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Light and Gravity
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Igor Khavkine
science forum Guru


Joined: 01 May 2005
Posts: 607

PostPosted: Fri Jun 02, 2006 10:44 am    Post subject: Re: Light and Gravity Reply with quote

Chalky wrote:
Quote:
I understand that the velocities of light and gravity are the same in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of the
same frequency will also propagate at the same reduced velocity through
the same solid or fluid material?

Or is this a question without a known answer, as yet?

The answer is known, and it is in the negative. To see that, you first
have to realize why light slows down when it enters a material medium.
The atoms and molecules that it encounters scatter the light in
different directions. In the empty space between atoms and molecule,
light still travels at a constant speed. However, because it's not
really traveling in a straight line any more, the effective speed of a
light ray gets reduced.

Different materials scatter light differently, that's why different
materials have different effective spees of light in them. Air scatters
light very little (compared to glass or water), so in it the speed of
light is very close to the speed of light in vacuum (but in glass or
water it is not). Now, what about gravity?

The basic principle apply, if there is scattering of gravitational
waves as they pass through a medium, their effective speed will be
slowed down. The amount of slow down will depend on the amount of
scattering. However, the latter depends on how strongly gravitational
waves interact with matter. Unfortunately, this interaction strength is
famously small (many orders of magnitude smaller than that for
electromagnetism). That is why huge experiments like LIGO and LISA are
necessary even to detect the existence of gravitational waves. So, in
conclusion, because gravity interacts with matter so weekly, the speed
of gravity will be reduced only imperceptibly as it passes through a
material medium.

Hope this helps.

Igor
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Greg Egan
science forum addict


Joined: 01 May 2005
Posts: 75

PostPosted: Fri Jun 02, 2006 10:45 am    Post subject: Re: Light and Gravity Reply with quote

Chalky wrote:
Quote:
I understand that the velocities of light and gravity are the same in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of the
same frequency will also propagate at the same reduced velocity through
the same solid or fluid material?

Or is this a question without a known answer, as yet?

C

EM waves excite motion in the electrons in the atoms of the media they
pass through, and the moving electrons in turn generate EM waves that
interact with the incoming wave. The net result of this is a delay in
the propagation of the wave through the medium.

Gravitational waves will excite very little motion in any media they
pass through, and what motion they do excite will generate very, very
tiny gravitational waves. So although a precise analysis of the
interactions of a gravitational wave with various kinds of media would
be a very complex task, you can be sure that gravitational waves will
pass through most media faster than EM waves will.
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Uncle Al
science forum Guru


Joined: 24 Mar 2005
Posts: 1226

PostPosted: Fri Jun 02, 2006 12:38 pm    Post subject: Re: Light and Gravity Reply with quote

Chalky wrote:
Quote:

I understand that the velocities of light and gravity are the same in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of the
same frequency will also propagate at the same reduced velocity through
the same solid or fluid material?

Or is this a question without a known answer, as yet?

Red light and blue light do not propagate at the same overall velocity
through transparent media - dispersion and Abbe number. Refractive
index arises from the electrical permittivity and magnetic
permeability of the medium (including vacuum). Gravitational waves
are not EM and won't care.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf
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Jonathan Thornburg -- rem
science forum beginner


Joined: 06 May 2005
Posts: 41

PostPosted: Fri Jun 02, 2006 12:38 pm    Post subject: Re: Light and Gravity Reply with quote

Chalky <chalkyspam@bleachboys.co.uk> wrote:
Quote:
I understand that the velocities of light and gravity are the same in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of the
same frequency will also propagate at the same reduced velocity through
the same solid or fluid material?

No. In ordinary optical glass, light propagates about about 2/3 c,
whereas gravitational radiation propagates at very close to c.

ciao,

--
-- "Jonathan Thornburg -- remove -animal to reply" <jthorn@aei.mpg-zebra.de>
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam
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FrediFizzx
science forum Guru


Joined: 01 May 2005
Posts: 774

PostPosted: Sat Jun 03, 2006 8:04 am    Post subject: Re: Light and Gravity Reply with quote

"Igor Khavkine" <igor.kh@gmail.com> wrote in message
news:1149170531.901276.88520@c74g2000cwc.googlegroups.com...
Quote:
Chalky wrote:
I understand that the velocities of light and gravity are the same
in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of
the
same frequency will also propagate at the same reduced velocity
through
the same solid or fluid material?

Or is this a question without a known answer, as yet?

The answer is theoretically known but not experimentally. And since the
gravitational interaction with matter is so weak, I don't think we will
know for sure experimentally for quite some time.

Quote:
The answer is known, and it is in the negative. To see that, you first
have to realize why light slows down when it enters a material medium.
The atoms and molecules that it encounters scatter the light in
different directions. In the empty space between atoms and molecule,
light still travels at a constant speed. However, because it's not
really traveling in a straight line any more, the effective speed of a
light ray gets reduced.

Different materials scatter light differently, that's why different
materials have different effective spees of light in them. Air
scatters
light very little (compared to glass or water), so in it the speed of
light is very close to the speed of light in vacuum (but in glass or
water it is not). Now, what about gravity?

The basic principle apply, if there is scattering of gravitational
waves as they pass through a medium, their effective speed will be
slowed down. The amount of slow down will depend on the amount of
scattering. However, the latter depends on how strongly gravitational
waves interact with matter. Unfortunately, this interaction strength
is
famously small (many orders of magnitude smaller than that for
electromagnetism). That is why huge experiments like LIGO and LISA are
necessary even to detect the existence of gravitational waves. So, in
conclusion, because gravity interacts with matter so weekly, the speed
of gravity will be reduced only imperceptibly as it passes through a
material medium.

If we haven't even detected gravitational waves yet, then how can the
answer be known for sure?

FrediFizzx

Quantum Vacuum Charge papers;
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.pdf
or postscript
http://www.vacuum-physics.com/QVC/quantum_vacuum_charge.ps
http://www.arxiv.org/abs/physics/0601110
http://www.vacuum-physics.com
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Cl.Massé
science forum Guru Wannabe


Joined: 24 Mar 2005
Posts: 149

PostPosted: Sat Jun 03, 2006 8:56 pm    Post subject: Re: Light and Gravity Reply with quote

"Chalky" <chalkyspam@bleachboys.co.uk> a écrit dans le message de news:
1149118095.245721.304860@j55g2000cwa.googlegroups.com

Quote:
I understand that the velocities of light and gravity are the same in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of the
same frequency will also propagate at the same reduced velocity through
the same solid or fluid material?

No, the coupling with matter causing the slow down isn't the same. In fact,
a photon doesn't cross a transparent medium, it is absorbed and reemitted
many times.

--
~~~~ clmasse on free F-country
Liberty, Equality, Profitability.
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Chalky
science forum beginner


Joined: 13 Jan 2006
Posts: 20

PostPosted: Mon Jun 05, 2006 9:01 am    Post subject: Re: Light and Gravity Reply with quote

Igor Khavkine wrote:
Quote:
Chalky wrote:
I understand that the velocities of light and gravity are the same in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of the
same frequency will also propagate at the same reduced velocity through
the same solid or fluid material?

Or is this a question without a known answer, as yet?

The answer is known, and it is in the negative. To see that, you first
have to realize why light slows down when it enters a material medium.
The atoms and molecules that it encounters scatter the light in
different directions. In the empty space between atoms and molecule,
light still travels at a constant speed. However, because it's not
really traveling in a straight line any more, the effective speed of a
light ray gets reduced.

I don't really understand this. If the light is scattered, how do we

still get a fixed refraction angle for a fixed incident angle, for the
whole light beam, to any depth, in a transparent solid?

For that matter, I don't really understand why (the other way round) a
reduction in speed causes the light beam to bend either. I could
understand this if the light was slowed down in the direction
perpendicular to the material surface but not in the direction
parallel, but this doesn't seem to make sense, because the light is
interacting with the medium in both directions.

I hope these further questions don't make me sound too dumb.

C
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Admral
science forum beginner


Joined: 07 Sep 2005
Posts: 22

PostPosted: Wed Jun 07, 2006 3:20 am    Post subject: Re: Light and Gravity Reply with quote

Chalky wrote:
Quote:
Igor Khavkine wrote:
Chalky wrote:
I understand that the velocities of light and gravity are the same
in GR theory.

Doe this mean that the velocities of EM and gravitational waves of
the same frequency will also propagate at the same reduced velocity
through the same solid or fluid material?

Or is this a question without a known answer, as yet?

The answer is known, and it is in the negative. To see that, you
first have to realize why light slows down when it enters a material
medium. The atoms and molecules that it encounters scatter the light
in different directions. In the empty space between atoms and
molecule, light still travels at a constant speed. However, because
it's not really traveling in a straight line any more, the effective
speed of a light ray gets reduced.

I don't really understand this.

Nobody does.

Quote:
If the light is scattered, how do we
still get a fixed refraction angle for a fixed incident angle, for the
whole light beam, to any depth, in a transparent solid?

"Scattering," in this sense, should not be taken literally; it not like that
a photon is a bullet that deflects when it hits something solid. A better
way to look at it is that the photon is absorbed by an electron inside the
transparent media, and then, at some later time, another photon is emitted
(by the same electron). This delay obviously slows down the propagation of
photons.

Quote:
For that matter, I don't really understand why (the other way round) a
reduction in speed causes the light beam to bend either.

OK, lets start with the concept of a "light beam", which somehow suggests a
train of particles that follows straight lines until they reach some
obstacle. In quantum electrodynamics, things are mouch more crazy than
that. In QED, light is essentially a statistical correclation between two
events: emission of energy by an electron, and at some other point in space
time, absorbtion of energy by an electron (and it could even be the same
electron; and even worse, the absorbtion could sometimes take place before
the emission). At these two events, light behaves like a particle - the
photon. What nature light takes between these two events, we know little
about (because light is unobservable in this interval), except that a
particular sort of wave mathematics is involved. This wave mathematics,
however, implies that if you set up your apparatus so that the region where
electrons are highly likely to absorb (and reemit) those photons is close to
a straight line (e.g., send light through a pin hole in a smoke filled
chamber), then any continuation of this line (e.g., through a prism) will be
of a form so that it seems that the light always takes a path between points
A and B that involves that least time of travel. I believe that Feynman's
"QED: The strange theory of light and matter" describes this part of QED
very intuitively (he gets the math through without really using math); you
might try go get a copy.

[...]

Quote:
I hope these further questions don't make me sound too dumb.

Not at all. These are the same questions that drove the science of physics
for the last 100 years.
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Igor Khavkine
science forum Guru


Joined: 01 May 2005
Posts: 607

PostPosted: Thu Jun 08, 2006 3:45 am    Post subject: Re: Light and Gravity Reply with quote

Chalky wrote:
Quote:
Igor Khavkine wrote:
Chalky wrote:
I understand that the velocities of light and gravity are the same in
GR theory.

Doe this mean that the velocities of EM and gravitational waves of the
same frequency will also propagate at the same reduced velocity through
the same solid or fluid material?

Or is this a question without a known answer, as yet?

The answer is known, and it is in the negative. To see that, you first
have to realize why light slows down when it enters a material medium.
The atoms and molecules that it encounters scatter the light in
different directions. In the empty space between atoms and molecule,
light still travels at a constant speed. However, because it's not
really traveling in a straight line any more, the effective speed of a
light ray gets reduced.

I don't really understand this. If the light is scattered, how do we
still get a fixed refraction angle for a fixed incident angle, for the
whole light beam, to any depth, in a transparent solid?

All of these phenomena are described properly by the wave theory of
light.

Quote:
For that matter, I don't really understand why (the other way round) a
reduction in speed causes the light beam to bend either. I could
understand this if the light was slowed down in the direction
perpendicular to the material surface but not in the direction
parallel, but this doesn't seem to make sense, because the light is
interacting with the medium in both directions.

These are all interesting questions and are worth persuing. However, it
might take you a little while to catch up on all the successively
better theoretical descriptions of light, that have been built up over
the last few hundred years.

First, lets recap what we see light do. It can travel in straight
lines. It can bend its direction when entering a refractive medium.
This behavior follows if we presume Fermat's principle holds: light
always travels along the shortest path, while each material is assigned
a multiplicative factor (the refractive index) which when prefixed to
the physical distance gives the optical distance within the matherial.

So far so good, but we see light do other things that don't follow from
Fermat's principle. For instance, light also propagates at a finite
speed and carries momentum. Here we can make an appeal to a corpuscular
theory of light. Say that a light beam is composed of many particles,
each carrying some momentum. And say that the refractive index of a
material actually represents a potential that the light corpuscules
feel. The the path of light can be described with Newton's equations
dp/dt = -grad V. The potential V is, for instance, constant throughout
a uniform optical material, but has different heights for different
materials. But if light is made up of corpuscules, why does it obey
Fermat's principle? This principle turns out to be a simple
consequences of the postulated equations of motion and form of
potential.

But we still see light do things that do not follow from the
corpuscular hypothesis. Most notably, we see diffraction. Here is where
the wave theory of light comes in. Suppose that light is described by a
wave obeying Maxwell's equations. Say also that different materials now
possess different permittivities and permeabilities, it is they that
now account for different refractive indexes in different materials.
Solve Maxwell's equations for a wave propagating around a barrier. The
solution reproduces diffraction effects (check!). But are all the other
properties of light described by the corpuscular theory reprodcued?
Yes. Presume that light beams are actually propagating wave packets of
small width and the the mean packet frequency is much smaller than the
size of geometric features around it. Then apply the so called eikonal,
aka WKB, aka geometric optics approximation to Maxwell's equations, the
corpuscular theory described above is precisely recovered (check!).

One could climb this ladder even higher. For instance, there is also a
theory that accounts for light's quantum behavior. But I won't go into
the details here. The important point I wanted to illustrate here is
the following. There are many possible answers to a question like "why
does light refract at a certain angle when passing from one material to
another?", depending on how high you want to climb the ladder of
explanations I started describing. You can stop at any given rung, you
can stop and be satisfied with the answer. That is, unless you are
trying to reconcile this answer with observations belonging to a higher
rung. It is very easy to get confused this way.

For instance, part of your confusion in the previous message is
probably due to trying to fit together an atomistic view of matter
together with the simple explanation that light slows down when it
enters an optical material. That's very hard to do. May have to foget
the atomistic picture and treat matter as continuous. Then, presuming
that the interaction of light with optical materials is reduced to an
effective optical potential, refraction is pretty well explained by
anything from Snell's law to a corpuscular theory. Alternatively, you
can give up the simple idea that light "slows down" inside a material
and accept that only a wave description gives you an adequate level of
detail. Then refraction (along with a great many other effects) is
explained by destructive and constructive interference of light waves
scattered from atoms.

Hope this clears some things up for you.

Igor
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Cl.Massé
science forum Guru Wannabe


Joined: 24 Mar 2005
Posts: 149

PostPosted: Sun Jun 11, 2006 1:28 am    Post subject: Re: Light and Gravity Reply with quote

"Chalky" <chalkyspam@bleachboys.co.uk> a écrit dans le message de news:
1149325856.061951.74510@h76g2000cwa.googlegroups.com

Quote:
I don't really understand this. If the light is scattered, how do we
still get a fixed refraction angle for a fixed incident angle, for the
whole light beam, to any depth, in a transparent solid?

The same reason why a light beam propagates in right line: interference.
Light scattered from different points are emitted by those points without
change of relative phase, the interference is then constructive only in the
direction of propagation.

Quote:
For that matter, I don't really understand why (the other way round) a
reduction in speed causes the light beam to bend either. I could
understand this if the light was slowed down in the direction
perpendicular to the material surface but not in the direction
parallel, but this doesn't seem to make sense, because the light is
interacting with the medium in both directions.

Again, interference is the explanation. Look at it in terms of wave
surfaces, surfaces where the phases are the same. In the medium, they are
nearer to on another than in vacuum, and they are continuous at the
interface.

Quote:
I hope these further questions don't make me sound too dumb.

To the contrary. The ability to pose questions is essential for a
researcher.

--
~~~~ clmasse on free F-country
Liberty, Equality, Profitability.
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Chalky
science forum beginner


Joined: 13 Jan 2006
Posts: 20

PostPosted: Mon Jun 12, 2006 10:40 pm    Post subject: Re: Light and Gravity Reply with quote

Cl.Massé wrote:
Quote:
"Chalky" <chalkyspam@bleachboys.co.uk> a écrit dans le message de news:
1149325856.061951.74510@h76g2000cwa.googlegroups.com

I hope these further questions don't make me sound too dumb.

To the contrary. The ability to pose questions is essential for a
researcher.

The answers to my earlier questions have certainly given me a better

understanding of light propagation in matter, and why gravitational
propagation is expected to be different. (Thanks). However, this does
tempt me to now pose one further question on the subject.

Given that the speed of gravitational propagation is not controlled by
the electrical permittivity and magnetic permeability within a material
medium, why is the speed of gravity in vacuum still equal to the square
root of the product of electrical permittivity and magnetic
permeability of free space?

C
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Timo Nieminen
science forum Guru Wannabe


Joined: 12 May 2005
Posts: 244

PostPosted: Wed Jun 14, 2006 6:59 am    Post subject: Re: Light and Gravity Reply with quote

On Mon, 12 Jun 2006, Chalky wrote:

Quote:
Given that the speed of gravitational propagation is not controlled by
the electrical permittivity and magnetic permeability within a material
medium, why is the speed of gravity in vacuum still equal to the square
root of the product of electrical permittivity and magnetic
permeability of free space?

Because that's the fastest that anything can go. Light goes that fast in
free space because photons are massless, or, if you prefer, light is a
massless wave. Since a gravitational wave is also massless as far as we
know (whether or not gravitational waves are quantised), it also travels
at the same speed.

The permittivity and permeability of free space are essentially unit
conversion constants that result from our defining D and H in terms of
sources (charge density and current density) and E and B in terms of force
exerted on charges and currents. See
http://groups.google.com/group/sci.physics/msg/02c5db2aca385146?hl=en&
for more details.

That the maximum speed of propagation of "things" is equal to
1/sqrt(permittivity*permeability) is a necessary consequence of photons
being massless, rather than an amazing coincidence.

Exercise for the interested reader:
In analogy with the Maxwell equations, use mass density to define a
"gravitational displacement" field, and a force law of gravitational
fields exerted on mass density to define the gravitational equivalent of
E. Then write down the units and value of the gravitational
"permittivity". Then, what must the gravitational "permeability" be? Can
you write down other two gravitational Maxwell-like equations yet? What
would a gravitational "Ohm's law" mean?

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
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Igor Khavkine
science forum Guru


Joined: 01 May 2005
Posts: 607

PostPosted: Wed Jun 14, 2006 6:59 am    Post subject: Re: Light and Gravity Reply with quote

Chalky wrote:
Quote:
Cl.Massé wrote:
"Chalky" <chalkyspam@bleachboys.co.uk> a écrit dans le message de news:
1149325856.061951.74510@h76g2000cwa.googlegroups.com

I hope these further questions don't make me sound too dumb.

To the contrary. The ability to pose questions is essential for a
researcher.

Or for anyone who's just curious.

Quote:
The answers to my earlier questions have certainly given me a better
understanding of light propagation in matter, and why gravitational
propagation is expected to be different. (Thanks). However, this does
tempt me to now pose one further question on the subject.

Given that the speed of gravitational propagation is not controlled by
the electrical permittivity and magnetic permeability within a material
medium, why is the speed of gravity in vacuum still equal to the square
root of the product of electrical permittivity and magnetic
permeability of free space?

Yes, it is. However, your proposition sounds much less strange if you
realize that both electrical permittivity and magnetic permeability are
1 in appropiate units. In the same units, the speed of light is also 1,
and so is the speed of gravity. It is not so strange that both light
and gravity have the same propagation speed. They just have the maximal
allowed speed, which in the right units, is the dimensionless constant
1.

The fact that both electromagnetic and gravitational waves travel at
maximal allowed speed, follows from the form of their equations of
motion. These are Maxwell's equations and linearized Einstein's
equations. So if we are sure that these equations hold true, then we
are sure that this fact also holds true. To understand the details of
the implication you have to know something about solutions to wave
partial differential equations and something about special relativity,
namely the difference between null, timelike, and spacelike vectors.

Hope this helps.

Igor
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I.Vecchi
science forum Guru Wannabe


Joined: 05 May 2005
Posts: 124

PostPosted: Thu Jun 15, 2006 4:39 pm    Post subject: Re: Light and Gravity Reply with quote

Timo A. Nieminen ha scritto:

Quote:
On Mon, 12 Jun 2006, Chalky wrote:

Given that the speed of gravitational propagation is not controlled by
the electrical permittivity and magnetic permeability within a material
medium, why is the speed of gravity in vacuum still equal to the square
root of the product of electrical permittivity and magnetic
permeability of free space?

Because that's the fastest that anything can go. Light goes that fast in
free space because photons are massless, or, if you prefer, light is a
massless wave. Since a gravitational wave is also massless as far as we
know (whether or not gravitational waves are quantised), it also travels
at the same speed.

The permittivity and permeability of free space are essentially unit
conversion constants that result from our defining D and H in terms of
sources (charge density and current density) and E and B in terms of force
exerted on charges and currents. See
http://groups.google.com/group/sci.physics/msg/02c5db2aca385146?hl=en&
for more details.

That the maximum speed of propagation of "things" is equal to
1/sqrt(permittivity*permeability) is a necessary consequence of photons
being massless, rather than an amazing coincidence.

...

I do not see how your argument can be reconciled with the Cherenkov
effect ([1]). Cherenkov radiation is induced by charged particles
travelling at superluminal speed (i.e. faster than photons) inside an
insulator. A clarification would be helpful.

IV

[1] http://en.wikipedia.org/wiki/Cherenkov_effect
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Uncle Al
science forum Guru


Joined: 24 Mar 2005
Posts: 1226

PostPosted: Thu Jun 15, 2006 4:39 pm    Post subject: Re: Light and Gravity Reply with quote

"Timo A. Nieminen" wrote:
Quote:

On Mon, 12 Jun 2006, Chalky wrote:

Given that the speed of gravitational propagation is not controlled by
the electrical permittivity and magnetic permeability within a material
medium, why is the speed of gravity in vacuum still equal to the square
root of the product of electrical permittivity and magnetic
permeability of free space?

Because that's the fastest that anything can go. Light goes that fast in
free space because photons are massless, or, if you prefer, light is a
massless wave. Since a gravitational wave is also massless as far as we
know (whether or not gravitational waves are quantised), it also travels
at the same speed.

The permittivity and permeability of free space are essentially unit
conversion constants that result from our defining D and H in terms of
sources (charge density and current density) and E and B in terms of force
exerted on charges and currents. See
http://groups.google.com/group/sci.physics/msg/02c5db2aca385146?hl=en&
for more details.

That the maximum speed of propagation of "things" is equal to
1/sqrt(permittivity*permeability) is a necessary consequence of photons
being massless, rather than an amazing coincidence.

Exercise for the interested reader:
In analogy with the Maxwell equations, use mass density to define a
"gravitational displacement" field, and a force law of gravitational
fields exerted on mass density to define the gravitational equivalent of
E. Then write down the units and value of the gravitational
"permittivity". Then, what must the gravitational "permeability" be? Can
you write down other two gravitational Maxwell-like equations yet? What
would a gravitational "Ohm's law" mean?

Rather than a proposed exercise, explicitly do it and post it. The
results immediately suggest experimental verification in both orthodox
(Gravity Probe B and mass currents) and heterodxx (below) venues.

Maxwell's equations contain vector cross products (generalized as
vector triple products). Left and right hands are *distinguishable*
(e.g., polarized light, optical gyrotropy of media; optical rotatory
dispersion and circular dichroism spectrometries). If you wrote a
corresponding theory of gravitation with spacetime torsion rather than
spacetime curvature,

1) Gravitation would look like a Lorentz force.

2) Torsion gravitation predictions would need be fundamentally
indistinguishable from those of metric gravitation - there is only one
observed reality - EXCEPT...

3) ...in matters of angular momentum of non-zero rest mass -
physical spin, massed particle quantized spin, and chirality.
Spacetime torsion is not symmetric to parity inversion (left- and
right-hand coordinate systems ). Test masses with any of those
polarized properties would pursue diastereoptoic minimum action vacuum
free fall trajectories. This could be modeled as interaction with a
chiral pseudoscalar vacuum background. A left foot is not detected by
a sock or a left shoe, only by a right shoe.

One can then physically decide between metric and teleparallel
gravitation by explicit experiment. Is the Equivalence Principle
violated by any of the three disjoint cases?

1) Physical spin. The body must be traveling relativistic vs. the
observer or it is only helicity that reverses with point of view.
Binary pulsars have equatorial velocities only ~20% of lightspeed.
Not nearly sensitive enough as an observed test case for both reasons.

2) Magnetism. The theoretically most magnetically polarized test
mass would be aligned undecatiplet manganese. Its active mass
fraction would be 5x10^(-5), polarized electron mass/total mass. The
most extreme real world magnets approach 10^(-7). Alignment of
nuclear spins is ruined by thermal disorder and the small size of
nuclear vs. Bohr magnetons, and nuclei are fermion composite
stuctures. Physical experiments are limited to about 50
parts-per-billion net polarized mass. There is nothing at which to
look. Nothing is observed at 10^(-13) difference/average sensitivity,

http://www.aip.org/pnu/2006/775.html

3) Geometric parity, chirality in all directions,

http://www.mazepath.com/uncleal/invert.gif

Quantitative geometric parity can be explicitly calculated for any set
of countable points (e.g., atoms) with overall finite moments of
inertia. A centimeter-scale periodic single crystal is self-similar
down to its 0.1 nm scale unit cell, allowing relative atom positions
to be known. Left-handed and right-handed quartz are maximially
parity divergent atom by atom. At least 99.9726% of quartz test mass
(nucleus' positions) is parity-active mass. Neutron diffraction and
x-ray diffraction give identical coordinates, so toss in all but the
valence electrons, too.

Do single solid spheres of left-handed and right-handed quartz locally
vacuum free fall along parallel trajectories? If they do not, metric
General Relativity is wrong at the founding postulate level though it
has never made a falsified prediction. Gravitation would be
demonstrated to be teleparallel with spacetime torsion rather than
spacetime curvature. Unlike physical and quantum spins, it would be a
large amplitude signal vs. experimental sensitivity.

Somebody should look.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf
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