WHEN picturing particles, freely confesses to physically incorrect thoughts. “With great respect to my forebears, I personally still do start off picturing marbles,” says Kaiser, a physicist at the Massachusetts Institute of Technology. “Little round objects spinning around and around.”
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The picture has intuitive appeal. Surely the fundamental particles that make up matter are tiny, indivisible objects with concrete properties such as position and mass? Since the dawn of quantum mechanics, the theory that governs their workings, we have known that particles do spin – or at least possess a property superficially similar to the rotation of spherical bodies that we call spin.
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Peer any closer, though, and you rapidly start losing your marbles.
The rot started in experiments that shot supposedly fundamental particles at other particles and saw them shatter – rather unlike what you would expect for indivisible units of matter. In fairly quick succession in the 20th century, atoms turned out to be nuclei orbited by electrons, nuclei turned out to be protons and neutrons, and protons and neutrons got subdivided into even smaller particles, known as quarks and gluons.
But those smaller particles are just smaller marbles, right? Perhaps, except that other quantum experiments show that these particles can sometimes pass through walls, move through space and time without taking a detectable path and be in more than one place at once.
The conclusion we must draw is as simple as it is baffling. Material particles are simultaneously also immaterial waves that, left to their own devices, have no particular position, and can be described only by a probabilistic “wave function”. “This is the quantum mechanical aspect of particles,” says particle theorist , also of MIT. “They have wave-like properties as well as this classic idea of a ball that’s in a definite space.”
It is a duality no one can quite explain. It is why chemistry textbooks sometimes depict an electron as a point and sometimes as a diffuse cloud orbiting an atomic nucleus.
And it contains a further conundrum. Not only are particles waves, waves are particles. Something as immaterial as a light wave is made of units, photons, that have no mass or charge but nonetheless can sometimes be said to be localised in one place, like an ethereal marble.
Ponds, not Marbles
These particles do not constitute matter, but are associated with fields that suffuse space and determine how matter particles behave. Photons, for example, embody the electromagnetic field that acts on electrically charged particles; the invisible Higgs field fills all of space to give particles their mass. Inject enough energy into this field, as researchers did at CERN near Geneva, Switzerland, in 2012, and you prove it exists by getting it to pop out a particle – the Higgs boson.
Ultimately, because matter works on levels we cannot directly see, both particles and fields are just imperfect mathematical crutches to support our understanding of it. But when it comes to the properties of matter, it’s fields and the forces they generate that call the shots. They determine why protons and neutrons huddle together in the atomic nucleus, and why electrons, whether diffuse or point-like, orbit it.
So here’s a clue: drop the marbles and think of reality as a pond. “The fundamental thing is the water,” says Wilczek, “and the particles are disturbances in that water.” Whatever those particles are.
This article appeared in print under the headline “How to think about… Particles”
