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

[skwinty (Steve)]

Here's a thought.
Lets assume that gravity is a geometry, and the carrier of this information is the graviton.
Now liken this geometry to a ruler. When you move the ruler, every point of the ruler will see the move instantaneously unlike a photon which would have to travel the moved distance.

Now, current thinking implies that empty space is not empty but full of dark energy, so perhaps the graviton is part of this dark energy and this dark energy forms the gravitational geometry.

Just a thought, use it or lose it

[Paddy (Patrick)]

Originally Posted by skwinty

Here's a thought.
Lets assume that gravity is a geometry, and the carrier of this information is the graviton.
Now liken this geometry to a ruler. When you move the ruler, every point of the ruler will see the move instantaneously unlike a photon which would have to travel the moved distance.

Now, current thinking implies that empty space is not empty but full of dark energy, so perhaps the graviton is part of this dark energy and this dark energy forms the gravitational geometry.

Just a thought, use it or lose it

Now here I will reveal my naivety. I'm not convinced about gravitons to carry this information, but then I don't know much and might be presumptuous in thinking my naivety might help to avoid a trap. I've just been reading Stephen Hawkings' "a briefer history of time". In his chapter on the unification of physics, he says "if "empty space were really completely empty - that would mean that all the fields such as gravitational and electromagnetic would have to be exactly zero.However the value of a field and its rate of change with time are like the position and velocity of a particle: the uncertainty principle implies that the more accurately one knows these quantities the less accurately one can know the other. So if a field in empty space were fixed at exactly zero then it would have both a precise value and a precise rate of change, in violation of that principle"

He seems to argue that in order to satisfy the uncertainty principle, gravity must be a quantum phenomenon and a quantum theory of gravity is necessary. But if it's not a quantum phenomenon, would it have to satisfy the principle? Is his logic saying that a principle of quantum physics must apply to gravity as an axiom? Maybe gravity is something fundamentally different.

[skwinty (Steve)]

Current thinking is that even macroscopic "reality" is a quantum phenomena.
Also note that Stephen Hawking says "if" space is really empty.
I dont believe that it is, but then thats only my belief.

[renormalised (Carl)]

Given the uncertainty principle, spacetime can't be completely empty. No matter if you try to remove all the energy and matter from the system, the fact that you can never know whether you've removed everything means that some energy residual still exists in the fabric of space. It's essentially what they call the residual field, or zero point energy, of spacetime.

You can readily observe it in the Casimir Effect.

[renormalised (Carl)]

You know, Einstein would be upset if he knew this, but there is two things which definitely travel faster than lightspeed....unequivocally.

1. bad news in the media, and
2. Politicians going back on their promises

[edwardsdj (Doug)]

Sorry to have been a bit harsh before. The rise of pseudo-science these days causes me so much concern I find it hard not to speak out sometimes.

Originally Posted by renormalised

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".

The reason we don't feel the gravitational field of the Sun is that the Earth is in free-fall around the Sun. Us inhabitants of the surface of the Earth are not in free-fall around the Earth so we experience the gravitational field of the Earth from our reference frame on the surface of the Earth.

When you go into free-fall (like jumping off a cliff or sitting in an orbiting space shuttle) you cease to feel the gravitational field of the Earth either.

The gravity well of the Sun is actually much stronger than that of the Earth. From a reference frame in free-fall, you just don't feel any gravitational field at all. This was Einstein's great insight (priciple of equivalence).

[renormalised (Carl)]

Originally Posted by edwardsdj

Sorry to have been a bit harsh before. The rise of pseudo-science these days causes me so much concern I find it hard not to speak out sometimes.



The reason we don't feel the gravitational field of the Sun is that the Earth is in free-fall around the Sun. Us inhabitants of the surface of the Earth are not in free-fall around the Earth so we experience the gravitational field of the Earth from our reference frame on the surface of the Earth.

When you go into free-fall (like jumping off a cliff or sitting in an orbiting space shuttle) you cease to feel the gravitational field of the Earth either.

The gravity well of the Sun is actually much stronger than that of the Earth. From a reference frame in free-fall, you just don't feel any gravitational field at all. This was Einstein's great insight (priciple of equivalence).

That's true, the reason why we feel it here on Earth is because we're living inside Earth's gravitational well. We feel an acceleration towards the Earth's centre as the "pull" of gravity. The Sun, being so far away, has minimal effect (although tell astrologers that)

I wouldn't exactly call jumping off a cliff as being in free fall!!!! The result at the end is never free

That's a given, greater mass = much larger curvature of space.

I'm just as concerned about pseudo-science as you are, however I'm more careful about whom I address as being one who talks pseudo-science. Much of particle physics and cosmology these days sounds like pseudo-science but you'd never address someone like Neil Turok as being a crackpot!!!! (even if he looks like the proverbial mad scientist).

[csb (Craig)]

Skwinty, I did read your assumptions again and still I must take a contradictory position.

But as this thread has progressed, I notice the matters as discussed are either too deep for my high school learnings or just too weird (no offence intended to any participants).

[skwinty (Steve)]

Hi Craig,
You are still missing the point by not reading the post correctly.
I said the force of gravity doesnt exist. It is a fictitious force like centrifugal force and the coriolus force.
I did not say that gravity does not exist.
Sure, in high school they teach about the force of gravity, but as I said, this is a mathematical construct to explain the effects of gravity. Now the formulae work and produce the correct answers to explain the effect as our inertial frames experience them.
When you are driving your car and you accelerate wildy down the road, your body and senses tell you that you are being forced into the back of your seat.
There is no "force" pushing you into the seat. Your inertia tries to keep your body the way it is and your car pushes into you as its accelerates. As you start to accelerate with the car the feeling of being "forced" into the seat goes away as you have now reached the sames velocity as the car, unless of course you have a special car which does not stop accelerating.

If your car was travelling at a constant speed, but turning then your car would be accelerating whilst turning and so you feel the effects of the "fictitious" centrifugal force. Once again the car is turning into your inertial path and not another force acting on your body.

[bird (Anthony Wesley)]

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.

If this were true then it gives a means for faster than light communication, violating pretty much all known laws of physics :-)

cheers, Bird

[edwardsdj (Doug)]

Originally Posted by renormalised

That's true, the reason why we feel it here on Earth is because we're living inside Earth's gravitational well. We feel an acceleration towards the Earth's centre as the "pull" of gravity. The Sun, being so far away, has minimal effect (although tell astrologers that)

I don't think you understood my resoponse. We are actually much deeper inside the gravity well of the Sun than that of the Earth. The reason we don't feel the effect of the Sun's gravity is that we live on the surface of the Earth which is in free-fall around the Sun.

This is the same reason that an astronaut on the Space Shuttle doesn't feel the gravitational effects of the Earth even though they are only a couple of hundred kilometers above the surface of the Earth - they are in free-fall around the Earth - in Einsien's language they are in an inertial reference frame.

From our reference frame on the surface of the Earth, we are not in free-fall relative to the Earth. In Einstein's language: we are not in an inertial reference frame. The space-time curvature induced by the mass of the Earth is thus experienced in this frame of reference as an acceleration which we call gravity.

To repeat: the gravity well of the Sun is vastly larger than that of the Earth at our position. We just don't experience it from our frame of reference in free-fall around the Sun. Likewise, the gravity well of the galaxy is vastly larger here than that of the Sun: we just don't experience it from this frame of reference because the whole Solar System is in fee-fall around the centre of the galaxy.

Originally Posted by renormalised

I wouldn't exactly call jumping off a cliff as being in free fall!!!! The result at the end is never free

Well the example Einstein used himself was being in an evelator car when the cable breaks. He realised that the occupants would be unable to determine if they were in free-fall or in intergalactic space.

In the same way, if you jump off a cliff, you feel weightless as you are in free-fall (neglecting effects of air resistance obviously). When you hit the ground, you are obviously no longer in free-fall. (Einstien himself only bothered to mention this obvious fact in his little book on relativity for the general public: http://en.wikisource.org/wiki/Relati...General_Theory).

This is the fundamental line of reasoning on which the general theory of relativity is based. Einstein realised this in 1907 but it wasn't until 1915 that he had got his head around the math enough to publish a complete physical theory theory of gravity, electomagnetics and motion - the General Theory of Relativity.

Originally Posted by renormalised

That's a given, greater mass = much larger curvature of space.

You have this correct.

Originally Posted by renormalised

I'm just as concerned about pseudo-science as you are, however I'm more careful about whom I address as being one who talks pseudo-science. Much of particle physics and cosmology these days sounds like pseudo-science but you'd never address someone like Neil Turok as being a crackpot!!!! (even if he looks like the proverbial mad scientist).

I have never said that Neil Turok is a crackpot. The Chair of Mathematical Physics at Cambridge University is most certainly not a "pseudo" or "fringe" scientist.

All of the things I have mentioned here (and the laws of termodynamics) were covered in a course called PY101 that I studied in my first year at university about 15 years ago. If you are genuinely concerned about psedo-science, I implore you to get your basic scientific facts straight before confronting the more "cutting-edge" areas of physics.

I assure you I am far from arrogant and I mean no disrespect to you. I do have the greatest respect for the scientific tradition as first described by Descartes and perfected by Newton and will jump to it's defense when provoked.

Take care,
Doug

[csb (Craig)]

Aha! I understand now, Steve.

I had realised that centrifugal force is actually an effect - from some reading I did before posting yesterday.

And so the same applies to the force of gravity.

I reread the post and I can see what you mean but I still seem to disagree with a few of your statements.

However this discussion is to esoteric for me (but quite interesting for a casual reader of Scientific American) so I will leave you guys to it.

Thanks again, Steve.

And Caleb, if you're still here, will you please edit & correct the typo in the header - what a shocker!

[Ian Robinson]

Originally Posted by bird

If this were true then it gives a means for faster than light communication, violating pretty much all known laws of physics :-)

cheers, Bird

Not really violating the laws - see the metaanalysis in the article I provided a link to.

The speed of gravity is not infinite while it is very much greater than c .... that's good - in physics infinities are not desireable .
The effect of gravity still obeys causality.
The models of multiple bodies in motion still work.

[astroron (Ron)]

i herd that the speed of gravity was the same as light.

He probably thinks we are all sheep for following this thread
Ron

[csb (Craig)]

Well, I know I am, what about ewe

[skwinty (Steve)]

Originally Posted by astroron

i herd that the speed of gravity was the same as light.

He probably thinks we are all sheep for following this thread
Ron

Well, Caleb is a good shepherds name

[renormalised (Carl)]

Originally Posted by edwardsdj

I don't think you understood my resoponse. We are actually much deeper inside the gravity well of the Sun than that of the Earth. The reason we don't feel the effect of the Sun's gravity is that we live on the surface of the Earth which is in free-fall around the Sun.

This is the same reason that an astronaut on the Space Shuttle doesn't feel the gravitational effects of the Earth even though they are only a couple of hundred kilometers above the surface of the Earth - they are in free-fall around the Earth - in Einsien's language they are in an inertial reference frame.

From our reference frame on the surface of the Earth, we are not in free-fall relative to the Earth. In Einstein's language: we are not in an inertial reference frame. The space-time curvature induced by the mass of the Earth is thus experienced in this frame of reference as an acceleration which we call gravity.

To repeat: the gravity well of the Sun is vastly larger than that of the Earth at our position. We just don't experience it from our frame of reference in free-fall around the Sun. Likewise, the gravity well of the galaxy is vastly larger here than that of the Sun: we just don't experience it from this frame of reference because the whole Solar System is in fee-fall around the centre of the galaxy.



Well the example Einstein used himself was being in an evelator car when the cable breaks. He realised that the occupants would be unable to determine if they were in free-fall or in intergalactic space.

In the same way, if you jump off a cliff, you feel weightless as you are in free-fall (neglecting effects of air resistance obviously). When you hit the ground, you are obviously no longer in free-fall. (Einstien himself only bothered to mention this obvious fact in his little book on relativity for the general public: http://en.wikisource.org/wiki/Relati...General_Theory).

This is the fundamental line of reasoning on which the general theory of relativity is based. Einstein realised this in 1907 but it wasn't until 1915 that he had got his head around the math enough to publish a complete physical theory theory of gravity, electomagnetics and motion - the General Theory of Relativity.



You have this correct.



I have never said that Neil Turok is a crackpot. The Chair of Mathematical Physics at Cambridge University is most certainly not a "pseudo" or "fringe" scientist.

All of the things I have mentioned here (and the laws of termodynamics) were covered in a course called PY101 that I studied in my first year at university about 15 years ago. If you are genuinely concerned about psedo-science, I implore you to get your basic scientific facts straight before confronting the more "cutting-edge" areas of physics.

I assure you I am far from arrogant and I mean no disrespect to you. I do have the greatest respect for the scientific tradition as first described by Descartes and perfected by Newton and will jump to it's defense when provoked.

Take care,
Doug

Doug, you seem to have a rather somber personality. My comment on the free fall off a cliff was prefaced right at the end of my sentence with a twist on words, yet you took it too literally!!! You need to lighten up a bit

It was meant to be humorous, in that you pay the price for hitting the ground.

I do understand your response. I did almost exactly the same course (PH101) nearly a decade before you did. Actually I finished uni several years before you started

Anyway, there's nothing wrong with what you have said except for this. Even though the Sun's gravitational well is much larger than the Earth's, we are subject to a greater acceleration towards the centre of the Earth than we are to the Sun. Sure, as part of the Earth system, we are in free fall about the Sun and don't feel the Sun's curvature of spacetime, but that's precisely the point. We are much closer to the Earth than we are to the Sun and from our vantage point, the curvature of spacetime the Earth generates is more intense than the Sun's. Hence we don't go floating off into space towards the Sun. It's the same for the Solar System. All the planets and assorted debris are well within the Sun's gravitational well. As such they can be considered as acting as one unit when considered w.r.t the Galaxy. The entire gravitational body is in free fall about the centre of the Galaxy. Despite the Galaxy having a vastly larger and more intense overall well, because we are much closer to the Sun than to everything else, we feel the curvature of space generated by the Sun far more so than we do the Galaxy's spacetime curvature. You hit the matter right on the spot with this statement....

From our reference frame on the surface of the Earth, we are not in free-fall relative to the Earth. In Einstein's language: we are not in an inertial reference frame. The space-time curvature induced by the mass of the Earth is thus experienced in this frame of reference as an acceleration which we call gravity.

We are not in free fall relative to the Earth. We feel the curvature of spacetime near the Earth as the "force" of gravity. We have no inertial frame of reference w.r.t. the Earth. However, the Earth (along with us and everything on it) is in free fall about the Sun and so there is an inertial frame of reference here. It's the same for the Solar System.... the planets feel the acceleration towards the Sun far more so than the presence of the spacetime curvature generated by the Galaxy. There is no inertial frame of reference for the Sun and planets on this scale, however, everything is in free fall about the Galaxy and therefore in an inertial frame w.r.t. the Galaxy.

It's also the reason why satellites and such don't feel the Earth's gravity. They're in free fall about the centre of the Earth because they're constantly in acceleration about the centre. They have an inertial frame of reference w.r.t. the Earth. They're also in free fall about the Sun at the same time, however, if the satellite fired its thrusters and broke the acceleration, it would fall towards the Earth rather than the Sun because at its distance from the Earth it feels a far greater spacetime curvature, hence gravitational acceleration towards the Earth's centre than it does with the Sun.

Once again, Doug, you too what I said took literally....about Neil Turok. In actual fact, he does look like the typical madscientist. Rather gorky and awkward looking and so he can be stereotyped in this fashion. Nerdy you could say. That's where the "crackpot" moniker comes into it. I agree, he's far from being crackpot, only because we can understand what he's on about, however most people would find him rather "out there".

The problem I see here, Doug, is that we have two completely different people. You seems to be very literal and orthodox in your way of thinking. Very analytical and "step by step" in your train of thought. In contrast, whilst I can think very literally, I can also think and visualise at tangents and sometimes I interpret things differently to you. To put it in another way, the both of us are in orbit about Einstein, but the only thing we have in common is the fact that the light from his torch reaches us at the same velocity, despite our differences

[Karls48 (Karl)]

Ian Robinson has posted very interesting link to the paper that doesn’t go well with currently accepted theories. Why all of you who disagree with it read the article and then explain why it is wrong or right in it instead constantly quoting the BOOK. Regardless how intelligent and innovative Einstein was he was not the God and as anyone he could be wrong in some of his assumptions. I take my hat off to the people like Renormalised and Ian Robinson because they can think for themselves. Forget what you learn in Uni 20 years ago, I have done it 40 years ago and lot of what I learn is not accepted truth today. Actually it is better to forget most of what you learn in any kind of school. The main reason for the education is to teach you how to conform and for some – how to think. If you want know definite answers to everything join one of thousands of religions that are around. Read the BOOK and it will give you all the answer to the mysteries of life and Universe.
How dare anyone to assume that we are even close to understood how the Universe works. Imposing limits on the speed and the information at our stage of civilisation is ridicules. We are just primitives who just few thousand years ago run around hitting each other with simple club The only think that make as progress were wars as we needed to invent better club. Well we still do it( hit each other) when we run out of bullets.
It has been discussed many times before that Scientists are not really free and impartial, they depend on funding and if they come up with some idea that is not in the line with current thinking, they will get none (unless it is of some use for military).
We are all amateurs. There is no economical pressure on us to influence our thinking. So keep questioning current theories when you come upon something that does not make sense in relation to something else.
To me Darwin was much greater scientist then Newton or Einstein as he plotted the path as where we come from, what we are and where are we going (and is a PUB going to be still open when we get there?) .
Again big thumbs up to the people such as Ian Robinson, Renormalised and others who are able to think for themselves.
Don’t get me wrong, is Einstein was right I got no problem with it. Its just there are too many inconsistencies in his theories. And I just don’t believe that we can place finites on anything at our stage of civilisation.

[Bassnut (Fred)]

I dont understand the details, but one thing puzzles me.

An outside observor sees infalling matter into a black hole slow down to a stop at the event horison (and its "information" be preserved, smeared over the event horison, as Hawkings has postualted recently). This implies that time, to an outside observor, at the event horison slows to a stop (matter is traveling at C). To the black hole then, the passage of time of the outside universe is instananeous, as it is to a photon. How can a black hole actually grow in mass beyond the event horison then, it hasnt got time to. Is the space inside the event horison empty?. Is all its mass smeared over the event horison?.

[skwinty (Steve)]

Hi Karl
For clarity, which book is constantly being quoted?