# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Vern on 25/02/2010 16:28:03

Title: Does gravity affect gravity?
Post by: Vern on 25/02/2010 16:28:03
The equation for acceleration contains time as an element.  Stars in galaxies experience acceleration toward the galactic centre. Massive objects in a gravity field experience the passage of time as being slower than if the gravity field were not there.

So it seems that gravity would have a negative affect upon gravity so that it would not follow a linear increase as mass increases.

Time dilation due to gravity should then be considered when calculating the expected speed of galactic rotation.

What am I missing?
Title: Does gravity affect gravity?
Post by: Geezer on 25/02/2010 23:23:57
Great question Vern!

Do you think it might be sufficient to account for the observed effects?
Title: Does gravity affect gravity?
Post by: JP on 26/02/2010 01:07:01
When dealing with cases where time dilation is going to become important, you're in the realm of general relativity rather than the far easier Newtonian gravity.  From what I recall, in general relativity, gravity can effect gravity, since it's a form of energy, and energy is what causes space-time to bend (which is the cause of gravity in GR).  The equations of GR take into account all of this and they do provide measurable differences from Newtonian gravity.
Title: Does gravity affect gravity?
Post by: Geezer on 26/02/2010 01:13:56
rather than the far easier Newtonian gravity.

Sheesh! Well, it was not that easy for some of us  [:)]
Title: Does gravity affect gravity?
Post by: Vern on 26/02/2010 11:56:49
I haven't seen gravity's affect on gravity taken into account either in the calculations for galactic spin rates or in black hole calculations. For example, this would prevent black hole formation.

Reality does not change from one calculation method, or realm, to another.

Quote from: Geezer
Do you think it might be sufficient to account for the observed effects?
That was the thought. However, I have no idea whether the affect would be great enough to account for all of it.

Quote from: JP
The equations of GR take into account all of this and they do provide measurable differences from Newtonian gravity.
The reason I feel that maybe GR does not take this into account is that it would be impossible to predict black hole formation and GR does predict black hole formation.
Title: Does gravity affect gravity?
Post by: LeeE on 26/02/2010 12:08:54
Gravity is not a form of energy.  Were it to be so then gravity would provide a perpetual and inexhaustible source of energy, seeing as, as far as we know, the mass that produces the gravity does not decay as a result of doing so.

Energy can be associated with gravity, but only by virtue of mass/energy equivalence of the mass producing the gravity, or by the potential energy of a mass raised against gravity.

It will take a Theory of Everything/Grand Unification Theory to more directly reconcile gravity with energy.
Title: Does gravity affect gravity?
Post by: Farsight on 26/02/2010 13:27:15
Vern: we were talking about something related to this at http://www.thenakedscientists.com/forum/index.php?topic=27444.msg297794#msg297794. In Einstein's The Foundation of The General Theory of Relativity page 185 he says "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". As JP said, there's an extra term for this, and on page 201 Einstein starts talking about an integral.

Put another way, a region of space in which we find a gravitational field has a different "energy density" than one that doesn't. The energy density increases as you approach a planet, then shoots up off the scale when you encounter the planet itself because of all the E=mc2 energy tied up as matter. I think the important thing to remember here is that a concentration of energy causes gravity, and since a gravitational field represents a region of space where there's some concentration of energy caused by a massive central concentration of energy, gravity has a positive effect upon gravity.

I'd be very surprised if this wasn't accounted for in galactic rotation. IMHO what isn't accounted for is the expansion of the universe. Einstein didn't quite get that right because he had a steady-state concept. I can't see how it would prevent black hole formation, but maybe we should talk about black holes separately.
Title: Does gravity affect gravity?
Post by: Vern on 26/02/2010 14:07:25
How do you get around the fact that the equation for acceleration has time as one of the multipliers. Acceleration determines the rate of galactic spin. Gravity causes massive objects to experience time as slowed. If you do the math and plot a graph of acceleration rate against mass increase you don't get a linear slope.
Title: Does gravity affect gravity?
Post by: Farsight on 26/02/2010 14:25:46
I don't know offhand Vern, that sounds like the difference between Newtonian mechanics and relativity. Can you give a pointer to something to look at in more detail?
Title: Does gravity affect gravity?
Post by: JP on 26/02/2010 16:03:33
Gravity is not a form of energy.  Were it to be so then gravity would provide a perpetual and inexhaustible source of energy, seeing as, as far as we know, the mass that produces the gravity does not decay as a result of doing so.

I admit, gravitational energy is subtle if you have to use GR.  But I think most people working in the field believe that gravity does have energy associated with it.  A simple example of why this should be so is gravitational waves: a system generating gravitational waves decreases its motion as it radiates these waves, which is what you would expect if the waves carried away energy.
More here:
http://scope.joemirando.net/faqs/Relativity/GR/energy_gr.html
and here:
http://www.physics.ucla.edu/~cwp/articles/noether.asg/noether.html

Title: Does gravity affect gravity?
Post by: Vern on 26/02/2010 19:02:17
Gravitational waves are part of a theory. We can't assume their existence and use that as evidence for something else.
Title: Does gravity affect gravity?
Post by: Vern on 26/02/2010 19:06:24
I don't know offhand Vern, that sounds like the difference between Newtonian mechanics and relativity. Can you give a pointer to something to look at in more detail?
One of the reasons I'm curious about this is that it seems to be simple and straight forward. The solution would be the sum of two vectors just like the Lorentz transforms. It would be interesting to see the slope of an acceleration curve plotted against mass increase.

What little I can do of the maths puts the slope in the direction of observed galactic spin anomaly.
Title: Does gravity affect gravity?
Post by: LeeE on 27/02/2010 13:30:17
A simple example of why this should be so is gravitational waves: a system generating gravitational waves decreases its motion as it radiates these waves, which is what you would expect if the waves carried away energy.

I'm happy to accept that there is energy in a gravity wave.
Title: Does gravity affect gravity?
Post by: Vern on 27/02/2010 21:03:41
If we can conclude that gravity does affect gravity we can preclude the existence of black holes and big bangs.

So it's not trivial. It's not settled science.
Title: Does gravity affect gravity?
Post by: JP on 28/02/2010 01:27:26
That's not true at all Vern.  The equations of general relativity predict black holes and those equations are nonlinear, which means that gravity effects itself.
Title: Does gravity affect gravity?
Post by: Vern on 28/02/2010 11:05:52
I know you say it. I haven't seen it demonstrated yet.

I have known for a long time that it is impossible to produce a model of black hole creation via an accretion disk when you include relativity phenomena. Some universities have produced studies of this and concluded the same thing.

I'll research for the studies. It's been awhile since I saw it.
Title: Does gravity affect gravity?
Post by: JP on 28/02/2010 15:56:58
I know you say it. I haven't seen it demonstrated yet.
The non linearity of gravity is built into general relativity, and it has been fairly rigorously tested.  If you don't believe general relativity, then that's a bigger that is outside of textbook physics.

Quote
I have known for a long time that it is impossible to produce a model of black hole creation via an accretion disk when you include relativity phenomena. Some universities have produced studies of this and concluded the same thing.

I'll research for the studies. It's been awhile since I saw it.
Please do.  However, it sounds like non-standard physics again, since general relativity does predict the formation of black holes.
Title: Does gravity affect gravity?
Post by: Farsight on 28/02/2010 16:18:05
There are a lot of interpretational issues with all this stuff Vern. Mass is a mess, because there's active gravitational mass, passive gravitational mass, relativistic "mass", inertial mass, and invariant mass. Gravity isn't too bright either, because when you read the original Einstein he talks about a non-constant gμv and curvilinear motion, and whilst he talks geometry he doesn't put the same emphasis on curved spacetime as we see now. Then with black holes there's the Oppenheimer-frozen-star "Weinberg Field Interpretation" but nobody really knows about it, and everybody tends to  think that the central-singularity Misner/Thorne/Wheeler "Geometrical Interpretation" is what relativity unambiguously says. Maybe this is where the problem lies?

Since there are some issues I guess one has to say it isn't settled science, but I'd say regardless of the interpretation, get enough mass together and you end up with this black thing, and if you or light or anything else gets too close, it's a one-way trip. At the same time I'd also say a concentration of energy causes gravity, and when we see a gravitational anomaly, we should look for an non-uniform energy density or inhomogeneous space before we jump to the conclusion that this extra energy exists as dark matter.
Title: Does gravity affect gravity?
Post by: Vern on 28/02/2010 16:34:15
I'll just let it rest.
Title: Does gravity affect gravity?
Post by: yor_on on 01/03/2010 09:58:19
Vern. Take a look at this are black holes forbidden math (http://scienceblogs.com/startswithabang/2009/11/are_black_holes_forbidden_math.php)

"Even setting aside the black hole issue in GR, the poster is incorrect saying that GR cannot handle point singularities. You can coordinate patch away many forms of singularities. And while in GR the space=time metric is not positive definite, and therefore not m-complete, we can show that it is b-complete, which is equivalent to m-complete in a pd metric.

So, even if black-holes contained a singularity, and their existence depended on such, GR still admits them as solutions."
Title: Does gravity affect gravity?
Post by: Soul Surfer on 01/03/2010 18:44:04
To go back to your original question Vern.  The small gravitational fields involved in the rotation of galaxies produce an extremely small effect on time it is only when you get very close to the event horizon of a black hole that time dilation becomes significant.
Title: Does gravity affect gravity?
Post by: Vern on 02/03/2010 10:12:33
The consensus seems to be that GR does take into account the affect that gravity has upon gravity. I still haven't seen the details of how this works. Simple arithmetic says it can't work. But GR ain't simple. (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fphotontheory.com%2Ffield.png&hash=cbf73b74c1cfd90cd99797545e88b161)

Title: Does gravity affect gravity?
Post by: yor_on on 02/03/2010 10:22:40
let's put it this way, for us to observe something as moving spatially one would expect energy to be involved somehow initially at least, so in that motto a gravity wave can be seen as needing 'energy'. But the wave we might see is like a bubble wandering inside a liquid, a local disturbance of a field, so I'm not that sure that gravity waves contain any 'energy' in themselves. Don't know how to express it really but the 'effect' we would observe is more due to the surrounding vacuum/densities adapting themselves than to the 'bubble' releasing some indefinable 'energy', it seems to me.
Title: Does gravity affect gravity?
Post by: Vern on 02/03/2010 10:31:04
I'm still not satisfied that we've got it right yet. But I am satisfied that I can't make a contribution to it.

Quote from: yoron
Vern. Take a look at this are black holes forbidden math
Interesting link. Simple maths forbid them. Elite maths understood only by those who have the power permit them.
Title: Does gravity affect gravity?
Post by: JP on 02/03/2010 10:58:08
The consensus seems to be that GR does take into account the affect that gravity has upon gravity. I still haven't seen the details of how this works. Simple arithmetic says it can't work. But GR ain't simple. (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fphotontheory.com%2Ffield.png&hash=cbf73b74c1cfd90cd99797545e88b161)

How I've heard it described is because the energy and momentum of a body tells space-time how to curve, and the curvature tells the body how to move (and moving gives it energy and momentum, meaning it therefore influences the curvature).

Title: Does gravity affect gravity?
Post by: yor_on on 03/03/2010 07:17:13
What forbids a black hole is the fact that it is a singularity Vern, 'Terra Incognito'. You can't divide with zero, so in that motto we need some really 'phreaky math' to come up with ideas for what might be in there. Seems like it's only string theory that have succeed with that, as yet?

If they are right? Well, as far as I know it's still a purely mathematical construction, without any real experimental evidence, but that might change, and maybe there is the possibility of indirect evidence too? Like things we can't really observe but still have good reasons to expect to exist, like vacuum energy.
Title: Does gravity affect gravity?
Post by: Vern on 03/03/2010 10:05:30
I suspect the answer will not come through string theory. When I look at Poincare's work, I get the feeling that he knew the answer to the final puzzle. The answer wasn't acceptable in his time. It's still not.
Title: Does gravity affect gravity?
Post by: jartza on 07/11/2010 06:58:30
The equation for acceleration contains time as an element.  Stars in galaxies experience acceleration toward the galactic centre. Massive objects in a gravity field experience the passage of time as being slower than if the gravity field were not there.

So it seems that gravity would have a negative affect upon gravity so that it would not follow a linear increase as mass increases.

Time dilation due to gravity should then be considered when calculating the expected speed of galactic rotation.

What am I missing?

The bigger the galaxy the more the space rotates with the galaxy.

No centrifugal force is generated when you rotate at the same rate that the space rotates.

Title: Does gravity affect gravity?
Post by: jartza on 10/11/2010 04:51:37

No centrifugal force is generated when you rotate at the same rate that the space rotates.

I wonder could it really be so that there's no centrifugal force?

Now let's shoot a relativistic projectile at a small black hole.

After the shot the black hole is moving backwards.

A pushing force between the projectile and the black hole caused the backwards motion.

A pushing gravitational force.

Also when dropping something into a black hole there is a pushing force. First there is a pulling force, then there is a pushing force. That's why the black hole does not change its speed when an object is dropped into it.