# HUB & accurate single measurement

1. Feb 9, 2013

### MHD93

Hello,

I recently read some texts which says that in a SINGLE measurement you can measure position and momentum to an arbitrary accuracy, assuring that that's not what HUB talks about.

That confuses me, because I learnt that HUB is there NOT simply because some property of QM prevents us from knowing them simultaneously accurately, BUT BECAUSE the particle really don't have such things as position and momentum at the same time.... ......... I'm confused..... HELP me!

2. Feb 9, 2013

### ZapperZ

Staff Emeritus
It doesn't have definite position and momentum BEFORE a measurement. But it doesn't mean that when you make the measurement, you can't make it with arbitrary accuracy. There's nothing to prevent you from making as accurate of a single measurement of those values, other than the technological limit of your instrument.

https://www.physicsforums.com/blog.php?b=4364 [Broken]

Zz.

Last edited by a moderator: May 6, 2017
3. Feb 9, 2013

### MHD93

Let's take the double slit experiment, you want to measure position and momentum when it's at the slit, say.
This is all before the measurement (that is, when the electron hits the screen).

or you mean the particle determines to have a specific past when you measure it in the present, ?

4. Feb 9, 2013

### ZapperZ

Staff Emeritus
Do you know what those $\Delta$'s mean in the HUP? They correspond to the variance and standard deviation of a value.

So what are the variance/standard deviation of ONE single measurement?

You are also now confusing superposition principle with the HUP by bringing up the double slit.

Zz.

5. Feb 9, 2013

### MHD93

Excuse me, I meant a single slit whose width determines the accuracy of position measurement..

So again, can we measure position and momentum at the slit at a later time?

6. Feb 9, 2013

### ZapperZ

Staff Emeritus
"momentum at the slit"?

The location of the slit is the "position". The width of the slit is the uncertainty in position.

AFTER it passed through the slit, you THEN measure its transverse momentum (i.e. momentum in the same direction as the slit's width).

I have no idea where this is going. However, this is all MOOT if you simply answer my question regarding the definition of each of the values in the HUP.

Zz.

7. Feb 9, 2013

### vanhees71

Perhaps this is what you refer to: