# How can you actually measure the electron position?

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1. Jun 1, 2015

### Tomishiyo

Standard quantum mechanics text-books discusses Born rule, which states that the probability of finding a particle in a certain region in space is given by

$$|\Psi ({\bf r},t)|^2d^3r$$

Thing is, I never have seen a discussion about how you can actually measure the particle position in a given system. Do you use x-rays? How the experiment actually works? Can you refer me to papers about it or describe the experiment with details?

2. Jun 1, 2015

### The_Duck

You can let the particle run into a photographic plate which produces a dot at the point of impact. Or you can shine light on the particle and look for reflections/scattering. That sort of thing.

3. Jun 2, 2015

### ZapperZ

Staff Emeritus
As has been stated, you let an electron hit a screen that will leave a mark or a signal. I can also make a very small slit, and if I detect a signal after the slit, I know that the electron passed through that position where the slit is. The uncertainty in the position corresponds to the width of the slit.

Zz.

4. Jun 2, 2015

### ftr

I think the OP meant the position of the electron in an atom.

5. Jun 2, 2015

### ZapperZ

Staff Emeritus
This is the OP's post (my emphasis)

Nowhere in there is there any indication that this is only for within an atom.

Zz.

6. Jun 2, 2015

### ftr

That is why I said I think, because he said "in a given system", atoms being the more interesting case. But anyway the OP himself can clarify if he wishes.

7. Jun 2, 2015

### ZapperZ

Staff Emeritus
"Interesting" is subjective. Electrons in a Luttinger Liquid system is more interesting to me. Yet, I try not to impose what I find "interesting" to the question.

Zz.

8. Jun 2, 2015

### Derek Potter

And yet, if you let a photon go through the same slit and detect it, you haven't measured its position. Or so I'm told.

9. Jun 2, 2015

### ZapperZ

Staff Emeritus
I have too! At the moment that photon passed through that slit, where do you think it is? 20 miles away?

I have dealt with this in my explanation of the HUP in another post

... or so I'm told.

Zz.

Last edited by a moderator: May 7, 2017
10. Jun 2, 2015

### bhobba

He is referring to the fact in QED for a photon position is not an observable.
http://www.mat.univie.ac.at/~neum/physfaq/topics/position.html
'The POVM does not allow one to talk about the position of a photon - which could exist only if the corresponding operator existed -, but only about the measured position: The photon is somewhere near the range of values established by the measurement, without any more definite statement being possible.'

In other words you can localise its field but getting an exact position is problematical.

Thanks
Bill

Last edited: Jun 2, 2015
11. Jun 2, 2015

### ZapperZ

Staff Emeritus
But within the context of this thread, i.e. the OP is asking how one actually measures the position of a particle, it is the act of measurement and the experiment that matters. As I've said, I can localize the position of a photon or an electron when it passed through such a slit. A whole slew of experiments depend on that being true. Otherwise, a lot of things will go wrong with our measurements.

Zz.

Last edited: Jun 2, 2015
12. Jun 2, 2015

### bhobba

Hi Zapper

For sure.

Thanks
Bill

13. Jun 2, 2015

### OCR

Why 20? ...if he were wrong, then one would have been enough...

Last edited: Jun 2, 2015
14. Jun 2, 2015