I heard that the speed of gravity was the same as light

[caleb]

what do you think, im in a rush to crusty demons so il have to explain later.

[Lee]

Gravity is a force between two masses ie you and the earth, so doesn't have a speed.

[skwinty (Steve)]

The force of gravity does not exist. This force was a newtonian construct to explain why apples fall from trees to the ground and why we don't fall of the earth.
The earth is in freefall around the sun (due to the curvature of space time caused by the suns mass) and bodies in free fall feel no "gravity". As far as the earth is concerned it is travelling in a straight line in curved space time.
ie the shortest distance between 2 points which is a straight line.
We dont fall off the earth due to the inertia and acceleration of the earth.

[Jazza (Jay)]

The effects due to gravity are however felt in such a way as if 'gravitons' moved at the speed of light.
i.e. binary pulsars are used as tests of gen relativity and show that the effect of one pulsar on the other is due to where it was before, not where it is now...

If that made any sense at all... *sigh*

[renormalised (Carl)]

Originally Posted by skwinty

The force of gravity does not exist. This force was a newtonian construct to explain why apples fall from trees to the ground and why we don't fall of the earth.
The earth is in freefall around the sun (due to the curvature of space time caused by the suns mass) and bodies in free fall feel no "gravity". As far as the earth is concerned it is travelling in a straight line in curved space time.
ie the shortest distance between 2 points which is a straight line.
We dont fall off the earth due to the inertia and acceleration of the earth.

Pretty good answer, Steve, but not quite correct. For every day situations, such as an apple falling from a tree, tripping over a crack in the pavement etc, gravity can still be called a force (although, technically, it still isn't....if you like to be pedantic about it). However, gravity is really no more than a curvature of the local spacetime field caused by a massive object, Caleb. As Steve said, the Earth and everything else in the Solar System, is in free fall about the Sun and is traveling along straight lines in curved space caused by the Sun's mass.

The reason why we don't feel the Earth's motion about the Sun, or fall off the Earth is due to the fact we're a part of the inertial system which is the Earth and its gravitational well. Really, we're far to small to notice as for all intents and purposes, everything on the planet is just the one object in the system.

However, if you were to produce some sort of rippling effect in the curved space around the Sun (or the Earth, for that matter), you would be generating a gravitational wave, and this wave would travel at the speed of light....it's characteristics are the same as for a light wave except the propagating agent in this case is a graviton and not a photon.

[TrevorW]

32 feet/sec squared

300,000 kilometers a second

No contest light wins

[Entropy]

If the speed at which gravity propagates was greater than c. Could this ever be confirmed by experiment anyway?

Wouldn't it be only if the speed of the propagation of gravity was equal to or less than c could we accurately measure it ?

[AndrewJ]

If the speed at which gravity propagates was greater than c. Could this ever be confirmed by experiment anyway?

Nup
It would be finished before you started it.
That would be horribly confusing

Andrew

[Ian Robinson]

Originally Posted by AndrewJ

Nup
It would be finished before you started it.
That would be horribly confusing

Andrew

You are assuming that time actually behaves the way Einstein though it might if particles exceeded c. It may not.

Maybe this will be of interest : re superluminary velocity of gravitational effect. http://www.metaresearch.org/cosmolog...of_gravity.asp where the Speed of Gravity is ³ 2x10power10 c (ie 20 billion x c) , for all intents and human purposes , that makes the speed of gravity essentially infinite. See Physics Letters A 250:1-11 (1998).

direct experimental verification in the laboratory that gravity propagates faster than light may now be possible. The protocol and preliminary results were reported in (Walker, 1997).

If this true , that's an extraordinary property.

Sorry if the maths is a bit heavy , requiring at least a knowledge of vector calculus and electromagnetism and relativity (2nd and 3rd year uni undergraduate level studies), but well, the proof is in the physics and maths involved in the meta-analysis.

[caleb]

yes, but i mean. If the sun spontanoiusly dissapeared it would be 8 minutes till we noticed the light source gone. But would the gravity dissapear straight away or 8 minutes after, as the light went.

[renormalised (Carl)]

Originally Posted by caleb

yes, but i mean. If the sun spontaneously disappeared it would be 8 minutes till we noticed the light source gone. But would the gravity disappear straight away or 8 minutes after, as the light went.

8 minutes later, same as the light...if you follow current understanding in physics.

[Ian Robinson]

Originally Posted by caleb

yes, but i mean. If the sun spontanoiusly dissapeared it would be 8 minutes till we noticed the light source gone. But would the gravity dissapear straight away or 8 minutes after, as the light went.

According to the paper below - if the sun disappeared , the earth would "feel" the effect virtually instanteously - there is no delay in the action at a distance of gravity , even if it would take another 8 minutes for us on earth to be aware the sun is not there anymore.

[renormalised (Carl)]

Originally Posted by Ian Robinson

You are assuming that time actually behaves the way Einstein though it might if particles exceeded c. It may not.

Maybe this will be of interest : re superluminary velocity of gravitational effect. http://www.metaresearch.org/cosmolog...of_gravity.asp where the Speed of Gravity is ³ 2x10power10 c (ie 20 billion x c) , for all intents and human purposes , that makes the speed of gravity essentially infinite. See Physics Letters A 250:1-11 (1998).


If this true , that's an extraordinary property.

Sorry if the maths is a bit heavy , requiring at least a knowledge of vector calculus and electromagnetism and relativity (2nd and 3rd year uni undergraduate level studies), but well, the proof is in the physics and maths involved in the meta-analysis.

Just had a look at the article, but I haven't read it properly yet. Very interesting and since it's come under the imprimatur of a trusted journal, it means that it holds some veracity....regardless of what pure relativists might think.

[acropolite (Phil)]

Originally Posted by TrevorW

32 feet/sec squared

Sorry Trev, that's simply the acceleration of a body falling within the earths gravitational field.

[renormalised (Carl)]

Originally Posted by TrevorW

32 feet/sec squared

300,000 kilometers a second

No contest light wins

Doesn't work like that.....one is just the gravitational acceleration of an object falling through Earth's spacetime curvature, the other is the velocity of a photon in a vacuum.

What Caleb wanted to know is how fast does gravity propagate at, and is it at the same velocity as light.

[renormalised (Carl)]

Originally Posted by Ian Robinson

According to the paper below - if the sun disappeared , the earth would "feel" the effect virtually instanteously - there is no delay in the action at a distance of gravity , even if it would take another 8 minutes for us on earth to be aware the sun is not there anymore.

Which is the really interesting part

Sort of throws current thinking into a spin

[Ian Robinson]

Originally Posted by renormalised

Which is the really interesting part

Sort of throws current thinking into a spin

It is interesting .

[skwinty (Steve)]

Originally Posted by skwinty
The force of gravity does not exist. This force was a newtonian construct to explain why apples fall from trees to the ground and why we don't fall of the earth.
The earth is in freefall around the sun (due to the curvature of space time caused by the suns mass) and bodies in free fall feel no "gravity". As far as the earth is concerned it is travelling in a straight line in curved space time.
ie the shortest distance between 2 points which is a straight line.
We dont fall off the earth due to the inertia and acceleration of the earth.

Originally Posted by renormalised
Pretty good answer, Steve, but not quite correct. For every day situations, such as an apple falling from a tree, tripping over a crack in the pavement etc, gravity can still be called a force (although, technically, it still isn't....if you like to be pedantic about it).

Help me out here renorm.
What part of my statement is incorrect?

My understanding is that Einsteins principle of equivalence states that gravity is equivalent to acceleration.

This implies that we do not realise or feel the earths acceleration as it wends its way around the sun and hence had to invent a fictitious force of gravity to explain the fact of apples falling from trees and rivers flowing downhill.

We choose to ignore the truth that our surrounds are moving relative to us.
Once set in motion, as in a car at a constant speed, our bodies inertia tends to keep us travelling in a straight line at a constant speed until the car turns. If we are not restrained in the car we are jammed against the car door. We attribute this to the force of gravity when in effect it the effect of inertia, or the cars acceleration in a different direction.

Gravity is more of a geometry rather than a force, and the speed of gravity would refer to the speed of the geometries propogation. Herein is the function of the hypothetical "graviton".
Mass tells spacetime how to warp and warped spacetime tells mass how to move and this becomes recursive ad infinitum. This is the origin of gravity, in my understanding.

Now, I suppose this is pedantic semantics but I find that in my attempts at understanding these principles, clarity is impeded by incorrect terms being applied to the fundamentals.

ie is "force" rather than for example "field" the correct word to describe gravitational effects.

[renormalised (Carl)]

I suppose it's all pedantic semantics, when you boil it down. I even said that technically, gravity's "force" on the Earth's surface is just a result of the geometry of warped spacetime (well, not in so many words). It may have been the way you worded your original that confused me. However, the main reason why we don't fall off the Earth is that the gravitational field we find ourselves in is much steeper than the one the Earth is in orbit about the Sun. We're sitting near the bottom of ours in a fairly warped spacetime so we feel the effects more so than what we would out in space between the Sun and Earth. It essentially renders us part of the Earth itself, so any change of inertia the Earth might feel in a change of its orbit wouldn't be passed onto us. We'd just "go with the flow".

Essentially we're too small to matter.

Unless, of course, it was a sudden and violent move....like Earth running into a brick wall (built by a Vogon construction fleet, of course), or some giant goober space entity used us as a golf ball.

[csb (Craig)]

Skwinty, sorry but I am going to tear your 'assumptions' to bits..in a friendly manner with no malice intended. For you are completely wrong in your whole statement. This is High school stuff.

I haven't backed my answers up but some basic reading will show my understanding is more correct.

Originally Posted by skwinty

The force of gravity does not exist. This force was a newtonian construc...

What!? How do you come to that conclusion?
Gravity does exist.

Originally Posted by skwinty

The earth is in freefall around the sun

The Earth is held to the Sun by gravity. It would actually fall into the Sun except it is moving fast enough to counteract the gravitational pull of the Sun and demonstrates Centrifugal Force.

Originally Posted by skwinty

due to the curvature of space time caused by the suns mass

I'm not sure but I think the curvature is caused by the gravitational force which comes from the Sun's mass.

Originally Posted by skwinty

As far as the earth is concerned it is travelling in a straight line in curved space

No, actually Earth is trying to travel in a straight line but due to the effects of inertia(which says Earth should continue in a straight line and not veer away) and the Sun's gravity(which stops Earth from moving further away), Earth travels in a circular pattern - demonstrating Centrifugal Force.

Originally Posted by skwinty

We don't fall of the earth due to the inertia and accerleration of the earth

We don't fall off the Earth due to gravity.