# Uncertainty principle

1. May 30, 2005

### Spastik_Relativity

a problem i had recently. ive always thought of the uncertainty principle as if you understand position you less understand velocity and vise versa. the model i know is that to know the exact poition of a particle you shine a light wave on it, the more accurate u want the position the higher frequency you need but the more disturbance in velocity you cause. the problem im having is my teacher tried to explain in used a single slit light experiment similar to the famous double split experiment. i just didnt quite understand how it works so if anyone could help me with this it would be much appreciated.

thanks

2. May 30, 2005

### whozum

Someone wrote an amazing explanation of the uncertainty principle that I read on here just a few days back, try searching for it.

3. May 30, 2005

### extreme_machinations

Single Slit Experiment Resulsts Are Caused By Diffraction Of Light Double Split Experiment Results Are Caused By Interference Of Light

4. May 30, 2005

### extreme_machinations

Hmm Mmmm
Intersting Topic For Pondering .

5. May 30, 2005

### ZapperZ

Staff Emeritus
I wrote something about this a while back, but couldn't find the right thread to refer to. So I'll make a reference to my journal entry below:

[11-15-2004 08:26 AM] - Misconception of the Heisenberg Uncertainty Principle

You may want to read this since I used specifically the single-slit diffraction phenomenon to convey the fact that the diffraction effects is the HUP staring at you right on your face.

Zz.

6. May 30, 2005

### whozum

Funny Zz, the explanation I was thinking of was your journal entry. Its great, by the way.

7. May 30, 2005

### ZapperZ

Staff Emeritus
Then maybe I'm psychic, or I've just invaded your mind. Either way, it's not good!

:)

Zz.

8. May 31, 2005

### Spastik_Relativity

ZapperZ, nice expliantion. its definately helped me further understand the HUP.

9. May 31, 2005

### dextercioby

And where would you draw the line (if there is such a thing) between the 2 classical phenomena...?Give a documented answer.

Daniel.

10. May 31, 2005

### marlon

i wouldn't know

yes professor...

marlon

11. Jun 2, 2005

### dorel1958

Both phenomena: diffraction and interference are effects of the waves who passes 1 or more splits.
The idea is that the same 'thing' seems TO US to bee a 'wave' some times and a 'particle' some other times.
In fact the concepts of wave and particle are mathematical MODELS in our minds that try to explain what is happening around us (and sometimes do that very well).
The fact is that in this case do not.
The really unpleasant fact is that we are no one intuitive MODEL (and now seems that we will never have) to explain this fact.

12. Jun 6, 2005

### Beholder

I have read Zapperz's misconception of the heisenberg uncertainty principle (and in magazines/books etc) and am still confused. What your saying when you describe how when the slit is made smaller the spread of the particles becomes increasingly spread out doesn't makes sense. So you say in classical mechanics you can predict things as accuratly as you want but in quantum mechanics this is not so? So what does this really mean?, what does it imply?

13. Jun 6, 2005

### DrChinese

I would say that's uncalled for, Beholder. You might try researching the subject before you begin your critique. ZapperZ's description is accurate.

14. Jun 6, 2005

### ZapperZ

Staff Emeritus
You will note that you CAN do this and test it for yourself! I'm not describing something a high-school physics student hasn't seen before. So if you don't think that the pattern you see spreads out as you make the slit smaller, then you have a serious issue with explaining experimental observation.

Also note that arguing against something because it doesn't make 'sense' to you isn't a valid argument in physics. A lot of things do not makes sense until you learn more about it. THEN they make perfect sense. Your "sense" changes with knowledge - it isn't static, nor perfect. The more you know, the more things make sense that didn't used to before. I urge you to never use that to counter something in physics, because frankly, that doesn't make much sense!

Zz.

15. Jun 7, 2005

### pmb_phy

That is a statistical principle and does not apply to any single measurement. This applies to more than position and momentum. You mentioned "velocity" but there is no "velocity" operator in QM. Its not a meaningful quantity in QM. One might be tempted to define a velocity operator by dividing the momentum operator by the mass of the particle. However the momentum of the particle is not the mechanical momentum p = mv, its the canonical momentum, which can be different. In fact for an electron moving in an EM field it is different.
That is correct.
The double slit experiment is often used to show that you can't say which hole the particle went through. What you're speaking of sounds to me very much like an experiment like the microscope experiment. Have you ever read about that one?

Pete

Last edited: Jun 7, 2005
16. Jun 7, 2005

### El Hombre Invisible

That's just pure comedy genius! :rofl: