The proton is such a complicated system. ~ German physicists Jan Bernauer and Randolf Pohl
Every atom has a nucleus that carries a positive charge by virtue of possessing at least 1 proton. The atomic number of an atom, its nuclide, refers to the number of protons in the nucleus.
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Like the base of a pyramid, the physics of protons provide the foundation of what is understood about matter. It is a tenuous base. Researchers cannot agree about the radius of the proton; somewhere around 0.88 femtometers, ±4%. 4% sounds small, but the discrepancy cannot ostensibly be explained by either experimental method or error. It may be that the uncertainty principle is exercising itself in a grandiose way, in that how big protons are depends upon how they are observed. But that is less uncertain than it is unlikely.
The situation is a bit tight inside a proton. Pressure in the center of a proton is 10 times greater than in the heart of a neutron star, which is as packed as atomic matter may be.
The quark-trio picture by which protons are painted is simplistic. In addition to these ever-present constituents, a swarm of transient particles churn within a proton. Meantime, gluons – the bosonic glue that holds protons together – ceaselessly careen between quarks.
The upshot of this bustle is that the properties of protons, and their neutral cousins, neutrons, are hard to get a handle on. Spin exemplifies the problem. Physicists have studied subatomic spin for decades, but it’s still not sorted out.
Like the Earth rotating on its axis, quantum particles act as if they are whirling at blistering speed. Because a rotating charge creates a magnetic field, this spin makes protons behave like tiny magnets. This property is key to the medical imaging procedure called magnetic resonance imaging, popularly known by its acronym: MRI.
But there’s no actual spin going on. Because fundamental particles like quarks don’t have a finite physical size, they can’t twirl. They just give the appearance that they do, yielding a proton spin of 1/2.
In 1987, physicists discovered that only a small fraction of the spin owed to the quarks inside. They then suspected gluons as being spin-meisters. No such luck.
The current tally is that gluons are responsible for only ~35% a proton’s spin. Quarks make up ~25%, leaving 40% unaccounted for.
We have absolutely no idea how the entire spin is made up. We maybe have understood a small fraction of it. ~ American nuclear physicist Elke-Caroline Aschenauer
Experimental physicists get little help from their theoretical counterparts when trying to unravel the proton’s perplexities. Quantum chromodynamics – the theory of the strong force transmitted by gluons – is a mathematical marvel of such complexity that its equations cannot be solved. Instead, theorists rely upon an approximate technique that roughly quantizes the quantum actors and their actions. Results only coarsely correspond with experimental measurements, which indicates that rough is not good enough.
The proton is not something you can calculate from first principles. ~ Elke-Caroline Aschenauer
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We have a lot of circumstantial evidence that something like unification must be happening. ~ Indian nuclear physicist Kaladi Babu
Protons seemingly live forever: a fact that physicists are reluctant to accept. The rub is that all of the various physical models which unify electromagnetism with the nuclear forces demand that protons give up the ghost some time.
Experiments show that a proton has a life expectancy of at least 1.6 x 1034 years. That may understate the situation, as proton decay has never been seen.
The proton is the most fundamental building block of everything, and until we understand that, we can’t say we understand anything else. ~ Scottish nuclear physicist Evangeline Downie
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Hydrogen-1 is the only instance of an atom comprising only a proton and an electron, as protons will not bind with each other: their electromagnetic repulsion is stronger than their nuclear strong force attraction.
Enter neutrons, the strong-force glue in atoms, holding protons together in a nucleus. The number of neutrons in an atom determines the isotope of an element.
Besides having selfsame spin, protons and neutrons have equivalent girth: 10–14 meters; and about the same mass: 1.7 x 10–24 grams.
(A 2010 measurement of the proton, using a muon racing around it as a metric, put the proton 4% smaller than previously found. The measurement was supposedly more accurate than those used before. The different result put quantum electrodynamics (QED), which describes how light and matter interact, in deep trouble, as it predicts a fatter proton. At of 2018, whether QED is off-base, or using a muon somehow mucked the result, or the experiments themselves were faulty, remains unknown.)