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Guest

Hi to all quantum physicists!
I>m learning QM for quite a time. Nevertheless there are still many
unclear items especially when I look back at them after some years.
One of these came to me soon as I meticulously stared upon the
Heisenberg Uncertainty Principle. I would be pleased to hear your
opinion about my suggestions. They are logically connected and
formulated as questions so I put numbers to the 'separate' items.

1. Does the Momentum (impulse) Conservation Law hold true for a
particle going thru a single slit? I think that as the particle can
change its impulse (in fact velocity direction) obviously the Law
doesn’t apply. Nevertheless I haven’t read about this in any QM
textbook.
2. Maybe this should be understood as a private case of the Mean Value
Theorem of Ehrenfest?
3. Of course it shouldn’t be accepted that part of the impulse is
transferred to the material of the slit holder because the particle
doesn’t interact with it (or am I wrong?)
4. On the other side MCL is a direct consequence of the homogeneity of
space. So QM and in particular HUP do insist that space is not
homogeneous? (in presence of a slit?) So is the space homogeneous in
QM or not? I suppose this is connected to the creation- annihilation
of the virtual particles?? But then HUP must automatically imply their
existence as necessary???? Although I have read many QM textbooks I
didn’t learn such a thing so far.

Thank you in advance for your considerations and discussion!
Ilian

Glen Herrmannsfeldt · Guest

ilper@abv.bg wrote:

[quote]I>m learning QM for quite a time. Nevertheless there are still many
unclear items especially when I look back at them after some years.
[/quote]
Feynman>s quote:

"I think I can safely say that nobody understands quantum mechanics."

http://en.wikiquote.org/wiki/Quantum_mechanics#Sourced

[quote]One of these came to me soon as I meticulously stared upon the
Heisenberg Uncertainty Principle. I would be pleased to hear your
opinion about my suggestions. They are logically connected and
formulated as questions so I put numbers to the 'separate' items.

1. Does the Momentum (impulse) Conservation Law hold true for a
particle going thru a single slit? I think that as the particle can
change its impulse (in fact velocity direction) obviously the Law
doesnâ€™t apply. Nevertheless I havenâ€™t read about this in any QM
textbook.
[/quote]
Yes, but momentum can transfer to/from the slit. If the slit
can move you have changed the problem from the uncertainty of
the particle to the uncertainty of the slit. If it can>t, then
you can>t measure the momentum transfer.

-- glen

J. J. Lodder · Guest

<ilper@abv.bg> wrote:

[quote]Hi to all quantum physicists!
I>m learning QM for quite a time. Nevertheless there are still many
unclear items especially when I look back at them after some years.
One of these came to me soon as I meticulously stared upon the
Heisenberg Uncertainty Principle. I would be pleased to hear your
opinion about my suggestions. They are logically connected and
formulated as questions so I put numbers to the 'separate' items.

1. Does the Momentum (impulse) Conservation Law hold true for a
particle going thru a single slit? I think that as the particle can
change its impulse (in fact velocity direction) obviously the Law
doesn>t apply. Nevertheless I haven>t read about this in any QM
textbook.
[/quote]
Momentum conservation always holds.
Your missing momentum is taken up by the material around the slit(s).

[quote]2. Maybe this should be understood as a private case of the Mean Value
Theorem of Ehrenfest?
[/quote]
No.

[quote]3. Of course it shouldn>t be accepted that part of the impulse is
transferred to the material of the slit holder because the particle
doesn>t interact with it (or am I wrong?)
[/quote]
Yes, you are wrong here.
All matter consists of charged particles,
which do interact with photons.
If the photons didn>t interact with the slitted screen
they would all go straigh ahead as if in vacuum,
and produce no interference pattern.
Conversely, you could make the screen very light,
and measure the momentum transferred to it
to predict where the photons will go.
According to QM this destroys the interference pattern.

[quote]4. On the other side MCL is a direct consequence of the homogeneity of
space. So QM and in particular HUP do insist that space is not
homogeneous? (in presence of a slit?)
[/quote]
No, see above.

[quote]So is the space homogeneous in QM or not?
[/quote]
Yes.

[quote]I suppose this is connected to the creation- annihilation
of the virtual particles?? But then HUP must automatically imply their
existence as necessary???? Although I have read many QM textbooks I
didn>t learn such a thing so far.
[/quote]
Virtual particle have no physical reality.
They are an artifact of the way we do perturbation theory,

Oh No · Guest

Thus spake J. J. Lodder <nospam@de-ster.demon.nl>
[quote]ilper@abv.bg> wrote:

Hi to all quantum physicists!
I>m learning QM for quite a time. Nevertheless there are still many
unclear items especially when I look back at them after some years.
One of these came to me soon as I meticulously stared upon the
Heisenberg Uncertainty Principle. I would be pleased to hear your
opinion about my suggestions. They are logically connected and
formulated as questions so I put numbers to the 'separate' items.

1. Does the Momentum (impulse) Conservation Law hold true for a
particle going thru a single slit? I think that as the particle can
change its impulse (in fact velocity direction) obviously the Law
doesn>t apply. Nevertheless I haven>t read about this in any QM
textbook.

Momentum conservation always holds.
Your missing momentum is taken up by the material around the slit(s).
[/quote]
At the slits, there is only a change in the wave function. This leads to
no missing momentum, or contribution from the slits. If there is a later
measurement of momentum, the wave function collapses, to a value
typically different from the value prior to passing through the slits.
But this change in momentum takes place at time of measurement. One may
conclude that the change in momentum comes from the measurement
apparatus, not from the material around the slit.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.teleconnection.info/rqg/MainIndex

Igor Khavkine · Guest

On Nov 12, 6:12 pm, Glen Herrmannsfeldt <g...@ugcs.caltech.edu> wrote:
[quote]il...@abv.bg wrote:
I>m learning QM for quite a time. Nevertheless there are still many
unclear items especially when I look back at them after some years.

Feynman>s quote:

"I think I can safely say that nobody understands quantum mechanics."
[/quote]
That quote has probably done more harm than good to education in
quantum mechanics. Nearly a century of successful, even spectacularly
so, application of QM speaks otherwise.

Now, I know that Feynman had a particular idea in mind having to do
with intuition and realistic interpretations, and he may even have had
a point. Unfortunately, that>s not how his words are generally
understood.

As a service to the physics community at large, I>ll start a meme that
I hope will propagate far and wide:

Screw Feynman! Anyone who works at it can understand quantum
mechanics.

Igor

Glen Herrmannsfeldt · Guest

Igor Khavkine wrote:
(snip, I wrote)

[quote]Feynman>s quote:

"I think I can safely say that nobody understands quantum mechanics."

That quote has probably done more harm than good to education in
quantum mechanics. Nearly a century of successful, even spectacularly
so, application of QM speaks otherwise.

Now, I know that Feynman had a particular idea in mind having to do
with intuition and realistic interpretations, and he may even have had
a point. Unfortunately, that>s not how his words are generally
understood.
[/quote]
I would say that we mostly understand it, but not completely,
maybe asymptotically is about right.

For one, what ever happened to quantum gravity?

http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics

There was an article a few years ago, something like the greatest
hoax ever pulled on students is teaching the Copenhagen
interpretation of quantum mechanics. I don>t remember where
I saw it, and some web searching didn>t find it.

-- glen

Oh No · Guest

Thus spake Igor Khavkine <igor.kh@gmail.com>
[quote]On Nov 12, 6:12 pm, Glen Herrmannsfeldt <g...@ugcs.caltech.edu> wrote:
il...@abv.bg wrote:
I>m learning QM for quite a time. Nevertheless there are still many
unclear items especially when I look back at them after some years.

Feynman>s quote:

"I think I can safely say that nobody understands quantum mechanics."

That quote has probably done more harm than good to education in
quantum mechanics. Nearly a century of successful, even spectacularly
so, application of QM speaks otherwise.

Now, I know that Feynman had a particular idea in mind having to do
with intuition and realistic interpretations, and he may even have had
a point. Unfortunately, that>s not how his words are generally
understood.

As a service to the physics community at large, I>ll start a meme that
I hope will propagate far and wide:

Screw Feynman! Anyone who works at it can understand quantum
mechanics.

[/quote]
Anyone can understand the mathematical structure of quantum mechanics,
and anyone can put their head in the sand by claiming that
interpretation is not the business of physics, but so long as
interpretation is an unresolved issue, I do not find it reasonable to
dismiss what Feynman said.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.teleconnection.info/rqg/MainIndex

Glen Herrmannsfeldt · Guest

Oh No wrote:
[quote]Thus spake Igor Khavkine <igor.kh@gmail.com
(snip)[/quote]

[quote]Screw Feynman! Anyone who works at it can understand quantum
mechanics.

Anyone can understand the mathematical structure of quantum mechanics,
and anyone can put their head in the sand by claiming that
interpretation is not the business of physics, but so long as
interpretation is an unresolved issue, I do not find it reasonable to
dismiss what Feynman said.
[/quote]
Yeah, what he said.

Also, being able to solve the equations is one thing,
explaining the meaning is another.

-- glen

Guest

On Nov 13, 10:15=A0am, nos...@de-ster.demon.nl (J. J. Lodder) wrote:
[quote]
Virtual particle have no physical reality.
[/quote]
I seem to be able to feel the virtual photons being emitted and
absobed when I hold 2 magnets and face them at each other, if I am not
feeling the physical reality of virtual particles then what is it?

Guest

There two opposite views on this thread.

1. momentum comes from/to the slit:

Glen Herrmannsfeldt wrote:

[quote]Yes, but momentum can transfer to/from the slit. If the slit
can move you have changed the problem from the uncertainty of
the particle to the uncertainty of the slit. If it can>t, then
you can>t measure the momentum transfer.

-- glen
[/quote]
J. J. Lodder wrote:
[quote]Momentum conservation always holds.
Your missing momentum is taken up by the material around the slit(s)

All matter consists of charged particles,
which do interact with photons.
If the photons didn>t interact with the slitted screen
they would all go straigh ahead as if in vacuum,
and produce no interference pattern.
Conversely, you could make the screen very light,
and measure the momentum transferred to it
to predict where the photons will go.
[/quote]
I have some objectioin to this.

If there is an interraction between the material of the slit and the
particles moving tru the slit then there must be loss not only in the
direction but also in the magnitude of the impulse. Secondly if we
narrow the slit this wouldn>t increase delta p because this would not
change the interraction in order to take effect and broaden delta p.

2. momentum comes from the measurement apparatus

Charles Francis wrote

[quote]At the slits, there is only a change in the wave function. This leads to
no missing momentum, or contribution from the slits. If there is a later
measurement of momentum, the wave function collapses, to a value
typically different from the value prior to passing through the slits.
But this change in momentum takes place at time of measurement.
One may conclude that the change in momentum comes from the
measurement apparatus, not from the material around the slit.
[/quote]
I have the following objections:

It turns out that the wave function is something which carry the
momentum to the measurement apparatus. But WF is not real (in
Copenhagen interpretatiion) so it can not do this.

================================================
I>m still thinking the momentum conservation law doesnt hold for a
single particle. Why not if this is permitted for the energy (at the
end they form a four momentum in SR).

Regards: Ilian

Igor Khavkine · Guest

On Nov 14, 5:53 pm, il...@abv.bg wrote:
[quote]There two opposite views on this thread.

1. momentum comes from/to the slit:

I have some objectioin to this.

If there is an interraction between the material of the slit and the
particles moving tru the slit then there must be loss not only in the
direction but also in the magnitude of the impulse.
[/quote]
You are right in general, but not in the idealized experiment being
considered here. The interaction with the slit is considered elastic,
hence energy and momentum magnitude conserving.

[quote]Secondly if we
narrow the slit this wouldn>t increase delta p because this would not
change the interraction in order to take effect and broaden delta p.
[/quote]
Your reasoning is flawed here, and your conclusion is false. Of course
the interaction changes when the slit is narrowed; there is now more
barrier material for the electron to bounce off of than before. And
delta-p increases because both the Schroedinger equation and the
Heisenberg uncertainty principle say so.

[quote]I>m still thinking the momentum conservation law doesnt hold for a
single particle. Why not if this is permitted for the energy (at the
end they form a four momentum in SR).
[/quote]
Again, your argument and conclusion here are flawed. For an isolated
system, even in special relativity, 4-momentum is conserved as a
whole, including its individual components. The key word here is
isolated.

Hope this helps.

Igor

J. J. Lodder · Guest

<johndevers@iprimus.com.au> wrote:

[quote]On Nov 13, 10:15=A0am, nos...@de-ster.demon.nl (J. J. Lodder) wrote:

Virtual particle have no physical reality.

I seem to be able to feel the virtual photons being emitted and
absobed when I hold 2 magnets and face them at each other, if I am not
feeling the physical reality of virtual particles then what is it?
[/quote]
You can feel nothing of the kind.
All you can feel is a force.
The rest is (mistaken) interpretation.
You should try not to confuse observation
and interpretation,

Jan

Guest

On 16 ÐÐ¾ÐµÐ¼, 22:06, Igor Khavkine <igor...@gmail.com> wrote:
[quote]On Nov 14, 5:53 pm, il...@abv.bg wrote:

There two opposite views on this thread.

1. momentum comes from/to the slit:
I have some objectioin to this.

If there is an interraction between the material of the slit and the
particles moving tru the slit then there must be loss not only in the
direction but also in the magnitude of the impulse.

You are right in general, but not in the idealized experiment being
considered here. The interaction with the slit is considered elastic,
hence energy and momentum magnitude conserving.
[/quote]
Well I admit the change in momentum would be inversly proportional to
the mass of the material of the slit holder and so it is normal to be
neglected. (Nevertheless some inside dissatisfaction about making this
QM an approximation to reality on its very beginning creeps in).

So more important is your secondary notice.

Igor Khavkine <igor...@gmail.com> wrote:

[quote]On Nov 14, 5:53 pm, il...@abv.bg wrote:

Secondly if we narrow the slit this wouldn>t increase
delta p because this would not change the interraction in > >order to take effect and broaden delta p.
[/quote]

[quote]Your reasoning is flawed here, and your conclusion is false. Of course
the interaction changes when the slit is narrowed; there is now more
barrier material for the electron to bounce off of than before. And
delta-p increases because both because both the Schroedinger equation and the
Heisenberg uncertainty principlethe espesially say so.

[/quote]
I dont agree. Consider a flux of just one particle. It sees just the
edge of the slit and it changes its direction. But what if the other
edge is near or far? I think this changes nothing (espesially in an
idealised picture of absolutely rigid walls). I dont thik there would
be more barrier material for the electron to bounce off. Note that
this is just a classical consideration of the slit. I>m not arguing
against what the Schroedinger equation and the Heisenberg uncertainty
principle say.
Alternatively one can think that the wave function now sees more
barrier material. But as it is (WF) unphysical (just a probability)
there can not be maintained that it makes an interraction leading to
broadening of delta p.

Of course the percentage of the electrons deflected towards electrons
not deflected would be greater for a narrow slit but this has nothing
to do with the imergence of electrons with greater deviation after the
slit. I cant see classicaly why there should be greater deviation for
some electrons when the walls are closer.

Regards: Ilian

noshellswill · Guest

On Mon, 17 Nov 2008 09:15:31 -0500, ilper wrote:

[Moderator>s note: Quoted text snipped. -P.H.]

[quote]Of course the percentage of the electrons deflected towards electrons
not deflected would be greater for a narrow slit but this has nothing to
do with the imergence of electrons with greater deviation after the
slit. I cant see classicaly why there should be greater deviation for
some electrons when the walls are closer.

Regards: Ilian
[/quote]
Rigid or not, if the (finite length) slit-walls are metallic, wouldn>t
the passing electron generate a current leading to (transient) dipole
radiation from the slit?

nss
******

Igor Khavkine · Guest

On Nov 17, 9:15 am, il...@abv.bg wrote:
[quote]On 16 ÐÐ¾ÐµÐ¼, 22:06, Igor Khavkine <igor...@gmail.com> wrote:
On Nov 14, 5:53 pm, il...@abv.bg wrote:

You are right in general, but not in the idealized experiment being
considered here. The interaction with the slit is considered elastic,
hence energy and momentum magnitude conserving.

Well I admit the change in momentum would be inversly proportional to
the mass of the material of the slit holder and so it is normal to be
neglected. (Nevertheless some inside dissatisfaction about making this
QM an approximation to reality on its very beginning creeps in).
[/quote]
This idealization is no worse than any other. Do you think neglecting
air drag unreasonably impacts your understanding of projectile motion?

[quote]Secondly if we narrow the slit this wouldn>t increase
delta p because this would not change the interraction in
order to take effect and broaden delta p.

Your reasoning is flawed here, and your conclusion is false. Of course
the interaction changes when the slit is narrowed; there is now more
barrier material for the electron to bounce off of than before. And
delta-p increases because both because both the Schroedinger equation
and the Heisenberg uncertainty principle espesially say so.

I dont agree. Consider a flux of just one particle. It sees just the
edge of the slit and it changes its direction. But what if the other
edge is near or far? I think this changes nothing (espesially in an
idealised picture of absolutely rigid walls). I dont thik there would
be more barrier material for the electron to bounce off. Note that
this is just a classical consideration of the slit. I>m not arguing
against what the Schroedinger equation and the Heisenberg uncertainty
principle say.
Alternatively one can think that the wave function now sees more
barrier material. But as it is (WF) unphysical (just a probability)
there can not be maintained that it makes an interraction leading to
broadening of delta p.

Of course the percentage of the electrons deflected towards electrons
not deflected would be greater for a narrow slit but this has nothing
to do with the imergence of electrons with greater deviation after the
slit. I cant see classicaly why there should be greater deviation for
some electrons when the walls are closer.
[/quote]
Your reasoning is still flawed. There are no individual trajectories
that pass near or far to the slit edges. QM only makes predictions
about statistical distribution of repeated measurements. A trajectory
is not a measurement outcome, hence not predicted by QM.

Whether you think the wave function is physical or not, it>s amplitude
squared is what predicts the measurement distribution. And the wave
function amplitude is determined by the Schroedinger equation. All
other reasoning is subsumed by this fact (as long as the situation
does not warrant switching to the Dirac equation or other
alternative).

Igor


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