| Topic: |
Science > Physics |
| User: |
"Iconoclast" |
| Date: |
16 Nov 2003 05:22:06 PM |
| Object: |
QM, particles and waves |
As a layperson, I am struggling with "Causality", maybe someone has an
answer for me.
The Uncertainty Principle postulates that it is inherently impossible to
simultaneously establish the momentum and position of an electron.
Is it also inherently impossible to determine if a photon exhibits as a
particle or as a wave? Or can it be both at the same time.
Thanks for any input.
--
Walter
The Happy Iconoclast www.rationality.net
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| User: "David Macmanus" |
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| Title: Re: QM, particles and waves |
17 Nov 2003 04:12:00 AM |
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"Iconoclast" <wer25@yahoo.com> wrote in message
news:yETtb.201481$ZH4.71734@twister.socal.rr.com
As a layperson, I am struggling with "Causality", maybe someone has an
answer for me.
The Uncertainty Principle postulates that it is inherently impossible to
simultaneously establish the momentum and position of an electron.
Is it also inherently impossible to determine if a photon exhibits as a
particle or as a wave? Or can it be both at the same time.
This is a bit of a can of worms you're going into here. It's very murky,
and it's difficult to get ot the bottom of it. Even the Heisenberg
Uncertainty principle is more complex than it first seems. Also, things
like the HUP have been discussed ad infinitum here so people aren't that
enthused about discussing it again. I've poked around at trying to
understand the whole thing a bit better, with not a lot of success. One
book you might try is "The Infamous Boundary: Seven Decades of Heresy in
Quantum Physics
D. Wick ". Here you can find out about the different ways of stating the
HUP. It tries to explain the difference between the Uncertainty
Principle and the indeterminacy principle, for example. It also looks at
the gamma ray microscope experiment which is basically an experiment for
trying to gather information about some particle or other - essentially
the problem is that when you try to 'measure' the particle's position,
you disturb it so everything becomes spoiled. There are some on this
newsgroup who will castigate you for even mentioning this experiment
because it is not really in the spirit of the HUP. The HUP is typically
stated in terms of "inherent" uncertainty, whereas the gamma ray
microscope is about dfisturbance caused by measuring. There is a
difference! Finding discussions of this difference is not so easy.
Wick's book also discusses a thing called "retrodiction" which is about
looking back into the past and being able to say that a particle had an
particular position AND momentum. See what I mean about it getting
murky? I've tried asking questions about retrodiction here and not got
much luck, so I still don't know whether it is a real thing or not! If
you ever find out about it let me know! By the way, one of the posters
impugns your statement of the HUP. What you state about it being
inherently impossible to
simultaneously establish the momentum and position of an electron is
correct, but it's not actually what the HUP "states", so you should be
careful there (physicists insist on the right language!).
As far as the photon is concerned, nobody would ever stick their neck
out and state is one thing or the other. It is what it is, a photon. If
you do experiments measuring wave phenomna on it you will find it
behaving like a wave; if you do paricle-type experiments on it you will
find it acting like a particle. You won't be able to win. It's not a
good idea to try and nail it down.
Well, I probably haven't been very helpful, except by making you aware
that you are wandering a minefield. I reckon you could study this stuff
for years and still not know what's going on.
By the way, things don't get any easier when you start looking at things
like entanglement and Bells' theorem.
Good luck,
David.
--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG
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| User: "sol" |
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| Title: Re: QM, particles and waves |
17 Nov 2003 04:17:17 PM |
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"David Macmanus" <macmanus@tripos.com> wrote in message news:<0f87c2d8c9b188f18fbeb9b64291cf47.35661@mygate.mailgate.org>...
"Iconoclast" <wer25@yahoo.com> wrote in message
news:yETtb.201481$ZH4.71734@twister.socal.rr.com
As a layperson, I am struggling with "Causality", maybe someone has an
answer for me.
The Uncertainty Principle postulates that it is inherently impossible to
simultaneously establish the momentum and position of an electron.
Is it also inherently impossible to determine if a photon exhibits as a
particle or as a wave? Or can it be both at the same time.
This is a bit of a can of worms you're going into here. It's very murky,
and it's difficult to get ot the bottom of it. Even the Heisenberg
Uncertainty principle is more complex than it first seems. Also, things
like the HUP have been discussed ad infinitum here so people aren't that
enthused about discussing it again. I've poked around at trying to
understand the whole thing a bit better, with not a lot of success. One
book you might try is "The Infamous Boundary: Seven Decades of Heresy in
Quantum Physics
D. Wick ". Here you can find out about the different ways of stating the
HUP. It tries to explain the difference between the Uncertainty
Principle and the indeterminacy principle, for example. It also looks at
the gamma ray microscope experiment which is basically an experiment for
trying to gather information about some particle or other - essentially
the problem is that when you try to 'measure' the particle's position,
you disturb it so everything becomes spoiled. There are some on this
newsgroup who will castigate you for even mentioning this experiment
because it is not really in the spirit of the HUP. The HUP is typically
stated in terms of "inherent" uncertainty, whereas the gamma ray
microscope is about dfisturbance caused by measuring. There is a
difference! Finding discussions of this difference is not so easy.
Wick's book also discusses a thing called "retrodiction" which is about
looking back into the past and being able to say that a particle had an
particular position AND momentum. See what I mean about it getting
murky? I've tried asking questions about retrodiction here and not got
much luck, so I still don't know whether it is a real thing or not! If
you ever find out about it let me know! By the way, one of the posters
impugns your statement of the HUP. What you state about it being
inherently impossible to
simultaneously establish the momentum and position of an electron is
correct, but it's not actually what the HUP "states", so you should be
careful there (physicists insist on the right language!).
As far as the photon is concerned, nobody would ever stick their neck
out and state is one thing or the other. It is what it is, a photon. If
you do experiments measuring wave phenomna on it you will find it
behaving like a wave; if you do paricle-type experiments on it you will
find it acting like a particle. You won't be able to win. It's not a
good idea to try and nail it down.
Well, I probably haven't been very helpful, except by making you aware
that you are wandering a minefield. I reckon you could study this stuff
for years and still not know what's going on.
By the way, things don't get any easier when you start looking at things
like entanglement and Bells' theorem.
Good luck,
David.
John Bell's Inequalities
http://worldcrossing.com/WebX?14@32.hAO0azHy6oI.1@.1dde418b/1
I am having ahard time of it, that if energy is considered and let us
say strings as a basis( a energy value)as a particle description, then
would we not say that position and momentum are calcuable?
Sol
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| User: "Eckard Blumschein" |
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| Title: Re: QM, particles and waves |
20 Nov 2003 08:13:10 AM |
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Somewhat off topic but hopefully appreciated:
In signal analysis, the HUP must also not be taken a gospel.
Having thoroughly dealt with causality, the natural zero of time and an
adequate integral transform, I managed to avoid the notorious trade-off
between spectral and temporal resolution of spectrogram.
Even our ears do not obey the HUP.
Eckard Blumschein
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| User: "Edward Green" |
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| Title: Re: QM, particles and waves |
16 Nov 2003 09:39:36 PM |
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"Iconoclast" <wer25@yahoo.com> wrote in message news:<yETtb.201481$ZH4.71734@twister.socal.rr.com>...
As a layperson, I am struggling with "Causality", maybe someone has an
answer for me.
The Uncertainty Principle postulates that it is inherently impossible to
simultaneously establish the momentum and position of an electron.
Is it also inherently impossible to determine if a photon exhibits as a
particle or as a wave? Or can it be both at the same time.
Many things which can be measured in quantum mechanics are known as
"observables". The uncertainty principle applies to certain pairs of
observables known in the trade as "canonically conjugate" (if I have
my jargon right), and an exact statement of it concerns the
statistical properties of a large number of observations on a
gaggletude of identical starting states (the technical term for
gaggletude is "ensemble") -- not on the simultaneous values of the
observables for a particular system.
That aside, "exhibiting as a particle" and "exhibiting as a wave" are
not canonically conjugate observables ... they are not even
"observables" in any usual sense ... so the uncertainty relation
simply does not apply. There is no measurement one can perform on a
photon which gives a value of "particlehood" or "wavehood" -- just
various situations which seem to emphasize one aspect of light's
behavior or the other.
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| User: "Sam Wormley" |
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| Title: Re: QM, particles and waves |
16 Nov 2003 06:38:54 PM |
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Iconoclast wrote:
As a layperson, I am struggling with "Causality", maybe someone has an
answer for me.
The Uncertainty Principle postulates that it is inherently impossible to
simultaneously establish the momentum and position of an electron.
Is it also inherently impossible to determine if a photon exhibits as a
particle or as a wave? Or can it be both at the same time.
http://www.arxiv.org/abs/quant-ph/0309016
The wave nature of biomolecules and fluorofullerenes
Authors: Lucia Hackermueller, Stefan Uttenthaler, Klaus Hornberger,
Elisabeth Reiger, Bjoern Brezger, Anton Zeilinger, Markus Arndt
Comments: 5 pages, 4 figures
Journal-ref: Phys. Rev. Lett. 91, 090408 (2003)
DOI: 10.1103/PhysRevLett.91090408
We demonstrate quantum interference for tetraphenylporphyrin, the
first biomolecule exhibiting wave nature, and for the
fluorofullerene C60F48 using a near-field Talbot-Lau
interferometer. For the porphyrins, which are distinguished by
their low symmetry and their abundant occurence in organic systems,
we find the theoretically expected maximal interference contrast
and its expected dependence on the de Broglie wavelength. For
C60F48 the observed fringe visibility is below the expected value,
but the high contrast still provides good evidence for the quantum
character of the observed fringe pattern. The fluorofullerenes
therefore set the new mark in complexity and mass (1632 amu) for de
Broglie wave experiments, exceeding the previous mass record by a
factor of two.
See: http://www.arxiv.org/abs/quant-ph/0309016
You might also enjoy:
Entanglement: The Greatest Mystery in Physics
Amir D Aczel
2002 John Wiley & Sons/Four Walls Eight
Windows 302pp 16.99/$28.00hb
There are two kinds of books about quantum
mechanics. There are those in which we learn
about abstract concepts such as Hilbert spaces,
state vectors and density matrixes, but where the
author never addresses - or only pays lip-service
to - the question of what quantum mechanics
actually means. This is the approach often taken in
textbooks. The other, quite opposite, approach
focuses on the interpretative question - drawing all
kinds of conclusions and analogies, talking about
telepathy and other mysteries, and perhaps even
claiming that quantum mechanics transcends
Western philosophy.
Neither approach is very helpful when one wants
to understand what quantum mechanics really
means in a deep philosophical sense. Amir Aczel's
new book on entanglement - falling as it does into
neither category - avoids such pitfalls.
Anton Zeilinger from the Institute of Experimental
Physics at the University of Vienna reviews the
book in the May issue of Physics World; email
Did you ever wonder "What the heck is a photon, anyway?"
http://math.ucr.edu/home/baez/photon/schmoton.htm
The American Institute of Physics Bulletin of Physics News
Number 626 February 26, 2003 by Phillip F. Schewe, Ben Stein, and James
Riordon
3600 ATOMS IN TWO PLACES AT ONCE. Bose Einstein condensates (BEC), clouds
of ultracold atoms which fall together into a single coherent state,
continue to be a marvelous working material for studying subtle quantum
effects. Last year physicists at the Max Planck Institute for Quantum
Optics (MPQ) managed to load a BEC of rubidium atoms into a
three-dimensional optical lattice, an artificial crystalline environment in
which crossing laser beams provide the forces needed to pinion atoms in the
3D equivalent of an egg crate. Moreover, by a delicate modification of the
laser light the resident atoms could be made to undergo a quantum transition
between two phases. In one phase the atoms constitute a superfluid: all the
atoms have a coordinated wave function, but the number of atoms in any one
"well" in the egg carton is unknown. In the other phase the atoms
constitute an insulator: the number of atoms in each well is known exactly
to be equal to one, but the atoms are all uncoordinated with respect to each
other (that is, they no longer can be considered a coherent quantum
material). These were the results as of a year ago (see Greiner et al.,
Nature, 3 January 2003.) Speaking at last week's meeting of the American
Association for the Advancement of Science (AAAS) in Denver, Immanuel Bloch
reported that he and his MPQ colleagues have exploited the fact that the Rb
atoms possess two magnetic substates and have succeeded, by a further
adjustment of the confining laser beams, to separate each atom into two
entangled spatially separated parts. The researchers are also attempting
to get the different diploid atoms (an average of 3600 per plane in the
lattice) to interact; one aim is to engineer an unprecedented degree of
quantum entanglement, possibly for computational purposes.
TUNABLE OPTICAL FIBERS. Optical fibers regularly carry billions of phone
conversations and other data transmissions every day and are a fundamental
part of optical sensing and numerous medical applications. The photonic
devices responsible for all this traffic are being made even more efficient
and versatile by handing over some of the switching and reconfiguring chores
to the fibers themselves---the trunk lines linking all the optical nodes. An
active optical fiber, which can tunably filter light at different
frequencies, has been created by infusing microfluidic plugs, spaced at
characteristic (periodic) intervals along the fiber, into air holes running
parallel to the passageway for the light at the center of the fiber (see
figure at http://www.aip.org/mgr/png/2003/180.htm ). The arrays of
microfluidic plugs along the light path serves as a diffraction grating for
producing the photonic-crystal effect. In other words, the presence of the
fluids is used to change the refractive index periodically, and hence the
transmission properties, of the fiber. The creators of this new
microstructured optical fiber (MOF), Charles Kerbage (OFS Laboratories in
Murray Hill, NJ; kerbage@ofsoptics.com) and Ben Eggleton (University of
Sydney, egg@physics.usyd.edu.au), say that this is the first time a tunable
grating has been achieved with microfluids, and that this provides (in
addition to the switchability) a very high index of refraction when compared
to conventional gratings. (Applied Physics Letters, 3 March 2003)
SHAKEN NOT STIRRED. The progression toward smaller and smaller electrical
and mechanical components presents tremendous challenges to engineers and
scientists as they strive to create devices on scales measured in microns
and nanometers. One solution may be to develop materials that automatically
arrange themselves in useful patterns. Now a collaboration of researchers
(Igor Aronson, aronson@msd.anl.gov, 630-252-9725) at Argonne National
Laboratory and Institute of Physics for Microstructures of the Russian
Academy of Sciences has developed a new method for encouraging microscopic
particles to self assemble into desired complex patterns. The technique is
inspired by the patterns formed in shaken mixtures of much larger granular
materials.
Numerous, beautiful experiments involving agitated containers of sand, ball
bearings, or other granular materials have shown that the combination of
gravity and inter-particle forces from collisions can lead to a rich variety
of patterns, ranging from particle-like localized excitations known as
oscillons to honeycomb shapes to chaotic swirls (Update 264). Other studies
have helped to explain why large and heavy brazil nuts sometimes rise to the
top in shaken containers of mixed nuts (Update 132). The new research
extends such experiments into microscopic regimes.
Rather than mechanically agitating tiny grains to create self assembled
patterns, however, the method relies on electrostatic fields to drive
metallic microparticles immersed in liquids. The researchers placed
120-micron bronze spheres in a mixture of toluene and ethanol trapped
between glass plates. The plates were coated with thin layers of transparent
conducting material, and an electric field of up to 3 kilovolts per
millimeter was applied between them. Particles that contacted the lower
plate acquired a charge and were repelled toward the upper plate. If the
upward electrostatic force is sufficient to overcome gravity, the particles
fly upward, contact the upper plate where their charge is reversed, and then
are forced back down again. In effect, the alternating charge on the
particles is analogous to shaking a container of macroscopic grains. As in
the classic granular material experiments, varying the conditions causes the
particles to form vortices, pulsating rings, honeycomb patterns, or other
structures (see figure at www.aip.org/mgr/png ). Ultimately, say the
researchers, studies such as this may allow us to design systems that
spontaneously self assemble into useful structures on increasingly tiny
scales. (M. V. Sapozhnikov, Physical Review Letters, upcoming article)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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| User: "Uncle Al" |
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| Title: Re: QM, particles and waves |
16 Nov 2003 06:36:08 PM |
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Iconoclast wrote:
As a layperson, I am struggling with "Causality", maybe someone has an
answer for me.
The Uncertainty Principle postulates that it is inherently impossible to
simultaneously establish the momentum and position of an electron.
No. Heisenberg says that the product of the uncertainties of
conjugate variables' simultaneous values cannot be less than a stated
very small number
Is it also inherently impossible to determine if a photon exhibits as a
particle or as a wave? Or can it be both at the same time.
This is irrelevant to Heisenberg. A photon is neither.
http://www.quantum.univie.ac.at/research/matterwave/c60/
The group wants to repeat the experiment with small thermostable
viruses.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
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