What makes measurement possible in the physical world?

[ConradDJ]

I want to argue that there’s something very basic about the structure of the physical world that’s taken for granted everywhere in physics, but isn’t actually described in any theory. The argument goes like this –

What does it take for any physical parameter to be observable? Take the mass of a particle – there are several ways to measure that. For example, if we know its charge, we can watch how the particle’s path gets deflected as it moves through a magnetic field. To do that we have to measure its charge and observe its position at certain times, so we have to be able to measure distances and time-intervals, as well as the strength and orientation of the field.

Very generally, every way that any physical parameter can be measured will always involve the measurement of several other parameters. There is no such thing as a simple isolated measurement. So the question is – in what kind of system is any kind of measurement possible?

What I want to get at here has nothing to do with whether a human being is involved. It’s a question of what’s needed in the physical situation itself so that the value of a certain parameter can be meaningfully defined, in that situation. The mass of the particle can’t be meaningfully defined except by reference to other parameters, whose values must also be meaningfully defined.

Now in the world we live in, there are obviously many observable parameters. For every one of them there are certain interaction-contexts through which they can be measured, all of which involve the measurement of other parameters, in other interaction-contexts. It’s not important to this argument exactly what a “measurement” or “observation” is. It’s enough that we know they’re possible... and that for anything to be in any way observable, there have to be other things that are also observable, in terms of still other things that are also observable.

This isn’t an infinite regress – I assume there are only a finite number of basic physical parameters defining each other. But there has to be a certain closure or completeness to the structure of the observable world, as a self-defining system. It seems to me that this inter-referential completeness is a very remarkable characteristic of our universe, and one that no theory I know of takes into account.

We tend to take it for granted that if something is real, then of course there will be some way to measure it. After all, if a particle has a certain property X that can’t be observed in any way, then it makes no difference to anything whether that property exists or not – it’s simply meaningless. That makes sense, but I don’t think it undercuts my argument.

It’s easy to see that not just any physical system supports the measurement of its own parameters. Imagine a Minimal Newtonian Cosmos consisting of simple point-particles scattered through Euclidean space and time. Say each particle has a certain mass, and no other characteristics, and that they interact with each other only through Newton’s gravity.

This seems like a mathematically well-defined system – but there’s no way actually to measure the distance between two particles, or how they move, or what their respective masses are. Interaction in this system may be lawful, but it doesn’t communicate any information about anything, since none of the parameters of the law are defined by the system itself.

My point is that in physics, we don’t require that our theoretical models define all their own parameters in terms of each other. We try not to introduce parameters that are – in our universe – unobservable. But we don’t try to model what it is about the structure of our universe that let's anything be observable.

I think this is probably why we haven’t come to any clear understanding about Relativity and Quantum theory, and the relationship between them. These theories both (in different ways) make measurement central, but we’re still building models that don’t consider what it takes to make any measurement possible.

 

[kote]

I like it, but I'm a little confused. I see the underline - can you clarify what you are asking though?

I will say that it all reminds me of arguments concerning language. How can we precisely define any word when we must use other undefined words to do so? Are we doomed to circular references and ambiguity? Is precision even possible?

Anyways I don't want an answer. I just thought it was interesting parallel. But what exactly are you asking? Are you looking for a grounding to the circular definitions of properties in terms of other properties?

 

[kote]

See http://www.nyu.edu/gsas/dept/philo/faculty/block/papers/MentalSemanticHolism.html#anchor822217 (It's just a passing overview, so I'm sure there's better literature to find)

One doesn't learn definitions of 'force', 'mass', 'kinetic energy', or 'momentum' in terms that are understood beforehand, for there are no such definitions. Rather, these terms are learned together (in conjunction with procedures for solving problems. As Quine and Putnam argued, local "definitions" in a scientific theory tend to be mere passing expository devices of no lasting importance for the theory itself. And this is quite ubiquitous in theories--a circle of interdefined theoretical terms none of which are definable in terms outside the theory. This fact motivates Lewis' proposal that scientific terms can be defined functionally in terms of their roles in a whole theory (see FUNCTIONALISM; SEMANTICS, CONCEPTUAL ROLE).

Also:

http://en.wikipedia.org/wiki/Confirmation_holism
http://en.wikipedia.org/wiki/Semantic_holism

 

[rewebster]

in what kind of system is any kind of measurement possible?


Those things which are known, and that we are capable of measuring through 'standards' we have set with the measuring tools we have created.

 

[brainstorm]

The OP started with a empirical approach to measurement and then degenerated (in my evaluative opinion) into a philosophical discussion of logical propositions and necessary conditions/consequences.

When I want to understand what measurement is and what makes it possible, I would start with a sample of concrete examples. To measure the mass of something, you place it on a scale where the fulcrum is exactly in between the end-points. Then you balance the scale with an certain amount of standardized mass-units on the other end, such as 1g weights. Measuring mass, thus, requires comparison based on the principle of a lever suspended in a homogeneous gravity context. One object going down is presumed to exert equivalent force to the opposite object being pushed up, hence the comparison becomes possible.

So things are measurable not because they exist, but because they are manipulated to generate results according to the logic of certain assumptions, which may also be verified through repeatable testing. If you don't believe a scale works, you can measure one object and then another of the same mass and then balance the two objects with each other instead of counterweights. This demonstrates generalizability among different instances of applying the same comparative logic.

If different things couldn't be compared in terms of the same comparative reference, they would not be measurable; or you would have to experiment until a stable reference was found.

Measurement is based on the logic of regularity and comparability.

Now my question is what the empirical facts are about this magnetic field used to measure particles. What is measured exactly and how?

 

[humanino]

If I want to measure a length with a 1 mm accuracy, I need a stick 1 mm long, and I will count how many of those sticks will cover the length I want to measure. So all I need is a reference stick. In the past, the reference stick was defined by an actual physical stick. Today, the definition of length is a spectral one, from quantum mechanics. In principle, it does not require anything to be previously defined. It is a (pure) number times the wavelength of an atomic transition. We can have experimental definitions for units which are purely spectral, and only require a pure integral number times a fundamental constant throughout the universe. This is not done yet for every unit, in particular, the mass is still defined from a physical object (IIRC).

On the theoretical side, we know the exact lists of parameters for our standard models of particle physics or cosmology for instance. Particle physics has 20 or so parameters. Speculations beyond the standard model amount to reducing this number of parameters, and this requirement is something explicit and well-understood in any serious research beyond the standard model of particle physics.

Magnetic fields can be used to measure the ratio of charge to momentum for a particle. This ratio is directly related to the curvature radius of the trajectory of the particle in the magnetic field.

 

[rewebster]

Now my question is what the empirical facts are about this magnetic field used to measure particles. What is measured exactly and how?

well, you're going to have to wait just for a while until the 'magnetic field' is explained

 

[brainstorm]

Empiricism and critical reason seems to be absent in these explanations of measurement. Are people capable of breaking these things down to the level of reviewable observations and reasoning, or is that too basic a level?

 

[rewebster]

Empiricism and critical reason seems to be absent in these explanations of measurement. 1) Are people capable of breaking these things down to the level of reviewable observations and reasoning, or 2) is that too basic a level?

1) yes

2) no

 

[brainstorm]

1) yes

2) no

Wonderful, but you don't provide an example to go with it.

 

[rewebster]

Wonderful, but you don't provide an example to go with it.

what kind of example were you looking for?

If your question is about the magnetic field, refer back to post #7

 

[kote]

To address a little more of the original question...

In your minimal universe, if the only force is gravity, then that is what must be used for measurement. Introduce a third low-mass particle and see how it behaves. You can see the mass and location of the first two particles by observing what happens to your measuring device, the third particle.

I know you wanted to avoid questions of measurement, but barring discussions of subjective knowledge, measurement just is interaction. Any system that undergoes an effect can be said to have measured whatever system it was that caused that effect. Measuring devices are physical systems designed to amplify effects to give us information about their causes.

And units, of course, are arbitrary in terms of any underlying framework. We pick units that are useful because of their relationship to certain aspects of nature that we observe (or to other units that we decide are more basic).

As to what generates observables from the void... who knows. In your example system, mass and location actually are observable because information about them can be communicated through gravity, which you allow for. Say your universe also has charge, though, but with no EM fields. Then you really would have a mathematically defined parameter with absolutely no method of being measured. There's no way to tell that there aren't such properties.

From QM we know that things like mass and spin are not consistently defined properties belonging to particles. It is very reasonable to question whether these properties can even be said to exist, or if there is some other basic layer that these manifest out of. The properties we have probably aren't even basic, but because they are measurable, they are all we have to work with. Since we can't investigate anything beyond the observable, we're really just stuck here. I think at this point the question reduces to why is there something rather than nothing? - what reason is there for anything to appear as an observable. That's a tough question, but at least there's been a lot of investigation into it.

So to answer why is anything measurable? I would refer to discussions of why is there something rather than nothing? I really think they reduce to the same question of why observables exist, or rather, why anything is observable. There is a technical difference on the matter of unobservable properties, but unobservable properties aren't what anyone is asking about when they ask why there's something rather than nothing. That technicality isn't a concern for any of the arguments.

Unless, of course, your issue is with the holism of scientific theories and properties. Then see above .

 

[brainstorm]

what kind of example were you looking for?

If your question is about the magnetic field, refer back to post #7

What I'm looking for? You answered my question that "yes, people are capable of breaking these things down to the level of reviewable observations and reasoning."

You shouldn't have to explain how a magnetic field works to explain how it can be applied to measure something else. You don't have to explain how gravity works to postulate that downward movement on one side of a scale is equal to upward movement on the other, and that the two balancing each other out means mass-equivalency.

 

[rewebster]

You shouldn't have to explain how a magnetic field works to explain how it can be applied to measure something else. You don't have to explain how gravity works to postulate that downward movement on one side of a scale is equal to upward movement on the other, and that the two balancing each other out means mass-equivalency.

well, if you don't explain how a magnetic field works or you don't explain how gravity works, then any results will be flawed to some extent, in the same way as if you don't know how a yardstick works (how to use it), it can only be used in a comparative sense, like the balance beam scale.

 

[humanino]

From QM we know that things like mass and spin are not consistently defined properties belonging to particles.

Mass and spin are precisely the constants required to define a particle in modern QM. Mass and spin define the representation of the proper orthochronous Lorentz group to which the particle belong.

 

[kote]

Can I ask if you agree with the argument that "why is anything measurable?" is functionally equivalent to "why is there something rather than nothing?" See above, but to recap, it's something like:

Why is anything measurable?
Why is anything observable?
Why is anything an observable?
Why do observables exist?
Why do observables exist rather than not existing?
Why is there something rather than nothing?

If you disagree that the above are equivalent, or at least equivalently motivated (and therefore analyzed), at which step do you think they differ and why?

 

[kote]

Mass and spin are precisely the constants required to define a particle in modern QM. Mass and spin define the representation of the proper orthochronous Lorentz group to which the particle belong.

And neither mass nor spin are persistently defined on a particle. There are particles with indefinite mass. There are particles with indefinite spin. Neither mass nor spin can therefore be an essential property of a particle, so the argument goes.*

If there are basic properties essential to the existence of particles, we haven't found them yet.

This conversation probably belongs somewhere else though. It wasn't terribly important to my argument.

(*Similarly, no other properties that we use in physics can be essential properties of particles.)

 

[brainstorm]

well, if you don't explain how a magnetic field works or you don't explain how gravity works, then any results will be flawed to some extent, in the same way as if you don't know how a yeardstick works (how to use it), it can only be used in a comparative sense, like the balance beam scale.

the comparative sense is all that is entailed in measurement. Measurement is nothing more than comparison of comparable things in terms of standardized units. You can measure length in terms of anything else, if you can keep moving one across the other without losing track of where the end of it was before you put it at the point again as a beginning. You can measure volume using any comparison if both can be submerged in the same fluid. The foundation of measurement is to understand the mechanics of the instrumentation.

 

[brainstorm]

Can I ask if you agree with the argument that "why is anything measurable?" is functionally equivalent to "why is there something rather than nothing?" See above, but to recap, it's something like:

Why is anything measurable?
Why is anything observable?
Why is anything an observable?
Why do observables exist?
Why do observables exist rather than not existing?
Why is there something rather than nothing?

If you disagree that the above are equivalent, or at least equivalently motivated (and therefore analyzed), at which step do you think they differ and why?

The title of the thread is "why is anything measurable," but it sounds like it should be "is measurability proof of existence?"

It's not. You can measure an imaginary unicorn with an imaginary yardstick and arrive at an imaginary length, but it doesn't mean the unicorn or the yardstick existed outside of your imagination.

 

[kote]

You can measure an imaginary unicorn...

Yeah, you can't measure imaginary things. You can pretend to, but that's totally irrelevant to anything we're discussing.

I'm trying to keep this on track here. I think we'd all appreciate a little effort.

 

[rewebster]

The 'knowns' that we use are 'accepted' for applied use.

---this is one reason why a lot of debates arise in these forums. The people that profess the accepted knowledge will present it to explain a question that arises. When the debate goes into the 'unknown' realm, like where this is, it can go more into what one believes.


Measurement seems to be limited down (or out) to the point where it stops at the 'next' assumed 'unknown'.



(PS---this thread is in philosophy)

 

[brainstorm]

The 'knowns' that we use are 'accepted' for applied use.

---this is one reason why a lot of debates arise in these forums. The people that profess the accepted knowledge will present it to explain a question that arises. When the debate goes into the 'unknown' realm, like where this is, it can go more into what one believes.

Somewhere in between what is known because it is received from a trusted authority and what is unknown but believed, there is what can be established through critical reason.

You don't have to "accept" "knowns" on the basis of applied use, although it can be helpful to for instrumental/practical purposes. You can also dissect why or how something can be known or applied. As you said, this is the philosophy forum - so philosophize.

 

[humanino]

And neither mass nor spin are persistently defined on a particle.

Yes, they are defined.

There are particles with indefinite mass.

No there is not.

There are particles with indefinite spin.

No there is not.

Neither mass nor spin can therefore be an essential property of a particle, so the argument goes.

This is basically chapter 1 of any good book on quantum field theory, the definition of a particle from the representation of Lorentz group it belongs to. You obviously have no idea what you are talking about. Do you know what a group representation is ? Do you know what the Lorentz group is ? Have you heard of Casimir operators ? For Pete's sake, look up "standard model" on wikipedia and you will find a list of particles, defined by their masses, spins, and a few other discrete quantum numbers such as parity.

Can you please name a particle whose spin would not be defined ? There is a basic superselection rule in quantum mechanics that we can not have coherent superpositions of states with different angular momenta.

 

[brainstorm]

Why doesn't anyone posting on this thread just provide some example of exactly how something is measured instead of jumping into discursive references to complex phemonena or other texts?

If you would just pick some particle with definite mass and definite spin, and explain how these are measured - the issue would be empirically clarified.

 

[rewebster]

Somewhere in between what is known because it is received from a trusted authority and what is unknown but believed, there is what can be established through critical reason.

You don't have to "accept" "knowns" on the basis of applied use, although it can be helpful to for instrumental/practical purposes. You can also dissect why or how something can be known or applied. As you said, this is the philosophy forum - so philosophize.

and I thought I was---my, my, my...



As far as any 'magnetic' anything to measure anything/something very, very small, we may know some about what a magnetic field can do, but it is an 'accepted' unknown in what it really is. So, doing anything with it is that last step in the 'accepted' unknown measurement.

 

[brainstorm]

As far as any 'magnetic' anything to measure anything/something very, very small, we may know some about what a magnetic field can do, but it is an 'accepted' unknown in what it really is. So, doing anything with it is that last step in the 'accepted' unknown measurement.

Why can't you just say what it is supposed to be doing when you're using it?

 

[rewebster]

Why can't you just say what it is supposed to be doing when you're using it?

well, (its a lot better if) you have to know what it is doing, before you can say what it's supposed to be doing.

 

[brainstorm]

well, 'its a lot better if' you have to know what it is doing, before you can say what it's supposed to be doing.

you didn't say either in your post. what is it doing? what is it supposed to be doing? what do you think it is doing? answers to any of these question would be a substantive starting point.

 

[rewebster]

you didn't say either in your post. what is it doing? what is it supposed to be doing? what do you think it is doing? answers to any of these question would be a substantive starting point.

yes, they would (were)

 

[brainstorm]

yes, they would (were)

ok, I didn't get you were purposefully trolling until now. You never intended to do anything more than circumvent substance did you?

 

[rewebster]

ok, I didn't get you were purposefully trolling until now. You never intended to do anything more than circumvent substance did you?

its against forum rules to post one's theories


if you think that thinking about these things are a good 'starting point' , I agree totally with you

 

[kote]

Can you please name a particle whose spin would not be defined ?

Huh? It's simple HUP. When particles have a defined location, they don't have a defined momentum... all that good stuff. I suppose I should clarify spin in a certain direction rather than just spin in general, but that's beside the point. Unless you are arguing that particles always have a defined spin in every direction, I don't see how you could disagree with the statement that some particles have undefined spin (in a given direction).

See http://arxiv.org/abs/quant-ph/0110102 for some published rigor on complementarity. Also http://arxiv.org/abs/quant-ph/9905042. As for particles in QFT: http://arxiv.org/abs/quant-ph/0103041. I don't claim to be a QFT expert, but I thought we were pretty set on HUP when it comes to particle properties, no? If it's about "essential properties," fine, that's a philosophical concept that my claim relies on. Hence the "so the argument goes" above.

I'd be happy to follow to the QM forum if you'd care to explain where I'm wrong here.

 

[brainstorm]

Huh? It's simple HUP. When particles have a defined location, they don't have a defined momentum... all that good stuff. I suppose I should clarify spin in a certain direction rather than just spin in general, but that's beside the point. Unless you are arguing that particles always have a defined spin in every direction, I don't see how you could disagree with the statement that some particles have undefined spin (in a given direction).

See http://arxiv.org/abs/quant-ph/0110102 for some published rigor on complementarity. Also http://arxiv.org/abs/quant-ph/9905042. As for particles in QFT: http://arxiv.org/abs/quant-ph/0103041. I don't claim to be a QFT expert, but I thought we were pretty set on HUP when it comes to particle properties, no? If it's about "essential properties," fine, that's a philosophical concept that my claim relies on. Hence the "so the argument goes" above.

I'd be happy to follow to the QM forum if you'd care to explain where I'm wrong here.

how is spin-direction observed? How is the particle identified in the first place, for that matter?

 

[humanino]

I suppose I should clarify spin in a certain direction rather than just spin in general, but that's beside the point.

Yes you should have clarified it because it is quite different. So particle still have a well defined spin, although they can have different spin components, good.

Now can you describe a situation where a particle has a undefined mass ? I suppose you are going to invoke the HUP again. That would not be a measurable particle then, but a virtual particle. Real particles always have a well-defined mass.

 

[brainstorm]

Yes you should have clarified it because it is quite different. So particle still have a well defined spin, although they can have different spin components, good.

Now can you describe a situation where a particle has a undefined mass ? I suppose you are going to invoke the HUP again. That would not be a measurable particle then, but a virtual particle. Real particles always have a well-defined mass.

Why does every post in this thread address theory instead of empirical methodology? Why can't people just explain how their observables are empirically observed and measured?

 

[ZapperZ]

Huh? It's simple HUP. When particles have a defined location, they don't have a defined momentum... all that good stuff. I suppose I should clarify spin in a certain direction rather than just spin in general, but that's beside the point. Unless you are arguing that particles always have a defined spin in every direction, I don't see how you could disagree with the statement that some particles have undefined spin (in a given direction).

See http://arxiv.org/abs/quant-ph/0110102 for some published rigor on complementarity. Also http://arxiv.org/abs/quant-ph/9905042. As for particles in QFT: http://arxiv.org/abs/quant-ph/0103041. I don't claim to be a QFT expert, but I thought we were pretty set on HUP when it comes to particle properties, no? If it's about "essential properties," fine, that's a philosophical concept that my claim relies on. Hence the "so the argument goes" above.

I'd be happy to follow to the QM forum if you'd care to explain where I'm wrong here.

This is a common misunderstanding of the HUP.

If I have ONE particle, and I measure it's position to the precision that's allowed by my instrument, I can then determine its momentum with arbitrary precision, again, that's allowed by my instrument. The accuracy of each of those measurement, one after the other, has nothing to do with the HUP. This is NOT the HUP.

I've described an example of this using the single-slit example a few times. The HUP says nothing about the accuracy of one measurement of position and one measurement of momentum. The uncertainty in each of those measurement is the instrumentation uncertainty, not the HUP. So to say that "... particles have a defined location, they don't have a defined momentum... " is incorrect. Each of those particle can have well defined position and well-defined momentum. It is the spread in the values of the positions and the values of the momentum, measured under identical conditions, that is governed by the HUP. If the spread in the values of the position is small, then the spread in the values of the momentum will be big. This means that your ability to predict what the next value of the momentum is is not very accurate.

One needs to carefully look at the mathematical expression for the HUP. This is not some handwaving argument. It is deeply rooted (as is with the rest of physics) in some underlying mathematical description. And if one does that, one can see the statistical nature of the HUP.

Zz.

 

[ZapperZ]

Why does every post in this thread address theory instead of empirical methodology? Why can't people just explain how their observables are empirically observed and measured?

My avatar shows where electrons hit a CCD screen. Due to the nature of the instrument (a hemispherical electron analyzer), the vertical axis corresponds to the energy that that electron has (very much like a mass spectrometer), while the horizontal axis is the momentum along a particular direction of the analyzer corresponding to the direction of the slit.

The justification for being able to designate those position are covered in any text on angle-resolved photoemission spectroscopy. In other words, these are extensively covered and well-established to allow us to deduce those "spots" on the screen to correspond to a particular set of observables, in this case, E and k.

It is strange that this is in the Philosophy forum.

Zz.

 

[brainstorm]

If I have ONE particle, and I measure it's position to the precision that's allowed by my instrument, I can then determine its momentum with arbitrary precision, again, that's allowed by my instrument.

How does the instrument measure position and momentum exactly?

 

[brainstorm]

The justification for being able to designate those position are covered in any text on angle-resolved photoemission spectroscopy. In other words, these are extensively covered and well-established to allow us to deduce those "spots" on the screen to correspond to a particular set of observables, in this case, E and k.

Why do you cite a text instead of just saying how it works?

 

[humanino]

How does the instrument measure position and momentum exactly?

I have already said how one can measure the momentum of a charged particle, using magnetic fields and trackers.

Take a photon now. An optical lens will select for a given deviation a given momentum. The position can simply be registered by a photographic plate. The slit or simply a punched hole will select a position without destroying the photon.

Alternatively, one could measure energy and position for an electron or a photon using a calorimeter. A calorimeter is simply a segmented stack of individual cells, each of which absorbing a certain amount of energy in the electromagnetic shower, usually recorded by collecting the amount of light out of the material. Typically for instance, one could use crystals. Alternatively one could use a stack of heavy material, such a lead, and scintillators, such as plastic. Then we call it a sandwich calorimeter.

Your question is very vague.

 

[brainstorm]

This seems to be getting closer, but it's still not at the level of the instrument's methodology and how one observation is linked to another to relate the unit to what is measured.

I have already said how one can measure the momentum of a charged particle, using magnetic fields and trackers.

You did? When?

Take a photon now. An optical lens will select for a given deviation a given momentum.

How? Because momentum determines the angle/path the light takes through the lens? Many things are being left implicit.

The position can simply be registered by a photographic plate. The slit or simply a punched hole will select a position without destroying the photon.

For what purpose? What is being measured exactly this way?

Alternatively, one could measure energy and position for an electron or a photon using a calorimeter. A calorimeter is simply a segmented stack of individual cells, each of which absorbing a certain amount of energy in the electromagnetic shower, usually recorded by collecting the amount of light out of the material.

How is an amount of light collected out of a material? Is this a measurement of units of a particular chemical change that represents a given amount of light energy? What is light affecting exactly in these "cells?"

Typically for instance, one could use crystals. Alternatively one could use a stack of heavy material, such a lead, and scintillators, such as plastic. Then we call it a sandwich calorimeter.

What would change in each or any of these materials that could be measured in terms of units?

Your question is very vague.

My question? Have you been reading how all the posts in this thread avoid actually describing the logic behind measurement by spouting off about the complexities?

 

[apeiron]

So to answer why is anything measurable? I would refer to discussions of why is there something rather than nothing? I really think they reduce to the same question of why observables exist, or rather, why anything is observable. There is a technical difference on the matter of unobservable properties, but unobservable properties aren't what anyone is asking about when they ask why there's something rather than nothing. That technicality isn't a concern for any of the arguments.

Unless, of course, your issue is with the holism of scientific theories and properties. Then see above .

The issue at the heart of this is the difference between treating reality as a system and reality as a construction.

Yes, we can model reality as a construction - a bunch of small localised stuff (events, substance, atoms, information) glued together to create additive effects. That works. Although it then introduces paradoxes, such as how did that local stuff get created in the first place? And oh, we also seem to need a global spacetime, a void, a vacuum, a dimensionality, for the stuff to do its constructing in.

So the success of mechanical modelling - gluing together particles, or masses, or energies, or force vectors, or even microstates - is undeniable. But it leaves unanswered some basic metaphysical questions. Which is why it can be such a puzzle over, well, how can we have the measured without also having a measurer?

You will say, I want to be a good mechanist, a good reductionist, and do away with everything but the measured local stuff. Yet I cannot get away from the nagging realisation that a measurer, a global context that makes a measurement meaningful, is also always required.

The best recent philosopher on these issues I believe is CS Peirce. And his theory of semiosis is exactly about this issue. What gives meaning to a sign?

The radical step he made (or was trying to make) was to frame things so that he was talking simultaneously about epistemology and ontology.

Semiosis is how we (as reality modelling creatures) dichotomise the world into our formal models and the measurements they entail.

And then pan-semiosis would be saying well, the world has that logic itself. The world self-organises into a model (the classical, decohered, prevailing state of the universe) and its measurements (the acts of decoherence that create the events, or particles, or bits, or however you chose to think about the local stuff, out of which the whole is being created).

So if Conrad's question is why is anything measurable? The answer would be that systems are measuring devices. The local stuff out of which they are constructed is also the local stuff which they are creating (via the causality of downward "observational" constraint).

It is all about reality modeled as a self-organising system. And to do that properly, you require Peirce's firstness, secondness and thirdness. Or what I normally talk about as vagueness, dichotomies and hierarchies (as Peircean terminology is even more opaque, and I also prefer to connect to the larger bodies of thought on these matters).

 

[apeiron]

Real particles always have a well-defined mass.

Do you mean a well-defined average mass? That would seem to be the more accurate statement in a QM context.

There would also be the GR view of "well-defined". We would be talking there about rest-mass? And so the local definite status flows from the fixed global measurement frame.

 

[brainstorm]

Interesting post, but I'm bracketing it in favor of getting to the empirical specifics of specific measurement instruments/techniques to see if people actually understand the basic logical principles their tools are based on.

So if Conrad's question is why is anything measurable? The answer would be that systems are measuring devices. The local stuff out of which they are constructed is also the local stuff which they are creating (via the causality of downward "observational" constraint).

Is this not vague to you? How can any or every "system" be a measuring device? Is such a "system" knowable according to you, or does that transcend the dichotomy of the system/construction approach where only in the mindset of constructionism can a system actually be dissected to its mechanics - and presumably you have some reason that you should not engage in such constructionist mechanical dissections. It's a clever cop out, but still a cop out, imo.

It is all about reality modeled as a self-organising system. And to do that properly, you require Peirce's firstness, secondness and thirdness. Or what I normally talk about as vagueness, dichotomies and hierarchies (as Peircean terminology is even more opaque, and I also prefer to connect to the larger bodies of thought on these matters).

Is "reality as a self-organizing system" the reason why any measurement instrument generates valid measurements without having any logic for how it measures what it is supposedly measuring?

 

[apeiron]

If there are basic properties essential to the existence of particles, we haven't found them yet.

The best candidate could be local gauge symmetries. In general, like me, you would seem to be taking a constraints-based soliton approach to concieving of particles. Well, what the wider observing system cannot constrain, is then by definition able to exist locally as a degree of freedom.

This is how intrinsic spin and inertial motion would have to arise - as properties that the system cannot see directly. They can only be measured intermittently as events - collisions or polarisations and other kinds of systems measurement.

But in general, properties would arise contextually in a systems (as in condensed matter approaches to particles) rather than being intrinsically existing. And so in existence even in the absence of a measuring context.

 

[kote]

The HUP says nothing about the accuracy of one measurement of position and one measurement of momentum. The uncertainty in each of those measurement is the instrumentation uncertainty, not the HUP.

Agreed.

So to say that "... particles have a defined location, they don't have a defined momentum... " is incorrect. Each of those particle can have well defined position and well-defined momentum.

Each particle can have a well defined position or a well defined momentum. Not both at the same time. You left out a critical "when" at the beginning of quoting me here.

It is the spread in the values of the positions and the values of the momentum, measured under identical conditions, that is governed by the HUP. If the spread in the values of the position is small, then the spread in the values of the momentum will be big. This means that your ability to predict what the next value of the momentum is is not very accurate.

It's not just about accuracy of predictions though. There are metaphysical implications. As you know, it's not just about our knowledge of position and momentum, it's about their existence as well defined properties. A particle with a well defined momentum does not exist at any specific location. I think it's misleading to talk about small vs larger spreads in this context. There is no reason to assume that during measurement properties have anything but sharp values ontologically. In practice, of course, you'll always have uncertainty. But as you mentioned, it doesn't need to be the HUP variety of uncertainty when just considering a single property by itself.

See http://arxiv.org/abs/quant-ph/0003074 for the math on properties having sharp values. From the abstract: "I shall show that the same subtle issues arise with respect to continuous quantities in classical and quantum mechanics; and that it is, after all, possible to describe a particle as possessing a sharp position value without altering the standard formalism of quantum mechanics." Notice that it was written by a philosopher (with tenure at Princeton) - these are the details philosophers care about.

I'll concede that mentioning mass didn't help my case any. It is clear, though, that (defined) location is a property that particles may lack. Per http://plato.stanford.edu/entries/essential-accidental/, if something can lack a property and still be the same thing, that property is said to be accidental and not essential. You could make a case against this distinction. As it's currently understood however, existing at a defined location is not an essential property of a particle.

 

[apeiron]

Each of those particle can have well defined position and well-defined momentum. It is the spread in the values of the positions and the values of the momentum, measured under identical conditions, that is governed by the HUP. If the spread in the values of the position is small, then the spread in the values of the momentum will be big. This means that your ability to predict what the next value of the momentum is is not very accurate.

What Kote said was:

Huh? It's simple HUP. When particles have a defined location, they don't have a defined momentum... all that good stuff.

And I can't see any real difference to what Kote meant.

Both Zapper and Humanino seem to be saying that well-defined uncertainty is the same, philosophically, as well-defined existence. Plainly it is not, otherwise QM would not be a challenge to classical conceptions of reality.

 

[ZapperZ]

Why do you cite a text instead of just saying how it works?

Because I will have to teach you the physics of photoemission spectroscopy, and I'm not good enough (nor do I have the patience) to do that on a public forum, when it took me 2 years to learn it myself. Furthermore, I believe that I HAD given you ample example of how the quantity is measured. What I said what the exact details of how the location on the detector corresponds to these quantities will require further digging into the physics.

I don't know why this is so difficult. When you look at the interference pattern on a screen, and then arrive at the frequency based on the location of the peaks, it is the same thing. So why is this that mysterious?

And why do you want everyone to spoodfeed you?

Zz.

 

[ZapperZ]

What Kote said was:


And I can't see any real difference to what Kote meant.

Both Zapper and Humanino seem to be saying that well-defined uncertainty is the same, philosophically, as well-defined existence. Plainly it is not, otherwise QM would not be a challenge to classical conceptions of reality.

Er.. this is what I said? I have no clue what you just said here.

Zz.

 

[ZapperZ]

Agreed.



Each particle can have a well defined position or a well defined momentum. Not both at the same time. You left out a critical "when" at the beginning of quoting me here.

I construct a single slit with width \Delta(x) So any particle that passes through that slit has an uncertainty in position equal to the width of that slit. Now, after the slit, the particle hits a detector at a position x1 measured from the centerline of the slit. The uncertainty of this measurement depends on the resolution of the detector. This is not the HUP. Knowing the distance from the slit to the detector, I can use the x1 position to arrive at the value of momentum along the x direction, i.e. p_x1. The uncertainty of this corresponds to the resolution of the detector. I can make the width as small as I want, it would not affect the uncertainty of the momentum.

Each of the measurement of the position (at the location of the slit) and the measurement of the momentum, has been made with arbitrary accuracy independent of each other. How are they not well defined "at the same time"? Besides, what is this "at the same time" business? In QM, there is an order of the operation of the operator. That's the whole point of non-commutativity. You operate first on one, and then the other. You never operate them "at the same time", so this issue here is non-existent. In this case, I operate my position operator first (by imposing the slit) and then I do the momentum measurement when it hits the screen. How soon or how late I do that doesn't matter, as long as I do one after the other.

It's not just about accuracy of predictions though. There are metaphysical implications. As you know, it's not just about our knowledge of position and momentum, it's about their existence as well defined properties. A particle with a well defined momentum does not exist at any specific location. I think it's misleading to talk about small vs larger spreads in this context. There is no reason to assume that during measurement properties have anything but sharp values ontologically. In practice, of course, you'll always have uncertainty. But as you mentioned, it doesn't need to be the HUP variety of uncertainty when just considering a single property by itself.

You need to be careful here. AFTER the particle passes through the slit, BEFORE it hits the detector that determines its momentum, it is fine to say that it has no specific momentum. This is because it can acquire a range of momentum, depending on how small the width is. The smaller the width of the slit, the larger the range of momentum it can have, and thus, you are not able to say with greater certainty of what momentum it will be WHEN you measure! However, if you look at my example, AFTER it hits the screen, it has a definite momentum!

But here's the next thing. If I do this only ONCE, i.e. one particle passes through the slit, and that one particle then hits the detector, where is the HUP here? I have, in my possession, a definite position and definite momentum values of that particle. Where, in all of this, is the HUP? Can you use the values that I've just obtained to find \Delta(x) and \Delta(p_x)?

Zz.