Can a free electron absorb a photon?

We can show that a free electron cannot absorb a light photon completely, by looking at the Conservation laws of Energy and Linear momentum in such a process. As it can be easily shown that such a process of a free electron absorbing a photon does not conserve energy and momentum simultaneously. Thus we can conclude that such a process is not possible.

However, partial absorption is possible (for e.g. Compton effect)
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[FortheLoveofPhysics]

1. Why pair production cannot occur in vacuum?
2. What is a Black Hole? Understanding the first ever image of a Black Hole

[rashisharma]

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[TerryBollinger]

While trying to give an example of how mass and momentum add under special relativity, I was baffled when my casually selected example of a Lorentz factor 2 electron ended up with a different relativistic momentum than an electron at rest absorbing a gamma photon of equal mass-energy. It was off by a factor of square root of three! Your excellent video that confirmed a suspicion that had only flitted through my mind: Despite QED consisting of nothing but electrons absorbing photons, those cases do not represent a real, fully conservative process at classical spacetime scales. For anyone interested in a deeper dive, in that same search I also encountered a lovely brief 4D mass-shell explanation of the imbalance by Michael Seifert. That one is findable by searching for the following quoted phrase, including the misspelling of satisfies: "The four-momentum of any real particle satisifies the relationship"

[sheroukel-sherbiny]

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Greetings from Egypt.

[nihaldesiscenes]

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[Snagabott]

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[jehovalchangsanchangsan]

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[eugedrimer]

Excelente! Un saludo desde Argentina :)

[PrivateSi]

As should be the case. Some incoming electrostatic field blips that make up the photon will bounce off the electrons outgoing field blips, some will be in sync and be absorbed.. A photon is a transverse wave pattern of longitudinal subspace field cell blips (moves half a cell with and back, sending out lateral vibes squeezing through the field while transferring its energy to the cell beyond, before returning)..
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Having the entire universe field blipping at light speed, with force changing direction and phase helps with conservation of photonic blip momentum in this solid space, but momentum can be conserved without this, simply via the rest of the universe trying to rebalance, pushing the excess energy = subspace charge imbalance forward so it can rebalance behind...
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It's a self-balancing system of +ve charge cells held together by -ve gas/space depending on your preference.. I go with -ve gas as it's more super-symmetrical with the emergent universe (protons and electrons in a dense plasma are like the subspace field, but larger).... Gas can flow, truly empty space can't do anything but suck...!

[andrewburbidge]

I hope it will be accepted that my comment is for the love of Physics although not dealing precisely with the question. The use of equations for photon and electron are relevant to the question regarding a transmission medium. I followed the treatment and pointed out something of interest about it:

Considering what these equations may mean regarding any such system as a transmission medium;
the first two equations seem to be based on the assumption that the energy of a photon is mc^2 as though related to the idea of rest mass although that isn't stated, seemingly by convention because no rest mass can be observed for a photon;
it may seem reasonable, in that a photon can be produced by some sort of acceleration to light speed away from the region of influence of an electron;
the energy is taken to be hν, with the assumption that energy is associated with a constant and with frequency of some kind of action;
then, the momentum of a photon is taken to be mc, equal to mc^2/c
and so can be written as hν/c

for the electron, the treatment for momentum seems to be the same
this gives a wave equation for the electron,
p = hν/c = h/λ

What philosophical statements can be made regarding photons and electrons in consideration of possible observation of a frame of reference? One would like to make a statement that can be useful in understanding Physics and which cannot easily be disproved, so that it is worth considering and can be taken further.

Because photons and electrons seem to be dealt with in the same way, if one considers a line of electrons of a structure being representative of its length, such as in an interferometer and also a pair of mirrors attached to such an interferometer, with laser light reflecting between them so as to form an interference pattern, what might one expect to observe when such an apparatus is moved at variable speeds parallel to a line defined by the line of electrons?

Would the apparent similar wave nature of electrons and photons mean that it might be very difficult to find evidence of a transmission medium from movement on that line, or even on any straight line? It is said that virtual photons mediate between particles in the electromagnetic interactions that define structures.

Would it be better, rather than to say that experiments were not able to find evidence of a transmission medium so there isn't one, to say instead that there might be one that is difficult to find?
Are the particles that are produced in collision experiments produced from a transmission medium?

Searching for more on the proposition that a photon has momentum, I found that there seems not to be complete agreement.
Photons have angular momentum, gained from the spin or orbital change of an electron. So, it seems the photon's wave nature would be closely related to that of the electron. The Japanese IKAROS space probe used a solar sail, proving that momentum change can occur under the influence of photons, without there being a proof of how that must have happened.

Should one assume that a photon has no linear momentum and that a free electron cannot absorb a photon because the electron's region of influence is too small but that in comparison, an electron maintains a large enough standing wave system around an atom for absorption to occur? The large inequality derived in the treatment here wouldn't arise.

If absorption and reflection of photons by a solar sail can occur without an associated momentum transfer, could it be that only sometimes there is resultant momentum transfer, possibly from emission of photons by a photoelectric effect with charge-balancing absorption from a cloud of electrons that accompany the spacecraft?

See Wikipedia: IKAROS
"JAXA scientists stated on 9 July 2010 that the measured thrust force by the solar radiation pressure on IKAROS' 196 m^2 sail is 1.12 millinewtons"

[elizabethreyna]

Professor this is the best and exelent video i ever found. Just a question👉👈. I do not undestand so far at all why is that a bound electron can absorb or take the whole energy of a photon. I do not understand the term recoil😢. What really hapoend inside the atom or the electron to be able to absorb the whole photon. How is that part of the nuclei absorbs 😢😢😢 please hope you can help me 🙏🙏🙏🙏🙏❣️

[mukutkumar]

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[pushpitakaushik]

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[najninmansuri]

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[una_paradoja]

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[vwcanter]

It's worth pointing out that a frequency of zero is identical to a static field. So the limit in which the "nu" goes to zero is indeed the case of a classical charge being accelerated by a static electric field. So it is not impossible at all.

[zzzoldik]

How about in feyman diagram qed said that posible some particle like electron absort and decay photon many time?

[cesars.semp.]

Without formulae, only words. Choose the system of reference where the electron is at rest after the absorption. The only energy is the rest mass of the electron. In this system of reference, you see the scenario before the absorption and count the rest mass of the electron and two more energies, both positive: that of the photon and maybe the kinetical energy of the electron. So energy is not conserved. You don't need to know any formulae, only that these energies are positive or zero. Choose your system of reference right and you will save a lot of calculations. Thanks.

[zakirhussain-jsku]

We observe transfer of energy & momentum from one object to another without any problem. Electron can't absorb a photon because it is an em wave. Atom absorbs photon since space b/w electron & nucleus is made up of em field which accommodates this additional em field by increasing its size.

[LastMinutePhysics]

The key word here is "completely". This explanation does not exclude the possibility that the electron absorbs some fraction of the photon's energy, whereby the new energy of the electron is in terms of the change in frequency of the photon. Interesting nonetheless, but a somewhat misleading title considering that it seems to imply, counterintuitively, that a charged particle would not interact with a propagating electromagnetic field, when in fact it does, just never in the case where the photon is completely absorbed. It is true that the photon must not be "completely" absorbed in order to conserve energy and momentum.