Clear Science!

We talked about how electromagnetic radiation or light displays characteristics of both waves and particles. Another question we were wondering about was whether or not these particles, photons, have mass. Since light is affected by a gravitational field, does that mean they have mass?

The answer is no, photons are believed to have no mass. To be affected by gravity, they would not need to have mass. Rather, light travels in a straight line through spacetime, which is distorted near a massive object like a planet, star, black hole, etc. Spacetime is a unified continuum taking into account both space and time. The concept of spacetime comes from the special theory of relativity. Special relativity was developed by Einstein, and famously also posits the equivalency of mass and energy, which everyone recognizes as E = mc².

Light traveling in a straight line through spacetime around a star curves, making the source of the light appear to be in a different place. This has been experimentally verified by observing stars around the sun during eclipses. The above graphic is taken from this website, which is throwing its own Clear Science on the question (check it out).

Electromagnetic radiation, i.e. light, can be thought of as both a wave and a particle. We’ve touched on this before concerning Planck’s idea that light has a minimum quantity, which is the basis of quantum mechanics. Einstein postulated that this minimum amount traveled in one direction and acted like a particle, called a photon.

Light unquestionably acts like a wave too, though. One property of waves is that they can interfere with each other: if one wave’s peak corresponds to another wave’s trough, they will cancel out. Albert Abraham Michelson invented the interferometer in the 1880’s, in which a beam of light is split in half, sent two different directions, and then recombined at the detector.

By varying the distances to the two mirrors, the peaks and troughs of the light waves can be superimposed in any way when the light beams recombine. For example, you could set them up to cancel out, or to add together again. If a single photon is put through this device, it will interfere with itself (wave behavior) as if it went both directions, but still be detected as a single photon (particle behavior). This makes no sense … that’s part of the point—however, it’s true.

And sgtrenegade, too. You’re right! People use concepts of both waves and particles to describe light. A lot of you wrote the Clear Science staff asking about this, but don’t worry … we’re coming to that …

Continuing with light, maroonflush and mikerickson have great questions about light’s wave/particle duality, and why light bends in a gravitational field.

So “light” is electromagnetic radiation, or EMR. And the different kinds of light seem different because of their wavelengths (λ). Okay great. But what exactly is waving? And going back to the question, how does it travel through the vacuum of space?

It’s a simultaneous waving of the electric and magnetic fields, at a 90 degree angle to each other. (Hence the name: electro-magentic radiation.) The blue wave (electric field) is going up and down, and the red wave (magnetic field) is going left and right.

These fields exist in the vacuum of space, and require no medium to propagate through. So, light can go anywhere. A very famous experiment called the Michelson-Morley Experiment was conducted at Case Western Reserve University in Ohio in 1887, and proved that light doesn’t propagate through any “stuff.”

Light is a wave (for one), but let’s back up a second and think about what light really is. The “light” we see is a very small slice of something know generally as electromagnetic radiation (EMR). Different kinds of EMR exist because of their wavelength. We’ve talked about wavelength before, and how it’s similar to frequency. Both things mean: how rapidly is the wave waving?

The entire range of possible wavelengths of EMR is called the “spectrum.” The plot above looks complicated, but it’s not. All that’s changing is that waves are rapid at the top and slow at the bottom.

The slice we call “visible light” is shown. The difference between colors is the wavelength of the wave. Radio and TV waves are the exact same thing as visible light, except much longer wavelength. (You could say they are redder than red.) Microwaves, X-rays, gamma-rays … it’s all EMR.

Excellent question, boomerangyoudoalwayscomeback! Light is indeed a wave, and the characteristics of that wave are what make light red.

However, who said a wave can’t exist in a vacuum?!