On the Quantum Nature of EM Waves

[Quarker]

In a double-slit experiment, an EM wave is said to travel all paths to the screen, even though only one path will appear at measurement. Do any of those paths include the possibility of the EM wave traveling backwards in time? Or being detected in a place that would imply faster than light travel? Or does “all” paths mean only those paths allowed by physical laws?

 

[PeterDonis]

In a double-slit experiment, an EM wave is said to travel all paths to the screen

No. If you're using QM--and really the path integral version of QM--in this way, you're not talking about EM waves any more. You're talking about quantum objects. An "EM wave" is a classical concept.

even though only one path will appear at measurement.

No, this is not correct. If you are running the experiment with a normal light source of normal intensity, an interference pattern will appear on the detector screen. That pattern is produced by many "paths".

If you run the experiment with the light source at sufficiently low intensity, you will see individual dots appear on the screen, one at a time, and over time, the impact points of those dots will build up the interference pattern. But any individual dot on the screen tells you nothing about any "path". All it tells you is the location of the dot on the screen.

Do any of those paths include the possibility of the EM wave traveling backwards in time?

So far we've been talking about non-relativistic QM, where there is no such thing. For the double slit experiment, this works fine--you can just look at the spatial paths from the source to the detector and assume that everything moves forward in time.

You could analyze the experiment using QFT, in which, at least on one viewpoint, there are nonzero amplitudes for paths (in spacetime, not space) that are spacelike. But even QFT does not predict that you'll ever see, for example, a dot on the screen at a time that would imply that something traveled faster than light from the light source to the screen.

Or does “all” paths mean only those paths allowed by physical laws?

It depends on what particular approximation to "physical laws" you're using. See above.

 

[Quarker]

So in the right conditions, quantum forces can create interference patterns with one another even though they exist at different times?

 

[PeterDonis]

So in the right conditions, quantum forces can create interference patterns with one another even though they exist at different times?

What do you mean by "quantum forces"?

 

[PeterDonis]

can create interference patterns with one another even though they exist at different times?

In the case where you're running the experiment one quantum at a time (because the source is at sufficiently low intensity), each individual quantum "interferes with itself"--the interference pattern is there in each individual quantum's wave function at the detector screen, and determines the probabilities for where it hits.

 

[Quarker]

In the case where you're running the experiment one quantum at a time (because the source is at sufficiently low intensity), each individual quantum "interferes with itself"--the interference pattern is there in each individual quantum's wave function at the detector screen, and determines the probabilities for where it hits.

It sounds like there can be an interference pattern without any interference actually taking place, just a wave function that for some odd reason makes it appear as if there is.

 

[Doc Al]

It sounds like there can be an interference pattern without any interference actually taking place, just a wave function that for some odd reason makes it appear as if there is.

Not sure why you think that. The wave function shows the interference -- it interferes with itself, as mentioned above.

 

[FactChecker]

It sounds like there can be an interference pattern without any interference actually taking place, just a wave function that for some odd reason makes it appear as if there is.

It might be better to say that parts of the wave, each part coming through an associated slit and spreading as a simple sphere, interfere with each other to create the total wave. The combined total wave is the only thing observable in an experiment result.

ADDED: The experimental results close to any one slit would be dominated by the simple wave from that slit.

 

[Quarker]

When conducting a double-slit experiment, is there any pattern that forms on the back of the screen containing the slits?

 

[PeterDonis]

When conducting a double-slit experiment, is there any pattern that forms on the back of the screen containing the slits?

That screen is not set up to record any such patterns, so experimentally the answer is that we don't even try to observe whether this happens or not.

Theoretically, I think the prediction would be that the probability of observing anything in the location you describe, even if it were set up to record such observations, is negligible.

 

[Quarker]

Theoretically, there shouldn’t be an interference pattern on the back of the screen containing the two slits, correct? Or would any such interference pattern be too dim to detect?

 

[PeterDonis]

Theoretically, there shouldn’t be an interference pattern on the back of the screen containing the two slits, correct? Or would any such interference pattern be too dim to detect?

See my post #10.

 

[bhobba]

It's conceptually easier if you think in terms of Quantum Field Theory (QFT)

An electron is literally a disturbance in an electron field that permeates all of space. There is no wave-particle duality, just disturbances in quantum fields. Particles, strictly speaking, do not exist - they are like knots in the field when thought of as a localised particle. That disturbance can spread out like a wave, go through the slits (or be 'collapsed' by the screen if it doesn't go through the slits), and interfere with itself. It will then be collapsed at a point on the screen behind.

Thats the intuition in terms of QFT. If you want something deeper at the undergrad level, there is a paper by Marcella:

https://arxiv.org/abs/quant-ph/0703126

Thanks
Bill

 

[Quarker]

That’s why I posted on this forum. Classically, the back of the screen should contain nothing of interest. But at the quantum level, an interference pattern on the back of the screen would be as if a negative energy was emerging from the areas of destructive interference of the classical interference pattern. It would appear as if light was traveling backwards in time. This sounds similar to the way the positron was originally predicted to be a negative energy. Except that a backward facing interference pattern would imply something more significant than a new antiparticle.

 

[PeterDonis]

at the quantum level, an interference pattern on the back of the screen

Which does not actually occur. But...

would be as if a negative energy was emerging from the areas of destructive interference of the classical interference pattern. It would appear as if light was traveling backwards in time. This sounds similar to the way the positron was originally predicted to be a negative energy. Except that a backward facing interference pattern would imply something more significant than a new antiparticle.

Where are you getting all of this from? Please note that personal speculation is off limits here.

 

[Michael Caputo]

"Where are you getting all of this from? Please note that personal speculation is off limits here."

But one must assume hypotheses come from somewhere; so are you curious enough to set up the experiment and then draw conclusions for the illustrious to gnaw at and potentially call you 'fool'? The question is valid if no one has looked at it. Wouldn't PeterDonis be surprised if you proved dark forces were afoot?

 

[Mark44]

But one must assume hypotheses come from somewhere; so are you curious enough to set up the experiment and then draw conclusions for the illustrious to gnaw at and potentially call you 'fool'? The question is valid if no one has looked at it.

That's fine and dandy, but Physics Forums is not the venue for presenting these hypotheses. Given that you are a brand-new member, perhaps you haven't read the forum rules that you agreed to abide by when you joined. From these rules (https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/):

We wish to discuss mainstream science. That means only topics that can be found in textbooks or that have been published in reputable journals.

 

[Nugatory]

But one must assume hypotheses come from somewhere; so are you curious enough to set up the experiment and then draw conclusions for the illustrious to gnaw at and potentially call you 'fool'? The question is valid if no one has looked at it.

The problem with this line of thinking is that the supply of unfounded hypotheses is infinite while resources available for experimentation are not. Thus, there is some level of judgment involved in choosing which experiments are likely to yield useful results: for example we will not find anyone doing experiments to test the hypothesis that the value of $G$ in $F_g=Gm_1m_2/r^2$ is different when one of the masses is a chunk of limestone instead of shale.

The hypothesis starting around post #14 of this thread seems (with no math it's not clear that it is a testable hypothesis) to be that there is some sort of "negative energy" effect that would appear behind the screen but not in any of the other countless experiments in quantum optics over the years. Sure, it's possible, and maybe a bit less far-fetched than the limestone/shale example... but that possibility is still far too speculative for a forum whose purpose is helping people understand what our current theories (as described by @bhobba and @PeterDonis above) say.

We can leave this subthread up, but further discussion of the PF policy here should be in a new thread in the forum feedback section - and please read some of the earlier discussion there, there's a lot of past history behind the policy,

 

[Paul Colby]

When conducting a double-slit experiment, is there any pattern that forms on the back of the screen containing the slits?

In classical diffraction theory (solving the classical em problem for a screen with slits) there are non-zero field present on the back side of the conducting screen. For points on the backside of the screen these are greatest near the slits and decay exponentially as one moves away from the slits on the backside.

 

[Michael Caputo]

Point.
I do not mean to rock the boat by my previous post. The inquiry by Quarker is/was legitimate. He just needed (apparently) Mr Colby to drop by with the answer. I simply encouraged Quarker to find out for himself if he is truly curious.

 

[Michael Caputo]

That's fine and dandy, but Physics Forums is not the venue for presenting these hypotheses. Given that you are a brand-new member, perhaps you haven't read the forum rules that you agreed to abide by when you joined. From these rules (https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/):

Yes, I read them. May I quote?
"Please treat all members with respect, even if you do not agree with them."
"Be civil, and productive and aim for high-quality participation."
"Our mission is to provide a place for people (whether students, professional scientists, or others interested in science) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community."

I was encouraging Quarker to investigate something of interest to Quarker.
I suppose I could have spent more time as a lurked before registering.
If I was truly out of line, I apologize.

 

[berkeman]

Yes, I read them. May I quote?
"Please treat all members with respect, even if you do not agree with them."
"Be civil, and productive and aim for high-quality participation."
"Our mission is to provide a place for people (whether students, professional scientists, or others interested in science) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community."

I was encouraging Quarker to investigate something of interest to Quarker.
I suppose I could have spent more time as a lurked before registering.
If I was truly out of line, I apologize.

This is an unfortunate response in two ways. First, you ignored my DM on how to use the PF Reply/Quote feature effectively. Why did you do that? Please re-read my DM about how to use the Reply/Quote feature here.

Second, you selected non-applicable parts of the PF Rules. Try this part instead:

Speculative or Personal Theories:
Physics Forums is not intended as an alternative to the usual professional venues for discussion and review of new ideas, e.g. personal contacts, conferences, and peer review before publication. If you have a new theory or idea, this is not the place to look for feedback on it or to ask for help developing and publishing it.

Soliciting endorsers for publication to arXiv and similar sites is not allowed.

For further explanation of our policy on personal theories and speculative posts and the history behind it, see the following entry in the Physics Forums FAQ:

https://www.physicsforums.com/threads/physics-forums-faq-and-howto.617567/#post-4664231

 

[Michael Caputo]

No sense moving this further off topic.