What's the Smallest Particle in the Universe?

"Forget about turtles; for all practical purposes, it's really particles all the way down. Whether as the protons and neutrons that help form chemical elements, the photons that we perceive as light or even the flows of electrons that power our smartphones, subatomic particles constitute essentially everything any of us will ever experience. Ironically, however, because they're so minuscule, the particles underpinning our everyday reality tend to escape our noticeand our comprehension."

"Consider the seemingly simple matter of their size, the very thing that makes them so alien. We're typically taught to imagine any and all particles as tiny, colorful spheres, as if they were solid things that we could lay a ruler alongside to determine their dimensions like we'd do for any other physical object in the world. But subatomic particles don't actually look like that at all."

Subatomic particles — including protons, neutrons, photons and electrons — make up essentially all observable matter and energy. Their extremely small scale makes them difficult to perceive and understand with everyday intuition. Visualizing particles as tiny solid spheres is misleading because many particle properties do not correspond to classical dimensions. For composite particles some broad size measures can be defined experimentally, but for smaller, ostensibly fundamental particles the idea of a definite radius or size breaks down. Different meanings of "small" (mass, radius, density, interaction range) yield different comparisons, so size assessments are context-dependent and experimentally defined.

Read at www.scientificamerican.com