# B Does the Uncertainty Principle work through barriers?

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1. Oct 27, 2016

### dccd286

I'm currently designing an experiment on Double slits for a high school science fair, but I'm not sure whether it's plausible or not. I'll be spending some of my savings for this, that's why I would like to hear some feedback from more-knowledgeable people first.

In this experiment, I am trying to find out whether Heisenberg's Uncertainty Principle would work through a barrier, such as a thin piece of cardboard or something. In order to do this, I will do three different "sub-experiments" within my experiment:
1. Double slit + a laser pointer
2. Double slit separated by a barrier + a laser pointer
3. Double slit separated by a barrier + 2 laser pointers

Note: The barrier I'm talking about will merely divide the beam from the laser pointer, but I will not put a barrier that would hinder the interference of light. Like this: http://imgur.com/a/oewPU

I'm thinking the first part of my experiment would serve as my "control" and the second one will be the "independent" experiment. With the first experiment, I would prove that the Uncertainty Principle is true. With the second one, I am trying to find out whether it works through barriers or not. And I'm thinking the third one is merely to see whether two separate laser pointers would interfere with each other.

I'm basing my experiment from Veritasium's videos on Double slits, and Uncertainty Principle.

Last edited: Oct 27, 2016
2. Oct 27, 2016

### houlahound

Not sure what this has to do with uncertainty principle. Can you draw a diagram of the set up and write a clear method including, your hypothesis, what you are going to measure, how you will analyse the data and what theory justifies your hypothesis and analysis.

I would not spend any money until you can do that.

3. Oct 27, 2016

### BvU

Second houla. For a link with the uncertainty principle a single slit demo is adequate and complicated enough.
For the double slit:
It seems to be unclear to you what interferes with what. Witness this 'through a barrier' idea. What would the barrier do to the 'interferers' ?

And if you want to have a visible pattern, a very thin piece of cardboard would already be too thick anyway!

Gather some more know-how on the subject first would be my advice.
Spend pocket money on a laser pointer to play with, not savings.

4. Oct 27, 2016

### Staff: Mentor

A double slit experiment with a laser as a source can be described completely using classical waves. The HUP does not enter anywhere.

5. Oct 27, 2016

### BvU

Was I wrong in thinking the HUP is equivalent to attributing wave characteristics to particles and the 1 slit experiment illustrates the wave characteristics of photon particles ? Like $\Delta x =$ slit width, $\Delta p \propto$ width of diffraction pattern ?

6. Oct 27, 2016

### dccd286

Just to be clear, I am not quite sure whether I understood everything correctly from the videos that I've watched, but my understanding is that:
HUP is in play with one slit because of ΔxΔp >= h/2pi because this tells us that if the slit is narrow enough, then the momentum has to increase in order to follow the principle. I am not quite sure whether this is still HUP, but to my understanding, based on the video "Single Photon Interference," HUP is in play with two slits because a photon passes through "both slits" at the same time.

7. Oct 27, 2016

### StevieTNZ

You may need to list these videos you have watched, to see if they're legit or garbage.

8. Oct 28, 2016

### David Lewis

I think OP wants to demonstrate there are an infinite number of possible trajectories (besides a straight shot) a photon can take and still make it to the screen. Most of the unlikely paths are immediately adjacent to each other and out of phase. When a barrier blocks the phase-canceling paths, the virtually impossible paths become certain, and light gets through.

9. Oct 28, 2016

### BvU

In the setup diagram it's hard to distinguish what it is the OP wants to block...

10. Oct 28, 2016

### dccd286

Sorry for the ambiguity, I don't have very deep knowledge on these kinda stuff, but what I had in mind was along the lines of what David Lewis mentioned

Oh and the videos that I watched from Veritasium are:
The Original Double Slit Experiment

Single Photon Interference

Heisenberg's Uncertainty Principle Explained

(I am not entirely sure of the legitimacy of these videos, or that I understood it the right way)

11. Oct 28, 2016

### BvU

Again: what would the barrier block, exactly ?

12. Oct 28, 2016

### houlahound

I think that's precisely what we are trying to determine from you.

13. Oct 28, 2016

### BvU

I briefly checked the three videos (boy, what a lot of noise) First one is interesting at 2:30 : imagine adjusting your thin cardboard to not block the sunlight from going through both slits and still do its -- it's what ?

Third one backs up my post #5, which is always nice. But dccd: post #6 makes me worry that you don't really understand it.
Then
in combination with post #1, suggests that you think you can prevent that with your barrier (am I right ?)

14. Oct 28, 2016

### Staff: Mentor

The videos of Veritasium I had come across before were okay, but that one on the HUP is awful. Again, what is observed is a purely classical wave phenomenon. Even if Nature was not quantum, and hence there was no HUP, the result would be the same.

To use a slit-type experiment to investigate the HUP, you would have at least to use particles, such as electrons, instead of light.

15. Oct 28, 2016

### Staff: Mentor

There is no position operator for photons, so I don't see how to get a position momentum uncertainty principle for them. On top of that, the momentum of photons is related to their frequency, and I don't think that a slit changes the color of a laser

16. Oct 28, 2016

### phinds

You are misinterpreting the equation. What is says is that the UNCERTAINTY in the momentum must increase, not that the momentum must increase.

17. Oct 28, 2016

### BvU

Well, they come through the slit, so the uncertainty in position at the slit is the slit width. And the uncertainty in the momentum in a direction perpendicular to the beam is proportional to the width of the central maximum.

I know it's all a matter of Fourier transforming that you can also do for water waves, but isn't the HUP coming from the same source ?

I had Richard Feynman convince me that photons are particles (pity I can't render the pronunciation). That they go real fast or have no mass doesn't matter all that much, or does it ?

18. Oct 28, 2016

### Staff: Mentor

Yes, it ultimately boils down to the fact that position and momentum are canonically conjugate. You can make an analogy between the light waves and the HUP, but you can't use light waves to demonstrate anything about the HUP, no more that you could do it with sound waves.

It actually does. You will find many people on PF warning against thinking of photons as little balls moving at the speed of light. As I said, there is no position operator for photons, so they are special particles. Below the level of QED, it is best to either consider light as simply an EM wave, or, for QM, consider massive particles.

19. Nov 1, 2016

### Jamison Lahman

You could "prove" the HUP by stating you have no idea where any electron is because that technically satisfies the criteria. Conversely, disproving it would require measurements of position and momentum on orders of magnitude more precise than a percent (h ≈ 6.6E-34).
Regardless, I am not sure I would be willing to accept the notion that light diffracting through a slit proves the HUP. They are similar in mathematical formulation however different in physical ramifications.