High School Is Quantum Interference Evidence of Multiple Universes?

[vanhees71]

Indeed, as I said somewhere else in this forum today: If you want religion, go to church. You find answers to your questions in physics!

 

[MichPod]

If you have no interest in learning QM from the experts like @vanhees71, then what is the point of your questions?

I have learned some parts of QM from books and online lectures. I doubt I could learn much from forum discussions. Now, if I am supposed here to ask a question and thankfully accept the answer, then thank you and let's think the case is closed.

 

[MichPod]

Also I think the main point was lost along the discussion. What was discussed is the "interpretation" of Aaronson which bhobba proposed, not the QM itself. So when I was asking my "how" questions that was related to Aaranson's approach and could not in general be clarified with a reference to regular QM.

 

[bhobba]

How do you of Aaronson explain the interference pattern in the double slit experiment if not by waves?

I have posted that one, and with great care as well. Or simply - read the paper.

Vanhees and I have had a long running 'discussion' about that paper - it has issues. To me they are minor - but he has a different view. See:
https://arxiv.org/abs/1009.2408

Scott taught at MIT for a long time - but just checked- recently he moved to UT Austin. I think he gave those lectures while at the University of Waterloo though just before going to MIT.

Thanks
Bill

 

[bhobba]

Also I think the main point was lost along the discussion. What was discussed is the "interpretation" of Aaronson which bhobba proposed, not the QM itself. So when I was asking my "how" questions that was related to Aaranson's approach and could not in general be clarified with a reference to regular QM.

To follow Scotts approach and explainb the double slit you need more than his lecture. That simply motivates wave-functions. How that is then used to derive the double slit - it doesn't give the detail mind you - the linked paper does - you need to read the first 3 Chapters of Ballentine.

I don't think I ever claimed a direct link between Scott's lecture and the double slit - I merely stated it's my preferred method to introduce students to what's happening in QM. I do not like the semi historical approach most take because you have to unlearn stuff just as the early pioneers had to.

Feynman's approach is better as well IMHO, but still prefer Scott's.

Thanks
Bill

 

[bhobba]

And besides, having presented this nice variation of the probability theory with complex amplitudes, what can we say about what kind of reality stays behind, say, negative amplitudes? Is it that our theory works just as a calculation tool or because it corresponds more or less to some reality?

Reality - what's that? Seriously its so laden with philosophical baggage its best banned from physics - but he we get queries along those lines all the time.

Physics is a mathematical model. How that relates to 'reality' - well you could probably spend a lifetime discussing that with some philosopher like Wittgenstein, and another lifetime, with a different answer answer with someone like Kuhn. To avoid this we generally don't discuss it here. As to interpretations of these 'negative' probabilities - we have a myriad of those - take your pick.

Thanks
Bill

 

[bhobba]

Well, that depends on my goals. If I am trying to see what QM may mean, then I definitely need some other things. I am not a physicist by profession, and while my level is defenitely below what is needed to work in physics, I at least can afford to myself to not conform with the "everything is awesome" attitude which is what is always common among the majority.

If you want to know what QM 'means' in the sense of just what interpretations we have - then here is the book to get:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

But to understand it you need first to know QM at least to the level of Griffiths:
https://www.amazon.com/dp/1316646513/?tag=pfamazon01-20

Vanhees, who actually teaches this stuff has a different recommendation I think - I can't recall it even though I think, like Griffiths, I have a copy.

So here is the recommendation from this thread:
Feynman Lectures
Griffiths
Schlosshauer

It will be a long road - but at the end you will understand various interpretations and their pro's and con's a lot better.

BTW nobody can tell you the reality behind QM - all we have is some conjectures - that all you can study. There is no actual answer - we know of today anyway.

Thanks
Bill

 

[MichPod]

BTW nobody can tell you the reality behind QM - all we have is some conjectures - that all you can study. There is no actual answer - we know of today anyway.

I think that is what I said and meant in the beginning of the thread, saying "nobody knows" [why interference happens], from which the argument with me started, and in continuation saying that learning of current books in QM will not resolve my questions.
And while it is true I have no knowledge of advanced level QM, not decoherence theory, nor yet crisp knowledge of QM at B.Sc. level, it was instructive to see how my position provoked an idea that I simply did not learn QM.

I again agree with you - learning some primitive picture then unlearning it for another one is problematic and that QM teaching follows historical way is problematic as well. No doubt. It is that I do not see how Aaronson's approach is better than others or more true than others.

 

[stevendaryl]

Indeed, as I said somewhere else in this forum today: If you want religion, go to church. You find answers to your questions in physics!

Just a comment: I don't see how calling someone's questions "religious" in any way helps. I assume you mean that the questions are not scientific, but why "religion"? You might as well call them "stand-up comedy" or "limericks"---those aren't scientific, either.

 

[bhobba]

You said, with regard to interference:

I guess, nobody knows.

I said:

We do - its just not common-sensical and can only really explained mathematically.

Now its become clear for you knowing is the reality behind the math - whatever that is - you will find even defining that is far from easy, let alone getting any kind of agreement. That's why we don't do it here. I think knowing is simply the mathematics - you think otherwise. That's fine - but you can't turn around, forgetting what has been explained in the tread, which is basically an elucidation of that, and why you can't really pin it down that easily, then claim its what you said from the start. You didn't. You were talking of knowing in the sense you meant it, which you didn't define. I talked about knowing being clear up front what I meant by knowing eg an explanation from a mathematical model.

The other point to note has anything been resolved in your view of knowing? No - and its obvious it never will until care is taken with the meaning of knowing etc etc. Then the answer is in terms of that agreed meaning. Good luck on getting such an agreement.

Why not do what I do - simply take knowing as deductions from a mathematical model. Simply take reality as what our model describe. Then these questions can be answered - but otherwise - its useless.

Thanks
Bill

 

[bhobba]

And why does the Fourer transform of the coordinate probability amplitudes is supposed to give the momentum probability amplitudes?

It does - but you need further development of the idea - see the first 3 chapters of Balentine. It doesn't fill in all the gaps - you will have to think a bit - a lot actually. We are here to help if you want to make that journey.

If you don't want to do that you will simply have to take our word for it.

Thanks
Bill

 

[stevendaryl]

My complaint with two-slit interference (and with discussions of interference, in general), is that there are two different types of interference, one of which is purely classical, and another of which is purely quantum-mechanical. I think it's easy to lose sight of what is uniquely quantum.

Maxwell's equations predict a two-slit interference pattern for light, so there is no quantum mystery about the pattern, itself.

 

[MichPod]

Some simple (hypotetical) analogy: some ancient scientist might have said that while we can predict solar eclipses, it's a non-scientific question to ask what reality causes them and that we DO know why they happen just because we have some periodical math model of eclipses.

I personally see much similarity between such a position and what was declared in this thread by many in relation to QM. It's not me who should explain what "understanding" and "knowing" means exactly, but rather people who declare that having a working math model is the only science we may need and other things along the same attitude. Now, I do realize that this is the mainstream approach in QM scientific community, I am yet not much used to the fact that "nobody knows" idea raises such an unconditional protest. Thinking twice, I would not put my first comment in the thread. This topic of discussion brings nothing to nobody. I just did not expect the argument will start as a result of my opinion.

 

[MichPod]

It does - but you need further development of the idea - see the first 3 chapters of Balentine

My promise to you - I will.
And I meanwhile am reading some introductory books on QFT which you recommended to me - which helps a lot!

Thank you.

 

[bhobba]

Reality, it seems to me, is what we measure. There are, incidentally, the same issues in classical gravitation, for example. How does the Earth know that the Sun is there? How does the Earth know the strength of the gravitational field and act accordingly? Both classical gravitation and QM are mathematical models that leave an underlying cause unexplained. There is, however, one interpretation that is too bizarre. That is that a particle acts sometimes like a wave and interferes with itself and sometimes mysteriously turns back into a point particle!

Of course. People like us that have studied physics take for granted such simplistic views of nature - reality is what we measure, its also what our theories describe - very simple views of things in terms that are almost trivial but raise no issues with our theories. However posters that haven't studied actual textbooks, not populations but proper textbooks, where such a view is taken for granted to the point its never even explained, you sort of pick it up as you go along, don't see what we see straight away. We don't read deep philosophical tomes discussing this stuff. Its simple ideas used to make sense of our theories. But people not exposed to this start to ask questions we really have not gone into in our studies. Really its not what science is worried about. Einstein for example became overly concerned about it and was basically lost as a front line researcher - he was a lonely figure at the end. Its not he didn't write interesting papers that raised interesting questions, it just not what we really worry about. If you do, you, like Einstein, tend to go down the gurgler except for rare cases like Bell.

I have pointed out often to take good old Euclidean geometry. Points are supposed to have no size and lines no widths. Yet all these theorems are proved with diagrams where such is not the case. How can that be? Its just common-sense so obvious nobody worries about it - well maybe some philosophers do - but virtually everyone exposed to it gets it and it doesn't even have to be explained. Its the same common-sense used here. It just works and fairly obviously so as well.

Thanks
Bill

 

[bhobba]

Maxwell's equations predict a two-slit interference pattern for light, so there is no quantum mystery about the pattern, itself.

That's true. When talked about in the QM section I take it to be the experiment done with electrons.

Thanks
Bill

 

[bhobba]

My promise to you - I will.

The issue will likely be getting the two axioms he uses. Think about it - but if you can't nut it out post back here.

Thanks
Bill

 

[bhobba]

It's not me who should explain what "understanding" and "knowing" means exactly,

I am afraid it is. There are all sorts of answers to that question. Do you know Penrose, yes the great Penrose, literally believes our equations are the reality and they live in an actual Platonic realm from which our world is just a shadow. Knowing is discovering those equations. That's just one 'weird' view (I actually at one time believed it but won't go into it here). There are all sorts of other views. For example Wittgenstein believed it all just a convention, we don't really know and understand in any usual sense at all, reality is that convention. Turing argued about that with Wittgenstein in a series of famous debates that occurred when he was part of a course Wittgenstein gave. Turing said it was nuts - if it wasn't true bridges could fall down, we can't be sure of anything, society would be in a terrible mess. Wittgenstein said - so? I could go on and talk about Kuhn and others - but the situation is obvious - you can't really talk about such things unless you are clear what you are talking about. Sciences answer is simple - like Turing - its just just a model and its just a really easy common-sense view that allow us to use our theroes. We don't go down that philosophy path in a serious way. If you do you get nowhere. Wittgenstein as great as he was didn't basically win WW2 by cracking the German code - Turing did - same with Feynman and Von-Neumann that worked on the atom bomb project. BTW it was Von-Neumann that basically invented the bomb - and Bethe can express best about Feynman (remember the great Fermi and Teller were in Beth's team - Teller didn't like working under Bethe - he thought he should be a division leader - not just a team leader):


Thanks
Bill

 

[MichPod]

I do not think the problem may be drawn as an opposition of true science and weird phylosophy. Take my analogy again - there is some physical reality behind solar eclipses yet one may just declare a question on reasons of eclipses as "phylosophy". That QM does not explain all the "why" and "how" is a very notmal situation in my view. It is normal that scientists do not know everything and it is actually an extremely interesting situation. Interesting and normal. What IMHO is not normal at all is to declare all such questions for which the answer is not known as relating to phylosophy and not to science. No doubt that phylosophy is mostly fruitless, but again, declaring every hard question as phylosophy is a nonsense. In the end, we do not only have solar eclipses predictions but kepler orbits as well as undefstanding of why kepler orbits exist. A question "why" has been answered by science many times outside of QM, but now QM scientists tend to declare such sort of questions as meaningless in the science domain.

 

[Lord Jestocost]

It's not me who should explain what "understanding" and "knowing" means exactly...

"To understand quantum theory, as opposed to merely knowing how to use it, we must answer two questions: 1. What does the wave function really mean? (the interpretation question); 2. What happens during a quantum measurement? (the measurement problem). The quantum reality crisis arises because physicists have no good answers for either of these questions." [Underline, LJ]

Nick Herbert in "Quantum Reality: Beyond the New Physics"

 

[MichPod]

But really, really after this long discussion, if a layman asks what is the quantum interfernce, what answer is possible other than:
"We do have a very powerful theory with which (among the other things) we can CALCULATE such an interference, but otherwise we do not know."
How is this possible answer wrong?

 

[PeroK]

But really, really after this long discussion, if a layman asks what is the quantum interfernce, what answer is possible other than:
"We do have a very powerful theory with which (among the other things) we can CALCULATE such an interference, but otherwise we do not know."
How is this possible answer wrong?

You asked me what is quantum interference and I gave you an answer. So, the answer isn't "I don't know".

If you ask me "why does nature exhibit an intrinsic randomness", then the answer to that is "I don't know".

 

[MichPod]

You asked me what is quantum interference and I gave you an answer. So, the answer isn't "I don't know".

If you ask me "why does nature exhibit an intrinsic randomness", then the answer to that is "I don't know".

Btw, I was not the topic starter.

Do I understand you right that the answer is along the following - the nature is random and this radomness is according to some very special sort of pobability theory (i.e. probability amplitudes instead of just probabilities of the regular probability theory) etc. Then how many QM physicists will agree with this explanation?

 

[PeroK]

Btw, I was not the topic starter.

Do I understand you right that the answer is along the following - the nature is random and this radomness is according to some very special sort of pobability theory (i.e. probability amplitudes instead of just probabilities of the regular probability theory) etc. Then how many QM physicists will agree with this explanation?

The "minimal statistical interpretation" is at the heart of QM. That's the core of mainstream QM.

There are also "interpretations" on top of this, which explain further in some way how things work. I learned the Copenhagen interpretation - wave function "collapse" etc.

There is general agreement that the interpretations are different ways for humans to makes sense of QM. It is possible that one interpretation will be proved correct and then our understanding may go deeper.

The statistical nature of quantum interference, though, is not really in doubt.

 

[PeterDonis]

Nick Herbert in "Quantum Reality: Beyond the New Physics"

This is a pop science book, not a textbook or peer-reviewed paper.

Everyone, please keep the discussion focused on physics, not philosophy, and on acceptable sources--textbooks and peer-reviewed papers--not pop science.

 

[bhobba]

Take my analogy again - there is some physical reality behind solar eclipses yet one may just declare a question on reasons of eclipses as "phylosophy".

Even after I explained just how terms like physical reality are very ill defined and loaded you simply don't seem to get it. We know a deeper explanation of solar eclipses than just the patterns they seem to follow that the ancients knew. Does that mean we know the physical reality - of course not.

Please, please, and again please stop making this fundamental and obvious error.

Physics is a mathematical model - its relation to this thing you called physical reality first needs a definition of physical reality many many of which exist, so many its useless. Think in terms of mathematical models (just like in Euclidean Geometry where you don't argue about what a point or line is 'in physical reality' - you just accept the obvious) - in your example Newton gave us a better mathematical model, Einstein an even better one - that's it - that's all. Do physicists believe we are getting closer to some truth about the world - of course - eg see Wienberg:
http://www.physics.utah.edu/~detar/phys4910/readings/fundamentals/weinberg.html

But it is not expressed in the terms you use which are very 'contentious' - its expressed in the language of math.

Note the similarity between Wittgenstein that Kuhn eventually degenerates into. Just like Turing Weinberg, correctly IMHO, takes exception to this view - 'All this is wormwood to scientists like myself, who think the task of science is to bring us closer and closer to objective truth.'

But the language of that objective truth is math and its correspondence so experiments can be done to check it, just like Euclidean Geometry, is not philosophical - but pretty easy to see using little more than common-sense and knowledge of the theory.

Thanks
Bill

 

[vanhees71]

Just a comment: I don't see how calling someone's questions "religious" in any way helps. I assume you mean that the questions are not scientific, but why "religion"? You might as well call them "stand-up comedy" or "limericks"---those aren't scientific, either.

Well, if you are asking why the world is as it is or what's "behind the phenomena" it's something at least touching on religious questions, and it's beyond what you can get answered by science, because science restricts itself to that part of human experience which is reproducibly and objectively observable and even quantatively measurable. By construction the outcomes of this method are independent of any opinion or worldview the researchers using it might be. That's also evident from the history of science. E.g., many ideas of the great discoveries in the physics of the 2nd half of the 20th century, mostly about quantum field theory (relativistic as well as non-relativistic) and many-body theory, have been quite independently developed in the eastern and the western part of the world then divided by the Cold War. Of course, both worlds where not completely isolated, but nevertheless many ideas and results were achieved independently in slightly different approaches but leading to precisely the same result. That's also why, by construction, there can never be any real tension between religious believes (be it in terms of one of the "world religions" or atheism or whatever type of believe you might think of) and science: It's just about different realms of human experience.

 

[vanhees71]

Physics is a mathematical model - its relation to this thing you called physical reality first needs a definition of physical reality many many of which exist, so many its useless. Think in terms of mathematical models (just like in Euclidean Geometry where you don't argue about what a point or line is 'in physical reality' - you just accept the obvious) - in your example Newton gave us a better mathematical model, Einstein an even better one - that's it - that's all. Do physicists believe we are getting closer to some truth about the world - of course - eg see Wienberg:
http://www.physics.utah.edu/~detar/phys4910/readings/fundamentals/weinberg.html
Bill

Physics is not a mathemtical model. A "mathematical model" I'd define as a certain set of axioms (e.g., the axioms of Euclidean geometry) which can be freely invented. Of course you are constrained by the fact that the axioms should not contradic themselves in an obvious way (although according to Gödel for any sufficiently interesting set of axioms you can never prove this consistency within this system of axioms itself), but otherwise you are pretty free to invent anything.

Physics is first of all an empirical science. It's about observation of phenomena in nature that show some regularity and pattern in the sense that you can reproduce these observations in an objective way, i.e., if you find something in a certain arrangement (in experiments that's called "preparation"), then you always find the same observational results (and be it only in a statistical sense). In the history of the modern sciences it turned out that you can make observations quantitative by defining measures for the most importantant quantities involved, starting from the geometrical quantity of length, the duration of times, and mass (as a measure for inertia). Already with this quite limited set you can describe everything what's now called "classical mechanics", and with the work of physicists like Galilei and Newton this could brought into an elegant mathematical system of "Natural Laws", now called Newtonian Mechanics. It's very powerful, giving us all the theoretical tools needed to construct all kinds of machines, including such a phantastic possibility as flying to the moon or landing a little lab at the spot of a tiny comet on the spot after a decade-long journey with some spectacular maneuvres all following Newtons laws as predicted, but nevertheless all this is based on observational facts, and the theory follows these observational facts, reducing the basic ones to an astonishingly simple handful of "fundamental laws" that finally can be cast into symmetry principles.

Of course, from this point on it's very mathematical, nearly like pure mathematics starting from a few axioms and building up a platonic world, but one must not forget that it's just the result of a lot of empirical evidence made ever more precise with the progress of technology of observation. As any empirical finding, it's always bound to be incomplete, and indeed the first evidence that Newtonian Mechanics cannot be the full truth are electromagnetic phenomena, which to a certain extent were found to be quite completely described by another set of fundamental laws, today called "Maxwell's Theory of Electromagnetism". It's plainly contradicting the very fundamental symmetry principles underlying Newtonian spacetime, and indeed after another struggle of about 50 years of many physicists, finally Einstein came to another better mathematical model called Special Theory of Relativity.

It's also true that without these mathematical formulation almost all physics we know to day, and which is crucial for our technological development (particularly quantum theory which is behind almost everything shaping our modern lives, particularly the laptop I'm typing this posting right now, the internet which let's me deliver it to PF where it can be read within a few microseconds or so all around the world), because often you get the idea for new experiments only through the mathematical conclusions within a given model of natural phenomena. In some sense an extreme example is the LHC (as far as I know the 2nd-most expensive experiment ever built up), which is the result of the quest for the final corner stone of the Standard Model of elementary-particle physics, the long-thought Higgs boson, which was predicted almost 50 years before it indeed has been unambigously discovered by ATLAS and CMS. I guess nearly every physicist around the world will remember what he or she did on Jul/04/2012, where the discovery was announced. I remember that we first had a seminar by a guest on another topic and then all watched the announcement of the Higgs discovery via the WWW.

So one must not forget that physics is about reproducible objective observations of nature, leading to astonishingly precise but always incomoplete mathematical models, but it's not math. If there is anything you can call "reality" in the sense of natural sciences it's the objective reproducibility of observations of nature. For sure, the mathematical models are NOT the "reality" in this sense but always incomplete pictures of it. The much I like Penrose's semipopular books (I've read some portions of "Road to Reality"), I cannot agree with his radical neoplatonism. He must have forgotten his time in the introductory and advanced science labs, where he should have learned that physics is finally an empircal science, not some system of purely mathematical axioms.

 

[bhobba]

Physics is not a mathemtical model. A "mathematical model" I'd define as a certain set of axioms (e.g., the axioms of Euclidean geometry) which can be freely invented.

I have a few books on mathematical modelling - its defined as something like what Wikipedia says:
https://en.wikipedia.org/wiki/Mathematical_model
A mathematical model is a description of a system using mathematical concepts and language.

You and I both know this, but since this is a beginner thread I think its important to explain what is meant by your comment about Euclidean geometry. Kant said Euclidean geometry was a-priori correct. However because in those days Kant was held in such high regard challenging him was something not to be taken lightly. Then Gauss, who in math was at least equally great as Kant, came along and showed not only do other geometries exist (I think some others had already done that as well) but logically they are just as consistent as Euclidean geometry. However because of Kant he did not publish - too scared. Later mathematicians like Riemann, who many thought of as Gauss's successor, had no such qualms, and he laid the foundation for the math Einstein used.

We freely choose which one to use depending on the objectives of our model. So you are 100% correct in pointing out the freely chosen nature of mathematical models. But, and I think this is the key point, how good a model conforms to experiment is its judge ie how good a description is it. That's what makes it science. For everyday things Euclidean geometry is a remarkably good description - but we know its wrong as readily attested to by GPS devices - unless they take into account GR they would not work.

I suspect like some of the concepts in this thread what a mathematical model is may be a bit mutable, so in future when I use it I will be clear what I mean - namely its a mathematical description of some system - how good a description it is, is judged by experiment.

So one must not forget that physics is about reproducible objective observations of nature, leading to astonishingly precise but always incomoplete mathematical models, but it's not math. If there is anything you can call "reality" in the sense of natural sciences it's the objective reproducibility of observations of nature. For sure, the mathematical models are NOT the "reality" in this sense but always incomplete pictures of it. The much I like Penrose's semipopular books (I've read some portions of "Road to Reality"), I cannot agree with his radical neoplatonism. He must have forgotten his time in the introductory and advanced science labs, where he should have learned that physics is finally an empircal science, not some system of purely mathematical axioms.

Like just about all you write, both true and expressed well. I think very few agree with Penrose, but its what he thinks, and its seductive - I believed it at one time until I understood what Gell-Mann said:
https://www.ted.com/talks/murray_gell_mann_on_beauty_and_truth_in_physics

One interesting point he makes, that all science advisers on this site know only too well, popularization's (except a few like Feynman) are invariably wrong, even some movies/documentaries about it are wrong. And many are written/done by famous physicists like Brian Greene - go figure.

Thanks
Bill

 

[vanhees71]

As far as I know, Gauss wasn't too afraid of Kant but didn't publish his results on non-Euclidean geometry because he thought he'd shock is contemporaries ;-)). Of course to day we know that physical space is not best described as a Euclidean affine manifold but spacetime as a whole as a pseudo-Riemannian (Lorentzian) one. Another clue about the genius of Gauss is that in fact he tried to check the Euclidicity of physical space by doing a careful triangulation of 3 mountains (indeed Gauss was earning his living as a geodesist precisely making a map of the kingdom of Hanover). He came to the conclusion that he cannot find any deviation from Euclidicity within the accuracy he could achieve with his instruments. Maybe that was another reason to keep silent about his mathematical discoveries concerning non-Euclidean geometry. Of course, he himself published important work on general differential geometry later, after Lobechevsky and Bolyai independently published their work on (I think hyperobolic) non-Euclidean geometry.