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)