Comments on FAQ - Does light have mass?
and...@attglobal.net
>
> 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.
> : 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.
> : In modern terminology the mass of an object is its invariant mass
> : which is zero for a photon.
>
> What is exactly the invariant mass of 10^24 molecules of hydrogen?
> Does invariant mass vary with temperature and chemical state?
>
Yes. The invariant mass is E/(c^2) where E is the total energy
of those 10^24 molecules in a reference frame where the total
momentum is zero.
Invariant does not mean independent of temperature or chemical
state. It means that it's independent of the relative velocity
of such a system and the observer. If you change the temperature,
it's not the same system.
> As far as I understand the relevant faq texts, invariant mass is
> a relative concept (not in the kinematic sense). Depending on the
> physical situation studied, the total mass is considered as composed
> of "invariant mass" and forms of energy. But it should be remembered
> that inertia and gravitational effects do not depend only on the
> "invariant mass" but on the sum of "invariant mass" and energy
> (i.e. total mass).
>
Sorry, but the implications of what you said are obscured
by the meaningless terminology. The mass of something is
the invariant mass. If it's a composite object, it's
E/(c^2) in the frame in which the total momentum of the
components is zero.
> : 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. Differernt
observers will measure the SAME object to have the same mass.
> : The definition of the invariant mass of an object is
> : m = sqrt{E^2/c^4 - p^2/c^2}.
>
> How is this formula applied in the case of a gas or in the case of a
> box containing very fast rotating objects?
>
Just take all the stuff in the box and measure it's total energy
including potential energy due to interactions and it's total
momentum.
> : Relativistic mass is equivalent to energy so it is a redundant
> : concept.
>
> It has only be renamed to "total mass", which admittedly is a better
> name.
>
It's still a misleading and confusing idea.
> : In the modern view mass is not equivalent to energy. It is just
> : that part of the energy of a body which is not kinetic energy.
> : Mass is independent of velocity whereas energy is not.
>
> What about this:
>
> In the modern view invariant restmass is not equivalent to
> restenergy. It is just that part of the restenergy of a body
> which is not thermal energy. Invariant restmass is independent
> of temperature whereas restenergy is not.
>
That's just not true. A hotter object has more invariant mass.
> : The mass is then independent of velocity and is closer to the old
> : Newtonian concept.
>
> Really? Most equations of classical physics (e.g. weight) depend
> rather on total mass than on invariant mass. E.g. inertia depends
> on total mass. Not even linear momentum of a rotating object depends
> on the object's invariant (non-rotational) mass.
>
Are you saying that the weight of something in Newtonian
gravity depends on it's velocity? That looks like what
you're saying to me. That's not true in Newtonian
gravity.
> : The interpretation most widely used is a compromise in which mass
> : is invariant and always has energy so that total energy is conserved
> : but kinetic energy and radiation does not have mass. The distinction
> : is purely a matter of semantic convention.
>
> It's a compromise making sense in several situations, but not more.
>
> : 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.
John Anderson
z@z
: 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.
: 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?
: In modern terminology the mass of an object is its invariant mass
: which is zero for a photon.
What is exactly the invariant mass of 10^24 molecules of hydrogen?
Does invariant mass vary with temperature and chemical state?
As far as I understand the relevant faq texts, invariant mass is
a relative concept (not in the kinematic sense). Depending on the
physical situation studied, the total mass is considered as composed
of "invariant mass" and forms of energy. But it should be remembered
that inertia and gravitational effects do not depend only on the
"invariant mass" but on the sum of "invariant mass" and energy
(i.e. total mass).
: 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).
: The definition of the invariant mass of an object is
: m = sqrt{E^2/c^4 - p^2/c^2}.
How is this formula applied in the case of a gas or in the case of a
box containing very fast rotating objects?
: Relativistic mass is equivalent to energy so it is a redundant
: concept.
It has only be renamed to "total mass", which admittedly is a better
name.
: In the modern view mass is not equivalent to energy. It is just
: that part of the energy of a body which is not kinetic energy.
: Mass is independent of velocity whereas energy is not.
What about this:
In the modern view invariant restmass is not equivalent to
restenergy. It is just that part of the restenergy of a body
which is not thermal energy. Invariant restmass is independent
of temperature whereas restenergy is not.
: The mass is then independent of velocity and is closer to the old
: Newtonian concept.
Really? Most equations of classical physics (e.g. weight) depend
rather on total mass than on invariant mass. E.g. inertia depends
on total mass. Not even linear momentum of a rotating object depends
on the object's invariant (non-rotational) mass.
: The interpretation most widely used is a compromise in which mass
: is invariant and always has energy so that total energy is conserved
: but kinetic energy and radiation does not have mass. The distinction
: is purely a matter of semantic convention.
It's a compromise making sense in several situations, but not more.
: 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?
The above example of the box containing photons has shown that photons
contribute in the same way to gravational effects as other energy forms
such as nuclear or thermal energy. If 'massless' particles generate
curvature of spacetime anyway, the question whether neutrinos are
declared massless or not becomes totally irrelevant to the dark matter
problem.
Wolfgang Gottfried G.
Liechteinstein / Europe
Aaron Bergman
>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.
How about "without mass". Maxwell's equations would be different
if the photon had a mass, for example.
>
>: 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?
All our theories allow it. The photon appears to be massless and
carries momnetum.
>
>: In modern terminology the mass of an object is its invariant mass
>: which is zero for a photon.
>
>What is exactly the invariant mass of 10^24 molecules of hydrogen?
Sum their energy momentum four vectors and take the Minkwoski
norm.
>Does invariant mass vary with temperature and chemical state?
With the above definition, yes.
>
>As far as I understand the relevant faq texts, invariant mass is
>a relative concept (not in the kinematic sense).
It's not relative -- it's invariant. That's where it comes from.
> Depending on the
>physical situation studied, the total mass is considered as composed
>of "invariant mass" and forms of energy. But it should be remembered
>that inertia and gravitational effects do not depend only on the
>"invariant mass" but on the sum of "invariant mass" and energy
>(i.e. total mass).
Actually, gravity depends on the full energy momentum tensor.
>
>: 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.
Not really. Read the last sentence of the paragraph.
> 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).
I don't see any point here.
>
>: The definition of the invariant mass of an object is
>: m = sqrt{E^2/c^4 - p^2/c^2}.
>
>How is this formula applied in the case of a gas or in the case of a
>box containing very fast rotating objects?
See above. When talking about multiple particles, things get a
little different. I'm using a definition (which may not be the
best) which is synonymous with center of mass energy. This has
the nice property of being frame invariant.
>
>: Relativistic mass is equivalent to energy so it is a redundant
>: concept.
>
>It has only be renamed to "total mass", which admittedly is a better
>name.
Huh?
>: In the modern view mass is not equivalent to energy. It is just
>: that part of the energy of a body which is not kinetic energy.
>: Mass is independent of velocity whereas energy is not.
>
>What about this:
>
> In the modern view invariant restmass is not equivalent to
> restenergy.
For a single particle, this is wrong. For multiple particles, one
has to be more specific in just what one means by rest mass.
>It is just that part of the restenergy of a body
> which is not thermal energy.
Thermal energy is just vibrations of molecules and similar stuff.
> Invariant restmass is independent
> of temperature whereas restenergy is not.
If you want to define restmass as the sum of the rest masses of
the individual particles, then this is true. If you use my
definition, then it is not.
>
>: The mass is then independent of velocity and is closer to the old
>: Newtonian concept.
>
>Really? Most equations of classical physics (e.g. weight) depend
>rather on total mass than on invariant mass.
No, it depends on a variety of factors.
>E.g. inertia depends
>on total mass.
Inertial depends on rate of change of momentum.
>Not even linear momentum of a rotating object depends
>on the object's invariant (non-rotational) mass.
Huh?
>
>: The interpretation most widely used is a compromise in which mass
>: is invariant and always has energy so that total energy is conserved
>: but kinetic energy and radiation does not have mass. The distinction
>: is purely a matter of semantic convention.
>
>It's a compromise making sense in several situations, but not more.
>
>: 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 speed of light in a medium is really just an effective
theory. Fundmentally, light still goes at c. Regardless, m^2 =
E^2 - p^2 is a definition.
There are situations (in plasmas, I think) where one can regard
the photon as gaining an effective mass, BTW.
>
>: 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's interesting? Because it might help us solve the
solar neutrino problem? Because it might provide evidence for
various GUTs? Because the rest mass of the neutrino would
consitute extra mass that could have a cosmological effect?
Howzat?
>
>The above example of the box containing photons has shown that photons
>contribute in the same way to gravational effects as other energy forms
>such as nuclear or thermal energy. If 'massless' particles generate
>curvature of spacetime anyway, the question whether neutrinos are
>declared massless or not becomes totally irrelevant to the dark matter
>problem.
No.
Aaron
--
Aaron Bergman
<http://www.princeton.edu/~abergman/>
Ben Wiens
What is the concrete meaning of 'massless' in the following context from a
FAQ: 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.
Ben Wiens reply:
Many of us in this newsgroup use the concept of relativistic-mass and
rest-mass. I point out in my web-booklet "Encyclopedia of Energy Science" at
http://www.benwiens.com that the definition of mass has changed over time.
Presently the word mass is usually defined as...the quantity of matter in a
body. This means that photons which do not contain matter are called
massless. To properly come to a conclusion about how mass should be defined
it is necessary to understand why we need the word mass to begin with. The
mass of a wavicle is...equivalent to what is called the inertia as well as
the gravitational attraction of a body. Inertia is...the resistance of a
body to any change in its state of motion. Gravitational attraction...is the
natural force of attraction between any two massive bodies. Because inertia
and the gravitational attraction are always proportional to each other, a
common term "mass" of a wavicle can be used in formulas to calculate the
forces that will be produced. Both matter wavicles such as atoms as well as
interactive wavicles such as photons have inertia and are affected by
gravity depending on their "mass". So it seems appropriate to use the common
term "mass" for all energy wavicles.
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.
* Sent from AltaVista http://www.altavista.com Where you can also find related Web Pages, Images, Audios, Videos, News, and Shopping. Smart is Beautiful
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 |
z@z
| Extracts from http://www.corepower.com/~relfaq/light_mass.html :
| : 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 explanation of light refraction by Huygens' principle is
a first-rate example of an effective (i.e. simple and elegant)
physical explanation.
This classical explanation of refraction requires that waves
propagate at different velocities depending on the refraction
coefficients of the media and that the waves can freely move
over distances larger than their wavelength.
In QED, the velocity of photons does not depend on constitutive
constants of the medium as in Maxwell's theory but is always c.
The progagation delays corresponding to refraction coefficients
are explained by regular absorption and reemission of photons.
The refraction coefficient of water at 20 degree Celsius is
n = 1.333 for visible light of a wave length of 590 nanometer.
So according to common sense physics, the wave length of photons
shrinks from around 590 nm to around 590 nm / 1.333 = 440 nm
when the photons penetrate water. But the diameter of water
molecules is only around 0.3 nm. So how can Huygen's principle
work, if photons are continuously absorbed and reemitted by
molecules?
The situation is especially strange as the path-integral method
of QED is essentially the same as Huygens principle. So in this
respect, QED is not even self-consistent!
A further problem is that water molecules are assumed to absorb
(and reemit at exactly the same frequency) a continuous spectrum
and not only discrete lines corresponding to discrete energy
levels.
Yes, I know, the work of Bohr, Heisenberg, Dirac, Feynman, and
others has demonstrated that only a fool can be naive enough to
believe that ordinary logic, conservation laws and similar common
sense principles are relevant to the strange world of quanta.
Sometimes I wonder whether Feynman, a gifted entertainer with
common sense, really believed in what he taught mankind. Maybe
virtual particles were at least partially intended as a good joke.
In claiming that particles can propagate backwards in time (from
the future to the past!) Feynman either ignores our philosophical
heritage or he makes fun of it (and of us).
But in proposing his path-integral method as an alternative to
Heisenberg's and Schroedinger's formulations of QM, Feynmen has
shown that QM is (apart from the Planck-Einstein-Bohr quantum
concept and from integrated experimental facts) essentially not
much more than an unwarranted generalization of the old principle
of Huygens.
Wolfgang Gottfried G.
Liechtenstein - 1999/11/11
Stephen
> Sometimes I wonder whether Feynman, a gifted entertainer with
> common sense, really believed in what he taught mankind. Maybe
> virtual particles were at least partially intended as a good joke.
No, they were intended to represent terms in a series. Feynman
diagrams, and the imagery of virtual particles used in these diagrams, are
a very useful tool for stating and solving problems.
--
"The end of our foundation is knowledge of causes,
and secret motions of things; and the enlarging of the bounds
of human empire, to the effecting of all things possible."
- Francis Bacon, "New Atlantis".
Gregory L. Hansen
Stephen <stephe...@deathtospam.hotmail.com> wrote:
>In article <80f4ee$pfq$1...@pollux.ip-plus.net>, "z@z" <z...@z.lol.li> wrote:
>
>
>> Sometimes I wonder whether Feynman, a gifted entertainer with
>> common sense, really believed in what he taught mankind. Maybe
>> virtual particles were at least partially intended as a good joke.
>
>
> No, they were intended to represent terms in a series. Feynman
>diagrams, and the imagery of virtual particles used in these diagrams, are
>a very useful tool for stating and solving problems.
I know where virtual particles came from, but I'm not so sure what the
thinking is on them in modern physics. People talk about virtual photons
carrying the electromagnetic force and virtual mesons carrying the weak
force, for instance. And there are experiments (I'm involved in one) that
even measure the coupling constants for those particles. The way it's
talked about sure makes it seem like there really are ghostly particles
flying around (not to mention ghost particles...). But I know they're
mathematical terms in a series, which makes me wonder if they've been
unjustly reified.
--
"That's not an avocado, that's a grenade!" -- The Skipper
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.
Gerry Quinn
The stuff fields are made of is neither particles nor waves, but has
aspects of both. I would say the particle aspects are as real as any
others. We know something leaks out over all of space anyway, so
something is flying around.
- Gerry Quinn
Charles Francis
>The situation is especially strange as the path-integral method
>of QED is essentially the same as Huygens principle. So in this
>respect, QED is not even self-consistent!
You have to distinguish self-consistency from conceptualisation. Self
consistency is a mathematical constraint, which is satisfied by qed. If
qed were saying that there really is a wave, then it would be
conceptually inconsistent. But it is only saying that there is a wave
formula which predicts experimental results. That is quite consistent.
>
>Sometimes I wonder whether Feynman, a gifted entertainer with
>common sense, really believed in what he taught mankind. Maybe
>virtual particles were at least partially intended as a good joke.
Feynman does not appear to have thought of them as virtual. That ideas
seems to come from theorists with a field theoretic inclination rather
than a particulate inclination.
>In claiming that particles can propagate backwards in time (from
>the future to the past!) Feynman either ignores our philosophical
>heritage or he makes fun of it (and of us).
>
No, he simply reports on a discovery with no other satisfactory
explanation. The being of the universe has no requirement to comply with
our philosophical heritage.
>But in proposing his path-integral method as an alternative to
>Heisenberg's and Schroedinger's formulations of QM, Feynmen has
>shown that QM is (apart from the Planck-Einstein-Bohr quantum
>concept and from integrated experimental facts) essentially not
>much more than an unwarranted generalization of the old principle
>of Huygens.
>
The path integral approach was billed as a mathematical trick,
equivalent to other approaches. It is only speculation that it may give
more insight.
--
Charles Francis
cha...@clef.demon.co.uk
Charles Francis
<glha...@steel.ucs.indiana.edu> writes
>In article <stephenwells-1...@mac008.joh.cam.ac.uk>,
>Stephen <stephe...@deathtospam.hotmail.com> wrote:
>>
>>
>>> Sometimes I wonder whether Feynman, a gifted entertainer with
>>> common sense, really believed in what he taught mankind. Maybe
>>> virtual particles were at least partially intended as a good joke.
>>
>>
>>diagrams, and the imagery of virtual particles used in these diagrams, are
>>a very useful tool for stating and solving problems.
>
>I know where virtual particles came from, but I'm not so sure what the
>thinking is on them in modern physics. People talk about virtual photons
>carrying the electromagnetic force and virtual mesons carrying the weak
>force, for instance. And there are experiments (I'm involved in one) that
>even measure the coupling constants for those particles. The way it's
>talked about sure makes it seem like there really are ghostly particles
>flying around (not to mention ghost particles...). But I know they're
>mathematical terms in a series, which makes me wonder if they've been
>unjustly reified.
>
complete and consistent theory of qed as particle interactions. This is
evidence of scientific caution, not unjust reification. But it is
possible to construct a complete and consistent theory of qed as
discrete particle interactions, and to show that this is a theory of
real particles.
http://xxx.lanl.gov/abs/physics/9905058
A Theory of Quantum Space-time
--
Charles Francis
cha...@clef.demon.co.uk
Tom Roberts
> Experimentally confirmed masslessness? What is the concrete meaning
> of 'massless' in this context.
Experimentally, the 1994 Particle Data Group's limit on the photon mass
is 3*10^-27 eV. For comparison, the electron's mass is 510999 eV, so
that is miniscule indeed.
> What is the concrete evidence that momentum can exist without mass?
The fact that light can push objects around, and yet the mass of the
photon is miniscule indeed.
> What is exactly the invariant mass of 10^24 molecules of hydrogen?
Your question is not well-posed. The invariant mass of 10^24 molecules
of hydrogen can be anything from 10^24 times the mass of of a hydrogen
molecule up. The invariant mass of a collection of objects depends upon
their masses and their velocities in their overall center-of-mass frame.
> Does invariant mass vary with temperature and chemical state?
Yes to both.
> As far as I understand the relevant faq texts, invariant mass is
> a relative concept (not in the kinematic sense).
You sure use words funny - nobody else considers it to be a "relative"
concept. In modern physics the term "relative" usually means "differs
depending upon which frame you use to look at it", and invariant mass
is definitely not "relative" in that sense.
> Depending on the
> physical situation studied, the total mass is considered as composed
> of "invariant mass" and forms of energy.
You need to be more precise. But yes, given two free atoms of mass M1
and M2, if they combine chemically their invariant mass will be less
than M1+M2. Of course during the process of their combining they will
release some energy, and in their center-of-mass frame this released
energy will equal M1+M2 minus the invariant mass of the molecule
(assuming the molecule decays to its ground state). Energy is conserved,
after all.
> But it should be remembered
> that inertia and gravitational effects do not depend only on the
> "invariant mass" but on the sum of "invariant mass" and energy
> (i.e. total mass).
It is significantly more complicated than you say.
> : Relativistic mass is equivalent to energy so it is a redundant
> : concept.
> It has only be renamed to "total mass", which admittedly is a better
> name.
No, it is not. Energy is the proper name for "relativistic mass",
because that is what it _IS_. That is, the usual "relativistic mass"
of an object is identical in value to the time component of the
object's 4-momentum; in the non-relativistic limit, this component is
equal to the object's rest mass plus its kinetic energy, and so this
component is called "energy" in relativity.
You really should sit down with an elementary textbook on relativity
and learn what SR really is, and not rely on your guesses and
misinterpretations. It is not possible to learn physics via "20
questions" as you seem to be attempting here. I suggest:
Taylor and Wheeler, _Spacetime_Physics_.
> In the modern view invariant restmass is not equivalent to
> restenergy.
Wrong - an object's invariant restmass _IS_ the energy of the object
in the frame in which it is at rest.
> It is just that part of the restenergy of a body
> which is not thermal energy.
Not true, either. See above. The object's restmass depends upon its
temperature. BTW temperature of an object is an invariant, and is
always measured in its rest frame. For an object composed of multiple
parts, its rest frame is really its center-of-mass frame; calling a
collection of molecules of gas an "object" is pushing the limits of
that word a bit.
> Invariant restmass is independent
> of temperature whereas restenergy is not.
Wrong. See above.
Again, you need to actually learn the vocabulary of modern physics.
Your guesses are completely inadequate.
> Most equations of classical physics (e.g. weight) depend
> rather on total mass than on invariant mass. E.g. inertia depends
> on total mass.
Again, completely wrong. In particular, classical physics contains
no invariant mass in the sense of relativity, because classical
physics is Galilean invariant, not Lorentz invariant.
> Not even linear momentum of a rotating object depends
> on the object's invariant (non-rotational) mass.
Classically, the (linear) momentum of an object is mv, rotating or not.
Mass is clearly in there. It appears you need to learn not only SR but
Newtonian mechanics as well.
> 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)
No. You need to learn some physics, and stop making apparently random
guesses about well-known phenomena.
> and that
> a photon transfers a part of its momentum to a lens when it gets into
> it.
This is true.
> If this is true, the general validity of m^2 = E^2/c^4 - p^2/c^2
> is refuted, isn't it?
No. You need to learn at least the basics of QED before you can hope to
understand this. I suggest:
Feynman, _QED_.
> But why then is the question whether neutrinos have mass or not so
> important?
Because the masses of the neutrinos are fundamental physical parameters
of the standard model, and knowledge of them is important in applying
it to experimental observations. remember that it is experimental
observations which have opened the question of their masses in recent
months.
> If 'massless' particles generate
> curvature of spacetime anyway, the question whether neutrinos are
> declared massless or not becomes totally irrelevant to the dark matter
> problem.
Only in your naive "sound bite" approach to physics. In actual
computations, the energy-momentum tensor for massless objects is
significantly different (i.e. traceless) than for massive objects,
and this has a profound effect on the question of dark matter. It is
still not known whether or not non-zero neutrino masses could account
for the dark matter problem.
Tom Roberts tjro...@lucent.com
Tom Roberts
> The explanation of light refraction by Huygens' principle is
> a first-rate example of an effective (i.e. simple and elegant)
> physical explanation.
Yes. And QED provides a one-level-deeper explanation of why Huygens'
principle is valid.
> In QED, the velocity of photons does not depend on constitutive
> constants of the medium as in Maxwell's theory but is always c.
> The progagation delays corresponding to refraction coefficients
> are explained by regular absorption and reemission of photons.
Sort of. It's significantly better to think of this as interference of
all possible interactions between the photons and the charged particles
of the medium (see below).
> The refraction coefficient of water at 20 degree Celsius is
> n = 1.333 for visible light of a wave length of 590 nanometer.
> So according to common sense physics, the wave length of photons
> shrinks from around 590 nm to around 590 nm / 1.333 = 440 nm
> when the photons penetrate water. But the diameter of water
> molecules is only around 0.3 nm. So how can Huygen's principle
> work, if photons are continuously absorbed and reemitted by
> molecules?
It is your "sound bite" approach to physics which is at fault. When
one considers the properties of photons in water as an interference
effect among all the electrons and protons in the water, the importance
of the diameter of the individual molecules vanishes. This is a _bulk_
effect, and not an individual-molecule effect.
If you think about it, this is a bulk effect classically as
well. Think more about it, and you will see that the accuracy
of the classical description pretty much requires this to be a
bulk (i.e. multiparticle) effect in QED.
> The situation is especially strange as the path-integral method
> of QED is essentially the same as Huygens principle. So in this
> respect, QED is not even self-consistent!
Not true. Learn what QED really is. As I have said before, you need to
actually _LEARN_ what these physical theories say before making such
conclusions.
> A further problem is that water molecules are assumed to absorb
> (and reemit at exactly the same frequency) a continuous spectrum
> and not only discrete lines corresponding to discrete energy
> levels.
Actually, to compute the behavior of photons in water, no atomic
transitions are used at all. Your model of "absorbtion and subsequent
emission" is blatantly violated in QED, because the emission can occur
_BEFORE_ the absorbtion (one must sum over all Feynman diagrams, and
each diagram includes an integral over all possible 4-momenta; when
expressed in space-time this implies that the interactions are not
well-ordered in time, and all possible orderings occur with equal
likelihood).
> In claiming that particles can propagate backwards in time (from
> the future to the past!) Feynman either ignores our philosophical
> heritage or he makes fun of it (and of us).
He did not ignore it, he (and the other founders of QED) discovered
that this "heritage" (as you call it) is not applicable here. Note
that that "heritage" was acquired in everyday experience, which does
not include quantum effects at all (directly). Only a fool would
expect/insist that our common-sense heritage would apply in regimes
far removed from the domain in which that heritage was acquired;
this applies to the quantum world, and to relativiity, and to extreme
low or high temperatures, and to extreme high or low pressures, and
to....
Your criticisms would be more effective if you actually _UNDERSTOOD_
what you are attempting to criticise. But then, your criticisms would
vanish.
Tom Roberts tjro...@lucent.com
Charles Francis
<tjro...@lucent.com> writes
>
>> In claiming that particles can propagate backwards in time (from
>> the future to the past!) Feynman either ignores our philosophical
>> heritage or he makes fun of it (and of us).
>
>He did not ignore it, he (and the other founders of QED) discovered
>that this "heritage" (as you call it) is not applicable here. Note
>that that "heritage" was acquired in everyday experience, which does
>not include quantum effects at all (directly).
Indeed, it would be a good thing if the anti quantum theorists and anti
relativists would work through the reasoning which shows that classical
ideas can be eliminated by thinking in terms of classical ideas, as the
founders of modern theories did, instead of dismissing as nonsense
arguments which they have not troubled themselves to understand.
--
Charles Francis
cha...@clef.demon.co.uk
Luc Bourhis
> "z@z" wrote:
> > Experimentally confirmed masslessness? What is the concrete meaning
> > of 'massless' in this context.
>
> Experimentally, the 1994 Particle Data Group's limit on the photon mass
> is 3*10^-27 eV.
This value is the one found by CHIBISOV from the measurement of the
Galactic magnetic Field. Taking into account other experiments, the 1999
Particle Data Group's limit is 2 10^-16 eV ...
> For comparison, the electron's mass is 510999 eV, so
> that is miniscule indeed.
... a value which does change this conclusion.
--
Luc Bourhis
Center for Particle Physics / University of Durham
United Kingdom
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
Jim Carr
:
: 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.
In article <7vqlu3$7pl$1...@pollux.ip-plus.net>
"z@z" <z...@z.lol.li> writes:
>
>Experimentally confirmed masslessness? What is the concrete meaning
>of 'massless' in this context.
That the measurement is consistent with zero to within
experimental uncertainties.
: 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?
Nonsequitur. The evidence spoken of in the first sentence is
experimental data on light pressure. The second statement says
that there is a theory that allows p = E/c when m=0.
: 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.
No, it confirms that 'invariant mass' is an invariant, if
you pay attention to the details or work out an example.
: The definition of the invariant mass of an object is
: m = sqrt{E^2/c^4 - p^2/c^2}.
>How is this formula applied in the case of a gas or in the case of a
>box containing very fast rotating objects?
By performing vector addition first (since that is actually
a statement about the length of a 4-vector.
: Relativistic mass is equivalent to energy so it is a redundant
: concept.
>It has only be renamed to "total mass", which admittedly is a better
>name.
Do you see Newton use mass in a way that makes it depend on v?
Do chemists? Engineers? No. Hence bad idea.
> In the modern view invariant restmass is not equivalent to
> restenergy. It is just that part of the restenergy of a body
> which is not thermal energy.
Nope.
: The mass is then independent of velocity and is closer to the old
: Newtonian concept.
>Really?
Yes.
>Most equations of classical physics (e.g. weight) depend
>rather on total mass than on invariant mass. E.g. inertia depends
>on total mass.
Which is invariant.
z@z
|>: 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.
|
| That the measurement is consistent with zero to within
| experimental uncertainties.
If 'massless' has no concrete and transparent meaning, then it is
irrelevant whether we say a neutrino or a photon has mass or not.
I'm interested in the concrete physical properties which are
associated with 'mass', and not whether it does or does not make
sense to call 'mass' a physical quantity arising in a complicated
and rather obscure theoretical framework.
At least in pre-Maxwellian physics, mass was considered
proportional to inertia and to gravitational effects and it was
subject to a conservation law.
|>: 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.
|
| No, it confirms that 'invariant mass' is an invariant, if
| you pay attention to the details or work out an example.
'Invariant mass' is misleading. 'Restmass' would be a better and
more honest name because 'invariant' only refers to the motion of
the mass center. 'Invariant mass' varies with temperature, rotation,
chemical state, electric polarization and so on.
|>: Relativistic mass is equivalent to energy so it is a redundant
|>: concept.
|
|> It has only be renamed to "total mass", which admittedly is a better
|> name.
|
| Do you see Newton use mass in a way that makes it depend on v?
| Do chemists? Engineers? No. Hence bad idea.
Do you see Newton use mass in a way that makes it depend on
temperature? Do chemists? Engineers?
A big problem of physics since the creation of the concept energy
in the nineteenth century is the split between absolute and
relative energy forms. The mass-energy-equivalence has further
aggravated this problem.
The currently 'official' solution states that only absolute energy
forms are equivalent to mass.
I think that the best solution consists in assuming that also
'relative' energy forms such as kinetic energy are absolute inasfar
as their absolute values depend on the motion relative to the
surrounding matter systems (particles) according to a simple
quantified version of Mach's principle.
I'm quite sure that such a view will replace the currently
prevailing one.
| > In the modern view invariant restmass is not equivalent to
| > restenergy. It is just that part of the restenergy of a body
| > which is not thermal energy.
|
| Nope.
Tom Roberts has cannibalized this out-of-context quote much more
effectively than you do here.
See his post http://www.deja.com/=dnc/getdoc.xp?AN=548331168
Cheers, Wolfgang
My previous posts of this thread:
http://www.deja.com/=dnc/getdoc.xp?AN=544192703
http://www.deja.com/=dnc/getdoc.xp?AN=546039265
http://www.deja.com/=dnc/getdoc.xp?AN=547437435
Jim Carr
>
>Many of us in this newsgroup use the concept of relativistic-mass and
>rest-mass.
Those would probably be the "many" that do not work with
relativity when doing physics, or read the journals regularly.
>I point out in my web-booklet "Encyclopedia of Energy Science" ...
You should read the section on relativistic kinematics by the PDG
or the book with the same title by Hagedorn. Then note that these
are the conventions used in physics today.