How can light be both a particle and a wave?

Question

I usually see myself as being pretty solid on my general science knowledge, but this one continues to stump me.

Light is photons, little particles that move through space… but then it’s also a wave, like the visible light portion of the electromagnetic spectrum? How can it be both? How would photons red shift over great distances? Do we just not know what light is, really?

In: 9

Answer 1

Think of a solid cylinder: if you look at it end-on it looks like a circle, and if you look at it from the side it looks like a rectangle, but a cylinder is neither, it just resembles them under certain circumstances. Similarly, rather than thinking of photons as either waves or particles, think of them as something else entirely which just has some particle-like behavior when you look at it one way, and some wave-like behavior when you look at it a different way.

Red-shifting happens because the universe is expanding—the space through which the photon is traveling is stretching. Imagine you’re walking at your usual pace, only the ground beneath you is continually stretching. As you swing your leg forward to take a next step, the stretching of the ground pulls your rear foot further back, causing you to step a longer distance each time. Now imagine the length of your steps is the wavelength of a photon being red-shifted.

We know a whole lot about light, like each photon is made up of an electric field and a magnetic field oscillating in unison, but at 90^o to each other; that all photons are created as the energy shed by electrons “jumping” to lower-energy orbitals around atomic nuclei; that photons do not obey the Pauli exclusion principle, meaning two or more photons can occupy the same locale and energy level simultaneously; we know they have zero rest mass; and we know their speed, c, is defined by two physical constants: the [vacuum permittivity](https://en.wikipedia.org/wiki/Vacuum_permittivity) and the [vacuum permeability](https://en.wikipedia.org/wiki/Vacuum_permeability) of free space. However, there’s likely a whole lot we have yet to learn about light.