From: jac@ds8.scri.fsu.edu (Jim Carr)
Newsgroups: sci.physics
Subject: Re: Defining Photons
Keywords: Relating photons E=MC^2 criticism
Date: 26 Jul 92 21:15:52 GMT
Reply-To: jac@ds8.scri.fsu.edu (Jim Carr)
Organization: SCRI, Florida State University

In article <3942@cruzio.santa-cruz.ca.us> snarfy@cruzio.santa-cruz.ca.us writes:
>
>>For massless particles, it is the case that E=pc.
>
>       Your  criticism is noted and forwarded to my sources.  However my
>       personal evaluation is that you are missing the point.  When  the
>       word  ``particle''  is both applied to that which has no mass and
>       to that which does have mass,  we  must  seriously  question  the
>       choice of words used, for the sake of clarity ,if nothing else .

For the sake of clarity, I will state that the standard model used by 
particle and nuclear physicists has adopted the word "particle" to 
describe the things that appear in the model, massless or not.  These 
particles appear to obey certain dynamical equations which include the 
relativistic kinematic equation E^2 = m^2 + p^2 (in units where c=1). 
(I get tired of typing c^4 etc all the time when it is unnecessary.)

These dynamical equations imply that these particles propagate according 
to a "wave" equation, so that there are experimental conditions where 
these particles act in a way analogous to our observations of macroscopic 
waves.  Other conditions lead to results analogous to our observations 
of the behavior of macroscopic particles.  

Both massive objects (say an electron) and massless ones (say a photon) 
can appear to have either property.  Rather than deal with the language 
problems associated with this duality, particle physicists adopted a 
simpler scheme where all of these things are called particles.  This 
is quite convenient once you get used to it.  It certainly solves the 
problem of what to call the neutrino when you do not know for sure if 
it has zero mass or a very tiny mass.

>        We  are  not here trying to ``apply'' the equatiom E=Mc^2 to the
>       photon. The stated purpose and idea  is  to  ``relate''  massless
>       ``particles''  to  the  massive  ones  in the M of E= Mc^2 . If a
>       photon has E and E has some relationship to M via E=Mc^2 (or  E^2
>       =  m^2c^4  + p^2c^2) then there must some ``relationship'' that a
>       photon  has  to  M  that  can  be  expressed  or  measured.   The

Yes.  You measure p and E and solve for m and get zero.

>       expression  E=pc  contains no reference to M so that's one reason
>       E=Mc^2 was used. The other reason is that we want E on  one  side
>       of the equals sign and not E^2.

That is because M=0.  If you want an equation for E, just use 
E = sqrt{ p^2 + m^2 }.  There is a certain advantage to using an 
equation that has been verified as precisely as this one. 

>>I find it odd that someone can spend a lot of time working on such a
>>document that was posted here and never take the time to read enough
>>of Einstein to know the actual equations he used.
>
>       If you are a college professor , the taxpayers  or  students  pay
>       you to enlighten and clarify issues pertaining to your specialty.

I am a research scientist, and they do, and I hope I have done so above.
I thought I did in the original post, but I guess not.  It is still true 
that Einstein wrote E^2 = m^2c^4 + p^2c^2 for the general case. You 
wrote about Einstein's papers as if you had read them, and thus I was 
a bit harder on you than I would be otherwise. 

>       Why then have you not yet found a word other than ``particle'' to
>       describe  that  which has no mass , to differentiate it from that
>       which does?.  

We once used the word wave, and found it failed when we looked at Compton 
scattering.  We now find it more useful to use an adjective instead, as 
in "massless particle".  I suppose someone could have made up a word, 
perhaps one as picturesque as quark, to be used instead of particle in 
a construction such as "massless qwerty", but if someone did it was not 
adopted by the international physics community.  This is a definition we 
all use, and it is used that way in this forum because it would make no 
sense to the physicists here to use any other word.

>                     I think it's because some of you  would  prefer  to
>       confuse people rather than teach them. The photon, as a concept ,
>       would  be  more accurately described as a ``phenomena'' given the
>       difficulty  you  have  manipulating  it  the  same   way   as   a
>       ``particle''   mathematically.  (multiplying  by  zero  ,for  one
>       thing).

It is a phenomena, one with very well understood properties.  There are 
no difficulties manipulating the relevant equations with m=0, although 
there are difficulties manipulating other equations that are inappropriate 
when photons are involved.  Indeed, some of those difficulties (in 
particular the "what would it look like if I were traveling at the 
speed of light" question) led directly to the relativity theory. 

>      I find it odd that you have a job.

I do not.  It would be odd if anyone were to hire me in this field 
if I did *not* know and speak the common language of the field. 

--
J. A. Carr                                    |  "The New Frontier of which I  
jac@gw.scri.fsu.edu                           |  speak is not a set of promises
Florida State University  B-186               |  -- it is a set of challenges."
Supercomputer Computations Research Institute |   John F. Kennedy (15 July 60)