When and why does an electron emit a photon?

[Daniel Hendriks]

Is this known in QFT or is this "spontaneous".

 

[DaveC426913]

Can you be a little more specific? An electron that's been excited into a higher orbital might emit a photon to drop back to its unexcited state.

 

[Daniel Hendriks]

Can you explain "might"? This is what I want to know.

 

[PeroK]

Is this known in QFT or is this "spontaneous".

A free electron emiting a photon is forbidden by conservation of energy.

 

[malawi_glenn]

Can you explain "might"? This is what I want to know.

There is a matrix element for that transition which you can compute, and then calculate the decay rate (half life if you will). Note that this can not happen for a free electron, as pointed out in the previous post.

 

[Daniel Hendriks]

So it is on luck? Or maybe it is not known I conclude.

 

[malawi_glenn]

So it is on luck? Or maybe it is not known I conclude.

Processes in quantum mechanics are deterministically random. We can compute the probabilities, but we can not predict what will come out in the experiemnt. Compare with radioactivity (have you studied radioactivity in school?)

 

[Drakkith]

So it is on luck? Or maybe it is not known I conclude.

We know that the transition will happen, we know the average duration before many of these transitions will occur, we know how much energy will be released, and we know various other facts about the transitions, but if you're asking an underlying 'why' you may be disappointed. I could say that the transition occurs because it is energetically favorable, or that it occurs because the laws of nature allow it, and I'm sure there are various ways of explaining it using quantum mechanical and mathematical details involving fields and charges. Beyond that, you start running into an ever deeper series of 'why' questions that can't be answered and, even if they could, would just result in another 'why' question.

 

[Daniel Hendriks]

Thanks. What does energetically favorable mean? What is wrong with saying we only know the average and amount of energy, but we don't have a clue what is happening? That's the first thing someone learning this stuff needs to know.

 

[PeroK]

Thanks. What does energetically favorable mean? What is wrong with saying we only know the average and amount of energy, but we don't have a clue what is happening? That's the first thing someone learning this stuff needs to know.

The first thing someone learning this stuff needs to know is the Schrödinger equation.

 

[Daniel Hendriks]

The Schrödinger equation doesn't describe photons. You need to understand before you start learning something, what you are going to learn and what the limitations are. Thats the whole problem with schools, you start but nobody even the teacher knows anything. That's why the system is failing.

 

[vanhees71]

Indeed, unfortunately to understand spontaneous emission, i.e., the transition of an electron in an excited state of an atom to a lower state under emission of a photon, is the most simple example for the necessity to quantize the electromagnetic field in addition to quantizing the description of the electron. The "reason" for the spontaneous emission is that the electron is not only bound to the atomic nucleus via the electromagnetic (Coulomb) interaction (which can be described in the semi-classical approximation with the em. field kept as a classical unquantized field) but also to the quantized em. radiation field. Since the electric and the magnetic field components cannot be determined at once for any state (that's a variety of Heisenberg's uncertain relation) the em. field is fluctuating. Even in the vacuum state of the em. field, i.e., the state, where no photons (radiation) is present, there are fluctuations of the em. field which interact with the excited electron, which triggers it to "spontaneously" go to a lower energy eigenstate and emit a photon. The fluctuations are inherently random, i.e., you cannot predict, when the transition will occur but you can give only probabilities for the rate such a transition occurs.

 

[Drakkith]

Thanks. What does energetically favorable mean?

Energetically favorable means that there is available energy that can be 'liberated' or 'moved' by the electron transitioning between a higher and lower energy level. In contrast, the transition of an electron from its ground state to a higher energy state isn't energetically favorable, as it would require that something add energy to the atom for this transition to happen.

What is wrong with saying we only know the average and amount of energy, but we don't have a clue what is happening?

But we do have a clue as to what is happening, unless you subscribe to the belief that, since physics can't know absolute truths, we can't say we know anything about the universe. Which I would disagree with.

The Schrödinger equation doesn't describe photons.

But it's a first step at understanding the math and models that we use to describe atoms and other quantum-scale systems, including photons.

You need to understand before you start learning something, what you are going to learn and what the limitations are. Thats the whole problem with schools, you start but nobody even the teacher knows anything. That's why the system is failing.

The problems of the school system is an extremely complex topic, with many major causes, but I seriously doubt that the failure to teach the limitations of knowledge is a significant factor.

 

[malawi_glenn]

Thats the whole problem with schools, you start but nobody even the teacher knows anything.

Are you suggesting that all teachers does not know anything? I mean they don't even know what date and time it is?
That is the most preposterous thing I have read in quite a while.

What has that anything to do with the topic of photon emission from energy transitions of electrons in bound states? It just came up as a pretty random quote.

 

[berkeman]

The Schrödinger equation doesn't describe photons. You need to understand before you start learning something, what you are going to learn and what the limitations are. Thats the whole problem with schools, you start but nobody even the teacher knows anything. That's why the system is failing.

Thread is closed for Moderation...

 

[berkeman]

The Schrödinger equation doesn't describe photons. You need to understand before you start learning something, what you are going to learn and what the limitations are. Thats the whole problem with schools, you start but nobody even the teacher knows anything. That's why the system is failing.

This is nonsense. Please check your PMs.

Thread will remain closed.