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Dec 17, 2002, 11:01:32 PM12/17/02

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In article <asfteu$jcm$1...@lfa222122.richmond.edu>

eb...@lfa221051.richmond.edu wrote:

eb...@lfa221051.richmond.edu wrote:

>By using both the words "unknown" and "unknowable," you seem to be

>suggesting that there's a distinction between them, but I can't figure

>out what the difference between "presently unknown" and "presently

>unknowable" is supposed to be.

Perhaps the distinction between "unknown" and "unknowable"

can be illustrated by the following examples from antiquity as well as the

present.

I. Examples from Antiquity

When Democritus posited his theory of atoms, the answer to the question

of whether or not atoms existed was unknown. It would remain unknowable for

about 2000 years, until technology was sophisticated enough for detection.

The circumference of the earth was also unknown, but was indeed

knowable. It was determined by Eratosthenes about 230 B.C. by means of

measuring and comparing the angles of shadows at different latitudes.

II Examples from the Present

Today, the answer to the question of whether life has ever existed on

Mars, while unknown, is essentially knowable, since there exist appropriate

methods of evaluation.

The nature of dark matter and dark energy, which is currently unknown,

will remain unknowable until appropriate means of detection are

developed.

Barbara Burger

Jan 6, 2003, 10:29:27 PM1/6/03

to

"Chuck and Barbara Burger" <bcbu...@rcn.com> wrote:

> When Democritus posited his theory of atoms, the answer to the question

> of whether or not atoms existed was unknown.

And it is still unknown.

> It would remain unknowable for about 2000 years, until technology

> was sophisticated enough for detection.

What was observed with the improving technology was the previously invisible

constituents of what is visible to the unaided eye.

The indivisible and indestructible Democretean primary elements were never

observed.

Ahmet Gorgun

Jan 13, 2003, 5:18:00 AM1/13/03

to sci-physic...@moderators.isc.org

In message <VjGR9.27646$p_6.2...@bgtnsc04-news.ops.worldnet.att.net>,

Ahmet Gorgun <ago...@att.net> writes

Ahmet Gorgun <ago...@att.net> writes

>"Chuck and Barbara Burger" <bcbu...@rcn.com> wrote:

>> When Democritus posited his theory of atoms, the answer to the question

>> of whether or not atoms existed was unknown.

I don't agree. Democritus teacher, Leucippus, posited the theory in

answer to the paradoxes of Zeno, and on the basis of rational thought,

as a way to get around the deep issues regarding the appearance of

infinity in a geometrical background space.

>The indivisible and indestructible Democretean primary elements were never

>observed.

But they have been observed now, electrons, quarks fulfil the role quite

accurately.

Regards

--

Charles Francis

Feb 5, 2003, 3:25:40 PM2/5/03

to

"Charles Francis" <cha...@clef.demon.co.uk> wrote:

> Ahmet Gorgun <ago...@att.net> wrote:

> >The indivisible and indestructible Democretean primary elements were never

> >observed.

>

> But they have been observed now, electrons, quarks fulfil the role quite

> accurately.

Are you saying that electron has absolutely no parts and you can prove that it

has absolutely no parts? Otherwise the electron is not the absolutely

indivisible elements that Democritus postulated.

Ahmet Gorgun

Feb 7, 2003, 7:46:30 AM2/7/03

to

Ahmet Gorgun <ago...@att.net> wrote in message

news:gik%9.18435$rq4.1...@bgtnsc05-news.ops.worldnet.att.net...

> Are you saying that electron has absolutely no parts and you can prove

> that it has absolutely no parts?

The prevailing theory and equations/description are all set up on electrons

being point particles.

What more proof does one need?

[Moderator's note: no compositeness has been seen up to energies

of at least 4 TeV:

http://www.slac.stanford.edu/pubs/snowmass96/PDF/NEW160.PDF

though this paper is from 1996, and the bound may be higher now. - jb]

Feb 12, 2003, 6:12:25 AM2/12/03

to sci-physic...@moderators.isc.org

"Ralph E. Frost" wrote:

> The prevailing theory and equations/description are all set up on electrons

> being point particles.

>

> What more proof does one need?

>

> [Moderator's note: no compositeness has been seen up to energies

> of at least 4 TeV:

Isn't there a significant difference between pointlike

(= no spatial extension) and composite (= made up of smaller particles)?

I can well conceive of extended (not pointlike) objects that cannot be divided

by any means (indivisible, not composite).

In this sense, an electron can well be regarded as an

extended indivisible particle (with an extension given by the region

where |psi|^2 is significant). More importantly, a photon, which cannot

be localized in space (according to Newton & Wigner),

can hardly be regarded as being pointlike in any geometric sense,

though it is probably indivisible.

Arnold Neumaier

Feb 12, 2003, 6:12:03 AM2/12/03

to

Charles Francis <cha...@clef.demon.co.uk> wrote in message

news:<oLjTjXAr...@clef.demon.co.uk>...

news:<oLjTjXAr...@clef.demon.co.uk>...

Well that's not the current view. Today, quarks and electrons are

considered to be 1-dimensional superstrings. Anyway, both Ahmet and

Charles are correct since they mean two different things. Obviously,

the theories of Leucippus and Democritus were not "correct". We have

never had a view of the Universe that was actually true, and we never

will. The purpose of physics is not to find out what's true, since

we'll never be able to do that. The purpose of physics is to think up

possible explanations that could possibly explain what you observe.

You can read "On the Nature of Things" written by Lucretius in the

first century B.C. for a good overview of the various theories of

particle physics in the classical world. You can see how they arrived

at theories through logical reasoning based on observation. Of course,

it's also obvious to us that none of their theories are true. Of

course, our view of the Universe, which is based on superstrings and

M-theory, is not true either, but that's not the point. We've never

had a view of the Universe that was actually true, and we never will.

That's not the purpose of physics. The purpose of physics is to come

up with theories that fit what we observe, and we've been remarkably

successful at doing that.

I don't think Charles and others defending Democritus are saying that

their theory of atoms, the way they imagined them, is actually true.

They were rather speaking in a broad sense, in that Democritus and

others were right in saying that macroscopic matter is made of

constituents too small to be seen, with empty space between them, as

opposed to the prevailing view that macroscopic was composed of

continuous solid matter. So that basic idea turned out to be right,

although obviously their view of the Universe, and their specific

theories, are obviously not correct. They were right in that

macroscopic matter is made of smaller constituents, but Democritus

imagined the smaller constituents being point particles, and today we

call them molecules, which are made of atoms, which are made of

electrons and atomic nuclei, which are made of protons and neutrons,

which are made of quarks, which we now consider to be actually

one-dimensional superstrings.

Jeffery Winkler

Feb 12, 2003, 3:10:29 PM2/12/03

to

In article <8a8c1f93.03020...@posting.google.com>,

Jeffery <jeffery...@hotmail.com> wrote:

>Charles Francis <cha...@clef.demon.co.uk> wrote in message

>news:<oLjTjXAr...@clef.demon.co.uk>...

>

>> In message <VjGR9.27646$p_6.2...@bgtnsc04-news.ops.worldnet.att.net>,

>> Ahmet Gorgun <ago...@att.net> writes

>>

Jeffery <jeffery...@hotmail.com> wrote:

>Charles Francis <cha...@clef.demon.co.uk> wrote in message

>news:<oLjTjXAr...@clef.demon.co.uk>...

>

>> In message <VjGR9.27646$p_6.2...@bgtnsc04-news.ops.worldnet.att.net>,

>> Ahmet Gorgun <ago...@att.net> writes

>>

>> >The indivisible and indestructible Democretean primary elements were never

>> >observed.

>>

>> But they have been observed now, electrons, quarks fulfil the role quite

>> accurately.

>

>

>Well that's not the current view. Today, quarks and electrons are

>considered to be 1-dimensional superstrings.

>> >observed.

>>

>> But they have been observed now, electrons, quarks fulfil the role quite

>> accurately.

>

>

>Well that's not the current view. Today, quarks and electrons are

>considered to be 1-dimensional superstrings.

Considered by some.

One should not forget that there is no experimental evidence of this,

nor a workable phenomenological model.

--

======================================================================

Kevin Scaldeferri Calif. Institute of Technology

The INTJ's Prayer:

Lord keep me open to others' ideas, WRONG though they may be.

Feb 15, 2003, 2:46:57 AM2/15/03

to

Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

news:<3e43b60b$0$9620$3b21...@news.univie.ac.at>...

news:<3e43b60b$0$9620$3b21...@news.univie.ac.at>...

> In this sense, an electron can well be regarded as an

> extended indivisible particle (with an extension given by the region

> where |psi|^2 is significant).

This is definitely _wrong_, according to current wisdom, and is a

not uncommon misconception. The wavefunction reflects the

indeterminancy of the particle's location, not it's spatial extension.

A "wavefunction wave" falling upon a screen of detectors can cause

only one detector to fire, not several.

> More importantly, a photon, which cannot

> be localized in space (according to Newton & Wigner),

> can hardly be regarded as being pointlike in any geometric sense,

> though it is probably indivisible.

This is already a much more subtle issue. I didn't form my final

opinion on the subject yet, but a priori it seems hard to imagine

how an object with a single position degree of freedom can be

spatially extended in a local theory. Possibly we're just using

the wrong variables: in 1+1D, the rapidity and the generator of

boosts are canonically conjugate (up to a constant, I think) and the

generator of boosts is the "proper coordinate": the coordinate at

time zero in the particle's own frame. This operator of course has

a (generalized) zero eigenstate which can be interpreted as the

state for which the particle passes through the origin, with full

honesty. In higher dimensions, though, it's more problematic as the

boost generators don't commute...

Best regards,

Squark

------------------------------------------------------------------

Write to me using the following e-mail:

Skvark_N...@excite.exe

(just spell the particle name correctly and change the

extension in the obvious way)

Feb 15, 2003, 3:30:04 AM2/15/03

to by lfa222122.richmond.edu id h1DEcDo29041, Thu

In message <8a8c1f93.03020...@posting.google.com>, Jeffery

<jeffery...@hotmail.com> writes

>Charles Francis <cha...@clef.demon.co.uk> wrote in message

>news:<oLjTjXAr...@clef.demon.co.uk>...

>>Democritus teacher, Leucippus, posited the theory in

>>answer to the paradoxes of Zeno, and on the basis of rational thought,

>>as a way to get around the deep issues regarding the appearance of

>>infinity in a geometrical background space.

>> >The indivisible and indestructible Democretean primary elements were never

>> >observed.

>> But they have been observed now, electrons, quarks fulfil the role quite

>> accurately.

>Well that's not the current view. Today, quarks and electrons are

>considered to be 1-dimensional superstrings.

That is only speculation, and it is a speculation which seems to me to

have born remarkably little fruit when one considers all the work which

has been done on it.

>Anyway, both Ahmet and

>Charles are correct since they mean two different things. Obviously,

>the theories of Leucippus and Democritus were not "correct". We have

>never had a view of the Universe that was actually true, and we never

>will.

I think it helps to loosen up a bit on what one means by "correct". It

is probably not within language to be absolute and literal, but if we

merely mean by correct "containing a great deal of truth" that is a far

more reasonable and workmanlike objective.

>The purpose of physics is not to find out what's true, since

>we'll never be able to do that.

I do hate this modern habit of prejudging the issue. We do not know we

will never be able to do that.

>You can read "On the Nature of Things" written by Lucretius in the

>first century B.C. for a good overview of the various theories of

>particle physics in the classical world. You can see how they arrived

>at theories through logical reasoning based on observation.

Actually you can't. Lucretius was a Roman poet writing about two hundred

years after Leucippus and Democritus, and he had little grasp of the

theory and no grasp of the logical problems the theory was intended to

circumvent. The essential idea was that that Zeno had shown that the

concept of "space" was badly flawed, and that there are serious problems

with infinity in physics. The model was intended to do away with that by

disposing of "space", and talking of the "void" meaning a non-existence,

a complete absence of properties. This idea was later picked up by

Descartes, who said that it does not make sense to talk of "space", and

reappears in the orthodox interpretation of quantum mechanics which

prohibits discussion of observable properties (such as position) between

measurements.

The atomic model was also intended to counter Aristotle's idea of

infinite subdivisibility. If there is no infinite subdivision it follows

that there is a smallest indivisible element. In so far as I can see we

have no reason even now to think that these central parts of the

original atomic model are not "correct" (I am not claiming here that

they are proven either).

>I don't think Charles and others defending Democritus are saying that

>their theory of atoms, the way they imagined them, is actually true.

Certainly I don't think that Democritus came up with an absolutely

precise and rigorous scientific theory, correct in every way. For

example when he discusses the "shape" of an atomic particles I would

have to substitute something like "Dirac spinor" and "Vector Boson".

Regards

--

Charles Francis

Feb 15, 2003, 3:34:09 AM2/15/03

to

>Well that's not the current view. Today, quarks and electrons are

>considered to be 1-dimensional superstrings.

I'm sorry, this is misleading. There is not a single piece of

experimental evidence for superstring theory; it's just a line

of research that's currently popular. A string theorist might say

"quarks and leptons are HYPOTHESIZED TO BE 1-dimensional superstrings",

but the phrase CONSIDERED TO BE suggests some sort of consensus

based on experimental evidence, and that doesn't exist.

It's also misleading to speak "the current view", as if there

were such a monolithic thing. Even within superstring theory there

are a large number of specific models battling for acceptance, and

no clear winner yet.

Feb 17, 2003, 7:00:37 AM2/17/03

to sci-physic...@moderators.isc.org

Squark wrote:

> Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

> news:<3e43b60b$0$9620$3b21...@news.univie.ac.at>...

> > In this sense, an electron can well be regarded as an

> > extended indivisible particle (with an extension given by the region

> > where |psi|^2 is significant).

> This is definitely _wrong_, according to current wisdom, and is a

> not uncommon misconception. The wavefunction reflects the

> indeterminancy of the particle's location, not it's spatial extension.

> A "wavefunction wave" falling upon a screen of detectors can cause

> only one detector to fire, not several.

But this does not contradict my statement. The latter is a macroscopic

consequence of the particle hitting the screen; the result must be analyzed

in terms of a complicated dynamics. An extended flood also breaks a dam

only at one place, that of least resistance...

To argue that something is _wrong_ (a logical category), one should give

a _logical_ argument, not a handwaving plausibility statement.

Arnold Neumaier

Feb 17, 2003, 6:10:35 PM2/17/03

to sci-physic...@moderators.isc.org

Squark wrote:

>

> Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

> news:<3e43b60b$0$9620$3b21...@news.univie.ac.at>...

>

> > In this sense, an electron can well be regarded as an

> > extended indivisible particle (with an extension given by the region

> > where |psi|^2 is significant).

>

> This is definitely _wrong_, according to current wisdom, and is a

> not uncommon misconception. The wavefunction reflects the

> indeterminancy of the particle's location, not it's spatial extension.

>

> Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

> news:<3e43b60b$0$9620$3b21...@news.univie.ac.at>...

>

> > In this sense, an electron can well be regarded as an

> > extended indivisible particle (with an extension given by the region

> > where |psi|^2 is significant).

>

> This is definitely _wrong_, according to current wisdom, and is a

> not uncommon misconception. The wavefunction reflects the

> indeterminancy of the particle's location, not it's spatial extension.

It is not just my ideosyncracy, but gives a quite useful and intuitive

geometric picture of microphysics, especially if one wants to

understand the meaning of everything in a relativistic context.

For example, discussing the localization of relativistic particles

in space-time,

D. Marolf and C. Rovelli, Relativistic quantum measurement,

Phys.Rev. D66 (2002) 023510, gr-qc/0203056,

say on p.7 (top right, of the archived version):

... the quantum particle has an intrinsic Compton ``extension''...

Arnold Neumaier

Feb 17, 2003, 6:15:19 PM2/17/03

to

"Ralph E. Frost" <ref...@dcwi.com> wrote in message

news:v43tf2h...@corp.supernews.com...

>

> Ahmet Gorgun <ago...@att.net> wrote in message

> news:gik%9.18435$rq4.1...@bgtnsc05-news.ops.worldnet.att.net...

>

> > Are you saying that electron has absolutely no parts and you can prove

> > that it has absolutely no parts?

>

> The prevailing theory and equations/description are all set up on

electrons

> being point particles.

>

> What more proof does one need?

news:v43tf2h...@corp.supernews.com...

>

> Ahmet Gorgun <ago...@att.net> wrote in message

> news:gik%9.18435$rq4.1...@bgtnsc05-news.ops.worldnet.att.net...

>

> > Are you saying that electron has absolutely no parts and you can prove

> > that it has absolutely no parts?

>

> The prevailing theory and equations/description are all set up on

electrons

> being point particles.

>

> What more proof does one need?

Let's apply the same logic to celestial mechanics: If we assume that planets

are point particles we can still predict their position with accuracy. From

this it does not follow that planets are indivisible. The same is true for

the electron. How do you prove that electron has no parts?

> [Moderator's note: no compositeness has been seen up to energies of at

> least 4 TeV:

> http://www.slac.stanford.edu/pubs/snowmass96/PDF/NEW160.PDF

> though this paper is from 1996, and the bound may be higher now. - jb]

This reference looks for compositeness of Quarks and Leptons not electrons.

Ahmet Gorgun

[Moderator's note: Electrons are leptons. -TB]

Feb 18, 2003, 4:37:12 PM2/18/03

to

Arnold Neumaier <Arnold....@univie.ac.at> wrote in message news:<3e50b4ea$0$14964$3b21...@news.univie.ac.at>...

> But this does not contradict my statement. The latter is a macroscopic

> consequence of the particle hitting the screen; the result must be analyzed

> in terms of a complicated dynamics. An extended flood also breaks a dam

> only at one place, that of least resistance...

> But this does not contradict my statement. The latter is a macroscopic

> consequence of the particle hitting the screen; the result must be analyzed

> in terms of a complicated dynamics. An extended flood also breaks a dam

> only at one place, that of least resistance...

But there is no dynamical theory of that sort to account for quantum

phenomena. Would there be one, there would be no need for quantum theory,

as everything would be classical. However, it is apparently impossible to

construct that sort of theory consistently with quantum mutli-particle

effects (i.e. EPR correlations).

> To argue that something is _wrong_ (a logical category), one should give

> a _logical_ argument, not a handwaving plausibility statement.

As I said, I'm only talking about current wisdom. In current theory, the

wavefunction does not express spatial extension and position measurements

always yield a non-ambiguous result. This theory is completely consistent

with experiment, and to counter it you would have to present an alternative.

The later is problematic in view of Bell's theorem, unless you're apt for

de Broglie - Bohm. In de Broglie - Bohm one has a pilot wave which might be

said to posses a spatial extension, but also a particle with a definite

coordinate.

Feb 19, 2003, 11:57:19 PM2/19/03

to

In message <3e43b60b$0$9620$3b21...@news.univie.ac.at>, Arnold

Neumaier <Arnold....@univie.ac.at> writes

>. More importantly, a photon, which cannot

>be localized in space (according to Newton & Wigner),

>can hardly be regarded as being pointlike in any geometric sense,

>though it is probably indivisible.

I don't agree with the conclusion, though there are certainly subtle

issues regarding the wave function of the photon. You cannot actually

measure the position of a photon, you can only measure the position of

the electron which absorbed the photon. This does do curious things to

the wave function, but not enough to say that the photon is not

pointlike.

For "pointlike in a geometrical sense" I follow one of the definitions

going back to Euclid, that a point is that which has neither length nor

breadth. This definition I think does hold up, though clearly any

definition which claims that a point has a position fails to make sense

in the quantum domain.

Regards

--

Charles Francis

Feb 19, 2003, 11:57:10 PM2/19/03

to

Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

news:<3e50f609$0$14964$3b21...@news.univie.ac.at>...> It is not just my idiosyncracy, but gives a quite useful and intuitive

> geometric picture of microphysics, especially if one wants to

> understand the meaning of everything in a relativistic context.

> For example, discussing the localization of relativistic particles

> in space-time,

> D. Marolf and C. Rovelli, Relativistic quantum measurement,

> Phys.Rev. D66 (2002) 023510, gr-qc/0203056,

> say on p.7 (top right, of the archived version):

> ... the quantum particle has an intrinsic Compton ``extension''...

One must not confuse the apparent "extended" nature of relativistic

quantum particles with the general notion of indeterminate position in

quantum mechanics. It is true that in relativist quantum mechanics the

notion of position is subtle and problematic, but note the "extension"

is of the size of the Compton wavelength, not the characteristic

length of the |psi(x)^2| distribution.

Feb 19, 2003, 11:57:07 PM2/19/03

to

Squark <fii...@yahoo.com> writes

>This is definitely _wrong_, according to current wisdom, and is a

>not uncommon misconception. The wavefunction reflects the

>indeterminancy of the particle's location, not it's spatial extension.

>A "wavefunction wave" falling upon a screen of detectors can cause

>only one detector to fire, not several.

Red rag to a bull, this.

What makes you believe that a 'wavefunction wave' falling on a screen of

detectors causing one to fire excludes the particle from being spatially

extended before then?

If we take a 'free' particle, we can indisputably send it through both

slits and perform many experiments that show it behaves as an extended

wavelike object. The single-photon diffraction pattern is a bit of a

giveaway on that score.

In fact the ONLY time we see it as a pointlike particle is when we

attempt to locate it's precise position. It should not be surprising

that 'locating it's precise position' results in seeing it at a point.

That is what 'locating it's precise position' means, after all.

I have discussed this point here many times and so far nobody has given

me a convincing argument as to why one should not correctly consider a

free diffracting particle as being an extended wave.

IMHO (albeit of a near total ignoramus) the pointlike qualities are a

function of the wavefunction-detector interaction which generally

requires localisation as part of the detection.

--

Oz

This post is worth absolutely nothing and is probably fallacious.

Note: soon (maybe already) only posts via despammed.com will be accepted.

Feb 20, 2003, 7:26:12 PM2/20/03

to

Oz <aco...@btopenworld.com> wrote in message news:<b31n73$gbg$1...@panther.uwo.ca>...

> In fact the ONLY time we see it as a pointlike particle is when we

> attempt to locate it's precise position. It should not be surprising

> that 'locating it's precise position' results in seeing it at a point.

> That is what 'locating it's precise position' means, after all.

>

> I have discussed this point here many times and so far nobody has given

> me a convincing argument as to why one should not correctly consider a

> free diffracting particle as being an extended wave.

> In fact the ONLY time we see it as a pointlike particle is when we

> attempt to locate it's precise position. It should not be surprising

> that 'locating it's precise position' results in seeing it at a point.

> That is what 'locating it's precise position' means, after all.

>

> I have discussed this point here many times and so far nobody has given

> me a convincing argument as to why one should not correctly consider a

> free diffracting particle as being an extended wave.

It all depends on definitions, as always. However, what we might expect

of a physical extended object is, for instance, the possibility the

measure it's state at every point separately. This cannot be done with

the quantum particle. There is no way you can measure the value of the

wavefunction at any given point.

Moreover, if you detect a quantum particle at a certain point, the rest

of the wavefunction "disappears" instantaneously, which would be

"action at a distance" if the wavefunction indeed represented an

extended object, which is another hint at the fact the situation is not

so.

Feb 21, 2003, 12:25:28 AM2/21/03

to sci-physic...@moderators.isc.org

Squark wrote:

> Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

news:<3e50b4ea$0$14964$3b21...@news.univie.ac.at>...

> > But this does not contradict my statement. The latter is a macroscopic

> > consequence of the particle hitting the screen; the result must be analyzed

> > in terms of a complicated dynamics. An extended flood also breaks a dam

> > only at one place, that of least resistance...

>

> But there is no dynamical theory of that sort to account for quantum

> phenomena. Would there be one, there would be no need for quantum theory,

> as everything would be classical. However, it is apparently impossible to

> construct that sort of theory consistently with quantum mutli-particle

> effects (i.e. EPR correlations).

No; this has nothing to do with EPR or hidden variables.

There *is* a dynamical quantum theory of multiparticle interaction,

namely the multiparticle Schroedinger equation.

The measurement process is the result of interaction of a single

quantum particle with a quantum multiparticle system (the detector),

and therefore should be described in these terms. Sometimes,

measurement is idealized as instantaneous reduction of the wave packet,

but this is well-known to be inaccurate, and hides what is going on

under the carpet. But sometimes, more realistic scenarios were discussed.

I have seen derivations of the path of a particle in a bubble chamber

(answering the question, 'why do the bubbles describe a path

although the particle has a wave function without well-defined position?'),

and in a similar way one must be able to study the interaction

of a particle with a photographic plate, although I haven't seen

anything about this.

The analogy with a dam is then quite reasonable -

the detector is a specially prepared unstable thermodynamic system with

an energy landscape with multiple local minima at the possible outcomes

of the measurement, and details of the microstate determine into which

of these local minima the system will fall when excited by an incident particle

and dissipating its energy. But it will fall only into one,

of course.

> > To argue that something is _wrong_ (a logical category), one should give

> > a _logical_ argument, not a handwaving plausibility statement.

> As I said, I'm only talking about current wisdom. In current theory, the

> wavefunction does not express spatial extension and position measurements

> always yield a non-ambiguous result.

This is the reduction of the wave packet of the 1930's, which puts

the particle into a position eigenstate, in which |psi|^2 indeed is a

delta function, hence pointlike also according to my recipe.

But it is pointlike only at the idealized measurement instant, not before!

Before the measurement, it is generally not in a position eigenstate,

hence has an extended |psi|^2 distribution, and therefore a spatial

extension. The act of measurement changes the shape of the wave

function, and hence its spatial extension.

But this is all heavily idealized; realistic measurements are neither

instantaneous, as required by von Neumann's orthodox theory, nor do

they localize perfectly in space. Many people think there is no

(nonunitary) reduction of the wave packet at all. There is a thick book

by Wheeler and Zurek (Quantum theory and measurement,

Princeton Univ. Press, Princeton 1983) with collected articles about

all this, displaying the full range of current uncertainty and lack of

wisdom.

Fortunately, there is also more recent stuff, e.g., an excellent book by

Braginsky and Khalili (Quantum measurement, Cambridge Univ. Press,

Cambridge 1992) on _real_ quantum measurements close to actual

(optical) experiments, and they talk for example (p.3 bottom) about

'the photon must have occupied a volume larger than the slit separation.'

I'd take this to be the current wisdom.

Arnold Neumaier

Feb 21, 2003, 12:25:24 AM2/21/03

to

Squark <fii...@yahoo.com> writes

>As I said, I'm only talking about current wisdom. In current theory, the

>wavefunction does not express spatial extension and position measurements

>always yield a non-ambiguous result. This theory is completely consistent

>with experiment, and to counter it you would have to present an alternative.

The two slit experiment with single particles refutes your statement.

How does this single particle go through both slits simultaneously?

Feb 21, 2003, 2:17:36 PM2/21/03

to

Squark <fii...@YAHOO.COM> writes

>Oz <aco...@btopenworld.com> wrote in message news:<b31n73$gbg$1...@panther.uwo.ca>

>...

>> In fact the ONLY time we see it as a pointlike particle is when we

>> attempt to locate it's precise position. It should not be surprising

>> that 'locating it's precise position' results in seeing it at a point.

>> That is what 'locating it's precise position' means, after all.

>>

>> I have discussed this point here many times and so far nobody has given

>> me a convincing argument as to why one should not correctly consider a

>> free diffracting particle as being an extended wave.

>

>It all depends on definitions, as always.

>Oz <aco...@btopenworld.com> wrote in message news:<b31n73$gbg$1...@panther.uwo.ca>

>...

>> In fact the ONLY time we see it as a pointlike particle is when we

>> attempt to locate it's precise position. It should not be surprising

>> that 'locating it's precise position' results in seeing it at a point.

>> That is what 'locating it's precise position' means, after all.

>>

>> I have discussed this point here many times and so far nobody has given

>> me a convincing argument as to why one should not correctly consider a

>> free diffracting particle as being an extended wave.

>

>It all depends on definitions, as always.

Indeed.

>However, what we might expect

>of a physical extended object is, for instance, the possibility the

>measure it's state at every point separately.

I'm not sure why you would expect that of a quantised wave.

Measuring it's state inevitably destroys the state, that's a feature of

QM. Not least it localises the particle, how else can you measure a

particle at a point?

>This cannot be done with

>the quantum particle. There is no way you can measure the value of the

>wavefunction at any given point.

Hmmm. Yes, and no. You can do it statistically by preparing particles in

the same state and measuring a lot of them. Actually you have little

choice but to use this method since detecting a particle destroys the

state (and usually the particle). Fortunately preparing a particle in a

given state is often very easy because particles are easily divided into

identical species (electron, photon, etc) so when bound deliver

identical wavefunctions (to first order). Physicists are real good at

preparing particles in required states to order.

>Moreover, if you detect a quantum particle at a certain point, the rest

>of the wavefunction "disappears" instantaneously, which would be

>"action at a distance" if the wavefunction indeed represented an

>extended object, which is another hint at the fact the situation is not

>so.

I'm not sure that this is indeed typically true, not that it would

matter that much if it did.

Take the emission of a photon by an atom. Typically this does not happen

'instantly', in fact the time required is very well known for many types

of emission. Time reverse this and you have an absorption and I can see

no reason why the absorption is 'instant' whilst the emission is 'slow'.

Why should there be any difference in basic mechanism for other

absorbers like silver halide film?

If you then proceed to entangled pairs and cite 'FTL information

transmission' then you have to explain what physical laws are being

broken to make this implausible. For the rest of the universe there is

no instantaneous passage of useful information, no breaking of any of

the conservation laws (quite the contrary) and all is perfectly well

with the world. Why should this be implausible?

A particle formulation, though, has serious problems with self-

interference and big problems when you start putting waves through

polarisers. Sure they can all be overcome by appropriate mathematical

jugglement and the mathematical techniques are well understood and

convenient, but it's much more straightforward to imagine the particle

as a simple quantised wave.

Then you will point to the 'pointlike' electron (say). It's so pointlike

we can diffract it and treat it perfectly happily as a wave. Aha, you

say, but when we fire really short wavelength particles at it we find it

increasingly behaves like a point. Is this so surprising? The wavelength

of a particle is momentum-dependent. The higher the momentum the shorter

the wavelength. The shorter the wavelength the smaller a particle looks.

It's a completely self-fullfilling prophecy that a particle looks

smaller when probed at higher energies. It doesn't contradict wave

formulation, it confirms it.

Aha, you say next, look at the derivation of electron properties

provided by pointlike QED. Surely this proves that electrons are

pointlike? Well, ignoring the cutoff (ie it's NOT a point) my

understanding is that QED is identical (should I say isomorphic or

something?) to wave-based formulations. So one could do the same

integration using a wave formulation, just it's probably very much

easier done following feynman.

Over to you .....

Feb 21, 2003, 2:22:40 PM2/21/03

to

Arnold Neumaier <Arnold....@univie.ac.at> wrote in message news:<3e538dee$0$14964$3b21...@news.univie.ac.at>...

> the detector is a specially prepared unstable thermodynamic system with

> an energy landscape with multiple local minima at the possible outcomes

> of the measurement, and details of the microstate determine into which

> of these local minima the system will fall when excited by an incident

> particle and dissipating its energy.

> the detector is a specially prepared unstable thermodynamic system with

> an energy landscape with multiple local minima at the possible outcomes

> of the measurement, and details of the microstate determine into which

> of these local minima the system will fall when excited by an incident

> particle and dissipating its energy.

But there is no such "microstate"! Such a microstate would be exactly

what is called "hidden variables".

> Before the measurement, it is generally not in a position eigenstate,

> hence has an extended |psi|^2 distribution, and therefore a spatial

> extension. The act of measurement changes the shape of the wave

> function, and hence its spatial extension.

You might have misunderstood my point. I perfectly know and agree

the wavefunction has spatial extension. The only thing I'm claiming

is that the physical object the information about which the

wavefunction represents, i.e. the quantum particle, cannot be

assigned the spatial extension of the |psi^2| distribution.

> But this is all heavily idealized; realistic measurements are neither

> instantaneous, as required by von Neumann's orthodox theory, nor do

> they localize perfectly in space.

As I said, relativistic effects are another issue, but the predict a

finite extension unrelated, in general, to the |psi^2| distribution.

In fact, it proves my point: the distribution is present both in

relativistic and non-relativistic QM, but the problem with describing

the quantum particle as point-like only arises in the relativistic

case.

> Fortunately, there is also more recent stuff, e.g., an excellent book by

> Braginsky and Khalili (Quantum measurement, Cambridge Univ. Press,

> Cambridge 1992) on _real_ quantum measurements close to actual

> (optical) experiments, and they talk for example (p.3 bottom) about

>

> 'the photon must have occupied a volume larger than the slit separation.'

>

> I'd take this to be the current wisdom.

I never read this book, and don't have the possibility to either browse it

or understand in what context was this phrase said. Therefore, you are

putting me in a somewhat unfair position here. In any case, arguing about

whether something is or is not the current wisdom is silly enough, and in

the circumstances it's hard to bring good evidence. Therefore, if you

don't beleive me what I'm saying _is_ current wisdom, I won't engage in an

arguement about it.

Feb 22, 2003, 12:59:39 AM2/22/03

to

Oz <aco...@btopenworld.com> wrote in message

news:<b34d84$fpq$1...@panther.uwo.ca>...> The two slit experiment with single particles refutes your statement.

>

> How does this single particle go through both slits simultaneously?

It doesn't. It goes through one of them, but it is undefined which.

Feb 22, 2003, 1:59:49 AM2/22/03

to

Squark <fii...@yahoo.com> writes

>One must not confuse the apparent "extended" nature of relativistic

>quantum particles with the general notion of indeterminate position in

>quantum mechanics. It is true that in relativist quantum mechanics the

>notion of position is subtle and problematic, but note the "extension"

>is of the size of the Compton wavelength, not the characteristic

>length of the |psi(x)^2| distribution.

Hmmm. Is it? Where is the electron in an outer orbital of a heavy atom?

Sure, you can probe for it with a high energy particle but this will

just give you some 'found' position but does not mean the particle was

'there' before it interacted with your probe.

OTOH I would _probably_ agree with you in situations like the S2->S1

orbital transition where psi is roughly a spherical shell because I feel

that the 'recoiling atom' is big and complex enough to result in de-

entanglement (I could be wrong here). In that case I might well consider

'our total knowledge of the position of the photon' to indeed be

spherical (and on average give splendidly accurate results) even though

the emitting atom did indeed recoil 'unobserved' to give a modestly

localised photon (at least in momentum).

There again, I am continually fireballed for this viewpoint.

Feb 22, 2003, 1:59:54 AM2/22/03

to

Arnold Neumaier <Arnold....@univie.ac.at> writes:

[NB Note I am in fact stunningly ignorant and not expert]

>No; this has nothing to do with EPR or hidden variables.

>

>There *is* a dynamical quantum theory of multiparticle interaction,

>namely the multiparticle Schroedinger equation.

>The measurement process is the result of interaction of a single

>quantum particle with a quantum multiparticle system (the detector),

>and therefore should be described in these terms. Sometimes,

>measurement is idealized as instantaneous reduction of the wave packet,

>but this is well-known to be inaccurate, and hides what is going on

>under the carpet. But sometimes, more realistic scenarios were discussed.

Cor! I do hope you are an expert because I have been arguing this for

years and being told I am simply ignorant (which is indeed true). Mind

you, you do put it in proper technospeak.

>I have seen derivations of the path of a particle in a bubble chamber

>(answering the question, 'why do the bubbles describe a path

>although the particle has a wave function without well-defined position?'),

>and in a similar way one must be able to study the interaction

>of a particle with a photographic plate, although I haven't seen

>anything about this.

The cloud chamber is a position detector. It must localise the particle

to detect it. The particle must thus have a (reasonably) well-defined

position (to within a few wavelengths).

>The analogy with a dam is then quite reasonable - the detector is a

>specially prepared unstable thermodynamic system with an energy

>landscape with multiple local minima at the possible outcomes of the

>measurement, and details of the microstate determine into which of

>these local minima the system will fall when excited by an incident

>particle and dissipating its energy. But it will fall only into one,

>of course.

Yeeees!!!

Been claiming exactly this for years!

It's worth also pointing out that typically the incoming wave is HUGE

compared to the number of local minima (if you want reasonable detector

efficiency). Consider the size of an absorbing silver halide molecule (a

few hundred pm) with light (of wavelength, let alone |psi^2|, of a few

hundred nm). It's likely (including thickness) that at least billions of

molecules become mutually entangled in a very complex way into a

wavefunction that must time-evolve into one excited silver hailde

molecule if the light is to be detected. Imagining this in a wavelike

formulation is quite easy.

>This is the reduction of the wave packet of the 1930's, which puts

>the particle into a position eigenstate, in which |psi|^2 indeed is a

>delta function, hence pointlike also according to my recipe.

Agreed.

>But it is pointlike only at the idealized measurement instant, not before!

>Before the measurement, it is generally not in a position eigenstate,

>hence has an extended |psi|^2 distribution, and therefore a spatial

>extension.

Agreed but see above.

>The act of measurement changes the shape of the wave

>function, and hence its spatial extension.

Agreed.

>But this is all heavily idealized; realistic measurements are neither

>instantaneous, as required by von Neumann's orthodox theory, nor do

>they localize perfectly in space. Many people think there is no

>(nonunitary) reduction of the wave packet at all. There is a thick book

>by Wheeler and Zurek (Quantum theory and measurement,

>Princeton Univ. Press, Princeton 1983) with collected articles about

>all this, displaying the full range of current uncertainty and lack of

>wisdom.

I'd be tempted to buy it if:

1) I could afford it.

2) I could understand it.

>Fortunately, there is also more recent stuff, e.g., an excellent book by

>Braginsky and Khalili (Quantum measurement, Cambridge Univ. Press,

>Cambridge 1992) on _real_ quantum measurements close to actual

>(optical) experiments, and they talk for example (p.3 bottom) about

>

> 'the photon must have occupied a volume larger than the slit separation.'

>

>I'd take this to be the current wisdom.

Ummm. There might just be one or two people here with a publication list

running into pages who would disagree ....

Trust me on this.

Er, photon thread, anybody?

[Only joking ..]

Feb 24, 2003, 3:35:57 AM2/24/03

to

Squark wrote:

> Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

news:<3e538dee$0$14964$3b21...@news.univie.ac.at>...

> > the detector is a specially prepared unstable thermodynamic system with

> > an energy landscape with multiple local minima at the possible outcomes

> > of the measurement, and details of the microstate determine into which

> > of these local minima the system will fall when excited by an incident

> > particle and dissipating its energy.

> But there is no such "microstate"! Such a microstate would be exactly

> what is called "hidden variables".

No; this is a misunderstanding.

In a thermodynamic description there is the classical

macrostate given by a few thermodynamic parameters

(namely the mass density and the temperature, etc), and many,

many microstates (namely quantum density matrices, or wave functions

if you idealize) consistent with this macrostate. Nothing about

hidden variables.

> > Before the measurement, it is generally not in a position eigenstate,

> > hence has an extended |psi|^2 distribution, and therefore a spatial

> > extension. The act of measurement changes the shape of the wave

> > function, and hence its spatial extension.

> You might have misunderstood my point. I perfectly know and agree

> the wavefunction has spatial extension. The only thing I'm claiming

> is that the physical object the information about which the

> wavefunction represents, i.e. the quantum particle, cannot be

> assigned the spatial extension of the |psi^2| distribution.

The particle _is_ the wave function - what else could it be? It has

no properties apart from that represented in the wave function (unless

you assume hidden variables); so one has every right to identify the

two. This is possible since both the particle and the wave function

are conceptual abstractions. The 'reality' is indiscernible...

> > But this is all heavily idealized; realistic measurements are neither

> > instantaneous, as required by von Neumann's orthodox theory, nor do

> > they localize perfectly in space.

> As I said, relativistic effects are another issue, but they predict a

> finite extension unrelated, in general, to the |psi^2| distribution.

What I said has nothing to do with relativistic or not; a

realistic measurement takes time, and this is discussed

by the experts independent of relativity. See, e.g., Wigner, 1976,

in: Wheeler and Zurek, Quantum Theory and Measurement, p. 284:

>>The fact that the measurement is of finite duration introduces

a more serious problem... The existence of this issue

reemphasizes that the quantum-mechanical description of

the measurement ... is a highly idealized description.<<

> In fact, it proves my point: the distribution is present both in

> relativistic and non-relativistic QM, but the problem with describing

> the quantum particle as point-like only arises in the relativistic

> case.

No, it is only that in the relativistic case you are _forced_ to that

conclusion while in the nonrelativistic case you have an option. But

even then, I think, visualization as extended is preferable.

> > Fortunately, there is also more recent stuff, e.g., an excellent book by

> > Braginsky and Khalili (Quantum measurement, Cambridge Univ. Press,

> > Cambridge 1992) on _real_ quantum measurements close to actual

> > (optical) experiments, and they talk for example (p.3 bottom) about

> >

> > 'the photon must have occupied a volume larger than the slit separation.'

> >

> > I'd take this to be the current wisdom.

> I never read this book, and don't have the possibility to either

> browse it or understand in what context was this phrase

> said. Therefore, you are putting me in a somewhat unfair position

> here.

Well, it is worth reading. It has _lots_ of information about

realistic quantum measurement. But for your convenience,

let me quote more extensively:

>>Experiments on the interference and diffraction of light,

when performed with very low intensities, revealed further that an

interference pattern (a classical, pure wave effect) shows up on

a photographic plate only when the number of photons falling on

the plate is very large. Each photon in such an experiment

is _completely_destroyed_ [original italic] (ceases to exist)

by interacting with the plate's silver chloride molecules.

When the photon is destoyed there appears somewhere on the

photographic plate an atom of free silver, which acts as an

embryo from which, by photographic developing, a small seed

of silver will grow. The silver embryo is much smaller than

an electromagnetic wavelength.

This is remarkable. In the interference process (e.g. in the

two-slit experiment of Fig. 1.1), [standard picture] the photon must

have been influenced by the locations of both slits, since the

interference pattern depends on the distance between them. This means

that the photon must have occupied a volume larger than the slit

separation. On the other hand, when it fell on the photographic plate,

the photon must have been localized into the tiny volume of the silver

embryo. Later the terms ''collapse of the wave function'' and

''reduction of the wave packet'' were used to describe such

localization.<<

> In any case, arguing about whether something is or is not

> the current wisdom is silly enough, and in the circumstances

> it's hard to bring good evidence.

Well, I gave good evidence by quoting from a current [1995]

book by experts on quantum measurement. Textbook wisdom is not

current in this case.

> Therefore, if you don't believe me what I'm saying _is_

> current wisdom, I won't engage in an argument about it.

I don't believe you since I think I am better informed, and

since I think judging extension by |psi|^2 also makes much

more sense than claiming a pointlikeness that is operationally

meaningless since it relates to occult properties of physical

objects apart from their state. Current or not, it is _wise_

to consider quantum particles as being extended.

Arnold Neumaier

Feb 24, 2003, 8:30:08 PM2/24/03

to

Squark <fii...@yahoo.com> writes

>Oz <aco...@btopenworld.com> wrote in message

>news:<b34d84$fpq$1...@panther.uwo.ca>...

>

>> The two slit experiment with single particles refutes your statement.

>>

>> How does this single particle go through both slits simultaneously?

>

>It doesn't. It goes through one of them, but it is undefined which.

>Oz <aco...@btopenworld.com> wrote in message

>news:<b34d84$fpq$1...@panther.uwo.ca>...

>

>> The two slit experiment with single particles refutes your statement.

>>

>> How does this single particle go through both slits simultaneously?

>

>It doesn't. It goes through one of them, but it is undefined which.

That's a most devious and unconvincing explanation.

You have to explain why going through one (but not the other) whilst

'unobserved' gives a diffraction pattern but going through one (when

observed) doesn't. Remembering that we are talking about single particle

diffraction here.

I'm sure there are sufficiently devious explanations for this but occham

forces me to reject them in favour of the simplest:

To diffract it must go through both.

This is effortlessly easy with a wave, heck it even gives the correct

pattern straight off.

Feb 24, 2003, 8:31:39 PM2/24/03

to

Squark <fii...@yahoo.com> writes

>Arnold Neumaier <Arnold....@univie.ac.at> wrote in message news:<3e538dee$0

>$14964$3b21...@news.univie.ac.at>...

>> the detector is a specially prepared unstable thermodynamic system with

>> an energy landscape with multiple local minima at the possible outcomes

>> of the measurement, and details of the microstate determine into which

>> of these local minima the system will fall when excited by an incident

>> particle and dissipating its energy.

>

>But there is no such "microstate"!

>Arnold Neumaier <Arnold....@univie.ac.at> wrote in message news:<3e538dee$0

>$14964$3b21...@news.univie.ac.at>...

>> the detector is a specially prepared unstable thermodynamic system with

>> an energy landscape with multiple local minima at the possible outcomes

>> of the measurement, and details of the microstate determine into which

>> of these local minima the system will fall when excited by an incident

>> particle and dissipating its energy.

>

>But there is no such "microstate"!

Eh? You mean you do not accept that silver halide molecules are quantum-

mechanical and exist in a state themselves? And that this state can

time-evolve with an incoming photon?

If I placed a single SH molecule in a light beam I'm sure some smart

physicist or two would be able to characterise the time-evolution of the

absorption of an incoming light quantum eventually. I wouldn't consider

this 'hidden', but 'currently unknown', because in principle it is

knowable.

Probably an individual one only has a small chance of trapping a photon,

but on a film we have billions of them. Billions of local minima

entangled with the incoming photon. All in slightly different states,

all co-evolving together until one gets lucky (or the photon goes

straight though).

>Such a microstate would be exactly

>what is called "hidden variables".

Eh? I have near total ignorance of 'hidden variables' but I don;t think

silver halide film is one of them. Technically I'm sure the required

details could be measured and probably already have been.

>> Before the measurement, it is generally not in a position eigenstate,

>> hence has an extended |psi|^2 distribution, and therefore a spatial

>> extension. The act of measurement changes the shape of the wave

>> function, and hence its spatial extension.

>

>You might have misunderstood my point. I perfectly know and agree

>the wavefunction has spatial extension. The only thing I'm claiming

>is that the physical object the information about which the

>wavefunction represents, i.e. the quantum particle, cannot be

>assigned the spatial extension of the |psi^2| distribution.

I agree in some circumstances (and I gave an example earlier).

Take an idealised electron emitter that emits a spherical distribution

of electrons. Without knowledge of the emission direction we describe it

as a spherical distribution |psi_t^2| and lo and behold everything works

out fine. Some people think this means each electron is spread over a

spherical shell but I don't.

I think we have individual more localised electrons (of order compton

wavelength) following |psi_e^2| but an awful lot of them (typically). We

can even screen out those with selected directions with an aperture.

This means, in almost certainly wrong notation but try and get my gist,

that summing the individual electron wavefunctions should total

|psi_t^2|.

I have a horrible feeling this doesn't quite work, because somewhere we

should need to include a function in |psi_t^2| to express our level of

ignorance. Oh, being realistic also because of my level of knowledge,

and for a whole host of technical reasons well beyond my ken.

What you argue, and I argue above, is a precise analogy to my argument

in the 'photon' threads when I was discussing atoms emitting photons

spherically (yer s2->s1 transition) that were seen by ted 100M years

later. Does it make sense to say it was emitted as a spherical wavefront

for 100M years? The expert opinion was that it does, I am not convinced

(at all).

OTOH if you are talking about a single electron diffraction pattern

where the slits are many wavelengths apart then it is precisely the

compton wavelength that is relevant. You will (eventually) get a

diffraction pattern and it will reflect the electron's compton

wavelength. I would imagine that the proportion of electrons that go

through both slits will decrease drastically as the slit spacing is

increased. To me that gives a measure of the lateral 'size' of the

electron. There isn't much 'electron' many compton wavelengths away from

the 'centre'. Remember, though, that I tried this analogy for 'photon

size' and got thoroughly (and repeatedly) duffed up by the great and the

good.

>> But this is all heavily idealized; realistic measurements are neither

>> instantaneous, as required by von Neumann's orthodox theory, nor do

>> they localize perfectly in space.

>

>As I said, relativistic effects are another issue, but the predict a

>finite extension unrelated, in general, to the |psi^2| distribution.

>In fact, it proves my point: the distribution is present both in

>relativistic and non-relativistic QM, but the problem with describing

>the quantum particle as point-like only arises in the relativistic

>case.

So drop it? Or reformulate it to give the right answers accepting that

it's just a model of the real wave set up for convenient integration.

Or is that too simple?

Feb 24, 2003, 8:53:19 PM2/24/03

to

In geometry, one has long left this kind of definitions since

they are circular. Instead, one specifies the axioms that one

wants a point to possess - mathematical properties.

In particular, one asks in the literature for localizability

- clearly, a point should be localizable. In the quantum optic

bilbe of mandel & Wolf, there are several pages devoted to the

impossiblility of localizing a photon (Section 12.11), and there

is also a significant literature about this elsewhere, just

because of its irritating nature.

Arnold Neumaier

Feb 25, 2003, 5:49:07 PM2/25/03

to

>>Oz <aco...@btopenworld.com> wrote:

>>>How does this single particle go through both slits simultaneously?

>>

>>It doesn't. It goes through one of them, but it is undefined which.

>>>How does this single particle go through both slits simultaneously?

>>

>>It doesn't. It goes through one of them, but it is undefined which.

I preffer to say that it goes through one *plus* it goes through the other.

"Plus", as a logical conective, is more of like "or" than like "and" (which

is sort of like "times").

The whole idea of the Hilbert space business is to let you use aritmatic on

things you would normally apply logic to. Logic (at least when applied in

the obvious way) dosen't seem to work in the quantum world, but -

miraculously - aritmatic *does*. The main diference is that numbers can be

negative.

Quantum mechanics, as usually expressed, allows imaginary numbers as well,

but that just makes the equations easier to write (Forier transforms and

all that).

> That's a most devious and unconvincing explanation.

So? Did the universe promise to be straightforward and convincing?

Ralph Hartley

Feb 25, 2003, 7:16:16 PM2/25/03

to

In article <b3775a$o33$1...@panther.uwo.ca>, Oz <aco...@btopenworld.com>

writes:

>Arnold Neumaier <Arnold....@univie.ac.at> writes:

[...]

>>There *is* a dynamical quantum theory of multiparticle interaction,

>>namely the multiparticle Schroedinger equation.

>>The measurement process is the result of interaction of a single

>>quantum particle with a quantum multiparticle system (the detector),

>>and therefore should be described in these terms. Sometimes,

>>measurement is idealized as instantaneous reduction of the wave packet,

>>but this is well-known to be inaccurate, and hides what is going on

>>under the carpet. But sometimes, more realistic scenarios were discussed.

>Cor! I do hope you are an expert because I have been arguing this for

>years and being told I am simply ignorant (which is indeed true). Mind

>you, you do put it in proper technospeak.

[...]

You might like Bohmian Mechanics. You will at least find some experts

who are "enemies of your enemies". Here is quote from a recent paper,

which starts by quoting Bell:

"...conventional formulations of quantum theory, and of quantum

field theory in particular, are unprofessionally vague and

ambiguous. Professional theoretical physicists ought to be able

to do better. Bohm has shown us a way." (Bell, 1987)

The problem, in other words, with orthodox quantum theory is not

that it fails to be intuitively founded, but rather that, with

its incoherent babble about measurement, it is not even well

formulated!

Bohmian Mechanics as the Foundation of Quantum Mechanics D. DÃ¼rr, S.

Goldstein, and N. ZanghÃ¬. arXiv: quant-ph/9511016

You can find this paper and other information about Bohmian Mechanics at

http://www.mathematik.uni-muenchen.de/~bohmmech/BohmHome/bmstartE.htm

Graham

--

Graham Jones, author of SharpEye Music Reader

http://www.visiv.co.uk

21e Balnakeil, Durness, Lairg, Sutherland IV27 4PT, Scotland, UK

Feb 26, 2003, 5:52:31 PM2/26/03

to

Arnold Neumaier <Arnold....@univie.ac.at> wrote in message news:<3e56a7e5$0$14448$3b21...@news.univie.ac.at>...

> Squark wrote:

>

> > Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

> news:<3e538dee$0$14964$3b21...@news.univie.ac.at>...

>

> > > the detector is a specially prepared unstable thermodynamic system with

> > > an energy landscape with multiple local minima at the possible outcomes

> > > of the measurement, and details of the microstate determine into which

> > > of these local minima the system will fall when excited by an incident

> > > particle and dissipating its energy.

>

> > But there is no such "microstate"! Such a microstate would be exactly

> > what is called "hidden variables".

>

> No; this is a misunderstanding.

> In a thermodynamic description there is the classical

> macrostate given by a few thermodynamic parameters

> (namely the mass density and the temperature, etc), and many,

> many microstates (namely quantum density matrices, or wave functions

> if you idealize) consistent with this macrostate. Nothing about

> hidden variables.

> Squark wrote:

>

> > Arnold Neumaier <Arnold....@univie.ac.at> wrote in message

> news:<3e538dee$0$14964$3b21...@news.univie.ac.at>...

>

> > > the detector is a specially prepared unstable thermodynamic system with

> > > an energy landscape with multiple local minima at the possible outcomes

> > > of the measurement, and details of the microstate determine into which

> > > of these local minima the system will fall when excited by an incident

> > > particle and dissipating its energy.

>

> > But there is no such "microstate"! Such a microstate would be exactly

> > what is called "hidden variables".

>

> No; this is a misunderstanding.

> In a thermodynamic description there is the classical

> macrostate given by a few thermodynamic parameters

> (namely the mass density and the temperature, etc), and many,

> many microstates (namely quantum density matrices, or wave functions

> if you idealize) consistent with this macrostate. Nothing about

> hidden variables.

If you consider the wavefunction as the "macrostate", the "details of

the microstate" which determine outcome of quantum measurements are

hidden variables. This is because the hidden variables, are, by

definition, unobserved quantities which deterministically determine

the outcome of quantum measurements.

> The particle _is_ the wave function - what else could it be? It has

> no properties apart from that represented in the wave function (unless

> you assume hidden variables); so one has every right to identify the

> two. This is possible since both the particle and the wave function

> are conceptual abstractions. The 'reality' is indiscernible...

Firstly, one cannot identify the two as in a multi-particle system

individual particles have no wavefunctions.

> What I said has nothing to do with relativistic or not; a

> realistic measurement takes time, and this is discussed

> by the experts independent of relativity.

Sorry, I misread "realistic" for "relativistic". Yes, realistic

measurements have various limitations, but I don't think it's

appropriate to draw conclusions from it about the

"pointlikeness", unless you can place a universal limit on the

accuracy such measurements can achieve.

> No, it is only that in the relativistic case you are _forced_ to that

> conclusion while in the nonrelativistic case you have an option. But

> even then, I think, visualization as extended is preferable.

This seems to be a subjective arguement, therefore I can hardly argue

against it.

> > > Fortunately, there is also more recent stuff, e.g., an excellent book by

> > > Braginsky and Khalili (Quantum measurement, Cambridge Univ. Press,

> > > Cambridge 1992) on _real_ quantum measurements close to actual

> > > (optical) experiments, and they talk for example (p.3 bottom) about

> > >

> > > 'the photon must have occupied a volume larger than the slit separation.'

> > >

> > > I'd take this to be the current wisdom.

>

> > I never read this book, and don't have the possibility to either

> > browse it or understand in what context was this phrase

> > said. Therefore, you are putting me in a somewhat unfair position

> > here.

>

> Well, it is worth reading. It has _lots_ of information about

> realistic quantum measurement. But for your convenience,

> let me quote more extensively:

>

> This is remarkable. In the interference process (e.g. in the

> two-slit experiment of Fig. 1.1), [standard picture] the photon must

> have been influenced by the locations of both slits, since the

> interference pattern depends on the distance between them. This means

> that the photon must have occupied a volume larger than the slit

> separation. On the other hand, when it fell on the photographic plate,

> the photon must have been localized into the tiny volume of the silver

> embryo. Later the terms ''collapse of the wave function'' and

> ''reduction of the wave packet'' were used to describe such

> localization.<<

I don't think this introductionary exposure of quantum mechanical

effects contains any attempt to accurately reflect on such issues as

whether the quantum particle is point-like or not. Again, I claim

two things must be distinguished: the point-like quantum particle,

which is a point without location - much like a point in a

non-commutative space, this as a philosophical notion is quite

different from the usual point - and the extended wavefunction.

Therefore, the notion "point-like" applies here in a sense different

from the usual, classical, sense. Nevertheless we would hardly

benefit from abandoning the notion, though it might make our

arguement ill posed: we don't agree on the definitions.

> I don't believe you since I think I am better informed, and

> since I think judging extension by |psi|^2 also makes much

> more sense than claiming a pointlikeness that is operationally

> meaningless since it relates to occult properties of physical

> objects apart from their state. Current or not, it is _wise_

> to consider quantum particles as being extended.

It only remains to me to quote Shakespeare:

"The fool doth think himself wise, but the wise man knows himself

to be a fool."

Feb 26, 2003, 5:52:44 PM2/26/03

to

Oz <aco...@btopenworld.com> wrote in message news:<b35u0g$gbo$1...@lfa222122.richmond.edu>...

> Squark <fii...@YAHOO.COM> writes

> >However, what we might expect

> >of a physical extended object is, for instance, the possibility the

> >measure it's state at every point separately.

>

> I'm not sure why you would expect that of a quantised wave.

> Measuring it's state inevitably destroys the state, that's a feature of

> QM.

> Squark <fii...@YAHOO.COM> writes

> >However, what we might expect

> >of a physical extended object is, for instance, the possibility the

> >measure it's state at every point separately.

>

> I'm not sure why you would expect that of a quantised wave.

> Measuring it's state inevitably destroys the state, that's a feature of

> QM.

In QFT, which exactly describes "quantized waves" (at least in the free

case), you can do it (ignoring subtleties, as always :-) ). This is

because operators at space-likes separations commute. And this is what

ensures locality of the whole theory. This, in fact, is the exact

reason I would expect that: locality.

> Hmmm. Yes, and no. You can do it statistically by preparing particles in

> the same state and measuring a lot of them.

Yes, but then you ain't measuring nothing: you prepared the particles in

this state, so you know the wavefunction already. Try doing the same

with an unknown quantum state!

> given state is often very easy because particles are easily divided into

> identical species (electron, photon, etc) so when bound deliver

> identical wavefunctions (to first order).

There's a famous theorem in quantum infomation theory saying that an

unknown quantum state cannot be duplicated. So things are not so

simple :-)

> Take the emission of a photon by an atom.

There's no "action at a distance here". The emission happens in a

single (usually ambiguous, of course) world-point.

> If you then proceed to entangled pairs and cite 'FTL information

> transmission' then you have to explain what physical laws are being

> broken to make this implausible.

There is no passage of information there. As is well known, the

whole beauty of EPR is that it doesn't allow you to transmit

information.

> Then you will point to the 'pointlike' electron (say). It's so pointlike

> we can diffract it and treat it perfectly happily as a wave.

Yes, and nevertheless it still fires a single detector. This

"pointlike" electron is not at all "pointlike" in the classical

sense. Well, what it's doing doesn't really make sense

classically! It's a sort of special "quantum pointlikeness" we're

talking about here, which still allows for the non-pointlike

wavefunction.

Feb 26, 2003, 7:13:47 PM2/26/03

to sci-physic...@moderators.uu.net

In message <gik%9.18435$rq4.1...@bgtnsc05-news.ops.worldnet.att.net>,

Ahmet Gorgun <ago...@att.net> writes:

Ahmet Gorgun <ago...@att.net> writes:

>"Charles Francis" <cha...@clef.demon.co.uk> wrote:

>> Ahmet Gorgun <ago...@att.net> wrote:

>> >The indivisible and indestructible Democretean primary elements were never

>> >observed.

>>

>> But they have been observed now, electrons, quarks fulfil the role quite

>> accurately.

>Are you saying that electron has absolutely no parts and you can

>prove that it has absolutely no parts? Otherwise the electron is not

>the absolutely indivisible elements that Democritus postulated.

Yes I am. The equation for a fundamental indivisible particle was

written down on purely theoretical grounds by Dirac in 1928. This

equation exhibits precisely the observed properties of the electron,

and indeed of the muon, the other leptons, the tau and the neutrinos,

and indeed of the quarks. In the case of gyromagnetic moment it has

the observed properties to at least 11 significant figures.

Regards

--

Charles Francis

--=_Turnpike_te3CmqR7XIX+47uI=

Content-Type: text/plain;charset=us-ascii;format=flowed

Regards

--

Charles Francis

--=_Turnpike_te3CmqR7XIX+47uI=--

Feb 26, 2003, 7:13:25 PM2/26/03

to

In message <b3egv0$2sd$1...@lfa222122.richmond.edu>, Oz

<aco...@btopenworld.com> writes

<aco...@btopenworld.com> writes

>You have to explain why going through one (but not the other) whilst

>'unobserved' gives a diffraction pattern but going through one (when

>observed) doesn't. Remembering that we are talking about single particle

>diffraction here.

>

>I'm sure there are sufficiently devious explanations for this but Occam

>forces me to reject them in favour of the simplest:

>

>To diffract it must go through both.

>

>This is effortlessly easy with a wave, heck it even gives the correct

>pattern straight off.

Yes, but I should like to see how you change your view when you really

grok ket space, especially as we are working through the two slits

example now. The "wave" which goes through the slits is only a

probability amplitude, and with probabilities we only say things may

happen, we do not say all possibilities happen at once in some measure.

Regards

--

Charles Francis

Feb 26, 2003, 7:13:45 PM2/26/03

to

Ralph Hartley <har...@aic.nrl.navy.mil> writes:

>Oz wrote:

>> Squark <fii...@yahoo.com> writes:

>>>Oz <aco...@btopenworld.com> wrote:

>>>>How does this single particle go through both slits simultaneously?

>>>It doesn't. It goes through one of them, but it is undefined which.

>I preffer to say that it goes through one *plus* it goes through the other.

>"Plus", as a logical conective, is more of like "or" than like "and" (which

>is sort of like "times").

Hmmm.

I don't see how 'plus' can equal 'or', nor 'and' equal 'times'.

Unless you chose to define them thus (which is cheating).

I don't see how going through one slit 'plus' going through the other

slit isn't the same as 'going through both slits'.

>The whole idea of the Hilbert space business is to let you use aritmatic on

>things you would normally apply logic to. Logic (at least when applied in

>the obvious way) doesn't seem to work in the quantum world, but -

>miraculously - aritmetic *does*. The main diference is that numbers can be

>negative.

I absolutely agree with that, and have agreed all along.

Equally I don't consider that taking y=x^2 to actually consist of

infinitesimally small slices of size dx means I can't equally take it as

a continuum. No matter how convenient slicing it up into bits is,

mathematically.

>Quantum mechanics, as usually expressed, allows imaginary numbers as well,

>but that just makes the equations easier to write (Forier transforms and

>all that).

>> That's a most devious and unconvincing explanation.

>So? Did the universe promise to be straightforward and convincing?

Absolutely not. I was applying Occam.

Feb 28, 2003, 4:28:07 PM2/28/03

to

In article <b37755$o32$1...@panther.uwo.ca>, Oz

<ozac...@despammed.com> wrote:

<ozac...@despammed.com> wrote:

>Squark <fii...@yahoo.com> writes

>>One must not confuse the apparent "extended" nature of relativistic

>>quantum particles with the general notion of indeterminate position in

>>quantum mechanics. It is true that in relativistic quantum mechanics the

>>notion of position is subtle and problematic, but note the "extension"

>>is of the size of the Compton wavelength, not the characteristic

>>length of the |psi(x)^2| distribution.

>Hmmm. Is it? Where is the electron in an outer orbital of a heavy atom?

>Sure, you can probe for it with a high energy particle but this will

>just give you some 'found' position but does not mean the particle was

>'there' before it interacted with your probe.

Squark didn't say that a particle has a specific definite location

even when its wavefunction is all smeared out! He's no dope; he

knows quantum mechanics just fine. That's why he mentioned the

"general notion of indeterminate position in quantum mechanics".

Here he's be pointing out, correctly, that this quantum-mechanical

effect has nothing to do with a different effect that comes in only

when you take relativity into account as well.

Namely: if you try to measure the position of a particle very

accurately, you'll need to hit it with stuff of such high momentum

that you'll start creating particle-antiparticle pairs. Since all

particles of a given sort are identical, this makes it impossible

to decide which particle you were measuring the position of!

This other effect becomes important around a distance scale called

the Compton wavelength, which is

hbar/mc

for a particle of mass m, and about 4 x 10^{-13} meters for an

electron.

But even this effect has NOTHING to do with what working particle

physicists mean when they say the electron looks pointlike rather than

extended. SURE, quantum mechanics is true. SURE, relativity matters.

But they're completely used to that. If you tell them about

this stuff they'll say "Ho hum, Oz - we learned all that in school

when we were kids!" They've factored all this into their equations

already.

When they (and I) say the electron is pointlike, they mean that it

doesn't give any indication of being a bound state of other particles...

it doesn't act composite... it acts the way a *fundamental* particle

should when you bounce other particles off it...

... at least down to a certain distance scale - or up to a

certain energy scale, in other words!

But, this distance scale is a lot less than 10^{-13} meters.

To see that the proton is composite, you have to go down to a

distance scale of about 10^{-15} meters. At this distance

scale it acts like a big bag of quarks, virtual quarks and

gluons.

People have gone down quite a bit further and still not seen any

deviations from pointlike behavior on the part of electrons.

(I don't know the the current best figure. Does anyone know?)

Feb 28, 2003, 4:27:34 PM2/28/03

to

Charles Francis <cha...@clef.demon.co.uk> wrote in message

news:b3jl7r$qbb$1...@panther.uwo.ca...

> In message <gik%9.18435$rq4.1...@bgtnsc05-news.ops.worldnet.att.net>,

> Ahmet Gorgun <ago...@att.net> writes:

>

> >"Charles Francis" <cha...@clef.demon.co.uk> wrote:

>

> >> Ahmet Gorgun <ago...@att.net> wrote:

>

> >> >The indivisible and indestructible Democretean primary elements were

never

> >> >observed.

> >>

> >> But they have been observed now, electrons, quarks fulfil the role

quite

> >> accurately.

>

> >Are you saying that electron has absolutely no parts and you can

> >prove that it has absolutely no parts? Otherwise the electron is not

> >the absolutely indivisible elements that Democritus postulated.

>

> Yes I am. The equation for a fundamental indivisible particle was

> written down on purely theoretical grounds by Dirac in 1928. This

> equation exhibits precisely the observed properties of the electron,

> and indeed of the muon, the other leptons, the tau and the neutrinos,

> and indeed of the quarks. In the case of gyromagnetic moment it has

> the observed properties to at least 11 significant figures.

And are you saying this was done with no inputs, and it works perfectly for

neutrinos?

Mar 3, 2003, 4:07:45 PM3/3/03

to

John Baez wrote:

>

> But even this effect has NOTHING to do with what working particle

> physicists mean when they say the electron looks pointlike rather than

> extended. SURE, quantum mechanics is true. SURE, relativity matters.

> But they're completely used to that. If you tell them about

> this stuff they'll say "Ho hum, Oz - we learned all that in school

> when we were kids!" They've factored all this into their equations

> already.

>

> When they (and I) say the electron is pointlike, they mean that it

> doesn't give any indication of being a bound state of other particles...

> it doesn't act composite... it acts the way a *fundamental* particle

> should when you bounce other particles off it...

>

> ... at least down to a certain distance scale - or up to a

> certain energy scale, in other words!

>

> But even this effect has NOTHING to do with what working particle

> physicists mean when they say the electron looks pointlike rather than

> extended. SURE, quantum mechanics is true. SURE, relativity matters.

> But they're completely used to that. If you tell them about

> this stuff they'll say "Ho hum, Oz - we learned all that in school

> when we were kids!" They've factored all this into their equations

> already.

>

> When they (and I) say the electron is pointlike, they mean that it

> doesn't give any indication of being a bound state of other particles...

> it doesn't act composite... it acts the way a *fundamental* particle

> should when you bounce other particles off it...

>

> ... at least down to a certain distance scale - or up to a

> certain energy scale, in other words!

Well, this is at least a clear definition of how the terminology is used.

So saying 'the electron is pointlike' is simply a convention for

saying 'the electron is indivisible' (at least down to a certain distance

scale), not meaning anything else? But if language provides two different

terms with different associated intuition, isn't it then better to

use these terms differently, especially when there are aspects of the

situation for which one term applies far more than the other?

I think the spin is a clear indicator of non-point behavior;

a point cannot spin.

At distances large compared to the extension of an extended body,

the only indicators of extendedness are the spin (detectable due to

angular momentum conservation) and electromagnetic radiation (which is

long range and gives away oscillation information), but both give no

information about the size of the extension. This holds for

macroscopic bodies as well as for microscopic bodies. Why should we

think these indicators become unreliable simply because the distance

is of the order of 10^{-13} or less, while it is reliable above that scale?

Wigner, in his 1939 classification of elementary particles defines one

as being (for practical purposes) indivisible (and not radiating),

irrespective of it being pointlike or not.

And he proves that the elementary particles in this sense

_all_ have an identical description - as an irreducible projective

representation of the Poincare group, with [in the absense of internal

symmetries] only two characteristic parameters, mass and spin/helicity.

So the fact that the electron or quarks are described in the standard

model by irreducible representations does not allow any inference at

all about their internal structure. We simply treat them as

structureless since this is adequate at the scale of present

experiments. But even if they are truly indivisible this does not

mean that they are pointlike. They are more like fuzzy clouds...

The situation is like in the characterization of [neutral]

stationary vacuum black holes by mass and angular momentum.

Real approximations to black holes will probably have internal

structure that is idealized away since it cannot be observed

without severe dangers for the observer.

And even in their idealized form they have a horizon,

and describing them as pointlike seems inadequate.

But, it seems to me, they are as indivisible as electrons.

Somewhere I read that Kerr holes are indeed related to the Dirac

equation. But I don't remember details.

Arnold Neumaier

Mar 3, 2003, 4:42:46 PM3/3/03

to

"Charles Francis" <cha...@clef.demon.co.uk> wrote:

> ...The equation for a fundamental indivisible particle was

> written down on purely theoretical grounds by Dirac in 1928. This

> equation exhibits precisely the observed properties of the electron...

This argument does not prove that electron is an indivisible elementary

particle.

Here's an equation which explains precisely the motions of point planets:

d^2x/dt^2 + mx/r^3 = 0. It does not follow from this equation that planets

are fundamental elementary particles. The same is true for the electron.

Ahmet Gorgun

Mar 4, 2003, 2:18:52 PM3/4/03

to sci-physic...@moderators.isc.org

In message <SDf8a.76979$zF6.5...@bgtnsc04-news.ops.worldnet.att.net>,

Ahmet Gorgun <ago...@att.net> writes

>"Charles Francis" <cha...@clef.demon.co.uk> wrote:

>

>> ...The equation for a fundamental indivisible particle was

>> written down on purely theoretical grounds by Dirac in 1928. This

>> equation exhibits precisely the observed properties of the electron...

>

>This argument does not prove that electron is an indivisible elementary

>particle.

Ahmet Gorgun <ago...@att.net> writes

>"Charles Francis" <cha...@clef.demon.co.uk> wrote:

>

>> ...The equation for a fundamental indivisible particle was

>> written down on purely theoretical grounds by Dirac in 1928. This

>> equation exhibits precisely the observed properties of the electron...

>

>This argument does not prove that electron is an indivisible elementary

>particle.

This was not an argument. It was a statement of historical fact.

>Here's an equation which explains precisely the motions of point planets:

>d^2x/dt^2 + mx/r^3 = 0. It does not follow from this equation that planets

>are fundamental elementary particles.

This equation had nothing to do with fundamental particles. What Dirac

did was seek out the equation for a fundamental particle.

Regards

--

Charles Francis

Mar 4, 2003, 2:24:51 PM3/4/03

to

Arnold Neumaier wrote:

> Somewhere I read that Kerr holes are indeed related to the Dirac

> equation.

Probably here: http://arxiv.org/abs/hep-th/0210103 .

Mar 4, 2003, 2:43:57 PM3/4/03

to

In message <v5r01ta...@corp.supernews.com>, Ralph E. Frost

<ref...@dcwi.com> writes

The inputs are the very general laws of quantum mechanics and special

relativity, and it works perfectly for neutrinos in so far as we are

able to measure properties of neutrinos.

Regards

--

Charles Francis

Mar 5, 2003, 2:54:47 PM3/5/03

to

Charles Francis <cha...@clef.demon.co.uk> writes

>Yes, but I should like to see how you change your view when you

>really grok ket space, especially as we are working through the two

>slits example now.

I'm trying. A few examples will probably help.

>The

>"wave" which goes through the slits is only a probability amplitude, and with

>probabilities we only say things may happen, we do not say all possibilities

>happen at once in some measure.

We shall see if it is incompatible with my current viewpoint.

I rather doubt that it will be.

The Bell inequality might do, but so far nobody has been able to

*explain* the details of difference to me. That is I am after the

difference in *mechanism* not a bunch of statistics.

Mar 5, 2003, 2:55:57 PM3/5/03

to

Squark <fii...@yahoo.com> writes:

>Oz <aco...@btopenworld.com> wrote in message

>news:<b35u0g$gbo$1...@lfa222122.richm ond.edu>...

>> Squark <fii...@YAHOO.COM> writes

>> >However, what we might expect

>> >of a physical extended object is, for instance, the possibility the

>> >measure it's state at every point separately.

>>

>> I'm not sure why you would expect that of a quantised wave.

>> Measuring it's state inevitably destroys the state, that's a feature of

>> QM.

>In QFT, which exactly describes "quantized waves" (at least in the free

>case), you can do it (ignoring subtleties, as always :-) ). This is

>because operators at space-likes separations commute. And this is what

>ensures locality of the whole theory. This, in fact, is the exact

>reason I would expect that: locality.

I presume that at some point francis will bring this up in the ket

thread. I have to say that the high jargon content means I don't

actually understand what you are saying.

>> Hmmm. Yes, and no. You can do it statistically by preparing particles in

>> the same state and measuring a lot of them.

>Yes, but then you ain't measuring nothing: you prepared the particles in

>this state, so you know the wavefunction already. Try doing the same

>with an unknown quantum state!

I already explained that.

>> given state is often very easy because particles are easily divided into

>> identical species (electron, photon, etc) so when bound deliver

>> identical wavefunctions (to first order).

>

>There's a famous theorem in quantum infomation theory saying that an

>unknown quantum state cannot be duplicated. So things are not so

>simple :-)

Hah! Indeed. See below.

>> Take the emission of a photon by an atom.

>

>There's no "action at a distance here". The emission happens in a

>single (usually ambiguous, of course) world-point.

But it's an unknown world-point, in an unknown direction.

It's a spherical wavefront until you measure the atom recoil, then it's

got a higher accuracy of momentum knowledge. The difference between the

two reflect your knowledge of the photon, both will give (statistically)

the same result. This has been discussed before.

>> Then you will point to the 'pointlike' electron (say). It's so pointlike

>> we can diffract it and treat it perfectly happily as a wave.

>

>Yes, and nevertheless it still fires a single detector.

I already pointed out that 'detection' is a complex QM process designed

to (in this case) that one and only one detector per particle fires.

It's the feature of the QM wave that you either detect (some property)

of the wave or you don't detect it at all. That nobody ever detected

half an electron is unremarkable.

>This

>"pointlike" electron is not at all "pointlike" in the classical

>sense. Well, what it's doing doesn't really make sense

>classically!

>It's a sort of special "quantum pointlikeness" we're

>talking about here, which still allows for the non-pointlike

>wavefunction.

It's the special 'quantum waveness' we are talking about here, which

allows for a quantised wave.

The difference being that a wave can go through both slits, whilst a

particle can't. Unless you allow it to exist as a probability wave that

looks just like the quantum wave, that way you can have half a particle

going through each slit (probably even a whole one through each slit).

A convenient mathematical fiction.

Mar 5, 2003, 8:15:14 PM3/5/03

to

In article <3e5feaf0$0$13932$3b21...@news.univie.ac.at>,

Arnold Neumaier <Arnold....@univie.ac.at> wrote:

Arnold Neumaier <Arnold....@univie.ac.at> wrote:

>John Baez wrote:

>> When they (and I) say the electron is pointlike, they mean that it

>> doesn't give any indication of being a bound state of other particles...

>> it doesn't act composite... it acts the way a *fundamental* particle

>> should when you bounce other particles off it...

>>

>> ... at least down to a certain distance scale - or up to a

>> certain energy scale, in other words!

>Well, this is at least a clear definition of how the terminology is used.

Great.

>So saying 'the electron is pointlike' is simply a convention for

>saying 'the electron is indivisible' (at least down to a certain distance

>scale), not meaning anything else?

Basically yes - a more precise answer is that when you measure

what happens when you throw electron at each other, the answer

matches that given by some quantum field theory in which the electron

is a fundamental field in the Lagrangian - in particular, the Standard

Model.

>But if language provides two different

>terms with different associated intuition, isn't it then better to

>use these terms differently, especially when there are aspects of the

>situation for which one term applies far more than the other?

In theory yes, but it's not my goal to reform how professional

particle physicists talk. I am happy just to understand what they

are saying, and to have them understand me.

In theory one can often gain precision by using a word in some nonstandard

ways. But, if most of ones target audience doesn't use it that way,

one winds up confusing more people than one helps. Of course this is

not a problem for "internal communications" - e.g. talking to oneself,

or within a small group of friends.

Mar 6, 2003, 4:39:29 PM3/6/03

to

"Ahmet Gorgun" <ago...@att.net> wrote in message

news:<SDf8a.76979$zF6.5...@bgtnsc04-news.ops.worldnet.att.net>...

> Here's an equation which explains precisely the motions of point planets:

> d^2x/dt^2 + mx/r^3 = 0. It does not follow from this equation that planets

> are fundamental elementary particles. The same is true for the electron.

news:<SDf8a.76979$zF6.5...@bgtnsc04-news.ops.worldnet.att.net>...

> Here's an equation which explains precisely the motions of point planets:

> d^2x/dt^2 + mx/r^3 = 0. It does not follow from this equation that planets

> are fundamental elementary particles. The same is true for the electron.

Spin is the tricky part. If electron were a composite particle, then

it should be possible to see an spin 3/2 state or so. Dirac equation

is about an spin 1/2 particle, thus one can said that this equation

assumes the particle is elementary. Or course, you can also say that it

just represents the lower spin state of a bigger particle.

Mar 7, 2003, 4:01:10 PM3/7/03

to

> news:<3e50f609$0$14964$3b21...@news.univie.ac.at>...

>

> > For example, discussing the localization of relativistic particles

> > in space-time,

> > D. Marolf and C. Rovelli, Relativistic quantum measurement,

> > Phys.Rev. D66 (2002) 023510, gr-qc/0203056,

> > say on p.7 (top right, of the archived version):

> > ... the quantum particle has an intrinsic Compton ``extension''...

>

> One must not confuse the apparent "extended" nature of relativistic

> quantum particles with the general notion of indeterminate position in

> quantum mechanics.

>

> > For example, discussing the localization of relativistic particles

> > in space-time,

> > D. Marolf and C. Rovelli, Relativistic quantum measurement,

> > Phys.Rev. D66 (2002) 023510, gr-qc/0203056,

> > say on p.7 (top right, of the archived version):

> > ... the quantum particle has an intrinsic Compton ``extension''...

>

> One must not confuse the apparent "extended" nature of relativistic

> quantum particles with the general notion of indeterminate position in

> quantum mechanics.

Indeterminate position only says you cannot locate a particle too accurately;

but of course one can locate it highly reliable within some region.

In fact, just within the region given by its extension. No different from

the location of the Moon, which one cannot give more precise than to the

order of its radius.

> It is true that in relativistic quantum mechanics the

> notion of position is subtle and problematic, but note the "extension"

> is of the size of the Compton wavelength, not the characteristic

> length of the |psi(x)^2| distribution.

The two things are on the same footing, as explained in detail ,e.g., by

L.L. Foldy and S.A. Wouthuysen, Phys. Rev. 78 (1950), 29-36 (the paper

with their transformation of the Dirac equation). In their 21 line

abstract they spend 4 lines on remarking,

``Some light is cast on the question of why a Dirac electron

shows some properties characteristic of a particle of finite

extension by an examination of the relationship between the new

and the conventional position operator.''

In the main text they repeatedly use without shame geometric language.

For example, on p.32 top left, they write:

``Psi' at a given point is constituted from contributions depending

on Psi over a neighborhood of dimensions of the order of a Compton

wave-length of the particle about the point. Thus a wave function

which in the old representation corresponed to a state in which

the particle was definitely located at one point, passes over in

the new representation into a wave function which apparently

corresponds to the particle being spread out over a finite region.''

(Here Psi = relativistic Dirac wave function, Psi' = unitarily transformed

wave function with correct - Pauli - nonrelativistic limit.)

'Extension' - 'being spread out' - 'being located': these are always used

in the sense of the region where |psi(x)|^2 is significant; and

'definitely' refers to the special situation where |psi(x)|^2 is

concentrated at a single point. The Compton wavelength is not the

generic size of the electron but simply the *additional* size of the blurring

due to relativistic effects, namely the minimal size of an optimally

localized particle. But unless measured, particles are usually much

more spread out = extended. For example, in a hydrogen atom, it is the

electron size, not the nucleus, that defines the size of the atom!

Field theory gives more ground for the extended view. In QED, there

are no particles but fields, and the electron field Psi(x) has the

physical meaning that omega(|Psi(x)|^2) is the charge density of an

electron in a state omega, the component j_0 of the conserved

4-current j=omega(Psi_bar gamma Psi). (If there are negative energy

contributions, one has to subtract these.) The current j is the _only_

thing a classical observer (i.e., a macroscopic object for which

a mean field description is adequate) can detect about an electron field,

since this is what couples (in a semiclassical mean field approximation)

to an electromagnetic field. Lots of calculations for real experiments

and real equipment are done in this way, showing that j and hence the

charge density j_0=omega(|Psi(x)|^2) is something tangible.

Now there is at least one case in which quantum field theory is

mathematically well understood, namely for free fields. Thus look

at a pure single particle state in a free spin 1/2 theory, universally

agreed as the right description of a single electron. It defines the

state omega(f) =<psi|f|psi>, where psi is the wave function of the particle.

Lo and behold, it turns out that omega(|Psi(x)|^2) is just the charge

density - and not a probability!!! (For relativistic spin 0 particles

one does not even have a meaningful probability concept.) The probability

interpretation of the Dirac wave function is simply a historical leftover...

Now take any nonzero solution of the free Dirac equation you like

(any of these is a valid wave function) - you'll see that the charge

density always has an extended support, with exception of at most one

moment in time (for special waves only) - this could be the moment

someone has measured its position. At all other times, the particle

is extended, sometimes over regions much bigger than the Compton length.

Having given good physical arguments for, and quoted five different

authors - several of them highly qualified to talk about the issue -

supporting the extendedness of elementary particles, I'd like to see

the facts (or authorities) on which you base your view that particles

are pointlike in some geometric sense different from indivisibility!

Arnold Neumaier

Mar 7, 2003, 4:03:03 PM3/7/03

to

I wrote my last mail too quickly and was hence a little sloppy;

sorry!

Arnold Neumaier wrote:

>

> localized particle. But unless measured, particles are usually much

> more spread out = extended. For example, in a hydrogen atom, it is the

> electron size, not the nucleus, that defines the size of the atom!

I meant; unless the position is measured.

> Now there is at least one case in which quantum field theory is

> mathematically well understood, namely for free fields. Thus look

> at a pure single particle state in a free spin 1/2 theory, universally

> agreed as the right description of a single electron. It defines the

> state omega(f) =<psi|f|psi>, where psi is the wave function of the particle.

> Lo and behold, it turns out that omega(|Psi(x)|^2) is just the charge

> density

I meant: it turns out that omega(|Psi(x)|^2) is just the |psi(x)|^2;

hence the squared amplitude is just the charge - and not a probability!

Mar 7, 2003, 4:03:31 PM3/7/03

to

Oz wrote:

>

> I'm not sure why you would expect that of a quantised wave.

> Measuring it's state inevitably destroys the state, that's a feature of

> QM. Not least it localises the particle, how else can you measure a>

> I'm not sure why you would expect that of a quantised wave.

> Measuring it's state inevitably destroys the state, that's a feature of

> particle at a point?

There are nowadays many measurements that do not destroy a state.

In particular, nondemolition measurements do not even change the state.

Quantum measurement theory and practice has advanced quite a lot since

the time of von Neumann.

Arnold Neumaier

Mar 7, 2003, 4:12:37 PM3/7/03

to

"Charles Francis" wrote:

> This was not an argument. It was a statement of historical fact.

I must have misunderstood your post. I thought that you were saying:

1. Write down an equation which describes the motions of a material particle

without describing its physical constitution.

2. Test the equation with an experiment.

3. If the equation saves the observations conclude that the assumed particle

is fundamental, elementary and indivisible.

Or, in symbols:

A = a point particle (structure undefined, diameter d = 0)

B(a,b,c,e,f...) = equation

C = fundamental particle (finite diameter, d = not zero, one indivisible

entity)

D = Experiment.

Then,

1. Given: A, B;

2. Test the equation with an experiment

B - D = 0 --> good residuals, equation saves the experiment.

3. Conclusion: A = C.

I say that B - D = 0 does not prove that A = C.

Please correct this interpretation of your statement so that I understand

what you are saying?

Ahmet Gorgun

Mar 7, 2003, 4:21:11 PM3/7/03

to

[The following reply from end of February apparently didn't

make it to the net]

make it to the net]

Squark wrote:

>

> Arnold Neumaier <Arnold....@univie.ac.at> wrote in message news:<3e56a7e5$0$14448$3b21...@news.univie.ac.at>...

> > The particle _is_ the wave function - what else could it be? It has

> > no properties apart from that represented in the wave function (unless

> > you assume hidden variables); so one has every right to identify the

> > two. This is possible since both the particle and the wave function

> > are conceptual abstractions. The 'reality' is indiscernible...

>

> Firstly, one cannot identify the two as in a multi-particle system

> individual particles have no wavefunctions.

Of course they have; else one couldn't do any quantum physics

without considering the wave function of the universe.

The state of a subsystem is commonly described by the trace

of the the state of a bigger system with respect to all

variables not belonging to the system. This gives a density matrix

which, however, is often to a good approximation of rank 1 and

hence defines the wave function of the subsystem. (If not, one

has to treat the state as a mixture of several wave functions.)

Sometimes, other procedures are used, too. For example, in a hydrogen atom,

you can separate the center of mass motion and end up with a wave function

of the electron (mixed with a little nucleus, reflected in the reduced mass).

Now everyone agrees that the hydrogen atom is an extended object.

Its radius is of the order of the distance from the nucleus at which

|psi(x)|^2 is still large. If not the |psi(x)|^2 of the electron, what then

could determine the atom's size?

> > > > Braginsky and Khalili (Quantum measurement, Cambridge Univ. Press,

> > > > Cambridge 1992) on _real_ quantum measurements close to actual

> > > > (optical) experiments, and they talk for example (p.3 bottom) about

> > This is remarkable. In the interference process (e.g. in the

> > two-slit experiment of Fig. 1.1), [standard picture] the photon must

> > have been influenced by the locations of both slits, since the

> > interference pattern depends on the distance between them. This means

> > that the photon must have occupied a volume larger than the slit

> > separation. On the other hand, when it fell on the photographic plate,

> > the photon must have been localized into the tiny volume of the silver

> > embryo. Later the terms ''collapse of the wave function'' and

> > ''reduction of the wave packet'' were used to describe such

> > localization.<<

>

> I don't think this introductionary exposure of quantum mechanical

> effects contains any attempt to accurately reflect on such issues as

> whether the quantum particle is point-like or not.

Other people use similar language in the middle of technical discussions.

For example, the standard quantum mechanics textbook by Messiah says

(at least in the German version, from which I translate; the original

is French, which I cannot read) in 20.5.8:

``In the nonrelativistic limit, the Dirac electron is described

not by a point charge but ... extension.''

(The German version of Bjorken and drell contains a similar remark after

(4.18), but the English version is formulated differently. So at least the

translators thought the electron to be extended.)

If it is so suggestive that people use it (by slip of the mouth, you'd have

to assume), then because it is the geometrically natural way of thinking

about what the formulas mean.

> Again, I claim two things must be distinguished: the point-like quantum

> particle, which is a point without location

why a point???

> - much like a point in a non-commutative space, this as a philosophical

> notion is quite different from the usual point -

Please provide details; I haven't seen _anywhere_ a definition of what

a point in a non-commutative space is. Since a non-commutative space

is a purely mathematical concept, there should be a precise definition

of a point, without the usual phiosophical difficulties associated with

reality. Fuzzy definitions in mathematics are meaningless.

> and the extended wavefunction.

> Therefore, the notion "point-like" applies here in a sense different

> from the usual, classical, sense.

in _which_ different sense? You haven't made it precise.

> Nevertheless we would hardly

> benefit from abandoning the notion, though it might make our

> arguement ill posed: we don't agree on the definitions.

I haven't seen you give a definition; so how can I agree?

> > I don't believe you since I think I am better informed, and

> > since I think judging extension by |psi|^2 also makes much

> > more sense than claiming a pointlikeness that is operationally

> > meaningless since it relates to occult properties of physical

> > objects apart from their state. Current or not, it is _wise_

> > to consider quantum particles as being extended.

>

> It only remains to me to quote Shakespeare:

> "The fool doth think himself wise, but the wise man knows himself

> to be a fool."

I know I am a fool ;-) scoring close to 200 points on the crackpot index

http://math.ucr.edu/home/baez/crackpot.html

Arnold Neumaier

Mar 7, 2003, 4:28:15 PM3/7/03

to

Oz <aco...@btopenworld.com> wrote in message news:<b45kod$dvb$1...@panther.uwo.ca>...

> Squark <fii...@yahoo.com> writes:

> >Yes, but then you ain't measuring nothing: you prepared the particles in

> >this state, so you know the wavefunction already. Try doing the same

> >with an unknown quantum state!

>

> I already explained that.

> Squark <fii...@yahoo.com> writes:

> >Yes, but then you ain't measuring nothing: you prepared the particles in

> >this state, so you know the wavefunction already. Try doing the same

> >with an unknown quantum state!

>

> I already explained that.

I'm not quite sure I know what you're talking about.

> >There's no "action at a distance here". The emission happens in a

> >single (usually ambiguous, of course) world-point.

>

> But it's an unknown world-point, in an unknown direction.

That's what I meant by ambiguous. So what?

> >Yes, and nevertheless it still fires a single detector.

>

> I already pointed out that 'detection' is a complex QM process designed

> to (in this case) that one and only one detector per particle fires.

> It's the feature of the QM wave that you either detect (some property)

> of the wave or you don't detect it at all. That nobody ever detected

> half an electron is unremarkable.

It is remarkable. When you detect the electron at a given point, you

cannot detect it at another even though its wavefunction was non-zero

there. This means the "wave" supposedly disappeared there once you

performed the measurement. This is "action at a distance" and it also

has problems when considering different frames of reference in special

relativity. That's why the wave interpretation is physically unsound

(again, it's still a matter of interpretation, so you are free to

choose your side, but mind the facts). That's also the problem

physicists had on the turn of the 19/20 centuries. That the electron

behives neither as a classical particle nor as a classical wave.

What I'm saying is that it's a "quantum particle" i.e. something that

has a point-like location, but only in a freaky "quantum" sense. Just

like a "quantum group" is both like and unlike a real group.

> The difference being that a wave can go through both slits, whilst a

> particle can't.

It doesn't go through both slits, it goes through either one, but it

is undetermined which :-)

Mar 7, 2003, 4:40:19 PM3/7/03

to

John Baez wrote:

>

> In article <3e5feaf0$0$13932$3b21...@news.univie.ac.at>,

> Arnold Neumaier <Arnold....@univie.ac.at> wrote:

>

> >So saying 'the electron is pointlike' is simply a convention for

> >saying 'the electron is indivisible' (at least down to a certain distance

> >scale), not meaning anything else?

>

> Basically yes - a more precise answer is that when you measure

> what happens when you throw electron at each other, the answer

> matches that given by some quantum field theory in which the electron

> is a fundamental field in the Lagrangian - in particular, the Standard

> Model.

>

> >But if language provides two different

> >terms with different associated intuition, isn't it then better to

> >use these terms differently, especially when there are aspects of the

> >situation for which one term applies far more than the other?

>

> In theory yes, but it's not my goal to reform how professional

> particle physicists talk. I am happy just to understand what they

> are saying, and to have them understand me.

>

> In article <3e5feaf0$0$13932$3b21...@news.univie.ac.at>,

> Arnold Neumaier <Arnold....@univie.ac.at> wrote:

>

> >So saying 'the electron is pointlike' is simply a convention for

> >saying 'the electron is indivisible' (at least down to a certain distance

> >scale), not meaning anything else?

>

> Basically yes - a more precise answer is that when you measure

> what happens when you throw electron at each other, the answer

> matches that given by some quantum field theory in which the electron

> is a fundamental field in the Lagrangian - in particular, the Standard

> Model.

>

> >But if language provides two different

> >terms with different associated intuition, isn't it then better to

> >use these terms differently, especially when there are aspects of the

> >situation for which one term applies far more than the other?

>

> In theory yes, but it's not my goal to reform how professional

> particle physicists talk. I am happy just to understand what they

> are saying, and to have them understand me.

So it is consitent to call an electron pointlike _and_ extended

in space. Strange terminology...

It seems to me that most paradoxes in quantum mechanics

are based on imprecise language inviting conflicting conclusions,

and cleaning up the language helps avoiding being misled by intuition.

After the paradoxes of set theory were discovered, mathematicians

worked hard to rectify the situation and after understanding how

to resolve the difficulties they reformed the way they were talking

about sets. Not to their disatvantage.

In physics, people don't seem to care. Maybe physics derive an

advantage from the fogginess of current quantum concepts since

it makes the subject mysterious and hence attractive for young

people? But it also attracts fancy New Age misinterpretations

like many worlds, which seems a dubious gain.

Arnold Neumaier

Mar 10, 2003, 2:39:11 AM3/10/03

to sci-physic...@moderators.isc.org

Squark <fii...@yahoo.com> writes

>It is remarkable. When you detect the electron at a given point, you

>cannot detect it at another even though its wavefunction was non-zero

>there.

It was non-zero there. It was non-zero there before it was detected.

It became zero during the process that resulted in a detection.

Remember it's a great big wave.

>This means the "wave" supposedly disappeared there once you

>performed the measurement.

Of course. The moving photon is now no more, instead we have an exited

atom in an orbital somewhere on the emulsion.

>This is "action at a distance"

Hardly. It's a great big wave, remember. The wavefunction time-evolves

when it interacts with the billions of potentially absorbing orbitals in

the emulsion.

>and it also

>has problems when considering different frames of reference in special

>relativity.

Tough. Doubtless entangled particles give even more of a problem.

There it can be over kilometers.

>That's why the wave interpretation is physically unsound

>(again, it's still a matter of interpretation, so you are free to

> choose your side, but mind the facts).