IS THIS the Heisenberg uncertainity principle?

[mahela007]

I read that the Heisenberg uncertainty principle arises for stuff like electrons simply because the observer affects the observed .
That is,because photons (or what ever the observer uses to detect the position) will affect the trajectory of the particle...
That doesn't make much sense to me.. (SURPRISE!!). Does that mean, that if no one "looks" at the electron , it will keep moving in a regular, predictable orbit? That doesn't seem likely, what will all of Schrodinger's work... So I think I've got it mixed up.

Let's see... I'll try to sum up my understanding of this subject and I hope you guys will be able to correct me.

1 The electron is fizzing around the nucleus.

2. If someone tries to look at it, then it's path is altered by incoming photons. (uncertainty principle)

3. Schrodinger came up with the wave function of electrons to predict where they will most probably be.

Now, to me, step 2 and 3 seem to be contradictory.
If nobody looks at it the electron would move in it's regular (predictable) orbit...and shrodingers equations would seem to be wrong.

One the other hand, if shrodinger's equation are right, that would mean that whether you looked or not the electrons wouldn't move in a predictable manner...

(please feel free to say so if you think I'm in a hopeless muddle. :confused: ) Please help me clear this up.

[jtbell]

I read that the Heisenberg uncertainty principle arises for stuff like electrons simply because the observer affects the observed .

No, it does not. This misconception is unfortunately very common in popular-science books and even in some introductory physics textbooks. The HUP can be derived from basic principles of QM without making any reference to an actual measurement.

See the following post for a simplified description of one way to do this (there are others):

http://www.physicsforums.com/showthread.php?p=1127553#post1127553

[jtbell]

1 The electron is fizzing around the nucleus.

2. If someone tries to look at it, then it's path is altered by incoming photons. (uncertainty principle)
[...]
If nobody looks at it the electron would move in it's regular (predictable) orbit...

The mathematics of QM as physicists use it in their work, does not contain the notion of "path" or "regular (predictable) orbit" of the electron. It simply does not address questions like "what is the electron 'really' doing before we make a measurement of its position, momentum, etc.?"

This is the subject of interpretations of QM which attempt to build a theory that lies "underneath" the QM that we actually use. A number of interpretations are viable in the sense that they make predictions which agree with standard QM (because they're constructed to do so!). However, there is as yet no way to distinguish between them experimentally, and all of them have features that some people consider distasteful, so people argue about them endlessly. Just look around on this forum! :rolleyes:

[mahela007]

No, it does not. This misconception is unfortunately very common in popular-science books and even in some introductory physics textbooks. The HUP can be derived from basic principles of QM without making any reference to an actual measurement.

See the following post for a simplified description of one way to do this (there are others):

http://www.physicsforums.com/showthread.php?p=1127553#post1127553

Thanks.. That clears up a lot of stuff.
But that is the phenomenon of "observer affecting the observed " called? Does is have a connection with the electron and it's wave function or is it just something separate that provides another reason why we can't find the electron?

[jtbell]

Certainly the measurement process can affect what is being measured, but this is in addition to the uncertainty that we calculate from the wave function.

[mahela007]

oh... That's exactly the statement I wanted to hear.
(by the way.. is this article (www.bbc.co.uk/dna/h2g2/A408638) wrong? the section titled "the principle")