What makes the interpretations of Quantum Mechanics so important?

[forcefield]

He doesn't mean what you think he means.

I can only guess what he means by the "quantum laws" and even then only vaguely. Could you please elaborate on my question then ?

 

[bhobba]

It's not bizarre, it's simply a misleading formulation you read quite often. I've never understood what the intention of the authors making it may be. It's said "a quantum state can be in a superposition". That's just a meaningless intellectual-sounding phrase.

That it must be so (ie a vector space) is even intuitive. Simply start with the idea a system is described in some way by a function. Especially when extended to Rigged Hilbert space virtually any function is an element of such a space eg take any two functions and its sum is another function, multiply it by c and it is also a function. Viewed this way it would strange its NOT a vector space. The real issue is the functions are complex - why is that? There is an answer but rather than me say it people might like to try a little research:
https://www.scottaaronson.com/democritus/lec9.html
Now how does probability come into it? Well there is this theorem called Gleason:
http://kiko.fysik.su.se/en/thesis/helena-master.pdf
You may not be able to spot the hidden assumption - I didn't when I first studied it. But then you read Weinberg, who, correctly says - the Born Rule can't be derived from within QM - so trusting someone like Weinberg you go looking for it. There are in fact a couple but the main one is non-contextuality. So you have this other assumption - non- contextuality. Its quite intuitive, but its an assumption. What if we reject it. That is the path that leads to things like DBB etc.

That is the purpose of interpretations - it illuminates exactly what is being assumed and what different assumptions lead to. Will it lead to progress? It already has - the essence of Non Contextuality is in the so called Kochen-Specker theorem (but not emphasized enough its a simple corollary to Gleason).

Interpretations are not a popular research area - it gets most of its traction on the internet - but progress is being made. But at a slow pace and as I have posted before I nowadays do not think it will in the end help in progressing Quantum Theory - it still has issues like quantum gravity, the triviality of QED (the whole standard model - excluding gravity - may in fact be trivial) plus other issues. To me moving that forward will be of greeter value. But of course science often does surprise.

Thank
Bill

 

[bhobba]

The laws don't change with the scale.

True - but their approximation valid at the larger scale can be different.

Thanks
Bill

 

[Michael Price]

True - but their approximation valid at the larger scale can be different.

Thanks
Bill

Yes, absolutely.

 

[vanhees71]

The laws don't change with the scale.

That's for sure not true. Not a single piece of physics is scale invariant according to the Standard Model of HEP!

 

[Michael Price]

That's for sure not true. Not a single piece of physics is scale invariant according to the Standard Model of HEP!

That is a different issue, and has nothing to do with the universality of quantum physics.

 

[vanhees71]

I also don't understand what scaling invariance should have to do with the universality of quantum physics. I just wanted to point out that the claim that anything is scaling invariant is simply wrong.

 

[Sophrosyne]

Summary: Provided it's correct that the interpretations of Quantum Mechanics can be neither proved nor disproved why then do researchers invest so much time and talent in this field?

How valid is the statement "It means physics is ultimately concerned with descriptions of the real world" in the realm of QM? Heretic question, what is "real" besides the outcome of the measurement?

I think what makes people uncomfortable is not understanding WHY it acts the way it does; not understanding the underlying mechanism. Wanting to know WHY something acts the way it does is not a bad question. It's perfectly reasonable. We may not be in a position to answer it yet, but that doesn't mean we can't keep asking it. Until then though, we are still just left with Feynman's "shut up and calculate" interpretation. It's all we got for now. But that's no reason to think that's all there is to it.

Who knows, maybe once we figure out the mechanism of why quantum mechanics works the way it does, it will open up very interesting new worlds and possibilities, and a whole bunch more questions.

What is clear is that we don't understand everything there is to understand about this stuff yet. Like Feynman said, learning to live with such discomfort of not knowing everything is part of being a scientist, and what keeps science always growing.

"The scientist has a lot of experience with ignorance and doubt and uncertainty, and this experience is of very great importance, I think...If we take everything into account — not only what the ancients knew, but all of what we know today that they didn't know — then I think that we must frankly admit that we do not know.
But, in admitting this, we have probably found the open channel.This is not a new idea; this is the idea of the age of reason. This is the philosophy that guided the men who made the democracy that we live under. The idea that no one really knew how to run a government led to the idea that we should arrange a system by which new ideas could be developed, tried out, and tossed out if necessary, with more new ideas brought in — a trial and error system. This method was a result of the fact that science was already showing itself to be a successful venture at the end of the eighteenth century. Even then it was clear to socially minded people that the openness of possibilities was an opportunity, and that doubt and discussion were essential to progress into the unknown. If we want to solve a problem that we have never solved before, we must leave the door to the unknown ajar...

Now, we scientists are used to this, and we take it for granted that it is perfectly consistent to be unsure, that it is possible to live and not know. But I don’t know whether everyone realizes this is true. Our freedom to doubt was born out of a struggle against authority in the early days of science. It was a very deep and strong struggle: permit us to question — to doubt — to not be sure. I think that it is important that we do not forget this struggle and thus perhaps lose what we have gained."
-Richard Feynman

Don't stop asking the questions, but be careful not to jump on any answer just because you have to have one. If we still don't know, it's OK to just leave the question open.

 

[timmdeeg]

Who knows, maybe once we figure out the mechanism of why quantum mechanics works the way it does, it will open up very interesting new worlds and possibilities, and a whole bunch more questions.

Wouldn't this imply a proof that e.g. Many Worlds really exist or that the wave function really collapses?

 

[RUTA]

Wouldn't this imply a proof that e.g. Many Worlds really exist or that the wave function really collapses?

I think an experiment like this one would have implications https://arxiv.org/abs/1705.04620

 

[PeterDonis]

I think an experiment like this one would have implications

It would certainly have implications if (admitted to be highly unlikely by the author) results were obtained that obeyed the Bell inequalities and disagreed with the predictions of QM.

I don't see what implications (other than "oh, well, QM is confirmed again") it would have if the results were that the Bell inequalities were violated and everything was just the same as in previous experiments.

 

[RUTA]

It would certainly have implications if (admitted to be highly unlikely by the author) results were obtained that obeyed the Bell inequalities and disagreed with the predictions of QM.

I don't see what implications (other than "oh, well, QM is confirmed again") it would have if the results were that the Bell inequalities were violated and everything was just the same as in previous experiments.

I’m thinking Kim et al delayed choice quantum eraser where the decision to erase or not is made by a human. That would have implications either way

 

[DarMM]

What would be the implications if it went the other way? As in over Tsirelson's bound.

 

[RUTA]

What would be the implications if it went the other way? As in over Tsirelson's bound.

I don’t know of any theory predicting that

 

[PeterDonis]

I’m thinking Kim et al delayed choice quantum eraser where the decision to erase or not is made by a human. That would have implications either way

Why would it have implications either way? If all that happens is that the predictions of QM are confirmed, what implications does that have?

 

[RUTA]

Why would it have implications either way? If all that happens is that the predictions of QM are confirmed, what implications does that have?

Do you know what that experiment is?

 

[PeterDonis]

Do you know what that experiment is?

I'm reasonably familiar with it. Why do you ask? If you think it should be obvious to me why your statement is true given a description of the experiment, it's not; that's why I asked the question.

 

[RUTA]

I'm reasonably familiar with it. Why do you ask? If you think it should be obvious to me why your statement is true given a description of the experiment, it's not; that's why I asked the question.

Yes it should be obvious that’s why I asked. I’m typing with my phone so I can’t give a detailed description here. I didn’t think it would be necessary that’s why I used that example

 

[RUTA]

Read section VI here https://arxiv.org/abs/1901.10825 if you don’t see the implications

 

[PeterDonis]

Read section VI here https://arxiv.org/abs/1901.10825

This section contains a very telling statement in its first paragraph: "a conscious agent who attempts to violate the probabilities of QM, as one might think a truly free conscious agent ought to be able to do". So basically you want to test whether conscious agents can violate the laws of physics? To me that's not even a consistent question to ask: the laws of physics are the laws of what actually happens, not what would have happened if a conscious agent wasn't present. Whatever conscious agents are, they can't "violate" the laws of physics because anything they do has to be describable using the laws of physics.

If I take that objection as meaning you couldn't possibly have meant that conscious agents might be able to violate the laws of physics, and instead read you as basically asking whether the laws of QM as we know them today are the correct laws of physics--so that finding violations of the predictions of QM in an experiment just means that we need to look for better laws of physics--then obviously any experiment that tests the predictions of QM will test this. But on that reading experiments with conscious observers like humans in them don't have any special status, so I don't see how that could be what you mean.

If, OTOH, I read you as asking the question I said above isn't even consistent, namely whether conscious agents can violate the laws of physics (the "real" ones, not just our best current approximations to them), and I put aside my objection that the question doesn't make sense, I guess I can see how an experiment with conscious observers in it would have "implications" for your own view either way, since if we did umpteen experiments with conscious observers that all confirmed the predictions of QM, that would be evidence against the hypothesis you are trying to test, namely that conscious agents can violate the laws of physics. But since I don't think the question even makes sense, for the reason I gave above, I don't see how it should be obvious to me that such an experiment would have implications either way. It might be obvious to you, but the question you are asking wouldn't even occur to me since to me it doesn't make sense.

 

[PeterDonis]

Read section VI here https://arxiv.org/abs/1901.10825

Btw, this section appears to me to misdescribe somewhat the delayed choice quantum eraser experiment such as Kim et al that you referred to. Your paper says:

experiments have been done where the ‘lens is inserted’ after the particles have hit the detector. This is called a “delayed choice quantum eraser experiment"

"The particles" without qualification is not correct. In the Kim et al experiment [1], the equivalent of "inserting the lens", namely passing the idler photon through beam splitters, happens after the signal photon has hit its detector--more precisely, the time when the idler photon reaches the beam splitters, assuming that the photons travel at the speed of light, is after the time when the signal photon hits its detector. (That assumption is actually not justified, strictly speaking, for a photon that is not detected in between the source and the beam splitters. I assume it's considered to be a good enough approximation in the given experiment, but I have not seen much discussion of this point.) But the idler photon certainly does not reach the beam spliiters after the idler photon hits a detector, which is what "the particles" without qualification in the quote above implies.

[1] https://arxiv.org/abs/quant-ph/9903047

 

[PeterDonis]

Read section VI here https://arxiv.org/abs/1901.10825

I note that the description I quoted from in my previous post just now is from the "dynamical" viewpoint, which you then contrast with your proposed "adynamical" viewpoint, according to which there is no mystery about delayed choice at all: the experimental results simply have to be consistent with the overall spacetime arrangement, which they are. I think you are correct that, from the adynamical viewpoint, there is no mystery at all: the results simply need to be consistent with the spacetime arrangement, and they are.

I think your description of the adynamical viewpoint also says the correct thing about "free will": people's choices are physical events and have to be consistent with the spacetime arrangement just like all other physical events. However, I don't see how an adynamical viewpoint is required for this: all that is required is basic physicalism, that conscious beings like us are physical things and everything they do has to be consistent with the laws of physics. From the physicalist viewpoint, as I said a couple of posts ago, an experimental result that showed that, for example, putting a conscious human in a delayed choice experiment made the results violate the predictions of QM, would not mean that conscious beings could violate the laws of physics; it would just mean our understanding of the laws of physics was incomplete.

 

[DarMM]

I don’t know of any theory predicting that

I didn't really expect so. It was just idly curious if we were allowing humans to alter the results of a Bell test in one direction had somebody came up with an idea if it was altered in the other direction. Of course it would be very difficult to make sense of such a thing.

 

[RUTA]

You’re missing the mystery of delayed choice entirely. And the Sci. Am. version captures precisely the same mystery as the Kim et al version. Let me ask you this, what would you experience as you switched the lens in and out? Some force moving your hand in accord with the previous outcome? Hypnotic? Or would you make choices like you do everyday and violate the QM predictions (which are not qualified for non conscious agents only)? Or would your freely made choices just happen to satisfy QM? Any and all options have profound implications for the way most physicists think.

 

[vanhees71]

Ehm, are we back in the time of scholastics, or is this still a forum on modern science as established since Galilei's and Newton's times?

A human being cannot act by some magic against the established natural laws, only because it's a human being. The natural laws are fulfilled for all matter in the universe, including that also humans and other living beings consist. Otherwise our models and theories were wrong and we had to adapt them to what's really observed. That's how science works: It checks the predictions of models and theories against real-world observations. So far there's not one single observation that contradicts the laws of QT. The socalled "quantum eraser experiment" has been realized in the lab, and always the predictions of QT have been found intact.

Recently there was also a check with randomizing the experimental setup by humans:

https://www.nature.com/articles/s41586-018-0085-3https://arxiv.org/abs/1805.04431

 

[PeterDonis]

what would you experience as you switched the lens in and out? Some force moving your hand in accord with the previous outcome? Hypnotic? Or would you make choices like you do everyday and violate the QM predictions (which are not qualified for non conscious agents only)? Or would your freely made choices just happen to satisfy QM? Any and all options have profound implications for the way most physicists think.

Your freely made choices would not "just happen" to satisfy QM. They would satisfy QM (assuming QM gives the correct laws of physics for this scenario) because you are a physical thing, and your freely made choices are physical events, and the laws of physics are the laws that describe physical events. As I said, this is basic physicalism.

Your experience as you switched the lens in and out would be the same as your experience of participating in any other kind of experiment where you had to try to just do things "at random", without any external trigger--or, for that matter, your experience of, say, just sitting in your chair and deciding to lift your hand without any external trigger. Can you violate QM by deciding to lift your hand at a "random" time without an external trigger?

If considering the above would really have "profound implications for the way most physicists think", then that just indicates to me that most physicists have not thought through the implications of physicalism. Which I actually doubt is the case; I suspect most physicists would not consider anything that I wrote above (or, for that matter, anything I've written in this thread) surprising.

 

[RUTA]

Your freely made choices would not "just happen" to satisfy QM. They would satisfy QM (assuming QM gives the correct laws of physics for this scenario) because you are a physical thing, and your freely made choices are physical events, and the laws of physics are the laws that describe physical events. As I said, this is basic physicalism.

Your experience as you switched the lens in and out would be the same as your experience of participating in any other kind of experiment where you had to try to just do things "at random", without any external trigger--or, for that matter, your experience of, say, just sitting in your chair and deciding to lift your hand without any external trigger. Can you violate QM by deciding to lift your hand at a "random" time without an external trigger?

If considering the above would really have "profound implications for the way most physicists think", then that just indicates to me that most physicists have not thought through the implications of physicalism. Which I actually doubt is the case; I suspect most physicists would not consider anything that I wrote above (or, for that matter, anything I've written in this thread) surprising.

What you are describing is superdeterminism and most scientists do not believe that. The majority belief is that the scientist is free to make settings independently of the laws being tested. That’s why the experiment described by Hardy is important

 

[PeterDonis]

What you are describing is superdeterminism

No, it isn't, it's physicalism. Saying that human "free choices" have to obey the laws of physics is not superdeterminism. See below.

The majority belief is that the scientist is free to make settings independently of the laws being tested.

No, the majority belief is that the scientist is free to make settings that are uncorrelated with the physical system being tested. That's a much weaker claim.

Your claim is that a majority of scientists believe that scientists can violate the laws of physics when setting up their experiments. I would need to see extremely strong documentation of that claim; it seems to me to be obviously false.

 

[RUTA]

No, it isn't, it's physicalism. Saying that human "free choices" have to obey the laws of physics is not superdeterminism. See below.

No, the majority belief is that the scientist is free to make settings that are uncorrelated with the physical system being tested. That's a much weaker claim.

Your claim is that a majority of scientists believe that scientists can violate the laws of physics when setting up their experiments. I would need to see extremely strong documentation of that claim; it seems to me to be obviously false.

I never said we can violate the laws of physics. That’s ridiculous. Answering delayed choice with physicalism is superdeterminism. You’re saying the choice of settings could not have been otherwise because of some physical force or cause.

 

[PeterDonis]

I never said we can violate the laws of physics. That’s ridiculous.

Then I have no idea what you mean when you say things like this:

The majority belief is that the scientist is free to make settings independently of the laws being tested.

If scientists can't violate the laws of physics, then how can they possibly be free to make settings independently of those laws?

If your answer is that the scientist isn't using the same laws to make the settings as the laws that are being tested, that won't work; the laws aren't compartmentalized like that. If you're testing QM, well, your brain has to obey QM, your experimental devices have to obey QM, etc. You can't separate the laws being tested from all the other laws.

Answering delayed choice with physicalism is superdeterminism.

This makes no sense unless you are equating physicalism itself with superdeterminism.

You’re saying the choice of settings could not have been otherwise because of some physical force or cause.

I'm saying no such thing. I am only saying the choice of settings has to obey, as in be consistent with, the laws of physics. Whether or not the laws of physics require that the choice of settings could not have been otherwise is a separate question that I have not taken a position on either way.

 

[PeterDonis]

You’re saying the choice of settings could not have been otherwise because of some physical force or cause.

Even if the laws of physics require this (which, as I said in my previous post, I have not taken a position on either way), that still does not imply superdeterminism. As I said several posts ago, the important thing is that the choice of settings is uncorrelated with the physical system being tested. If the process of choosing the settings is deterministic but chaotic, that would be sufficient to make the choice of settings uncorrelated with the physical system being tested.

 

[RUTA]

Even if the laws of physics require this (which, as I said in my previous post, I have not taken a position on either way), that still does not imply superdeterminism. As I said several posts ago, the important thing is that the choice of settings is uncorrelated with the physical system being tested. If the process of choosing the settings is deterministic but chaotic, that would be sufficient to make the choice of settings uncorrelated with the physical system being tested.

But the choice of settings in delayed choice is correlated perfectly with a previous outcome, that’s what “delayed choice” means. If you still don’t understand it, I can’t help you.

 

[PeterDonis]

the choice of settings in delayed choice is correlated perfectly with a previous outcome

In the Kim et al delayed choice experiment, the "choice" of which photon detector detects the idler photon is correlated with whether or not interference is detected for the corresponding subsample of detections of the signal photon. But the experimenter doesn't make that choice.

In a (hypothetical) delayed choice experiment in which humans were used, a human could, I guess, flip some sort of switch that would take the place of the beam splitters that route the idler photon in either the "which path detection" or "no which path detection" direction. Then the human's choice of whether which path information was available for that idler photon would be correlated with whether or not interference is detected for the corresponding subsample of detections of the idler photon.

We already know that the original experiment, with no humans and beam splitters, violates the Bell inequalities and confirms the predictions of QM. So let's consider the two possibilities for the results of the experiment with humans:

(1) The results are different from before: they obey the Bell inequalities and violate the predictions of QM. Obviously this would have implications, but what implications? Would we say humans can violate the laws of physics? Obviously not. We would say there is something we don't understand about the laws of physics, and we would start looking for ways to explore whatever is going on in the experiment with humans present that affects the results.

(2) The results are the same as before. In this case I can't see that we would have learned anything new, except that humans being involved in the experiment doesn't affect the results (which is what I think a majority of physicists would expect anyway). Humans are physical things, just like beam splitters, so having a human choose which direction each idler photon goes in is no different from having a beam splitter "choose". The human is just a much more complicated and expensive way of doing the same job.

If you still don’t understand it

I think I understand the delayed choice experiment and what the implications would be in cases 1 and 2 above just fine.

I don't understand whether you are trying to argue that even in case 2 above, having humans in the experiment somehow has implications that having just beam splitters in it doesn't, or whether you are only saying that having humans in the experiment would have implications in case 1 above (which I agreed above that it would). If you are trying to argue the former, I don't understand your argument: why is having a human's choice of which direction to send the idler photon in being correlated with the signal photon detection data any different from having a beam splitter's "choice" correlated the same way?

 

[Mentz114]

But the choice of settings in delayed choice is correlated perfectly with a previous outcome, that’s what “delayed choice” means. If you still don’t understand it, I can’t help you.

Apparently nothing is erased and nothing is delayed. I like this paper by Ruth Kastner.

https://arxiv.org/abs/1905.03137

 

[charters]

But the choice of settings in delayed choice is correlated perfectly with a previous outcome, that’s what “delayed choice” means. If you still don’t understand it, I can’t help you.

In the paper, you write:

"The question from our dynamical perspective is, How do the particles ‘know’ whether or not the lens will be inserted? And, if they do not ‘know’ whether the lens will be inserted or not, how do they ‘know’ whether or not to create the interference pattern?"

"Let us now bring the conscious agent into the picture by imagining it is a conscious agent inserting the lens (or not) in the experimental set-up. The question from our dynamical perspective is, What will I experience if I am the agent deciding whether or not to insert the lens? If the predictions of QM are to hold, then my decision must always be in accord with the particle’s behavior at the detection screen and that event occurred before I made the decision. Assuming QM holds, will I feel mentally ‘coerced’ into making the appropriate choice? Will I feel some ‘physical force’ moving my hand against my will?"

So you are suggesting that for a signal photon hitting at a given spot on D0 - call it x1,y1 - the probability that it will have an "erased" partner can be different from it having an "unerased" partner. Like you think there are spots on D0 that are the peaks and troughs of the interference pattern, such that some mysterious force would have to compel the (spacelike separated) human to choose to insert or remove the lens on these runs of the experiment, so that the statistical correlations are correct.

The logic is valid, but the premise is wrong. You are forgetting about the $\pi$ phase shift between the two "erased" detectors, which have exactly inverse peaks and troughs in their interference patterns at D0. Crucially, this means that for every impact spot on the D0 screen, the probability of the idler detection at D1+D2 = D3+D4, no matter what. See Pisanty's answer here with nice diagrams: https://physics.stackexchange.com/q...-the-delayed-choice-quantum-eraser-experiment

So, if x1,y1 is in the trough for the D1 interference pattern, this does NOT mean it is now less likely the idler is going to be erased - it only means that if the idler is erased, it is more likely to be seen at D2, rather than D1.

So, when we remember the phase shift, there is no need for the human's will to be swayed by the prior D0 measurement. Every D0 measurement is equally consistent with the human inserting the lens or not - the only thing that is conditioned on specific D0 outcomes is whether, after the choice to erase, outcomes are weighted to D1 or D2 (or whether unerased cases are weighted to D3 or D4). This is just basic EPR correlations.