Is superposition widely accepted?

[EskWIRED]

I've been wondering and asking questions about entanglement lately.

I am very dissatisfied with the answers I've been getting - not necessarily because any of the answers were incorrect, but more likely, because the answers were of the sort which reminded me that i was asking about interpretations, while quantum physics really, or strictly, or maybe only, supplies answers to questions about the probable outcome of measurements.

So here i go again, with a very basic, conceptual question that may help me to understand my dissatisfaction and confusion.

My understanding is that prior to measurement, two entangled particles are in a state of superposition. Neither is, for example, spin-up or spin-down. Instead, their state is that they are BOTH spin-up AND spin-down. ONLY after a measurement are they in a coherent state.

Is that an accepted truth? Or is it a "mere" interpretation of probabilities?If that is an accepted truth, that "really" the particles are not (yet) in a coherent state, then it seems that measurement of one "really" affects the state of the other.

But the answers I get seem to indicate that prior to measurement of the nearby particle, the distant particle is in a definite state, which is revealed by the local measurement. Is that true? Or were both in a state of superposition prior to the first measurement?

Or is "superposition" an interpretation? Are there any theories which deny that unmeasured particles are in a state of superposition?

 

[mfb]

My understanding is that prior to measurement, two entangled particles are in a state of superposition. Neither is, for example, spin-up or spin-down. Instead, their state is that they are BOTH spin-up AND spin-down. ONLY after a measurement are they in a coherent state.

Is that an accepted truth? Or is it a "mere" interpretation of probabilities?

If it were just up and down, you could interpret it as probability. Entanglement goes further, you are free to choose the direction in which you measure the polarization. And that cannot be interpreted as probability any more.

If that is an accepted truth, that "really" the particles are not (yet) in a coherent state, then it seems that measurement of one "really" affects the state of the other.

I guess you mean incoherent here. They are in a coherent state before a measurement is done.
That depends on the interpretation now.

But the answers I get seem to indicate that prior to measurement of the nearby particle, the distant particle is in a definite state, which is revealed by the local measurement.

That is certainly wrong, unless the distant particle has been measured.

Is that true? Or were both in a state of superposition prior to the first measurement?

I think that was your starting point? That you have entangled particles?
It is possible to have un-entangled photons, but then all those questions are not interesting.

Or is "superposition" an interpretation? Are there any theories which deny that unmeasured particles are in a state of superposition?

I don't think so.

 

[ModusPwnd]

I think you should forget about "truth" and "really" when it comes to science. That is the realm of religion, opinion and wider philosophy. Science is more about observations, models, predictions and descriptions.

Note that "superposition" is a basic concept that applies even where entanglement does not. The simple double slit experiment appeals the the superposition of the particle to explain the interference pattern. Superposition is a key component of quantum theory and it absolutely necessary to make the powerful kinds of predictions and descriptions that quantum theory is able to do. In that sense its just as "real" and any other piece of powerful scientific theory. But again, I would caution against thinking about science along the lines of "truth" and "really".

 

[stevendaryl]

My understanding is that prior to measurement, two entangled particles are in a state of superposition. Neither is, for example, spin-up or spin-down. Instead, their state is that they are BOTH spin-up AND spin-down. ONLY after a measurement are they in a coherent state.

Is that an accepted truth? Or is it a "mere" interpretation of probabilities?

It's pretty much an integral part of the quantum mechanics of spin (it's true of other properties, as well, but it's a little more complicated to show in those cases).

Spin is relative to a direction. The mathematics of spin works this way:

If a particle has spin-up in the z-direction, then it is in a superposition of spin-up and spin-down in the x or y direction.

 

[EskWIRED]

OK. Thanks for all of the replies.

So do i understand correctly that the state of a distant entangled particle is in superposition prior to it's local cousin being measured? And that it is no longer in superposition, but rather, in some definite state after the local measurement?

And do i need to study the concept of superposition more deeply, to disabuse me of the notion that it refers to some real, physical state of matter? Might my problem stem from a basic misunderstanding of what is meant by superpositon?

 

[DrChinese]

And do i need to study the concept of superposition more deeply, to disabuse me of the notion that it refers to some real, physical state of matter?

If it were representative of specific values, then the nature of an observation on a particle here would not affect the correlation statistics for an observation on an entangled particle there. But short of FTL action, Bell's Theorem tells us that there is not a statistical distribution of values that will match experiment in these cases.

 

[golnat]

I think the biggest misconception about entanglement is that the two particles are in these different states. The idea is that they are both part of the same state, and this is why measuring one forces us to understand the observables of the other. Basically, since both particles are considered part of the same state, measuring one will collapse the state and tell you about the other particle as well. QM doesn't answer any old question that seems reasonable from our very classical understanding of the universe.

As for superposition: this is much more fundamental than even physics. It comes about from the fact that two solutions to a differential equation may be added to yield yet another solution. This is the principle behind a lot of the math that is used to develop QM and many other branches of physics.

 

[bhobba]

Neither is, for example, spin-up or spin-down. Instead, their state is that they are BOTH spin-up AND spin-down.

Superposition is built right into the foundations of QM - its about as accepted as you can get.

However semantics like what you expressed above most certainly are not. In QM saying a quantum system has any property outside a measurement context is not really what the theory is about - its about the results of measurements, observations, etc - not what it is otherwise. Saying it is both spin up and spin down is not correct - it doesn't have properties like that until measured.

Don't worry too much though - it takes a while to get used to this type of thinking - I have read a LOT of books on QM and thought I knew this stuff pretty well until I posted on this forum - I fell into similar semantic problems saying things like particles are literally in two positions at once. It was wrong, and I needed to be corrected. It takes a little while and practice to think correctly regarding QM.

Keep at it - slowly but surely what's going on will dawn - as much as its possible to understand what going on in QM is possible anyway.

Thanks
Bill

 

[Fiziqs]

My understanding is that prior to measurement, two entangled particles are in a state of superposition. Neither is, for example, spin-up or spin-down. Instead, their state is that they are BOTH spin-up AND spin-down. ONLY after a measurement are they in a coherent state.

At the risk of being called out for technical inaccuracies, I often hear people question how something can be in two states at once. How can it be both spin up and spin down at the same time? Or a zero and a one, as in quantum computers. I like to point out the wave-particle duality of matter. People are generally accepting of the fact that light can act like either a particle or a wave. They don't have a problem with that. Well electrons and other things can act like waves too. A particle has a definite state, but a wave doesn't. Have you ever seen a wave that was all peaks and no troughs? If it ain't got both, it ain't a wave. A wave by definition is both up and down at the same time.

So if you can imagine that a particle can at times act like a wave, then perhaps it would be easier to understand, how it can be in two opposite states at once. That's what makes it a wave. With entangled particles, if you measure one particle to be up, the other will always be down. So the idea of superposition isn't so difficult to understand, if you just think of the particle as acting like a wave.

I realize that I've probably only made things more confusing, and that I'm not really giving the correct explanation of a particle in superposition, but the idea might be helpful to some of us less educated people.

 

[StevieTNZ]

Given the superposition state |state>=|O> + |E> (|O> photon on ordinary path, |E> photon on extraordinary path) [the photon has just come out of a birefringent crystal] I state what GianCarlo Ghirardi says on what superposition is:

the assertion "the photon is in the superposition |O> + |E> is logically different from all the following statements:
1. the photon propagates itself along path O or along path E or;
2. the photon follows both O and E or;
3. the photon follows other paths.

You can tell why 2 is true. The photon exists as a potentiality, not an actuality, in both paths. Its not acting as a particle located in some place in the universe. It doesn't exist in the universe as a particle.

That might clarify the issue.

 

[bhobba]

A particle has a definite state, but a wave doesn't.

When a quantum object is LIKE a wave (meaning it is wavelike when expanded in the basis relevant to the context being examined) it is in just as definite a state as when it isn't.

A state is definite - its expansion in a certain basis is measurement context dependent.

I know you mention you may not be technically accurate, but it must be said a lot of semantic confusion surrounding this stuff makes matters worse in trying to understand what's going on. Even though its difficult I believe it necessary to be accurate.

Thanks
Bill

 

[EskWIRED]

Given the superposition state |state>=|O> + |E> (|O> photon on ordinary path, |E> photon on extraordinary path) [the photon has just come out of a birefringent crystal] I state what GianCarlo Ghirardi says on what superposition is:


You can tell why 2 is true. The photon exists as a potentiality, not an actuality, in both paths. Its not acting as a particle located in some place in the universe. It doesn't exist in the universe as a particle.

That might clarify the issue.

It is often stated that photons (for example) are both waves and particles. It seems to me that while they sometimes mimic the objects of these concepts, and while they sometimes behave in a manner similar or identical to these objects, they nevertheless cannot possibly be either waves or particles. I say this because in important respects, such objects do NOT behave like a wave or a particle would behave.

Is it at all useful to conceptualize things this way? That they are NOT waves and that they are NOT particles, but something else altogether? Will this mode of analysis lead to insight or confusion?

 

[ModusPwnd]

That's the mainstream interpretation, they they are not waves nor particles but something else entirely that acts a little like each. When you say you often hear that photons are both waves and particles that is probably somebody being ignorant or lazy. You generally won't hear that in scientific circles.

 

[bhobba]

It is often stated that photons (for example) are both waves and particles. It seems to me that while they sometimes mimic the objects of these concepts, and while they sometimes behave in a manner similar or identical to these objects, they nevertheless cannot possibly be either waves or particles. I say this because in important respects, such objects do NOT behave like a wave or a particle would behave.

Is it at all useful to conceptualize things this way? That they are NOT waves and that they are NOT particles, but something else altogether? Will this mode of analysis lead to insight or confusion?

It is not a classical wave or classical particle. I will repeat it again - it is not a wave or particle in the classical sense.

It is a quantum object - that's it - that's all. If it has the observable property of a definite position then it is considered a particle - but a quantum particle.

It's a strange but true fact that all of QM is derivable from just two axioms - you can find the detail in Ballentine:
https://www.amazon.com/dp/9810241054/?tag=pfamazon01-20
http://www-dft.ts.infn.it/~resta/fismat/ballentine.pdf

The so called wave particle duality follows from Schrodinger's equation which follows from the Galilean POR - exactly as classical mechanics is derived from Galilean Relativity and the Principle Of Least Action (PLA). The two axioms of QM imply the PLA, so really non relativistic QM is based on exactly the same fundamental principle as Classical Mechanics. The essence of QM is not the wave particle duality - its the two axioms detailed in Ballentine.

Thanks
Bill

 

[bhobba]

That's the mainstream interpretation, they they are not waves nor particles but something else entirely that acts a little like each. When you say you often hear that photons are both waves and particles that is probably somebody being ignorant or lazy. You generally won't hear that in scientific circles.

Indeed.

Check out the FAQ - it comes up often enough to have its own entry:
https://www.physicsforums.com/showthread.php?t=511178
'So there is no duality – at least not within quantum mechanics. We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn't mean that in physics, or in the working of physicists, such a duality has any significance.'

What we think of as the wave-particle duality (ie that in certain physical situations wavelike solutions naturally occur) in fact follows from more fundamental principles that lie at the foundation of QM.

As mentioned previously you will find the detail in Ballentine.

I also want to point out its easy to be confused by this stuff - even people who have taken undergraduate courses in QM from books like Griffiths (and it is a good book - just a bit pricey) get confused. What's really going on is found in more advanced books like Ballentine - and even then you have to think a bit - he doesn't spell it all out - but that's the way with more advanced textbooks in any subject - they assume - what's the word used - maturity . Translation - you need to persevere and think.

Thanks
Bill

 

[vanhees71]

One cannot stress enough that particularly photons are a very bad example to start to learn quantum mechanics, because as a massless quantum with spin 1, it's more complicated than massive quanta. There is not even a position operator for photons in the strict sense!

"Wave-particle duality" is an old-fashioned idea which is obsolete for nearly 90 years now and should not be taught anymore. Unfortunately, many textbooks start with photons in an old-fashioned inadequate way and with "wave-particle duality" for electrons. This then sticks with the students's minds and one has a hard time to forget this wrong ideas again, when learning about modern quantum theory later.

 

[bhobba]

One cannot stress enough that particularly photons are a very bad example to start to learn quantum mechanics, because as a massless quantum with spin 1, it's more complicated than massive quanta. There is not even a position operator for photons in the strict sense!

"Wave-particle duality" is an old-fashioned idea which is obsolete for nearly 90 years now and should not be taught anymore. Unfortunately, many textbooks start with photons in an old-fashioned inadequate way and with "wave-particle duality" for electrons. This then sticks with the students's minds and one has a hard time to forget this wrong ideas again, when learning about modern quantum theory later.

Indeed.

But guess what - that's the way its taught in the first brush of typical HS courses on it. That was my first exposure. Then I read this book called - In Search Of Schrodinger's Cat. It raised more questions than it answered but said Dirac and Von Neumann was the books to answer them. Right - good books for sure - and I learned a lot - but for me, having a math rather than physics background, it raised all sorts of issues like what the hell is this Dirac Delta function and why the hell weren't physicists using Von Neumann's approach which sent me down the road of Rigged Hilbert spaces and what not. I emerged the other side after sorting it out and then came across Ballentine and was impressed - very impressed. Everything was clear. But its a graduate text.

IMHO what we need is a undergrad and even high school version of Ballentine - anyone want to step up to the plate?

Thanks
Bill

 

[Naty1]

some really good insights already posted!

My understanding is that prior to measurement, two entangled particles are in a state of superposition. ... their state is that they are BOTH spin-up AND spin-down. .

Is that an accepted truth? Or is it a "mere" interpretation of probabilities?[/QUOTE]

it's a 'mere' interpretation of the math; what is real is what we measure...the rest is math, our best model of what we think underlies the measurement. We measure particles; the underlying fields and virtual particles, for example, are mathematical theory.

The famous quote still reigns: "SHUT UP AND CALCULATE' ...We may disagree on what the all the math means, but we can arrive at the proper probabilistic quantum answer...

Quantum entanglement refers to the mathematics of QM. What it 'means' remains a mystery so far without complete explanation. The math explains what we observe, not precisely why.

 

[EskWIRED]

Quantum entanglement refers to the mathematics of QM. What it 'means' remains a mystery so far without complete explanation. The math explains what we observe, not precisely why.

Is anyone besides me less enamored with uantum physics the more one learns about it?

I read secondary sources and short papers by the originators, and I get fascinated. I think of the implications and mysteries revealed by the experiments. I put two and two together and search for understanding.

And then, when I get to the crux of the matter, people who study this stuff and who have mastered it tell me that nobody has a clue as to what is going on, and that the entire discipline is centered solely on calculating numbers with no real understanding of any underlying mechanisms and no real insight into what any of it means.

My interest in physics stems from wondering about the nature of reality - both our perceived consensual reality and any underlying reality that escapes our senses.

But with uantum physics, it seems, those who are in the trenches care little about, and are uick to point out that they know nothing about, any underlying reality that escapes our measurements.

I dunno. I'm increasingly disappointed. Thanks for listening.

 

[Nugatory]

But with uantum physics, it seems, those who are in the trenches care little about, and are uick to point out that they know nothing about, any underlying reality that escapes our measurements.

Dude, get that "q" key fixed

My perspective, which may be somewhat influenced by a side interest in the history of science:
Being quick to point out that we don't understand the underlying reality is, I think, well-placed humility. It signifies not that people don't care about it but rather that the people who know the most know how much we still don't know. Indeed, I view the enormous volume of work on interpretations of quantum mechanics, produced by some of the top thinkers of the past century, as very compelling evidence that people care, a lot.

It's true that when we're using QM as a predictive tool in day-to-day work, we tend to be a bit impatient with discussions of interpretations. But again I don't see this as evidence of disinterest in the problem so much as disinterest in the conversation when no one has anything new to say. After all, when someone does come up with a major insight so there is something new to say, attention and excitement is quickly rekindled. We saw this happen when the Copenhagen interpretation was first solidified, when Bell discovered his theorem (and stimulated a wave of experimentation), when decoherence was discovered. There's no reason to expect that we're at the end of that road.

 

[craigi]

I think you're right to be unsatisfied with quantum physics. The fact that it's a piece of fundamental physics, with incredible predictive power, but no one really knows what it means, is a serious problem.

I disagree with an earlier poster. The underlying reality is what inspires us to extrapolate a model and to know its bounds. The fact that no one really understands quantum physics manifests itself just there. We don't really know where quantum physics ends. We presume that it just fades out when we get outside the domain of the really small with the exception of a few special, carefully prepared cases. That's completely unsatisfactiry in a theory. We should be able to reach a good understanding of exactly where it's applicable and why it convegeres to something simpler in the other cases.

There is no universally accepted interpretation of the maths. The Copenhagen interpretation is the one that I believe is still most frequently taught, but has problems in that it can't explain what it is that constitutes a measurement. How can we be satisfied with that? "It's like this", "but what does that mean?", "shut up about that". You wouldn't accept that of any other theory so why accept it of quantum physics?

 

[stevendaryl]

I think you're right to be unsatisfied with quantum physics. The fact that it's a piece of fundamental physics, with incredible predictive power, but no one really knows what it means, is a serious problem.

Well, the perplexing thing about quantum mechanics is that pursuing a deep understanding of what it means (A) doesn't seem to accomplish anything, and (B) doesn't seem to be necessary. One might expect that if there is some area of science that is poorly understood, that that would leave an opening for a breakthrough that can lead to greater understanding and better, more predictive, theories. But in the case of quantum mechanics, we don't lack for predictive power---as a matter of fact, we have a really hard time finding situations where our current theories don't already make accurate predictions (the extremes of high energy and huge distances and very strong gravity).

So even though the foundations of quantum mechanics are very murky, there is not a lot of reward for making them clearer.

 

[DennisN]

But again I don't see this as evidence of disinterest in the problem so much as disinterest in the conversation when no one has anything new to say. After all, when someone does come up with a major insight so there is something new to say, attention and excitement is quickly rekindled.

That is my impression too. I'm not a physicist, but I try to follow the QM branch - and other physics branches - as good as I can, and my impression is quite like what Nugatory said.

The fact that it's a piece of fundamental physics, with incredible predictive power, but no one really knows what it means, is a serious problem.

Sort, of perhaps, but I would probably not call it a "problem", but "interesting". We could put quantum mechanics in perspective and compare it with, let's say, gravitation; Newton's gravitation from ca 1687 was something which implied a bizarre instantaneous action-at-a-distance mechanism, which nobody really knew what it meant/how it worked. Nevertheless, it was the leading model (and successfully used) until General Relativity appeared in 1916 - and it's still used! And as the case is today we could say we don't really have a full understanding of gravitation (singularity issues, short distances issues, no complete/experimentally confirmed theory of quantum gravity etc.). My point is that this "problem" is not unique to quantum mechanics.

You wouldn't accept that of any other theory so why accept it of quantum physics?

I can't say I agree with this. In science it's usual that we work with theories/models within their domain of applicability without demanding a deeper understanding of "why they work?" or "how do things really work beyond this?". Some of the current important frontiers of physics are quantum field theory/the Standard Model and general relativity. We have to accept these as the working theories/models until we have (theoretical and observational) reasons to believe otherwise. That is how science works.

 

[mfb]

Physical theories are never meant to describe how reality actually is, this is nothing special for quantum mechanics. It is just a bit more unintuitive there, as the theory does not come with an obvious way how the world could "really be".

Newton proposed a universal law of gravitation, without any explanation "why" or "how" this force would act. Einstein formulated general relativity based on a curved spacetime - that does not mean that spacetime has to "be" curved - it is just a very good model, the predictions are in agreement with the experiments.

 

[bhobba]

Is anyone besides me less enamored with uantum physics the more one learns about it?

I think people go through various phases.

First is how can nature be like that. Part of the reason for that is at the beginner level of say a typical high school physics course what's actually happening is not explained too well. For example they talk about the wave particle duality which when you go deeper into it is not a duality at all - it's neither classical particle or wave - simply quantum stuff - but we all must start somewhere.

The next phase is you study books like Griffith that teach the mathematical machinery and how to apply it but skirts what's going on.

The final phase is you study a proper book like Ballentine - Quantum Mechanics - A Modern Development. Here you will find a correct axiomatic treatment emphasizing what it actually means. That is when you get the proper understanding. For example you see its a theory about what measuring apparatus (in a general sense) tells us about the quantum world. Many of the misconceptions boils down to understanding this rather simple observation - but really understanding it.

It takes time, you can't really shorten it, and even at the end of it issues remain, but you reach an accommodation where its not quite as mysterious as it was at the beginning.

If you are just starting out I suggest two sources - Feynmans - QED - The Strange Theory Of Light And Matter and Hughs - Structure And Interpretation Of Quantum Mechanics. But that is just a start - you should work your way up to a book like Ballentine.

But with uantum physics, it seems, those who are in the trenches care little about, and are uick to point out that they know nothing about, any underlying reality that escapes our measurements.

You really need to study Ballentine. He goes very deeply into its meaning and actually reaches a rather interesting conclusion - Einstein was right - Bohr was wrong. But this is what Einstein ACTUALLY believed - not the stuff reported in the popular press. Intrigued - study the hook - its worth it.

After that - and it will take you a while to get through it - study Decoherence and the Quantum-to-Classical Transition by Schlosshauer:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Its not a book about interpretations or the meaning of QM per-se but about decoherence that has had a big influence on that question in modern times and in explaining that discuses its interpretive implications.

As far as what QM actually means goes there are a number of highly qualified posters on this forum who know about and are deeply interested in that issue. But to really engage them its necessary to have the background of a book like Ballentine. BTW - I am not one of them - I am just a retired guy with a backgound in math interested in this stuff.

Thanks
Bill

 

[bhobba]

There is no universally accepted interpretation of the maths. The Copenhagen interpretation is the one that I believe is still most frequently taught, but has problems in that it can't explain what it is that constitutes a measurement. How can we be satisfied with that? "It's like this", "but what does that mean?", "shut up about that". You wouldn't accept that of any other theory so why accept it of quantum physics?

Well at least you understand the REAL issue with Copenhagen - it accepts the existence of measurements and of a classical common sense world they reside in - but since that world is all quantum how does a theory that assumes such to explain what that 'such' is composed of actually accomplishes such a lifting yourself up by your bootlaces feat is a BIG issue - I hasten to add it doesn't invalidate it or anything like that - but it is a major blemish that should be corrected.

People are not satisfied with it - research is ongoing and progress has been made - eg decoherence.

But you must understand not everyone is interested in foundational issues. And one of the annoying things I find as a person interested in discussing such things is to discuss it properly you need be familiar with more than the kiddy versions of QM presented in the popular press, and even in some undergraduate texts. For axample when I speak of a state in QM I mean a positive operator of unit trace as implied by Gleason's theorem - yet many still think a state is an element of a vector space - it isn't really - but less advanced texts (I call them at the kiddy level) aren't too forthcoming at explaining that.

Those that often want to discuss it seem to be more interested in the philosophy of it. But IMHO the issue is not philosophical - its understanding what QM says in the first place - which is why I always recommend Ballentine - he explains it very well - as well as the crux of the whole interpretational issue. For example its really based on two axioms - but the interesting thing is the second axiom more or less follows from the first via Gleasons theorem (strictly speaking it doesn't - but seeing why requires some deep thought tied up with non-contextuality - still it makes it seem very very reasonable). So basically you really need to come to grips with one axiom - and that's it. Therein lies the crux of all quantum weirdness. Yet few seem aware of it.

Thanks
Bill

 

[craigi]

I can't say I agree with this. In science it's usual that we work with theories/models within their domain of applicability without demanding a deeper understanding of "why they work?" or "how do things really work beyond this?". Some of the current important frontiers of physics are quantum field theory/the Standard Model and general relativity. We have to accept these as the working thieories/models until we have (theoretical and observational) reasons to believe otherwise. That is how science works.

This really depends upon what we mean by 'accept'. We accept the predicitive power of the model but I think anyone learning quantum physics should be unsettled by its lack of completeness. We can convince ourselves that it doesn't matter, but exploring how to complete the theory not only has value in itself, it tells us where to look for new phenomena that aren't yet well explained by the model.

All these models were born out of a dissatisfaction with our current understanding of reality. To wish that away when we get to quantum physics is defeatist, particularly when it's the most unsatisfactory, yet successful theory ever produced by physics.

 

[bhobba]

This really depends upon what we mean by 'accept'. We accept the predicitive power of the model but I think anyone learning quantum physics should be unsettled by it's lack of completeness. We can convince ourselves that it doesn't matter, but exploring how to complete the theory not only has value in itself, it tells us where to look for new phenomena that aren't yet well explained by the model.

Yes and no.

All the stuff you said above is debatable ie even if it is or is not complete.

That's the real issue - its not really science because science is based on correspondence with experiment and lacking that is really philosophy which by its very nature is a quagmire of endless debate that resolves nothing.

Thanks
Bill

 

[craigi]

Yes and no.

All the stuff you said above is debatable ie even if it is or is not complete.

That's the real issue - its not really science because science is based on correspondence with experiment and lacking that is really philosophy which by its very nature is a quagmire of endless debate that resolves nothing.

Thanks
Bill

I think it's better descibed as theoretical physics than philosophy, but it's not too long ago that scientists were popularly described as natural philosophers.

 

[bhobba]

I think it's better descibed as theoretical physics than philosophy, but it's not too long ago that scientists were popularly described as natural philosophers.

Hmmmm. Maybe.

I think theoretical physics is more along the lines of what you would find in say a book like Landau's Mechanics. It's a supremely elegant and beautiful way to treat mechanics and those who are natural theoretical physicists are enamoured - it doesn't do anything different than you will find in your typical mechanics textbook, but its treatment is strikingly beautiful and elegant. Those of a theoretical bent often react to it along the lines of the following unashamed review I will post because I think it illustrates what really is the crux:

'If physicists could weep, they would weep over this book. The book is devastatingly brief whilst deriving, in its few pages, all the great results of classical mechanics. Results that in other books take take up many more pages. I first came across Landau's mechanics many years ago as a brash undergrad. My prof at the time had given me this book but warned me that it's the kind of book that ages like wine. I've read this book several times since and I have found that indeed, each time is more rewarding than the last.

The reason for the brevity is that, as pointed out by previous reviewers, Landau derives mechanics from symmetry. Historically, it was long after the main bulk of mechanics was developed that Emmy Noether proved that symmetries underly every important quantity in physics. So instead of starting from concrete mechanical case-studies and generalising to the formal machinery of the Hamilton equations, Landau starts out from the most generic symmetry and dervies the mechanics. The 2nd laws of mechanics, for example, is derived as a consequence of the uniqueness of trajectories in the Lagragian. For some, this may seem too "mathematical" but in reality, it is a sign of sophisitication in physics if one can identify the underlying symmetries in a mechanical system. Thus this book represents the height of theoretical sophistication in that symmetries are used to derive so many physical results.'

If you react like that then you are of a theoretical bent. BTW its how I reacted - I was struck dumb by that book.

If you are of a theoretical bent you realize when something has cut to the heart of the issue. Landau did in Mechanics. Its the same when you are exposed to GR. You realize even though it is mathematically much more sophisticated than Newtonian gravity it cut to the heart of the issue and many are also enamoured by its supreme beauty. In fact Landau believed that was the true test of if you are a theoretical physicist - did you immediately recognize GR for the extremely beautiful and elegant theory it is.

IMHO we have a bit to go before the same can be said of QM - but progress has been made.

Thanks
Bill

 

[DennisN]

[...] but exploring how to complete the theory not only has value in itself, it tells us where to look for new phenomena that aren't yet well explained by the model.

Sounds ok to me. Particularly if it's made in combination with suggestions on how to test for new phenomena (experiments).

All these models were born out of a dissatisfaction with our current understanding of reality. To wish that away when we get to quantum physics is defeatist, particularly when it's the most unsatisfactory, yet successful theory ever produced by physics.

Dissatisfaction? Hmm, well, quantum mechanics was born out of experiment observations that did not fit the models at that time (e.g. blackbody radiation (the "ultraviolet catastrophe") and the photoelectric effect). So it wasn't a case of any philosophical dissatisfaction - it was scientific dissatisfaction; the observations did not fit the models.

Defeatist?

I do not wish anything away, I am far from a defeatist, haha, I believe in the progress of science; there are many things to explore and solve - not only in quantum mechanics. What I tried to explain was that this "problem" you mentioned is not unique to quantum mechanics. And please note: there is currently NO theory of everything. This automatically means that there are "problems" in all branches of physics (they have domains of applicability). And I think it is quite likely - considering the history of science - that if/when, let's say, quantum mechanics or general relativity get replaced by some new theory - guess what - there will be "problems" and issues with this new theory as well (but I can't be certain of this, of course), that's my 2 cents.

And, once again, I think it would be interesting to hear Richard Feynmans words (in the clip I mean specifically 0:00-1:30 and 3:20-4:10 concerning his views on science and nature in general, and doubt and uncertainty):

https://www.youtube.com/watch?v=3zi699WzAL0

 

[craigi]

Sounds ok to me. Particularly if it's made in combination with suggestions on how to test for new phenomena (experiments).



Dissatisfaction? Hmm, well, quantum mechanics was born out of experiment observations that did not fit the models at that time (e.g. blackbody radiation (the "ultraviolet catastrophe") and the photoelectric effect). So it wasn't a case of any philosophical dissatisfaction - it was scientific dissatisfaction; the observations did not fit the models.

Defeatist?

I do not wish anything away, I am far from a defeatist, haha, I believe in the progress of science; there are many things to explore and solve - not only in quantum mechanics. What I tried to explain was that this "problem" you mentioned is not unique to quantum mechanics. And please note: there is currently NO theory of everything. This automatically means that there are "problems" in all branches of physics (they have domains of applicability). And I think it is quite likely - considering the history of science - that if/when, let's say, quantum mechanics or general relativity get replaced by some new theory - guess what - there will be "problems" and issues with this new theory as well (but I can't be certain of this, of course), that's my 2 cents.

And, once again, I think it would be interesting to hear Richard Feynmans words (in the clip I mean specifically 0:00-1:30 and 3:20-4:10 concerning his views on science and nature in general, and doubt and uncertainty):

https://www.youtube.com/watch?v=3zi699WzAL0

Yet it was Feynman himself who described the measurement problem as the central mystery of quantum physics.

 

[DennisN]

Yet it was Feynman himself who described the measurement problem as the central mystery of quantum physics.

And your point is...?

 

[bhobba]

And your point is...?

Same here.

But even aside from that I am not so sure that was Feynman's view - it most certainly was that the double slit experiment contained the central mystery - but that it was the so called measurement problem can't recall him ever saying. IMHO that isn't the central mystery because every interpretation has a different take on it - the central mystery is we have so many interpretations, each suck in their own unique and different way, and we have no way to decide experimentally between them.

I do know later on in life Feynman was very attracted to Decoherent Histories as championed by the guy in the office next door - Murray Gell-Mann. Feynman evidently would sit in the back of lectures on it and ask some very illuminating and penetrating questions about it that showed he understood it only too well.

Thanks
Bill

 

[DennisN]

- the central mystery is we have so many interpretations, each suck in their own unique and different way, and we have no way to decide experimentally between them.

Yes, this is one of the main points which makes it interesting to me too . By the way, I really don't know very much about Feynman in detail, but I've noticed I very much agree with his general approach to science.