# A thought experiment with Heisenberg's Uncertainty Principle [duplicate]

Possible Duplicate:
Could the Heisenberg Uncertainty Principle turn out to be false?

# Thought Experiment

Ponder, for a moment, if I had a cube with 10cm sides which I'll name The Box. By some unexplained means (future technology, magic, the work of deities, insert whatever you want), The Box had a special property: it would determine with absolutely zero uncertainty* the momentum of every particle inside of it (and only those inside of it). You can imagine it hooked to a computer with a listing of each particle and it's momentum, or whatever you like.

Edit: I've been informed that perhaps zero uncertainty is unattainable in the momentum, so if this is the case then perhaps simply "Known with enough certainty such that the combination of the momentum uncertainty with the size of the box yields $\sigma_m \sigma_x < { ħ \over 2}$, or in other words, the pair are known with more certainty than the Heisenberg's Uncertainty Principle says can be known."

• What would this device be likely to cause in our universe, with our physical laws? Since by the Heisenberg's Uncertainty Principle (or the wavefunction commutation) their positions cannot be known, then it would seem to follow that I can no longer know they are in The Box, or indeed anywhere in the universe. But I know they are in The Box, because they are on the screen. Even if some small amount of uncertainty about The Box's location can be shown, because I have perfect certainty of the momentum, nothing about it's position can be known (by the HUP/WC) - so knowing anything about it is a violation.

• Would this cause all particles to leave The Box once it was activated, leaving a perfect vacuum, devoid of even virtual particles (since once they popped into existence, I could determine their momentum and that they were inside The Box)?

• If it caused a perfect vacuum inside of it, then it would seem that such a box would indeed not break HUP/WC at all - since I would still not have any knowledge of momentum and position of any particles. Could one argue that it is compatible with our laws of physics then, provided that activating it caused the expellation of every particle inside of it?

• What would the existence of a region of true vacuum do to the surrounding matter, if anything at all?

• Or would nothing out of the ordinary happen whatsoever, and it just turns out that the box 'beats' the HUP / defies the wavefunction commutation? Or perhaps, "forces classical behavior inside The Box"?

• Would the confirmed existence of such a box force the reconsideration of QED Theory? As in "Well, that checks out, so I guess Quantum Theory wasn't a complete model of our universe"? Or would we just refine it, changing the details of the HUP/WC somewhat? (Stated another way: would we still have QED if HUP was proven false)

• Other alternatives to consider? Details that I missed in formulating the thought experiment? Helpful suggestions?

Note: if you're not interested in considering the thought experiment, then don't. Wild conjectures will be fine, and the purpose of the question is to assist my understanding of the implications of the HUP/WC. "The Box couldn't exist" or "That can't happen" isn't helpful or relevant. I'm asking that consideration of it's existence and validity is already accepted (it's accurate and not just making things up), not asking for how to build one or why we haven't made it yet.

# Edit

It looks like a number of the comments are getting at this central point, but not explaining it - just asserting it. Please add an answer to provide a description of what you mean, and it would likely be the answer I'm looking for:

"if you really assume nothing else than that you have a Universe in which HUP refuses to hold, it's clearly too little information to deduce anything else. But in the very same comment, you're also asking whether QED would have to be discarded. You bet. All of modern physics would have to be discarded." - Luboš Motl

What do you mean by 'modern'? Does Gravity have to go? Relativity? Big bang cosmology? Rocket Science? The Laws of Thermodynamics? String Theory? All of QM? I want to know what specifically will be contradicted, and why.

Now, please don't strawman my experiment into HUP not holding at all, everywhere - in other words, HUP being false in all cases, places, and times - I'm not asserting that (just doubting the ultimate certainty of the truth of HUP). At a minimum, HUP does not hold inside The Box, yet it very well may hold everywhere else in the universe. Given this, it seems to me that QM would not necessarily be contradicted, and could easily be saved on a basic level into something analogous to Particles can be represented by a wavefunction everywhere that is not inside of The Box, and thus QM is not contradicted, just refined.

Given this case, is it still your position that the very existence of The Box invalidates all of 'modern physics'? Please be clear on what that means.

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I guess it would just be a "classical physics" box, and would suggest a universe where physics was just that. –  Alec S Dec 14 '12 at 10:04
Consider this thought experiment; 3+2=77, pigs can fly, the moon is made of cheese and 'everything everyone has ever learned is false', then what is the price of a BigMac? –  user1708 Dec 14 '12 at 10:05