A little help? Sol I? Dan O? Anyone???

[Dancing David]

Well, I copped a speil and a link to a philosophical document, and was told



I didn't understand what he meant...

https://www.pprune.org/5789444-post80.html

It seems a little strange to me.

ETA: OMFSM, it gets worse

http://plato.stanford.edu/entries/causation-counterfactual/#Lew197CouAna

 

[Dilb]

Insofar as I can parse that, I disagree with it completely. The idea that the only real things are those we can (in principle) measure not only has not been discredited, it's one of the most powerful ideas around. It has repeatedly proven its worth over the last century (the equivalence principle being an excellent example).

As near as I could understand, the philosophy objects to it because it defines physical things in terms of measurements. For example, the length of something is defined by the time it takes for light to cross it. The problem being that we can also measure length other ways, like by using a ruler. So we apparently have defined both "ruler length" and "optical length", without allowing for an underlying "true length" that both measurements are supposed to be equal to.

Of course, it's only philosophers who came up with this objection, and think of this as an actual problem. Somehow they went from "real things must be measurable" to "models are useless", pointing out things like how you never directly measure wavefunctions, or the thought experiments Einstein used to come up with relativity.

Apparently psychologists were also unhappy that this didn't somehow turn what they were doing into hard science, though I really don't follow that train of logic.

 

[Dancing David]

Hey don't blame psychologists some of us are behaviorists!

You can only observe behavior, no more, no less.

 

[Floyt]

Inquiring minds want to know... where else in physics do we find application for the equivalence principle? Of the top of my head I can't come up with anything but gravitation & rotational frame of reference.

 

[Vorpal]

Inquiring minds want to know... where else in physics do we find application for the equivalence principle? Of the top of my head I can't come up with anything but gravitation & rotational frame of reference.

I can't either, but it should be noted that it's just a particular example that's especially relevant to the OP because the question was initially gravitational. The general situation:

The view of science that quantities which you cannot discriminate with instruments are the same thing is about the only philosophy of physics which has been definitively discredited in the last hundred years. So it is rather inappropriate to try it on here, don't you think?

denies that observationally indistinguishable things should be taken as fundamentally equivalent whenever possible. I can't read this as anything other than a straight-up denial of Ockham's razor, which would be used (for example) whenever there was a theory unifying phenomena that were initially thought to be disparate (in the case of the equivalence principle, this would be gravitation and inertia).

----

As near as I could understand, the philosophy objects to it because it defines physical things in terms of measurements. For example, the length of something is defined by the time it takes for light to cross it. The problem being that we can also measure length other ways, like by using a ruler. So we apparently have defined both "ruler length" and "optical length", without allowing for an underlying "true length" that both measurements are supposed to be equal to.

I doubt that even deserves characterization a philosophical argument. There is an operational definition of a unit of length, so the former would be the "true length", while the alternative and easier methods, such as rulers, would be approximations. This is straight-forward even by standards of philosophy.

Somehow they went from "real things must be measurable" to "models are useless", pointing out things like how you never directly measure wavefunctions, or the thought experiments Einstein used to come up with relativity.

Wavefunctions are just as measurable as position, momentum, energy, etc.--i.e., in just the same sense as anything any other observable*. In the position basis, its amplitude-squared is a probability distribution of position measurements, and thus measurable in the statistical sense over many identically prepared particles (i.e., not conceptually worse than any other observable), while its phase of the wavefunction represents the probability flux, which is empirically recoverable from measurements over different times.

As for Einstein's thought experiments to argue something like that, I don't see the point, because what they did was ultimately the exact reverse of what that person is arguing for (i.e., the equivalence principle treats observational equivalence as indicative of fundamentally identical phenomena).

[size=-1]* In QM's language, the |ψ(x)|² is the expected value of the projection operator to the eigenspace of the position operator with eigenvalue x. Since this operator is self-adjoint, it represents an observable.[/size]

 

[sol invictus]

Of course, it's only philosophers who came up with this objection, and think of this as an actual problem. Somehow they went from "real things must be measurable" to "models are useless", pointing out things like how you never directly measure wavefunctions, or the thought experiments Einstein used to come up with relativity.

Wavefunctions can certainly be observed. Here's an image of some.

Inquiring minds want to know... where else in physics do we find application for the equivalence principle? Of the top of my head I can't come up with anything but gravitation & rotational frame of reference.

The equivalence principle is a statement about gravitation, so I'm not sure what you're asking.

 

[Dilb]

Wavefunctions can certainly be observed. Here's an image of some.

I don't agree with the complaint, but I think they'd respond by saying that the image is, at best, a measure of the position-projected wavefunction amplitude. Since the complex-valued wavefunction is not directly being measured, it's not "real". It then follows that since physicists do use wavefunctions, operationalism can't be right about real things being measurable.

From the Stanford Encyclopedia of Philosophy

Bridgman actually acknowledged from early on that there were good theoretical concepts that were not amenable to direct operationalization, illustrating the point with the example of stress and strain inside a solid body (1927, 53–54), and the wavefunction in quantum mechanics (Bridgman in Frank 1956, 79). Bridgman saw clearly that these theoretical concepts only had indirect connections with physical operations, but he did not see any problems with that. He in fact went so far as to say: “there need be no qualms that the operational point of view will ever place the slightest restriction on the freedom of the theoretical physicist to explore the consequences of any free mental construction” (Bridgman in Frank 1956, 79).[/url]

Ultimately I can't see how operationalism could apply to airplanes in any useful sense.

 

[Floyt]

The equivalence principle is a statement about gravitation, so I'm not sure what you're asking.

I guess that answers the question - I didn't know whether it was specific to relativity theory, or whether it was a wider principle that applied to gravitation, among other things.

 

[sol invictus]

I don't agree with the complaint, but I think they'd respond by saying that the image is, at best, a measure of the position-projected wavefunction amplitude. Since the complex-valued wavefunction is not directly being measured, it's not "real". It then follows that since physicists do use wavefunctions, operationalism can't be right about real things being measurable.

Except that we can and do measure both the amplitude and phase of wavefunctions. It's done all the time, in many different contexts, in any experiment that measures any kind of QM interference.

You see, if you were correct and we couldn't measure the phase, it would mean something very important - it would mean that the theory was overcomplete, and that knowledge would likely point to a simpler and more powerful formulation of it. It's that kind of thinking that led to general relativity, and a subtle version of it underlies gauge theories (and therefore our entire modern understanding of forces and particles).

That's why I object to strongly to the absurd claim (not yours) that this kind of thinking has been "discredited". That couldn't be further from the truth. It has proven repeatedly to be the sharpest philosophical tool available to physicists.

 

[Vorpal]

I don't agree with the complaint, but I think they'd respond by saying that the image is, at best, a measure of the position-projected wavefunction amplitude. Since the complex-valued wavefunction is not directly being measured, it's not "real".

The imaginary part of the wavefunction directly corresponds to the probability flux, it being a gradient scaled by the amplitude and some constants. If you can measure the wavefunction at a specific time, all you've to do is measure it at several different times to recover the phase, up to some overall factor But this last bit is no worse than, and in many situations exactly corresponding to, potential energy being defined only up to some arbitrary additive factor. Thus, it's well-defined even in cases without interference.

If the wavefunction amplitude is operationally defined--and since it corresponds to an observable (in QM's sense), I don't see why not--then so is the entire wavefunction.

 

[ponderingturtle]

Hi guys,

I need some help from a physics Guru, even if it is to tell me I'm wrong so I can drop it.

On an Aviation forum, a discussion came up regarding the forces on an aircraft in a turn.

As you'd be aware, and aircraft in a 60deg Angle of Bank turn experiences 2g. People started refering to this as the Aircrafts "Apparent Weight".

I chimed in and said it was meaningless calling it "Apparent Weight" as it was indestiguishable from "Real" weght. Basically the the opposite force felt as a reault of the aircrafts acceleration was the same thing as that due to Gravity, so it was incorrect to label them as different things.

I got pretty soundly shouted down, and one posted invoked that I was using "Operationalsim" which was out-moded.

What are the physics heavy-weights opinions?

Thanks

Well this gets into that weight is not a strictly defined term in physics. It is normally associated with the normal force, in which case you could say that any plane that is flying is weightless as gravity is being counter acted by non normal forces.(objects in the plane would still have weight being subjected to the normal force from the floor or chair).

It seems that they might be thinking of real weight more as the planes mass than weight. As you can assume 1g fairly effectively in the environment that they are concerned with this is not all that bad.