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.