| Topic: |
Science > Physics |
| User: |
"tetrahedron" |
| Date: |
05 Jan 2005 03:41:34 PM |
| Object: |
Analogy Electromagnetism vs.Gravitation |
Wow, so many answers! I must have raised a hot topic. I've been
researching on the subject a little, but I'm in disadvantage not
knowing quantum mechanics. I found the explanation someone gave using
spin of the difference between the quantum field theories for EM and
gravitation deep but fascinating.
OK, on to something else. After last time's mistakes, I don't want to
push too far my assumptions. Something both EM and gravitation agree
on is the speed of light c as the speed limit in the universe. I was
wondering
1) Is c a purely experimental value or is it predicted exactly by
theory? Does it perhaps result from other "fundamental" physical
quantities? Or does it follow from geometrical considerations?
2) What about epsilon_0?
3) How is it possible to prove in GR that c is the speed limit? c
comes from EM. Is there something kin to a definition of light in GR?
Or does it simply fall under the forms of energy?
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| User: "Oliver Keating" |
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| Title: Re: Analogy Electromagnetism vs.Gravitation |
06 Jan 2005 12:43:37 AM |
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"tetrahedron" <jarynth@yahoo.com> wrote in message
news:de044e77.0501051341.7523a11c@posting.google.com...
Wow, so many answers! I must have raised a hot topic. I've been
researching on the subject a little, but I'm in disadvantage not
knowing quantum mechanics. I found the explanation someone gave using
spin of the difference between the quantum field theories for EM and
gravitation deep but fascinating.
OK, on to something else. After last time's mistakes, I don't want to
push too far my assumptions. Something both EM and gravitation agree
on is the speed of light c as the speed limit in the universe. I was
wondering
1) Is c a purely experimental value or is it predicted exactly by
theory? Does it perhaps result from other "fundamental" physical
quantities? Or does it follow from geometrical considerations?
2) What about epsilon_0?
Please, can we remember that Physics is a science and as such no values
whatsoever can be "predicted exactly by theory", all values are
experimental. Even the famous 3.1415926...
What we can say is that such constants appear to be related, epsilon_0 and
mu_0 and c by a consistent theory. In fact, the way our system of units is
set up is that mu_0 and c are *defined* to be certain values, and epsilon_0
is a measured value. The reason for having such definitions is that a system
of physical units is not an abstract concept but must be tied into physical
quanitities.
3) How is it possible to prove in GR that c is the speed limit? c
comes from EM. Is there something kin to a definition of light in GR?
Or does it simply fall under the forms of energy?
I have forgotten - GR is monstorously complicated.
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| User: "" |
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| Title: Re: Analogy Electromagnetism vs.Gravitation |
06 Jan 2005 01:16:13 AM |
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In article <41dcde18$0$74273$ed2619ec@ptn-nntp-reader03.plus.net>, "Oliver Keating" <oliver.keating@NOSPAMPLEASE.ic.ac.uk> writes:
"tetrahedron" <jarynth@yahoo.com> wrote in message
news:de044e77.0501051341.7523a11c@posting.google.com...
Wow, so many answers! I must have raised a hot topic. I've been
researching on the subject a little, but I'm in disadvantage not
knowing quantum mechanics. I found the explanation someone gave using
spin of the difference between the quantum field theories for EM and
gravitation deep but fascinating.
OK, on to something else. After last time's mistakes, I don't want to
push too far my assumptions. Something both EM and gravitation agree
on is the speed of light c as the speed limit in the universe. I was
wondering
1) Is c a purely experimental value or is it predicted exactly by
theory? Does it perhaps result from other "fundamental" physical
quantities? Or does it follow from geometrical considerations?
2) What about epsilon_0?
Please, can we remember that Physics is a science and as such no values
whatsoever can be "predicted exactly by theory", all values are
experimental. Even the famous 3.1415926...
No, this one isn't. It is a mathematical constant, not a physical
one.
What we can say is that such constants appear to be related, epsilon_0 and
mu_0 and c by a consistent theory. In fact, the way our system of units is
set up is that mu_0 and c are *defined* to be certain values, and epsilon_0
is a measured value. The reason for having such definitions is that a system
of physical units is not an abstract concept but must be tied into physical
quanitities.
Since epsilon_0, mu_0 and c are (as you say) related, it is striclty
impossible for two of them to be defined and the third being measured.
3) How is it possible to prove in GR that c is the speed limit? c
comes from EM. Is there something kin to a definition of light in GR?
Or does it simply fall under the forms of energy?
I have forgotten - GR is monstorously complicated.
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: Analogy Electromagnetism vs.Gravitation |
07 Jan 2005 03:57:06 PM |
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tetrahedron <jarynth@yahoo.com> wrote:
[...] Something both EM and gravitation agree
on is the speed of light c as the speed limit in the universe. I was
wondering
1) Is c a purely experimental value or is it predicted exactly by
theory? Does it perhaps result from other "fundamental" physical
quantities? Or does it follow from geometrical considerations?
The speed of light is a dimensionful constant. That is, it's not
a number; it's some number of meters per second, a different number
of feet per nanosecond, yet a different number of furlongs per
fortnight. Geometry and relativity don't know about meters or
seconds, so they can't predict a number of meters per second. The
speed of light can only be predicted if you have some other quantity
with the same units to compare it to.
(I wish I remember where I heard this quote: "The speed of light has
the value it does because we have chosen a unit of distance about the
length of an arm and a unit of time about the length of a heartbeat.")
[...]
3) How is it possible to prove in GR that c is the speed limit? c
comes from EM. Is there something kin to a definition of light in GR?
Or does it simply fall under the forms of energy?
The fundamental speed is the "GR speed limit," which comes from the
underlying structure of spacetime. It happens to be true that light
travels at this speed. This didn't have to be the case -- one could
imagine an alternative universe in which the photon had a mass, in
which case the two speeds would be different. The fact that the speed
limit is called the "speed of light" is a historical accident, not
anything deeper.
(On the other hand, *if* you know that the speed of light is the same
in every inertial frame, then the two speeds have to be the same.
That comes from basic geometry; you can only have one speed that is
independent of the velocity of the observer.)
Steve Carlip
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| User: "Ken S. Tucker" |
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| Title: Re: Analogy Electromagnetism vs.Gravitation |
07 Jan 2005 04:32:47 PM |
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To Tetra, Steve says,
"The fact that the speed
limit is called the "speed of light" is a historical accident, not
anything deeper."
But so much of our physics, math and geometry sprung
from astronomical siteings, using light, and now we find
the most consistent way to survey uses radar and lasers,
terrestrially, so yeah we have evolved to that standard.
Ken
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| User: "Uncle Al" |
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| Title: Re: Analogy Electromagnetism vs.Gravitation |
05 Jan 2005 04:45:36 PM |
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tetrahedron wrote:
Wow, so many answers! I must have raised a hot topic.
Idiot.
Google
"Kaluza-Klein" 70,200 hits
Read instead of pulling it out of your *****.
I've been
researching on the subject a little, but I'm in disadvantage not
knowing quantum mechanics. I found the explanation someone gave using
spin of the difference between the quantum field theories for EM and
gravitation deep but fascinating.
Get educated before you attempt to make a call. Crack a book, do a
Web search, just goddamned stop squealing tripe. How do you
differentiate what is in the refereed literature from what a fellow
turkey gobbles?
[snip]
--
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: "Bjoern Feuerbacher" |
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| Title: Re: Analogy Electromagnetism vs.Gravitation |
06 Jan 2005 08:46:39 AM |
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tetrahedron wrote:
Wow, so many answers! I must have raised a hot topic. I've been
researching on the subject a little, but I'm in disadvantage not
knowing quantum mechanics. I found the explanation someone gave using
spin of the difference between the quantum field theories for EM and
gravitation deep but fascinating.
OK, on to something else. After last time's mistakes, I don't want to
push too far my assumptions. Something both EM and gravitation agree
on is the speed of light c as the speed limit in the universe.
No, EM (you probably mean Maxwell's equation, right?) does not
say anything about c being a speed limit. That would be Special
Relativity. And since SR is a subset of General Relativity,
it's not wonder that GR (you meant this by "gravitation", right?)
also says that.
Or did you meant to say that both the speed of electromagnetic
and of gravity waves is c?
I was wondering
1) Is c a purely experimental value or is it predicted exactly by
theory?
In modern physics, it is considered as an artifact of our choice
of units, I would say.
Does it perhaps result from other "fundamental" physical
quantities? Or does it follow from geometrical considerations?
No to both.
2) What about epsilon_0?
Also only an artifact of our choice of units. Have you ever
seen Gaussian units?
3) How is it possible to prove in GR that c is the speed limit?
By noting that SR is a subset of GR.
OTOH, if you want to show that the speed of gravity waves is
c, then try looking into a textbook on GR. The calculation
is straightforward, but much too long for a newsgroup post.
c comes from EM.
Originally, yes. Einstein's postulate that c should be
invariant incorporated it into SR, and then almost automatically
into GR.
Is there something kin to a definition of light in GR?
Or does it simply fall under the forms of energy?
Sorry, I don't understand these questions.
Bye,
Bjoern
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