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Author Topic: Does Gravity do any work?  (Read 69540 times)

Offline Geezer

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« Reply #75 on: 28/12/2009 19:44:23 »
While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.

I wish we really could explain what causes the force. I think it's fair to say we really don't know at this point. However, we are very good at describing the force in terms of the effects it produces.
 

Offline LeeE

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« Reply #76 on: 29/12/2009 13:48:28 »
While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.

I wish we really could explain what causes the force. I think it's fair to say we really don't know at this point. However, we are very good at describing the force in terms of the effects it produces.

I think it sort of depends at what level of causality we're talking about.

We can say, for example, that gravity is caused by the presence of non-zero rest-mass matter, but we can't say why non-zero rest-mass matter appears to result in gravity.

An intrinsic problem with top-down analysis is that it can only result in an abstract, which can only be defined in terms of itself i.e. it is what it is and it does what it does because it is what it is.  As you burrow further down through the analysis, each time you reach a new lower, or more fundamental level, you are able to say why the higher level does what it does, defined in terms of the new lower level, but the lowest level can only be defined in terms of what happens.

For example, if we say that the presence of non-zero rest-mass matter results in gravity, and that gravity is the consequence of the mediation of gravitons, we've explained how non-zero rest-mass matter results in gravity by defining it in terms of gravitons, but we're now stuck with explaining why the presence and behaviour of gravitons manifests itself as gravity.

(Edited to remove a redundant 'at' in the penultimate paragraph)
« Last Edit: 30/12/2009 13:04:27 by LeeE »
 

Offline om

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« Reply #77 on: 29/12/2009 16:26:23 »
Those interested in the role of gravity in the cosmos should go to

http://www.thenakedscientists.com/forum/index.php?topic=9197.0

To recap a small part:

1. Gravity is a nuclear force since essentially all mass is in the nucleus.

2. Gravity is a weak force over long distances.

3. Gravity is a very strong force over very short distances, but

4. Gravity cannot overcome repulsive forces between neutrons to convert neutron stars into black holes.

http://arxiv.org/pdf/0905.1667v1

In the Sun and in the cosmos, dynamic competition between long-range attractive gravitational forces and short-range repulsive forces between neutrons powers the objects that release energy and fill interstellar space with Hydrogen, a neutron-decay product.

If the universe is finite, then neutrons themselves may be the particle-sized black holes that were made in a Big Bang and compressed into massive, highly energetic neutron stars.

If the universe is infinite, then it may oscillate between expansion as interstellar space is filled with Hydrogen from neutron decay, and contraction after the neutron stars have evaporated and gravitational forces become dominant over repulsive forces between neutrons.

With kind regards,
Oliver K. Manuel
http://myprofile.cos.com/manuelo09
 

Offline yor_on

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« Reply #78 on: 29/12/2009 20:11:43 »
Yep LeeE. It's very frustrating :)

Or if you accept the Higgs field then gravity 'accumulates' around mass. But where would that field come from, and why would it work at all. What's guaranteeing mass to exist even if that field existed? When we see two phenomena directly related to each other it is easy to wonder what came 'first', or did they came 'together'?

That's one of the reasons why I like 'emergences', as it allows them to come 'together' begetting new 'property's' (water to ice)

But if we look at it as having a 'beginning' involving 'forces' then we treat it as a chain of occurrences from a beginning to an end, and then that first 'force' must contain it all, in some manner of speaking, as from it all other will come.

Maybe there are other ways to look at it too?
 

Offline questioner

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« Reply #79 on: 29/12/2009 22:16:42 »
Is gravity doing work if the energy that creates the action is within the atom to start with.
 

Offline Geezer

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« Reply #80 on: 29/12/2009 23:59:48 »
Is gravity doing work if the energy that creates the action is within the atom to start with.

Gravity can do work on the mass of an atom, but I don't believe there has been any change in the energy within the atom.
 

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« Reply #81 on: 30/12/2009 02:12:23 »
Is gravity doing work if the energy that creates the action is within the atom to start with.

Gravity can do work on the mass of an atom, but I don't believe there has been any change in the energy within the atom.
If gravity is a weak force and does not change the energy within the atom but affects the equilibrium of the atom.
Think of an atom moving into space away from the earth, as the gravity signal weakens the atom regains equilibrium.
 

Offline Geezer

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« Reply #82 on: 30/12/2009 07:12:58 »
If gravity is a weak force and does not change the energy within the atom but affects the equilibrium of the atom.
Think of an atom moving into space away from the earth, as the gravity signal weakens the atom regains equilibrium.

That's an interesting idea. Would I be correct in saying that you are proposing that atoms are subject to "stress" in a gravitational field?
 

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« Reply #83 on: 30/12/2009 09:57:05 »
If gravity is a weak force and does not change the energy within the atom but affects the equilibrium of the atom.
Think of an atom moving into space away from the earth, as the gravity signal weakens the atom regains equilibrium.

That's an interesting idea. Would I be correct in saying that you are proposing that atoms are subject to "stress" in a gravitational field?
Perhaps, how else would you explain it's reaction at varying distances to the earth. Maybe the gravity signal is undetectable by us at this stage.
 

Offline LeeE

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« Reply #84 on: 30/12/2009 13:25:04 »
Yep LeeE. It's very frustrating :)

Or if you accept the Higgs field then gravity 'accumulates' around mass. But where would that field come from, and why would it work at all. What's guaranteeing mass to exist even if that field existed? When we see two phenomena directly related to each other it is easy to wonder what came 'first', or did they came 'together'?

That's one of the reasons why I like 'emergences', as it allows them to come 'together' begetting new 'property's' (water to ice)

But if we look at it as having a 'beginning' involving 'forces' then we treat it as a chain of occurrences from a beginning to an end, and then that first 'force' must contain it all, in some manner of speaking, as from it all other will come.

Maybe there are other ways to look at it too?

One of the main things I've been playing with is a bottom-up synthetic approach as an alternative to the top-down analytic approach: instead of analysing downwards through the hierarchy structure from the top, towards the bottom-level fundamental abstract, you start with the bottom-level fundamental abstract and try to synthesise the hierarchy structure upwards.  It's an interesting exercise.
 

Offline questioner

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« Reply #85 on: 30/12/2009 22:14:01 »
Yep LeeE. It's very frustrating :)

Or if you accept the Higgs field then gravity 'accumulates' around mass. But where would that field come from, and why would it work at all. What's guaranteeing mass to exist even if that field existed? When we see two phenomena directly related to each other it is easy to wonder what came 'first', or did they came 'together'?

That's one of the reasons why I like 'emergences', as it allows them to come 'together' begetting new 'property's' (water to ice)

But if we look at it as having a 'beginning' involving 'forces' then we treat it as a chain of occurrences from a beginning to an end, and then that first 'force' must contain it all, in some manner of speaking, as from it all other will come.

Maybe there are other ways to look at it too?

One of the main things I've been playing with is a bottom-up synthetic approach as an alternative to the top-down analytic approach: instead of analysing downwards through the hierarchy structure from the top, towards the bottom-level fundamental abstract, you start with the bottom-level fundamental abstract and try to synthesise the hierarchy structure upwards.  It's an interesting exercise.
If you want to start at the bottom forget about gravity forming around mass and consider the existence of star dust. A material that planets and stars form around.
 

Offline LeeE

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« Reply #86 on: 31/12/2009 18:05:28 »
If you want to start at the bottom forget about gravity forming around mass and consider the existence of star dust. A material that planets and stars form around.

Actually, stuff like gravity and mass are relatively high-level secondary phenomenon viewed from where I've been starting from while playing with bottom-up approaches.  Star dust is just ordinary matter, which is even further up the hierarchy.
 

Offline Farsight

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« Reply #87 on: 05/01/2010 00:18:08 »
I've gone top-down, and it all seems to work out pretty easily. What causes forces is geometry. But it's dynamical geometry, and whilst it's really simple, people don't understand it. I expect that one day it will be taught, but until then it's somehow alien and unwelcome. It's rather odd actually.  
 

Offline LeeE

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« Reply #88 on: 05/01/2010 14:03:05 »
Lol - such certainty... ;D

Is there not something oxymoronic in claiming that "it's really simple" yet "people don't understand it?
 

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« Reply #89 on: 06/01/2010 02:37:45 »
It's true Lee. People have this amazing ability to reject something simple even when they themselves can't  elucidate any flaw, simply because it doesn't fit with what they know.
 

Offline Geezer

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« Reply #90 on: 06/01/2010 06:23:00 »
Then kindly elucidate.
 

Offline LeeE

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« Reply #91 on: 06/01/2010 17:07:41 »
Actually, I was being a bit facetious re the oxymoron; it's only natural that people require overwhelming evidence to accept a replacement for accepted hard-won, and hard-learned knowledge.
 

Offline Farsight

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« Reply #92 on: 07/01/2010 15:30:24 »
No problem Lee. The trouble is that all that hard learning isn't always supported by evidence. Then when somebody presents some evidence to challenge it, some people refuse to admit it as evidence. They have become so convinced of something they've learned, that can no longer be impartial and open-minded. We're familiar with this sort of thing from religious folk, or those who have adopted some ideology, but we tend to think it doesn't apply to scientists. Sadly, I'm afraid it's a human trait, and all those affected by it are convinced that they are not.   

Geezer, here's something I prepared earlier. There's more, but let's see how you get on with this for now. 

 

Offline Farsight

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« Reply #93 on: 07/01/2010 16:17:08 »
People tend to think the speed of light is constant, and Einstein said it. It isn't exactly true. Yes, he started with this as a postulate in 1905, but in 1911 he wrote On the Influence of Gravitation on the Propagation of Light, where you can see his ideas evolving as he talks about c = c0 (1 + Φ/c˛). Then in 1916 in section 22 of Relativity: The Special and General Theory he talks further:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light)".

People tend to see the word velocity in the 1920 translation without seeing the context. Many skip over his reference to "one of the two fundamental assumptions", and don't see that he's talking about a serious issue with the SR postulate of the constant speed of light. Many do not realise that Einstein didn't speak English in 1916, and what he actually said was die Ausbreitungsgeschwindigkeit des Lichtes mit dem Orte variiert. This translates into the speed of light varies with the locality. He was saying the speed varies with position, hence the reference to that postulate. And what he also said, is that this causes the light to follow a curvilinear path like a car veers when the near-side wheels encounter mud at the side of the road.



People often react badly this. Einstein talking about the variable speed of light does not fit with the relativity they've been taught. They don't appreciate that relativity today is something different to Einstein's relativity. People think Einstein told us about curved spacetime, but when you read The Foundation of the General Theory of Relativity it's simply not there. Yes, he talks about geometry and curvature and space-time, but he's giving the equations of motion, through space. He doesn't talk about "motion through spacetime". You can't move through spacetime, it's just the mathematical space where we plot our lines. There's other things that people aren't taught. Such as how he was still derided by many theoreticians even in 1923. You can see a reference to this on page 53 of Graham Farmelo's Dirac biography The Strangest Man:

"At that time, Cunningham and Eddington were streets ahead of the majority of their Cambridge colleagues, who dismissed Einstein's work, ignored it, or denied its significance".

Many people don't know that despite the media accolades and public adulation, Einstein drifted out of the mainstream from 1927 when he fell out with Bohr and others over quantum mechanics. They don't know that General Relativity was a "cottage industry" until the sixties, when the Golden Age changed it significantly:

"The Golden Age of General Relativity is the period roughly from 1960 to 1975 during which the study of general relativity, which had previously been regarded as something of a curiosity, entered the mainstream of theoretical physics. During this period, many of the concepts and terms which continue to inspire the imagination of gravitation researchers (and members of the general public) were introduced, including black holes and 'gravitational singularity'. At the same time, in closely related development, the study of physical cosmology entered the mainstream and the Big Bang became well established... A number of simultaneous paradigm shifts characterize the Golden Age of general relativity. First and foremost, the Big Bang became the canonical cosmological model. Other paradigm shifts included a growing appreciation of the: Role of curvature in general relativity; Theoretical importance of the black holes; Importance of geometrical machinery and levels of mathematical structure, especially local versus global spacetime structure; Overall legitimacy of cosmology by the wider physics community".

Nor do most people know that in 1949 Einstein and Godel worked out that time is cofounded with motion through space, not with space. It's there in A World without Time: The Forgotten Legacy of Godel and Einstein by Palle Yourgrau. But perhaps the signal most important thing most people don't know, is that whilst aether is a taboo word which is most definitely out of the mainstream, Einstein's gave his Leyden address in 1920. And the title is Ether and the theory of relativity. There's Einstein, talking about space and calling it an aether: 

"Mach’s idea finds its full development in the ether of the general theory of relativity. According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that ‘empty space’ in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty".

All in all it adds up to something rather surprising. Relativity has always been the Cinderella of modern physics, despite his vast reputation, Einstein was hardly in the mainstream at all, and his understanding of gravity isn't mainstream any more. What's especially surprising is how similar it is to the way Newton described it in Opticks:

Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies in empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines? ...Is not this medium much rarer within the dense bodies of the Sun, stars, planets and comets, than in the empty celestial space between them? And in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer?" queries 20 & 21

The language is different, but the underlying concept is the same. The energy tied up as the matter of a planet "conditions" the surrounding space to create a non-constant gμν along with a gradient in c which causes curvilinear motion. To many people this is unacceptable, because it isn't what they've been taught. It doesn't matter that it comes from Einstein and Newton and is supported by experimental evidence, they refuse to believe it.   



Show them two astronauts carrying parallel-mirror light clocks at different locations, and they will refuse to admit what the different readings on those light clock is telling them. They'll talk about coordinate speed and time dilation and spacetime curvature, anything to avoid what's in plain view: in a place where the gravitational potential is lower, the light goes slower.   
 

Offline LeeE

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« Reply #94 on: 07/01/2010 16:48:10 »
I don't think it's as simple as saying that the speed of light 'c' is not constant, for it's a measure of movement along two axis which themselves are not constant.  The speed of light will always be locally measured as being 'c', but different observers, in different localities will disagree upon how much space and time was covered during the measurement.

For example, if space-time were linear then a distant angular measurement, from a known distance, would give the same distance between two spatially separate points as a direct local measurement between them, however, due to the differing shape of space-time between the local observer making the direct measurement, and the distant observer deriving the distance via the angular displacement, they'll get different results.
 

Offline Farsight

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« Reply #95 on: 07/01/2010 18:05:40 »
Lee: yes, different observers can get different results, and all observers will measure their local speed of light to be 299,792,458 m/s. But you're the single observer looking at two astronauts, each holding a one-metre parallel-mirror laser light clock. One's down near a neutron star, holding his light clock flat to avoid radial length contraction. The other astronaut is well out in space. Let's say you have excellent telescopes, high-speed cameras, and recording facilities. You can put the two astronauts up side-by-side on a split screen, and in slow motion you can even see the laser light reflecting back and forth between their parallel mirrors. What will you notice?
 

Offline PhysBang

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« Reply #96 on: 08/01/2010 14:50:34 »
Lee: yes, different observers can get different results, and all observers will measure their local speed of light to be 299,792,458 m/s. But you're the single observer looking at two astronauts, each holding a one-metre parallel-mirror laser light clock. One's down near a neutron star, holding his light clock flat to avoid radial length contraction. The other astronaut is well out in space. Let's say you have excellent telescopes, high-speed cameras, and recording facilities. You can put the two astronauts up side-by-side on a split screen, and in slow motion you can even see the laser light reflecting back and forth between their parallel mirrors. What will you notice?
Well, when you collect the tapes later, you will see that each clock operates identically. One cannot beam real-time information from the two locations without some distortion that makes this exercise useless.
 

Offline PhysBang

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« Reply #97 on: 08/01/2010 14:53:59 »
Then in 1916 in section 22 of Relativity: The Special and General Theory he talks further:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light)".
And how does he go about showing that sspecial relativity does not ahve an unlimited domain? He uses curved spacetime, where special relativity holds in local manifolds that are combined together through differential geometry. If one does not understand this, then one cannot hope to understand contemporary gravitational theory.
 

Offline Farsight

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« Reply #98 on: 09/01/2010 17:43:59 »
The recording facilities are at your single observer's location, PhysBang. It's not sufficient to claim there's some kind of distortion between you and the astronauts that invalidates the evidence you observe. You can give each astronaut their own recording facilities, and see from afar that astronaut 1's tape moves slower, just like his light moves slower. When you retrieve their tapes you see that tape 1 has recorded say 999 feet as opposed to 1000 feet for astronaut 2, and that this tallies with what you saw during the experiment. It adds up to direct observable evidence that in a location where the gravitational potential is lower, the light goes slower.

Einstein uses differential geometry, but he doesn't actually mention curved spacetime in The Foundation of the General Theory of Relativity. You can interpret the curvilinear motion as curved spacetime, but you mustn't let this distract you from the evidence that's in accord with what Einstein actually said. Here's a corrected translation from section 22 of Relativity: The Special and General Theory:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the speed of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the speed of light varies with location."

Einstein's non-constant guv is observable as a non-constant speed of light in GPS and the Shapiro delay, as well as in the Gedankenexperiment I've described here.   
 

Offline Geezer

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« Reply #99 on: 09/01/2010 18:21:01 »
Are we drifting from the original question here? Should we split some of this great discussion into a new or different topic (assuming I can figure out how to do that without fouling everything up!)

What does everyone think?
 

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