Gravity question

[Mookow]

Originally posted by: sao123
Gravity is instantanious for this reason...

Gravity fields are continuous and infinite. Since every particle in the universe exerts a gravity field on every other particle in the universe...All particles are already subjected to all the gravitational fields that exist in the universe. There is no area without gravity (only microfractional in magnitude). The only thing that ever changes is the magnitude of the gravitational field. Since there is no beginning or no end..there can be no how soon does gravity take effect... And since the magnitude vector field is continuous in D^3...all changes are instantaneous

What about the conversion of matter into energy? When matter is converted, or created, the "net gravity" of the universe should change, so the net force of gravity isnt constant in time. Also, during the beginning of the Big Bang (if you accept the theory), there was a lot less protons sitting around, and a lot more photons flying about, and unless a photon exerts gravitational force on objects (I know it can be affected by the gravity of other objects), the "net gravity" of the universe should have been lower.

Its true that, as far as science knows, gravitational fields have no limit to their extent (I put that qualifier in since, after all, we only have a theory about the basic properties of gravity), however, I dont see how you translate this to gravity having an instant effect. LEts say you have two masses, A which is on point X, and B which is on point Y, seperated by some big distance D. Now, if A all of a sudden is accelerated perpendicular to the line XY, why precisely would B instantly be accelerated along line AB (as it changes), rather than along XY for some portion of time? In other words, if we talk about gravity as curving space, what is the rate at which the curve of space can be changed?

Heres a proof by contradiction for you.

In order for gravity to have a speed at which it acts on something, there would have to be a point at which it is not acting.

1) Meaning that there has to be a point where gravity does not exist.

2)However gravity acts as G = 1/D^2. As D-> infinity...G approaches zero, yet never reaches it. Therefore 1 is false.

3)Since there can be no area where gravity does not effect. The speed at which gravity acts on a moving particle is now irrelevent because gravity has always been in effect everywhere the particle has been since the particle in question came into existence.

4)The laws of conservation of mass and energy say you cant create matter or energy only convert between them.
4b)Gravity acts upon mass & energy alike. Therefore since the particle has always existed in one form of mass or energy which it was always affected by gravity.

5)So the speed of gravity does cannot exist.

In part 3, yes a particle has always been in contact with gravitational force, always will... however, that doesnt equal being in contact with a static gravitational force, ie you havent shown (or at least I dont see) how this proves that when the gravitational field changes, the particle instantly is subject to those changes

 

[FrenchTickler]

Originally posted by: Mookow
What speed does the "force of gravity" act at? IE, is it instantaneous, or does it apply itself at a speed lower than 'c'?

You mean the acceleration of gravity? On earth the acceleration of gravity is 9.81 meters per second squared

Originally posted by: RossGr
Gravity does not accelerate light. Light follows space-time geodesics, that is the shortest path throgh space time. Light merely follows the contures of space-time. It is not the path of light that is skewed but our perspective of space-time.

Bravo, you hit it right on the nose. It's funny (sad really) how little people know about black holes. Most people I know think black holes just suck in anything and everything that comes close to it. Which for one isn't true because there are only push and pull forces, and second it's pretty much damn near impossible to get "sucked" in by a black hole because the radius of the star is so small, you would have to get extreemly close to enter the gravitional acceleration where you couldn't escape it.


Ryan

 

[Wiktor]

My theory on why gravity is not instantaneous (because I believe it is not), is quite simple.

If it was, we could send "information" at faster-than-light speeds (infinite speeds), by changing gravity on one side and detecting the changes on the other side - with no (in perfect conditions) time delay.
I think this should never be possible, otherwise we can end up with instant processors (if computing time can be reduced to how long information must circle around in the processor), teleportation (if we have a machine to disassemble a human into, say particles, store info about all that in memory, then transmit it by modulating gravity, and then use that information to recreate the human), probably time travel and free energy too (or maybe not that far )

I think Einstein's special theory of relativity also applies to information, and as such it cannot travel/be transmitted faster than it would take to use light particles (photons) for example. In fact the whole relativity theory would be rather pointless if that wasn't the case: length contraction (one of the relativistic effects) would never happen if we could use "gravity field to see" instead of something that travels at the speed of light. Same with time delay!

So: if the Sun disappeared from space right now - we would still have the gravity field for approx. 8 minutes.
Of course that has nothing to do with the question whether gravity is infinite field or not, that doesn't matter here. (It may be infinite in it's function approximation - and still be quantified in reality which would lead to a limited gravity field, it is possible).

 

[Mingon]

Bravo, you hit it right on the nose. It's funny (sad really) how little people know about black holes. Most people I know think black holes just suck in anything and everything that comes close to it. Which for one isn't true because there are only push and pull forces, and second it's pretty much damn near impossible to get "sucked" in by a black hole because the radius of the star is so small, you would have to get extreemly close to enter the gravitional acceleration where you couldn't escape it.

But then all that we know (about black holes) is 'hear say' and as such whilst you are feeling smug in the knowledge that you know lots about black holes you have yet to realise the earth flat

 

[glugglug]

Originally posted by: Wiktor
My theory on why gravity is not instantaneous (because I believe it is not), is quite simple.

If it was, we could send "information" at faster-than-light speeds (infinite speeds), by changing gravity on one side and detecting the changes on the other side - with no (in perfect conditions) time delay.

Here is why those 2 conditions (instaneous gravity and instantaneous information transfer) do NOT have to go together:

In order to change the properties of the gravitational field associated with an object, that object, or a piece of it, must move, or if you are trying to use that for information transfer, it must change it's state of movement, in other words accelerate. For the object to accelerate, it must be interacting with a forcefield produced by other object(s), and the other object(s) must experience the exact opposite change in total momentum (Newton's 3rd law).

From a distance (relative to the object(s) observed interacting with each other), the changes in the gravitational field picked up by the object trying to read this information exactly cancel out.

 

[Ameesh]

i asked my professor in college this and his answer (simply put) was 3E8 m/s

 

[Wiktor]

Originally posted by: glugglug

Here is why those 2 conditions (instaneous gravity and instantaneous information transfer) do NOT have to go together:

That is true, I agree.
But there maybe many other ways to alter gravity field, what you described is just one. Another, mentioned here already, is to use matter-energy equivalence (E=mc^2) and by transforming matter into energy (into photons, which are believed to have a zero mass and no gravity around them) - change the "total" gravity.

Of course this is all just a logical construction...

 

[DrPizza]

Originally posted by: Wiktor
My theory on why gravity is not instantaneous (because I believe it is not), is quite simple.

If it was, we could send "information" at faster-than-light speeds (infinite speeds), by changing gravity on one side and detecting the changes on the other side - with no (in perfect conditions) time delay.
I think this should never be possible, otherwise we can end up with instant processors (if computing time can be reduced to how long information must circle around in the processor), teleportation (if we have a machine to disassemble a human into, say particles, store info about all that in memory, then transmit it by modulating gravity, and then use that information to recreate the human), probably time travel and free energy too (or maybe not that far )

You're right, but suppose gravity was instantaneous, how could you instantaneously change the gravitational field? You would have to move mass, which requires an impulse = F*t, that is, you couldn't instantaneously move a mass.

I think Einstein's special theory of relativity also applies to information, and as such it cannot travel/be transmitted faster than it would take to use light particles (photons) for example. In fact the whole relativity theory would be rather pointless if that wasn't the case: length contraction (one of the relativistic effects) would never happen if we could use "gravity field to see" instead of something that travels at the speed of light. Same with time delay!

Great point!

So: if the Sun disappeared from space right now - we would still have the gravity field for approx. 8 minutes.
Of course that has nothing to do with the question whether gravity is infinite field or not, that doesn't matter here. (It may be infinite in it's function approximation - and still be quantified in reality which would lead to a limited gravity field, it is possible).[/quote]

 

[rjain]

You're right, but suppose gravity was instantaneous, how could you instantaneously change the gravitational field? You would have to move mass, which requires an impulse = F*t, that is, you couldn't instantaneously move a mass.

What if the gravitational field were space-time itself? (Which is what GR says.)

Then a change in the gravitational field would simply be a shift in the curvature of space-time. I'm not sure how you get moving a mass out of changing a gravitational field. Mass moves along the gravitational field and shapes it.

 

[rjain]

But there maybe many other ways to alter gravity field, what you described is just one. Another, mentioned here already, is to use matter-energy equivalence (E=mc^2) and by transforming matter into energy (into photons, which are believed to have a zero mass and no gravity around them) - change the "total" gravity.

Of course this is all just a logical construction...

You can't use mass-energy equivalence. It's just a physical law. Matter is energy and energy is matter. Photons DO have gravity "around" them (they both create and respond to gravity), since they have mass (they can never be at rest). Changing the "total" gravity in that way would be a violation of the law of conservation of mass-energy.

Also, the explanation you were responding to didn't imply any way of altering the gravitational field, it determined what changes would involve the transmission of information and then figured out what the implications of those changes would be.

 

[rjain]

Its true that, as far as science knows, gravitational fields have no limit to their extent (I put that qualifier in since, after all, we only have a theory about the basic properties of gravity), however, I dont see how you translate this to gravity having an instant effect.

AFAIK, GR doesn't require that the speed of gravity be instantaneous. It's just that we couldn't measure anything that would give an indication as to its speed at the time. Some experiments have been done recently that vaguely indicate that the speed of gravity is finite and may be close to c, but there's a lot of uncertainty in those measurements.

Paper discussing the speed of gravity

 

[Noriaki]

Originally posted by: Wiktor(into photons, which are believed to have a zero mass and no gravity around them)

Even if photons do have 0 mass (I agree they do), the photons resulting from converting an atom to EM radition exert the same gravity that that atom did.

Energy curves spacetime just as well as matter does.

E=mc^2 tells us quantitatively how much energy we get from converting mass to energy, but there is more too it than that. Energy curves spacetime (exihibits gravity) as well as matter does.



As to grativity propogating at a given speed (believed to be c), think of it this way.

You have a rubber sheet stretched over a rectangular wooden frame. Now push your finger into the center of the rubber sheet, as you push your finger with more force you stretch the sheet further, it takes time for your stretching to propogate towards the corners.

Gravity is the same way, it curves spacetime the way you stretch rubber. Suddenly you change gravity at a position in spacetime's rubber sheet, it takes time for the curvature to propogate outward from that positon.


If I were to magically remove the sun, it would take some time before the curvature of spacetime "flattened out" in our vicinity, spacetime wouldn't instantly loose all curvature resulting from the now vanished sun.

 

[Finnkc]

9.8 meters/sec. = accel of object due to gravity. (terminal velocity due to air friction resistance aside)

it's all about equilibrium folks ...

 

[Mookow]

Originally posted by: Finnkc
9.8 meters/sec. = accel of object due to gravity. (terminal velocity due to air friction resistance aside)

it's all about equilibrium folks ...

a.) You presume you are Earth when you state that number, it by no means is a univerable applicable constant

b.) I fail to see how that has anything to do with the actual topic/conversation in this thread.

 

[rjain]

As to grativity propogating at a given speed (believed to be c), think of it this way.

You have a rubber sheet stretched over a rectangular wooden frame. Now push your finger into the center of the rubber sheet, as you push your finger with more force you stretch the sheet further, it takes time for your stretching to propogate towards the corners.

Gravity is the same way, it curves spacetime the way you stretch rubber. Suddenly you change gravity at a position in spacetime's rubber sheet, it takes time for the curvature to propogate outward from that positon.


If I were to magically remove the sun, it would take some time before the curvature of spacetime "flattened out" in our vicinity, spacetime wouldn't instantly loose all curvature resulting from the now vanished sun.

I thought the question was not if gravity propagates, but how fast the changes in the gravitational field (a.k.a. the curvature of space-time) propagate. Just because photons propagate through space-time at c doesn't mean that disturbances in space-time itself must be constrained to that same speed.

 

[grant2]

Originally posted by: RadioactiveHamzter
I remember hearing about how they measured the speed of gravity, by comparing the gravitational effect of Jupiter with it's actual position. The managed to confirm that gravity travelled at least 95% the speed of light (don't remember the exact value, it was somewhere in that region though).

The speed was claimed to be exactly C (speed of light)

http://www.nrao.edu/pr/2003/gravity/

You can see a bit about the peer criticism of this experiment here:

http://www.space.com/scienceastronomy/gravity_speed_030116.html

And then there's another older article written in 1998 by Tom Van Flandern, who claims the speed of gravity must be > 2x10^10 C (crazy faster than the speed of light) .. I haven't read the whole article but I don't really buy what I HAVE read:

http://www.ldolphin.org/vanFlandern/gravityspeed.html

 

[sao123]

After having read all three articles, I must state that my beliefs are more aligned with the theories presented in the third article as being more accurate than your "widely accepted theories".

 

[rjain]

Originally posted by: sao123
After having read all three articles, I must state that my beliefs are more aligned with the theories presented in the third article as being more accurate than your "widely accepted theories".

Which "widely accepted" theories? Everyone has a different idea of what's widely accepted, and I have no idea which theories he thinks are widely accepted, assuming that you do...

 

[rjain]

Originally posted by: grant2
And then there's another older article written in 1998 by Tom Van Flandern, who claims the speed of gravity must be > 2x10^10 C (crazy faster than the speed of light) .. I haven't read the whole article but I don't really buy what I HAVE read:

http://www.ldolphin.org/vanFlandern/gravityspeed.html

Have you come up with a way in which angular momentum can be conserved when there is a finite propagation delay for gravitational fields?

Also remember that the measurements of celestial mechanics involved in this article are rather simple and have been perfected for the past 10,000 years. On the other hand, the Jupiter light-bending article uses new techniques that haven't been well-tested (it's the first time they've ever been used).

 

[Pudgygiant]

So could gravity pull something with nearly zero mass at nearly the speed of light? Assuming an infinite distance and full gravitational strength the whole time of course.

 

[rjain]

Originally posted by: Pudgygiant
So could gravity pull something with nearly zero mass at nearly the speed of light? Assuming an infinite distance and full gravitational strength the whole time of course.

What's "full" gravity? Do they sell half-orders of it, too? Can I get it as a platter?

 

[Mookow]

Originally posted by: rjain

Originally posted by: Pudgygiant
So could gravity pull something with nearly zero mass at nearly the speed of light? Assuming an infinite distance and full gravitational strength the whole time of course.

What's "full" gravity? Do they sell half-orders of it, too? Can I get it as a platter?

Platter or the basket size

To answer Pudgy's question:

Gravity can be strong enough to prevent light from exiting a black hole. That's a case of a object with zero rest mass moving at the speed of light, but at a fairly close distance, being pulled by a gravitational force. Same basic thing applies to you sitting at your computer... your the resulting gravity from your mass exerts a pull on the photons exiting your monitor.

To answer your question as its written:
You need to clarify. Funny things happen at infinite distance (or would, if such a thing exists). However, assuming it was in contact with a gravitational field at a near infinite distance, the hypothetical particle would feel a force due to the gravitational force of X, where X stands for any object you want it to. However, this force would be of such a small magnitude that you couldnt detect without, say, observing the path of the object for a long, long, LONG time and watching it circle X, assuming there is nothing else in this near infinite volume to exert a force on the particle.

Anyway, you need to clarify your question... I had to make some assumptions, and some of them probably changed it from what you wanted it to be

 

[grant2]

Originally posted by: rjain
Have you come up with a way in which angular momentum can be conserved when there is a finite propagation delay for gravitational fields?

Also remember that the measurements of celestial mechanics involved in this article are rather simple and have been perfected for the past 10,000 years. On the other hand, the Jupiter light-bending article uses new techniques that haven't been well-tested (it's the first time they've ever been used).

Maybe i'm just a big dummy but I don't really know what "angular momentum" means, let alone why it's not being conserved. Could you explain it to me in small simple words?

The author points out that all celestial mechanics are based on the premise that gravity travels instantaneously... you can't use a system to disprove itself, that's illogical.

The author also claims that gravity has to travel faster than light to escape itself. That makes no sense, it's like saying water cannot float on top of an ocean because a rock cannot.

 

[rjain]

Originally posted by: grant2

Originally posted by: rjain
Have you come up with a way in which angular momentum can be conserved when there is a finite propagation delay for gravitational fields?

Also remember that the measurements of celestial mechanics involved in this article are rather simple and have been perfected for the past 10,000 years. On the other hand, the Jupiter light-bending article uses new techniques that haven't been well-tested (it's the first time they've ever been used).

Maybe i'm just a big dummy but I don't really know what "angular momentum" means, let alone why it's not being conserved. Could you explain it to me in small simple words?

It's the rotational equivalent of momentum, using rotational rates and moments of inertia instead of velocities and masses.

The author points out that all celestial mechanics are based on the premise that gravity travels instantaneously... you can't use a system to disprove itself, that's illogical.

No, he points out that that's how people are taught to do the calculations because otherwise, they are wrong.

The author also claims that gravity has to travel faster than light to escape itself. That makes no sense, it's like saying water cannot float on top of an ocean because a rock cannot.

No, that has to do with black holes. Phtons can't escape the gravity of a black hole. How can gravitons get stuck in a black hole and still transmit gravitational forces? They obviously don't follow the same rules photons do.

 

[silverpig]

Whoa, I can't belive I'm this late coming into this thread.

Gravity travels at c. The reason why black holes don't trap themselves with their gravity is because gravitons are the messenger particles (theorized) that transmit the gravitational force. They are not limited to the curvature of spacetime. In order for them to be so, gravitons must be able to interact with themselves (like, one graviton has mass and sends a graviton to another one with mass thereby affecting it).

You guys should do a google search on a project called LIGO. They are trying to detect gravitational waves from supernovae. The instruments are insanely sensitive, and they are having many problems right now with the huge noise of traffic on highways many miles away blowing out all their equipment with noise. The device works by splitting a laser beam into two perpendicular beams and then sending these beams down mile long tunnels a few times via reflecting them off of very massive (many tons) mirrors. The gravitational waves, say travelling in the x direction, will compress space in the y direction and expand it in the z direction, then expand it in the y direction and compress it in z in an oscillation. The oscillation can be detected by the difference in time it takes for two photons in the laser beam to run their perpendicular courses (as one's path will be shorter than the other's). It's kinda cool cause the frequency band they are looking at is in the 40 - 1000 Hz range IIRC, which is right in the range of human hearing. For fun they set up a speaker to the detector just so they could hear the signal. Two scientists were doing some trouble shooting down at the end of one of the tunnels and their voices were heard on the speaker they set up. The instrument was so sensitive that it could detect the miniscule distances that the sound waves produced by the scientists' voices moved the massive (remember, they weigh a few tons each) mirrors. Crazy.