| Topic: |
Science > Physics |
| User: |
"" |
| Date: |
26 Mar 2005 11:54:10 AM |
| Object: |
Photon Energy...Huh? |
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't get
farther.
Thanks.
BP
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| User: "PD" |
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| Title: Re: Photon Energy...Huh? |
28 Mar 2005 09:07:31 AM |
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wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it
aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't get
farther.
Thanks.
BP
A much better definition of energy is that quantity which, when added
to other contributions to the energy, is conserved in a closed system.
A similar but independent definition applies to momentum.
In *low-speed* applications, it turns out that (1/2)m*v^2 is a good
approximation to the kinetic energy contribution to the total energy,
and that m*v is a good approximation to the single-particle
contribution to linear momentum. But definition? No.
PD
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| User: "" |
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| Title: Re: Photon Energy...Huh? |
28 Mar 2005 02:17:15 PM |
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In article <1112022451.154874.42250@z14g2000cwz.googlegroups.com>, "PD" <pdraper@yahoo.com> writes:
bruce_pearson@hotmail.com wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it
aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't get
farther.
Thanks.
BP
A much better definition of energy is that quantity which, when added
to other contributions to the energy, is conserved in a closed system.
That's a tad circular. Try "the end point variation of the action
under time translation" or, alternatively, "the generator of
infinitsimal time translations".
A similar but independent definition applies to momentum.
Just the same, in fact, with time translation replaced by space
translation.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "PD" |
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| Title: Re: Photon Energy...Huh? |
28 Mar 2005 02:49:19 PM |
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wrote:
In article <1112022451.154874.42250@z14g2000cwz.googlegroups.com>,
"PD" <pdraper@yahoo.com> writes:
bruce_pearson@hotmail.com wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in
"Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it
aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't
get
farther.
Thanks.
BP
A much better definition of energy is that quantity which, when
added
to other contributions to the energy, is conserved in a closed
system.
That's a tad circular. Try "the end point variation of the action
under time translation" or, alternatively, "the generator of
infinitsimal time translations".
I'm not sure mine's so circular, but it's also not so useful. Yours is
more useful, but it offers little physics insight to the untrained.
I think my point to the neophyte would be this:
In a closed system (where this can easily be defined), we note that
there are certain quantities which, when added up for the system, turn
out to remain constant in the system, no matter what happens inside the
system. This is quite a remarkable statement, especially the last
clause. Why this is so is not completely understood, but it is true
nonetheless. The best we can do is say that these "conservation laws"
are related to symmetries exhibited by the laws that govern the set of
forces acting within the system, but this really only changes the
question from why those quantities are conserved to why nature exhibits
those symmetries, though the latter seems to have more intuitive
appeal. The trick, of course, is figuring out how to calculate the
quantities which, when added up, are conserved. Historically, this has
amounted to a combination of stumbling on the right thing (or something
very close to the right thing) or knowing some details about the system
(e.g. the Lagrangian of the system, which in turn tells you something
about the symmetries of the system).
The fundamental issue is "Does how to calculate it amount to a
definition?" I maintain not, otherwise all these folks that say
"momentum is m*v" would argue they have a definition. Likewise, coming
up with a guaranteed-to-work recipe for how to calculate it under any
conditions also presumes knowing more about the system (e.g. the
Lagrangian). I favor a *definition* that describes why the quantity is
important, but that's just me.
PD
A similar but independent definition applies to momentum.
Just the same, in fact, with time translation replaced by space
translation.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the
same"
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| User: "" |
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| Title: Re: Photon Energy...Huh? |
28 Mar 2005 03:20:28 PM |
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In article <1112042959.748229.190980@o13g2000cwo.googlegroups.com>, "PD" <pdraper@yahoo.com> writes:
mme...@cars3.uchicago.edu wrote:
In article <1112022451.154874.42250@z14g2000cwz.googlegroups.com>,
"PD" <pdraper@yahoo.com> writes:
bruce_pearson@hotmail.com wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in
"Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it
aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't
get
farther.
Thanks.
BP
A much better definition of energy is that quantity which, when
added
to other contributions to the energy, is conserved in a closed
system.
That's a tad circular. Try "the end point variation of the action
under time translation" or, alternatively, "the generator of
infinitsimal time translations".
I'm not sure mine's so circular, but it's also not so useful. Yours is
more useful, but it offers little physics insight to the untrained.
Yes, I admit. The circularity in yours comes from defining energy by
"... when added to other contributions to energy...". So, it requires
to have at least one entity already defined as energy before you can
start checking what else qualifies. Not that there is anything
inherently wrong with this, historically that's the way the process
worked (starting with kinetic energy, then identifying one after
another its "relatives").
I think my point to the neophyte would be this:
In a closed system (where this can easily be defined), we note that
there are certain quantities which, when added up for the system, turn
out to remain constant in the system, no matter what happens inside the
system. This is quite a remarkable statement, especially the last
clause. Why this is so is not completely understood, but it is true
nonetheless. The best we can do is say that these "conservation laws"
are related to symmetries exhibited by the laws that govern the set of
forces acting within the system, but this really only changes the
question from why those quantities are conserved to why nature exhibits
those symmetries, though the latter seems to have more intuitive
appeal. The trick, of course, is figuring out how to calculate the
quantities which, when added up, are conserved. Historically, this has
amounted to a combination of stumbling on the right thing (or something
very close to the right thing) or knowing some details about the system
(e.g. the Lagrangian of the system, which in turn tells you something
about the symmetries of the system).
The fundamental issue is "Does how to calculate it amount to a
definition?" I maintain not, otherwise all these folks that say
"momentum is m*v" would argue they have a definition. Likewise, coming
up with a guaranteed-to-work recipe for how to calculate it under any
conditions also presumes knowing more about the system (e.g. the
Lagrangian). I favor a *definition* that describes why the quantity is
important, but that's just me.
Oh, I think we're in agreement here.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "Fred Chen" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 11:40:44 PM |
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wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it
aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't get
farther.
Thanks.
BP
In my undergraduate physics education, we were warned of the confusion
regarding photon's "mass". The photon has zero <<rest>> mass. It will
have momentum and energy regardless. The photon momentum is given in
the other posts.
Unfortunately momentum is taught earlier than special relativity, so
people first learn momentum = mass * velocity. This is NOT a general
definition of momentum; it's actually the approximation in the limit of
zero speed. Here velocity has to be much less than the speed of light,
and the mass is the rest mass of the particle or object. The rest mass
is the mass at zero speed.
As the speed of the particle approaches c (speed of light), the ratio
of momentum to velocity increases. This has often led to references to
"relativistic mass". However, this terminology is not often favored,
e.g., see http://en.wikipedia.org/wiki/Relativistic_mass, but don't let
this be your final reading.
When E=mc^2 is applied to a photon, m is not the rest mass of the
photon; the photon rest mass is zero. "Massless" really means
"rest-massless". For a photon, E/c^2=m would be an equivalent or
effective mass of the photon, which would only be useful in discussing
gravitational effects.
Regards,
Fred
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| User: "" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 12:55:47 AM |
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Fred if light has mass only by virtue of its speed
since all light has the same speed shouldnt all
light weigh the same?
If its motion that gives it mass consider lights wave motion. That
motion has to do with its mass/energy.
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| User: "bz" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 08:46:40 AM |
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wrote in news:1111906547.964965.93220
@g14g2000cwa.googlegroups.com:
Fred if light has mass only by virtue of its speed
since all light has the same speed shouldnt all
light weigh the same?
If its motion that gives it mass consider lights wave motion. That
motion has to do with its mass/energy.
It is not its speed that gives it the mass, it is its energy that gives it
mass/momentum.
An x-ray photon at a frequency of 30 PHz(1e-8m)(2e-17J)(2.2e-32gm) has much
higher energy and thus much more mass/momentum than an VLF photon at a
frequency of 30 kHz(10e3m)(2e-29J)(2.2e-43gm).
--
bz
please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.
bz+sp@ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap
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| User: "Fred Chen" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 01:07:24 AM |
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The photon's gravitational mass will vary with the energy of the photon.
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| User: "Mark Martin" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 02:19:02 PM |
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wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it
aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't get
farther.
Good question. Not dumb at all. E = mc^2 is only a special form of the
whole energy equation. The whole thing is E^2 = p^2c^2 + m^2c^4. The
term saying p^2c^2 relates energy to momentum, and m^2c^4 connects it
to mass. Photons don't have intrinsic mass, but they do have momentum.
In fact, anything with momentum also has energy. Anything with mass
also has energy. But not everything has both momentum and mass.
By the way, a photon can have so-called "relativistic mass", because it
does have energy. But relativistic mass has turned out to be a very
confusing word to many people, and so it's use has been losing ground
for several decades. Most physicists today use the word 'mass' only
when speaking of intrinsic, mass, the mass that's leftover when an
object has no relative motion.
-Mark Martin
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| User: "" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 08:16:47 PM |
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Well, every definition i find for momentum requires mass for it's
calculation...what now?
.
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| User: "Mark Martin" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 09:13:42 PM |
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wrote:
Well, every definition i find for momentum requires mass for it's
calculation...what now?
Given E^2 = p^2c^2 + m^2c^4, the energy of a photon is given by
E = pc
and so
p = E/c
An indpendant formula for photon energy, by Louis DeBroglie, is
E = hf (h = Planck's constant, f = photon's frequency)
Thus, p = hf/c. Furthermore, f, is equal to c/w, with w = photon's
wavelength. So photon momentum becomes finally
p = hw
-Mark Martin
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| User: "FrediFizzx" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 09:32:20 PM |
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"Mark Martin" <qed100@hotmail.com> wrote in message
news:1111893222.850295.147270@o13g2000cwo.googlegroups.com...
| wrote:
| > Well, every definition i find for momentum requires mass for it's
| > calculation...what now?
|
| Given E^2 = p^2c^2 + m^2c^4, the energy of a photon is given by
|
| E = pc
|
| and so
|
| p = E/c
|
| An indpendant formula for photon energy, by Louis DeBroglie, is
|
| E = hf (h = Planck's constant, f = photon's frequency)
|
| Thus, p = hf/c. Furthermore, f, is equal to c/w, with w = photon's
| wavelength. So photon momentum becomes finally
|
| p = hw
Ya missed a division there.
p = h/w = h/lambda where lambda is wavelength. w in ascii is usually
omega, angular frequency, for this application as in E = hbar*w
FrediFizzx
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.pdf
or postscript
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.ps
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| User: "Mark Martin" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 11:25:46 AM |
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FrediFizzx wrote:
Ya missed a division there.
p = h/w = h/lambda where lambda is wavelength. w in ascii is usually
omega, angular frequency, for this application as in E = hbar*w
Ah, yes. Thanks for pointing that out. Loose keyboards can sink ships.
-Mark Martin
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| User: "Gregory L. Hansen" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 08:48:03 PM |
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In article <1111889807.644676.164080@l41g2000cwc.googlegroups.com>,
<bruce_pearson@hotmail.com> wrote:
Well, every definition i find for momentum requires mass for it's
calculation...what now?
Recognize the scope of the freshman physics book. Lagrangians and the
generalized definition of momentum will be found in intermediate mechanics
books.
--
"Will we be suturing the *****?"
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| User: "Jon Bell" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 01:07:26 AM |
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In article <1111889807.644676.164080@l41g2000cwc.googlegroups.com>,
<bruce_pearson@hotmail.com> wrote:
Well, every definition i find for momentum requires mass for it's
calculation...what now?
Look harder. ;-)
--
Jon Bell <jtbell@presby.edu> Presbyterian College
Dept. of Physics and Computer Science Clinton, South Carolina USA
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| User: "" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 03:32:17 AM |
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In article <1111889807.644676.164080@l41g2000cwc.googlegroups.com>, writes:
Well, every definition i find for momentum requires mass for it's
calculation...what now?
Well, high school definitions may require this, but that's about it:-)
Momentum is *not* defined as mv. The momentum for a specific case,
this of massive particle with assumed Galilean invariance happens to
be mv. Same as energy is not defined as kx^2/2 (where k a spring
constant and x the extension from equilibrium) even though in a
special case (this of a compressed spring) energy is given by this
expression. There is a difference between general definition and a
derived result for a special case.
The general definition of momentum is based on the Least Action
Principle and it can be formulated as:
1) Linear momentum is the end point variation of the action under
spatial translations.
or
2) Linear momentum is the generator of spatial translation
transformations.
or
3) Linear momentum is the gradient of the Lagrangian with respect to
velocity.
In general (other than some subtleties) the three definitions are
equaivalent.
Note, that already within the framework of classical physics (i.e.
before the advent of relativity) electromagnetic radiation has
momentum, even though it has no mass.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "Sam Wormley" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 02:02:34 PM |
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wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it aquire
energy without aquiring mass??
Here is a definition of Energy
http://scienceworld.wolfram.com/physics/Energy.html
Please look over the page as it may answer your question.
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| User: "Rene Tschaggelar" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 02:51:46 PM |
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wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
=20
Since, e=3Dmc^2. and a photon is a massless particle, how does it aquir=
e
energy without aquiring mass??
=20
I'm sure a dumb question, but, unless I understand this I won't get
farther.
The energy of a photon is hv, with h being the Plank
constant (6,6626=B710-34J) and v being the frequency.
Since a photon is not infinitely long in time, it cannot
have an infinitely small linewidth, but we leave that
for now.
As to the physical size of a photon nothing should be said
at the moment either. Is it 1 wavelength in diameter ?
Or perhaps 1000 wavelengths?
Rene
--=20
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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| User: "FrediFizzx" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 04:17:49 PM |
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"Rene Tschaggelar" <none@none.net> wrote in message
news:4245cb61$0$1150$5402220f@news.sunrise.ch...
bruce_pearson@hotmail.com wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't get
farther.
The energy of a photon is hv, with h being the Plank
constant (6,6626·10-34J) and v being the frequency.
Since a photon is not infinitely long in time, it cannot
have an infinitely small linewidth, but we leave that
for now.
As to the physical size of a photon nothing should be said
at the moment either. Is it 1 wavelength in diameter ?
Or perhaps 1000 wavelengths?
Our naive research shows that a free space photon might be somewhere in
a volume with the radius of lambda/2pi and 2 wavelengths long -- as an
expectation value. The volume being the one related to its energy
density. Relative to a particular observer. This is the kind of
strange stuff one gets when taking quantum objects into real space. We
think the solution has to lie in the concept of dual spacetimes where a
modified Dirac Sea is one of the spacetimes. A preliminary attempt to
explain this is at the link below if interested.
FrediFizzx
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.pdf
or postscript
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.ps
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| User: "" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 08:40:27 PM |
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=3DThe energy of a photon is hv, with h being the Plank
constant (6,6626=B710-34J) and v being the frequency.=3D
This was a wonderful insight. no doubt.. but. is no answer to my
question, "How does something with no mass aquire energy?"
BP
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| User: "FrediFizzx" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 09:24:47 PM |
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<bruce_pearson@hotmail.com> wrote in message
news:1111891227.716450.104470@z14g2000cwz.googlegroups.com...
=The energy of a photon is hv, with h being the Plank
constant (6,6626·10-34J) and v being the frequency.=
h should be ~6.626·10^-34 Joule second
| This was a wonderful insight. no doubt.. but. is no answer to my
| question, "How does something with no mass aquire energy?"
Because it is just energy to start with. There is no "aguire" here.
Mass is an elusive concept, IMHO. Perhaps you can fill in the question
marks with a proper mathematical expression that would apply to all
fundamental particles?
mass = ????
So far we usually do something like,
mass (proton) ~= 1836 x mass(electron)
FrediFizzx
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.pdf
or postscript
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.ps
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| User: "John Sefton" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 06:07:02 AM |
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wrote:
=The energy of a photon is hv, with h being the Plank
constant (6,6626·10-34J) and v being the frequency.=
This was a wonderful insight. no doubt.. but. is no answer to my
question, "How does something with no mass aquire energy?"
BP
First comes energy.
If the energy has movement in 3D it
acquires mass. If only in 2D it
does not.
Mass is an attribute of energy.
John
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| User: "John Sefton" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 03:47:35 PM |
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Rene Tschaggelar wrote:
bruce_pearson@hotmail.com wrote:
Physics Newbie here...
The words "energy" and "photon" are often put together, as in "Photo
energy".
Since, e=mc^2. and a photon is a massless particle, how does it aquire
energy without aquiring mass??
I'm sure a dumb question, but, unless I understand this I won't get
farther.
The energy of a photon is hv, with h being the Plank
constant (6,6626·10-34J) and v being the frequency.
Since a photon is not infinitely long in time, it cannot
have an infinitely small linewidth, but we leave that
for now.
As to the physical size of a photon nothing should be said
at the moment either. Is it 1 wavelength in diameter ?
Or perhaps 1000 wavelengths?
Rene
Sometimes people wqant the amplitude of
the wave.
Unknown as is any physical description.
And yet, some are satisfied with that.
John
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| User: "" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 05:33:42 PM |
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You have to assume that only *some* energy has
mass!!!
So they have the burden to explain that!!!
Can anybody do it?
I didnt think so.
Mitch -- Light Falls --
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| User: "Uncle Al" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 05:49:59 PM |
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wrote:
[snip crap]
Hey idiot, how may ISPs need we kill you at to shut you up?
FOaD
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf
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| User: "John Sefton" |
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| Title: Re: Photon Energy...Huh? |
26 Mar 2005 07:24:08 PM |
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Uncle Al wrote:
macromitch@internetCDS.com wrote:
[snip crap]
Hey idiot, how may ISPs need we kill you at to shut you up?
FOaD
..
Whaddaya mean 'we' white man?
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| User: "OsherD" |
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| Title: Re: Photon Energy...Huh? |
27 Mar 2005 01:14:45 AM |
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From Osher Doctorow
Well said! It is one of the first principles of psychology to be able
to distinguish between one and two (or more) stimuli, such as "I" and
"we". It is called psychological discrimination, and psychological
integration is the process of finding what is common to two or more
stimuli. A course in psychology and maybe a half dozen in philosophy
should be prerequisites for physics, chemistry, and mathematics majors.
The philosophy course should include Ethics. I also recommend
Military service to teach courtesy since most people except the most
introverted don't seem to learn it otherwise, even those desperate to
get a Degree (mostly for jobs).
In the last Presidential election, I was disappointed to see almost
half the voters vote for an abstract "Plurality" (humanity,
My-Ethnic-Group-At-Any-Price, Sexual-Choice-At-Any-Price,
My-Job-At-Any-Price) during a time of War when a Terrorist victory
would yield no jobs for anybody except undertaker. Our two-party
system over-emphasizes either the Plurality (Democrats) or the
Individual (Republicans) rather than equally and highly valuing both
including the Individual in every Plurality and the Pluralities to
which each Individual belongs. In addition, many people think that
the only Individual is Number One ("Me").
Osher Doctorow
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| User: "Y.Porat" |
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| Title: Re: Photon Energy...Huh? |
29 Mar 2005 11:23:47 AM |
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energy is
mass in motion
even th e photon is mass in motion
let no matemathics idot to confise you
it is just simple as that !!!
could not be simpler !!! .
Y.Porat
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