Light determines what we see and know. But what is it?
One of the oldest light debates revolved around whether it’s composed of waves or particles. Newton considered light a particle. Other early physicists deemed it a wave. It took physicists of the late 19th and early 20th century to somewhat settle the matter by declaring that light can manifest as either, depending on who’s asking. Set up an experiment with light-beams and mirrors and we find that light is a wave of energy; the juxtaposed beams produce telltale interference patterns just like swells crossing on the ocean.
But shoot electrons at a metal and electrons flake off like debris from a ricocheting bullet; only particles create damage in such a manner. This is the photoelectric effect for which Einstein won the 1921 Nobel prize. Light seems to change characteristics to fit the situation, and thus can be visualized as either a wave, a particle, or better yet, an amalgam — a “packet” of waves.
Each “wave packet” goes by the name of photon, an entity that has no rest mass; it weighs nothing. But since energy and mass are two sides of the same coin, the fact that light has energy proves it must possess an equivalent mass. However, to compare apples with apples we always compare objects only when they’re stationary. Otherwise, they have kinetic energy that gives them equivalent mass. Trouble is, if you could force light to stand still, it would vanish. So we say it has no rest mass. It cannot even exist when it’s not traveling, which zooms to the far end of peculiar.
There had always been hints that light might be unique. Back in the mid-19th century when the brilliant Scottish physicist James Maxwell realized that electricity and magnetism, thought until then to be separate phenomena, were really two facets of the same entity, Maxwell correctly saw light as an entity composed of an electrical pulse and, simultaneously oscillating at right angles, a magnetic pulse. It’s a union of those two. To this day, we speak of it as electromagnetic radiation.
But it’s its speed that truly holds our fascination. Though it greatly slows down when traversing transparent media like glass or air, its speed in a vacuum is 186,282.38 miles a second. Which amounts to circling Earth nearly eight times in a second, or going a foot each nanosecond, or billionths of a second. Wave to someone across the street, 50 feet from you, and you’re seeing each other as you were 50 billionths of a second ago. The light from something 5.88 trillion miles away requires a year to reach you, so we call that distance a light-year. No star is that nearby, and those filling our late summer sky typically lie between 20 and 2,000 light-years away. The farthest naked-eye object, the fuzzy elliptical blob straight overhead at 3:30 a.m., the Andromeda galaxy, is 2.5 million light-years from us.
This story has a strange ending. Einstein was certain that since gravitational and other effects also travel at light-speed, this speed represents the true velocity and upper limit for everything in the cosmos. But as quantum theory gained acceptance a century ago, physicists found its effects happen far faster than light-speed. In fact, they’re limitless. When two “entangled” particles communicate with each other, the effects unfold instantaneously. So nowadays we acknowledge a connectedness between objects in the universe, exchanges that occur in real time, simultaneously, with no delay whatsoever no matter the distance.
One conclusion is that space and time do not really exist. There is neither past nor future in the actual universe, nor any reliable gap separating anything from everything else. Time and space are human-created, like our numbering system. We carry them around with us like turtles with shells.
Thus our present view is of a cosmos more intimate and interconnected than previously imagined. And it was light that opened the door to all this new intimate if bizarre strangeness.