Undergrad Diffracted photon/partice momentum

[negative]

when a single photon is supposed to form a diffraction pattern, they hit the detector by different angles at the slit.

so then what cancels this photon's momentum change? what happens to the photon's energy/frequency?what measurements has been done to confirm the answer?

 

[Paul Colby]

I think your question is based on a fallacy. Photons, single or otherwise, pass through both slits. By "pass" I mean the EM (quantum) field exists in both apertures in the case described.

 

[PeterDonis]

what cancels this photon's momentum change?

What momentum change? The photons aren't classical billiard balls following classical trajectories. They are quantum objects. If you don't measure their momentum, there is no "momentum change". (And if you do measure the momentum, you know which slit each photon goes through, so the interference pattern disappears.)

 

[jtbell]

The total momentum of photon + slit-apparatus is conserved.

(provided no external forces (interactions) act on the slit-apparatus, in which case you either take those forces into account, or enlarge the definition of "slit-apparatus" to include the "source" of those forces)

This is just a macroscopic version of a scattering experiment with a photon and some other particle, e.g. Compton scattering, in which the target electron recoils in such a way as to conserve energy and momentum.

 

[negative]

I think your question is based on a fallacy. Photons, single or otherwise, pass through both slits. By "pass" I mean the EM (quantum) field exists in both apertures in the case described.

a double slit and single slit experiments are different. still this question applies to both.

What momentum change? The photons aren't classical billiard balls following classical trajectories. They are quantum objects. If you don't measure their momentum, there is no "momentum change". (And if you do measure the momentum, you know which slit each photon goes through, so the interference pattern disappears.)

in the case of double slit, and single slit experiments, photons, and electrons have momentum, in the direction of their movement. direction of movement changes. any change in the direction is supposed to result in a change of momentum.

The total momentum of photon + slit-apparatus is conserved.

(provided no external forces (interactions) act on the slit-apparatus, in which case you either take those forces into account, or enlarge the definition of "slit-apparatus" to include the "source" of those forces)

This is just a macroscopic version of a scattering experiment with a photon and some other particle, e.g. Compton scattering, in which the target electron recoils in such a way as to conserve energy and momentum.

in a macroscopic point of view, if you hit something with a hammer, even if it doesn't move, it heats up. sudden small forces transfer energy. a photon is supposed to either split or be affected by a force of unknown nature ( gravitational forces for example ) to have it's trajectory bent.

 

[PeterDonis]

in the case of double slit, and single slit experiments, photons, and electrons have momentum, in the direction of their movement. direction of movement changes. any change in the direction is supposed to result in a change of momentum.

No. You're still thinking of the photons as little billiard balls. They're not. They don't even have definite trajectories in the experiment, so it's meaningless to say that they have a "change of direction". All you can say is that they go through both slits and end up at the detector.

 

[negative]

No. You're still thinking of the photons as little billiard balls. They're not. They don't even have definite trajectories in the experiment, so it's meaningless to say that they have a "change of direction". All you can say is that they go through both slits and end up at the detector.

you can, because you can measure the time which takes for them to get to the detector. assuming we add the timer to the experiment...

 

[Paul Colby]

a double slit and single slit experiments are different. still this question applies to both.

yes, and it is good to ask how they differ. The EM field is a quantum mechanical system. At energies of a few ev with slits and such, this system becomes well modeled by one with distinct boundary conditions on the fields. The experiment, whether is has one slit or two, differ in these impose boundary conditions on the EM field. The question you raise assumes that one may measure the location and momentum of a quanta of energy exchanged with the detection device. These details need to be fleshed out and can't simply be left hanging. For example, I could use a lens system to collimate light from one side of the aperture and not the other. A photon collected would then have a momentum in the direction of the acceptance cone of the collimator. However, this collimator, just by being there, will eliminate the portion of the aperture effectively removing it from the interaction. The collimator changes the EM field or system being observed.

[Edit] Incidentally, this type of thought experiment was a very famous component in the Bohr-Einstein debates. Hint: Einstein lost.

 

[Drakkith]

you can, because you can measure the time which takes for them to get to the detector. assuming we add the timer to the experiment...

How? A photon is destroyed upon measurement, so how can you measure a single photon twice to get a time of flight?

 

[negative]

How? A photon is destroyed upon measurement, so how can you measure a single photon twice to get a time of flight?

you can always measure a single photon twice. at the point of emission and the point of it's destruction.
the emission system that you use, can implement a timer, the detector can implement a timer. you know the speed of light. you put a straight line from the emission source to the slit, then to the spot that it hit the detector.
this trajectory is the shortest that passes from the slit. and it (supposedly) fits the time that it took for the photon to reach the target.
any other trajectory either doesn't pass through the slit or means exceeding the speed of light.

 

[PeterDonis]

you can, because you can measure the time which takes for them to get to the detector. assuming we add the timer to the experiment...

If you have a source that tells you when it emits a particle, yes, you can measure the time. But that does not tell you the momentum. It only tells you the time of flight. You still don't know, because you're not measuring, the trajectory of the particle (and changing the experiment so you measure the trajectory will destroy the interference pattern).

you know the speed of light. you put a straight line from the emission source to the slit, then to the spot that it hit the detector.

Photons do not have to travel in straight lines. Nor do they have to travel at the speed of light. Once more: photons are not little billiard balls. They are quantum objects. They have nonzero amplitudes to do things that are classically forbidden, like travel in non-straight-line trajectories or travel at speeds other than the speed of light.

 

[PeterDonis]

The OP is simply repeating incorrect statements without responding to issues raised, and the original OP question has been answered. Thread closed.