A paradox about Heisneberg Uncertainty ?

[Klaus_Hoffmann]

a paradox about Heisneberg Uncertainty ??

Let be a particle, we measure the position at time 't' x(t)

We measure the position after an infinitesimal chnage t-->t+dt so x(t+dt) is obtained.

then as an approximation we can make (definition of derivative and velocity)

x(t+dt)-x(t)=dtp(t) so we can obtain the momentum at time 't' p(t) avoiding uncertainty

 

[HallsofIvy]

Where is the paradox? You now know the average velocity between t and t+ dt. You do NOT know the velocity before t because in measuring the postion you have changed the velocity when you measured the postion. You do NOT know the velocity after t+ dt because you changed the velocity when you measured the position. The average velocity you have contains no useful information.

 

[ZapperZ]

Let be a particle, we measure the position at time 't' x(t)

We measure the position after an infinitesimal chnage t-->t+dt so x(t+dt) is obtained.

then as an approximation we can make (definition of derivative and velocity)

x(t+dt)-x(t)=dtp(t) so we can obtain the momentum at time 't' p(t) avoiding uncertainty

The HUP says nothing about the measurement of a single observable. you can measure the position and momentum of one particle as accurately as you are capable of.

The HUP comes in when you make the NEXT AND SUBSEQUENT x(t) and x(t+dt) measurements. The p(t) that you measure will depend very much on how well you measured x(t). The more certain you are of x(t), the larger a range of values you will measure for x(t+dt), which will result in a larger range of p(t) values. This will occur even if you have an identical initial conditions and you simply repeat the identical experiment. This is the HUP.

Zz.

 

[StatusX]

Incidentally, in relativistic quantum mechanics, if you carry out the procedure you described, you're guaranteed to measure the particle traveling at an average velocity of c. This is becausen the momentum is uniformly distributed after the first measurement of position, and so "most" of the possible states lie at very high momenta, whose cooresponding velocities all approach the speed of light.

 

[nrqed]

Let be a particle, we measure the position at time 't' x(t)

We measure the position after an infinitesimal chnage t-->t+dt so x(t+dt) is obtained.

then as an approximation we can make (definition of derivative and velocity)

x(t+dt)-x(t)=dtp(t) so we can obtain the momentum at time 't' p(t) avoiding uncertainty

Yes, but the key point is that the HUP syas something about the spread of values in the observables. It does not say anything about a single measurement.

The whole point is that if you start with a second particle with exactly the same quantum state (same wavefunction) and you repeat exactly the procedure you just described, you will get a completely different result, in general. That's the key point. In classical physics, of course, one would get reproducible results (the same conditions will yield the same results, within th euncertainty of the measurements performed, which may in principle be reduced without any limit). In QM, repeating the same experiment to the same quantum system will yield different results every time, in general. The spread of the values in position and momenta will satisfy the HUP.

 

[Klaus_Hoffmann]

then you mean that there's also some uncertainty involving

[x(t),x(t+dt)]=ih where [] is the commutator

 

[nrqed]

then you mean that there's also some uncertainty involving

[x(t),x(t+dt)]=ih where [] is the commutator

Well, I don't think it is possible to express it this way because I don't see what would be the meaning of putting them in the order x(t) x(t+dt). What would that mean?

What I *am* saying is that the value of x(t+dt) is not uniquely defined even for a given x(t) and a specific initial wavefunction.

But to get to the uusal form of the HUP with x and p_x andto relate this to a comutator [x,p], the value of "p" must be obtained by different means, by using momentum conservation in a collision, say. That's at least my understanding, maybe others will have some clarifying comments to make.

regards

 

[ueit]

Let be a particle, we measure the position at time 't' x(t)

We measure the position after an infinitesimal chnage t-->t+dt so x(t+dt) is obtained.

then as an approximation we can make (definition of derivative and velocity)

x(t+dt)-x(t)=dtp(t) so we can obtain the momentum at time 't' p(t) avoiding uncertainty

There is no dificulty in finding both position and momentum of a particle with any accuracy you like. The problem is that you cannot use the result to make predictions, because the measurement alters the system. As Heisenberg said, HUP does not apply to the past.

 

[meopemuk]

then you mean that there's also some uncertainty involving

[x(t),x(t+dt)]=ih where [] is the commutator

I think that this conclusion is basically correct, although the actual form of the commutator is different. The operator x(t) is connected to x(t=0) = x by the time evolution equation

x(t) = e^{\frac{i}{\hbar}Ht} x e^{-\frac{i}{\hbar}Ht}

where H is the Hamiltonian whose commutator with x is [x,H] = i \hbar v, where v = p/m is the operator of velocity. (I assumed that there are no interactions.) Using this commutator one can obtain, as expected

x(t) = x + vt

so that

[x(0), x(t)] = [x, v]t = \frac{i t \hbar}{m}

Eugene.

 

[lightarrow]

There is no dificulty in finding both position and momentum of a particle with any accuracy you like. The problem is that you cannot use the result to make predictions, because the measurement alters the system. As Heisenberg said, HUP does not apply to the past.

Can you please explain this better? HUP is often expressed saying: "it's not possible to measure x and p simultaneously with arbitrary accuracy".

 

[MarkeD]

You can measure only one varaible at a time to any degree of accuracy. IF you get an accurate measurement of momentum, you will have a large uncertainty of position.

A nice metaphor is that the HUP is like taking a photograph of a moving car; you can have a fast shutter speed and accurately measure the position f the car but know little on how fast it was going, if you use a slower shutter speed you can measure the speed via the blur on the photo but have little knowledge on its exact position.

 

[ZapperZ]

You can measure only one varaible at a time to any degree of accuracy. IF you get an accurate measurement of momentum, you will have a large uncertainty of position.

A nice metaphor is that the HUP is like taking a photograph of a moving car; you can have a fast shutter speed and accurately measure the position f the car but know little on how fast it was going, if you use a slower shutter speed you can measure the speed via the blur on the photo but have little knowledge on its exact position.

Er.. this is not quite right. The problem in your example is that you are using inappropriate method to measure different things. The HUP never say anything about using the SAME technique to measure something.

First of all, there is a sequence of measurement, i.e. you measure position first, and then momentum, or you measure momentum first and then position. Both do not give you the same result because of non-commutation. But more importantly, the HUP, if you look carefully at the expression, requires the knowledge of the AVERAGE values of each of the observable, ie. an ensemble of measurements, not just one. This means that for a single measurement of the position, followed by the single measurement of the momentum, there's nothing that restricts the accuracy of each of those measurements. The accuracy of each of those measurements depends on the accuracy of the instruments, and they are independent of each other. I could use a slit of size \Delta (x) to measure the position and then look at where the particle hit the detector/screen behind the slit to measure the transverse momentum. The size of the slit and the size of the spot on the detector are independent of each other and are not governed by the HUP. Both of these sizes determine the uncertainty of that single measurement of position and single measurement of the momentum. These are not the HUP.

Zz.

 

[country boy]

Let be a particle, we measure the position at time 't' x(t)

We measure the position after an infinitesimal chnage t-->t+dt so x(t+dt) is obtained.

then as an approximation we can make (definition of derivative and velocity)

x(t+dt)-x(t)=dtp(t) so we can obtain the momentum at time 't' p(t) avoiding uncertainty

This "paradox" can be posed just as well without the infinitesimal part. Make the first position measurement and then later make the second position measurement. You can make each of these measurements as accurately as you want to and, with the times, you also have the velocity (and momentum) to arbitrary accuracy. The catch is that each position measurement introduces its own momentum uncertainty (the HUP position uncertainty is not the distance between the measurements). The momentum uncertainty introduced by the second measurement is independent of the first measurement.

It is easy to see this with waves and diffraction. If a wave encounters a slit of dimension similar to its wavelength, it will spread out (diffract). If another slit is placed downstream, the small flat portion of the wavefront that encounters the second slit will spread out again. The diffraction at each slit does not depend on what happened previously. [Note that if the slits are large compared with the wavelength, the wave will be collimated, not spread out. That corresponds to the classical limit.]

 

[country boy]

Can you please explain this better? HUP is often expressed saying: "it's not possible to measure x and p simultaneously with arbitrary accuracy".

This can be also be explained in terms of diffraction at a slit (please see my previous post). The width of the slit is the uncertainty in position lateral to the direction of travel. The angular spread (diffraction width) of the emerging wave is given by the ratio of the wavelength to the slit width. This angle corresponds to the lateral uncertainty of the momentum. So the uncertainties of both the position and momentum are determined "simultaneously" at the slit.

 

[lightarrow]

This can be also be explained in terms of diffraction at a slit (please see my previous post). The width of the slit is the uncertainty in position lateral to the direction of travel. The angular spread (diffraction width) of the emerging wave is given by the ratio of the wavelength to the slit width. This angle corresponds to the lateral uncertainty of the momentum. So the uncertainties of both the position and momentum are determined "simultaneously" at the slit.

Yes, but ( probably my english wasn't very good), I intended to ask ueit to explain his phrase:

"There is no dificulty in finding both position and momentum of a particle with any accuracy you like. "

since it would seem in contradiction with the fact we can't simultaneously measure position and momentum with arbitrary accuracy.

 

[ZapperZ]

Yes, but ( probably my english wasn't very good), I intended to ask ueit to explain his phrase:

"There is no dificulty in finding both position and momentum of a particle with any accuracy you like. "

since it would seem in contradiction with the fact we can't simultaneously measure position and momentum with arbitrary accuracy.

This is NOT a fact. It is a misunderstanding of the HUP. We can "simultaneously" measure position and momentum with arbitrary accuracy (up to our present-day technology), if by "simultaneous", you mean a single measurement of position, and then followed by a single measurement of the momentum of that particle. The HUP never stated anything about an instantaneous and simultaneous measurement of both position and momentum. If it does, then the order of the commutation relation won't make a difference.

Zz.

 

[jostpuur]

This is NOT a fact. It is a misunderstanding of the HUP. We can "simultaneously" measure position and momentum with arbitrary accuracy (up to our present-day technology), if by "simultaneous", you mean a single measurement of position, and then followed by a single measurement of the momentum of that particle. The HUP never stated anything about an instantaneous and simultaneous measurement of both position and momentum. If it does, then the order of the commutation relation won't make a difference.

Zz.

The way you use term "simultaneous" seems confusing. If you first measure the position, and after it the momentum, then the measurements were not simultaneous.

"There is no dificulty in finding both position and momentum of a particle with any accuracy you like. "

since it would seem in contradiction with the fact we can't simultaneously measure position and momentum with arbitrary accuracy.

You can find the position with arbitrary accuracy, and after it you can find the momentum with arbitrary accuracy, but you cannot know them both with arbitrary accuracy simultaneously.

 

[ueit]

Can you please explain this better? HUP is often expressed saying: "it's not possible to measure x and p simultaneously with arbitrary accuracy".

Sure. Just place a detector at a very large distance from a particle source and switch on the source for a small time. The accuracy in position is only limited by the detector resolution while the accuracy in momentum can be made arbitrary small by increasing the distance between the source and detector and/or decreasing the time when the source is on. However, the determined values cannot be used to make predictions about how the particle will evolve after detection, but you know them how they were in the past.

 

[ZapperZ]

The way you use term "simultaneous" seems confusing. If you first measure the position, and after it the momentum, then the measurements were not simultaneous.

That is why I clarified what I meant. Note that an instantaneous, simultaneous measurement is meaningless and irrelevant as far as the HUP is concerned. So this issue doesn't even matter. That is why I stated clearly what I meant as "simultaneous". In addition, I'm not the one who brought up the "simultaneous" issue.

Zz.

 

[country boy]

That is why I clarified what I meant. Note that an instantaneous, simultaneous measurement is meaningless and irrelevant as far as the HUP is concerned. So this issue doesn't even matter. That is why I stated clearly what I meant as "simultaneous". In addition, I'm not the one who brought up the "simultaneous" issue.

Zz.

Sorry, I'm not sure what you mean either. The textbooks I've looked at refer to either "simultaneously" or "at the same instant." The wording may be a little imprecise, but the uncertainty principle does make a statement about the degree to which two quantities can be measured accurately at the same time.

 

[Anonym]

The textbooks I've looked at refer to either "simultaneously" or "at the same instant." The wording may be a little imprecise, but the uncertainty principle does make a statement about the degree to which two quantities can be measured accurately at the same time.

Let forget about paradoxes for a while and let us try to understand what W.Heisenberg explain (for example, W. Heisenberg “The Physical Principles of the Quantum Theory”, Dover Publications, 1930 (the lectures given at the University of Chicago); it is first hand presentation of CI with N.Bohr removed).

Let return to normal discussion in physics: please, tell me what you want to know (measure), what the set-up you intend to use, what your physical system under test is, what the state of your system is before the measurement and what the state of your system is after the measurement. After receiving your answers we will continue about collapse in space and in time: notion of simultaneous (if you will use the usual Hilbert space math. formalism, think about that x*p is not self adjoint operator and, therefore, not observable).

Regards, Dany.

 

[ZapperZ]

Sorry, I'm not sure what you mean either. The textbooks I've looked at refer to either "simultaneously" or "at the same instant." The wording may be a little imprecise, but the uncertainty principle does make a statement about the degree to which two quantities can be measured accurately at the same time.

But there is a difference between the order of the operation, is there not?

I mean, in principle, there need not be any "time lag" in measuring x and then p, but which one comes first does make a difference. There isn't a "composite operator" that measure x and p together, instantaneously. What does not matter is how long is the time interval between x and p, because, again, in principle, once x has been measured, then the minimum spread in p is "set in stone". So one could measure p "almost instantaneously" after x, and would not be able to escape the HUP. But the fact that there has to be a "one after the other" sequence means that it does matter which one comes first.

Zz.

 

[country boy]

But there is a difference between the order of the operation, is there not?

I mean, in principle, there need not be any "time lag" in measuring x and then p, but which one comes first does make a difference. There isn't a "composite operator" that measure x and p together, instantaneously. What does not matter is how long is the time interval between x and p, because, again, in principle, once x has been measured, then the minimum spread in p is "set in stone". So one could measure p "almost instantaneously" after x, and would not be able to escape the HUP. But the fact that there has to be a "one after the other" sequence means that it does matter which one comes first.

You are right, the order does matter. I think the key is that an "order" of operation does not imply an "order" in time. The operators xp and px are just producing numbers from the state function. But they have no time sequence.

 

[lightarrow]

This is NOT a fact. It is a misunderstanding of the HUP. We can "simultaneously" measure position and momentum with arbitrary accuracy (up to our present-day technology), if by "simultaneous", you mean a single measurement of position, and then followed by a single measurement of the momentum of that particle. The HUP never stated anything about an instantaneous and simultaneous measurement of both position and momentum. If it does, then the order of the commutation relation won't make a difference.

Zz.

When we talk about accuracy of measurements in this context, we essentially talk about standard deviations of the measurements, or they estimators s:

s = \sqrt{\frac{1}{(n-1)}\cdot\sum_{i=1}^n (X_i - X_m)^2}

where X_i is the i-esim measurement of the variable X and X_m is its aritmetical average.

For 1 measurement, n=1 so

s = \infty.

So it's not possible to talk about "arbitrary accuracy" in one single measure.

 

[ZapperZ]

When we talk about accuracy of measurements in this context, we essentially talk about standard deviations of the measurements, or they estimators s:

s = \sqrt{\frac{1}{(n-1)}\cdot\sum_{i=1}^n (X_i - X_m)^2}

where X_i is the i-esim measurement of the variable X and X_m is its aritmetical average.

For 1 measurement, n=1 so

s = \infty.

So it's not possible to talk about "arbitrary accuracy" in one single measure.

No, you're making a typical mistake that most students who are studying things like Fourier series make. When you do n=0, you have to rederive the whole expression. This is because your numerator is also zero.

That expression doesn't work for only one measurement. There are no statistics of any kind. The accuracy of a single measurement depends on the instrument you use to measure it, not on the "spread" of the statistics of many measurements, because you haven't done many measurements.

Zz.

 

[country boy]

...For 1 measurement, n=1 so

s = \infty.

So it's not possible to talk about "arbitrary accuracy" in one single measure.

Also note that as n goes to infinity s approaches zero. That means the spread in the measurements becomes arbitrarily small. But the uncertainty principle imposes a limit on the spread of a large number of measurements.

 

[lightarrow]

Also note that as n goes to infinity s approaches zero. That means the spread in the measurements becomes arbitrarily small. But the uncertainty principle imposes a limit on the spread of a large number of measurements.

s approaches zero? It approaches sigma (standard deviation), not zero.

 

[country boy]

s approaches zero? It approaches sigma (standard deviation), not zero.

Ah yes. Thanks. But in your context the standard deviation has to due with the accuracy of the measurements. That is not what HUP deals with.

[Added later] I've thought about this a little more, so let me edit it. The uncertainty principle puts limits on how well we can know the values of two conjugate quantities at the same time. These limits are independent of the accuracy of our measuring apparatus. The measurement accuracy can be much lower than the HUP uncertainty limit or it can be much higher. In the latter case, for example, the position of arrival of a single photon at a measurement screen can be known very accurately, but the position of the next photon is not accurately known to better than the HUP uncertainty.

I'm not sure that this is different from what you are referring to, and it may sound a bit trivial, but it is important to keep in mind the distinction between measurement accuracy and the fundamental uncertainty imposed by the wave nature of matter.

 

[lightarrow]

Ah yes. Thanks. But in your context the standard deviation has to due with the accuracy of the measurements. That is not what HUP deals with.

[Added later] I've thought about this a little more, so let me edit it. The uncertainty principle puts limits on how well we can know the values of two conjugate quantities at the same time. These limits are independent of the accuracy of our measuring apparatus. The measurement accuracy can be much lower than the HUP uncertainty limit or it can be much higher. In the latter case, for example, the position of arrival of a single photon at a measurement screen can be known very accurately, but the position of the next photon is not accurately known to better than the HUP uncertainty.

Yes, that's clear to me, but I was wondering how it's possible to define the instrument's measurement accuracy as the uncertainty in a single, unique measurements: if we make many measurements we have a non-vanishing uncertainty; but, the less measurements we perform, the less the accuracy, so the accuracy is at its least value for a single measurement.

 

[ZapperZ]

Yes, that's clear to me, but I was wondering how it's possible to define the instrument's measurement accuracy as the uncertainty in a single, unique measurements: if we make many measurements we have a non-vanishing uncertainty; but, the less measurements we perform, the less the accuracy, so the accuracy is at its least value for a single measurement.

Have you ever done an experiment? Try one. Pick a ruler, and measure a length of something. Now, what is the uncertainty in that length? That is the uncertainty in that single measurement and it depends entirely on the accuracy of the instrument. This is not the HUP. I can make it more or less accurate by changing instrument, regardless of what was measured before, or what will be measured after.

I think you still have a strong misconception of the HUP. Maybe you might want to read what I wrote a while back on this.

Zz.

 

[lightarrow]

Have you ever done an experiment? Try one. Pick a ruler, and measure a length of something. Now, what is the uncertainty in that length? That is the uncertainty in that single measurement and it depends entirely on the accuracy of the instrument. This is not the HUP. I can make it more or less accurate by changing instrument, regardless of what was measured before, or what will be measured after.

I think you still have a strong misconception of the HUP. Maybe you might want to read what I wrote a while back on this.

Zz.

So, if I make only one measurement I can have more accuracy than with more measurements? This seems nonsense to me.

 

[Cthugha]

So, if I make only one measurement I can have more accuracy than with more measurements? This seems nonsense to me.

You are showing a general misconception of the HUP.
If you do one measurement, you get a result, which shows an accuracy corresponding to the accuracy of your measuring equipment.
If you are repeating these measurements under exactly the same circumstances (same initial state) you will find, that

a) for each of the measurements you get a result, which corresponds to the accuracy of your measuring equipment.
b) there is still a spread in your results, which is not due to the accuracy of your measuring equipment, although the initial state was the same at every measurement.

Case b) is what the HUP is about. The results are not reproducible exactly. Although the result of each measurement was exact, there is an intrinsic spread due to the quantum nature of what you are trying to measure.

 

[country boy]

When we talk about accuracy of measurements in this context, we essentially talk about standard deviations of the measurements, or they estimators s:

s = \sqrt{\frac{1}{(n-1)}\cdot\sum_{i=1}^n (X_i - X_m)^2}

where X_i is the i-esim measurement of the variable X and X_m is its aritmetical average.

For 1 measurement, n=1 so

s = \infty.

So it's not possible to talk about "arbitrary accuracy" in one single measure.

I have another question about how you seem to be applying the standard deviation. The calculation of the variance or standard deviation requires all of the measurements to have been completed so that a mean can be determined. Alternatively, I guess you could have some prior knowledge of the mean from a previous experiment or from a theoretical prediction. Either way, without some prediction of the outcome you cannot use the standard deviation on a single measurement, can you?

By the way, since the mean of a single measurement is the measurement itself, the numerator and denominator are both zero, so the standard deviation is indeterminate (see ZapperZ's comments). That means formula still works, though, because in fact you can't determine s from one measurement.

It's interesting that you could start with the prediction of a mean derived from the wavefunction and then calculate something like the standard deviation (with n instead of n-1) as you accumulate measurements. The standard deviation should then be greater than or equal to the spread predicted from the wavefunction. This probably only has value as a mental exercise.

 

[ZapperZ]

So, if I make only one measurement I can have more accuracy than with more measurements? This seems nonsense to me.

It's nonsense because you didn't understand what I just said.

The accuracy of a single measurement depends on the accuracy of your instrument. Now do you dispute this? Let's get that out of the way first.

Now, if you make repeated measurement of the SAME thing (i.e. of the width of the slit, for example), then you will have an average value, and a standard deviation of the width of the slit based on the repeated measurement. However, each of the measurement in your data set is accompanied by its own uncertainty based on the uncertainty of a single measurement. This is the uncertainty that I'm talking about, and it has nothing to do with the HUP. That is the uncertainty of the instrumentation. If I was using a ruler to measure the width of the slit, I get a certain amount of uncertainty. If I instead switch to a micrometer caliper, then I have a smaller uncertainty in that measurement because I just switched to a more accurate derive that measures a length. So now I've just reduced the uncertainty of my length measurement with no regards to what is going on before and after that measurement. What is nonsense is to confuse that with the HUP.

Zz.

 

[lightarrow]

It's nonsense because you didn't understand what I just said.

The accuracy of a single measurement depends on the accuracy of your instrument. Now do you dispute this? Let's get that out of the way first.

Now, if you make repeated measurement of the SAME thing (i.e. of the width of the slit, for example), then you will have an average value, and a standard deviation of the width of the slit based on the repeated measurement. However, each of the measurement in your data set is accompanied by its own uncertainty based on the uncertainty of a single measurement. This is the uncertainty that I'm talking about, and it has nothing to do with the HUP. That is the uncertainty of the instrumentation. If I was using a ruler to measure the width of the slit, I get a certain amount of uncertainty. If I instead switch to a micrometer caliper, then I have a smaller uncertainty in that measurement because I just switched to a more accurate derive that measures a length. So now I've just reduced the uncertainty of my length measurement with no regards to what is going on before and after that measurement. What is nonsense is to confuse that with the HUP.

Zz.

ZapperZ, it was very clear what you intended with accuracy of a single measurement (= accuracy of the instrument) and the fact HUP concerns repeated measurement of the same thing.

What I wanted to discuss are 2 different things:

1. Independently on HUP, but talking about measurements in general, I don't understand how is possible to define the accuracy of a single measurement as the accuracy of the instrument - let's say 0.01 mm of something - if, repeating many times that measure with the same instrument, we have a standard deviation of 10 mm, for example. It was just a statistical consideration. Certainly, with only one measurement we have nothing else than the instrument's accuracy, but, IMO, we should never estimate the experiment accuracy from one only measurement, we should make at least two of them if we want to give a reasonable value of it (reasonable in the sense I wrote).

2. It's not clear to me if it's possible or not (as you say) to measure x and p of, let's say, a photon, simultaneously. This is because, infact, I'm now more confused on what we mean with "measure of position" and "measure of momentum".

Let's set, e.g., the following situation:

A plane, monocromatic electromagnetic wave traveling along the +z direction hits a screen with a slit, which width is d along the x direction (and D>>d in the y direction) and then hits a final screen parallel to the first and at distance L from it (so, a very standard arrangement).

According to what is written in some books (see, for example, A.Messiah - ch. 4, par.13), the light passing through the slit correspond to a position measurement of photons, with \Delta x = d and to a momentum measurement with \Delta p_x = p\cdot\sin\alpha where \alpha is the angle of diffraction. Now I ask, since this is not stated: does it mean that the position measurement is performed before the momentum measurement?

Thank you for your patience.

 

[lightarrow]

I have another question about how you seem to be applying the standard deviation. The calculation of the variance or standard deviation requires all of the measurements to have been completed so that a mean can be determined. Alternatively, I guess you could have some prior knowledge of the mean from a previous experiment or from a theoretical prediction. Either way, without some prediction of the outcome you cannot use the standard deviation on a single measurement, can you?

By the way, since the mean of a single measurement is the measurement itself, the numerator and denominator are both zero, so the standard deviation is indeterminate (see ZapperZ's comments). That means formula still works, though, because in fact you can't determine s from one measurement.

Yes, I agree.

 

[ZapperZ]

ZapperZ, it was very clear what you intended with accuracy of a single measurement (= accuracy of the instrument) and the fact HUP concerns repeated measurement of the same thing.

What I wanted to discuss are 2 different things:

1. Independently on HUP, but talking about measurements in general, I don't understand how is possible to define the accuracy of a single measurement as the accuracy of the instrument - let's say 0.01 mm of something - if, repeating many times that measure with the same instrument, we have a standard deviation of 10 mm, for example. It was just a statistical consideration. Certainly, with only one measurement we have nothing else than the instrument's accuracy, but, IMO, we should never estimate the experiment accuracy from one only measurement, we should make at least two of them if we want to give a reasonable value of it (reasonable in the sense I wrote).

I don't quite understand this. Take a ruler that has, as the smallest scale, 1 mm tick marks. You can accurately measure by eye, a length of as close to -0.5 mm in accuracy, but with an uncertainty of +/- 0.5. So you can read a length of 10.0 mm with +/- 0.5. That is the accuracy of that length measurement. You can make 100,000 measurement, but that is as accurately as you can make each of the single measurement.

Now, there could be a spread in that measurement, since in this case, it is done by eye. Someone else can come in and instead, would say it is 10.5 mm. Another person says it is 10.0 mm, another says it is 11.0 mm. etc... etc. NOW you have a statistical spread in the measurement. The spread would correspond to the standard deviation of all the measurement. If this spread follows a normal distribution, you'd expect that the average value would be closest to the 'actual' value and the standard deviation to get smaller and smaller as the sampling data gets larger. But the uncertainty in each of the measurement remains the same!

In high energy physics experiments, a form of the standard deviation is often cited as the "confidence level" whenever the statistics of a spectrum is given. However, in other experiments such as in condensed matter (i.e. photoemission, tunneling, etc.), we often cite the RESOLUTION as the uncertainty in the measurement. This means that this is the uncertainty that is present in one single measurement, and it is the best resolution of energy, momentum, etc. that the instrument has. You can take a gazillion measurement (and often we do when we're dealing with a stream of electrons hitting a CCD), but you cannot get any better than the instrument's resolution.

This is not the HUP.

Zz.

 

[country boy]

...it was very clear what you intended with accuracy of a single measurement (= accuracy of the instrument) and the fact HUP concerns repeated measurement of the same thing.

There seem to be two kinds of measurement going on here. First, the measurement on the screen of the location of a single event. This position can be measured many times and the statistics will produce a progressively more accurate value for the position. In this case, the uncertainty approaches zero; as the number of measurements increases, the knowledge of the location improves without limit. The standard deviation among these measurements gives the measurement accuracy, presumably set by the measuring device or technique. So there is a "probability distribution" associated with the measurement.

Second, a large number of arrival events are measured. In this case we are determining, for a given slit width, the distribution of arrival positions on the screen. Again, the statistics of a large number of measurements will yield a progressively more accurate determination of the average arrival location (the center of the distribution on the screen). We can also calculate the standard deviation of the set of measurements. But now we notice that a smaller slit has the effect of making the standard deviation larger, exceeding our measurement accuracy for a single event. The standard deviation now gives the QM uncertainty, determined by the QM probability distribution. The standard deviation will be inversely proportional to the slit width.

The same statistical tools are used in both regimes, but they are measuring different things.

 

[lightarrow]

I don't quite understand this. Take a ruler that has, as the smallest scale, 1 mm tick marks. You can accurately measure by eye, a length of as close to -0.5 mm in accuracy, but with an uncertainty of +/- 0.5. So you can read a length of 10.0 mm with +/- 0.5. That is the accuracy of that length measurement. You can make 100,000 measurement, but that is as accurately as you can make each of the single measurement.

Now, there could be a spread in that measurement, since in this case, it is done by eye. Someone else can come in and instead, would say it is 10.5 mm. Another person says it is 10.0 mm, another says it is 11.0 mm. etc... etc. NOW you have a statistical spread in the measurement. The spread would correspond to the standard deviation of all the measurement. If this spread follows a normal distribution, you'd expect that the average value would be closest to the 'actual' value and the standard deviation to get smaller and smaller as the sampling data gets larger. But the uncertainty in each of the measurement remains the same!

A machinery makes steel balls; they should all be of the same diameter, but no machinery is perfect: the first ball is 10mm +/- 0.01 mm, the second is 10.5 mm +/- 0.01 mm, then 9.2 mm +/- 0.01 mm,...ecc.

The instrument accuracy (the instrument used to measure the ball's diameter) is 0.01 mm, the standard deviation of the balls dimension is, let's say, 1mm (computed on the entire population of balls made by the machinery).

If I perform a single measure on a single ball, I should say the accuracy is 0.01mm; if I perform more measures on more balls I discover that the accuracy is much less (= 1 mm).

So, should I perform 1 only measurement (on a single ball), to be sure to have the maximum accuracy? Certainly not.

 

[ueit]

2. It's not clear to me if it's possible or not (as you say) to measure x and p of, let's say, a photon, simultaneously. This is because, infact, I'm now more confused on what we mean with "measure of position" and "measure of momentum".

In the example I've given you, the detection is a measurement of both position and momentum. The position is the spot on the screen, the momentum is calculated from the two position measurements (the position of the source and that of the spot).

 

[ZapperZ]

A machinery makes steel balls; they should all be of the same diameter, but no machinery is perfect: the first ball is 10mm +/- 0.01 mm, the second is 10.5 mm +/- 0.01 mm, then 9.2 mm +/- 0.01 mm,...ecc.

The instrument accuracy (the instrument used to measure the ball's diameter) is 0.01 mm, the standard deviation of the balls dimension is, let's say, 1mm (computed on the entire population of balls made by the machinery).

If I perform a single measure on a single ball, I should say the accuracy is 0.01mm; if I perform more measures on more balls I discover that the accuracy is much less (= 1 mm).

So, should I perform 1 only measurement (on a single ball), to be sure to have the maximum accuracy? Certainly not.

And who said you should? In the example I gave you, BOTH types were used under difference circumstances. Did you miss that? No one said you should use one over the other. But my main point still stands, which is the uncertainty in a SINGLE measurement remains the same and independent of what was measured earlier or later.

And besides, what does this have anything to do with the HUP? That is my whole point - they don't!

Zz.

 

[lightarrow]

And who said you should? In the example I gave you, BOTH types were used under difference circumstances. Did you miss that? No one said you should use one over the other. But my main point still stands, which is the uncertainty in a SINGLE measurement remains the same and independent of what was measured earlier or later.

And besides, what does this have anything to do with the HUP? That is my whole point - they don't!

Zz.

Ok.
What about question n. 2 in my post n.35?

 

[lightarrow]

Which is what you consider the better way to express HUP?

 

[ZapperZ]

Ok.
What about question n. 2 in my post n.35?

But I thought I already answered that in the link that I gave to my blog entry? Besides, you never explained what you mean by "simultaneous". I had to clearly define what *I* meant by that, but no one else here who used that word seems to do that.

OK, one more time, and then I'm done with this one.

You measure the momentum AFTER the light passed through the slit. You do this by look at where the photon hit the screen/detector, and then deducing the transverse momentum it has gained in the x-direction. This means that in terms of the sequence of operators, the position operator came first, and the momentum operator came next. That's all we care about. So now you have x, p, and the uncertainty in x from the slit width. You still have no uncertainty in p that corresponds to the one referred to in the HUP. All you have in terms of the uncertainty in p as this point is the instrumentation uncertainty, i.e. how big was the spot on the detector that gives you the single-measurement uncertainty in p. But as I have said before, this is not part of the HUP. The spread that enters in the HUP isn't known yet from experimental observations.

So how do we get the HUP-related uncertainty in the p? You do this experiment many, many times. You'll see that if the slit width is small enough, the position where the photon hit the screen will have a very large spread. If you bin the results, you'll have roughly a gaussian spread centered directly behind the slit. The smaller the slit, the larger the spread. It is this spread that gives you the HUP uncertainty for p. It fits exactly in what the HUP predicted. The smaller the slit (i.e. the more I know about the position of the photon that got through), the less I can predict what x-component of the momentum will be of that photon that got through.

Zz.

 

[lightarrow]

But I thought I already answered that in the link that I gave to my blog entry? Besides, you never explained what you mean by "simultaneous". I had to clearly define what *I* meant by that, but no one else here who used that word seems to do that.

I thought it was clear: to me, it means "Exactly at the same time". I know you had already answered to it, I wanted to be sure of it.
Thank you to have clarified this, because it's not so clear in the books.

 

[ZapperZ]

I thought it was clear: to me, it means "Exactly at the same time".

But why is this so important and so relevant here? That has never been explained to me when I asked the first time. I have already indicated that this is a non-issue as far as the HUP is concerned, simply from the fact that the operators' order of measurement makes a whole lot of difference. So insisting on something being measured "exactly as the same time" introduces something that isn't contained in the HUP. So why are you requiring it?

Zz.

 

[reilly]

If you work it out, you'll find that x(t+dt) does not commute with x(t) -- formally, use the "time" unitary boost, exp(-i H t). So, this means given an, an x(0), any x(t+dt) is possible. Thus it will take many observations to get a mean value for x(t+dt). And the best one can do for v, or p, is an average value. No problem with the HUP;.
Regards,
Reilly Atkinson

 

[lightarrow]

If you work it out, you'll find that x(t+dt) does not commute with x(t) -- formally, use the "time" unitary boost, exp(-i H t). So, this means given an, an x(0), any x(t+dt) is possible. Thus it will take many observations to get a mean value for x(t+dt). And the best one can do for v, or p, is an average value. No problem with the HUP;.
Regards,
Reilly Atkinson

Ok, but, as also nrqed wrote in post n.7, what does the operator x(t)x(t+dt) mean, if AB means: "the operator B acts first, in the time, and then acts A"?

 

[jVincent]

So here a simple view of it:
classical view: Momentum is carried in little matter-lumps called particles which at any given time are located at _one_ place in space, and have _one_ definite momentum.

Now in QT momentum is carried in waves, which are not localized in space.
one wave is _everywhere_ and has _one_ definite momentum.

So we see a problem since in our world, things aren't everywhere, but only "one" place, so we seek out a method of creating a localized particle. We do this by adding up these waves, thus adding up different discrete momenta. So a particle is suddenly described as a wave-package.
This makes up the _one_ location particle, but this particle has infinite many momenta, otherwise it would not be at only one location.

This "uncertainty" principle doesn't have anything to do with measurements, it's simple a fact of the composition of our world. Thus no measurement can be made that breaks this "uncertainty"

In the classical world we can't measure the position of a car to be two different places at the same time, simply because if we do then we come to the conclusion that it wasn't the same car, since things can't be at to places at once. Even if we find a apparatus that measures position with uncertainty down to the nm scale, we still can't perform this feat, because it has nothing to do with uncertainty, but is related to the nature of what we are measuring.