1. I understand that light can be affected by gravitational lensing and am familiar with the mechanics of such.

My questions :::

Is gravity itself suseptable to lensing?

Is gravity cumulative? Not in the sense of greater density = greater gravity ::: but rather 10 identical black holes align along an axis, is gravity stronger along said axis and weaker than areas outside of axis ::: or no difference at all??

2.

3. I'm going to have to guess no. If gravity could be "bent" like light, then gravity would fail to escape a black hole and the black hole wouldn't exhibit gravitational attraction. This question seem's to approach paradox to me. I'm interested to hear an analysis presented by someone who understands GR.

4. Originally Posted by GiantEvil
I'm going to have to guess no. If gravity could be "bent" like light, then gravity would fail to escape a black hole and the black hole wouldn't exhibit gravitational attraction. This question seem's to approach paradox to me. I'm interested to hear an analysis presented by someone who understands GR.
I am not quite sure what the question means, and I suspect that the OP doesn't know either. But gravitational energy enters into the stress-energy tensor, and gravity does interact with itself. That is one reason for the existence of non-trivial solutions of the field equations of GR. This is sometimes phrased as "gravity gravitates."

It is also one of the difficulties with attempts at quantum field theories. Unlike the case with photons, gravitons would interact with gravitons.

5. gravitational energy enters into the stress-energy tensor,
"gravity gravitates."
I get exactly what your saying, and it is so hugely strange it warp's my mind. It's a good thing I gotta go to work, I can think about this without being distracted by book's or my computer.
EDIT: OK I'm back. If gravity gravitates, then is it possible to have a "gravity hole"? Like a black hole but gravity is incapable of escaping the event horizon.

6. 6nqpnw. Gravity can be cumulative in that the more mass exerting gravity over a given area, the greater the effects of gravity. However, since gravity does not affect gravity (which is how it escapes a black hole), it cannot be distorted in the sense you mean though other powerful sources can move the centre of attraction.

With a line of black holes, if sufficiently close to do what you want, they will just attract each other and coalesce to form one giant black hole.

7. Originally Posted by Cyberia
However, since gravity does not affect gravity .
Totally wrong, as usual.

8. I am trying to understand this "gravitation of gravity". So far I've got; Gravity is energy so consequently it enters into the stress-energy tensor, therein gravity is confined to the space-time geodesic.

I've found this; http://en.wikipedia.org/wiki/Stress-...m_pseudotensor. I only vaguely understand it and don't know if it's maybe crap.

Obviously gravity gravitates, but how does it do so without bending the universe into a FUBAR-pretzel? An explanation, a link, a clue? Somebody help, please!

9. Originally Posted by GiantEvil
therein gravity is confined to the space-time geodesic.

!
That makes no sense.

A geodesic is a curve in spacetime, a curve of maximum local distance (note maximal, not minimal as in the case of Riemannian geometry).

Light follows a geodesic. So do other bodies in the absence of forces other than gravity.

10. You're making the assumption that the gravity of the black hole is due to the swallowed mass/singularity inside the event horizon, Giant Evil, and needs to 'escape' the black hole in order to affect things outside the event horizon.
When the initial collapse occurrs, space/time is 'spherically bent' (I don't quite know how to explain in words) around the collapse (singularity or whatever) and what is actually affecting things external to the event horizon, is actually the curvature of space/time. That is what we call gravity in GR.
Now maybe DrR can elaborate, I know I cannot. But maybe that's one of the problems with a quantum theory of gravity that uses gravitons to propagate the gravitational effect. In the vicinity of a black hole the gravitons would be constrained just as photons, the propagators of EM force, are

11. Originally Posted by MigL
Now maybe DrR can elaborate, I know I cannot. But maybe that's one of the problems with a quantum theory of gravity that uses gravitons to propagate the gravitational effect. In the vicinity of a black hole the gravitons would be constrained just as photons, the propagators of EM force, are
So far as I know the quantum gravity crowd is not far enough along to have encountered that problem yet. I don't know how that would work.

12. But gravitational energy enters into the stress-energy tensor, and gravity does interact with itself. That is one reason for the existence of non-trivial solutions of the field equations of GR. This is sometimes phrased as "gravity gravitates."
Light follows a geodesic. So do other bodies in the absence of forces other than gravity.
So gravity "bends" gravity, but not along the geodesic?

Thanks for your answer Migl. I'm going to stick with "spherical cow's" for the moment and avoid black holes, well okay, maybe not. If the initial mass responsible for a black hole collapses into a singularity, then wouldn't all black holes be equivalent regardless of initial mass?

My current understanding of these things is leading to paradox. Obviously my understanding is deficient, I'm trying to reach the far shore.

13. Originally Posted by GiantEvil
So gravity "bends" gravity, but not along the geodesic?
Go read it again -- particularly the definition of a geodesic.

14. For now I'll be satisfied with the qualitative description that gravity "bend's" gravity and worry about understanding the particulars later. Thanks guys.

15. So it sounds as though a graviton's geodesic path can be altered by gravity, since given "gravity gravitates."

Can 2 units of gravity [gravitons?] (each from a different source) occupy the same point in space/time?

Can 2 units of gravity (again, each from a different source) travel along an identical geodesic?

::: @MigL :::
Are you saying that gravity is generated from the curvature of space/time around the event horizon (external), rather than from the singularity (internal)? Bizarre. Doesn't that violate the definition of gravity as mass is a requirement for gravity; event horizons are energy, right?

::: <shakes fist @Cyberia> :::
Fine. Instead of 10 BHs, say there's a binary BH system in a stable whirl orbit. Could a concentration of gravity (graviton beam?) be produced following the axis of separation outside the BHs' orbits?

16. Originally Posted by 6nqpnw
So it sounds as though a graviton's geodesic path can be altered by gravity, since given "gravity gravitates."

Can 2 units of gravity [gravitons?] (each from a different source) occupy the same point in space/time?
No one knows if gravitons exist. But if they do, they are bosons, not fermions, and therefore would not be subject to the
Ppauli exclusion principle

Originally Posted by 6nqpnw
Can 2 units of gravity (again, each from a different source) travel along an identical geodesic?
A geodesic is a curve in spacetime, so if the gravitons originate at different points their geodesic paths would have to be different. This is just like saying that the time dependent path of a bus from New York to Los Angeles is different from that of a bus from Buffalo to San Francisco.

Originally Posted by 6nqpnw
::: @MigL :::
Are you saying that gravity is generated from the curvature of space/time around the event horizon (external), rather than from the singularity (internal)? Bizarre. Doesn't that violate the definition of gravity as mass is a requirement for gravity; event horizons are energy, right?
The event horizon isn't anything physical, it is just a boundary. Gravirt IS (not is generated by) curvature if spacetime. It is not curvature "around the event horizon". In the case of the black hole it is a result of the mass/energy of the entire black hole, none of which is in the event horizon.

Forget about the singularity. It quite likely is not a physical thing.

Originally Posted by 6nqpnw
::: <shakes fist @Cyberia> :::
Fine. Instead of 10 BHs, say there's a binary BH system in a stable whirl orbit. Could a concentration of gravity (graviton beam?) be produced following the axis of separation outside the BHs' orbits?
You can safely ignore Cyberia. He has no idea what he is talking about.

17. Boatswains, pheromiones, Pauli Shore, gravirt <giggle> ::: that's alot for me to chew on.

However, on the quick:

Originally Posted by DocRock
This is just like saying that the time dependent path of a bus from New York to Los Angeles is different from that of a bus from Buffalo to San Francisco.
I'm askin' if those two buses can deviate from their paths onto a single road prior to reaching said destinations (for any given distance)? Not only the same path / direction / speed / time, but also the same point in spacetime?

Given the nature of gravitons (the pressumed product of gravity) as bosons, the short answer should be 'yes,' right? Cuz that's what lasers are (concentrated light = photons = bosons : concentrated gravity = gravitons = bosons) or something like that.

The point I was making with MigL is if the gravity isn't coming from within the event horizon, then where else could enough mass exist to generate the immense gravitational effects of BHs (forgetting the singularity).
You're making the assumption that the gravity of the black hole is due to the swallowed mass/singularity inside the event horizon
I was trying to state that the dense collection of mass within an event horizon MUST BE the source of the gravity.

18. I'm going to guess that GR and QG are just simply not compatible. It's got to be one or the other.
In GR gravity IS the geodesic. The geodesic is determined by a thing called the stress-energy tensor(?). Since E=Mc^2 the stress-energy tensor considers mass and energy(including gravitational energy)(?). I understand that there is a difference in how the time coordinate and the spacial coordinates are treated, but that is where my understanding end's in that matter.
In QG there is an exchange of particles(gravitons) and would seem no need for a geodesic(?).
It seem's to be similar to the situation of electromagnetism and classical dynamics before GR. Two variant set's of laws to describe physics.

There are certainly things out there(in space) that are not visible and act like holes. Other than that, black holes are strange beyond my own conception.

19. Originally Posted by GiantEvil
.
In GR gravity IS the geodesic.
no

Originally Posted by GiantEvil
.
The geodesic is determined by a thing called the stress-energy tensor(?).
It is not THE geodesic. There are lots of geodesics. In fact the nature of the famous "sigularities" of black holes and the big bang is a failure to be time-like geodesically complete.

You are getting yourself badly confused by failure to understand what a geodesic is.
http://en.wikipedia.org/wiki/Geodesic

Originally Posted by GiantEvil
.
Since E=Mc^2 the stress-energy tensor considers mass and energy(including gravitational energy)(?).
And other things.

Originally Posted by GiantEvil
.
I understand that there is a difference in how the time coordinate and the spacial coordinates are treated, but that is where my understanding end's in that matter.
In QG there is an exchange of particles(gravitons) and would seem no need for a geodesic(?).
The difference between local time coordinates and local spatial coordinates is breflected in the metric in GR.

Since there is no QG it is difficult to answer your question. But any successful QG will have to contain GR as a limit in ordinary circumstances.

I
Originally Posted by GiantEvil
.
t seem's to be similar to the situation of electromagnetism and classical dynamics before GR. Two variant set's of laws to describe physics.
Can't agree.

Originally Posted by GiantEvil
.
There are certainly things out there(in space) that are not visible and act like holes. Other than that, black holes are strange beyond my own conception.
huh?

20. A geodesic is a curve in spacetime, a curve of maximum local distance (note maximal, not minimal as in the case of Riemannian geometry).
A geodesic is a curve in spacetime, so if the gravitons originate at different points their geodesic paths would have to be different. This is just like saying that the time dependent path of a bus from New York to Los Angeles is different from that of a bus from Buffalo to San Francisco.
Gravirt IS (not is generated by) curvature if spacetime.
I'm not trying to call you out on anything here Doc, but this is where I got the impression that "Gravity is the geodesic". If A=B, and B=C, then A=C.
I'm going to guess that "geodesic" refers to paths taken by objects(photons, planet's, bus's, etc...) on the geometric structure of space-time called a "manifold"(?).
There are certainly things out there(in space) that are not visible and act like holes. Other than that, black holes are strange beyond my own conception.
I was trying to say "I believe in black holes even though I know little about them" in some way that wasn't incriminatingly stupid.

21. Thanks for clarifying my points DrR, but it looks like some of us still don't get it. According to the most accurate and complete theory of gravity that we currently have, the force that we experience and call gravity is the curvature of space/time,and an object will follow that curvature, or geodesic as per GR.
Whether that curvature is due to a visible object such as the Sun or a planet, a concentration of energy, or even an oject which undergoes gravitational collapse to become a black hole. The information left to us at that point is mass (determines the size of the Swartzchild radius or event horizon and proportional entropy), its charge and spin or ang. momentum. ALL other information is forever lost to us as to the size,shape, composition, colour, etc. of the original object.

22. Originally Posted by GiantEvil
I'm not trying to call you out on anything here Doc, but this is where I got the impression that "Gravity is the geodesic". If A=B, and B=C, then A=C.
I'm going to guess that "geodesic" refers to paths taken by objects(photons, planet's, bus's, etc...) on the geometric structure of space-time called a "manifold"(?).
.
no

One way to look at it is that curvature is reflected in the rate at which nearby geodesics diverge, and that divergence is gravity.

You are not going to understand this unless you do some reading and begin to understand the concept of "manifold" and "geodesic". You might try reading The Road to Reality by Roger Penrose.

23. Alright ::: lemme paraphrase to see if I grasp what u guys are puttin' out.

Let's roll a few bowling balls on a trampoline. The trampoline IS gravity; the bowling balls affect gravity (by warping / curving it) rather than producing it. Gravity is not emitted, it's already there; where there's spacetime, there's gravity. Which makes sense, 'cuz gravity never completely diminishes: strength is proportional to distance from mass - infinite.

Black holes bend this curvature in on itself; as if one of the bowling balls were so dense, it creates a conical hole in the trampoline...a hole that infinitely grows in depth and width because the mass infinitely grows more dense as more stuff falls in.

Therefore, for all intents and purposes, a black hole IS a 'gravity beam.'

24. Originally Posted by 6nqpnw
Alright ::: lemme paraphrase to see if I grasp what u guys are puttin' out.

Let's roll a few bowling balls on a trampoline. The trampoline IS gravity;
No

Originally Posted by 6nqpnw
the bowling balls affect gravity (by warping / curving it) rather than producing it. Gravity is not emitted, it's already there; where there's spacetime, there's gravity.
No

Gravity requires that spacetime be curved. There is nowhere in the universe where there is no spacetime -- the universe is spacetime and spacetime is the universe.

Originally Posted by 6nqpnw
Which makes sense, 'cuz gravity never completely diminishes: strength is proportional to distance from mass - infinite.
No

Gravity, in the far field obeys an inverse square law

Originally Posted by 6nqpnw
Black holes bend this curvature in on itself; as if one of the bowling balls were so dense, it creates a conical hole in the trampoline...a hole that infinitely grows in depth and width because the mass infinitely grows more dense as more stuff falls in.
No. No. NO.

If there is anything correct in this paragraph it is very elusive.

Originally Posted by 6nqpnw
Therefore, for all intents and purposes, a black hole IS a 'gravity beam.'
NO

Congratulations, you have misunderstood literally everything.

25. <buries face in hands>

I am so ashamed.

26. Let's roll a few bowling balls on a trampoline. The trampoline IS gravity; the bowling balls affect gravity (by warping / curving it) rather than producing it. Gravity is not emitted, it's already there; where there's spacetime, there's gravity. Which makes sense, 'cuz gravity never completely diminishes: strength is proportional to distance from mass - infinite.
As DrRocket says, spacetime itself is not gravity, curved spacetime is. It is like a hard drive. An empty hard drive has no data in and of itself. A pattern has to exist on the magnetic discs for data to be present.

27. Originally Posted by DrRocket
Originally Posted by Cyberia
However, since gravity does not affect gravity .
Totally wrong, as usual.
Are you a creationist?

I post on religious forum and come across such answers all the time, from people who know what they say is god's own truth, so they feel they do not need o provide any evidence. Just a blank statement of infallibility.

They do not work well on science forums as you should have found out by now.

So while gravity can of course be cumulative, your evidence that gravity can trap gravity in a black hole is......?

28. Originally Posted by DrRocket
You can safely ignore Cyberia. He has no idea what he is talking about.
And yet you are unable to explain why I am wrong so are limited to empty statements and insults regarding what I say.

29. Originally Posted by Cyberia
Originally Posted by DrRocket
You can safely ignore Cyberia. He has no idea what he is talking about.
And yet you are unable to explain why I am wrong so are limited to empty statements and insults regarding what I say.
You have yet to say anything that can be taken seriously.

30. Originally Posted by Cyberia
Originally Posted by DrRocket
Originally Posted by Cyberia
However, since gravity does not affect gravity .
Totally wrong, as usual.
Are you a creationist?

I post on religious forum and come across such answers all the time, from people who know what they say is god's own truth, so they feel they do not need o provide any evidence. Just a blank statement of infallibility.

They do not work well on science forums as you should have found out by now.

So while gravity can of course be cumulative, your evidence that gravity can trap gravity in a black hole is......?
What garbage.

What I have said is that gravity itself contributes to the stress-energy tensor of general relativity. That is sometimes stated as "gravity gravitates". It is reflected in the existence if non-trivial vacuum solutions of the Einstein field equations.

Maybe you should stop making such idiotic statements.

31. Originally Posted by Cyberia
Originally Posted by DrRocket
You can safely ignore Cyberia. He has no idea what he is talking about.
And yet you are unable to explain why I am wrong so are limited to empty statements and insults regarding what I say.
See above for just one of many examples demonstrating that you are wrong.

As to WHY you are wrong, it is quite obvious that you are ignorant of basic science. Since you have stated that you are quite aware of resources for learning basic science, one is forced to conclude that you lack the capacity to learn and understand the subject matter.

One can be ignorant without being stupid, but it clearly is enabling.

32. Okay. Enough now with the entertaining personal quarrels. Unless we get back to the original topic of this thread, I will close it.

Dishmaster
(Moderator)

33. Originally Posted by KALSTER
As DrRocket says, spacetime itself is not gravity, curved spacetime is.
What is the difference between gravity curving spacetime and the effect of gravity influencing something directly? No one talks of spaceheat as the sun's heat warms the Earth.

34. Again, gravity does not curve spacetime, gravity is curved spacetime. Origami swans don't fold the paper, origami swans are folded paper.

35. How does spacetime become "curved"?

36. Originally Posted by DrRocket
So while gravity can of course be cumulative, your evidence that gravity can trap gravity in a black hole is......?
I asked for evidence that gravity can adversely affect gravity and you wander off down a side street:

What I have said is that gravity itself contributes to the stress-energy tensor of general relativity. That is sometimes stated as "gravity gravitates". It is reflected in the existence if non-trivial vacuum solutions of the Einstein field equations.
More gravity is like trying to drown the Atlantic Ocean with water.

37. Originally Posted by Cyberia
How does spacetime become "curved"?
Mass/energy does it. Dunno.

38. Originally Posted by KALSTER
As DrRocket says, spacetime itself is not gravity, curved spacetime is. It is like a hard drive. An empty hard drive has no data in and of itself. A pattern has to exist on the magnetic discs for data to be present.
That made it -click- ::: thx Kal. I nearly had myself convinced that DocRock and MigL were being completely contradictory and was about to throw their useful feedback out the window.

39. Originally Posted by Cyberia
How does spacetime become "curved"?
see "general relatvity"

40. Gravity is the effect that matter has on spacetime. A black hole doesn't pull light back in on itself because photons are energy / massless; gravity and light don't interact directly. Within the Schwartzschild radius, light follows a geodesic of spacetime that gravity has curved back on itself.

And I've noticed a few slams regarding singularities. Are those folks suggesting that there is no matter within the event horizon of black holes? ...that matter is somehow 'vaporized' into energy that can't find a geodesic to follow that leads it out of the black hole's grip?

I anxiously await validation ::: like an abused child.

41. Originally Posted by 6nqpnw
And I've noticed a few slams regarding singularities. Are those folks suggesting that there is no matter within the event horizon of black holes? ...that matter is somehow 'vaporized' into energy that can't find a geodesic to follow that leads it out of the black hole's grip?

I anxiously await validation ::: like an abused child.
There is almost certainly matter within the event horizon. That has nothing to do with a singularity.

There is no spacetime geodesic leading from inside the event horizon to outside of it.

Nobody knows what happens to matter deep in the interior of a black hole. That will probably take a unified theory of gravity and quantum theory -- a theory that does not yet exist.

42. If the black hole was extremely large, we're talking galaxy size here, the tidal forces experienced upon crossing the event horizon would be much less severe than a smaller black hole and you could possibly survive 'spghettification' for a little while. You would have time to see the horizon coming up along side you and your view of the exterior universe would be a gradually shrinking circle over your head ( going in feet first ) until the tidal forces stretch you head-foot and compress you from sides, ie make spaghetti out of you.
As far as we know the only thing in your future, at this point, is the centre of the event horizon. Your timeline leads there and only there. So, yes there is matter inside the event horizon, but at some point, we cease to have a valid description of the state of this matter because we know of no mechanism which will stop the final compression into a point, or singularity. The infinitely dense point, or singularity, however, leads to other problems and this makes a lot of physicists unconfortable with the singularity, and they are actively looking for ways to avoid it. As DrR pointed out, however, we may have to wait for a Quantum Gravity theory before we reach an understanding of the deep interior of black holes.

43. Never bought the grav. lensing postulation, it would be more logically with plasma/gas lensing, but that's just my personal view.

44. Originally Posted by HexHammer
Never bought the grav. lensing postulation, it would be more logically with plasma/gas lensing, but that's just my personal view.
What do you mean? It is not a postulate anymore. It is verified by theory, observation and experiment.

45. Originally Posted by Dishmaster
Originally Posted by HexHammer
Never bought the grav. lensing postulation, it would be more logically with plasma/gas lensing, but that's just my personal view.
What do you mean? It is not a postulate anymore. It is verified by theory, observation and experiment.
It never was a postulate. It was a phenomena predicted by general relativity. Observational confirmation is more evidence supporting the validity of general relativity.

46. Originally Posted by DrRocket
It never was a postulate. It was a phenomena predicted by general relativity. Observational confirmation is more evidence supporting the validity of general relativity.
To my knowledge it has never been proved, only theorized. That observation counts for nothing as they have only seen what they "want to see", with a mere piece of glass I can do the excat same effect.

47. Originally Posted by HexHammer
Originally Posted by DrRocket
It never was a postulate. It was a phenomena predicted by general relativity. Observational confirmation is more evidence supporting the validity of general relativity.
To my knowledge it has never been proved, only theorized. That observation counts for nothing as they have only seen what they "want to see", with a mere piece of glass I can do the excat same effect.
"Proof" applies to mathematics, but not to science.

General relativity, which is supported by a great deal of experimental evidence, predicts gravitational lensing. Gravitational lensing has been observed many times.

Unless you are accusing the scientists reporting lensing of fraud, the fact that you can mimic it with optical trickery or computer software is irrelevant. If you are making such an accusation then you need hard evidence.

Who, specifically do you accuse of fraud ?

http://csep10.phys.utk.edu/astr162/l...s/lensing.html

http://astro.berkeley.edu/~jcohn/lens.html

48. DrRocket where do I accuse anyone of fraud, now you jump to conclusions, spaer me please.

49. Originally Posted by HexHammer
You most certainly imply it in your last sentence, quoted above.

Why spare you ?

50. Originally Posted by DrRocket
Originally Posted by HexHammer
You most certainly imply it in your last sentence, quoted above.

Why spare you ?
You are seeing things, what I imply is things like simplemindedness, naivity and group think ...etc.

51. Originally Posted by HexHammer
To my knowledge it has never been proved, only theorized. That observation counts for nothing as they have only seen what they "want to see", with a mere piece of glass I can do the excat same effect

Originally Posted by HexHammer
Originally Posted by "DrRocket
You most certainly imply it in your last sentence, quoted above.
Originally Posted by HexHammer
]You are seeing things, what I imply is things like simplemindedness, naivity and group think ...etc.

These are the facts.

I am not seeing things. I am reading things.

52. LOL?

Dude, I'll spare myself a headache and just put you on ignore.

53. Back on topic :::

Since the effect of gravity is infinite in range and bends spacetime, then light geodesics from all distant galaxies are effectively distorted. Right?

54. One at a time, please ::: jeez. Guess I'm not making any sense...again.

Is it possible to reproduce the redshift of distant galaxies with a single gravity source so strong and undetectably distant that it stretches / elongates the path of light from said galaxies on its multi-million year jouney to our telescopes? And do be gentle explaining 'why not' as I bruise easily.

55. Originally Posted by 6nqpnw
One at a time, please ::: jeez. Guess I'm not making any sense...again.

Is it possible to reproduce the redshift of distant galaxies with a single gravity source so strong and undetectably distant that it stretches / elongates the path of light from said galaxies on its multi-million year jouney to our telescopes? And do be gentle explaining 'why not' as I bruise easily.
What is "strong and undetectably distant" other than an oxymoron ?

56. Originally Posted by 6nqpnw
Is it possible to reproduce the redshift of distant galaxies with a single gravity source so strong and undetectably distant that it stretches / elongates the path of light from said galaxies on its multi-million year jouney to our telescopes? And do be gentle explaining 'why not' as I bruise easily.
Simply put, the effect that gravity can have on the redshift (or blueshift) of light is relative, between the source and the emitter.

If light has to "climb out" of the steep gravity well of a massive object, and is subsequently detected after it "falls into" the less steep gravity well of a less massive object, it will show a redshift. And vice-versa.

Light emitted from a neutron star would look redshifted to us. Light emitted by the Sun would look blueshifted to an observer near a neutron star. The difference in the gravity around the emitter and the source is what causes the shift.

Now, if light passes a massive object on its journey, what happens? The light is blueshifted as it falls into the gravity well, and is redshifted again, by the same amount, as it climbs out again. So there is no net difference.

It is only the gravity around the source, when compared to the gravity around the detector, that causes gravitational redshift.

Unless, that is, you consider the gravitational density of the whole universe, in the past, when compared to now. If the universe were denser in the past, then perhaps we can think of cosmological redshift as due to a change in the gravitational density of the universe, over time. The light from distant galaxies has been climbing out of a young dense universe and today is detected in an older, less dense universe. The change in the gravitational density is due to the expansion of the universe.

57. Originally Posted by DrRocket
[What is "strong and undetectably distant" other than an oxymoron ?
The same as 'dark matter' and 'dark energy' ::: figments of the imagination.

Originally Posted by SpeedFreek
Now, if light passes a massive object on its journey, what happens? The light is blueshifted as it falls into the gravity well, and is redshifted again, by the same amount, as it climbs out again. So there is no net difference.
Awesome, awesome, awesome explanation. Thx for your time, bud. While I don't have the credentials or expertise to verify the validy of your statements, let's assume you're right and move forward with this thought experiment.

With your quoted statement in mind and completely ignoring expanding space altogether, could a distant galaxy and the observer move towards one another and still observe a redshift if the light is influenced (not neccesarily passes) by a massive object?

58. Originally Posted by 6nqpnw
Originally Posted by SpeedFreek
Now, if light passes a massive object on its journey, what happens? The light is blueshifted as it falls into the gravity well, and is redshifted again, by the same amount, as it climbs out again. So there is no net difference.
Awesome, awesome, awesome explanation. Thx for your time, bud. While I don't have the credentials or expertise to verify the validy of your statements, let's assume you're right and move forward with this thought experiment.

With your quoted statement in mind and completely ignoring expanding space altogether, could a distant galaxy and the observer move towards one another and still observe a redshift if the light is influenced (not neccesarily passes) by a massive object?
If we ignore expanding space, and a distant galaxy and observer are moving towards one another, each would measure a blueshift in the spectrum of the light emitted from the other.

How can light be influenced by the gravity of a massive object unless it passes through the gravitational field of that massive object? If it passes through, it comes out the same as it was when it went in....

59. Take a quick look at my profile pic (thumbnail, avatar, w/ev, the pic to the left) ::: notice the orange ultraSMBH (top) and the two galaxies (bottom). Say the 2 galaxies are several million years apart and 'falling' towards the orange ultraSMBH; thus, the 2 galaxies are getting relatively closer (your blueshift).

My question is at what point (if any) would the light geodesic redshift (to the observer) as they both approach the ultraSMBH? My thought is a few million years is a lot of time for gravity to stretch...nah...increase the length of a geodesic, resulting in a redshift proportional to its distance. And since gravity is stronger with closer proximy to its source, then over time the geodesics would continue to lengthen at an increasing rate (perceived as accelerating expansion to the observer).

60. You seem to be mixing up a few concepts there. Redshift is only proportional to distance when the expansion of the universe is taken into account, which is not the case in your example.

There is redshift or blueshift due to relative motion through space (Relativistic Doppler effect).

There is redshift of blueshift due to the difference in gravitational potential of the source when compared to the observer (Gravitational red/blue shift).

And there is redshift due to the expansion of the universe (Cosmological redshift).

61. There's no doubt that I'm mixin' all sorts of things.

When we first observed the unexpected redshift from distant galaxies, we concluded that the universe is expanding. I've often wondered if what we're observing could be an optical illusion, leading to the aforementioned thought experiment.

So, could a strong and undetectably distant gravity source pucker spactime to the extent of accelerated elongation of all light geodesics from all distant galaxies?

62. Originally Posted by 6nqpnw
So, could a strong and undetectably distant gravity source pucker spactime to the extent of accelerated elongation of all light geodesics from all distant galaxies?
No, a strong and "undetectably distant" gravity source could not affect "all light geodesics from all distant galaxies".

To even begin to understand why, a good start would be to gain some understanding of how our past light-cone is defined.

63. I agree with you, but for different reasons ::: and I'm failing to find how our past light cone supports your response.

64. Well, if you understand the concept of the past light cone, you should understand that any event that can have had a causal effect on objects on our past light cone, is also on (or inside) our light cone. If an event is outside of our light cone, that event is in the future of the objects we see on our light cone and cannot have affected them as we see them.

So, no, an undetectably distant gravity source (i.e. outside our light cone - its light hasn't reached us yet), cannot have had any effect on anything we can detect.

65. Originally Posted by SpeedFreek
...an undetectably distant gravity source (i.e. outside our light cone - its light hasn't reached us yet)
I gotcha; I'm on the same page now and don't disagree with ya. Let me push this forward by replacing the gravity source's light cone (does not emit light) with a gravity cone that our observable universe has existed well within (past, present and future). In other words: we can feel it, we can observe its effects around us, but we can't see it (the source).

While approaching the gravity source would the cosmological redshift occur?

66. Originally Posted by 6nqpnw
Let me push this forward by replacing the gravity source's light cone (does not emit light) with a gravity cone that our observable universe has existed well within (past, present and future). In other words: we can feel it, we can observe its effects around us, but we can't see it (the source).

While approaching the gravity source would the cosmological redshift occur?
No. For many reasons.

Cosmological redshift is pretty much the same in all directions, as we look across increasing distance, where distance equals time. For your model to work at all, your undetectable gravity source (that has always been affecting everything) has to have been affecting everything in the universe by the same amount, at any given time. When we look at the same cosmological redshift in any direction, we are looking at objects whose light was emitted when the universe was the same age. Thus, there can be no direction to your source - the amount of influence it exerts is equal, across the whole universe. It therefore has to be a uniform gravitational field, if anything.

Now, for your undetectable source to remain at all coherent in terms of producing a redshift, it has to be exerting less and less gravitational influence on the whole universe, as the history of the universe progresses. Hence, we would be looking back in time to places where there was "more gravity".

So, how can there be no direction to the source? Why does this unseen gravitational influence that pervades the whole universe seem to be decreasing over the age of the universe?

Your idea leads to a universe with a changing "background" gravitational density but no expansion. How can the universe remain static, if it is not expanding? What effect would your changing uniform gravitational field have on the contents of the universe, bearing in mind that the universe has to be at a critical density not to contract, and even then it needs the help of a cosmological constant?

Then, of course, there is the whole business of "outside" the universe...

67. If the gravitational constant, G, was time varying, there would be no need for universal expansion and associated 'fixes' like Guth's inflation. Maybe 6nqpnw should consider this idea instea of a massive but undetectabe gravitational source.

68. Originally Posted by MigL
If the gravitational constant, G, was time varying, there would be no need for universal expansion and associated 'fixes' like Guth's inflation.
Are you completely sure about that?

69. Originally Posted by MigL
If the gravitational constant, G, was time varying, there would be no need for universal expansion and associated 'fixes' like Guth's inflation. Maybe 6nqpnw should consider this idea instea of a massive but undetectabe gravitational source.
Show it! What would be the reason for a variable G? What is the evidence for that, which distinguishes it from other explanations?

70. Originally Posted by Dishmaster
Originally Posted by MigL
If the gravitational constant, G, was time varying, there would be no need for universal expansion and associated 'fixes' like Guth's inflation. Maybe 6nqpnw should consider this idea instea of a massive but undetectabe gravitational source.
Show it! What would be the reason for a variable G? What is the evidence for that, which distinguishes it from other explanations?
Showing it will be tough. Just figuring out what it would mean will be a challenge. Since GR tells us that time is only a local concept, just which clock would govern gravity ?

71. What I meant was, if G ( gravitational constant ) was larger in the past, say 13 billion yrs ago, and has been continuously getting smaller ( or vice-versa ), would that account for universal red shift ???

This is meant as a discussion point not a valid proposition or theory. Can it account for expansion or the universal red shift which we currently attribute to expansion ??
Can it account for the isotropy and flatness of space along with the lack of domain boundary solitons ( monopoles ) that Guth's inflation does ??
Can it replace the Big Bang Inflationary theory ??

72. Originally Posted by MigL
What I meant was, if G ( gravitational constant ) was larger in the past, say 13 billion yrs ago, and has been continuously getting smaller ( or vice-versa ), would that account for universal red shift ???

This is meant as a discussion point not a valid proposition or theory. Can it account for expansion or the universal red shift which we currently attribute to expansion ??
Can it account for the isotropy and flatness of space along with the lack of domain boundary solitons ( monopoles ) that Guth's inflation does ??
Can it replace the Big Bang Inflationary theory ??

You miss the point. There are lots of problems with this theory, but first you have to define what it means.

Whose clock determines the past, or how G changes ? Remember that time is not universal. Does this somehow imply a spatial variation of G, since time here and time there are different ? Do observers in relative motion see different values of G ? If so, then GR goes out the window.

73. Well, let's take a speculative journey.
Currently G is considered a constant, but is it independant or dependant ? If independant, it is a set value for this space/time manifold. If dependant then something about space/time determines its value, ie change conditions and you change G.
I would imagine you are familiar with Mach's principle which loosely states that inertia is determined bu the garvitation of all mass/energy of the universe; and I would add my own thought that it is determined by all the mass/energy in the causally connected universe ( not by distant regions that cannot affect us due to finite c ). What if we speculate that G is also determined by all the mass/energy of the causally connected universe? According to the inflationary big bang theory causal domains have changed during the life of the universe, the greatest change occurring during the inflationary period. So that would be one mechanism for change in the value of G. I'm sure that with the right 'assumptions' many more can be thought of.
Note that I'm not saying this is correct or a theory. I'm just exploring a flight of fancy, and hope others will join in. It certainly has less holes in it than an undetectable, distant, massive gravitational source.

74. Related research topic: The angular diameter - distance relationship / The Tolman test for surface brightness.

75. Originally Posted by MigL
Well, let's take a speculative journey.
Currently G is considered a constant, but is it independant or dependant ? If independant, it is a set value for this space/time manifold. If dependant then something about space/time determines its value, ie change conditions and you change G.
I would imagine you are familiar with Mach's principle which loosely states that inertia is determined bu the garvitation of all mass/energy of the universe; and I would add my own thought that it is determined by all the mass/energy in the causally connected universe ( not by distant regions that cannot affect us due to finite c ). What if we speculate that G is also determined by all the mass/energy of the causally connected universe? According to the inflationary big bang theory causal domains have changed during the life of the universe, the greatest change occurring during the inflationary period. So that would be one mechanism for change in the value of G. I'm sure that with the right 'assumptions' many more can be thought of.
Note that I'm not saying this is correct or a theory. I'm just exploring a flight of fancy, and hope others will join in. It certainly has less holes in it than an undetectable, distant, massive gravitational source.
Einstein tried to make GR "Machian". It isn't, but has some Machian flavor. The problem with your loose statement of Mach's principle is that all statements are loose. No one, including Mach, has ever made it precise in any useful, testable way.

Your idea has the same problem of ban inability to define it. It sounds nice until you stop waving your hands and try to state what it really means in precise terms. Then you find that you can't do it.

Note that I did not say that "it" is wrong. I said that you have not defined what "it" is.

Yep, it's better than "an undetectable, distant, massive gravitational source" which is an oxymoron -- if it were a massive gravitational source it would be detectable. Black holes are detectable, you just can't see them. Their gravitational effect is detected.

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