Re: [asa] Results of Cameron's Survey

From: George Murphy <>
Date: Mon Jun 29 2009 - 22:10:56 EDT

David -

The question about the reality of virtual particles isn't the same as that about the possibility of simultaneous position & momentum values. The latter is forbidden in QM by the fact that position & momentum are represented by operators that don't commute. (Mathematically that's where the uncertainty principle comes from.) If you could have x = 5 and p = 3 (in appropriate units) then you'd have px = 15 = xp. The existence of virtual particles is sometimes explained in terms of the energy-time uncertainty principle. If there's not enough energy to make a real particle, a virtual particle (or pair, &c) can exist for a very short time if the product of that time & the energy are less than h.

We can observe the indirect effects of virtual particles. These give rise, e.g., to shifts in atmoic energy levels that can be observed & whose magnitude is in good agreement with theory (Lamb shift). Furthermore, application of sufficient energy can "realize" virtual particles. That's one way of describing the Hawking radiation from a black hole. When virtual pairs form near the event horizon, the gravitational field can pull them apart, one of the particles falling into the hole - thuis decreasing its mass - & the other escaping & constituting the Hawking radiation.

I'm not sure what you mean about muon decay. It usually goes into an electron, a muon neutrino and an electron anti-neutrino, though there are some other rare decay modes.


  ----- Original Message -----
  From: David Clounch
  To: George Murphy
  Cc: Iain Strachan ;
  Sent: Monday, June 29, 2009 12:33 PM
  Subject: Re: [asa] Results of Cameron's Survey

  George wrote:

  "but is a statement that position-momentum pairs that violate it don't exist. "

  George, my question is (because I don't know the rules) is this true for virtual particles as well?
  The reason I ask is it seems to me virtual particles exist ontologically but not in a way we could ever observe.

  [an aside]
  My son was telling me about muon decay and how it produces a number of virtual particles, and there are always a neutrino/anti-neutrino virtual pair produced, but these almost always destroy each other. But about every millionth time they don't and the neutrino exits. And these neutrinos are always left-handed. He was trying to tell me about the fact that all neutrinos are left handed but there is no law or rule of nature as to why it should be that way - that nobody knows why. My point is there are at least 10^6 virtual pairs for every real neutrino.
  [end aside]

  Obviously scientists believe in virtual particles. But, if we can never observe them, are they really there? Aren't they beyond the edge of MN? aren't they just something we pretend are there so we can get a logical reason for things we are able to see (the observed products).

  I agree with your statement as applied to real particles. Its not just about limits of knowledge, but is a real statement about existence. I'm just wondering if the same QM rules apply to virtual particles.

  If virtual particles are just a game and don't have ontological existence, then under the rules of MN aren't they really in the same category as other beyond the (playground) edge topics (like ID is alleged to be)? If the answer is yes, then are teachers allowed to discuss these areas where God plays dice?

  On Mon, Jun 29, 2009 at 11:10 AM, George Murphy <> wrote:

    Iain -

    This gets into the murky area of the interface between classical chaos theory & quantum mechanics, where I claim no great expertise. Since quantum mechanics is linear there's no "quantum chaos" in a straightforward sense. But in specifying the initial conditions more & more precisely for a classical system, as you suggest below, you'll eventually get to the limit specified by the uncertainty principle, & below that point you can't go. With a strong interpretation of QM even God can't because the uncertainty principle is not just about limits on what we can measure but is a statement that position-momentum pairs that violate it don't exist.


      ----- Original Message -----
      From: Iain Strachan
      To: George Murphy
      Sent: Monday, June 29, 2009 11:13 AM
      Subject: Re: [asa] Results of Cameron's Survey

      On Mon, Jun 29, 2009 at 3:52 PM, George Murphy<> wrote:
> You can put me down with Terry's #4. (Or, if only the 3 answers given
> originally are allowed, as is usually the case with standardized tests, I'll
> take #3 "under protest.")
> I would not absolutely rule out the idea of front-loading of design (#2) but
> it seems to me that hardwiring the detailed outcome of any physical process
> into its initial conditions billions of years in advance is just the sort of
> thing that chaos theory - which is more precisely "sensitivity to initial
> conditions" - rules out for systems of any complexity (i.e., nonlinearity).


      Just a quick query here. Is it not the case that it's not ruled out so much as non-computable

      For example if I try to integrate the third order non-linear differential equations for the Lorenz Attractor then I experience that if a slight change is made to the initial values of the state variables, then after a certain time, two runs that are otherwise the same diverge and bear no resemblance to each other. But the smaller the initial delta, the longer it will take to diverge. However, divergences can be observed even at the machine precision level, for example if I change X0 to X0*(1+eps) where eps is the smallest constant so that in the machine 1+eps > eps. (In standard double precision arithmetic eps has the value 2.2204e-016).

      But if one had a machine with billions of bits of precision for the arithmetic, instead of 53 (double precision arithmetic has 53 bits for the mantissa, 10 for the exponent and 1 for the sign, making 64 in total), then it's clear that macroscopic changes in the outcome will only appear after an immensely long time because the corresponding value of eps is so much smaller.

      So I wouldn't have said it was ruled out unless God only uses 64 bit arithmetic!


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Received on Mon Jun 29 22:11:51 2009

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