Second of two parts

Physicists have a lot in common with Ponce de León and Bono de U2. After decades of searching, they are no longer young. And they still haven’t found what they were looking for.

In this case, the object of the physicists' search is SUSY. SUSY is not a real person or even a relevant source for aging in any way. It is a mathematical framework based on principles of symmetry that could help physicists better explain the mysteries of the universe. Many experts believe that the particles predicted by SUSY are the weakly interacting massive particles or WIMP, which supposedly make up the invisible “dark matter” that lurks throughout the cosmos.

So far, SUSY has been a disappointment. Despite multiple heroic quests, SUSY remained hidden. Maybe it’s a mathematical mirage.

If SUSY turns out to be a myth, it won’t be the first time that symmetry leads science to a wild pursuit of WIMP. Reasoning from the symmetry of circular motion originally suggested the existence of a new form of matter in space more than two millennia ago. Devotion to that symmetry has blinded science to the true nature of the solar system and planetary motion for the next 19 centuries.

You can blame Plato and Aristotle. In its day, ordinary matter supposedly consisted of four elements: earth, air, fire, and water. Aristotle constructed an elaborate theory of motion based on these elements. He insisted that they naturally moved in straight lines; the earth and water moving downward (toward the center of the world), the air and fire moving upward. In the sky, however, Aristotle noticed that the motion seemed to circulate, while the stars revolved around the night sky. “Our eyes tell us that the heavens revolve in a circle,” he wrote in About the Skies. Since the four known elements moved in a straight line, Aristotle deduced that the heavens must consist of a fifth element, called the ether, absent on Earth but predominant in space.

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Plato, for theoretical rather than observational reasons, had already insisted that the symmetry of circularity meant perfection, and therefore circular motion should be required in the heavens. And so for centuries, the assumption that celestial motion must be circular has maintained a strangulation for natural philosophers trying to understand the universe. As early as the 16th century, Copernicus was willing to deposit Aristotle's Earth in the middle of everything but still believed that the Earth and other planets revolved around the sun with a combination of circular motions. It was another half-century before Kepler established that planetary orbits are elliptical and not circular.

Aristotle’s belief in an exotic form of matter in space is not so different from the image that scientists paint in the skies today, albeit in a rather more rigorous and sophisticated theoretical way. Astronomers believe that dark matter predominates in space; it is inferred that it exists from gravitational effects that alter the movements of stars and galaxies. And physicists have determined that dark matter cannot (for various non-circular reasons) be made of the same ordinary matter found on Earth.

SUSY particles have long been one of the most popular proposals for the identity of this cosmic dark matter, based on more complicated notions of symmetry than those available to Plato and Aristotle. And since the early twentieth century, symmetry mathematics has generated an astonishing number of scientific successes. From Einstein’s relativity to the theory of particles and elemental forces, symmetry considerations now form the core of nature’s understanding of science.

These mathematical forms of symmetry are more elaborate examples of symmetry that are commonly understood: a change that leaves things as before. A perfectly symmetrical face looks the same when a mirror swaps left to right. The appearance of a perfect sphere does not change when you turn it to see the other side. Rotate a snowflake by any multiple of 60 degrees and you will see the same snowflake.

Similarly, more sophisticated mathematical frameworks, known as symmetry groups, describe aspects of the physical world, such as time and space or the families of subatomic particles that make up matter or transmit forces. Symmetries in the equations of such mathematics can even predict previously unknown phenomena. Symmetry in the equations describing subatomic particles, for example, revealed that for each particle nature allowed an antimatter particle, with opposite electric charge.

In fact, all known particles of matter and force fit perfectly into the mathematical patterns described by symmetry groups. But none of those particles can explain dark matter.

SUSY particles as a possibility of dark matter emerged in the 70s and 80s, when theorists proposed an even more advanced system of symmetry. That mathematics, called supersymmetry (hence SUSY), suggested the existence of an even "super" particle for each known particle: a force-particle partner for each matter particle and a matter-particle partner for each force particle. It was a mathematically elegant concept and solved (or at least improved on) some other annoying theoretical problems. Also, of the super companion particles he predicted, the lightest (whatever it was) seemed to be a perfect dark matter WIMP.

Unfortunately, efforts to detect WIMP (which should be hitting Earth all the time) have barely managed to find any. An experiment that claimed that WIMP detection appears to be on unstable ground: a new experiment, using the same method and materials, reports no such evidence. And attempts to produce SUSY particles in the world’s most powerful particle accelerator, the Large Hadron Collider, were also thwarted.

Therefore, some physicists resigned from SUSY. And perhaps supersymmetry was as misleading as Greek infatuation with circular motion. But the truth is that SUSY is not a theory that can be killed by a single experiment. It is a more nebulous mathematical notion, a framework within which many specific theories can be constructed.

“You can’t kill SUSY because it’s not a thing,” said physicist Patrick Stengel of the International School of Advanced Study in Trieste, Italy, at a conference in Washington, DC, in 2019. “It’s not an idea that can kill. It's basically just a framework for a lot of ideas. "

At the same conference, Katherine Freese's physicist at the University of Texas at Austin noted that there was never a guarantee that the Large Hadron Collider would discover SUSY. “Even before the LHC was built, there were a lot of people who said,‘ Well, you may not get enough energy, ’” he said.

Therefore, SUSY can be an example of symmetry that leads physics to success. But just in case, physicists pursued other possibilities of dark matter. An old suggestion that has recently received renewed interest is a hypothetical light particle called axion (SN: 24/03/20).

Of course, if there are agions, fans of symmetry could still rejoice; the motivation for proposing the action to begin with was to solve a problem with another form of symmetry.