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Re: [Phys-l] Teaching Special Relativity

--- On Sat, 7/4/09, John Denker <jsd@av8n.com> wrote:
 "Consider the folks in Plato's cave.  They can directly measure the shadows, but they cannot directly measure the real objects.  I have to go with Plato on this one;  the objects, not the measurable shadows, are the physicist's "reality"."

   So, if one is getting frozen to death because of cold in a shadowed area, would you say to him/her that there is really nothing to worry about, because the shadow is not the physicist's reality?
  
"On 07/04/2009 07:49 AM, Moses Fayngold wrote in part:

... And how does one actually measure the invariant (rest) mass of a photon?

1) We can infer the mass in the usual way:  We can directly determine
the energy and momentum of the photon.  We can then construct the
(energy, momentum) 4-vector.  The gorm of this 4-vector, i.e. the
dot product of the 4-vector with itself, is a Lorentz invariant and
is (as always) equal to -m^2.  Theory and experiment show that m is
zero for photons.

1a) The theory is unequivocal.

1b) If you want high accuracy, the experiment is tricky, because it
involves a small difference between large numbers, but there are
ways to alleviate this problem."

  Sounds good to me. Then why not apply the same argument to the relativistic mass? Measure the momentum of an object, and its velocity. Take the ratio of their magnitudes. You will get the relativistic mass. The theory (and experiment!) is unequivocal and no more complicated than in the above quotation. 

">> Based on this, should we discard the notion of the invariant mass
for photons and use only their relativistic mass?

No"

Fine. Then, if I reverse my original question and ask "should we discard the notion of the relativistic mass for photons and use only their invariant mass? - I would expect from you the same answer - "No". That would be the only possible result of rigorous argument without inconsistencies or double standards.

Moses Fayngold,
NJIT

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