Meyrin, Switzerland: The world's biggest and most expensive time machine is running again.

Underneath the fields and shopping centres on the French-Swiss border outside Geneva, in the Large Hadron Collider, the subatomic particles known as protons are zooming around a 27-kilometre electromagnetic racetrack and banging into one another at the speed of light, recreating conditions of the universe when it was only a trillionth of a second old.

Some 5000 physicists are back at work here at CERN, the European Organisation for Nuclear Research, watching their computers sift the debris from primordial collisions in search of new particles and forces of nature, and plan to keep at it for at least the next 20 years.

Science is knocking on heaven's door, as the Harvard physicist Lisa Randall put it in the title of her book about particle physics.

But what if nobody answers? What if there is nothing new to discover? That prospect is now a cloud hanging over the physics community.

It has been five years and more than seven quadrillion collisions of protons since 2012, when the collider discovered the Higgs boson, the particle that explains why some other elementary particles have mass. That achievement completed an edifice of equations called the Standard Model, ending one significant chapter in physics.

A 2015 bump in the collider data hinted at a new particle, inspiring a flood of theoretical papers before it disappeared into the background noise as just another fluke of nature.

But since then, the silence from the frontier has been ominous.

"The feeling in the field is at best one of confusion and at worst depression," Adam Falkowski, a particle physicist at the Laboratoire de Physique Theorique d'Orsay in France, wrote recently in an article for the science journal Inference.

"These are difficult times for the theorists," Gian Giudice, the head of CERN's theory department, said. "Our hopes seem to have been shattered. We have not found what we wanted."

What the world's physicists have wanted for almost 30 years is any sign of phenomena called supersymmetry, which has hovered just out of reach like a golden apple, a promise of a hidden mathematical beauty at the core of reality.

Theorists in the 1970s posited a relationship between the particles that carry forces, like the photon that conveys electromagnetism or light, and the basic constituents of matter, electrons and quarks.

If the theory of supersymmetry is correct, there should be a whole new set of elementary particles to be discovered, so-called super-partners of the quarks and the electrons and the other particles we already know and love. Clouds of them left over from the Big Bang, moreover, could make up the mysterious dark matter that astronomers say constitutes a quarter of the universe and whose gravitational pull controls the fates of galaxies.

Colliders get their mojo from Einstein's equivalence of mass and energy. When a pair of protons collide in the Large Hadron Collider, they recreate a smidgen of the original Big Bang that jump-started the cosmos. Whatever forms of matter can be made from that bank of energy - particles and forces that held sway when the universe was young - can reappear and briefly strut their stuff through labyrinths of electronic detectors and computers.

Every time colliders get a little more energy to spend, scientists get access to realms of time, nature and possibility we have never experienced, and we get a little closer to the mathematical bones of reality.

The Large Hadron Collider was designed to collide protons with energies of seven trillion electron volts apiece. That was enough, physicists knew, to discover the Higgs or to prove that it was wrong.

Many theorists had also hoped that supersymmetrical particles would show up when the Large Hadron Collider was finally turned on in 2010. Indeed the mystery particles could have shown up even earlier, in the collider's predecessors, according to some versions of the theory.

As a headline in The New York Times put it in 1993: "315 Physicists Report Failure in Search for Supersymmetry."

So far they are still failing. In May, a new analysis by the 3000 physicists monitoring the big Atlas detector (one of two main detectors in the CERN tunnel) reported no hints of superparticles up to a mass of almost two trillion electron volts.

In other experiments, meanwhile, increasingly sensitive efforts to capture the putative dark matter particles drifting in space (and through our bodies) have also come up empty, and theorists have started turning to more complicated ideas for what nature might be doing in the dark.

Last year, some scientists gathered in Copenhagen to pay off bets, with bottles of expensive cognac, they had made that supersymmetry would appear by now.

"Many of my colleagues are desperate," said Hermann Nicolai of the Max Planck Institute for Gravitational Physics in Potsdam, Germany. "They have invested their careers in this."

The idea that the Large Hadron Collier would discover the Higgs boson but nothing else has long been physicists' worst nightmare. Among other things, it would leave them with no explanation for their greatest achievement: the Higgs itself.

According to CERN, the long-sought boson, the keystone to the Standard Model, weighs 125 billion electron volts, or as much as a whole iodine atom. But that is too light, according to theoretical calculations. The mass of the Higgs should be some thousands of quadrillion times as high.

The cause is quantum weirdness, one principle of which is that anything that is not forbidden will happen. That means the Higgs calculation must include the effects of its interactions with all other known particles, including so-called virtual particles that can wink in and out of existence.

Theorists have to doctor their equations for the Higgs and other numbers to come out right under the Standard Model.

But when the alleged supersymmetric particles are inserted in the mix, a miracle occurs. They cancel out the effects of the other particles, leaving the Higgs with a perfectly finite, normal mass. This is the way nature should be, they say.

Supersymmetry is such a general idea that there is always another version that can be proposed.

Not everybody is ready to give up on supersymmetry or to pay off bets.

Gordon Kane, a superstring theorist at the University of Michigan who is well known in the community for his optimism about supersymmetry, said his calculations predicted that the lightest superparticle should show up around about 1.6 trillion electron volts once enough data was properly analysed. "Sadly," he wrote in an email, "the experimenters have not done realistic searches".

Another staunch supporter is John Ellis, a veteran CERN theorist and professor at Kings College London, whose office at the lab displays a cardboard skeleton holding a sign that says this is what happened to the last person who criticised "Susy," short for supersymmetry. "Obviously I'm disappointed Susy didn't show up when the LHC was turned on," he said, adding that there were still plenty of chances for it to show up.

Guido Tonelli, a professor at the University of Pisa in Italy who was one of the leaders of the Higgs hunt, said, "For a while we thought we could discover the Higgs and new physics at the same time -that was very exciting." But he said he did not share his colleagues depression that it did not happen: "The fact that the Higgs fits the Standard Model means new physics is farther up the energy scale. We know it is there, we just don't know if it is tomorrow or the next decade."

He added, "We need to explore; don't be timid."

By the end of 2018, the collider will have logged some 15,000 trillion collisions. If something does not show up by then, Giudice said, it will be time to go back to the drawing board.

"It's a high point of research when we have confusion," he said. "Certainly this is a moment of confusion."

"Confusion," he explained, "means an opportunity for new ideas."

Among the other ideas, Giudice suggested with a few quick squiggles and scrawls on this blackboard, is that the Higgs mass is fixed not by some deep symmetry principle, but rather by the continuing dynamics of fields and forces. As the universe expands and evolves during the Big Bang, the Higgs field, of which the boson is an expression, undergoes phase transitions, like water turning to ice. At some point, it gets stuck.

"What fixes the value of the Higgs is the history of the universe," he said. But that would make the Higgs field unstable over very long time frames - much longer than the age of the universe - and could eventually collapse, dissolving what we think of as reality.

Another possibility, which is anathema to many card-carrying Einsteinians, is that these problematic numbers are due to random chance. There are virtually an infinite number of possible universes with different Higgs masses, but only one that has the capability of evolving into stars, planets, us.

CERN has begun laying plans for a truly giant successor to the Large Hadron Collider: It would be 100 kilometres around and collide protons at 100 trillion electron volts. China is also exploring a "Great Collider" along those lines.

At 14 trillion electron volts, the Large Hadron Collider would either find the Higgs boson or something else because the Standard Model broke down at those energies.

The Future Circular Collider, as CERN refers to it, has no such specific purpose because under the Standard Model, that higher energy range is barren of new particles - a desert in the parlance. But nobody really believes that the Standard Model, with no mention of gravity, is the last word about the universe.

There are trillions upon trillions of proton smash-ups to go before we sleep.

One encouraging hint has come from recent CERN studies of a weird short-lived little particle called a B-meson, which among other things flips back and forth from being itself and its antimatter opposite trillions of times a second. According to the Standard Model, these particles should have an equal chance of producing electrons as their fat cousins the muons, when they decay in certain ways. However, measurements at the CERN collider have shown a definite propensity for the mesons to underproduce muons, as reported at CERN in April.

The same quantum weirdness that blows up the theoretical mass of the Higgs might also be at work here, physicists say, hinting at a new very massive particle called a leptoquark. Or it could just be a fluke.

"Needless to say, if these signals hold up then it would be an extremely big deal, but it is too soon to say," said Guy Wilkinson, an Oxford professor who is the spokesman for the LHCb collaboration.

It was only six years ago that the collider was on the verge of ruling out the Higgs boson, at least as prescribed by the Standard Model. Scientists prepared to explain to the public why failing to find the Higgs boson would be more exciting than finding it: another chance at creative confusion.

It was just then, of course, that a small bump appeared in the data charts that would turn out to be the elusive boson.

"Nature might be more subtle than we think it is," said Joel Butler, a physicist at the Fermi National Accelerator Laboratory, who leads one of the CERN detector teams.

"It took 50 years to find the Higgs," he said, standing beside his multistory detector, known as CMS, 300 feet underground one morning.

"Patience is clearly a virtue in physics," he added.

The New York Times