What is an Elementary Particle?

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When I recently asked a dozen particle physicists what a particle is, they gave remarkably diverse descriptions. They emphasized that their answers don’t conflict so much as capture different facets of the truth.

“We say they are ‘fundamental,’” said Xiao-Gang Wen, a theoretical physicist at the Massachusetts Institute of Technology. “But that’s just a [way to say] to students, ‘Don’t ask! I don’t know the answer. It’s fundamental; don’t ask anymore.’”

A Particle Is a ‘Collapsed Wave Function’

A Particle Is a ‘Quantum Excitation of a Field’

A Particle Is an ‘Irreducible Representation of a Group’

‘Particles Have So Many Layers’

Particles ‘Might Be Vibrating Strings’

A Particle Is a ‘Deformation of the Qubit Ocean’

‘Particles Are What We Measure in Detectors’

“At the end of the day, quantum gravity has some mathematical structure, and we’re all chipping away at it,” Engelhardt (physicist at MIT) said. She added that a quantum theory of gravity and space-time will ultimately be needed to answer the question, “What are the fundamental building blocks of the universe on its most fundamental scales?” — a more sophisticated phrasing of my question, “What is a particle?”

In the meantime, Engelhardt said, “‘We don’t know’ is the short answer.”
 
Twelve fundamental particles making up everything in the universe. All discovered using particle accelerators. I presume there could be more. At eighty six, I don't give a big rats (you fill in the blanks).
 


For several years, the particle physicists who study nature’s fundamental building blocks have been in a textbook Kuhnian crisis.
The crisis became undeniable in 2016, when, despite a major upgrade, the Large Hadron Collider in Geneva still hadn’t conjured up any of the new elementary particles that theorists had been expecting for decades.

The swarm of additional particles would have solved a major puzzle about an already known one, the famed Higgs boson. The hierarchy problem, as the puzzle is called, asks why the Higgs boson is so lightweight — a hundred million billion times less massive than the highest energy scales that exist in nature. The Higgs mass seems unnaturally dialed down relative to these higher energies, as if huge numbers in the underlying equation that determines its value all miraculously cancel out.

The extra particles would have explained the tiny Higgs mass, restoring what physicists call “naturalness” to their equations. But after the LHC became the third and biggest collider to search in vain for them, it seemed that the very logic about what’s natural in nature might be wrong. “We are confronted with the need to reconsider the guiding principles that have been used for decades to address the most fundamental questions about the physical world,” Gian Giudice, head of the theory division at CERN, the lab that houses the LHC, wrote in 2017.

Researchers are increasingly zeroing in on what they see as a weakness in the conventional reasoning about naturalness. It rests on a seemingly benign assumption, one that has been baked into scientific outlooks since ancient Greece: Big stuff consists of smaller, more fundamental stuff — an idea known as reductionism. “The reductionist paradigm … is hard-wired into the naturalness problems,” said Nima Arkani-Hamed, a theorist at the Institute for Advanced Study in Princeton, New Jersey.

Now a growing number of particle physicists think naturalness problems and the null results at the Large Hadron Collider might be tied to reductionism’s breakdown. “Could it be that this changes the rules of the game?” Arkani-Hamed said. In a slew of recent papers, researchers have thrown reductionism to the wind. They’re exploring novel ways in which big and small distance scales might conspire, producing values of parameters that look unnaturally fine-tuned from a reductionist perspective.
 
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