Comments on FAQ - Does light have mass?
z@z
| z@z wrote:
| > Extracts from http://www.corepower.com/~relfaq/light_mass.html :
| >
| > : The photon is a massless particle. According to theory it has energy
| > : and momentum but no mass and this is confirmed by experiment to
| > : within strict limits.
| >
| > Experimentally confirmed masslessness? What is the concrete meaning
| > of 'massless' in this context.
|
| In SR, E^2 = (p*c)^2 + (m*(c^2))^2
|
| You measure the energy and momentum and if E = p*c, then the
| particle is massless.
No other concrete meaning than the one resulting from postulating
the validity of an equation?
Neither energy nor momentum are frame independent. Could we be
certain that the equation E = p*c holds in all frames if its
validity were experimentally confirmed in one frame?
| > : Even before it was known that light is composed of photons it was
| > : known that light carries momentum and will exert a pressure on a
| > : surface. This is not evidence that it has mass since momentum can
| > : exist without mass.
| >
| > What is the concrete evidence that momentum can exist without mass?
|
| Photons, if you use the SR relation above.
That's not concrete evidence but an argument from definition.
[snip]
| > : If we now return to the question "Does light have mass?" this
| > : can be taken to mean different things if the light is moving
| > : freely or trapped in a container.
| >
| > This statement only confirms that 'invariant mass' is a relative
| > concept. If we consider the container and the radiation as a
| > unity, we get only invariant mass and no energy. But we also can
| > distinguish between "invariant mass" of the box and the energy
| > corresponding to its contained radiation (and temperature).
|
| No, it's invariant in the sense defined above.
Assume the emergence of a pair of photons propagating in opposite
directions. 'Invariant mass' is a relative (philosophical sense)
concept insofar as it depends on us whether we consider the two
photons as a unit having 'invariant mass', or whether we consider
them as two separate photons without 'invariant mass'.
| Different observers will measure the SAME object to have the same mass.
This statement shows that measurements themselves are often
rather meaningless whereas their interpretation is crucial. Modern
theoretical physics like medieval theology (which like physics
today was the 'hardest' science) is rather an interpretational than
an experimental science.
[snip]
| > : A massless particle can have energy E and momentum p because mass is
| > : related to these by the equation m^2 = E^2/c^4 - p^2/c^2 which is
| > : zero for a photon because E = pc for massless radiation.
| >
| > What about photons in media with refraction coefficients n > 1. I
| > suppose that the momentum of a photon is then p = f*h/(c*n) and that
| > a photon transfers a part of its momentum to a lens when it gets into
| > it. If this is true, the general validity of m^2 = E^2/c^4 - p^2/c^2
| > is refuted, isn't it?
| >
| > : The energy and momentum of light also generates curvature of space-
| > : time so according to theory it can attract objects gravitationally.
| >
| > But why then is the question whether neutrinos have mass or not so
| > important?
|
| Because it would tell you something about neutrinos, not massless
| particles in general. There are experiments that measure neutrinos
| from the sun that would be simpler to explain if neutrinos
| were massive. I can't think of any experiment that would be
| easier to explain if photons were massive.
According to classical physics, gravity and weight are
attributes of mass and not of energy. Therefore photons have
mass (corresponding to the total energy wrt e.g. the CMBR frame)
at least insofar as they generate curvature of space-time.
In the case of neutrinos however, it is assumed that their total
mass does not generate curvature of space-time. Therefore the
question whether they have also 'invariant mass' is so important
to current theories.
CONCRETE statements concerning total and invariant mass are:
In the case of PHOTONS, TOTAL MASS is responsible for
gravitational effects.
In the case of NEUTRINOS, INVARIANT MASS is responsible
for gravitational effects.
In the case of ordinary matter, the situation is very
complicated and nobody really knows ...
Gruss, Wolfgang
Thread backwards:
http://www.deja.com/=dnc/getdoc.xp?AN=544243243 (ande452)
http://www.deja.com/=dnc/getdoc.xp?AN=544192703 (z@z)
Bernd Schuller
"z@z" wrote::
>
> John Anderson wrote:
> | z@z wrote:
>
> | > Extracts from http://www.corepower.com/~relfaq/light_mass.html :
> | >
> | > : The photon is a massless particle. According to theory it has energy
> | > : and momentum but no mass and this is confirmed by experiment to
> | > : within strict limits.
> | >
> | > Experimentally confirmed masslessness? What is the concrete meaning
> | > of 'massless' in this context.
> |
> | In SR, E^2 = (p*c)^2 + (m*(c^2))^2
> |
> | You measure the energy and momentum and if E = p*c, then the
> | particle is massless.
>
> No other concrete meaning than the one resulting from postulating
> the validity of an equation?
Physically, the masslessness of the photon is equivalent to the infinite
range of the electromagnetic interaction. As a consequence, the best
experimental
limits come from astrophysics.
> Neither energy nor momentum are frame independent. Could we be
> certain that the equation E = p*c holds in all frames if its
> validity were experimentally confirmed in one frame?
Yes. That's what special relativity is about. The equation
E^2=(pc)^2+(mc^2)^2 is
frame independent.
> | > : Even before it was known that light is composed of photons it was
> | > : known that light carries momentum and will exert a pressure on a
> | > : surface. This is not evidence that it has mass since momentum can
> | > : exist without mass.
> | >
> | > What is the concrete evidence that momentum can exist without mass?
> |
> | Photons, if you use the SR relation above.
>
> That's not concrete evidence but an argument from definition.
No. It's an experimental fact, not an argument.
> [snip]
> This statement shows that measurements themselves are often
> rather meaningless whereas their interpretation is crucial. Modern
> theoretical physics like medieval theology (which like physics
> today was the 'hardest' science) is rather an interpretational than
> an experimental science.
What do you mean by "modern theoretical physics" ? Special relativity ??
Come on ! Where have you been these last 100 years ?
>
> [snip]
> According to classical physics, gravity and weight are
> attributes of mass and not of energy. Therefore photons have
> mass (corresponding to the total energy wrt e.g. the CMBR frame)
> at least insofar as they generate curvature of space-time.
>
> In the case of neutrinos however, it is assumed that their total
> mass does not generate curvature of space-time. Therefore the
> question whether they have also 'invariant mass' is so important
> to current theories.
According to classical physics, most of the experiments concerning
relativity
(and quantum physics) can't be explained at all. That's why S.R. is
around in
the first place...
> CONCRETE statements concerning total and invariant mass are:
>
> In the case of PHOTONS, TOTAL MASS is responsible for
> gravitational effects.
>
> In the case of NEUTRINOS, INVARIANT MASS is responsible
> for gravitational effects.
>
> In the case of ordinary matter, the situation is very
> complicated and nobody really knows ...
In *all* cases:
The Energy-momentum tensor is responsible for gravitational effects.
Though the situation
*is*very complicated...
CU,
Bernd.
Anj
asking my science teacher this morning if something could
have kenetic energy and no mass. My example was photons.
She just told me photons had not yet been proved.
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Matthew Nobes
> you basically took the words out of my mouth. I was just
> asking my science teacher this morning if something could
> have kenetic energy and no mass. My example was photons.
> She just told me photons had not yet been proved.
I don't know what you were responding to here, but tell your science
teacher that she needs to go back to school.
Tell her to look up the fields of quantum optics, or quantum
electrodynamics. (and ask for your money back)
-------------------------------------------------------------------------------
|Matthew Nobes
|c/o Physics Dept.
|Simon Fraser University
|8888 University Drive
|Burnaby, B.C.
|Canada
www.geocities.com/CollegePark/campus/1098 |
Jim Carr
You appear to be addressing an article written by "z" that
mainly twists words, making little effort to convey the
core ideas of invariance in relativity and the role of
experiment in physics.
>I was just
>asking my science teacher this morning if something could
>have kenetic energy and no mass. My example was photons.
>She just told me photons had not yet been proved.
Both your teacher and "z" are in error. I suggest you read
the Relativity and Physics FAQs (and perhaps review some of
the philosophy of science regarding the use of the word
"proved" in the context of experiment) on these subjects.
--
James A. Carr <j...@scri.fsu.edu> | Commercial e-mail is _NOT_
http://www.scri.fsu.edu/~jac/ | desired to this or any address
Supercomputer Computations Res. Inst. | that resolves to my account
Florida State, Tallahassee FL 32306 | for any reason at any time.
Anj
yet"
It was a quote.
I don't think you can complain about people twisting words
around, you turn my question:
Does a particle have to have a mass to have kenetic energy
, into something else.
Oh and for a matter of interest what is photonic energy
other than the energy of a photon?
remember i am only 14