so far evaded detection because they’re a good deal more massive than their known counterparts, thus requiring more powerful collisions for their copious production.

A wealth of calculations strongly suggests that the collider will have that power.

The discovery of sparticles would be a monumental achievement, taking us far beyond Einstein by establishing a deep link between nature’s forces and the particles of matter. Such a discovery also has the potential to advance our understanding of dark matter — the abundant matter that permeates space but does not give off light and hence is known only through its gravitational influence. Many researchers suspect that dark matter is composed of sparticles.

Transdimensional Particles

A tantalizing idea considered since the early part of the last century is that the universe might have more than the three spatial dimensions of common experience.

In addition to the familiar left/right, back/forth and up/down, physicists have contemplated additional directions that are curled up to such a small size that they’ve so far eluded discovery.

For many years Einstein was a strong proponent of this idea. He had already shown that gravity was nothing but warps and curves in the familiar dimensions of space (and time); the new idea posited that nature’s other forces (for example, the electromagnetic force) amounted to warps and curves in additional, as yet unknown, spatial dimensions. Difficulties in applying the idea mathematically resulted in Einstein ultimately losing interest. But decades later, string theory revived it: the mathematics of string theory not only requires extra dimensions but has shown how to resolve the issues that flummoxed Einstein.

And now, remarkably, there’s a chance — albeit a small one — that the collider may find evidence for the extra dimensions. Calculations show that some of the debris produced by the proton collisions may be ejected out of our familiar spatial dimensions and crammed into the others, a process we’d detect by an apparent loss of the energy the debris would carry.

The unknown is just how powerful the collisions need to be for this process to happen, a number itself determined by another unknown: just how small the extra dimensions, if they exist, actually are. The more tightly they’re curled, the harder it would be to cram anything in them and so the more energetic the required collisions.

Should the Large Hadron Collider have the power necessary to reveal extra dimensions of space — to overturn our belief that length, width and height are all there is — that would rank as one of the greatest upheavals in our understanding of the universe.

Micro Black Holes

Now for the possibility that’s generated the fuss.

Recent work in string theory has suggested that the collider might produce black holes, providing physicists with a spectacular opportunity to study them in a laboratory.

The common conception is that black holes are fantastically massive astrophysical bodies with enormous gravitational fields. But in reality, a black hole can have any mass. Take an orange and squeeze it to a sufficiently small size (about a billionth of a billionth of a billionth of a meter across) and you’d have a black hole — with the mass of an orange.

Physicists have realized that the collider’s proton-proton collisions might momentarily pack so much energy into such a small volume that exceedingly tiny black holes may form — black holes even lighter than the one theoretically created by the orange, but black holes nevertheless.

Why might one worry that this would be a problem? Because black holes have a reputation for rapacity. If a black hole is produced under Geneva, might it swallow Switzerland and continue on a ravenous rampage until the earth is devoured?

It’s a reasonable question with a definite answer: no.

Work that made Stephen Hawking famous establishes that tiny black holes would disintegrate in a minuscule fraction of a second, long enough for physicists to reap the benefits of having produced them, but short enough to avoid their wreacking any havoc.

Even so, some have worried further that maybe Dr. Hawking was wrong and such black holes don’t disintegrate. Are we willing to bet the fate of the planet on an untested insight? And that question takes us to the crux of the matter: the collisions at the Large Hadron Collider have never before occurred under laboratory settings, but they’ve been taking place throughout the universe — even here on earth — for billions of years.

Cosmic rays — particles wafting through space — constantly rain down on the earth, the other planets and the wealth of stars scattered throughout the galaxy, with energies far in excess of those attainable by the Large Hadron Collider. And since these more powerful collisions haven’t resulted in astrophysical calamities, the collider’s comparatively tame collisions most assuredly won’t either.

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Should any of the particles described above be produced at the Large Hadron Collider, from Higgs particles to black holes, corks will rightly pop in physics departments worldwide. But the most exciting prospect of all is that the experiments will reveal something completely unanticipated, something that forces us to rethink our most cherished explanations.

Confirming an idea is always gratifying. But finding what you don’t expect opens new vistas on the nature of reality. And that’s what humans, including those of us who happen to be physicists, live for.

Brian Greene, a professor of math and physics at Columbia, is the author, most recently, of “Icarus at the Edge of Time.”