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I've learned at school, that the electromagnetic radiation consists of photons. And all the frequencies of the wavelengths of photons define electromagnetic spectrum. The lower wavelengths are ionizing radiation. In the middle of the spectrum there is the visible light, and above that infrared, microwave, and radio waves.

But the thing nobody told me, is that: How is it possible to imagine a wave? I don't think, that the photon is physically moving sideways, and that is called the waving. But what is it then?

The explanation of the wavelength is, that it is a spatial period of the wave—the distance over which the wave's shape repeats. But what is this wave exactly? How can a single photon has it?

Iter Ator
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    Possible duplicate of http://physics.stackexchange.com/q/74384/ – garyp Jul 30 '16 at 11:22
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    That question is not answered – Iter Ator Jul 30 '16 at 11:24
  • A photon is not an object, it's a property of the interaction of the wave with matter. Therefor an electromagnetic wave is not made out of photons, even though it is reasonable to say that a certain number of photons can be found while making measurements on the wave. – CuriousOne Jul 30 '16 at 11:27
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    Focus on the waves; study how radio waves are generated and detected. Then consider the same for light. The photon is the quantum of the electromagnetic field, and is obtained mathematically by quantization of the field modes. It's useful when quantum effects are important, but if classical waves are able to explain what is happening, photons only make the description more complicated. – Peter Diehr Jul 30 '16 at 11:29
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    Don't take the drawings and pictures of any physical process too literally, they are only models and aids to give you an idea of what is going on. They are always based on classical concepts, as it is impossible to visually imagine the true picture of a lot of modern physical concepts. –  Jul 30 '16 at 11:40
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    Possible duplicate of What is the relation between electromagnetic wave and photon? – John Rennie Jul 30 '16 at 14:55
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    It's hard to beat @Anna's answer. Here's my two cents (and that's about what it's worth). What's a photon? We don't know what it is, except when an electron is accelerated, or jumps from one orbit to another, we say a photon was either absorbed or emitted. What is an electromagnetic wave? We say its energy is nothing more or less than the probability of a photon appearing - i.e. that an electron moves. I know, that begs the question: what is an electron? The whole thing is circular. – Mike Dunlavey Jul 30 '16 at 15:23

2 Answers2

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I've learned at school, that the electromagnetic radiation consists of photons.

The electromagnetic wave is described by the solution of classical maxwell's equation which has a sinusoidal dependence for the electric and magnetic fields perpendicular to the direction of motion of the wave. It is called a wave for this reason and the frequency is the repetition rate of the sinusoidal pattern.

Quantum mechanically the classical wave is an emergent phenomenon. It is built up by photons with an energy associated with the observable frequency of the emergent classical beam, E=h*nu.

A photon only has this energy definition and a spin 1 h orientation either in its direction of motion or against it, where h is Planck's constant. The beam built up in the image, is built up by the individual photons ( middle image).

embeam and photon

This happens because the quantum mechanical wavefunction of a photon has the E and B information in its complex form, ( a solution of a quantized maxwell equation) and the superposition of photons builds up the classical fields with the frequency nu.

And all the frequencies of the wavelengths of photons define electromagnetic spectrum.

The frequencies of the classical electromagnetic spectrum define the energy of the photon, h*nu, not the photon the frequency, because it is only the probability of detection distribution of a single photon that "waves" in space, not the photon itself.

How is it possible to imagine a wave? I don't think, that the photon is physically moving sideways, and that is called the waving. But what is it then?

One need not imagine the photon as a wave. Only the probability of detecting it, as seen in this answer.

The explanation of the wavelength is, that it is a spatial period of the wave—the distance over which the wave's shape repeats.

This is true for the emergent classical wave.

How can a single photon has it?

A single photon has only a detection probability distribution that "waves", as explained above. It is not a wave.

Community
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anna v
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  • Why is a single photon not a wave? – velut luna Jul 30 '16 at 15:33
  • A single photon has never been registered as a wave. It leaves an (x,y) point interaction foot print as in the single photon detection in the link http://physics.stackexchange.com/questions/269077/how-many-photons-are-needed-to-make-a-light-wave/269098#269098 . It is the accumulation of photons that shows interferences characteristic of a wave nature, but the accumulation is a probability distribution for many measures of photons. – anna v Jul 30 '16 at 18:43
  • I think that depends on what you mean by "wave". – velut luna Jul 31 '16 at 01:58
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    @AlphaGo a wave was first defined in water and then in acoustics and electromagnetism. It is a periodic variation of energy in space and time that is fitted with sinusoidal functions. These functions are solutions of what are called wave differential equations . Hence the identification with waves of the solutions of the shrodinger equation, except it is not energy that is waving, but probability. – anna v Jul 31 '16 at 03:06
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Maxwell's equations have certain stationary states. We can obtain these so called modes and each classical waveform can be built as a linear combination of these modes.

In process called second quantization, we (hand wavingly) put particles into these modes. These particles are photons. Each mode can have 0, 1, 2 photons.

But there is more: we know from the uncertainity principle that no dynamic degrees of freedom can be absolutely confined, since that would imply infinite momentum. That holds for the coefficient of this electromagnetic mode as well, and hence there is always vacuum fluctuations of the electromagnetic field.

In other words, each mode can be represented as a quantum oscillator. (One derives an equation of motion for a mode, and realizes that some quantities behave like momentum and some like position). Quantum 101 tells that the modes of a quantum oscillator are quantized.

Now, we can have these modes in weird shaped cavity and hence we can have very structured modes with indefinte momentum. However, usually photons are measured in far field of the sample such that they have definite momentum and energy.

So, photon does not oscillate to any direction. Photon is an "occupation" of an electromagnetic mode which oscillates.

One more analogy to help to think about this: One could take a vibrating string and solve it's fundamental mode (say 440Hz). If this would be quantized, one can nerver find the string at rest due to uncertainity principle. Further, we will find that the string can only have quantized amount of energy. In other words, the magnitude of the vibrations is quantized. In other words, we can count how many (integer) energy quanta there are in the string. Let's all this quanta a vibron. Now, that is essentially the same thing to electromagnetism and phonons save Lorentz invariance, special relativity on masslesd particle, commutation relations of spin 1-particles, and some other compex stuff out of scope.

Mikael Kuisma
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