Electron absorbs photon, but what happens to the photon?

[paddyc]

TL;DR

If the electron absorbing the photon causes it to change from waveform to particle. Then what causes the photon to change from waveform to particle when observed ?

If the electron absorbing the photon causes it to change from waveform to particle ? Then what causes the photon to change from waveform to particle when observed ? (That is exposed to another photon).

 

[Nugatory]

TL;DR Summary: If the electron absorbing the photon causes it to change from waveform to particle. Then what causes the photon to change from waveform to particle when observed ?

It doesn’t. The wave-particle duality idea, that things would act like a wave until they’re observed and then start acting like a particle, was abandoned a century ago with the development of the modern theory of quantum mechanics.

If the electron absorbing the photon causes it to change from waveform to particle ? Then what causes the photon to change from waveform to particle when observed ? (That is exposed to another photon).

A photon isn’t what you’re thinking - you’ll hear it described as a “particle of light”, but that phrase is very misleading. What’s actually going on: electromagnetic radiation always propagates as waves. These waves transfer energy and momentum to any matter they interact with (think ocean waves smashing into the shore). However, if we measure what is going on at a sufficiently small scale we find that the energy is always transferred in discrete amounts at a single point; we call that amount of energy a photon and say that “a photon was detected at that point”. So all that’s happening is that one unit of energy that had been in the electromagnetic radiation is now in the electron.

The best way of catching up with the past century is to work through a decent quantum mechanics textbook. However, that’s a lot of work and comes with some fairly daunting math requirements. Although not a substitute for a real textbook, you could give Giancarlo Ghiradi’s layman-friendly book “Sneaking a look at God’s cards” a try.

 

[paddyc]

Not to flog a dead horse, but does your above explanation hold true when the experiment is done with only photon/s not photons and electrons ?

 

[paddyc]

Would you be able to explain the superposition to me ? That is, does the unobserved waveform also have properties of a particle until observed ?

 

[PeroK]

Would you be able to explain the superposition to me ? That is, does the unobserved waveform also have properties of a particle until observed ?

Superposition is the characteristic of all vectors that a single vector can be expressed as the sum of two or more other vectors.

The force of gravity, for example, is a vector directed vertically downwards. But, if we study the motion of an object sliding on an inclined plane, then we can decompose gravity into a normal force and a force tangential to the plane. In other words, a vertical force is a superposition of normal and tangential forces.

Superposition of physical states is at the heart of quantum mechanics.

A photon is a specific state of the quantum electromagnetic field. It's neither a classical wave nor a classical particle.

 

[PeroK]

Also, an electron cannot absorb a photon. It would violate energy-momentum conservation. An atom, however, can absorb a photon. The absorption represents a transfer of energy from the electromagnetic field to the atom. A photon in this sense represents the quantum of energy thus transferred.

 

[bhobba]

Would you be able to explain the superposition to me ? That is, does the unobserved waveform also have properties of a particle until observed ?

One issue here is what a photon is, is much more subtle than given in introductory texts and even some intermediate-level ones.

I have given the link before, but here is what a photon is:

https://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

A photon is an excitation of the EM quantum field that permeates all of space. The field can, for example, interact with an atom (also called a perturbation), and the excitation is destroyed, but an electron is in a higher energy state. The frequency such can occur at can be calculated using Fermi's Golden Rule:

https://quantum.lassp.cornell.edu/lecture/fermis_golden_rule

Thanks
Bill

 

[Demystifier]

"Electron absorbs photon, but what happens to the photon?"

It gets destroyed, after the absorption the photon does not longer exist.

 

[sophiecentaur]

"Electron absorbs photon, but what happens to the photon?"

It gets destroyed, after the absorption the photon does not longer exist.

That's one way of perhaps making this stuff easier but ,imo, that is still too 'mechanistic' a description. The photon, as an entity, is much more subtle than that. When EM energy is transferred from a source to a 'detector' the photon is just a way of describing what happens and, in the delay between source and detector, we have to use the photon as an easy way to think of what's going on. Between the two events, the energy could have arrived somewhere entirely different because the photon really doesn't exist anywhere or at any time until the event actually occurred. Where it arrives is random and is determined by the statistics of the situation. i.e. it "arrives(/d)" at the most likely place. I heard a recorded talk by Feinman which was pretty much along those lines.

It amazes me to think that this sort of explanation was arrived at long before I was born yet ideas like 'wave particle duality' are still bandied about these days. I guess the more up to date explanations are even less approachable. Popular Science will eventually catch up, but maybe after a new. better model turns up.

 

[hokhani]

Also, an electron cannot absorb a photon. It would violate energy-momentum conservation. An atom, however, can absorb a photon. The absorption represents a transfer of energy from the electromagnetic field to the atom. A photon in this sense represents the quantum of energy thus transferred.

Does photon have wave function?

 

[PeroK]

Does photon have wave function?

In QFT (Quantum Field Theory) photons are described by the quantized EM field. Or, perhaps it's better to say that the Quantum Field defines the number and state of the photons.

A photon can't be described by a wavefunction satisfying the Schrodinger equation - as that equation applies to a particle with mass. However, you can generalise the idea of a wavefunction to describe the behaviour of a photon - even if this is not precisely what QFT does.

 

[Demystifier]

Does photon have wave function?

Yes, but its absolute value squared is not exactly the probability density of photon position.

 

[hokhani]

Yes, but its absolute value squared is not exactly the probability density of photon position.

Is this wave function calculated by Hamiltonian?

 

[Demystifier]

Is this wave function calculated by Hamiltonian?

The QED Hamiltonian determines the time-dependence of the state $|\psi(t)\rangle$. But once you know $|\psi(t)\rangle$, you don't longer need the Hamiltonian to determine the wave function $\psi({\bf x},t)$ from the one-photon state $|\psi(t)\rangle$.

 

[sophiecentaur]

One issue here is what a photon is,

Absolutely - but the
is thread is 'Highschool' level and this level of discussion could confuse someone with Highschool Maths and Physics. Lets face it- it confuses many with degree level qualifications although they may not all realise it. (Mea culpa, at times)

 

[vladvic]

photon is not a particle per se, it's just a minimal fraction of electromagnetic energy that can be transferred to/from the particle, thus this is a quantum of the electromagnetic field energy that we can observe

 

[PeroK]

photon is not a particle per se, it's just a minimal fraction of electromagnetic energy that can be transferred to/from the particle, thus this is a quantum of the electromagnetic field that we can observe

The photon is a gauge boson, which is a particle is standard terminology:

https://en.wikipedia.org/wiki/Gauge_boson

 

[vladvic]

The photon is a gauge boson, which is a particle is standard terminology:

https://en.wikipedia.org/wiki/Gauge_boson

Yes, that is true, however standard model is a kind of abstraction that helps with calculations, in this sense it is definitely a particle. But ontologicaly photon is a quantum, i. e. minimal observable fraction of something (in case of the photon - EM energy). This doesn't though mean that it is some sort of a solid thing that can be used in this kind of speculations, this is a fraction of electromagnetic energy.
In other words, this is not a "thing", this is a measure.

 

[PeroK]

Yes, that is true, however standard model is a kind of abstraction that helps with calculations, in this sense it is definitely a particle. But ontologicaly photon is a quantum, i. e. minimal observable fraction of something (in case of the photon - EM energy). This doesn't though mean that it is some sort of a solid thing that can be used in this kind of speculations, this is a fraction of electromagnetic energy.
In other words, this is not a "thing", this is a measure.

The photon is a spin-one particle. It is not solely a quantum of energy. It carries a quantum of energy. Kinetic energy, per se, cannot be polarised. Kinetic energy and polarisation are properties of a photon. Which is a particle, by definition of what a particle is.

 

[vladvic]

The photon is a spin-one particle. It is not solely a quantum of energy. It carries a quantum of energy. Kinetic energy, per se, cannot be polarised. Kinetic energy and polarisation are properties of a photon. Which is a particle, by definition of what a particle is.

I am not to discuss terminology, I do agree that photon is a particle in the standard terminology. I am just trying to stress the fact, that ontologicaly it is a measure.
Let's assume, there is an EM wave which energy is less than a photon. It has polarization and kinetic energy. Its energy just is not enough to make it observable, since it is unable to change quantum state of any other particle. It would have energy and polarization, but it would not be a particle. What is the difference? The amount of energy it carries.

 

[PeroK]

I am not to discuss terminology, I do agree that photon is a particle in the standard terminology. I am just trying to stress the fact, that ontologicaly it is a measure.
Let's assume, there is an EM wave which energy is less than a photon. It has polarization and kinetic energy. Its energy just is not enough to make it observable, since it is unable to change quantum state of any other particle. It would have energy and polarization, but it would not be a particle. What is the difference? The amount of energy it carries.

There is no minimum possible energy. Energy, moreover, is frame dependent. A photon has no intrinsic, absolute energy. The measured energy depends on the four-velocity of the observer. No photon has too little energy to be measured. Only relative to the observer and their measurement apparatus.

Your posts are based on some fundamental misunderstandings.

 

[vladvic]

There is no minimum possible energy. Energy, moreover, is frame dependent. A photon has no intrinsic, absolute energy. The measured energy depends on the four-velocity of the observer. No photon has too little energy to be measured. Only relative to the observer and their measurement apparatus.

Your posts are based on some fundamental misunderstandings.

Again, I am not questioning the standard interpretation.
I am fully aware of the fact that the energy depends on the observer.
I am talking about the ontological sense of what a photon is, it is a measure of the energy/state exchange between particles, which in some sense is a definition of a gauge boson, and why they are "gauge".

 

[gentzen]

The photon is a spin-one particle.

I am not to discuss terminology, I do agree that photon is a particle in the standard terminology

I am talking about the ontological sense of what a photon is

I guess you both are making the third step before the first. If you are in a high energy particle physics context and study an effect like Compton scattering, then Lorentz covariance is relevant (and useful) and the photon behaves as a spin-one particle.

But if you are in a quantum chemistry or condensed matter physics context, then the Coulomb gauge

The Coulomb gauge (also known as the transverse gauge or the radiation gauge) is used in quantum chemistry and condensed matter physics and is defined by the gauge condition (more precisely, gauge fixing condition) $$\nabla\cdot{\bf A}({\bf r}, t) = 0.$$ It is particularly useful for "semi-classical" calculations in quantum mechanics, in which the vector potential is quantized but the Coulomb interaction is not.

is the way to go, and the relevant interactions are with the quantized photon field.

For example, if you have an isolated molecule in an excited state, you could approximate/compute stimulated emission and absorption by looking at interactions with given electromagnetic plane-waves of fixed frequency and direction. And you could approximate/compute spontaneous emission by representing the quantized photon field as a superposition of energy eigenstates for given periodic boundary conditions (and let the periodic pitch go to infinity).

Both the Coulomb gauge and the periodic boundary conditions need a given intertial reference frame, which is just taken as the rest frame of the molecule.

And this must be the first step, namely to describe how photons behave in your context. The second step might be to investigate how photons behave in different context, see whether you can find a unified perspective, and maybe also look at the drawbacks of such a unified perspective, compared to just accept different contexts. Only as a third step, it makes sense to try to investigate possible "underlying ontologies".

 

[gentzen]

One issue here is what a photon is, is much more subtle than given in introductory texts and even some intermediate-level ones.

I have given the link before, but here is what a photon is:

https://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

https://web.archive.org/web/2025062...u/torre/3700_Spring_2015/What_is_a_photon.pdf

 

[sophiecentaur]

Also, an electron cannot absorb a photon. It would violate energy-momentum conservation.

Yer but no but; A system with an electron in space with or without an electric field can 'absorb' a photon. The notion of a totally isolated electron is pretty meaningless.

 

[PeterDonis]

Thread closed for moderation.