Justification for no properties before measurement

[lightarrow]

What I’m looking for is something like a knock-down reply to the objection: ‘It’s just a matter of ignorance. Of course there are properties, we’re just unable to access or observe them.”

Sorry, I haven't read all the post after this so don't know if it's already been written.
I think your request is, in general, impossible, but you could answer something like what writes Griffiths at the beginning of his QM book: consider a particle in a state described by a wavefunction which is, essentially, a train of waves along some spatial directions, with a lot of cycles: which is the "position" of this train of waves? You could take the beginning of the train as well as the end or any other value in between.

--
lightarrow

 

[mikeyork]

Conclusion: there are NO SUCH POSSIBLE VALUES. Ignorance has nothing to do with it, you can't even make up values by hand that make it work.

Yes, there is a huge distinction between ignorance and unknowability. A quantum superposition represents the latter. Suppose someone else had already projected a superposition onto their chosen basis and thereby measured an observable. A subsequent observer may still be ignorant of this result, but the result is now knowable by communication with the other observer and any subsequent measurement must be consistent with the original.

 

[Mentz114]

Ok thank for the answer. For my understanding.

The corresponding quantum angular momentum L is also called spin angular momentum !?

View attachment 217960
Best regards
Patrick

Have a look at this article about intrinsic spin.

https://en.wikipedia.org/wiki/Spin_(physics)

and

http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html

 

[Fra]

I do have a physics background, but would like to be as non-technical as possible, hence my asking the question very simply and plainly (also, that’s a good exercise, I think, for anyone: to have the philosophical foundations clear in your head). The reason is, as you have guessed, my audience. I’m not sure about what amount of the math formalism of QM they’re familiar with.

I see, that sounds good that you have a physics background.

I have spent enough time on the philosophical foundations myself, so i have my own views quite clear in my head.. And I agree its important. But to convey it so someone else is a different challenge.

I appreciate your help very much. If you don’t mind my asking, though, why do you put “philosophy training” in quotes?

Good question :) The truth is that its a bad habit of mine, i find myself often put lots of things in quotation even when it doesn't always make sense. Sorry about this.

But i think what i meant in this case is that philosophical training could mean two things to me, either its simply that spend time on the thinking about foundations in a philosophical manner and train yourself to analyze things (I think physicists does this too to some extent), and then there is the philosophical training that means studying traditional philosophy as whatever they do at philosophy departments, where the emphasis isn't the physics but the process of analysis.

I have also a physics background but i never ever formally studied any philosophy whatsoever. Except I read some random books, like poppers terrible book, some books on history of probability theory. My own philosophical contemplation has arised and been driven strictly from open problems and interpretational issues in physics. This is why my experience that the "philosophy of physics" as per physicists which are then formally "amateur philosophers" are quite different from that of professional philosophers tha. But I think you can get a good combo if you combine background in both.

I actually remember one old physics teacher i had, I had him in both analytical mechanics and then in QM1 or QM2 (dont remember), and while obviously beeing a physicist he also had a formal background in philosophy (ie. having studied philosophy at philosophy department) and he had a distinguished ability to understand fuzzy questions in a way that some other teachers that were more narrow minded couldn't. He couldn't necessarily ANSWER the questions as they were admittedly open questions, but he did understand and acknowledge the questions, that others either didn't understand or pretended to not understand. The latter thing is a very bad thing todo to students. Some researchers may be clever but may be less suitable to teach. I have even experience cases where teachers almost get offended when getting deep questions from undergraduates that they could not handle as some even said straight out "an undergraduate are not supposed to ask these things" etc.

/Fredrik

 

[romsofia]

I'm in the same place you are currently, doing a PhD program and am questioning a lot of things fundamentally. I'm a relativist at heart, and I know that "hidden variable" theories are pretty much dead, and that's just something I'll have to accept currently, but I digress.

The best answer that I've found is that "quantum information" is not coupled to the environment. As I understand it, it's once you MAKE a measurement does this "pure" state become "mixed" with the environment. The only way for me to believe this, is to believe that the states DO have some inherent information, but until we actually act on it, we can't say what it is. I don't like it, but that's life. Physics doesn't care if I like something or not, it is what it is.

Now, the last part is the only way I can, personally, come to terms with it. If there is no inherent information, and we act on it, the state has to be ready to show me what it knows, right? Otherwise, it'd be saying the information comes from no where.

I think to explain it to a non-physics audience would be to relate it to cards. You know that if you deal someone cards, it will be of 4 suits, this or that, etc. Similar to particles, we know when dealing with certain particles they will follow these properties. Just like in cards, however, whether the person has a jack, a queen, etc won't be known until we ask. This can be related to quantum mechanics, we won't know the properties of the state of the particle until we make a measurement. I think the key idea for them to understand is that until you act on a state, you won't know. But to say there is no inherent information is just silly to me the more I think about it.

 

[Quandry]

Measurements are the evidence, so the position that particles don't have properties before they're measured is simply sticking to the evidence and not going beyond it at all. So asking for evidence of a position that just amounts to believing nothing beyond the evidence doesn't seem right.

"Absence of evidence is not evidence of absence" - Carl Sagan
We should not confuse no evidence of effect and evidence of no effect.
If there are particles in the universe, then they have properties be they known, unknown, or even unknowable.
If there are no particles until some interaction creates them, then they have no prior properties and so cannot be knowable until after the interaction.
We can only state that particles have properties if they have been experimentally observed.
We can only state that they do not have properties if it has been experimentally observed that they do not.
Given that the latter would appear to be impossible to achieve (given the definition of the issue) we cannot state whether particles have properties or not prior to 'measurement'.

 

[PeterDonis]

If there are particles in the universe, then they have properties be they known, unknown, or even unknowable.

We can only state that particles have properties if they have been experimentally observed.

These two statements are not consistent.

We can only state that they do not have properties if it has been experimentally observed that they do not.

How would you experimentally observe that a particle does not have a property? Or, as you say:

Given that the latter would appear to be impossible to achieve

...then it is meaningless to talk about what we could or could not state if it happened.

 

[Quandry]

These two statements are not consistent.

They are not meant to be consistent. They are two separate statements. One says if there are then they exist. The other says that we can only state what we know.

How would you experimentally observe that a particle does not have a property? .

I was not offering a method doing that, and it seems that no-one has worked it out yet,

Or, as you say
...then it is meaningless to talk about what we could or could not state if it happened.

It is never meaningless to talk about what you do or don't know. In this case we know that we cannot know what the state was before we performed the measurement.

Measurements are the evidence, so the position that particles don't have properties before they're measured is simply sticking to the evidence

Evidence resulting from measurements is not valid evidence of the state before measurement.

 

[bhobba]

That is an excellent answer, and a very accessible explication of Bell’s inequality. Thanks.

Yes it is - but also remember the counter example - Bohmian Mechanics. It has exactly those strange properties Dr Chinese talks about. So really you can't win this one - you can simply put up reasonableness arguments.

Thanks
Bill

 

[PeterDonis]

They are not meant to be consistent.

Then which one do you think is right? They can't both be. Or do you think they're both wrong?

 

[PeterDonis]

Evidence resulting from measurements is not valid evidence of the state before measurement.

I didn't say it was.

 

[microsansfil]

Have a look at this article about intrinsic spin.
http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html

Thank. Here an answer of my interrogation : https://physics.stackexchange.com/questions/216216/spin-and-angular-momentum

the Stern-Gerlach experiment shows that spin, like angular momentum, carries a magnetic moment. The conclusion is that the electron's spin is a quantum degree of freedom of the nature of angular momentum that carries a magnetic moment. It characterizes the electron's state independent of its position(or momentum)-dependent wave function, or as you observed, it is intrinsic. The orbital angular momentum, on the other hand, concerns the spatial wave function and is the analog of the classical angular momentum.

Best regards
Patrick

 

[Quandry]

Then which one do you think is right? They can't both be. Or do you think they're both wrong?

The first one cannot be wrong, because it is a statement of if. However, if you mean that in the case that there are protons they could have no properties then that, of course, cannot be true. The very existence defines a need for properties (based on experimental evidence that everything we know that exists has properties).
However, defining a need for properties does not define what these are (although we could make some basic assumptions). To be able to state (empirically) what these properties are, requires that they be experimentally observed. To state empirically that they do not have properties requires that the lack of properties must be experimentally observed.

Both of my statements are true.

The purpose was to encourage the OP to consider that the postulate that "the position that particles have no properties before they’re measured?" assumes evidence that has not been discovered.

 

[Quandry]

I didn't say it was.

Then I misinterpreted your comment

 

[DrChinese]

How would you experimentally observe that a particle does not have a property?

This is a great point in the context of this thread. Clearly, there is no experimental proof that non-commuting properties cannot have simultaneously well-defined values. (You also cannot disprove the existence of God by experiment.)

The "backup plan" for that is the no-go theorems, of which Bell is the best known. Again, we no there are no value sets that fit the predictions of QM. That's as good as it gets.

 

[vanhees71]

Non-commuting observables can have simultaneously determined values, if there is a common eigenvector. An example are the three components of angular momentum $\hat{\vec{J}}$ for $J=0$ (i.e., $\hat{\vec{J}}^2 |J=0,M=0 \rangle =0$). Obviously for this state $\hat{\vec{J}} |J=0,M=0 \rangle=0$.

However, only if the self-adjoint operators representing two variables are commuting, you have a complete orthonormal set of simultaneous eigenvectors and only then we consider these observables as compatible, i.e., for any possible value $a$ of the observable $A$ you can simultaneously also determine the value of observable $B$ to be any possible value $b$. This is, e.g., the case if the system is prepared in the state described by the common eigenvector $|a,b \rangle$ of the operators $\hat{A}$ and $\hat{B}$.

 

[PeroK]

We can only state that they do not have properties if it has been experimentally observed that they do not.
Given that the latter would appear to be impossible to achieve (given the definition of the issue) we cannot state whether particles have properties or not prior to 'measurement'.

In a sense Bell's theorem does provide an experiment to show that certain properties no not exist until they have been measured - by an ingenious use of statistical analysis.

You did say "appear" to be impossible!

 

[itfitmewelltoo]

All too often, people want to jump to the philosophy before having learned the details of experiment, mathematics, and theory. This leads to every manner of absurd statements.

There is an old joke asking the difference between a mathematician and a philosopher. A mathematician needs a pencil, paper, and a waste basket. A philosopher needs only a paper and pencil. (Present company not included, of course).

Feynman has a lecture series in which he does a fairly good job of explaining QM to a general audience.

My sense is that, at a most fundamental level, space and time must be "fuzzy". If it did not, nothing would move, space and time would not exist. If a particle we located at some infinitely specific location in space, it could never then exist in the location at an infitesimal distance away. Somehow, the span of space must connect.

Time, as we understand it, does not exist. It isn't some thing separate from the dimensions of space. In all measures that I am aware of, time is a measurement of distance. We measure time by the occurrence of a periodic event that occurs in some location. The hands of a clock rotate about it's center. A minute is the movement of the second hand about the distance of the circumference. A day is the movement of the Sun from a point in the sky to the same place again as the Earth makes one revolution on it's axis. In every measure of time is a measure of some periodic motion. Time is simply that things have changed. The existence of time as some thing, distinct in it's existence from the objects and the extent that they inhabit is an illusion of how we conceptualize our environment.

Those that presume to philosophize about physics, before having learned the measures and mathematics of physics, also tend towards a certain "absoluteness" in their perception. They see there to be some underlying absolute amd fundamental properties that explain the why of the universe. In the larger body of physics, there are no fundamental properties. Everything in physics is a comparison of one thing to another thing. Physics is a collection of correlations of one thing to something else. At some point, reality is simply irreducible. There are no further underlying properties of hidden variables to further explain "why?".

So, as I see it, at the finest level to which space is, point A is simply not distinguishable from point B. A point A, at x meters cannot exist as distinct and separate from a point B at x+0.000000000001 meters. Heisenberg uncertainty are actually coupled pairs so this is may be oversimplification. Never the less, points A and B cannot be distinct because if this were so, an object at point A could not move to point B. Points in space, A and B, are connected. We know they are connected. We can move from one place to the next. Quantum uncertainty is simply the natural outcome of space having an extent.

That being said, I have a trash can around here somewhere.

 

[Quandry]

Clearly, there is no experimental proof that non-commuting properties cannot have simultaneously well-defined values. (You also cannot disprove the existence of God by experiment.)

I am not sure that this statement is true in all cases (the first part, that is). I am sure that an experiment can be designed to show that any property that requires time as part of its definition cannot be measured without taking time to do so, hence ensuring that simultaneity (dictionary definition) cannot be achieved.

Not disagreeing, just commenting.

 

[PeterDonis]

My sense is that, at a most fundamental level, space and time must be "fuzzy".

This is a common speculation, but we have no evidence one way or the other, and there is no current theory with this property--trying to find such a theory and make it testable is a primary motivation for current research in quantum gravity. In the absence of evidence or theory, there's not much we can discuss about it.

 

[bahamagreen]

First question: what is the reason/evidence, EXACTLY, for the position that particles have no properties before they’re measured? I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen? I’m looking for an answer that I could present to, say, a roomful of non-physics people.

EXACTLY? :)

Perhaps those here that know more may answer whether this analogy is OK for "a roomful of non-physics people"... I offer it "as is" so feel free to comment and mark it up...

Fourier had a mathematical insight that waves might be decomposed into a collection of sine waves.
Fourier transformation is the name for this operation. Fourier synthesis is the inverse (using waves to compose another wave).

The wave forms used for decomposition and synthesis don't have to be sine waves; they may be any arbitrary wave themselves.
Sine waves are used because the math is less complex, but a wave may be decomposed into sine waves, triangle waves, saw-tooth waves, etc.
Likewise, sine, triangle, saw-tooth, etc. waves may be used to compose a wave.

The point here is that when one decomposes a wave, one has to chose a "measurement" wave, usually a sine wave.
If you chose the sine wave you are measuring the "sinewaveness" of the subject wave.
If you chose a square wave or impulse wave, you are measuring the subject wave's "squarewaveness" or "impulsewaveness".

The properties or attributes of the subject wave are depending on the "measurement" wave selected to decompose the subject wave.
The choice of measurement wave is like asking a specific question, and getting a particular answer.

The choice of measurement wave is the equivalent of the experimental design and conditions - determining the property measured.
The simple measurement waves allow for experiments that can be performed and properties that can be understood.
(As if sine waves measure momentum and impulse waves measure position - simple waves and simple properties).

There is an infinite list of possible measurement wave forms for which no possible real experimental design and construction can be imagined.
And the results would be the measurement of an indefinitely complex unimaginable abstract property of the subject wave.

So, if the subject wave has all potential decompositions (properties) corresponding to all potential measure wave selections (tests)...

Does it hold all potential properties waiting to be tested?
Does it have no properties until tested?
Does it have no properties; all properties being within the experimental setup?
Will the classical philosophical foundation meaning of "properties" need to be reviewed or updated?

 

[bhobba]

So, as I see it, at the finest level to which space is, point A is simply not distinguishable from point B. A point A, at x meters cannot exist as distinct and separate from a point B at x+0.000000000001 meters. Heisenberg uncertainty are actually coupled pairs so this is may be oversimplification. Never the less, points A and B cannot be distinct because if this were so, an object at point A could not move to point B. Points in space, A and B, are connected. We know they are connected. We can move from one place to the next. Quantum uncertainty is simply the natural outcome of space having an extent.

Your post has so many misconceptions I don't even know where to begin. But let's start with real numbers. Between any two real numbers lies another real number. A point can exist as distinct, separate and between one .000000000001 meters away. It's basic math. And QM places no limit whatsoever on the accuracy you can measure a particles position - but that is getting off topic and needs a thread to discuss exactly what the Uncertainty Principe says.

Time as we know it does not exist? In all measures you are aware it its simply a measure of distance - well we do have this thing called an atomic clock with a digital readout.

I think we pretty well know what time is:
http://www.informationphilosopher.com/solutions/scientists/feynman/past_and_future.html

Simply its what a clock measures - and as explained above what it measures is basically the tendency of things to go from order to disorder because there are many more disordered states than ordered ones. Entropy goes one way - and that is time.

There are many more misconceptions - but really it needs its own thread - so if you want to discuss it start a new one and try to stay on topic in this one.

Thanks
Bill

 

[ggraham76]

EXACTLY? :)

Perhaps those here that know more may answer whether this analogy is OK for "a roomful of non-physics people"... I offer it "as is" so feel free to comment and mark it up...

Fourier had a mathematical insight that waves might be decomposed into a collection of sine waves.
Fourier transformation is the name for this operation. Fourier synthesis is the inverse (using waves to compose another wave).

The wave forms used for decomposition and synthesis don't have to be sine waves; they may be any arbitrary wave themselves.
Sine waves are used because the math is less complex, but a wave may be decomposed into sine waves, triangle waves, saw-tooth waves, etc.
Likewise, sine, triangle, saw-tooth, etc. waves may be used to compose a wave.

The point here is that when one decomposes a wave, one has to chose a "measurement" wave, usually a sine wave.
If you chose the sine wave you are measuring the "sinewaveness" of the subject wave.
If you chose a square wave or impulse wave, you are measuring the subject wave's "squarewaveness" or "impulsewaveness".

The properties or attributes of the subject wave are depending on the "measurement" wave selected to decompose the subject wave.
The choice of measurement wave is like asking a specific question, and getting a particular answer.

The choice of measurement wave is the equivalent of the experimental design and conditions - determining the property measured.
The simple measurement waves allow for experiments that can be performed and properties that can be understood.
(As if sine waves measure momentum and impulse waves measure position - simple waves and simple properties).

There is an infinite list of possible measurement wave forms for which no possible real experimental design and construction can be imagined.
And the results would be the measurement of an indefinitely complex unimaginable abstract property of the subject wave.

So, if the subject wave has all potential decompositions (properties) corresponding to all potential measure wave selections (tests)...

Does it hold all potential properties waiting to be tested?
Does it have no properties until tested?
Does it have no properties; all properties being within the experimental setup?
Will the classical philosophical foundation meaning of "properties" need to be reviewed or updated?

Thx bahamagreen. I’ll consider your post very carefully.

 

[ggraham76]

Hi Graham

As promised here is my suggested reading list for your purposes.

1. Quick Calculus:
https://www.amazon.com/dp/0471827223/?tag=pfamazon01-20
2. Theoretical Minimum
https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20
3. Susskind - Quantum Mechanics::
https://www.amazon.com/dp/0465036678/?tag=pfamazon01-20
4. Structure And Interpretation Of QM:
https://www.amazon.com/dp/0674843924/?tag=pfamazon01-20
5. Quantum Probabilities:
https://www.scottaaronson.com/democritus/lec9.html
http://math.ucr.edu/home/baez/bayes.html
6. Consistent Histories - It'a a modern interpretation favored by Gell-Mann and towards the end by Feynman - sometimes categorized as Copenhagen done right:
http://quantum.phys.cmu.edu/CQT/index.html

I think that's enough to start with - get back with any questions.

Thanks
Bill

Bill, thank you so much: you went to a lot of trouble to help me. Very appreciative.

 

[SemM]

Hi. I’m a PhD student at University of Toronto, in philosophy of physics.
I have a number of questions which are quite fundamental: just trying to get everything clear in my head in the most succinct way possible, so I’m sorry if the questions seem remedial.

If a particle with it's wave-component travels through space, either in free state or bound state to a nucleus (oscillating), it has an energy that is unmeasured and cannot be measured unless it's state is excited by the measurement-equipment you might use (electron microscope or X-ray or similar). I think that is perhaps what some mean that it can't be measured. It's not like a tennis ball flying through the room, which you can track with light, an electron or a particle shifts state as soon as the observer (light) hits it and before you "saw it", so therefore it's really impossible to know what it was before. You can calculate what it was before though.

 

[Urs Schreiber]

Mathematically, the state space of quantum mechanics is not a simplex

Maybe you meant to say "point" or "0-simplex" instead of "simplex"? A 0-simplex is a point, a 1-simplex is an interval, a 2-simplex is a filled triangle, a 3-simplex is a filled tetrahedron, and so on. See simplex.

That said, the space of quantum states is a convex space (see here). While also each $n$-simplex is a convex space, the space of quantum states is not an $n$-simplex for any $n$. Unless indeed we have the completely trivial system for which there is only the zero-state, so that the state space is the point, hence the 0-simplex.

 

[atyy]

Maybe you meant to say "point" or "0-simplex" instead of "simplex"? A 0-simplex is a point, a 1-simplex is an interval, a 2-simplex is a filled triangle, a 3-simplex is a filled tetrahedron, and so on. See simplex.

That said, the space of quantum states is a convex space (see here). While also each $n$-simplex is a convex space, the space of quantum states is not an $n$-simplex for any $n$. Unless indeed we have the completely trivial system for which there is only the zero-state, so that the state space is the point, hence the 0-simplex.

What I wrote seems to be the same as what you wrote.

 

[Urs Schreiber]

What I wrote seems to be the same as what you wrote.

No, how? But it's not a big deal. I just thought you might not know the definition of "simplex", so I pointed it out.

 

[Fra]

(*)

"Absence of evidence is not evidence of absence" - Carl Sagan

I would like to defend PeterDonis original comment and object to this response as I think it trivializes away a possibly deeper aspect that is relevant to the OT.

The Statement(*) is only unambigously true in the context of deductive logic, where by evidence we mean "proof", in the sense that absense of a proof of statement, surely does not disproove it. And think this is what you meant as well.

But to what extent is this relevant to QM?

In a more general inference (induction or abduction), such as when you need to determine the odds for certain events, in order to choose a rational action, then absense of certain information certainly influenes the chosen action in a way that does imply that "absense of events" simply render these these events less probably as per the condtitional inference and thia has observable consequences.

Moreover, we do not know if understanding the causal parts of laws of physics in its most fundamental sense as "deductive logic" is right. My personal insights tell me rather that this is likely wrong, in fact it
makes no sense to me at all.

Surely insights is not a formal argument but others fringes these ideas as well. See Lee Smolin (reality of time and evolution of law). Also check out his the principle of precedence. https://arxiv.org/abs/1205.3707.

What I am hinting is that the ACTION of a quantum system, might be determined by this quantum systems lack of confirmed information about its own environment (read lack of a specfic interation history, lack of preparation etc). And vice versa. In this case, the absense or presence of certain information might even be the key to understand quantum causality.

Or maybe not - my point in any case is i
expect subtle things like to not escape any philsophical analysis of QM foundations. This issue may be neither black nor white.

/Fredrik

 

[Lord Jestocost]

... the position that particles have no properties before they’re measured? I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen?

Such a position was never part of the Copenhagen approach. The "Copenhagens" would merely ask: What is a PARTICLE? I hope the following will help:

"In classical physics the aim of research was to investigate objective processes occurring in space and time, and to discover the laws governing their progress from the initial conditions. In classical physics a problem was considered solved when a particular phenomenon had been proved to occur objectively in space and time, and it had been shown to obey the general rules of classical physics as formulated by differential equations. The manner in which the knowledge of each process had been acquired, what observations may possibly have led to its experimental determination, was completely immaterial, and it was also immaterial for the consequences of the classical theory, which possible observations were to verify the predictions of the theory.
In the quantum theory, however, the situation is completely different. The very fact that the formalism of quantum mechanics cannot be interpreted as visual description of a phenomenon occurring in space and time shows that quantum mechanics is in no way concerned with the objective determination of space-time phenomena." [Bold, LJ]

Werner Heisenberg in “The development of quantum mechanics”