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?
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When a photon interacts with an atom, three things can happen:
elastic scattering, the photon keeps its energy and changes angle
inelastic scattering, the photon gives part of its energy to the atom and changes angle
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:
Bill N
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Árpád Szendrei
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When what is at zero velocity? If you are asking "When the photon is at zero velocity, what is its mass?" then the answer is we have no idea, because its mass is currently not determinable. If you are asking what happens to the object it collides with when the object is at zero velocity, then the answer is the photon pushes it very slightly, and the object is given a velocity from the collision. This has been exploited in several methods of interstellar travel known as photonic propulsion
Alex Ambruso
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