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clipka wrote:
> David H. Burns schrieb:
>> Yes and I used to think that it was Einstein who showed the velocity
>> of light to be constant
>> in all frames of reference. But it wasn't he started with that. I was
>> not able to find where
>> this idea actually came from. Any ideas?
>
> Been around a while. People looking for the "aether" though which light
> waves were assumed to propagate, by measuring subtle changes in the
> difference of c depending on direction (a) parallel to the earth's
> movement on its orbit and (b) orthogonal to it, found that these subtle
> changes were apparently... well, /very/ subtle indeed. Actually too
> subtle to be measured, if there were any at all. At any rate, "aether"
> theory would have predicted a lot more.
There were (are ?) several different interpretations of the results of these
experiments: (1)that the "light bearing aether" didn't exist, (2) it
moved with the earth,
(3) it exists but is theoretically undetectable, and perhaps more.
Didn't the "Lorentz-Fitxgerald"
contraction also come out of this as an attempt to preserve the aether?
But that shows up
in Einstein's relativity too.
>
> That's how this whole "no absolute frame of reference" thing started.
I think it started long before that even as far back as Newton (?)
>
> By the way, Einstein did /not/ show that light was travelling at
> constant speed - that was Maxwell -
So the idea that the speed of light is a constant comes from Maxwell. Does
simply "fall out" of his equations or is it an assumption or what?
Come to think of it, isn't the speed of any wave in an unchanging
unmoving medium
constant and dependent on the properties of the medium?
>nor did he show that this was the
> case for every frame of reference - that was the experimenters (who
> didn't really show it for /every/ frame of reference of course, but for
> enough FoR to provide reason enough to seriously toy around with this
> idea).
The idea that the velocity of light is a fundamental constant is
something dictated by
theory or maybe better a fundamental assumption of Einstein's theory.
>
> All Einstein did was do just that: Take the "c appears to be constant in
> all frames of reference" as a given for argument's sake, and see what
> weird predictions he'd wind up with.
I think two of his basic assumptions were (in my words):
1) The speed of light is constant in all frames of reference.
2) Other than that, the observations made from within any frame of
reference are valid
only within it.
>
> It was only the experimenters who showed that Einstein was right in his
> predictions, and therefore it is prudent to assume that he was also
> right in his initial presumptions.
Of course the "light bearing aether" survives in a way as the
"electrical and magnet fields of space".
David
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David H. Burns wrote:
> moved with the earth,
Fairly easy to disprove by observing things outside the solar system, IIRC.
> (3) it exists but is theoretically undetectable, and perhaps more.
> Didn't the "Lorentz-Fitxgerald"
> contraction also come out of this as an attempt to preserve the aether?
It was to explain why it was undetectable, IIRC.
> So the idea that the speed of light is a constant comes from Maxwell. Does
> simply "fall out" of his equations or is it an assumption or what?
It's based on the charge to mass ratio of the electron, IIRC. I used to be
able to calculate what the speed of light would be for different charge to
mass ratios, but that was back in high school.
The fact that it was based only on charge and mass and not the relative
speed of the observer is what made it a "constant".
> Come to think of it, isn't the speed of any wave in an unchanging
> unmoving medium
> constant and dependent on the properties of the medium?
Nope. Your wake never catches up with you when you're in a boat.
> The idea that the velocity of light is a fundamental constant is
> something dictated by
> theory or maybe better a fundamental assumption of Einstein's theory.
My understanding is that "the speed of light" showed up in a number of
physics equations unrelated to light as such. Thus, it had to be a constant,
or there would be an adjustment in those equations to account for the
observer's motion. Again, this is just my vague memory.
> I think two of his basic assumptions were (in my words):
>
> 1) The speed of light is constant in all frames of reference.
>
> 2) Other than that, the observations made from within any frame of
> reference are valid
> only within it.
The third being "the same thing happens regardless of the frame of reference
from which you look at it." I.e., what *actually* happened doesn't change
depending on how you look at it.
> Of course the "light bearing aether" survives in a way as the
> "electrical and magnet fields of space".
A field isn't aether. A field is a technical term that basically means "a
collection of vectors with certain properties." It's not a physical thing,
but a mathematical thing.
--
Darren New, San Diego CA, USA (PST)
Understanding the structure of the universe
via religion is like understanding the
structure of computers via Tron.
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David H. Burns wrote:
> clipka wrote:
>> That's how this whole "no absolute frame of reference" thing started.
> I think it started long before that even as far back as Newton (?)
>>
>> By the way, Einstein did /not/ show that light was travelling at
>> constant speed - that was Maxwell -
>
> So the idea that the speed of light is a constant comes from Maxwell. Does
> simply "fall out" of his equations or is it an assumption or what?
> Come to think of it, isn't the speed of any wave in an unchanging
> unmoving medium
> constant and dependent on the properties of the medium?
As far as I understand it it's a bit misleading (although technically
correct I suppose) to claim that Maxwell "showed" that the speed of
light is a constant. A constant speed of light does fall out of
Maxwell's equations, but I believe that the original interpretation was
that they were only correct when considered by an observer at rest with
respect to the aether (this is where clipka's next sentence about
different reference frames comes in).
Historically speaking I think that the reasoning actually happened the
other way around. From Maxwell's equations he determined that
electromagnetic waves propagate at a particular speed, which happened to
match the speed of light, and thereby inferred that light was actually a
form of electromagnetic radiation.
>> All Einstein did was do just that: Take the "c appears to be constant
>> in all frames of reference" as a given for argument's sake, and see
>> what weird predictions he'd wind up with.
>
> I think two of his basic assumptions were (in my words):
>
> 1) The speed of light is constant in all frames of reference.
>
> 2) Other than that, the observations made from within any frame of
> reference are valid
> only within it.
I think a better way of phrasing these might be as follows:
1) All the laws to physics are invariant to the velocity of the observer
2) This includes Maxwell's equations (and thus light must have a
constant speed independent of the velocity of the observer.)
Side note: I am not a physicist, so take everything with a grain of salt.
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David H. Burns schrieb:
> There were (are ?) several different interpretations of the results of
> these
> experiments: (1)that the "light bearing aether" didn't exist, (2) it
> moved with the earth,
> (3) it exists but is theoretically undetectable, and perhaps more.
> Didn't the "Lorentz-Fitxgerald"
> contraction also come out of this as an attempt to preserve the aether?
> But that shows up
> in Einstein's relativity too.
Indeed. Maybe starting with the concept of "frame dragging" first (the
"aether" moving along with the earth) might have resulted in the same
equations. That's just pure speculation though.
> So the idea that the speed of light is a constant comes from Maxwell. Does
> simply "fall out" of his equations or is it an assumption or what?
It indeed "falls out" of his equations that electromagnetic waves in a
vacuum must travel exactly with the same speed as had already been
measured for light (which actually was the first hint that light /is/ an
electromagnetic wave).
> Come to think of it, isn't the speed of any wave in an unchanging
> unmoving medium
> constant and dependent on the properties of the medium?
It is [*]. But every medium (other than space) also provides a
"standard" frame of reference: For instance, water waves on a stream
will move at the same speed in all directions relative to an observer
drifting on that very stream (the "standard" frame of reference), but
relative to an observer on the bank they will move faster downstream
than upstream.
This is not so for light: No matter whether you're a stationary (hah!)
outside observer or zipping along at near-lightspeed, you'll always see
the light go at some 300,000 km/s relative to you.
[* Well, it is not /strictly/ true, unless the direction and
polarization of the wave is also a given, as the speed of a wave may be
anisotropic regarding these parameters; for instance, there are
materials out there in which the speed of light depends on polarization,
causing what is known as "birefringence"; but taking these constraints
into account as well, you're right.]
> The idea that the velocity of light is a fundamental constant is
> something dictated by
> theory or maybe better a fundamental assumption of Einstein's theory.
The latter, exactly. Or, to be even more precise, it's the idea that the
assertions "the velocity of light in vacuum is a fundamental constant"
and "there is no preferred frame of reference with respect to the
propagation of light in vacuum" are not mutually exclusive.
> I think two of his basic assumptions were (in my words):
>
> 1) The speed of light is constant in all frames of reference.
That was /the/ basic assumption of his theory of special relativity, yes.
> 2) Other than that, the observations made from within any frame of
> reference are valid
> only within it.
I'm not sure what exactly you mean by that. Maybe the "you can't tell
whether your own frame of reference is being accelerated or at rest"
assumption of general relativity?
> Of course the "light bearing aether" survives in a way as the
> "electrical and magnet fields of space".
Yup.
Although the magnetic field, as we have just seen, appears to be only an
illusion caused by relativistic effects in the electric field.
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clipka wrote:
> Although the magnetic field, as we have just seen, appears to be only an
> illusion caused by relativistic effects in the electric field.
I haven't actually gone through the math on this one but my limited
understanding is that this isn't necessarily the case. In the case
where there exists a reference frame in which you can analyze the
situation with electrostatics then you can derive the electromagnetic
forces in another reference frame with special relativity, but that by
no means implies that there exists any frame which allows you do do away
with magnetic forces altogether.
I think that this paper gives this argument, although it's a bit lacking
in details for my taste:
http://www.iop.org/EJ/abstract/0143-0807/29/5/002
I'm instead inclined to have the take on it that it nicely illustrates
why electricity and magnetism are not separate forces, but rather two
difference aspects of the same underlying field.
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Darren New schrieb:
>> moved with the earth,
>
> Fairly easy to disprove by observing things outside the solar system, IIRC.
Not really. If light slows down or speeds up as it approaches the earth
(due to the "aether" exhibiting "local movement"), how could you
possibly tell the difference?
> It's based on the charge to mass ratio of the electron, IIRC. I used to
> be able to calculate what the speed of light would be for different
> charge to mass ratios, but that was back in high school.
Now that's a puzzler to me: Isn't the electron known to have two
"brothers" (muon and tauon) that differ from it only in mass?
So if that is so, then in a manner of speaking we /do/ have electrons
with different charge-to-mass ratios. Yet they all interact with the
same electromagnetic field, in which the propagation speed of
disturbances must obviously be independent of the particular particle
that caused them.
How can that possibly work out with the propagation speed of those
disturbances being dependent on the charge-to-mass ratio of the
electron? And how come that despite such a striking correlation
scientists have still not managed to find out how particles obtain a
mass at all, if the respective formula would be right before their eyes,
giving the electron a mass depending directly on its charge and the
speed of light?
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Kevin Wampler schrieb:
> I think that this paper gives this argument, although it's a bit lacking
> in details for my taste:
>
> http://www.iop.org/EJ/abstract/0143-0807/29/5/002
I'm not buying that, literally ;-)
(How I hate this practice of making scientific papers available only for
money...)
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clipka wrote:
> Kevin Wampler schrieb:
>> I think that this paper gives this argument, although it's a bit
>> lacking in details for my taste:
>>
>> http://www.iop.org/EJ/abstract/0143-0807/29/5/002
>
> I'm not buying that, literally ;-)
>
> (How I hate this practice of making scientific papers available only for
> money...)
Me too, I didn't realize that it was access-restricted. The title was
"Lorentz contraction and current-carrying wires" in case you can find it
elsewhere. It's not a research paper anyway, and they just give an
example where what appears an a purely magnetic force in one frame can't
be reduced to a purely electric force, but only to different mixes of
electric and magentic forces.
I'll just quote a bit from the introduction and the conclusion:
"This sets the scene for the following argument. Imagine a charged
particle q moving parallel to the current-carrying wire at the electron
drift velocity1 . In the ion frame, the interaction between the wire and
the charged particle is described in terms of magnetostatics. In the
electron frame, this interaction is described in terms of
electrostatics—see figure 1. Special relativity is then invoked to show
that the two expressions can be transformed into one another.
Unless warned explicitly, a student following this line of thought may
well be left with the impression that either electrostatics or
magnetostatics is redundant. In this paper, I will use the closely
related example of the force between two parallel wires carrying equal
currents in the same direction to illustrate that both electrostatics
and magnetostatics need to be retained."
...
"This derivation shows that the force between two parallel
current-carrying wires is purely magnetostatic in the rest frame of the
wires, but is a combination of electrostatic and magnetostatic forces in
the electron frame. This result should serve to dispel any notion of
redundancy of electrostatics or magnetostatics. While the derivation is
primarily aimed at the introductory undergraduate level, it could also
serve to limit the risk of seeing electromagnetic fields as something
that can only be done in the context of antisymmetric second-rank
tensors in higher level courses."
I don't have any idea how much to trust the author's conclusions though,
and I've had an irritatingly difficult time finding a definitive answer
on Google from a source I feel I can trust more.
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clipka wrote:
> Not really. If light slows down or speeds up as it approaches the earth
> (due to the "aether" exhibiting "local movement"), how could you
> possibly tell the difference?
They'd change apparent angles, methinks. I'm sure there's something clever
that could be figured out. :-)
> Now that's a puzzler to me: Isn't the electron known to have two
> "brothers" (muon and tauon) that differ from it only in mass?
Dunno. Probably.
> in which the propagation speed of
> disturbances must obviously be independent of the particular particle
> that caused them.
Don't ask me. I just know what the formula says. Or knew. IIRC.
I think the charge is based on probabilities of interactions between photons
and electrons. (I.e., the definition of "charge" is "how much the particle
interacts with the photon" or some such.)
--
Darren New, San Diego CA, USA (PST)
Understanding the structure of the universe
via religion is like understanding the
structure of computers via Tron.
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clipka wrote:
>> Come to think of it, isn't the speed of any wave in an unchanging
>> unmoving medium
>> constant and dependent on the properties of the medium?
>
> It is [*]. But every medium (other than space) also provides a
> "standard" frame of reference: For instance, water waves on a stream
> will move at the same speed in all directions relative to an observer
> drifting on that very stream (the "standard" frame of reference), but
> relative to an observer on the bank they will move faster downstream
> than upstream.
>
> This is not so for light: No matter whether you're a stationary (hah!)
> outside observer or zipping along at near-lightspeed, you'll always see
> the light go at some 300,000 km/s relative to you.
We can't get out of the medium to observe light waves as we can with
water waves.
> [* Well, it is not /strictly/ true, unless the direction and
> polarization of the wave is also a given, as the speed of a wave may be
> anisotropic regarding these parameters; for instance, there are
> materials out there in which the speed of light depends on polarization,
> causing what is known as "birefringence"; but taking these constraints
> into account as well, you're right.]
I have heard the apparent decrease in the velocity of light is explained
by the interference of light
re-emitted by the material so as to give the appearance of a decrease in
velocity. But the "true" velocity
of the light remains that in free space. (I did not altogether
understand this and may have it wrong.)
>> I think two of his basic assumptions were (in my words):
>>
>> 1) The speed of light is constant in all frames of reference.
>
> That was /the/ basic assumption of his theory of special relativity, yes.
>
>> 2) Other than that, the observations made from within any frame of
>> reference are valid
>> only within it.
>
> I'm not sure what exactly you mean by that. Maybe the "you can't tell
> whether your own frame of reference is being accelerated or at rest"
> assumption of general relativity?
That may be what I was getting at, but also the fact that the
"relativistic" effects
such as time dilation or increase in mass that I would observe in an object
(say a space ship) moving with 99.99% the speed pf light relative to me
are *not*
observed by its occupants and my observation is no more (or no less)
valid than theirs.
A lot of the paradoxes of relativity seem to arise from treating the
observations from two
or more frames of reference as if they were made from the same frame of
reference.
David
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