Heisenberg and quantum mechanics

[ZapperZ]

It simply seems that we have a fundamentally different interpretation about what those elementary particles are.

To me they are waves, they are not little "balls".
So to explain paths by some sort of Newtonian mechanics does not make sense, and it actually does not work.
Look at momentum, can anybody with a straight face explain to me how a particle could have a momentum that is an imaginary number in space-time? Or a fractional spin?

I think a wave interpretation makes more sense, waves that spread out over time and operate non-locally.

This seems completely irrelevant to the current discussion.

*I* can tell you how "particles" can have fractional spin via emergent properties. That is how we set up the Laughlin wavefunction in describing the fractional charge and fractional quantum hall effect.

You still have not produced a single citation on how you are justified in connecting the non-locality of quantum entanglemnt with non-locality of "forces", or are you adament in insisting that (i) you never made such claims or (ii) you no longer want to make that connection?

Zz.

 

[MeJennifer]

You still have not produced a single citation on how you are justified in connecting the non-locality of quantum entanglemnt with non-locality of "forces", or are you adament in insisting that (i) you never made such claims or (ii) you no longer want to make that connection?

When we talk about EPR with photons for instance we talk about forces. photons represent forces!
It seems you misunderstood me, I don't claim that some unknown forces communicate at the non-local level.

So what are your thoughts about the matter, do you think photons are particles, little balls? Are they point sizes? Do they really spin? How can they have fractional spin?

I think the whole particle approach that started with Einstein was a mistake, sure we can make the math work and create any emerging property or virtual particle to "explain" it.

To me the wave approach makes much more sense. But of course I cannot prove it, but on the other hand you cannot prove it is a particle either. But I don't claim, and I suppose you don't either that we have an answer for all the questions in QM.

So then we can simply discuss this fascinating topic and even agree to disagree in a friendly and respectable way as far as I am concerned.

 

[MeJennifer]

You still have not produced a single citation on how you are justified in connecting the non-locality of quantum entanglemnt with non-locality of "forces", or are you adament in insisting that (i) you never made such claims or (ii) you no longer want to make that connection?

When we talk about EPR with photons for instance we talk about forces. photons represent forces!
It seems you misunderstood me, I don't claim that some unknown forces communicate at the non-local level.

So what are your thoughts about the matter, do you think photons are particles, little balls? Are they point sizes? Do they really spin? How can they have fractional spin?

I think the whole particle approach that started with Einstein was a mistake, sure we can make the math work and create any emerging property or virtual particle to "explain" it.

To me the wave approach makes much more sense. But of course I cannot prove it is really just waves, but on the other hand you cannot prove it is a particle either. But I don't claim, and I suppose you don't either that we have an answer for all the questions in QM.

So then we can simply discuss this fascinating topic and even agree to disagree in a friendly and respectable way as far as I am concerned.

 

[ZapperZ]

When we talk about EPR with photons for instance we talk about forces. photons represent forces!

Really?

What is the difference between the photons that you see as ordinary light, and the "photons" that are carriers of EM interaction in QED? Are you seriously telling me you see ZERO difference between the two?

It seems you misunderstood me, I don't claim that some unknown forces communicate at the non-local level.

The statements you have made that I quoted indicate otherwise.

So what are your thoughts about the matter, do you think photons are particles, little balls? Are they point sizes? Do they really spin? How can they have fractional spin?

Can you give me a citation of "fraction spin" for photon?

Secondly, please do a search on photon sizes on here. It has been discussed ad nauseum. Look in Einstein's papers, and even in QM and tell me where the property of "size" was ever associated with a photon. You might as well ask if it has a degree of saltiness.

I think the whole particle approach that started with Einstein was a mistake, sure we can make the math work and create any emerging property or virtual particle to "explain" it.

To me the wave approach makes much more sense. But of course I cannot prove it is really just waves, but on the other hand you cannot prove it is a particle either. But I don't claim, and I suppose you don't either that we have an answer for all the questions in QM.

Fine. IF you can explain qualitatively AND quantitatively (i) resonant photoemission (ii) angle-resolved photoemission and (iii) multiphoton photoemission experiments, then come talk to me that the wave picture can explain everything and as well as the photon picture. You are not the first to come this way touting such claims. But each time I asked for these people to put their money where their mouths are in coming up with a description that matches those 3 phenomena, they ran with their tails in between their legs. So I now ask you to do the same and come up with such a description to justify your claim that the wave picture is as good.

So then we can simply discuss this fascinating topic and even agree to disagree in a friendly and respectable way as far as I am concerned.

then create your own thread and not hijack an existing one. This appears to be nothing more than a diversion away from you having to justify what you have said earlier.

Zz.

 

[MeJennifer]

You are not the first to come this way touting such claims. But each time I asked for these people to put their money where their mouths are in coming up with a description that matches those 3 phenomena, they ran with their tails in between their legs.

Well frankly I am not surprised if you treat everybody the way you treat me.

then create your own thread and not hijack an existing one. appears to be nothing more than a diversion away from you having to justify what you have said earlier.

Ok, now you are simply rude, the floor is yours.

 

[ZapperZ]

Well frankly I am not surprised if you treat everybody the way you treat me.

I only treat people like that who have no qualm in making outrageous claim while being ignorant of the current understanding. To thnk that the photoelectric effect, in its primitive form, is the ONLY standard bearer for "photons" is ridiculous. The QM description has been used, and used successfully, to describe all those experiments that I've described. Where are the wave picture descriptions?

Without such a thing, how can one even begin to claim that the photon description is wrong and the wave picture is correct? It makes no rational sense.

Ok, the floor is yours.

Thank you. And you continue to ignore any of my request for citation to back any of your claims. You still don't see any difference between the QED photons and ordinary photons?

Zz.

 

[Mickey]

Light is much more likely to take a straight path. QED photons have an equal probability of taking any path to any destination (but not an equal probability of reaching any specific destination). http://jhunix.hcf.jhu.edu/~blee27/software/qedlens/index.htm

I give the floor back to whomever. :)

 

[yossell]

I think the point that momentum OR position can be made as accurately as you like is correct, but I still have some uncertainties about how the HUP should be interpreted.

This is from Feynmann, Lectures in Physics, vol III, section 1.8

"This is the way Heisenberg stated the uncertainty principle originally: if you make the measurement on any object, and you can determine the x-component of its momentum with an uncertainty dp, you cannot, at the same time, know its x-position more accurately than dx = h/dp, where h is a definite fixed number given by nature...The uncertainties in the position and momentum of a particle at any instant must have their product greater than Planck's constant".

In this version, the question as to whether the measurements are made simultaneously or at the same instant seems to be important, supporting moving finger's take on things.

Feynmann goes on to say that it is a special case of a more general uncertainty principle that 'one cannot design equipment in any way to determine which of two alternatives is taken, without, at the same time, destroying the pattern of interference'.

I'm not sure what to make of this second version (it seems pretty vague to me), but again 'at the same time' appears in its statement.

Could it be that there are variations on the uncertainty principle in the phyiscs literature and that different members have different versions in mind?

 

[ZapperZ]

I think the point that momentum OR position can be made as accurately as you like is correct, but I still have some uncertainties about how the HUP should be interpreted.

This is from Feynmann, Lectures in Physics, vol III, section 1.8

"This is the way Heisenberg stated the uncertainty principle originally: if you make the measurement on any object, and you can determine the x-component of its momentum with an uncertainty dp, you cannot, at the same time, know its x-position more accurately than dx = h/dp, where h is a definite fixed number given by nature...The uncertainties in the position and momentum of a particle at any instant must have their product greater than Planck's constant".

In this version, the question as to whether the measurements are made simultaneously or at the same instant seems to be important, supporting moving finger's take on things.

But your interpretation contradicts QM. How do you explain the significance of the commutating relations of observables in QM? Why would it matter if A or B operates on a wavefunction FIRST?

The very fact that AB is not identical to BA implies that the "order" of operation is crucial. If A and B can be determined simulaneously in a single measurement, then A and B commutes! You have this for non-degenerate plane wave states and you measure the momentum and get the energy at the same time, because p and E commutes! But tell me how you would measure p and x "simultaneously". In the paper that moving finger cited, you'll notice that they are using the same single-slit scenario where the momentum is determined AFTER the slit. Is this what we are all calling "simultaneous"?

Feynmann goes on to say that it is a special case of a more general uncertainty principle that 'one cannot design equipment in any way to determine which of two alternatives is taken, without, at the same time, destroying the pattern of interference'.

I'm not sure what to make of this second version (it seems pretty vague to me), but again 'at the same time' appears in its statement.

Could it be that there are variations on the uncertainty principle in the phyiscs literature and that different members have different versions in mind?

Again, this is a confusion between HUP and superposition principle. When you have the ability of a particle to go through a number of paths, QM description describes this as a superposition of all possible paths. This is not the HUP. HUP and superposition are two different, but connected, phenomena of QM.

Zz.

 

[yossell]

Dear Zz

"Your Interpretation contradicts QM"
Well, I didn't mean to endorse any particular interpretation in my post (I was trying to be very non-confrontational. Apologies if I seemed rude). I was merely quoting Feynman about HUP, and his words do seem to support this interpretation. But it may be that Feynman has mis-spoken here or that there are different versions of the principle in the literature.

But it's not just Feynmann: A.I.M. Rae, Quantum Mechanics (Undergraduate Text book): "This relation is known as the Heisenberg Uncertainty Principle. According to quantum mechanics it is a fundamental property of nature that any attempt to make simultaneous measurements of position and momentum are subject to this limitation". p.12.
Bohm: Quantum Theory, section 3 "On the Interpretation of the Uncertainty Principle", says "the momentum and position cannot even exist with simultaneously and perfectly defined values".

This isn't meant to tell against your own interpretation of HUD at all - but there's an indication that, at least in some of the serious literature, the explanation of the principle does seem to be sympathetic with the thought that it's the simultaneous possession or discovery of such quantities that is ruled out by the HUP.

Indeed there are parts of these books that support Zz's view: when Rae enters into a more mathematical discussion in terms of commutators (pp 71-2), he talks of a series of measurements rather than individual measurements, and simultaneity seems to disappear from his discussion. He introduces something called 'the generalised uncertainty principle', but the generality seems to come from the fact that it involves observables other than position and momentum - which wouldn't wholly explain the difference.

I agree that there's no experiment to measure p and x simultaneously - if there were, QM would be in trouble - but that's completely compatible with the version of HUP in terms of simultaneity. Indeed, if there were such an experiment, then that version of HUP would be untenable. Certainly, this version of the HUP seems compatible with the experiments you cite earlier showing that we can find the position and then the later momentum of an object with arbitrary accuracy. Since the object has these properties at different times, there is no conflict.

I do take your point that the signficance of the commutation relations needs explaining on the earlier view and it's not obvious what the right thing to say here is. Could it be that there is a difference between the time at which an experiment takes place and the information the experiment tells us about the time at which an object possessed a particular property? For instance, in the case of two compatible physical observables, if one quantify is measured a subsequent measurement of the other quantity will have a completely predictable result and will leave the wave function unchanged (or so my textbook tells me). Since the quantities are compatible, the later measurement merely reveals the property the system had all along. Indeed, the later measurement was actually unecessary since the quantity could have been predicted from the original measurement. In this case, Quantum mechanics allows us to know the simultaneous possession of the two observables. In cases of non-commuting operators, however, since the wavefunction changes on the second measurement we no longer have reason to think suppose that the measurement is non-disturbing and thus that we are merely revealing a property that the system had all along.

If this makes sense, then it may well be that different interpretations of HUP sit with different interpretations of QM itself. If one has something like a collapse interpretation, and one thinks that the only properties that an object determinately has are the ones given by the eigenfunctions of that state, then it may be that the interpretation of HUP involving simultaneity makes sense. If one has something like an ensemble interpretation of QM, which some comments of yours suggested you held, then it may be that the best way to make sense of HUP is in terms of repeated experiments on similarly prepared systems.

But I say this with no great confidence, and your challenge on how to interpret non-commuting observables is a good one.

Best

 

[ZapperZ]

Dear Zz

"Your Interpretation contradicts QM"
Well, I didn't mean to endorse any particular interpretation in my post (I was trying to be very non-confrontational. Apologies if I seemed rude). I was merely quoting Feynman about HUP, and his words do seem to support this interpretation. But it may be that Feynman has mis-spoken here or that there are different versions of the principle in the literature.

No, I mean your interpretation of QM and what Feynman wrote contradicts QM.

But it's not just Feynmann: A.I.M. Rae, Quantum Mechanics (Undergraduate Text book): "This relation is known as the Heisenberg Uncertainty Principle. According to quantum mechanics it is a fundamental property of nature that any attempt to make simultaneous measurements of position and momentum are subject to this limitation". p.12.
Bohm: Quantum Theory, section 3 "On the Interpretation of the Uncertainty Principle", says "the momentum and position cannot even exist with simultaneously and perfectly defined values".

But you need to figure out here what is meant by "simultaneous" as implied by classical mechanics. Remember that this is a manifestation of the non-commuting principle of QM. This is important! In fact, this commutation relation has often been called the First Quantization. It is what most undergraduate studies when they deal with something that looks like [A,B].

In classical mechanics, there is nothing to prevent a "simultaneous" knowledge of any set of variables with arbitrary accuracy. In QM, this is only true when you have two observables that obey the relationship in such a way that [A,B]=0. If I know of A, I automatically know of B, to equal accuracy, without having to perform a second measurement on B. But this isn't true for when [A,B] != 0. Here, a measurement of A tells you nothing about your ability to predict what B is. In fact, the more accurate you know about A, the less is your ability to predict B with the same accuracy. THIS is what Feynman and most QM text means as a "simultaneous" knowledge. It doesn't mean that you measure both observables simultaneously, even if you can. In the time-independent formulation, for example, QM makes no provision to how long after one measurement is made that the 2nd should be performed. There's no time element in the ordering of observables A and B here.

Zz.

 

[wavemaster]

...what Feynman wrote contradicts QM.

Wow, that's whom I'd call a braveheart.
Fine, but which is the last word?

In fact, the more accurate you know about A, the less is your ability to predict B with the same accuracy.

If I have a very fine CCD, I can measure where the particle hit the detector to very high accuracy! This accuracy has nothing to do with how fine I measure \Delta x!

BTW, there should be limitations on your CCD (and our technology) by uncertanity principle, so I suppose it's impossible to make a device that measures momentum of a particle with perfect accuracy. Err.. something like that:

...Then people sat down and tried to figure out ways of doing it, and nobody could figure out a way to measure the position and momentum of anything --a screen, an electron, a billard ball, anything-- with any greater accuracy. Quantum mechanics maintains it's perilous but still correct existence.

which is unlikely the previous discussion:

To be fair to superweirdo, what he should have said is “we cannot simultaneously know to arbitrary precision both the position and the momentum”

Yes we can!

The precision of a SINGLE MEASUREMENT of position and momentum is limited only via our technology.

Don't get me wrong (yikes, this topic has recently turned into an emotional one recently), I'm following the thread for a while, and I have great doubts on the topic, "the uncertanies of uncertainty principle", now, so I'll watch closely how this discussion will be settled.

 

[ZapperZ]

Wow, that's whom I'd call a braveheart.
Fine, but which is the last word?

This is what I wrote:

No, I mean your interpretation of QM and what Feynman wrote contradicts QM.

Maybe it wasn't structured properly, but in no way was I implying that what Feynman wrote contradicts QM. What I meant was the her interpretation that the quote from Feynman appears to contradict what I said to be QM isn't correct! There's nothing contradictory here. The definition of what "simultaneous" means isn't clarified, and certainly does not imply an immediate measurement of both observables. This strict condition isn't require for the HUP.

BTW, there should be limitations on your CCD (and our technology) by uncertanity principle, so I suppose it's impossible to make a device that measures momentum of a particle with perfect accuracy. Err.. something like that:

There maybe is, but before I get to that limit, I can improve the accuracy of my CCD (and my momentum measurement) INDEPENDENT of the size of the slit. This clearly does not follow the HUP description, and thus my argument that the accuracy of one measurement of position and one measurement of momentum of that particle has nothing to do with the HUP.

Zz.

 

[wavemaster]

Maybe it wasn't structured properly, but in no way was I implying that what Feynman wrote contradicts QM. What I meant was the her interpretation that the quote from Feynman appears to contradict what I said to be QM isn't correct!

Ouch! Sorry! I'm not a native English speaker, that's probaby why an quite-ambioguous sentence turned into disaster in my hands!

There maybe is, but before I get to that limit, I can improve the accuracy of my CCD (and my momentum measurement) INDEPENDENT of the size of the slit.

Hmmm. Right. But then, the width of slit isn't measure of it's position at the instant of momentum measurement. Let's see what I've understood from your single-slit experiment: I can measure the position of a particle some time, and after a year I can measure it's momentum at great precision. The problem is, this's not trying to know the particles momentum and position at a given time.

Well, I suppose two measurements should take place in an infinitesimal time intervals to get the real works. Of course the order would matter, but won't either way agree with HUP?

Anyway, i wonder what your answer is for my question "but which is the last word?" in my previous post. Mathematically, standard deviation means nothing to me for one, single particle. But intutively, a measurement should disturb particle (after, say, a peak in momentum space that follows from wavefunction collapse, what would position measurement yield?). Also, having seen the great masters' words about measurements on one, single particle (such as "nobody could figure out a way to measure the position and momentum of anything --a screen, an electron, a billard ball, anything-- with any greater accuracy"), it feels more likely that there's something to it. Something I fail to see. And I'm desperately searching for it.

 

[ZapperZ]

Ouch! Sorry! I'm not a native English speaker, that's probaby why an quite-ambioguous sentence turned into disaster in my hands!

No, it was also probably due to the poor way that I constructed the sentence. Often my fingers lag behind what my brain is telling them to type.

Hmmm. Right. But then, the width of slit isn't measure of it's position at the instant of momentum measurement. Let's see what I've understood from your single-slit experiment: I can measure the position of a particle some time, and after a year I can measure it's momentum at great precision. The problem is, this's not trying to know the particles momentum and position at a given time.

Well, I suppose two measurements should take place in an infinitesimal time intervals to get the real works. Of course the order would matter, but won't either way agree with HUP?

Anyway, i wonder what your answer is for my question "but which is the last word?" in my previous post. Mathematically, standard deviation means nothing to me for one, single particle. But intutively, a measurement should disturb particle (after, say, a peak in momentum space that follows from wavefunction collapse, what would position measurement yield?). Also, having seen the great masters' words about measurements on one, single particle (such as "nobody could figure out a way to measure the position and momentum of anything --a screen, an electron, a billard ball, anything-- with any greater accuracy"), it feels more likely that there's something to it. Something I fail to see. And I'm desperately searching for it.

I seem to be tackling the same issue on two different theads here. Let's see if your question an be answered by what I've written here:

https://www.physicsforums.com/showpost.php?p=1046959&postcount=64

Zz.