| Topic: |
Science > Physics |
| User: |
"George Kinley" |
| Date: |
14 Jan 2005 04:42:27 AM |
| Object: |
Light and gravit |
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
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| User: "Old Man" |
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| Title: Re: Light and gravit |
14 Jan 2005 10:21:55 PM |
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"George Kinley" <georgekinley@hotmail.com> wrote in message
news:nmNFd.34881$g4.647017@news2.nokia.com...
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
Why doesn't the matter in a galaxy collapse into its central
black-hole ?
Orbital angular momentum.
For a hyperbolic trajectory (escape trajectory), a body of
mass, m, a distance, R, from a black-hole of mass, M,
must possess orbital angular momentum, L, exceeding
L > L_escape = sqrt[ (2 G M m) R ]
That's from Newtonian gravity. For GTR, there's an
extra term that's responsible for orbital precession
of massive bodies and for null geodesics for light.
Except for the case of a head-on collision ( L= 0 ),
light possesses orbital angular momentum WRT the
central black-hole. Light follows a null geodesic that's
due to the extra term described above.
[Old Man]
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| User: "Bill Hobba" |
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| Title: Re: Light and gravit |
15 Jan 2005 12:14:13 AM |
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"Old Man" <nomail@nomail.net> wrote in message
news:gcSdnRRewcr6B3XcRVn-hg@prairiewave.com...
"George Kinley" <georgekinley@hotmail.com> wrote in message
news:nmNFd.34881$g4.647017@news2.nokia.com...
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
Why doesn't the matter in a galaxy collapse into its central
black-hole ?
Orbital angular momentum.
For a hyperbolic trajectory (escape trajectory), a body of
mass, m, a distance, R, from a black-hole of mass, M,
must possess orbital angular momentum, L, exceeding
L > L_escape = sqrt[ (2 G M m) R ]
That's from Newtonian gravity. For GTR, there's an
extra term that's responsible for orbital precession
of massive bodies and for null geodesics for light.
Except for the case of a head-on collision ( L= 0 ),
light possesses orbital angular momentum WRT the
central black-hole. Light follows a null geodesic that's
due to the extra term described above.
[Old Man]
Just seconding Randy's note - very good answer. Thanks for posting it.
Thanks
Bill
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| User: "Randy M. Dumse" |
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| Title: Re: Light and gravit |
14 Jan 2005 10:42:32 PM |
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"Old Man" <nomail@nomail.net> wrote in message
news:gcSdnRRewcr6B3XcRVn-hg@prairiewave.com...
Orbital angular momentum.
[Old Man]
A very nice answer! Deeply insightful.
--
Randy M. Dumse
Caution: Objects in mirror are more confused than they appear.
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| User: "Tom Roberts" |
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| Title: Re: Light and gravit |
15 Jan 2005 11:57:20 AM |
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Old Man wrote:
"George Kinley" <georgekinley@hotmail.com> wrote in message
news:nmNFd.34881$g4.647017@news2.nokia.com...
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
It is the density of matter+energy that basically determines whether
gravitational collpse occurs or not, and galaxies have densities far
below the value for which a global event horizon would form around the
entire galaxy. Local regions of galaxies, however can indeed exceed this
density, and objects that look the way black holes are predicted to look
are found at the centers of many galaxies (including the Milky Way). So
light that avoids capture by any local black hole within the galaxy can
escape (as long as it is not absorbed by other objects).
Why doesn't the matter in a galaxy collapse into its central
black-hole ?
Orbital angular momentum.
For a hyperbolic trajectory (escape trajectory), a body of
mass, m, a distance, R, from a black-hole of mass, M,
must possess orbital angular momentum, L, exceeding
L > L_escape = sqrt[ (2 G M m) R ]
That's from Newtonian gravity.
[I assume a Schwarzschild black hole, as you implicitly did.
Note R must be > 2GM/c^2 everywhere on the path for your
condition to hold. Note also that the black holes at the
centers of galaxies are NOT Schwarzschild black holes --
they rotate. But that complicates things too much for a
discussion here, and the basic conclusions remain valid.]
Note this is the condition for a trajectory of an object approaching
from far away to escape. An object that finds itself at some value of R
(>2GM/c^2) can escape radially outward with zero angular momentum if it
is kicked outward hard enough (or if is a rocket strong enough to
accelerate away).
EM radiation emitted radially outward from any R > 2GM/c^2 will escape
(but will be redshifted depending on R). This is important to know,
because infalling matter invariably hits other infalling matter, and for
a large black hole such collisions generate lots of radiation (much of
it X-rays and gamma rays). It is the radiation of such infalling matter
collisions that we see from around objects that look like black holes.
Note that the kinetic energy of a single proton falling into a million
solar mass black hole is much larger than the energy released per proton
from nuclear fusion inside stars, so such massive black holes can be
enormous radiation producers.
Tom Roberts tjroberts@lucent.com
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| User: "Ken S. Tucker" |
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| Title: Re: Light and gravit |
15 Jan 2005 02:20:22 PM |
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Did Tom just kill baryon conservation?
Please do the math Tom, drop a brick
into the sci-fi black-hole, figure baryon
conservation holds (or no), then red-shift
the outbound energy.
Hint: think a super ball bouncing off
pavement. <shrug>
What is the source of all this radiation
you claim?
Ken
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| User: "Tom Roberts" |
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| Title: Re: Light and gravit |
17 Jan 2005 05:26:47 PM |
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Ken S. Tucker wrote:
Did Tom just kill baryon conservation?
Good heavans, no. GR never had baryon conservation. It doesn't even have
baryons....
Please do the math Tom, drop a brick
into the sci-fi black-hole, figure baryon
conservation holds (or no), then red-shift
the outbound energy.
But there is no "outbound energy" -- that brick is falling in. Only if
it collides with other infalling matter will there be any "outbound
energy" (and you didn't mention any).
Indeed, this sort of scenario is one reason why GR has no global
conservation of energy (the brick leaves the manifold)....
Hint: think a super ball bouncing off
pavement. <shrug>
Black holes are not at all like "pavement".
What is the source of all this radiation
you claim?
In my previous post I was quite clear: infalling matter colliding with
other infalling matter.
Tom Roberts tjroberts@lucent.com
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| User: "Ken S. Tucker" |
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| Title: Re: Light and gravit |
17 Jan 2005 06:01:55 PM |
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Ok Tom,
Suppose we set-up a scenario as you suggest, two bricks
in orbit about a n-star (BH if you want), persuming the bricks
are moving relativistically, (pick your %), but in opposite
directions and collide...bang, lots of noise.
Three (at least) create radiation
1) electron to electron (aka chemical)
2) nuclear to nuclear (aka atomic)
3) particle to particle (aka baryon decay)
Given the red-shift from the location of the energy event,
(near the *horizon*) which of these three above gives, the
radiation output you describe as spectacular?
Regards
Ken S. Tucker
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| User: "Old Man" |
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| Title: Re: Light and gravit |
14 Jan 2005 10:05:13 PM |
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"George Kinley" <georgekinley@hotmail.com> wrote in message
news:nmNFd.34881$g4.647017@news2.nokia.com...
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
Why doesn't the matter in a galaxy collapse into its central
black-hole ?
Orbital angular momentum.
For a hyperbolic trajectory (escape trajectory), a body of
mass, m, a distance, R, from a black-hole of mass, M,
must possess orbital angular momentum, L, exceeding
L > L_escape = sqrt[ (G M m) R ]
That's from Newtonian gravity. For GTR, there's an
extra term that's responsible for orbital precession
of massive bodies and for null geodesics for light.
Except for the case of a head-on collision ( L= 0 ),
light possesses orbital angular momentum WRT the
central black-hole. Light follows a null geodesic that's
due to the extra term described above.
[Old Man]
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| User: "N:dlzc D:aol T:com \dlzc" |
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| Title: Re: Light and gravit |
14 Jan 2005 05:39:35 PM |
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Dear George Kinley:
"George Kinley" <georgekinley@hotmail.com> wrote in message
news:nmNFd.34881$g4.647017@news2.nokia.com...
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
The effect of gravity drops off roughly as 1/r^2. So light has no issues
with this. The velocities you imagine for these galaxies are low, and
gravity has aeons to do its job.
Some observer elsewhere in the Universe might see "gravitational lensing"
of light that passes by us. But most likely not.
The average density of the "Virgo supercluster", of which the Milky Way is
a part, is less than that of the 'atmosphere' where the ISS orbits. We are
smoke, observing smoke...
David A. Smith
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| User: "Uncle Al" |
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| Title: Re: Light and gravit |
14 Jan 2005 04:20:12 PM |
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George Kinley wrote:
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
1/r^2
--
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: "Last Timer" |
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| Title: Re: Light and gravit |
15 Jan 2005 02:00:01 PM |
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Uncle Al wrote:
George Kinley wrote:
Hi,
If our galaxy is rotating around other group of galaxy under
influenece
of combined gravity of galaxies, which mean that combined gravity
of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel
under
such emmence gravity
1/r^2
So you are saying light is made of particles with mass or just a whimsy
that decides to visit a blackhole because the source it came from made
it so?
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| User: "Old Man" |
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| Title: Re: Light and gravit |
16 Jan 2005 11:42:48 PM |
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"Last Timer" <dakshing64@yahoo.com> wrote in message
news:1105819201.846299.326440@c13g2000cwb.googlegroups.com...
Uncle Al wrote:
George Kinley wrote:
Hi,
If our galaxy is rotating around other group of galaxy under
influenece
of combined gravity of galaxies, which mean that combined gravity
of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel
under
such emmence gravity
1/r^2
So you are saying light is made of particles with mass or just a whimsy
that decides to visit a blackhole because the source it came from made
it so?
Uncle Al is aware of null geodesics whereof the photon's
effective potential is given by ( G = c = 1 )
W_eff = [ 1 / r^2 ] [ 1 - 2M / r ]
for r >> 2M , W_eff = 1 / r^2 (as per Uncle Al)
Which has the same radial dependence as that of orbital
angular momentum. Replacing r with r' such that,
r' = r / M, the photon's trajectory depends only upon
the initial impact parameter.
[Old Man]
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| User: "" |
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| Title: Re: Light and gravit |
15 Jan 2005 12:53:05 AM |
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George Kinley wrote:
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
Your question doesn't make sense. Where did you get hold
of the basis for your "question"?
John Anderson
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| User: "" |
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| Title: Re: Light and gravit |
15 Jan 2005 01:08:51 AM |
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George Kinley wrote:
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
Your statement "that combined gravity of
all the galaxy should be more then gravity of individual black hole"
is naive.
Gravity is a local force that can be stronger or weaker
at any location. The effect of gravity NEAR the event
horizon of a black hole is much stronger than the effect
much farther away. The effect of distant galaxies FAR from
the event horizon is much stronger than the effect of the black
hole.
Look at Newtonian gravity. The effect of gravity at the surface
of the earth is much weaker than the gravity of the sun that
keeps the earth in orbit. Otherwise, we would be pulled off
the earth into the sun.
John Anderson
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| User: "glbrad01" |
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| Title: Re: Light and gravit |
18 Jan 2005 06:11:56 AM |
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<ande452@attglobal.net> wrote in message news:41E8C183.5A62@attglobal.net...
George Kinley wrote:
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
Your statement "that combined gravity of
all the galaxy should be more then gravity of individual black hole"
is naive.
Gravity is a local force that can be stronger or weaker
at any location. The effect of gravity NEAR the event
horizon of a black hole is much stronger than the effect
much farther away. The effect of distant galaxies FAR from
the event horizon is much stronger than the effect of the black
hole.
Look at Newtonian gravity. The effect of gravity at the surface
of the earth is much weaker than the gravity of the sun that
keeps the earth in orbit. Otherwise, we would be pulled off
the earth into the sun.
John Anderson
Galactic clusters have no centralized blackhole holding them together yet
their gravity--as a single cluster unit without any single center of gravity
to the cluster--draws galaxies from a greater distance than can be accounted
for by any galaxy's gravity that is constituent to the galactic cluster.
This means the cluster itself must have a local gravity all its own, with a
reach all its own, raised in plane from the gravities of the galaxies that
constitute it.
Also, galaxies in cluster do not appear to orbit one another, nor do the
clusters as wholes appear to rotate in any way. Therefore nothing should
keep those galaxies from immediately rushing together and merging to become
one titanic blackhole. Yet something does keep them from it. Some kind of
harmonic force--other than angular momentum--keeps them civil in their
groupings of many. The gravity of a lot of galaxies in cluster acting upon
each galaxy in cluster, in the scheme of clustering, seems to act generally
to prevent mergers, therefore any final merger, rather than dictating
merger.
Brad
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| User: "Ken S. Tucker" |
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| Title: Re: Light and gravit |
18 Jan 2005 04:24:48 PM |
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Hi Brad
I studied the Virgo cluster a few years back and found the center
contains large spherical galaxies, tending threw ellipticals to
spirals. It appears (subjectively) the spiral is causal by the g-field
of the larger sphericals, rather like the Roche limit.
If you happen to study that send me an email,
dynamixs@uniserve.com, set x=c.
Ken
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| User: "Sam Wormley" |
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| Title: Re: Light and gravit |
14 Jan 2005 04:17:31 PM |
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George Kinley wrote:
Hi,
If our galaxy is rotating around other group of galaxy under influenece
of combined gravity of galaxies, which mean that combined gravity of
all the galaxy should be more then gravity of individual black hole
present in the centre of every galaxy, so how does light travel under
such emmence gravity
But the distances are great and gravity obeys Newton's Inverse Square Law
http://scienceworld.wolfram.com/physics/Gravity.html
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