Physics: Are Photons Particles or Electromagnetic Waves? How Can They Be Both?

11/14/2017 03:55 pm ET
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DigitalVision/Getty Images

Why do we call photons 'particles'? Aren't they all electromagnetic waves? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Viktor T. Toth, IT pro, part-time physicist, on Quora:

Why we call photons 'particles'? Aren't they all electromagnetic waves?

In the classical theory of electromagnetism, Maxwell’s, there are no photons. There are only continuous electromagnetic waves, their behavior governed by Maxwell’s beautiful equations. These waves do have energy and momentum, by the way; there is no need for a particle concept there, the classical theory of continuous fields can deal with energy and momentum just fine.

However… Maxwell’s classical theory of electromagnetism is just not good enough. It does not fully agree with observation. The first specific observation was the photoelectric effect. The classical theory would imply that if a light is sufficiently dim, it will not dislodge electrons. In reality, even very dim light was able to dislodge electrons, provided its frequency was sufficiently high.

This led Einstein to propose that the electromagnetic field itself consists of quanta. That is to say, at any given frequency, “excitations” of the electromagnetic field come in set, countable units. This basically assigns a minimum energy level to light of a given frequency: the energy of one unit of excitation. This explanation was found to be in good agreement with observational data and eventually (though there was much skepticism initially; some even thought Einstein made a blunder) earned Einstein his sole Nobel prize.

It is these units of excitations of the electromagnetic field that we call photons. Like other quantum particles, they are not really “particles” like miniature cannonballs. Rather, they represent the smallest indivisible unit of interaction with the field. Whenever the field exchanges energy and momentum with its environment, the “particle” determines the smallest unit of energy and momentum that can be transferred.

When the field interacts with the outside world, the interaction may be localized in space, confined to a small volume. In these cases, the “particles” indeed behave like particles in the conventional sense. But at other times, the field’s excitations are “spread out” over a large volume, about which there is nothing particle-like. Even so, when the field interacts with something else, its excitations will go up or down one “particle” unit at a time.

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