For centuries, scientists thought the universe was filled with a mysterious substance called the luminiferous ether, a medium they believed carried light waves through otherwise empty space.
In 1887, physicists Albert A. Michelson and Edward W. Morley decided to experimentally test the ether hypothesis by measuring the speed of light in different directions. If the ether did exist, then a beam of light aligned with the Earth’s movement around the sun should move faster than a beam of light traveling in the opposite direction, pushing against a hypothetical “ether wind.”
“We can imagine that at the time, it was obvious to everybody that they would detect the ether, because what else could it be?” says Flip Tanedo, an associate professor of theoretical physics at the University of California, Riverside.
But their experiment found no variation in the speed of light, delivering a famous null result that challenged the ether hypothesis and forced a reckoning within the global physics community.
It’s easy to be dismissive of an era in which scientists worked hard to test a hypothesis that turned out to be incorrect, says Matthew McCullough, a theoretical physicist at CERN. But when scientists disprove a favored theory, they’re forced back to the drawing board in a way that winds up deepening our understanding of physical reality.
Today, particle physicists find themselves in a new epoch in which null results outweigh discoveries. Modern “ether” moments are forcing the particle physics community to revisit their favorite theories and reconcile with some difficult truths.
Rethinking SUSY
The Standard Model is our best framework for understanding the universe at the subatomic level. First developed in the 1960s, it has been tried and tested countless times, and it has proven to accurately explain the fundamental particles and their interactions.
In the 1960s and ’70s, multiple physicists developed an extension to the Standard Model called supersymmetry (or SUSY for short). SUSY became one of the most promising candidates for a unified theory of particle physics.
According to SUSY, every matter particle in the Standard Model—such as the electron and each kind of quark—has a hidden, corresponding supersymmetric force-carrying partner. Likewise, every force-carrying particle has a supersymmetric matter partner.
This symmetry offered elegant solutions to several unresolved problems in physics. For instance, it provided a viable candidate for dark matter; explained the stability of protons, which do not seem to decay; and allowed for the unification of fundamental forces at high energies.
In the 1990s, researchers came together from across the planet to build the largest machine humanity has ever produced, the Large Hadron Collider. LHC physicists had two main goals: Find the elusive Higgs boson, which helps explain why some particles have mass while others don’t. And find supersymmetry.
In 2010, when the LHC began collecting data, many physicists felt that they were on the precipice of unlocking nature’s secrets. “We switched on the LHC and we were on a mission,” says Maurizio Pierini, a physicist at CERN.
Many expected the discovery of SUSY before that of the Higgs boson, Pierini says. They waited for the multitude of SUSY partner particles to appear.
Two years after the LHC started collecting data, physicists announced that they had discovered the Higgs boson. But despite numerous searches, supersymmetry was nowhere to be found.
“When it comes to discovering SUSY, it’s like we set out to win a 100-meter race only to find we were running a marathon,” Pierini says.
The Higgs is known as the “final piece of the Standard Model.” But without something beyond it, like supersymmetry, to address unanswered questions, the Standard Model seemed like a castle without a foundation. “We are in a moment of maximum confusion,” Pierini says.
Even the Higgs left scientists scratching their heads.
“If I take a step back and try to appreciate this construction called the Standard Model, I'd say we have to be lucky for the Higgs mass to be what it is,” says Tanedo. “Even though the Standard Model makes sense by itself, if there was anything else like dark matter, quantum gravity, or neutrino mass, they should make the Higgs mass heavier.”
The mass of the Higgs boson, measured as 125 giga-electronvolts, sits precisely at the energy scale where the Standard Model of particle physics is expected to break down, Pierini says. “It is the smoking gun of new physics.”
The specific value of the Higgs mass strongly suggests that physicists should find new particles in the surrounding range, somewhere around 100, 200 or 300 GeV. But, Pierini says, “We have searched and there is only smoke.”
Thinking outside the box
With so many unanswered questions in physics and cosmology, most physicists think that there is more to discover, but not in the form of the SUSY particles they originally imagined, Pierini says. “At this point, it’s very unlikely that you will have the kind of discovery we were expecting 20 years ago.”
LHC physicists have done “an incredible amount of work—some of it beyond what anyone had anticipated,” he says. “SUSY just hasn’t appeared.”
Physicists are now turning their attention to more complex and less obvious possibilities, McCullough says. “Experiments can be a real catalyst for progress by putting theorists in a very uncomfortable place where the only way to escape is to think outside the box.”
One idea is that the LHC is producing SUSY particles, but ones with long lifetimes that can travel anywhere from a few centimeters to a few kilometers before decaying and becoming visible to detectors. These long-lived particles would leave unusual signatures, which scientists could easily misinterpret as the result of a detector malfunction. Scientists are developing a variety of ways to look for them.
To investigate the ongoing mystery of dark matter, physicists are building experiments to detect a variety of potential candidates, including ultralight axions, which would act less as particles and more as waves.
In the absence of SUSY, scientists are pushing physics forward. By reexamining fundamental assumptions and refining their approach to experiments, they are laying the groundwork for the next big breakthroughs, especially if the answers are completely unexpected.
These efforts include continuing to collect data with the LHC, which, after this run, will be transformed into the High-Luminosity LHC. Before the end of its tenure, it is expected to grow the LHC dataset tenfold.
Pierini says, “We still have a long way to go.”
