# Double slit and measurement

• B
Science Advisor Gold Member laymanB said
LOL. I don't have such qualifications. I am just a layman, as per my moniker, who wants to accurately understand what modern science says. Thanks for trying to promote me though.

DrChinese
Gold Member
2. If we turn one of the polarizers by 1 degree and repeat the experiment ?
How would the interference pattern change , on the detection screen.

3. If we continued to change the polarize by 1 degree , until the angle between the polarizers were 45 degrees.
How would the pattern then look.

4 Finally if we get to PERPENDICULAR polarizers we would get the 2 distinct ''collapsed" pattern.

My actual question , is , at what point can we say that the 'knowing which way' the photon went through , caused "collapse".
If indeed such a physical phenomenon of ''collapse'' really exists?

I understand that the angle between the polarizers determine the probability outcome of the results , but where would , if any "collapse"
fit into this setup, if any?
Great questions!

Yes, our knowledge of "which slit" can be varied from 0% to 100% simply by rotating the polarizers a little at a time. Thus the visible pattern shifts from interference to no interference. Like many quantum phenomena, these blended setups are a bit counter-intuitive. (For example you can have photon pairs that are 50% entangled, or high but limited certainty for both position and momentum.)

So at what point do we cause collapse? Is there a tipping point? I guess when we have a better physical model of collapse (assuming there is such a thing), we could answer that. In the meantime: there is no specific point at which collapse can be said to occur, but it can vary as I described.

PeterDonis
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2019 Award
At which point does observer A (human or artificial, organic or inorganic, conscious or otherwise, as long as it records relevant evidence) say that observer B (human or artificial, organic or inorganic, conscious or otherwise, as long as it records relevant evidence etc.) has collapsed all relevant wave functions, including her/s own?
Before we can address this question, we first need to figure out whether "collapse" is something that actually happens at all (as opposed to being an artifact of the way humans currently undertstand QM, which is incomplete). Which means we need to come up with some kind of physical model of "collapse" that makes predictions different from the predictions of standard QM, and then run the experiments to see which one is right.

There are some proposed "collapse" models (e.g., the GRW model) that at least move in this direction, but so far, AFAIK, every one that leads to some kind of experimental test is already falsified by our current knowledge (for example, AFAIK all of the versions of the GRW model that are currently testable would, if they were true, have led to different results than were actually observed in lots of QM experiments we've already run). But that doesn't change the requirement (for a model that makes testable predictions different from standard QM). It just means the requirement is very difficult to meet, which means it will probably be quite some time before we can make real progress in this area.

Boing3000
Gold Member
When going beyond this by “smuggling in” classical ideas and conceptions, one begins to produce pseudo-science which is at the end based on personal psychological prédispositions (implicit assumptions).
You don't seem to realize that you are yourself smuggling unscientific misconception into your meta analyse of the meta analyse of QM.
Let's stick to non-pseudo meta science. This is QUANTUM mechanic. All statements about QM are made about classical FACT (not ideas, nor conceptions). Those facts are some "beep" here and there, in so many form, in so many totally classical apparatus. I call them event(s).

Some commenters here on PhysicsForums don’t seem to grasp that their “interpretations” merely reflect their “feelings” of what “relality” “ought to be”.
Actually, I have learned a lot here reading people being able to embrace MANY "interpretations", and use them to expand knowledge and probe hypothesis. Both about new experimental testing, or even simply on the internal coherence of the theory.

In case you need your "ensemble interpretation", that's o.k.. But, please, be aware that's your personal "feeling" of what "ought to be out there".
Your personal feeling seems to be that there "ought to be" no better explanation out there. I found that hypothesis very un-scientific.

You don't throw an "ensemble" into a double slits. There is no such things as an "ensemble" in nature. Your implicit interpretation is the source of the "consciousness" nonsense. The experiment start with the first quanta and ends with the last one. The ensemble is in ones head.
Thinking that all that happens in between is merely non-exciting and should be ignored, and only some probability assignable to ensemble element is existing, is a strong (and even if useful) but too chastising statement.

Lord Jestocost

Your implicit interpretation is the source of the "consciousness" nonsense.
What the heck is now a "classical FACT"? And what is exactly my "implicit interpretation"?

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Boing3000
Gold Member
What the heck is now a "classical FACT"? And what is exactly my "implicit interpretation"?
"Quantum phenomena do not occur in a Hilbert space. They occur in a laboratory"

Your implicit interpretation is that QM can tell something about what a photon will do when send into two slit. QM cannot. Saying that nobody should be interested by this problem is "your" interpretation, called "shut-up and calculate", that posit the extravagance of the existence of some probability space (and nothing else).
But there is a chance I get you wrong...

vanhees71
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2019 Award
I guess, by "classical fact" some philosphers mean simply observed measurement results. Instead of calling them in the straight-forward way they make in attempt to sound more intellectual rather than using clear understandable language ;-)). SCNR.

vanhees71
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2019 Award
"Quantum phenomena do not occur in a Hilbert space. They occur in a laboratory"

Your implicit interpretation is that QM can tell something about what a photon will do when send into two slit. QM cannot. Saying that nobody should be interested by this problem is "your" interpretation, called "shut-up and calculate", that posit the extravagance of the existence of some probability space (and nothing else).
But there is a chance I get you wrong...
QM, or rather QED, precisely tells you what a photon will do when send through a double slit. Hitting the detector/photo plate, it will leave a spot with a probability given by the corresponding probability distribution for hitting the photoplate at the observed position. With a single photon, of course you cannot check, whether the predicted probability distribution is correct. For that you have to create an ensemble of single photons large enough to let you measure the corresponding pattern on the photoplate and check whether it confirms your prediction.

Boing3000
Gold Member
QM, or rather QED, precisely tells you what a photon will do when send through a double slit. Hitting the detector/photo plate, it will leave a spot with a probability given by the corresponding probability distribution for hitting the photoplate at the observed position.
Exactly, and I find quite bizarre that you qualify "hitting the plate (anywhere) and leave a spot", a precise statement.

With a single photon, of course you cannot check, whether the predicted probability distribution is correct.
That's the point, isn't it ?

For that you have to create an ensemble of single photons large enough to let you measure the corresponding pattern on the photoplate and check whether it confirms your prediction.
This is fine, and useful "mind trick". Certainly one that will ultimately be useful to build some fancy macroscopic apparatus, or obtain useful information from a huge number of event.

Now, imagine you are that cat, in that box. You need a theory to help you escape alive. I don't think even you will be satisfied to bet on bets.

vanhees71
Gold Member
2019 Award
It's not a mind trick but an observational fact. QED is among the best confirmed theories of physics ever!

zonde
Gold Member
QM, or rather QED, precisely tells you what a photon will do when send through a double slit.
What a photon does is realm of interpretations. I think you ascribe too much to bare QM and QFT.

vanhees71
Gold Member
2019 Award
I ascribe as much to QM and QFT what these theories predict concerning measurable quantities. In the case of a typical double-slit experiment using single photons it predicts the detection probability at any position on the screen, no more no less. This is the minimal interpretation of the formalism that is consistent with observations.

Lord Jestocost
Your implicit interpretation is that QM can tell something about what a photon will do when send into two slit. QM cannot.
My attitude towards quantum theory is a simple one: Quantum theory cannot answer questions regarding the intrinsic nature of “phenomena” which we think to “observe”.

vanhees71
Gold Member
2019 Award
What do you mean by "intrinsic nature of phenomena"? Imho QT precisely describes what we observe. If this was not the case, there'd be a kind of crises, hopefully leading to a new even better theory. This happened exactly around 1900 when it became clear that classical physics cannot describe what has been observed around this time (black-body spectrum, specific heat of solids at low temperatures, spectral lines of atoms etc. etc.) and that a new theory had to be worked out, leading finally to modern QT around 1925-27 (Heisenberg, Born, Jordan with matrix mechanics, Schrödinger with wave mechanics, Dirac with the general formalism and field quantization for the em. field).

Lord Jestocost
What do you mean by "intrinsic nature of phenomena"?
Saying "quantum theory cannot answer questions regarding the intrinsic nature of 'phenomena'" means nothing else than saying "quantum theory deals only with object-subject relations". You cannot dissect it further on.

Boing3000
Gold Member
It's not a mind trick but an observational fact. QED is among the best confirmed theories of physics ever!
A theory IS a mind trick, by definition. An excellent mind trick which is very useful for a particularly precise (and limited if you are that cat) domain of application.
A theory is not an observational fact, a theory is confirmed by facts.

Whatever your philosophical inclination might be, one century ago, we observed some new facts about nature, that is, it comes in small packets. We don't have a quantum (singular) theory. We have a quanta (plural) theory. An excellent and powerfully theory.

Interpretation are not only born from the vanity of some people to impose there implicit misconception upon nature. A photon will pass trough two slit and ends up on the screen. And as far as facts go, only individual photon do that. Thus trying to explain what happens with that photon is quite legit.

Pretending that those individual event are just happenstance, and the only thing that can be made sense of is some ensemble behavior is really a strange claim.

Nature knows what to do with each and every individual photon. Shouldn't we be able to understand "how" it does that trick ?

vanhees71
Gold Member
2019 Award
There is one overall framework called quantum theory. It has different applications and ranges from relativistic quantum field theory for the highest energies in particle and nuclear physics to non-relativistic approximations at lower energies.

Natural sciences aim at describing Nature, as far as objective facts are concerned, not to understand "how she does that trick" (whatever trick you mean). This is done in terms of theories describing quantitatively what can be observed. What a photon is, is clearly defined in QED, and QED's predictions are confirmed by measurements at very high accuracy. There's no "trick" behind the behavior of photons. It's very well described by QED.

Nature knows what to do with each and every individual photon.
I would say that slightly different, perhaps "Individual photons behave how nature allows them to behave."
Shouldn't we be able to understand "how" it does that trick ?
Proposing a hypothesis about how that occurs is one thing, verifying it scientifically is another. If we could prepare photons in identical states and send them off to hit a screen at an identical location 100% of the time would seem to me to make quantum probability obsolete... but it seems to me nature doesn't care where photons hit a screen, only that they do hit somewhere. Like a classical example, if you roll a ball up a sphere with just enough force to make it to the top, nature doesn't care which direction it rolls down, only that it does roll down.

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Boing3000
Gold Member
There's no "trick" behind the behavior of photons.
That much is totally unknown. I have no idea how you can display that much confidence. Whatever is "behind the curtain" exist, whatever name you choose to describe nature.

It's very well described by QED.
It's clearly not on an individual Quantum basis. That QED uses a beautiful "trick" with a probability space of quasi infinite dimension made of complex value, is fine and unquestioned by anybody.
I suppose it is intellectually valid to say: let's stop science there and let's forget about the goal to describe nature better, because one can only deal in probability upon ensemble.
But it is intellectually dishonest to pretend to prove the absence of something without backing up with math and facts.

Whatever is "behind the curtain" exist, whatever name you choose to describe nature.
That's the crux of the problem though. Whatever is "behind the curtain" can't logically exist by classical, local means in a single universe. It can't add up to the verified predictions of QED. There's no other consistent way to get the same results.

That's the crux of the problem though. Whatever is "behind the curtain" can't logically exist by classical, local means in a single universe. It can't add up to the verified predictions of QED. There's no other consistent way to get the same results.
Trivially (?) If there is some other *real* classical mechanism which feeds the apparent universe - including our inclinations to make particular measurements at particular times, then this type of "conspiracy theory" could never be contradicted by any experiment. If our universe is (super) deterministic, then any non-locality demonstrating experiment, since it takes place in some light cone, can't be guaranteed to be non-local or illogical, eh?

If our universe is (super) deterministic, then any non-locality demonstrating experiment, since it takes place in some light cone, can't be guaranteed to be non-local or illogical, eh?
That would be quite a feat of nature... to make everything seem legit and predictable on macroscopic scales, yet to be non-local and probabilistic on microscopic scales. Just because some omnipotent point of view has some privileged perspective? It still does not form a consistent basis because there is no "light-cone" that can encompass the results to provide a c limit to information that has been collaborated. It can't fit in our 4 dimensional understanding of a single macroscopic universe no matter how you slice it unless something is "pulling strings" just for the sake of confusing us.

That would be quite a feat of nature... to make everything seem legit and predictable on macroscopic scales, yet to be non-local and probabilistic on microscopic scales. Just because some omnipotent point of view has some privileged perspective? It still does not form a consistent basis because there is no "light-cone" that can encompass the results to provide a c limit to information that has been collaborated. It can't fit in our 4 dimensional understanding of a single macroscopic universe no matter how you slice it unless something is "pulling strings" just for the sake of confusing us.
Yes I guess at some point you have to decide which illogical and unlikely possibility one prefers - the non-local non-causal one or the superdeterministic conspiracy theory.

PeterDonis
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