What makes measurement possible in the physical world?

[kote]

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.

All I'm saying is that it's possible for a particle to not have a defined momentum while still being a particle. Similarly, it's possible for a particle to not have a defined location while still being a particle. Therefore, location and momentum are not essential properties of particles.

I agree with you on everything else you said.

 

[ZapperZ]

All I'm saying is that it's possible for a particle to not have a defined momentum while still being a particle. Similarly, it's possible for a particle to not have a defined location while still being a particle. Therefore, location and momentum are not essential properties of particles.

Show me the exact phenomenon where this is occurring. You'd notice that I gave you a specific illustration on what I was trying to emphasize.

Zz.

 

[brainstorm]

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.

This post wasn't about spoonfeeding or learning photoemission spectroscopy. It was about why things are measurable. I have been trying to get people to provide clear reasoning about how their instrumentation measures at the level of direct empirical data.

When you look at the interference pattern on a screen, and then arrive at the frequency based on the location of the peaks

Take this sentence, for example. What generates the interference pattern you speak of? What is interfering with what and how do you know that it is?

I'm trying to get at the precise comparison taking place in measurement and what the medium/media are linking the different empirical observations. For example, when volume is measured by displacement of fluid, the fluid is the medium and the level of the fluid in the container is measured in increments of length. So when one object submerged causes the water-level to rise 1cm, its volume is half that of an object that causes the water-level to rise 2cm. The comparison is made between the two volumes of water displaced by the different objects, not by comparison of the objects themselves directly. It is reasoned that the water will not dissolve into the water, and as such all displaced water will correspond to the volume of the submerged object. Likewise it is assumed that no more water will be displaced than the volume of the object. As such, comparing the volumes of displaced water is regarded as analogous to comparing the volumes of the objects themselves.

 

[kote]

Show me the exact phenomenon where this is occurring. You'd notice that I gave you a specific illustration on what I was trying to emphasize.

Zz.

Apologies - I thought you agreed that particles don't always have defined momentum. We know that when a particle has a sharp momentum, its position isn't just hidden by uncertainty, it's undefined... right? I'd feel a little silly walking through Bell to show that there is no defined momentum when you define location.

Can I show you a positive situation which demonstrates that this is the case? No, I'm not sure that's possible. We have indirect mathematical proof from Bell though. This is all assuming, of course, that QM is valid.

http://arxiv.org/abs/quant-ph/0603277 (Foundations of Physics) is another more recent and concise version of the same argument:

A very simple illustration of the Bell-Kochen-Specker contradiction is presented using continuous observables in infinite dimensional Hilbert space. It is shown that the assumption of the existence of putative values for position and momentum observables for one single particle is incompatible with quantum mechanics.

...

If the predictions of quantum mechanics are correct, then we have proved that position and momentum observables can not be assigned any context independent value. The proof presented here involves elementary observables of one single particle and provides a very simple illustration of the Bell-Kochen-Specker contradiction.

Context dependent putative values are not prohibited and all attempts to replace standard quantum mechanics by some form of hidden variables theories must necessarily include the context dependence in the deterministic assignment of values to the observables. This necessity makes such deterministic theories less appealing. One of the main reasons for developing hidden variables theories was to bring the quantum world closer to the classical expectations but the necessary contextuality goes in the other direction.

So the specific situation in which a particle doesn't have a location, is any time context isn't being created for it. Any time position isn't being measured, it doesn't exist in a well defined way.

This is even a much stronger claim than is required to show that location (or momentum) isn't essential. All that is required for location to be accidental is that it is possible that at one point in time, anywhere in the universe, there might exist a particle without a well defined location.

 

[kote]

The above uses http://arxiv.org/abs/quant-ph/9912108.

However, we shall show here how direct Kochen-Specker arguments for complementarity between position and momentum can be given that are entirely independent of the uncertainty relation and its interpretation.

 

[apeiron]

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.

It is the impossibility of measuring both aspects of a particle at a single point of spacetime which does make a particle not well-defined in this argument about measurement. It is what puts a theoretical limit on measurement itself.

I'm not sure what you motivation can be here, but you are trying to dodge behind semantics.

Just focus on answering the question for a point in spacetime, not a point in space, or a point in time. Or does the difference need further explaining?

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!

And quite clearly the discussion was about a single measurement at a single point in spacetime, not a measurement at one time, then a second measurement at another.

After it hits the screen, you could retrospectively impute that the particle had a definite momentum. And using a single slit apparatus, you would know what it managed to squeeze through (and so that it did not get deflected before passing the screen).

It seems strangely evasive of you that you are even using the single slit story here rather than the twin slit one. Although it does help to conceal the QM issues that were being discussed of course.

 

[GeorgCantor]

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.

Wouldn't this cause an interference pattern(i.e. uncertainty about position, essentially what kote was implying)?

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"?

It's not at the same time. The particle didn't pass the slit and hit the detector 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.

You don't operate them at the same time, because it's impossible, as per HUP.

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.

I am sure kote agrees with this but he was talking about a particle having a definite position and momentum at the same time(not measuring them one after another).

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.

Make the slit opening small enough and you'll start seeing interference(the electron/photon will start interferening with itself, even though you'd need more particles to be sent one after another to make it visible). That's where the HUP is shown experimentally, but you know this stuff better than myself.

 

[GeorgCantor]

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.

This is misleading. You should have said "Real particles always have a well-defined mass"... from a particular frame of reference. Otherwise, mass is not a defined property and since we are discussing the philosophical implications, this is not a minor point.

 

[apeiron]

This seems a pretty unambiguous statement...

Heisenberg Uncertainty Relation

The term Heisenberg uncertainty relation is a name for not one but
three distinct trade-off relations which are all formulated in a more or
less intuitive and vague way in Heisenberg’s seminal paper of 1927 [1].
These relations are expressions and quantifications of three fundamental
limitations of the operational possibilities of preparing and measuring
quantum mechanical systems which are stated here informally
with reference to position and momentum as a paradigmatic example
of canonically conjugate pairs of quantities:

(A) It is impossible to prepare states in which position and momentum
are simultaneously arbitrarily well localized. In every state,
the probability distributions of these observables have widths that
obey an uncertainty relation.

(B) It is impossible to make joint measurements of position and momentum.
But it is possible to make approximate joint measurements
of these observables, with inaccuracies that obey an uncertainty
relation.

(C) It is impossible to measure position without disturbing momentum,
and vice versa. The inaccuracy of the position measurement
and the disturbance of the momentum distribution obey
an uncertainty relation.

http://philsci-archive.pitt.edu/archive/00004112/01/HUR_Busch_Falkenburg_philsci.pdf

 

[apeiron]

Experimental verification of HUP with fullerene molecules and single slit diffraction...

The Heisenberg uncertainty principle for material objects is an essential corner stone of quantum mechanics and clearly visualizes the wave nature of matter. Here, we report a demonstration of the Heisenberg uncertainty principle for the fullerene molecule C70 at a temperature of 900 K. We do this by showing the increase in molecular momentum spread after passage through a narrow slit with a variable width down to 70 nm. We find good quantitative agreement with the theoretical expectation.

http://agnes.dida.physik.uni-essen.de/~backhaus/Quanten/Arndt/PRAHeisenberg.pdf

 

[ZapperZ]

This post wasn't about spoonfeeding or learning photoemission spectroscopy. It was about why things are measurable. I have been trying to get people to provide clear reasoning about how their instrumentation measures at the level of direct empirical data.

And I thought I did. My avatar was taken exactly from the raw output of an instrument.

Take this sentence, for example. What generates the interference pattern you speak of? What is interfering with what and how do you know that it is?

This is why we have to know the physics of not only the phenomenon (interference), but also the physics of the instrumentation. I've described the physics of the instrumentation (i.e. the spectrometer). The physics of the phenomenon (photoemission) is covered in books.

As far as I can tell, I've given you exactly what you wanted. Short of you actually doing the experiment yourself (which frankly I think everyone should), I'm not sure what else you would want.

Zz.

 

[ZapperZ]

Apologies - I thought you agreed that particles don't always have defined momentum. We know that when a particle has a sharp momentum, its position isn't just hidden by uncertainty, it's undefined... right? I'd feel a little silly walking through Bell to show that there is no defined momentum when you define location.

Can I show you a positive situation which demonstrates that this is the case? No, I'm not sure that's possible. We have indirect mathematical proof from Bell though. This is all assuming, of course, that QM is valid.

http://arxiv.org/abs/quant-ph/0603277 (Foundations of Physics) is another more recent and concise version of the same argument:



So the specific situation in which a particle doesn't have a location, is any time context isn't being created for it. Any time position isn't being measured, it doesn't exist in a well defined way.

This is even a much stronger claim than is required to show that location (or momentum) isn't essential. All that is required for location to be accidental is that it is possible that at one point in time, anywhere in the universe, there might exist a particle without a well defined location.

Let's go back to the basics, shall we? This is covered in standard intro QM that everyone has to take as a physics major.

The definition of the HUP of an observable is (drum roll):

\Delta(A) = \sqrt{<A>^2 - <A^2>}

I claim that, for the HUP, a single measurement of ANY observable has an uncertainty of ... ZERO!

Can you tell me where this is wrong?

Now, after you consider that, you might want to start considering that you are missing the key aspect of what I've said, which is the quantity one gets IN A MEASUREMENT. From what I've been reading, it appears that you are think of the superposition of all possible outcome before a measurement that has the effect of contradicting classical realism. This isn't the issue and this isn't what I'm arguing against (heaven knows I've been trying so hard to describe all the Schrodinger Cat states experiment that clearly shows such violation of realism).

Now, as far as having some observables simultaneously, can you please tell me, when I find the commutator of two non-commuting observables, such as [A,B], what exactly does that tell you about the sequence of measurement of those observable? Are you able to show me an example of two non-commuting observables that are measured simultaneously in a single measurement?

Zz.

 

[ZapperZ]

Make the slit opening small enough and you'll start seeing interference(the electron/photon will start interferening with itself, even though you'd need more particles to be sent one after another to make it visible). That's where the HUP is shown experimentally, but you know this stuff better than myself.

I'm not sure how this answered my question. The "interference" is part of the description of QM. The SPREAD in the available values of the momentum increase according to the HUP. The HUP itself doesn't contain all the details of the pattern that one can see if one does this with many photons. All the HUP does is give a rough idea of how well one can predict the next one, or how much of a spread in values one will get. This is why the HUP has been elevated by some people to a stature that it doesn't deserve. It isn't a "principle" at all, but rather a consequence of the wavefunction and the observable.

Zz.

 

[ZapperZ]

It seems strangely evasive of you that you are even using the single slit story here rather than the twin slit one. Although it does help to conceal the QM issues that were being discussed of course.

But we're talking about the HUP at the SIMPLEST level. I can talk about the 2-slit as well, but it is no longer as easy because now, you have two locations of x, and each of them has its own slit width, etc. It confuses those who are trying to follow the HUP applied to it, but if you notice, the envelop of the interference pattern remains identical to that of the single slit (this is a popular undergraduate optics question). So I lose no generalities by tackling the single slit as an example.

And quite clearly the discussion was about a single measurement at a single point in spacetime, not a measurement at one time, then a second measurement at another.

After it hits the screen, you could retrospectively impute that the particle had a definite momentum. And using a single slit apparatus, you would know what it managed to squeeze through (and so that it did not get deflected before passing the screen).

Then show how you are able to perform [A,B] operation if A and B are non-commuting.

Zz.

 

[apeiron]

But we're talking about the HUP at the SIMPLEST level. I can talk about the 2-slit as well, but it is no longer as easy...

The simplest example for demonstrating the essential weirdness of QM is the twin slit experiment (or so Feynman said). That is where you are compelled to adopt a non-classical model such as the path integral.

You can reason from twin slits that the path integral also applies to the single slit case. But I have never seen anyone try to demonstrate that the path integral approach arises in single slit experiments, then derive the more "complicated" twin slit case from it.

Perhaps you can provide cites to support your argument here?

 

[ZapperZ]

The simplest example for demonstrating the essential weirdness of QM is the twin slit experiment (or so Feynman said). That is where you are compelled to adopt a non-classical model such as the path integral.

You can reason from twin slits that the path integral also applies to the single slit case. But I have never seen anyone try to demonstrate that the path integral approach arises in single slit experiments, then derive the more "complicated" twin slit case from it.

Perhaps you can provide cites to support your argument here?

The simplest demonstration of the SUPERPOSITION concept is the 2-slit experiment. The simplest demonstration of the HUP is the single-slit experiment. You can't use the same phenomenon to demonstrate ALL of the weirdness of QM. Try using the 2-slit experiment to demonstrate a bipartite quantum entanglement, for example.

I've described how the single-slit shows the HUP clearly in this thread, i.e. in how the lateral momentum RANGE increases as the slit becomes smaller. I thought this is a clear example already? If you wish for a QM derivation of the single-slit phenomenon, the Marcella paper that I've cited several times (I'll find the exact reference again if you haven't heard about it) in the QM forum has it in painful detail. You can apply the HUP using the wavefunction used in that formulation.

Zz.

 

[ConradDJ]

Thanks for all your comments!... I had no idea this thread would go to four pages before I could get back to my computer a mere 24 hours later.

First, this was posted in the philosophy forum because the argument is independent of the specifics of physical measurement processes. If the argument has merit, then I think it is important to look at how actual measurements are made. But as a starting-point, I only want to emphasize the obvious fact that the measurement of any parameter depends on the measurement of others.

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?

There is a parallel with language, as an inter-referential system, but with an important difference. In human language words refer not only to other words but also to all the stuff we talk about, out there in the world. When I say “tree” I can point to a tree. In physics there is no “outside world” – the observational meaning of any physical parameter has only the observation of other physical parameters to refer to. So there is a closed / complete inter-referential system here in a much stronger sense.

My purpose in emphasizing this is not to cast any doubt on what we can measure. It’s that we know something very remarkable about the structure of the physical world – namely that it provides measurement-contexts for all its own parameters. So, we should take this into account in trying to work out a fundamental physical theory. We should figure out how to construct a model whose parameters can all be defined in terms of each other.

I don’t think this will be easy, and to begin with it’s not very clear what it involves. The good news is that we have a working example of a self-defining system right in front of us, and we know an incredible amount about how it works. In other words, if we can clarify the question – which I take to be a philosophical issue – we have a whole lot of very detailed information about the physics of measurement to help us get to the answer.

Right now, the purpose of physics is to construct an elegant mathematical model that duplicates the structure of what we observe – that gives us all the right particles with their masses and charges and spin, etc. That's completely right. My point is that the model should also eventually duplicate the aspect of physical structure that makes every one of these things observable.

 

[ConradDJ]

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.

... 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.

But since we have no electromagnetic field in this model, we can not “see” the location of our particles. From the standpoint of any particle in this system, the most that can be observed is an acceleration in some direction. Even if we were to assume the particle has memory and an internal clock and inertial reference-frame, and so can track how this acceleration-vector changes over time, there’s no way that information could be used to determine the locations and masses of any other particles. That was the point of his illustration -- even though we can "stand outside" it and define the locations of the particles from an absolute standpoint, from inside the system itself, nothing is observable.

So I agree with you that “measurement just is interaction.” In the right context, any physical interaction constitutes a measurement. But we know from QM that interactions occur all the time (in entangled superpositions) without “measuring” anything – just because the “right context” of other interactions is missing.

So the question I'm raising is -- what makes our universe different from the minimal one? How is it that in our universe adequate information is in fact physically available to determine so much about what's in the world?

I think in the big old Misner/Thorne/Wheeler book on Gravitation there’s a construction that shows how to define spacetime intervals observationally using only gravity and light. Of course, you also need something that can detect gravity and light, but leaving that aside – the point is that if you have only gravity or only light, it’s impossible in principle to measure a spacetime interval.

If so, then space and time are only measurable in our universe, because we have these two essentially different kinds of long-distance interaction. We can use light to observe the motions of the planets, and we can measure gravity locally and use that information to interpret our visual information on how the planets move.

The traditional way physics is done, ideally we want a “unified” model in which gravity and electromagnetism turn out to be the same thing. But if measurement is actually a basic feature of our world, then the difference between gravity and light is important. Instead of eliminating this difference in our fundamental theory, we would want to explain exactly what role this difference plays in the referential structure of physics.

 

[ZapperZ]

Thanks for all your comments!... I had no idea this thread would go to four pages before I could get back to my computer a mere 24 hours later.

First, this was posted in the philosophy forum because the argument is independent of the specifics of physical measurement processes. If the argument has merit, then I think it is important to look at how actual measurements are made. But as a starting-point, I only want to emphasize the obvious fact that the measurement of any parameter depends on the measurement of others.

But with all due respect to philosophers, have they measured anything as a standard practice?

I'm a firm believer that you can't simply read about things to know what it is. You actually have to DO and practice it. I'm an experimentalist, so knowing what I'm measuring, and how I'm able to deduce that it is what it is, is extremely important. In a previous part of my career, a central issue of what I measure (related to my avatar) is whether an 'electron' that we measure in a solid is truly a well-defined particle, or weather such a concept (i.e. quasiparticle) is valid when there is a huge electron-electron interaction the that solid. So knowing what we measure and what that quantity represents is, and has always been, a central issue in physics, especially experimental physics. So that's why I am very puzzled that this is asked in the philosophy forum.

Zz.

 

[rewebster]

But with all due respect to philosophers, have they measured anything as a standard practice?

I'm a firm believer that you can't simply read about things to know what it is. You actually have to DO and practice it. I'm an experimentalist, so knowing what I'm measuring, and how I'm able to deduce that it is what it is, is extremely important. In a previous part of my career, a central issue of what I measure (related to my avatar) is whether an 'electron' that we measure in a solid is truly a well-defined particle, or weather such a concept (i.e. quasiparticle) is valid when there is a huge electron-electron interaction the that solid. So knowing what we measure and what that quantity represents is, and has always been, a central issue in physics, especially experimental physics. So that's why I am very puzzled that this is asked in the philosophy forum.

Zz.

you do lean toward the idea that just THINKING about things can be unproductive.


So, from that perspective, Einstein didn't know what he was doing and shouldn't have any attention paid to his ideas because HE didn't measure anything.

From your perspective, as an applied/experimental physicist, can you see that IDEA of anything most often comes first, rather than the experiment?----a lot of people HAVE ideas and CAN'T do experiments. Are they to be discounted for that reason?

 

[rewebster]

So the question I'm raising is -- what makes our universe different from the minimal one? How is it that in our universe adequate information is in fact physically available to determine so much about what's in the world?

...

The traditional way physics is done, ideally we want a “unified” model in which gravity and electromagnetism turn out to be the same thing. But if measurement is actually a basic feature of our world, then the difference between gravity and light is important. Instead of eliminating this difference in our fundamental theory, we would want to explain exactly what role this difference plays in the referential structure of physics.

The 'physics' of a lot of things are still unknown. Even when a 'theory of everything' comes out, there will still be questions and doubts.

We assign names and measures on things first to things that are most concrete and available to us for inspection that we have an interest. After that, its a guessing game (until the right 'guess' makes it a little more concrete).

 

[ConradDJ]

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.

the comparative sense is all that is entailed in measurement. Measurement is nothing more than comparison of comparable things in terms of standardized units.

This makes sense. Traditionally we just assume that things exist with certain definite properties, and that there are ways to observe each of these properties. So then, the only issue with measurement is that of finding a consistent way to compare observed properties with each other.

In the context of quantum physics, measurement takes on a different meaning, because the traditional assumptions are at best questionable. It turns out that physical properties generally do not have determinate values except when an interaction occurs that can communicate information about the property in question. Further, it’s not enough that an interaction occur – because in QM, interactions generally result in the entanglement of the two interacting systems, not in the so-called “collapse of the superposition”.

Exactly what is “enough” to constitute a measurement is the great difficulty with QM, and of course there’s a vast literature on this. I think probably the reason this has been so hard to resolve is that the “inter-referential” aspect of measurement I’m trying to focus on here hasn’t been taken seriously.

My argument doesn’t depend on QM. Even if the world were really as classical physics imagined it, any measurement of a physical quantity would still depend on a context of measurements of other physical quantities. And the ability of the universe to make some parameters observable would still be something remarkable.

However, QM does change this picture. Basically, QM provides strong evidence that ALL determinate physical quantities are observable (no “hidden variables”). To me this very strongly suggests that the inter-referential aspect of the structure of physics is in some way fundamental.

It’s not just that our universe happens to be built in such a way that some of its parameters are measurable in terms of others. Instead, it appears that the physical reality of our world in some way depends on measurement. Things only have definite characteristics to the extent that information about those characteristics is communicated to other things. So the physical structure that supports communication-contexts seems to be one of the most basic things we need to understand.

 

[brainstorm]

And I thought I did. My avatar was taken exactly from the raw output of an instrument.

The image seems to be an expression of some instrument being affected by some physical phenomenon, but it is not clearly defined what, how, and why. Maybe there are too many layers of complexity to provide direct sequences of actions and the logic of what is being measured and how.

This is why we have to know the physics of not only the phenomenon (interference), but also the physics of the instrumentation. I've described the physics of the instrumentation (i.e. the spectrometer). The physics of the phenomenon (photoemission) is covered in books.

I have little doubt that anything discussed on any website is not also covered in books. I'm just trying to get simple direct explanations of what is empirically observed and how it is measured. That way I can analyze why the thing measured is measurable, a la the thread title.

As far as I can tell, I've given you exactly what you wanted. Short of you actually doing the experiment yourself (which frankly I think everyone should), I'm not sure what else you would want./QUOTE]

I've given examples by describing the logic of measuring mass with a scale and that of measuring volume through water-displacement. I would like the same level of explanation for any other measurement process. I suspect the reason I'm asking a lot is because there's a lot of philosophy and mediality between what is directly empirically observable and what is believed to be measured by the process.

That basically proves my point that reasoning and logic the basis for measurement, but I like to see the connection with actual empirical observations of the phenomena and how the instruments work. Only at that level of dissection am I satisfied that I can trace the exact sequence of events that lead one thing to be compared and comparable with another in terms of standardized units, i.e. measured.

 

[ConradDJ]

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.

Kote -- I don’t agree that mass and spin are not “consistently defined” – but apparently what you mean is that they can’t be defined precisely as absolute, intrinsic properties of particles, since they’re subject to quantum indeterminacy. What you’re looking for as “basic” would be underlying properties that are absolutely definite in the nature of particles (or fields or whatever) “in themselves”.

Traditionally this is what philosophers and scientists have understood to be “basic”. What we observe is complex and seemingly chaotic – the goal has been to reduce it to a minimal set of simple, changeless facts. The ideal theory would describe an underlying reality that is not observable, but which accounts for everything we observe.

Against this traditional approach I would argue – even if there exists at the bottom of things a precisely well-defined reality, which precisely determines all the phenomena we observe, there is still an important aspect of the world that it can not account for – namely, that things are observable.

As argued above, for any particular physical parameter to be observable, there needs to be a context consisting of observations of several other parameters. It’s not clear exactly what is required in the structure of this observation-context, but one thing we know is not required is the “underlying reality”. Because this is not something that can be observed, it can make no contribution to the observational context.

To put it another way – the structure of observable phenomena has to be able to define itself whether or not there exists any underlying determinative “basis”. In principle, anything measurable has to be able to be defined exclusively in terms of other things that are measurable. We know that’s true because we can in fact measure things, and in doing so we do not have access to a non-phenomenal reality beyond what we can measure.

Now QM at least suggests that there may be no intrinsically well-defined reality at the bottom of things. That would mean that the inter-referential structure of observable phenomena is itself what’s “basic” in the physical world.

I think this puts a different spin on your question about “why something rather than nothing” – but I can’t take it any further today.

Thanks again -- Conrad

 

[GeorgCantor]

For me the most important idea behind the developments of twentieth-century physics and cosmology is that things don't have intrinsic properties at the fundamental level; all properties are about relations between things.

 

[brainstorm]

For me the most important idea behind the developments of twentieth-century physics and cosmology is that things don't have intrinsic properties at the fundamental level; all properties are about relations between things.

That sounds very grand, but it is also very vague. What is an intrinsic property except something's relation to itself or an observer? How can an intrinsic property be distinguished from a relational one? Without examples these are fruitlessly abstract issues.

 

[GeorgCantor]

But with all due respect to philosophers, have they measured anything as a standard practice?

Zz.

There is the measurement part and the interpretational part. I believe philosophers and philosophy-minded physicists are much better at the latter than intrumentalists. In fact, i and probably most philosophers, find instrumentalism dull and unrewarding.

 

[GeorgCantor]

That sounds very grand, but it is also very vague. What is an intrinsic property except something's relation to itself or an observer? How can an intrinsic property be distinguished from a relational one? Without examples these are fruitlessly abstract issues.

They are not vague in the slightest to those who understand the important lessons of quantum theory and general relativity.

 

[baywax]

My question when it comes to measurement is... where do you stop? I mean, firstly, in incremental measurements the increments continue between increments... so that between every increment there is an infinite amount of increments. These steps may not be observable to our instruments of measurement but, logically and numerically speaking they are there. So, as I said... where do you stop... there's always a "rounding off" of a measurement... but that could be the difference between reality and some imaginary measurement.

 

[brainstorm]

There is the measurement part and the interpretational part. I believe philosophers and philosophy-minded physicists are much better at the latter than intrumentalists. In fact, i and probably most philosophers, find instrumentalism dull and unrewarding.

The empiricism of measurement is only instrumental in the sense that you bracket the reality of what your measuring in order to analyze the logical connection between observations and synthesis.

What is much duller and unrewarding to me is when discussions of observability and measurement degenerate into insistence that the existence of reality is either a necessary precondition or a proven fact of empiricism. It is irrelevant whether anything is real or not. Access to data and reason is what it is, regardless of reality-status.

 

[brainstorm]

My question when it comes to measurement is... where do you stop? I mean, firstly, in incremental measurements the increments continue between increments... so that between every increment there is an infinite amount of increments. These steps may not be observable to our instruments of measurement but, logically and numerically speaking they are there. So, as I said... where do you stop... there's always a "rounding off" of a measurement... but that could be the difference between reality and some imaginary measurement.

Precision is relative to practical application. You're not measuring something to establish absolute truth about its traits. You are trying to answer questions or make predictions about its behavior.

This is what makes science inherently philosophical. Without theoretical reasoning about how a certain methodology addresses a certain question, you're just processing data or generating descriptive imagery (art).

A rounded-off measurement is not necessarily imaginary. It is just a question of sufficiency for the particular practical application. Good, clear scientific questions can be answered with relatively imprecise measurements. Where greater precision is relevant, it must be applied for the question to be answered.

 

[humanino]

I believe philosophers and philosophy-minded physicists are much better at the latter than intrumentalists.

Interesting personal opinion, but how informed ? How many "instrumentalists" do you know ? From what I read in this very subforum, self-appointed philosophers generally have a much poorer understanding of science than the average instrumentalist.

 

[Freeman Dyson]

In a word, patterns. Why are there patterns? I don't know... Einstein thought this the weirdest thing about the universe. That it was comprehensible. It could be made sense of. To a degree at least..

We can't define anything precisely. If we attempt to, we get into that paralysis of thought that comes to philosophers… one saying to the other: "you don't know what you are talking about!". The second one says: "what do you mean by talking? What do you mean by you? What do you mean by know?"

-Feynman

 

[brainstorm]

In a word, patterns. Why are there patterns? I don't know... Einstein thought this the weirdest thing about the universe. That it was comprehensible. It could be made sense of. To a degree at least.

Since when is science about recognizing or describing patterns? Patterns may be the basis for recognizability of observables, but science itself is about asking critical questions about what is observed and applying systematic investigation to answering them.

Theory and methodology are critical open processes that are subject to reason the same as any other part of scientific processes. The correct answer to "why does the instrument generate measurement X?" is never, "because it's accurate." Measurement processes have to be predicated on sound reasoning about the relationship between observations and instrumentation. Without that reasoning, you could be measuring the average girth of unicorns by interpreting the patterns on your itunes visualizer.

 

[ZapperZ]

The image seems to be an expression of some instrument being affected by some physical phenomenon, but it is not clearly defined what, how, and why. Maybe there are too many layers of complexity to provide direct sequences of actions and the logic of what is being measured and how.

If you look at T. Valla et al., Science 285, 2110 (1999), you'll have a complete explanation for the energy and momentum represented in the figure. This 2D images are now quite common in ARPES measurement that allows us to make a direct measurement of both the dispersion of a solid.

I have little doubt that anything discussed on any website is not also covered in books. I'm just trying to get simple direct explanations of what is empirically observed and how it is measured. That way I can analyze why the thing measured is measurable, a la the thread title.

As far as I can tell, I've given you exactly what you wanted. Short of you actually doing the experiment yourself (which frankly I think everyone should), I'm not sure what else you would want./QUOTE]

I've given examples by describing the logic of measuring mass with a scale and that of measuring volume through water-displacement. I would like the same level of explanation for any other measurement process. I suspect the reason I'm asking a lot is because there's a lot of philosophy and mediality between what is directly empirically observable and what is believed to be measured by the process.

That basically proves my point that reasoning and logic the basis for measurement, but I like to see the connection with actual empirical observations of the phenomena and how the instruments work. Only at that level of dissection am I satisfied that I can trace the exact sequence of events that lead one thing to be compared and comparable with another in terms of standardized units, i.e. measured.

But at some point, things does not stay that simple. For example, the deduction of the mass of a neutrino, for example. This comes in via a very interesting and non-trivial process of flavor mixing. In other words, you don't really "weigh" the mass of a particle. There's a whole lot of physics involved in such a deduction.

But even using a mass spectrometer, for example, to deduce the mass of something, will require you to know some basic classical E&M. A very common experiment in an undergraduate physics laboratory is the measurement of the ratio of e/m using some thermionic source in a uniform magnetic field. Again, the "location" of where the particle hits the screen corresponds to some mass, the same way it is done in my avatar, and the same way one deduces the frequency of the light based on the location of maxima/minima of the interference pattern.

So I'm not sure why you have an issue with all this, assuming that you are well-aware of such things already.

Zz.

 

[brainstorm]

Again, the "location" of where the particle hits the screen corresponds to some mass, the same way it is done in my avatar, and the same way one deduces the frequency of the light based on the location of maxima/minima of the interference pattern.

So I'm not sure why you have an issue with all this, assuming that you are well-aware of such things already.

You call it "an issue" as if you are defensive about something being questioned.

I am just trying to get the descriptions of measurement instruments and reasoning/deductions down to the level where they are falsifiable. I'm not doing this because I want to falsify them, per se, although shouldn't I want to if they are in fact falsifiable?

The reason is that as long as the description remains at the level of complexity and avoidance of putting the critical details on the table, there's no way to falsify or verify that what is presumed to be measured is actually being measured and how. Instead it's like a game of, "how long are you going to keep asking questions until you give up and accept this as the truth?"

That's not science.

 

[ZapperZ]

You call it "an issue" as if you are defensive about something being questioned.

I am just trying to get the descriptions of measurement instruments and reasoning/deductions down to the level where they are falsifiable. I'm not doing this because I want to falsify them, per se, although shouldn't I want to if they are in fact falsifiable?

The reason is that as long as the description remains at the level of complexity and avoidance of putting the critical details on the table, there's no way to falsify or verify that what is presumed to be measured is actually being measured and how. Instead it's like a game of, "how long are you going to keep asking questions until you give up and accept this as the truth?"

That's not science.

Science also involves the meticulous study of all the available knowledge. I gave you an exact reference. There's nothing to hide here. You're welcome to read it and learn from it if you wish, or not. You continue to ask for things that have been given.

Many of us have to put in a lot of effort to understand these things. There are no shortcuts. I spent at least one whole year as a postdoc just simply learning about the electron analyzer that are used in those ARPES experiments. None of these were handed to me in a platter, and they can't be. The process of learning and using these things is crucial in one's understanding of it.

So if you want to see if such a thing is falsifiable, you have to have an intimate understanding of it in the first place. What's wrong with such a concept?

Zz.

Zz.

 

[apeiron]

...the point is that if you have only gravity or only light, it’s impossible in principle to measure a spacetime interval.

Does a universe have to be measurable to exist? My answer, based on Peircean philosophy and systems science, is yes. A system could be considered a self-measuring device. Another way of saying self-organising. A system has to be self-consistent to persist as a dissipative process.

This approach also makes specific claims about the nature of that measurement structure. In particular here, there has to be emergently a dichotomisation of scale. You need a context to measure an event (and synergistically, many measured events add up to construct your measuring context - this second fact is also what the total theory has to capture, and what makes QM incomplete).

Now you highlight here a dichotomy between gravity and EM. You need one to be of different scale to the other to offer a realm in which measurement can take place. And I would argue further, that the two scales must be as far apart as possible. They must be the actual local~global limits of the system in question. You cannot stand in the middle of things and measure them properly. You have to stand right at the edge.

The obvious local~global dichotomy is then absolute rest~lightspeed. You cannot go any slower than absolute rest (QM uncertainty intrudes of course, so the value of absolute rest is asymptotic). And you cannot go faster than lightspeed (again an asymptotic story for massive particles).

So where you are talking about a difference between gravity and EM as being the contrast that allows measurement, I believe what is really at the back of your thinking is the scale contrast between restmass and lightspeed interactions.

Clearly, gravity is a lightspeed interaction itself. But measurable changes in gravitational potential are due to the local motions of masses - so tied to their capacity to be at rest and unchanging.

However, while this restmass~lightspeed dichotomy is essential to the kind of complex universe we find ourselves in, is it still possible to imagine a simpler case that is still a self-measuring system?

I think this is so (though I am happy to hear arguments otherwise). If we imagine a universe in which there was no CP asymmetry to prevent all massive particles radiating away into a pure bath of lightspeed photons, then could this universe exist? Is there anything in theory to prevent it?

I am presuming there would be no gravity fields, no measurable localised gravitational potential differences, because there would be no localised concentrations of mass. The radiation would have a gravity associated with it, but it would be all evenly spread out and so flat - a featureless field and so not observable.

Charley Lineweaver talks about this kind of thing...see p71 on the blackbody radiation that would arise just simply due to residual QM considerations in a de sitter universe with cosmological event horizons.

http://www.mso.anu.edu.au/~charley/papers/LineweaverChap_6.pdf

What would be the measurable here, as I understand it, is any lingering differences in local temperature. Lineweaver says if you have a cosmological constant (a global or general action) then you also have a minimal residual temperature that would be measureable at locales.

So rather than using EM to measure gravity (or whatever), the minimal measurement in this view of the heat death universe would be the global continued expansion (the cosmological constant and the event horizons it creates) vs the locally cooling residual action of photons "the wavelength of the visible universe".

This is the most minimal concept of the universe as a system that I have come across. Lineweaver seems to be developing the idea quietly with Davies (who is the most systems-sympathetic thinker among prominent cosmologists also I feel). It has not been published in an upfront way as cosmological theory yet. So perhaps it does not really fly.

This is why it would be nice to get some other opinions here.

But Conrad, if you are seeking a toy model of what minimal self-measurement would look like, the heat death universe would seem to be it. Especially if CP asymmetry and the persistence of mass is taken as an arbitrary feature of possible universes (it may always be inevitable for some reason - such as gauge symmetry breaking principles - of course).

 

[kote]

Let's go back to the basics, shall we? This is covered in standard intro QM that everyone has to take as a physics major.

The definition of the HUP of an observable is (drum roll):

\Delta(A) = \sqrt{<A>^2 - <A^2>}

I claim that, for the HUP, a single measurement of ANY observable has an uncertainty of ... ZERO!

Can you tell me where this is wrong?

Have you read anything I posted? I was very clear in making this exact claim. This has absolutely nothing to do with the status of observables when they aren't actively being measured, which is what we are talking about.

I have no idea why you're asking me for an example of two non-commuting observables measured simultaneously. I've been saying all along that it's impossible to do this. That's the entire point. Who are you trying to argue with?

Do you claim that all particles have, at all times, a well defined position? If not, I can't find anything I've actually said that you should be disagreeing with.

 

[apeiron]

But at some point, things does not stay that simple. For example, the deduction of the mass of a neutrino, for example. This comes in via a very interesting and non-trivial process of flavor mixing. In other words, you don't really "weigh" the mass of a particle. There's a whole lot of physics involved in such a deduction.

Yes, another good example of a soliton-inspired, constraints-based, approach. The universe as the measuring device does not have the resolution to limit the neutrino's existence to its minimal configuration. It "exists" as a mixture.

I like the kind of approach suggested by Carl Brannen.

Abstract: The spin of a free electron is stable but its position is not. Recent
quantum information research by G. Svetlichny, J. Tolar, and G. Chadzitaskos
have shown that the Feynman position path integral can be mathematically
dened as a product of incompatible states; that is, as a product
of mutually unbiased bases (MUBs). Since the more common use of MUBs is
in nite dimensional Hilbert spaces, this raises the question \what happens
when spin path integrals are computed over products of MUBs?" Such an
assumption makes spin no longer stable. But we show that the usual spin-1/2
is obtained in the long-time limit in three orthogonal solutions that we associate
with the three elementary particle generations. We give applications to
the masses and mixing matrices of the elementary fermions.

http://www.brannenworks.com/Gravity/EmergSpin.pdf

Yes, he is an amateur and the paper is not yet into publication. But it is the kind of reasoning that I am endorsing. He also has support from respectable sources...

http://dorigo.wordpress.com/2007/10...ers-by-authors-who-think-im-a-complete-idiot/

 

[rewebster]

Clearly, gravity is a lightspeed interaction itself. But measurable changes in gravitational potential are due to the local motions of masses - so tied to their capacity to be at rest and unchanging.

I believe that's an assumption/hypothesis which hasn't any data to support it.

 

[apeiron]

I believe that's an assumption/hypothesis which hasn't any data to support it.

OK, the experimental verification is still in question (http://www.space.com/scienceastronomy/gravity_speed_030116.html ).

But it is a reasonable assumption in most eyes. And there is some data, even if it is being questioned.

Or are you offering an argument that it has some different value? I would be interested in the shape of that argument.

 

[rewebster]

OK, the experimental verification is still in question (http://www.space.com/scienceastronomy/gravity_speed_030116.html ).

But it is a reasonable assumption in most eyes. And there is some data, even if it is being questioned.

Or are you offering an argument that it has some different value? I would be interested in the shape of that argument.

Can't...

the forum doesn't allow personal theories to be posted

 

[Evo]

Locked pending moderation.