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Heisenberg's Uncertainty vs Virtual Particle Fluctuations

  1. Dec 21, 2012 #1
    I notice that many physicists say that virtual particle fluctuations occur in space "because Heisenberg's Uncertainty Principle allows them"

    Is this really the best form of reasoning?
    Isn't it actually the other way around - that because virtual particle fluctuations happen, then there is a Heisenberg Uncertainty?

    Is the distinction I'm making trivial? To me, it's about Causality - the fluctuations create uncertainty, and not the other way around. Otherwise, it's like putting the cart before the horse and saying my alarm clock failure happens because my pay is docked for tardiness, rather than saying my pay is docked for tardiness after my alarm clock failure.

    Shouldn't Occam's Razor apply here? Why do we trivialize Causality by claiming fluctuations happen as a consequence of uncertainty, rather than claiming uncertainty is a consequence of fluctuations?

    I want to know if this convoluted reasoning was the result of our limited measurement capabilities in measuring vacuum fluctuations to begin with. It seems to me that because we couldn't really measure vacuum fluctuations before, that we had to turn to statistics as the measurement of last resort in order to describe the world of quantum mechanics. And then within the confines of statistics and uncertainty we then saw that the vacuum fluctuations were possible.

    But now that we know that vacuum fluctuations happen, why are we not able to re-order our knowledge to state that Heisenberg's Uncertainty is the result of Vacuum Fluctuations? Why do we persist in still claiming that the Vacuum Fluctuations are the result of Uncertainty?

    Shouldn't physics continually try to condense or deconstruct everything into what's most intuitive and logical? Isn't that the whole reason for physics in the first place? Why should we continue to stay hostage to our previous inability to measure things that we can now acknowledge we see?

    Am I being a nitpick here?
  2. jcsd
  3. Dec 21, 2012 #2
    The Uncertainty Principle comes from a more basic mathematical understanding of quantum mechanics, the idea that you cannot simultaneously determine a wave's location and wavelength, whereas particle fluctuation is more of a physical phenomenon, allowed by the UP, which is why uncertainty seems to be the more fundamental characteristic of the two, and thus is cited as the reason for vacuum fluctuations.

    I think, in the end, the uncertainty principle is more general that the quantum fluctuation phenomenon, in that it is observed in quantum systems where the idea of vacuum fluctuations doesn't really come into play. Maybe someone with more knowledge of QFT could prove me wrong though.
  4. Dec 21, 2012 #3
    What if we could construct our own theoretical universe where particle fluctuations don't exist?

    Wouldn't this be a universe devoid of waves or Uncertainty? If so, then wouldn't that amount to waves themselves being a consequence of basic fluctuations, which then gives rise to the Uncertainty principle as you've said? Can we even have wave behavior without the fluctuations, allowing the Uncertainty Principle to exist independently of those fluctuations? If waves can't exist without the fluctuations, I don't see how the Uncertainty Principle can.

    I guess I'm sounding Chicken-and-Egg here, but it bothers me that we so arbitrarily discard Causality to embrace non-causal quantum mechanics. It seems to me this originally occurred because we'd hit a roadblock in our ability to measure and analyze small-scale phenomena, and so we had to reach out to statistics and probability to make further progress in our understanding. This eventually allowed us to deal with and further probe the nature of the small-scale effects we couldn't previously see. But now that we have learned much more about the small-scale effects, why can't we once again revert back to a Causal understanding of them? To me, such a revision is part of basic Economy of Thought (aka Occam's Razor). Why do we choose to maintain a needlessly convoluted understanding when we don't have to?
  5. Dec 21, 2012 #4
    Because there is no causal understanding of them. The theory is not needlessly convoluted, it is actually very good.

    I think you have confused the relationship between theory and reality, where the reality is in some loose sense the "cause" of the theory, with the lack of causality WITHIN the theory. I.e., just because we have a theory because of a corresponding reality does not necessarily mean the theory itself must be causal.
  6. Dec 21, 2012 #5
    The point is: can you build such a universe?
    If your particles are billiard balls, then probably you can, but it will be a totally different universe from the one we live in. It will display different phenomena.
    If your particles have wavelike properties, uncertainty will be unavoidable even if you didn't introduce fluctuations by hand (for the same exact reason why you can't pinpoint where an ocean wave begins).

    Because it doesn't work. What you are suggesting is that there are small-scale effects we don't know about that generate the apparent randomness. The problem is there's no way for such effects to yield the behaviour we see, at least if they are local. Google Bell's theorem.
  7. Dec 21, 2012 #6
    But scientists talk about virtual particle fluctuations all the time. So these fluctuations seem to be an accepted mainstream fact. It's just that people claim they are a consequence rather than a cause of Uncertainty, which I find bizarre. They seem to have put the cart before the horse, only because the cart was discovered first.

    Regarding Bell's Inequality, the random "noise" that masks the signal should be due to the virtual fluctuations I was mentioning. Perhaps there are more components to the virtual particle fluctuations than we can see (Einstein's "hidden variables"?) but it seems reasonable and logical to assume that the noise is caused by something. Meanwhile, fact that Bell's Inequality happens at all seems to indicate that the correlation/signal would be useful if it weren't for the seemingly random noise. It's only because the noise is unavoidably there that the signal isn't useful.
  8. Dec 21, 2012 #7
    Look, the only real problem here is that our words, the language we use, do not do justice to the actual theory, which is accurately described with mathematics.

    Sure, if someone says that the the virtual particle fluctuations happen because of Heisenberg's uncertainty principle, they are wrong. They just spoke poorly. What they mean is well known: that the theory permits those fluctuations, which are experimentally observed, and this is a testament to the quality of the theory.

    I really think you are repeatedly confusing yourself with your use of the word cause. Of course there may be something that causes the fluctuations, but the nature of the fluctuations CAN STILL BE PROBABILISTIC. It's like how an electron dropping to a lower energy level emits a photon, the photon travels as a wave, but when it interacts with another particle, boom it is a photon again. Yes, their was a cause of the photon's emission, but a PROBABILISTIC theory still governs the behavior of the photon whilst it is a wave!

    Just because there is a cause of something does NOT mean that the BEHAVIOR of the thing must obey causal relationships. The BEHAVIOR (which is what QM describes) can be and in many cases is PROBABILISTIC.
  9. Dec 22, 2012 #8
    In physics, it is often not a good question to ask which of two descriptions of a theory is "more fundamental". Is the Lagrangian principle "more fundamental" than Newton's axioms, or is it the other way round?
    In QM; you can show that the uncertainty principle is correct using some standard set of axioms, so you can derive it and say "it's less fundamental". You could possibly change your set of axioms to include the uncertainty principle and then it would be "more fundamental". These questions are almost meaningless to reality.
    The quantum fluctuations (which are not really "fluctuations" anyway) can be derived easily as a consequence of our favorite ways of writing dorn the fundamentals of the theory (e.g., the lagrangian of a QFT), so they seem to be "more fundamental", but that is just our way of looking at the problem.

    Read chapter 2 and 3 of Feynman's "Character of physical law" (you can also find the videos of the lectures somewhere on the web, I think).
  10. Dec 22, 2012 #9

    Vanadium 50

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    Also, I notice confusion between "scientists" and "writers of popularizations on science". The two are not the same.
    Last edited: Dec 22, 2012
  11. Dec 22, 2012 #10
    I have never felt quite comfortable with how people talk about virtual particles. They arise as a result of perturbation theory. I don't see any reason to think of them being anything more than a useful mathematical curiosity. The same thing applies to vacuum fluctuations. I would appreciate if someone could convince me otherwise.
  12. Dec 22, 2012 #11
    This question is badly posed. However, I think I know what you are asking. You are asking what a universe governed by classical physics would be like. You are asking what the most fundamental differences would be between a fully classical universe and the physical one we live in.

    My conjecture is that the biggest deviation between that universe and ours would be the absence of thermal equilibrium. If the universe were fully classical, with no quantum fluctuations, objects would continuously cool without limit.

    There would be no absolute zero temperature. There would be no third law of thermodynamics, and the second law of thermodynamics would be badly posed. Entropy itself would be an ambiguous concept.

    The so called ultraviolet catastrophe shows us what a world without quantum fluctuations would be like. In order for a black body cavity to be in thermal equilibrium, it would have to absorb an infinite amount of energy. However, there is no such thing as infinite energy. What would happen is that the black body cavity would keep on absorbing energy over an infinite time without coming to equilibrium. So there would be no such thing as a black body spectrum, not even a classical spectrum.

    The problem with a classical world is that everything has an infinite number of degrees of freedom. There are always finer and finer gradations in physical quantities. Every thing would be continuous, and nothing would be discrete. So the conserved quantities would distribute themselves over finer and finer length scales.

    There would be waves in a classical world. Just as you observe in nature, there would be sound waves, light waves, and other types of waves. Without a particle nature, these waves would be unstable. A long wavelength wave would be broken up by nonlinear interaction into superimposed short wavelength waves. Then these short wavelength waves would break up into shorter wavelength waves. This could go on forever in a classical universe.

    Waves would dominate physics. However, the waves would be "fragile". Wavelengths would keep getting shorter and shorter with no limit. Amplitudes would get shorter and shorter with no limit. There wouldn't even be atoms. The existence of atoms is a manifestation of the quantization of matter waves. So the classical universe would be completely fluid, never pausing long enough to observe it.

    The classical experiments in quantum mechanics would show different experimental results. However, I can’t think of a single experimental fact so important as the fact that systems come to thermal equilibrium. There would be no experiment that could really be finished in a finite amount of time.

    In a classical world, there would be no system that would truly be at rest. At best, systems could come to metastable states with periodic motion.

    Random fluctuations would be replaced by periodic fluctuations. Maybe random fluctuations would be replaced by chaotic fluctuations. However, these types of motion can sometimes be even harder to determine than random fluctuations.

    The “ergodic hypothesis” would be garbage. Since no system could come to thermal equilibrium, no system has to satisfy the ergodic hypothesis. Without the ergodic hypothesis, a lot of problems would be numerically harder.

    Certain things would be more certain in a classical universe. Life would be simpler in some ways without quantum fluctuations. However, certain things would get even harder to determine in a classical universe. “Classical fluctuations” become much more intractable without thermal equilibrium.

    Here are some links, quotes, and my interpretation of them with regards to the classical universe.

    “The ultraviolet catastrophe, also called the Rayleigh–Jeans catastrophe, was a prediction of late 19th century/early 20th century classical physics that an ideal black body at thermal equilibrium will emit radiation with infinite power.”

    Since there would be no such thing as infinite power, even in a classical universe, the ultraviolet catastrophe implies that there would be no thermal equilibrium.

    “In the limit T0→0 this expression diverges. Clearly a constant heat capacity does not satisfy Eq.(12). This means that a gas with a constant heat capacity all the way to absolute zero violates the third law of thermodynamics.
    The conflict is solved as follows: At a certain temperature the quantum nature of matter starts to dominate the behavior. Fermi particles follow Fermi-Dirac statistics and Bose particles follow Bose-Einstein statistics. In both cases the heat capacity at low temperatures is no longer temperature independent, even for ideal gases.”

    If there was no quantum nature of matter, there could not be a limit like “T0→0”. There would be no temperature where “the entropy of a crystal is always zero”.
    Last edited: Dec 22, 2012
  13. Dec 22, 2012 #12
    Why do you feel that "virtual particle fluctuations", cause uncertainty? Do you truly have an understanding of what particle fluctuations are? Do you believe they better explain observed phenomenon in the single and double slit experiments than the uncertainty principle?

    No one here believes virtual particles are caused by the UP, rather, they are allowed by it. The UP can be thought of as a sort of quantum form of momentum and energy conservation, which explains why we observe some quantum phenomenon that seem to violate classical conservation principles, SUCH AS quantum fluctuations. There is not really any causality involved. The uncertainty principle is a kinematic theory, but we need dynamic theories to explain why things happen the way they do.
  14. Dec 23, 2012 #13
    Rather than feeding me rhetoric, why not just explain why virtual fluctuations could not cause uncertainty? Give me the disproof, please.

    Fine, but I want to know whether the idea/belief that virtual fluctuations were occurring emerged from the Uncertainty Principle. Otherwise, how did we come to believe that virtual fluctuations were occurring?

    Yes, I understand that there's conservation, but within that conservation there is a latitude for fluctuation, and I want to know whether this can be more economically re-phrased as classical physics by giving precedence to the fluctuation over the Uncertainty Principle as the basis for Quantum Mechanics.
  15. Dec 23, 2012 #14

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    Sorry, but "here's my idea - it's your responsibility to prove it wrong" is a) not how science is done, and b) not how PF operates.

    Thread closed.
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