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Re: [Phys-l] velocity-dependent mass (or not)

Hi all-
I try to teach students to use operational definitions of physical quanttities. The lifetime of muons is the answer you get from meaqsuring the decay lengths of a bunch of incident muons. The mass of an electron is the answer you get from measuring the radius of curvature of and electron track in a magnetic field.
The results of these meassurements depend on the speed of the particle being measured. It is useful, therefore, to refer these measurements to common standards, that is, the values in the PDB.
The process of comparing measured values with PDB values is useful for the identification of particles detected in an experiment.
Beyond this, I am unwilling to go. As long as the referent is unambiguous, I care not what you call it. A rose by any other name is just as fragrant, no matter what Einstein may have called it.

On Sun, 28 Jun 2009, Moses Fayngold wrote:

--- On Sun, 6/28/09, Jack Uretsky <> wrote:
"Hi all-
    Time dilatation is a corpse?"

This is what most of the opponents of relativistic mass say.

" Measure the lifetimes of some of the muons that are bombarding you in cosmic rays.  The answer you get will be greater than the value in the Particle Data Book, and will depend upon the velocity of the muon. Time dilation is a real observable."

  I am afraid the respect to simple and well known scientific evidence is, at best, obsolete if not dead at the hands of the same opponents.

  I truncated the text below to what included my argument, not John Denker's original one, which was sent unanswered due to glitch in my PC. My completed response was sent in my second message.

Moses Fayngold

On Sun, 28 Jun 2009, Moses Fayngold wrote:

--- On Sun, 6/28/09, John Denker <> wrote:
"It doesn't "come from" anywhere.  You shouldn't assume it needs to
"come from" anywhere.  Asking where it comes from has no physical
significance, because mass is not conserved.  There is no reason
why it should be."

   It comes from the past. If an isolated system (e.g. positronium) had a certain rest mass before reaction (e.g., annihilation), it has the same rest mass after reaction. In case of annihilation, the system of emerging two photons has the rest mass exactly equal to the initial mas of positronium. The system has changed beyond recognition, but its rest mass remains as before. The rest mass of the new state comes from the rest mass of the initial state.This is what conservation means.

  " Energy is conserved.  Rest energy (by
  itself) is not conserved.  There is no reason why it should be".

The reason is that the rest energy is also energy. Would you deny this?

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