Amateur question about the Double Slit Experiment

[ynotpup]

How did you find PF?: Google

In the double slit experiment (using single electrons/photons fired one at a time), I wonder what pattern would be displayed on the backboard if it was conducted without any observation at all during or subsequent to the event. Perhaps there is nothing on the backboard at all although we can never know or for that matter nobody can ever know

 

[.Scott]

That's a good question - because it can tip you off to the nature of QM.

If you could somehow manage to detect the fact that the screen was struck by a photon or electron, but kept the location of the strike completely unmeasured and unrecorded, then there would be no precise screen location where the strike occurred.

It can get a lot wilder than that. If you witnessed the pattern on the screen, but then you, the screen, and all recordings of the event then dropped past the event horizon of a black hole, could someone outside the black hole claim that a pattern ever existed? This is why there is such interest in the Black Hole paradox. Allowing information to permanently "leak" from the universe would have truly profound implications.

 

[ynotpup]

That's a good question - because it can tip you off to the nature of QM.

If you could somehow manage to detect the fact that the screen was struck by a photon or electron, but kept the location of the strike completely unmeasured and unrecorded, then there would be no precise screen location where the strike occurred.

It can get a lot wilder than that. If you witnessed the pattern on the screen, but then you, the screen, and all recordings of the event then dropped past the event horizon of a black hole, could someone outside the black hole claim that a pattern ever existed? This is why there is such interest in the Black Hole paradox. Allowing information to permanently "leak" from the universe would have truly profound implications.

I mean ...does the electron/photon exist in our reality if it isn't observed, it only ever potentially exists

 

[renormalize]

I mean ...does the electron/photon exist in our reality if it isn't observed, it only ever potentially exists

Can you describe an empirical observation or experiment which would distinguish between an electron that actually "exist(s) in our reality if it isn't observed" versus one that "only ever potentially exists"? If not, you're asking a question about philosophy, not physics.

 

[.Scott]

I mean ...does the electron/photon exist in our reality if it isn't observed, it only ever potentially exists

As part of your experiment, you were firing off photons or electrons one at a time.
For this question, we don't really need the double slit. We can just fire off photon or electrons at a screen.
If one of those particles was not fired off, it is not part of the experiment. So, within the context of this experiment, "all" of the particles exist within our universe. And during the experiment, there will be no loss of electric charge or mass/energy - they aren't going to leave our universe.

So, exactly what happens to them? Well, "exactly" nothing because our universe doesn't support "exactly".

To make things simple, we will presume that all of the particles reach the target screen and are detected and reported. Anything less than that gets complicated because you need to get into things like "What is a measurement?".
But instead of a predictable trajectory, each particle will evolve under the constraints of the Heisenberg Uncertainty Principle. And this constrains the forms of information we can collect from the screen. If we decide we want to know the very precise screen location where the particle landed, we will know very little about when or how forcefully it landed - and vice versa.

It's not that the information is unknown or was lost to another universe, it's just that the gun never fired it off with what one might consider "complete" information.

 

[Matterwave]

How did you find PF?: Google

In the double slit experiment (using single electrons/photons fired one at a time), I wonder what pattern would be displayed on the backboard if it was conducted without any observation at all during or subsequent to the event. Perhaps there is nothing on the backboard at all although we can never know or for that matter nobody can ever know

"Observation" is a rather loose term. Within the standard Copenhagen interpretation, it is not given much more sharpness than that it corresponds to a self adjoint operator. This is just a mathematical definition. What *physically* counts as an observation is only given vague descriptions like that it corresponds to a classical event, or that it's macroscopic, (above the so-called "Heisenberg Cut", whatever that means).

Questions of the type "what happens if we don't observe it?". Get quickly down the rabbit hole of QM interpretation.

All I think I can say is, if you count the screen as a position detector and the screen as making many position measurements, then according to Copenhagen, what you would expect is a series of dots would be on the screen. If you do not count it as a detection event then you would only say that the entire experimental apparatus is now described by a single wave function (or state vector).

One other thing you might say would be that Copenhagen makes no predictions for experiments where "you do not observe it" since the entire framework is based around "what we see when we observe/measure things".

Other interpretations might say something different but it would get us down the path of QM interpretation discussions.

 

[PeterDonis]

without any observation at all during or subsequent to the event.

I'm not sure what you mean here. If what you are calling the "backboard" is present at all, whatever is emitted from the source will end up hitting it. What kind of experimental setup are you imagining?

 

[Roberto Pavani]

In the double slit experiment (using single electrons/photons fired one at a time), I wonder what pattern would be displayed on the backboard

The experiment already assumes a source, a double slit and a screen. If particles reach the screen, they alter its physical state. Whether a human later looks at the result is a separate question.

AFAIK, quantum mechanics predicts an interference pattern whenever no which-path information is available. A conscious observer is not required.

If the screen later fades, is destroyed, or even falls into a black hole, that does not change the prediction that an interference pattern was recorded on the screen while the experiment took place.

 

[Liam_]

How did you find PF?: Google

In the double slit experiment (using single electrons/photons fired one at a time), I wonder what pattern would be displayed on the backboard if it was conducted without any observation at all during or subsequent to the event. Perhaps there is nothing on the backboard at all although we can never know or for that matter nobody can ever know

This comes from a misapprehension of the meaning of “observation”. Whether you look at it or not is irrelevant because that is not what it means.
In quantum mechanics, an observation is not a conscious person looking at the experiment. It is a physical interaction that transfers information into another system. In a typical double slit experiment, that happens when the particle strikes the detection screen. The particle interacts with the atoms in the screen, creating a record of its arrival. Information about that event spreads into the surrounding environment, a process known as decoherence.
If there are no which-path detectors and coherence is maintained until the particle reaches the screen, then firing particles one at a time will still build up the interference pattern, even though nobody has looked at it.
Of course theres a lot of nuance in the background depending on whether you subscribe to the Copenhagen, MWI or another camp, but that’s mostly irrelevant to your question

 

[Matterwave]

This comes from a misapprehension of the meaning of “observation”. Whether you look at it or not is irrelevant because that is not what it means.

The apprehension comes because the word is indeed ambiguous, and there has been a sea of literature and thought experiments which show the ambiguity of the word -- Schroedinger's cat is probably the most famous example.

Even if we take your definition "a physical interaction that transfers information into another system" -- this can mean 2 particles entangling. I don't think Copenhagen would say two particles entangling leads to collapsed states of both particles via the Born rule.

 

[DEvens]

In the double slit experiment (using single electrons/photons fired one at a time), I wonder what pattern would be displayed on the backboard if it was conducted without any observation at all during or subsequent to the event. Perhaps there is nothing on the backboard at all although we can never know or for that matter nobody can ever know

I wonder what you mean by "without any observation." What changes to the standard situation are you contemplating to have a situation "without any observation?"