How can light be both a wave and a particle?

1. A disturbance traveling through a medium by which energy is transferred from one particle of the medium to another without causing any permanent displacement of the medium itself.
2. A graphic representation of the variation of such a disturbance with time.
3. A single cycle of such a disturbance.

SDStaff Chronos and SDStaff Ed reply:

EDITOR’S NOTE: Due to the complexity of this topic, we’re going to handle it tag team style. First we’ll hear from Chronos, who actually has some idea what he’s talking about, and then from Little Ed, who doesn’t.

SDStaff Chronos replies:

It used to be thought that waves needed to have a medium, so once the evidence of light’s wavelike nature began pouring in, physicists started talking about a substance called the “ether,” the medium through which light travels. One minor problem: The ether doesn’t exist. Or, at least, if it does, it behaves exactly as if it doesn’t, which amounts to the same thing.

The key experiment in disproving the ether was performed by Albert Michelson and his graduate student Edward Morley in 1887. Michelson wasn’t trying to disprove the ether: His experiment was designed to measure the speed of the Earth through the ether. Surprisingly, he found no evidence that the Earth was moving at all with respect to the ether. Even more surprisingly, he found this exact same result at all times of day and all times of the year, and the Earth certainly can’t be at rest at every point in its orbit. Furthermore, Michelson was a master of precise experiments (he won the Nobel Prize in 1907 primarily for his work in establishing standards of measurement), and he knew that his apparatus, the Michelson interferometer, should have been easily capable of detecting the motion of the Earth. From there, it was all downhill for the ether: No experiment has found evidence for it, and a considerable number have found evidence against.

So, if there is no ether, then what is a light wave a disturbance of? It’s not a disturbance of any substance, but of electromagnetic fields. If you have charged objects at rest, they produce an electric field. If the charges are moving in a regular way, you get a magnetic field. If the charges change the way they’re moving, the electromagnetic field changes, too, but it doesn’t do so instantaneously. It’ll change first close to the charges, with the changes spreading out around them. This spreading change in the field is a light wave.

So you already see how light can be a particle, and it can be a wave, too. But how can it be both at once? Well, it can’t, really. Some experiments will definitely show that light is a particle: Einstein’s Nobel Prize was actually for the photoelectric effect, where he showed that light carries energy in discrete amounts, a certain amount per particle. And some experiments also show that light is definitely a wave: It exhibits interference patterns, which is something that only waves do. But there is no experiment which can show both. Whether light is a wave or a particle depends on how you’re looking with it. This may seem odd, but there are many examples of this sort of duality, even outside of physics. The same person can be a father, son, brother, nephew, or uncle, depending on who’s asking, for instance. So it is with light.

SDStaff Ed comments:

I have a feeling this answer isn’t going to satisfy the questioner. We say light isn’t a disturbance of any substance, but of electromagnetic fields. The obvious question is, well, what’s an electromagnetic field? I’ll be honest and say I don’t really get this myself. You sprinkle iron filings on a piece of paper above a magnet and you see the lines of force, but what are they? Wrinkles in the space-time continuum? It’s easy to imagine that light waves are really waves of photons, that is, packets not of matter but of energy, but years of exposure to the “wave/particle duality” paradox have persuaded me it can’t be that simple.

‘Nother thing, possibly related. In helping my kids with their science homework, I see something about electric fields propagating at right angles to magnetic fields. I don’t get that either. Is an electromagnetic wave a two-dimensional entity? Does it propagate strictly in a plane? This question seems to have some practical consequence, since it explains how polarized sunglasses work, etc., but damned if I understand it.

One last thing. I may be mixing up my enigmas here, but is there any relationship between the wave/particle duality business, the Heisenberg uncertainty principle, and Planck’s constant? Seems to me I recall some discussion along those lines once but it could just be the drugs.

SDSTAFF Chronos ripostes:

To address your questions:

1. I can’t really explain what an electric field is, beyond saying that it’s an electric field. I could add a little more detail about what it does (an electric field is generated by a charge, among other things, and it exerts a force on any other charge). In addition, you can also generate electric and magnetic fields from each other: A changing electric field generates a magnetic field, and a changing magnetic field generates an electric field. This is what’s happening with light, and why you can get a significant electromagnetic field so far away from the charges generating it.
2. As for the directions of these fields, for a uniform beam of light traveling in some direction (usually referred to as a plane wave), the electric field will always be in one plane, and the magnetic field will always be in a different plane at right angles to the electric field plane. Both the electric and magnetic fields are perpendicular to the direction the wave is moving. So, for instance, you might have the beam traveling in the X direction, the electric field in the Y direction, and the magnetic field in the Z direction. You might also have the beam in X, the electric in Z, and the magnetic in Y, or any angle in between. That angle is the polarization angle. This does get more complicated if you have a wave spreading out in three dimensions, and in that case, you usually don’t have any single polarization. Individual photons can have any old polarization, then.
3. This does relate to the uncertainty principle. If you know the position well, then the thing (photon, or electron, or whatever) behaves like a particle, and if you know the momentum well, then the thing behaves like a wave. You can’t know both well, so it can’t behave like both a particle and a wave at the same time.

SDSTAFF Ed humbly concedes:

I guess what we need to say, to address the guy’s question, is that electromagnetic fields are just a property of space–they’re not made up of particles, they don’t require a medium, etc. Just one of those Fundamental Mysteries you have to accept.

SDStaff Chronos and SDStaff Ed, Straight Dope Science Advisory Board

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