Heisenberg principle: how can you find an electron's speed without finding it's location?

[Ronald Hyde]

How can you be uncertain? if you are measuring an area that is only the size of an electron, then if the electron is anywhere other than in the path of the detector, it will not be detected.

 

And so what is the size of an electron? And how exactly do you build an apparatus with an opening of that size?

It's very easy to propose experiments that are impossible to perform in practice.

[ElasticCollision]

And so what is the size of an electron? And how exactly do you build an apparatus with an opening of that size?

It's very easy to propose experiments that are impossible to perform in practice.

 

If the size of an electron is unknown, then it seems my original question could have been cleared up extremely quickly.

[swansont]

How can you be uncertain? if you are measuring an area that is only the size of an electron, then if the electron is anywhere other than in the path of the detector, it will not be detected.

Classically, yes, but not in QM. There is no well-defined location and momentum when measured simultaneously. Plus, how do you measure the location of a single point?

[derek w]

If the size of an electron is unknown, then it seems my original question could have been cleared up extremely quickly.

 

 

 

The double slit experiment,proves that the electron does not simply go from point A to point B.

[ElasticCollision]

The double slit experiment,proves that the electron does not simply go from point A to point B.

 

You're confusing electrons with photons.

[derek w]

You're confusing electrons with photons.

 

 

 

No,the double slit experiment produces the same result with electrons.

And all sub atomic particles,atoms and some molecules such as Buckyballs.

Edited October 17, 2012 by derek w

[ElasticCollision]

No,the double slit experiment produces the same result with electrons.

And all sub atomic particles,atoms and some molecules such as Buckyballs.

 

In any case, you've gone way off subject.

[Pugdaddy]

What you want to know is the instantaneous speed as you reduce the distance between point B and point A. If you make a measurement at two points in space you always get average speed between them. What we can't do in reality is reduce the distance between measurement to 0 and still get some meaningful answer.

 

Maybe a better way to put it is the definition of "speed" is distance over time. As you move your measured points closer and closer, both distance and time go to 0. that leaves 0 distance divided by 0 time.

[derek w]

If you know the electron passed a detector at point A (x,y,z)=0,then 1 second later passed a detector a point B (x=10,y=0,z=0).

Then your argument is that it has travelled 10 metres along the x axis in 1 second and should thereby in 1 second time arrive at point C (x=20,y=0,z=0).

There would be an uncertainty about where point C is.

[Pugdaddy]

No that is not what I mean. If the electron passes detector A (x,y,z)=0, then 1 second later passes detector B (x=10,y=0,z=0). That gives you the average speed (10 m/s) of the electron between point A and point B, but does not tell you how fast the electron was moving when it passed point A.

[derek w]

I agree with you,it's ElasticCollision you need to convince.And you don't know it's momentum after point B.

[Pugdaddy]

It is also resolution. If we think of the electron as a wave propagating through space at a specific wavelength. If we measure it's position by bouncing a photon, also propagating through space, at a specific wavelength off it, the wavelength of the photon must be smaller than the wavelength of the electron or we will get a fuzzy picture. The smaller the wavelength of the photon the better the resolution and the better we know the position. Problem is that the smaller the wavelength of the photon the higher the frequency of the photon and the higher the energy it carries. So the finer we resolve the position the more the photon changes the momentum of the electron when it bounces off it. That means better we know position the less we know the momentum.

[derek w]

Yep,I think you have summed it up pretty well there.

 

One question is the frequency of the photon changed when it bounces of the electron?

Edited October 17, 2012 by derek w

[ElasticCollision]

I agree with you,it's ElasticCollision you need to convince.And you don't know it's momentum after point B.

 

Firstly you're looking at it the wrong way. You don't need to convince me, you need to try and educate me. I'm not saying I don't believe the Heisenberg principle, I'm saying that I don't understand how the idea I have put forward wouldn't work.

I get that detecting an electron changes it's path. But if you manage to eventually detect one that goes through point A and then point B, despite it's change in path, you will have still found it's exact position for that incredibly brief period of time and you could use the time taken for it to move from point A to point B to infer it's speed.

[derek w]

Firstly you're looking at it the wrong way. You don't need to convince me, you need to try and educate me. I'm not saying I don't believe the Heisenberg principle, I'm saying that I don't understand how the idea I have put forward wouldn't work.

I get that detecting an electron changes it's path. But if you manage to eventually detect one that goes through point A and then point B, despite it's change in path, you will have still found it's exact position for that incredibly brief period of time and you could use the time taken for it to move from point A to point B to infer it's speed.

 

 

 

And of what use is it to know this if it has changed.

But this goes back to what i said in post(4) about the large hadron collider.

Edited October 17, 2012 by derek w

[ElasticCollision]

And of what use is it to know this if it has changed.

But this goes back to what i said in post(4) about the large hadron collider.

 

-Not answering my question.

-Asking questions without a question mark.

-Asking questions before answering mine.

-Not explaining what "it" is.

-Referring back to something which hadn't solved anything anyway.

-Causing the entire conversation to go in circles.

Good day to you Sir.

[swansont]

It is also resolution. If we think of the electron as a wave propagating through space at a specific wavelength. If we measure it's position by bouncing a photon, also propagating through space, at a specific wavelength off it, the wavelength of the photon must be smaller than the wavelength of the electron or we will get a fuzzy picture. The smaller the wavelength of the photon the better the resolution and the better we know the position. Problem is that the smaller the wavelength of the photon the higher the frequency of the photon and the higher the energy it carries. So the finer we resolve the position the more the photon changes the momentum of the electron when it bounces off it. That means better we know position the less we know the momentum.

One needs to differentiate between the observer effect, which is what you describe here, and the Heisenberg Uncertainty Principle, which is the topic of the thread.

[derek w]

Whether it's the observer effect or the uncertainty principle,both come down to a superposition of waves.

[imatfaal]

Whether it's the observer effect or the uncertainty principle,both come down to a superposition of waves.

 

Derek - regarding the difference between measurement-disturbance effect and the uncertainty principle you might like to read this thread in which an experiment which showed that weak measurement CAN skirt the measurement-disturbance effect but CANNOT avoid the uncertainly principle.

 

http://www.sciencefo...inty-principle/

 

What the experimenters showed was that Heisenburg's initial measurement-disturbance relationship was incorrect. But this relationship has been ignored for a while - the mathematical formalism of quantum mechanics has shown than Heisenberg Uncertainty principle is intrinsic and not a product of measurement. This experiment does not touch the HUP - and I think most physicists still think that the HUP is correct and will not be shown to be avoidable through weak measurement. To be clear the measurement-disturbance idea was a bit of an old chestnut - wavelengths of light small enough to determine position are energetic enough to disturb momentum. As AJB said above the HUP follows from the non-commutative nature of certainoperators; this allows quantum physicists to be sure that the HUP is not just bad measurement

 

What the UofT researchers showed was that weak measurement can be so weak that the measurement would not be disturbing enough to create the uncertainty - but they do not deny that the uncertainty is still there. Lee Rozema the lead author was quoted saying "The quantum world is still full of uncertainty, but at least our attempts to look at it don't have to add as much uncertainty as we used to think!"

[derek w]

So the uncertainty is a question of the scale of the superposition of waves in the electrons environment?

For example in a copper wire with a voltage applied across it,the scale of the superposition of waves would be high,therefore the uncertainty would be high.

Where as in a vacuum the scale of superposition of waves would be very low but not zero,therefore the uncertainty would be low but not zero.

[swansont]

So the uncertainty is a question of the scale of the superposition of waves in the electrons environment?

For example in a copper wire with a voltage applied across it,the scale of the superposition of waves would be high,therefore the uncertainty would be high.

Where as in a vacuum the scale of superposition of waves would be very low but not zero,therefore the uncertainty would be low but not zero.

It's not really an issue of superposition, it's that there is a wavefunction that describes position or momentum and they are Fourier transforms of each other. You cannot make one arbitrarily small without making the other one large, which limits simultaneous measurement and is not the same as uncertainty imparted by a measurement.

[ElasticCollision]

Crikey, so for asking a question and pointing out how very unhelpful someone is being in answering it, I get negative reputation.

Doubt I'll be using this site much more now, awful.