Is a photon emitted in a particular direction?

[Green dwarf]

TL;DR Summary

Does a photon emitted from a lone atom move off in one direction or in a superposition of all directions?

Reference: https://www.physicsforums.com/forums/quantum-physics.62/post-thread

If a lone hydrogen atom in space emits a photon, is it emitted in a particular direction or in a superposition of all directions?

 

[DrChinese]

If a lone hydrogen atom in space emits a photon, is it emitted in a particular direction or in a superposition of all directions?

The basic answer is "all directions", although there are always caveats. For instance, such photon is only detected in a single place to the exclusion of all others. Experiments demonstrate this to very high accuracy.

https://arxiv.org/abs/1412.7790
"A single quantum particle can be described by a wavefunction that spreads over arbitrarily large distances, but it is never detected in two (or more) places. This strange phenomenon is explained in quantum theory by what Einstein repudiated as "spooky action at a distance": the instantaneous nonlocal collapse of the wavefunction to wherever the particle is detected."

 

[Green dwarf]

Thank you DrChinese. Your answer makes sense. If, instead of emitting a photon, the electron is knocked out, does the electron travel on all directions too? Would the same be true of the proton? What about a larger atom or a molecule or a speck of dust or a pebble?

 

[phinds]

Thank you DrChinese. Your answer makes sense. If, instead of emitting a photon, the electron is knocked out, does the electron travel on all directions too? Would the same be true of the proton? What about a larger atom or a molecule or a speck of dust or a pebble?

Quantum objects, such as electrons and photons, do not have deterministically defined trajectories as they would if classical physics were applicable. They all have probabilistic positions unless/until detected. This is a basic part of quantum mechanics.

Larger, macro sized objects, technically have the same probabilistic characteristics as quantum objects, BUT ... since they are conglomerations of quantum objects, the probabilistic nature of the individual constituents is tiny compared to the overall size so classical mechanics gives a good description of their position/trajectory.

For example, a pebble is in one place, but an electron in a particular atom of one molecule of the pebble is in a probablity cloud around that atom.

 

[PeroK]

Thank you DrChinese. Your answer makes sense. If, instead of emitting a photon, the electron is knocked out, does the electron travel on all directions too?

Your wording here is too loose. If you have a system where an event like electron emission may take place, then until you measure it, the system is in a superposition of states. This results in the electron being detected in various directions with various probabilities.

Just cannot say that the electron went in all directions.

What about a larger atom or a molecule

The same applies to these.

or a speck of dust or a pebble?

These systems have so many atoms and molecules that they themselves don't exhibit elementary QM behaviour.

A pebble could be ejected randomly by a classical, macroscopic process; but not by an elementary QM process such as nuclear decay.

 

[Green dwarf]

Let's say a uranium atom floating in space emits an alpha particle. Presumably, the alpha particle travels in a superposition of all possible directions until it is detected. (I'm guessing that a 'detection' can be an interaction with another particle, not just an interaction with a conscious observer.) At the moment of detection, it changes from being in a uniform probability distribution in a shell around the uranium atom, to being in a particular place, let's say a light year away from the atom in a direction towards a particular star.

I'm wondering about the alpha particle's effect on the gravitational field, which it seems should be dependent upon the alpha particle's position. Does the gravitational field change instantly right across the light-year-radius sphere around the original position of the uranium atom the moment the alpha particle is detected? Maybe the answer to that question requires a quantum theory of gravity ??

 

[DrClaude]

Gravitational interaction is sufficient to localize the particle.

(Note that quantum gravity is about the quantization of gravity. The effect of classical gravity on quantum systems can be studied within current quantum theory.)

 

[Green dwarf]

Gravitational interaction is sufficient to localize the particle.

(Note that quantum gravity is about the quantization of gravity. The effect of classical gravity on quantum systems can be studied within current quantum theory.)

Thanks DrClaude. Presumably all particles interact constantly with gravity. Would that mean that they are always localised?

 

[DrClaude]

Thanks DrClaude. Presumably all particles interact constantly with gravity. Would that mean that they are always localised?

All interactions lead to some localization, but with a varying degree. For instance, it is not sufficient to give which way information in double slit experiments with electrons. But in you example of emission of an alpha particle, it would indicate in which region the particle was emitted, even if it is not reduced to a precise location.

 

[Green dwarf]

All interactions lead to some localization, but with a varying degree. For instance, it is not sufficient to give which way information in double slit experiments with electrons. But in you example of emission of an alpha particle, it would indicate in which region the particle was emitted, even if it is not reduced to a precise location.

Thank you. That's a new idea to me - some localisation. Going back to my original question about the photon, would the same apply there, so that all directions are not equally probable?