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Photons are electromagnetic wave fields that travel at the speed of light. When they are absorbed by an object, their momentum and energy is transferred to the absorbing object. What is that equivalent mass when it is at zero velocity?

Qmechanic
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Ken
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  • Photons are not fields. They are irreversible energy exchanges between the electromagnetic field and external systems that we usually call "source" and "detector". The classical electromagnetic field is the statistical average of many such exchanges... under certain conditions. But to say that a photon has a field is not correct. It is the field that emerges from many photons. Photons are also only defined for the case of the vacuum fields. In matter coupled excitations of the em field and matter have different names (e.g. polariton for photon/dipole). – FlatterMann Mar 29 '23 at 18:13
  • What is the mass of a moving photon that has a wavelength and momentum before it is absorbed and ceases to exist? Is its mass proportional to its wavelength? – Ken Mar 30 '23 at 22:11
  • There are no photons in the free field. That is also a common misunderstanding of the theory. A "photon" is the amount of energy, momentum and angular momentum that is being emitted into the field or absorbed from the field. That does not mean that photons are the atomistic constituents of em fields. The totality of all interactions of the field with the environment has to fulfill the local conservation laws and every single interaction has to exchange one Planck unit of angular momentum, but that is not sufficient to construct the field from individual "moving photons". – FlatterMann Mar 30 '23 at 23:04
  • To try and clarify for a non-physicist, what are the properties of a photon? – Ken Mar 31 '23 at 23:38
  • A single photon has energy, momentum and angular momentum. That's it. – FlatterMann Mar 31 '23 at 23:50

2 Answers2

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When a photon interacts with an atom, three things can happen:

  1. elastic scattering, the photon keeps its energy and changes angle

  2. inelastic scattering, the photon gives part of its energy to the atom and changes angle

  3. absorption, the photon gives all its energy to the absorbing atom, and the absorbing electron moves to a higher energy level as per QM

Now you are specifically asking about 3., when the photon interacts with the atom so that all the photons energy transforms into the kinetic energy of the atom's absorbing electron. As per QM, the electron moves to a higher energy level.

Photons do not have rest mass. Their energy is proportionate to their frequency. Photons do not have a rest mass. When the photon gets absorbed, it ceases to exist as a photon, it transforms into the kinetic energy of the absorbing atom's absorbing electron.

There are cases, when the photon gets absorbed by a molecule, in the case of higher energy photons, they do deeper into the material, and transfer their energy into the vibrational energy of the molecule (heat up).

It is important to note that the photon not only interacts with the electron, but the whole atomic system.

So when measured locally, and the photon travels in vacuum towards the atom, it travels at speed c. When it gets absorbed, it ceases to exist as photon, and transforms into the kinetic energy of the electron.

It is a common misconception to think about the photon as slowing down before getting absorbed. As long as the photon exists, it travels at speed c in vacuum when measured locally. As soon as it gets absorbed, it ceases to exist as photon, and it does not have a speed.

The photon does not have a rest mass, and we cannot measure its mass when at rest, because the only way to interact with it is to absorb it (except in 1. and 2. where it only gives up part of its energy).

By the way, the electron does have a rest mass, but we cannot measure its mass when its spatial speed is 0. The rest mass is calculated in theory, and the experiments all support this data.

You might be thinking about relativistic mass, but that is not used commonly anymore. The reason for that is in relativity, mass is not a conserved quantity.

Here you can read about that more:

Relativistic mass of photon

Bill N
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Árpád Szendrei
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  • Mass is a conserved quantity within a system: $m^2c^4=E^2-p^2c^2$ is Lorentz invariant across reactions. Sum of individual masses is not a conserved quantity. – Bill N Mar 18 '19 at 20:49
  • For higher energy photons, photon absorption can result in an electron-positron pair which didn't exist before.
    • – Bill N Mar 18 '19 at 20:51