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Author Topic: How do we know that gravity is an attractive force?  (Read 8096 times)

Offline PhilS

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Can someone convince me that Gravity is not a pushing force.
It strikes me that gravity is a bit like air molecules, rushing about in all directions with probably an even distribution throughout the universe.
If 2 large objects (galaxies) for instance are a considerable distance apart these gravitons would act on them, pushing them further apart (maybe even at an accelerating rate). This being so, the gravitons must give up some of their energy to accelerate the galaxies. Thus the gravitons are slowed down by passing through a large object
On the otherhand, when 2 objects are close together (me standing on terra firma) the earth slows down the gravitons coming from below, so that the faster gravitons from above push me downwards.
« Last Edit: 15/02/2016 22:07:56 by chris »


 

Offline chris

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Re: Gravity
« Reply #1 on: 22/11/2004 10:18:13 »
So why would planets orbit the sun, and the moon our planet then ?

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Offline Ultima

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Re: Gravity
« Reply #2 on: 22/11/2004 10:50:27 »
The reason galaxies are moving away is to do with the expansion of the universe not gravity isn't it?????

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Offline neilep

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Re: Gravity
« Reply #3 on: 22/11/2004 12:08:35 »
I thought the planets were kept in check due to Chris's Charisma :-)

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Offline gsmollin

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Re: Gravity
« Reply #4 on: 22/11/2004 15:58:58 »
quote:
Originally posted by PhilS

"Can someone convince me that Gravity is not a pushing force.
It strikes me that gravity is a bit like air molecules, rushing about in all directions with probably an even distribution throughout the universe."

Is this like vacuum energy? Where does the "even distribution" come from? Are these primordial particles, and if so, at what point were they formed?

"If 2 large objects (galaxies) for instance are a considerable distance apart these gravitons would act on them, pushing them further apart (maybe even at an accelerating rate)."

Wouldn't the forces from the "even distribution" be "even", and not pose a net force on the distant objects? Where is the repulsion coming from?

"This being so, the gravitons must give up some of their energy to accelerate the galaxies. Thus the gravitons are slowed down by passing through a large object"

What are the initial and final velocities of "gravitons" as they pass through an object? How much energy does a graviton posses, and how much energy does it give up passing through a massive object?

"On the otherhand, when 2 objects are close together (me standing on terra firma) the earth slows down the gravitons coming from below, so that the faster gravitons from above push me downwards."

It sounds like the earth is shielding you from the gravitons below. If the earth were the density of a neutron star, located 3900 miles (6500 km) below your feet, as you stand now on terra firma, it would attract you towards it with a force of 1G, just as it does now. However, the earth would be a tiny sphere a few meters in diameter, and would not stop or slow down too many gravitons, because they could just shoot by the shrunken earth and strike you, pushing you away from the earth.

I think you are trying to formulate a gravitational law that would explain dark energy, by making gravity become repulsive at large distances. This problem has been tackled before, even by Einstein. You need to postulate a set of consistent laws for your theory, that are testable hypothesis. Then maybe we can discuss it in a meaningful manner.
 

Offline chris

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Re: Gravity
« Reply #5 on: 23/11/2004 10:15:48 »
quote:
Originally posted by neilep

I thought the planets were kept in check due to Chris's Charisma :-)



I'm glad you said charisma and not 'ego' !

Chris

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Offline PhilS

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Re: Gravity
« Reply #6 on: 24/11/2004 23:07:13 »
Not convinced at all.
Ultima's answer that galaxies moving away from each other is due to the expansion of the universe seems to agree with itself. If the number of galaxies in the univers is reasonably constant (Yes I know new ones are being formed) and the universe is roughly defined as the outer bounary of all the galaxies ( a sort of sphere), then if the universe expands all the galaxies by definition must move away from each other.

Chris has missed the point. If gravity is this universal pushing force and a large object like the sun is nearby (whats 93,000,000 miles between friends) that body will attenuate the push from the direction of the sun. So the net force on the earth from all other directions other than the direction of the sun will result in a net force pushing the earth towards the sun.

gsmollin has a point. Maybe the graviton was formed at the time of the big bang. If so presumably it would have spread out as far as the galaxies. Maybe the distribution is even within this boundary, but thins at the extremities. So the force pushing the galaxies towards the center of the univers would be less that the force pushing away.
No thoughts on velocities or energy loss.
Your Neutron Star is more difgficult to explain, but I suppose it depends on the density of the gravitons and the opacity of a neutron star.
 

Offline Raedon

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Re: Gravity
« Reply #7 on: 25/11/2004 07:28:01 »
Last time I checked, gravity is matter grouping, it's not like matter as it's instant and not governed by space/time just acts on it (seems to be anyway or black holes couldn't eat light.)

  If the universe was just two atoms of hydrogen just popped into this silly space/time millions of billions of light years apart they would fall toward each other instantly.




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Offline Sandwalker

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Re: Gravity
« Reply #8 on: 25/11/2004 16:59:14 »
Phils

In your discussion the net force on the earth would be a push, all gravity acting on the earth other that that of the sun would be close to, if not, equilibrium.

The average density of spacetime in all directions within in the light cone of the earth/universe existance, acting on the earth would be the same, (like the microwave background), this would mean that if the gravity was a push force then the net result of the sun on the earth would be a push and push us away from the sun.

Lightcone - A 3D cone-shaped portion of spacetime (2D space) containing all (past) locations from which light could arrive at a particular location within spacetime (the tip of the cone). The tip can be reached from points inside the cone at the speed of light or slower, and from points on the surface of the cone only at the speed of light. Since nothing can travel faster than light, points outside the cone cannot in any way affect the tip. (For 3D Space the shape would be a 4D sphere).

Raedon
Not if gravity is a product of warped space.
 

Offline Raedon

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Re: Gravity
« Reply #9 on: 25/11/2004 20:32:56 »
quote:

Raedon
Not if gravity is a product of warped space.




Expain.

 are you saying that the gravity of two, warping space/time (by gravity) atoms, light years from each other would not come together the instant they were created?  No dark matter. Nothing but empty infinite vacuum and 2 atoms.  I'm sure they would.

  I think if you say, "warped space" it should be space/time and beyond that you are warping the math used up to now.  

  I could be cracked because I have not observed anything but the lack  of space/time/ego and I failed college math because it does not add up so..




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Offline Sandwalker

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Re: Gravity
« Reply #10 on: 25/11/2004 23:32:44 »
Pardon, Yes warped spacetime, fell into euclidian space there for a moment!

To be frank I do not know! I have no evidence for the speed of the effect of gravity, just a measure of its pull and the decrease of its strength inversely proportional with distance.

All I'm saying is that if gravity is a parameter/product of warped spacetime then we have no problem with the graviton and black holes, its a paradigm shift, akin to your space/time/ego trip, or lack of it.

The warping of spacetime producing gravity is no different than saying m=E/c^2 or E/c=m*c or c^2=E/m they are all valid formulea of Einstiens classic (I hope), just expressing the relationships differently.

Matter produce gravity effect warped spacetime?
Matter warps spacetime effect gravity?
Acceleration produces gravity effect warped spacetime?
Acceleration warps spacetime effect gravity?



« Last Edit: 25/11/2004 23:41:09 by Sandwalker »
 

Offline Alexh

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Re: Gravity
« Reply #11 on: 29/11/2004 00:02:24 »
Hmm - I think this was (almost) sorted out recently with the some experiment measuring a quasar moving behind Jupiter proving that Einstein was right.

According to Newton, Gravity works instantaneously - Einstein modified this in Special Relativity when he stated that nothing NOTHING can travel faster than light.

According to the mad-haired one, if the sun were to disappear (ok a thought experiment) then the earth would continue around in orbit for 8 minutes, after which we would then move off in a tangent.

Gravitons (if they exist) can only move at c.  I believe this is now confirmed (with some remaining doubters).

newbielink:http://www.ldolphin.org/vanFlandern/gravityspeed.html [nonactive]
« Last Edit: 29/11/2004 00:03:26 by Alexh »
 

Offline chris

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Re: Gravity
« Reply #12 on: 29/11/2004 13:22:57 »
thanks Alexh, that's really interesting.

Chris

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Offline gsmollin

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Re: Gravity
« Reply #13 on: 30/11/2004 01:24:54 »
I read the paper in that link (http://www.ldolphin.org/vanFlandern/gravityspeed.html). It is really interesting, and raises some interesting questions about astro-mechanics. There is one simple point I really disagree with, however: The orbiting-spacecraft tether. I quote:

..."Moreover, contrary to what the rubber sheet analogy implies, an orbiting body such as a spacecraft orbiting the Earth is not following the curvature of space near the Earth. As we remarked earlier, two spacecraft some distance apart in the same orbit could stretch a tether between them and pull it taut, thereby describing a straight line through space different from their orbital path."...

The orbiting spacecraft cannot do this. The tether between them remains curved, following the curved space required by general relativity. Really, this is simple. The tether is in orbit, just like the spacecraft. Every point on the tether must follow the orbit at that point, to stay in orbit. If the tether is actually pulled straight between the two spacecraft, the center of the tether will be at a lower altitude than each of the two spacecraft. An object at a lower altitude has to travel in a faster orbit than one in a higher orbit, in order to remain in orbit. Since the tether is attached to the orbiting spacecraft, it cannot travel faster, so as to remain in orbit. So the center of the tether now has to be supported by the spacecraft, similar to a rope held between two people on the earth's surface. Pulling the tether straight, will put a force on both spacecraft, and they will move closer to each other in their mutual orbit, until the tether rises into the proper curved shape again. If the tether is kept straight by pulling on it, the two spacecraft will be pulled togther.

I know this is a nit in the body of the paper, but it bothered me, since it was wrong, in something I actually understood. So it makes me doubt the rest of it.
 

Offline PhilS

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Re: Gravity
« Reply #14 on: 01/12/2004 15:53:31 »
Thanks to all contributers, a lot of interesting replies.
I think Sandwalkers comments about the light cone missed the point. Yes the forces on the earth would be pretty well in equilibriun axcept in the direction of the sun, put here the pushing force will be attenuated so that the net force is towards the sun.
The contributions abou speed were interesting. I was under the impression that nothing could be accelerated beyond the speed of light due to the infinite energy reqiirements, but what happens if something is travelling faster than light like a graviton. What laws of physics apply. What happens to its mass. I presume it still will have energy and be able to push ordinary matter around
 

Offline gsmollin

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Re: Gravity
« Reply #15 on: 03/12/2004 16:36:32 »
Some clarification on my previous post, and a little reflection on the gravity paper:

I implied that the tether would rise back to the orbital altitude by itself. It would not. Having been pulled below the orbital altitude, and restrained so it cannot increase orbital speed, to assume a new, lower orbit, it will remain below the spacecraft, supported by them. This supporting force will pull the spacecraft together, and the tether will drop lower and lower below them, until the two spacecraft butt together, and the tether is straight down. This would be the new stable point. In fact, it is so stable it can be used to stabilize the spacecraft against attitude perturbations. In that case we call it "gravity-gradient stabilization".

OK, but what does this all mean to the referenced paper, which is essentially about the speed of gravity? The author was trying to say that gravity was not curving space, because the shortest distance between two points was not a geodesic, but the straight line prescribed by the tether.

I am no GR expert, but I did not think that this was what GR was saying about orbits. It was saying that in the case of gravity, Newton's second law had to be changed from "A body moves in a straight line at constant velocity unless acted upon by an outside force." to "A body moves in a geodesic curve when gravity is in effect, unless acted upon by an outside force."

The spacecraft-tether experiment illustrates this. With no outside force applied, the spacecraft and tether move in a geodesic orbital path around the earth. If this is disturbed by pulling the tether straight, a force will appear, acting on the spacecraft and tether until the geodesic orbital curve is re-established.

I think this result is in agreement with GR.
 

Offline Raedon

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Re: Gravity
« Reply #16 on: 05/12/2004 02:04:56 »
If light can be held by gravity then gravity does not follow "the crazy haired one"  It has to work without time or effect space/time beyond the speed of light.  If gravity could just hold light at light speed then black holes would be so bright no one could see as nothing would pass the horizon.  

EDIT: this may be flawed thought.
« Last Edit: 05/12/2004 02:16:51 by Raedon »
 

Offline Raedon

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Re: Gravity
« Reply #17 on: 05/12/2004 02:14:12 »
So.. We are talking about a force that is working on a D beyond left/right up/down.  I still feel Space/Time are two parts of the same D so what is this observable D?


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Offline dedaNoe

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Re: Gravity
« Reply #18 on: 30/01/2005 10:47:46 »
Newton had only wrong impression of Archimedes' lever when he came up with his gravity. What Newton imagined as gravity is just another lever. The forces must act with respect to some distance from some fixed point. That distance is causing the strengthening while the force is causing displacement. The force is colinear with the tangent of the trajectory of the body while the distance is along the conecting line between the centers of interacting bodies.

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Re: Gravity
« Reply #18 on: 30/01/2005 10:47:46 »

 

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