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Re: [Phys-l] Absolute four-momentum of massless particles

Some people prefer to phrase the 2nd postulate of SR (special relativity), less in terms of the speed of light or massless particles but in terms of a speed limit of causation. I guess this is in part due to the reasons the John mentions below.


Joel Rauber, Ph.D 
Professor and Head of Physics
Department of Physics
South Dakota State University
Brookings, SD 57007
605.688.5428 (w)
605.688.5878 (fax)

-----Original Message-----
From: [mailto:phys-l-] On Behalf Of John Denker
Sent: Friday, October 01, 2010 12:45 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Absolute four-momentum of massless particles

On 09/30/2010 08:23 PM, Derek McKenzie wrote:
On what basis do we declare a particle to have zero mass anyway?

What means "declare"? Strictly speaking, I reckon you
can declare anything you want.

At the other extreme, if you want to /prove/ that the
photon mass is zero, then you're probably out of luck.
According to Popper's view, which is nowadays a rather
conventional view in the scientific community, no theory
ever gets proved right. Roughly speaking: wrong theories
can be disproved, but valid theories always remain open
to further testing.

There is of course more to the story than that, but we
don't need to pursue it right now.

popular argument seems to be that assigning any positive mass leads
to a contradiction, but that only convinces me that the mass is
either zero for that particle OR meaningless.

We can do much better than that. A massless photon
means that electromagnetism is an infinite-range
interaction. A positive mass would dictate the range
of the interaction via the Yukawa formula. To say
the same thing in slightly different words, you would
see a departure from the 1/r^2 law.

This has been checked experimentally on the laboratory
length-scale and on cosmological length scales. In
all cases that have been checked, the exponent n in
the 1/r^n law is 2 within the precision of measurement,
and some of the measurements are good to one part in
10^12. Reference: Jackson; also

There is no 11th commandment that requires the photom
mass to be zero. Maybe tomorrow somebody will discover
that the photon has a mass ... but if so, it must be
reeeally small.

Note that neutrinos were for many years believed to be
massless, but are now believed to have a very small but
nonzero mass. This demonstrates that it is possible to
reclassify a particle from massless to non-massless.
Such a reclassification is not hard to carry out; it's
not like we've never seen a massive particle before.
On the other hand, such a reclassification is very
unusual and newsworthy.
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