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  1. #1 Gravity 
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    These are questions I have. Some might have answers already, others might not. If you know the answer please write it in the comments. If you have personal thoughts on the matter please write it as well.

    1)What is gravity?
    2)Does it affect everything equally?
    3)If it affects everything equally, why?
    -if #2 is correct-
    4)Does it really affect everything equally?
    5)Does it only act upon mass? why?


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    Quote Originally Posted by ravimi View Post
    These are questions I have. Some might have answers already, others might not. If you know the answer please write it in the comments. If you have personal thoughts on the matter please write it as well.

    1)What is gravity?
    2)Does it affect everything equally?
    3)If it affects everything equally, why?
    4)Does it really affect everything equally?
    5)Does it only act upon mass? why?
    Answer to Question #1) Gravity is a force. The magnitude of the force between two point particles whose first particle has mass m1 and whose second has mass m2 is

    F = Gm1 m2 /r////r2

    This can be generalized to arbitrary distributions of matter

    2) No. The force on a body depends on the mass of the body.

    4) No.

    5) Yes. That is a law of physics which mean we know its true but we don't know why its true.


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    Thank you for commenting, I like the thoroughness of your answer for question #1. Also, I'm not sure if there might have been an understanding for question 2 due to lack of explanation on my behalf but what I meant to ask was: does gravity affect everything, containing mass, the same way (not referring to how much mass there is [which definitely has an effect], but the existence of mass itself in an object). I guess in a way I am asking if there are different types of mass... like there are different types of cake. Maybe I'm thinking to much into it haha. Let me know if you understand and have an answer or if I'm just rambling.
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    As I understand it gravity is the distortion in space caused by mass. The fact of this distortion causes an attractive force to exist between any two masses. The attractive force is dependent on the amount of the masses involved.
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    Quote Originally Posted by Sealeaf View Post
    As I understand it gravity is the distortion in space caused by mass. The fact of this distortion causes an attractive force to exist between any two masses. The attractive force is dependent on the amount of the masses involved.
    Spacetime curvature is just another word for gradients in gravitational force. A gradient in the gravitational force is not the same thing as the gravitational force. Therefore spacetime curvature is not the same thing as the gravitational force which means that spacetime curvature is not the same thing as the gravitational force. That's just another way of saying that spacetime curvature is not gravity.
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    Quote Originally Posted by pmb View Post
    5) Yes. That is a law of physics which mean we know its true but we don't know why its true.
    No, this is incorrect. According to GR, all forms of energy are a source of the gravitational field, not just mass. It also acts on forms of energy other than mass - e.g. the photon has a vanishing rest mass, yet light paths are still bent in the presence of gravitational fields.
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    Quote Originally Posted by pmb View Post
    Spacetime curvature is just another word for gradients in gravitational force. A gradient in the gravitational force is not the same thing as the gravitational force. Therefore spacetime curvature is not the same thing as the gravitational force which means that spacetime curvature is not the same thing as the gravitational force. That's just another way of saying that spacetime curvature is not gravity.
    No. Gradients in the force are tidal forces. Tidal forces are not the sum total of spacetime curvature.

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    I am confused - the gradient is defined as a differential operator that sends a SCALAR field to a vector field. So that, for the scalar field one says that is a vector field.

    For sure, force is a vector, and as such can be defined at every point in spacetime i.e. it may be represented as a vector field.

    But the implication seems to be that the gravitational field is a scalar field.

    Is it? I rather think not

    Am I wrong? Have I misunderstood?
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by pmb View Post
    5) Yes. That is a law of physics which mean we know its true but we don't know why its true.
    No, this is incorrect. According to GR, all forms of energy are a source of the gravitational field, not just mass. It also acts on forms of energy other than mass - e.g. the photon has a vanishing rest mass, yet light paths are still bent in the presence of gravitational fields.
    The question was 5)Does it only act upon mass? why?

    By definition, that on which gravity acts is given the name passive gravitational mass. The photon has zero proper mass as you say. Yet is has non-zero gravitational mass by virtua of E = mc2

    As Feyman explains it in the Feynman Lectures V-I, page 7-11, section 7-8, first paragrah
    In the Einstein relaivity theory, anything which has energy has mass-mass in the sense that it is attracted gravitationally. Even light, which has an energy, has a "mass." When a light beam, which has energy in it, comes past the sun there is an attraction on it by the sun. Thus the light does not go straight, but is deflected.
    Quote Originally Posted by PhysBang View Post
    No. Gradients in the force are tidal forces. Tidal forces are not the sum total of spacetime curvature.
    You have it wrong. There is a 1-to-1 relationship between the tidal force tensor and the Reimann tensor. If there one vanishes then so does the other and if one is non-zero then so is the other. The Newtonian version is here
    Tidal Force Tensor

    For proof that tidal gradients and gravitational gradients are the same one merely has to follow the dervationof the equation of geodesic deviation. See
    Geodesic Deviation


    Kip Thorne had that in mind when he wrote it in his book Black Holes & Time Warps on page 111
    Therefore, spacetime curvature and tidal gravity must be precisely the same thing, expressed in different languages.
    For those of you who don't recognize Kip Thorne's name, he is one of the worlds leading experts in general relativity.
    Last edited by pmb; June 7th, 2012 at 04:33 PM.
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    Ya know, this site in general, and this sub-forum in particular, would be far more interesting if posters responded in their own words, with their own arguments As opposed, say, to linking to sites whose crediantials may or may not be known - or even encouraging members to buy and read texts which may or may not be reliable. It is a DISCUSSION forum, for heaven's sake - let's discuss rather than link
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    Quote Originally Posted by Guitarist View Post
    I am confused - the gradient is defined as a differential operator that sends a SCALAR field to a vector field. So that, for the scalar field one says that is a vector field.

    For sure, force is a vector, and as such can be defined at every point in spacetime i.e. it may be represented as a vector field.

    But the implication seems to be that the gravitational field is a scalar field.

    Is it? I rather think not

    Am I wrong? Have I misunderstood?
    First let us be clear about something. Let

    = gravitational potential of gravitational field

    U = m = gravitational potential energy of particle in field.

    Then

    F = -U = - (m ) = -m = gravitational force of particle in gravitattional field

    In Newtonian physcs some people refer to as the gravitational field. The object they use is the gravitational potential. This allows as to speak of the field it self rather than requiring the mass be part of it.

    This yields a smooth transition to general relativity where the quantity defining the gravitational field is no longer a scalar but is now a second rank tensor, called the metric tensor of spacetime, which is a set of potentials guv which are components of the metric tensor g.
    Last edited by pmb; June 7th, 2012 at 03:35 PM.
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    This test for bending of light (deflection of a stars position) shows sort of a lense effect to me (as when seeing something trough one side of a lense).
    But I never saw proof if this was from the sun or from the moon in front of it.

    The moon may be little but it,s much closer range from us. Due to closer distance with lenses same deflection of angle gives more deflection for the position of what,s behind the lense. And less angular deflection has same effect for this position.

    So is there any positive proof from testing that rules out the effect of the moon for this ?
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    Quote Originally Posted by Ghrasp View Post
    This test for bending of light (deflection of a stars position) shows sort of a lense effect to me (as when seeing something trough one side of a lense).
    But I never saw proof if this was from the sun or from the moon in front of it.

    The moon may be little but it,s much closer range from us. Due to closer distance with lenses same deflection of angle gives more deflection for the position of what,s behind the lense. And less angular deflection has same effect for this position.

    So is there any positive proof from testing that rules out the effect of the moon for this ?
    Lenses of glass have nothing to do with gravitational lenses, so your post is meaningless. And it's gibberish besides.
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    Quote Originally Posted by Ghrasp View Post
    But I never saw proof if this was from the sun or from the moon in front of it.
    Gravitational lensing is caused by any mass. The effect of the moon is too small to be detected (the mass of the sun is about 3x107 times greater than the moon and even then the deflection is very small). The measured deflection by the sun is just what is predicted by general relativity (this was one of the first tests of GR).

    Also, gravitational lensing is measured for other objects as well. The degree of lensing is exactly what is predicted by GR.
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    Quote Originally Posted by Guitarist View Post
    Ya know, this site in general, and this sub-forum in particular, would be far more interesting if posters responded in their own words, with their own arguments
    I did respond in my own words with my own arguments. Those are my web pages. Each URL has my name in it. Just place your cursor over it and look at the lower bar and you'll see my name peter.m.brown.

    There is a very good reason I do this. And every time I refer to a text there is a very good reason why I do so. In this case Kip Thorne is one of the worlds leading authority in general relativity and his words should be headed, especially when it comes to definitions. And anybody who knows physics at all should know who Richard Feynman is.

    Guitarist - If you don't know who these authors are you can simply ask.

    Many of the best physics texts I know of, i.e. literature which aims to educate, uses quotes to emphasize the material. That particular quote I found and recommended to the author ended up in the text I was proof reading. I see no reason why its considered good practice to use quotes in hysics texts and otherwise for people who are discussing physics. I recommended the quote for the GR text that came out back in 2000 and see no reasono not to recommend the quote in a post here.
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    Error
    Last edited by pmb; June 7th, 2012 at 04:26 PM. Reason: error
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    So not taken in consideration then ?
    Last edited by Ghrasp; June 8th, 2012 at 09:27 AM.
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    Quote Originally Posted by Ghrasp View Post
    One direktion angledeflection could make a line of sight curve but not result in a position shift. This is geometrical quite easy to see and illustrate with a paper and pencil.
    I'm not quite sure what you mean by this. But the deflection of light causes an apparent shift in position. This is due to the mass of the sun. (The moon has far too little mass to have a measurable effect.)

    Here is a nice explanation comparing the path of light (a) without the sun present, (b) as predicted by Newton and (c) as predicted by GR: Deflection and Delay of Light
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    Quote Originally Posted by Ghrasp View Post
    This test for bending of light (deflection of a stars position) shows sort of a lense effect to me (as when seeing something trough one side of a lense).
    But I never saw proof if this was from the sun or from the moon in front of it.
    What do you mean by "never saw proof"? I only read about the experiments myself. Is that what you mean? You never saw the moon's contribution mentioned? If that's what you meant then there's good reason for it. There wasn't any. And lack of observations never get published.

    The amount of delfection of starlight by the sun was calculated before it was observed. When a solar eclipse occured it was compared with the calculated deflection and the amounts matched. Further amount of delfectiom by the moon wouldn't be detected because the mass of the moon is far too small to have caused an additional amount of deflection. Therefore what was observed was in agreement with what was calculated. The moon's gravitational field was never taken into account due to the small amount of its mass.

    The closeness of the moon only worked against any defelection being taken into account since the light needs spatial distance to deflect. The greater the distance the greater the beam displacement from the axis.

    The deflection of starlight by the moon without first being delfected by the sun would be much easier to do. Had it been done it would be undetectable. Scientists don't look for things like that. I.e. theory says not to expect any deflection. Common sense tells us the same thing. There is thus no reason to measure what we don't expect to detect.
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    What do you mean by "never saw proof"? I only read about the experiments myself. Is that what you mean? You never saw the moon's contribution mentioned? If that's what you meant then there's good reason for it. There wasn't any. And lack of observations never get published.
    I mean I read about the position for this without the moon and sun and with both the sun and the moon.

    Only with the sun its impossible to registrate this star 'from heavy sunlight but what if it,s just the moon in between this star and the earth. That would give a comparance to proof the moon has no significant influence and also confirm or not confirm the theory. But maybe the moonlight allready is too strong to see this star with a telescope...That,s why I asked if it has been done.

    And does a curved timespace on itself give a change of position for a star ?

    Or is it just an angle that has to be made with a telescope for registrating this star when the moon and sun are in between ? If that,s the case the whole universe angles the same relative to the telescope. It would see the position for all the stars and the moon and the sun change with same angle if it had a broader view. The position of the star relative to other stars would stay the same then.

    Similar idea with a waterhose and at the end a sprinkler for sprinkling the garden. If the waterhose is curved the sprinkler at the end angles to the watertap at the other end and the garden and me. But nothing changes position just from that.

    The theorie would still predict how much the telescope has to be angled then but the interpretation that this is relevant for the position at which it is seen is not obvious or apparent to me.
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    Quote Originally Posted by Ghrasp View Post
    What do you mean by "never saw proof"? I only read about the experiments myself. Is that what you mean? You never saw the moon's contribution mentioned? If that's what you meant then there's good reason for it. There wasn't any. And lack of observations never get published.
    I mean I read about the position for this without the moon and sun and with both the sun and the moon.

    Only with the sun its impossible to registrate this star 'from heavy sunlight but what if it,s just the moon in between this star and the earth. That would give a comparance to proof the moon has no significant influence and also confirm or not confirm the theory. But maybe the moonlight allready is too strong to see this star with a telescope...That,s why I asked if it has been done.
    This is so poorly written, I have no real clue what you are trying to say. I'll take a wild guess. The mass of the moon is so small, the bending of starlight is too small to measure (smaller than the error bars in the measurement). Meanwhile, the sun's much larger mass bends the starlight enough to easily measure during an eclipse.

    And does a curved timespace on itself give a change of position for a star ?
    Well, no, it doesn't cahange the actual position of the star, which is many light years away. However, it does change the observed (measured) position of the star in the sky.

    Or is it just an angle that has to be made with a telescope for registrating this star when the moon and sun are in between ? If that,s the case the whole universe angles the same relative to the telescope. It would see the position for all the stars and the moon and the sun change with same angle if it had a broader view. The position of the star relative to other stars would stay the same then.
    it has nothing to do with the telescope. The telescope just magnifies it to make the displacement easier to measure. If your eyes were precise enough, you would see the same thing.

    Similar idea with a waterhose and at the end a sprinkler for sprinkling the garden. If the waterhose is curved the sprinkler at the end angles to the watertap at the other end and the garden and me. But nothing changes position just from that.
    Huh?

    The theorie would still predict how much the telescope has to be angled then but the interpretation that this is relevant for the position at which it is seen is not obvious or apparent to me.
    Again, forget about the telescope. It has nothing to do with the change in measure position.
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    Quote Originally Posted by MeteorWayne View Post
    Again, forget about the telescope. It has nothing to do with the change in measure position.
    I think I know where he's comming from. It appears that he's thinking that when making an observation of a star that you point the telescope in a direction in which a ray from the star travels down the tube of the telescope, thus retaining the star in the center position of his screen. When the light is deflected the trelescope has to point in a different direction so that the new, deflected rays are now traveling down the center of the tube.

    However it's harder for me to explain in words than it would be to imagine all his in a diagram.
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    My question is kinda tied to the OP so I hope its not to ill mannered to ask here instead of making a new thread.

    How fast does gravity work? If the sun disappeared it takes 8 minutes untill the light stops as Ive understood it. But what about gravity? Will it also take 8 minutes (Speed of light) for the earth to be affected by the diappearing sun - or will the disappearance of the sun on a gravitational level affect the earth instantly?
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    Quote Originally Posted by Raziell View Post
    My question is kinda tied to the OP so I hope its not to ill mannered to ask here instead of making a new thread.

    How fast does gravity work? If the sun disappeared it takes 8 minutes untill the light stops as Ive understood it. But what about gravity? Will it also take 8 minutes (Speed of light) for the earth to be affected by the diappearing sun - or will the disappearance of the sun on a gravitational level affect the earth instantly?
    Yes, changes in gravity propagate at the speed of light. (Although, obviously, the Sun couldn't just disappear )
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    Any propagation is limited by it's medium.
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    I have to say I never expected my questions to attract these kind of replies= 20% understood, 60% what the hell, 20% wtf. lol Keep it up, I am reading all the posts and am giving each their deserved time and focus
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    Quote Originally Posted by Geo View Post
    Any propagation is limited by it's medium.
    I like this answer a lot, it makes great sense to me, but it makes me want to ask, is it really true?
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    F = Gm1 m2 /r2 is only an experience formula never revealing essential .
    1, G is probably a variable but not a constant .
    2, And the formula ignored the response velocity .
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    When any wire or fibre is tordated it shortens. That means with a torsionbalance (for G) the moving parts of the balance are lifted a little in vertical direktion not just moving in a horizontal plane.

    So lets say M1 is supplied by the weights on the arm and M2 by the non moving weights.

    First the weights M 2 are not there for calibrating the instrument (m2=0). Then M2 weights are placed.

    The fibre twists 醤d shortens. with this shortening/lifting for M 1 the energy (potential energy ; the fibre tordated with the gravitational energy add up) for the balance (all parts except weights for m2) is obviously increased just from placing the weights for M2.
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    Quote Originally Posted by Emdrive View Post
    F = Gm1 m2 /r2 is only an experience formula never revealing essential .
    1, G is probably a variable but not a constant .
    2, And the formula ignored the response velocity .
    That's because that formula comes from classical mechanics. If you use the full model of gravity ( General Relativity ), you don't have those problems.
    Btw, "G" has thus far been shown to be constant.
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    Quote Originally Posted by ravimi View Post
    Quote Originally Posted by Geo View Post
    Any propagation is limited by it's medium.
    I like this answer a lot, it makes great sense to me, but it makes me want to ask, is it really true?
    Yes. The speed of propagation is determined by the medium in which the propagation takes place. In the case of light in vacuum, this medium is the electromagnetic field, and the speed of light is determined by that vacuum's permittivity and permeability.
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    Quote Originally Posted by Markus Hanke View Post
    the full model of gravity ( General Relativity ), you don't have those problems.
    Btw, "G" has thus far been shown to be constant.
    1,Should I add the part of "cosmological constant" or not?
    2,You can search it on internet. Several French scholars once reported about it after detection.
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    Quote Originally Posted by Emdrive View Post
    1,Should I add the part of "cosmological constant" or not?
    It doesn't matter - your point was that the original formula from classical mechanics provides for an instantaneous action of gravity ( you are correct in saying that ). General Relativity however clearly shows that changes in the gravitational field propagate at the speed of light - that prediction is independent of the value of the cosmological constant.

    2,You can search it on internet. Several French scholars once reported about it after detection.
    I would appreciate it if you could provide references, please. I could not find anyone of any standing in scientific circles who asserts that G is not a constant, except of course the usual assortment of cranks and crackpots.
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    Quote Originally Posted by Markus Hanke View Post
    "G" has thus far been shown to be constant.
    How ? With a torsionbalance on the moon the total weight on the fibre (gravity with the moon) is less and the balance would torque easier towards M2's.
    The way G is meassured (this is still done classic) it would show an increase for it,s value when g decreases. Fz would also decrease.

    Maybe Fz . r^2 / M1.M2.G = 1 (thus constant) would be closer but not G.

    As a sidenote : Newton never tought of a torsionbalance as a situation that relates to this equation in a logic consitent manner.

    The classic horizontal component force I see in the balance between m1,s and M2's is more like cohesion in a horizontal plane. Similar as surface tension, gluon effect maybe made vissible on a macroscale.

    A modern gravity effect maybe can be seen in a deflection for the vertical angle to normal (from this gluon effect between m1 and m2's ?). But it sure (to me) has too much forces (at least two) mixed up to make it fundamental.
    Last edited by Ghrasp; June 17th, 2012 at 04:26 AM.
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    Quote Originally Posted by Ghrasp View Post

    How ? With a torsionbalance on the moon the total weight on the fibre (gravity with the moon) is less and the balance would torque easier towards M2's.
    The way G is meassured (this is still done classic) it would show an increase for it,s value when g decreases. Fz would also decrease.

    Maybe Fz . r^2 / M1.M2.G = 1 (thus constant) would be closer but not G.

    As a sidenote : Newton never tought of a torsionbalance as a situation that relates to this equation in a logic consitent manner.

    The classic horizontal component force I see in the balance between m1,s and M2's is more like cohesion in a horizontal plane. Similar as surface tension, gluon effect maybe made vissible on a macroscale.

    A modern gravity effect maybe can be seen in a deflection for the vertical angle to normal (from this gluon effect between m1 and m2's ?). But it sure (to me) has too much forces (at least two) mixed up to make it fundamental.
    I am really not sure what you are on about. Nowadays the measurement of G is done using precision atom interferometers - the latest measurement was done as recently as 2007 :

    ^ J. B. Fixler; G. T. Foster; J. M. McGuirk; M. A. Kasevich (2007-01-05), "Atom Interferometer Measurement of the Newtonian Constant of Gravity", Science 315 (5808): 74–77, Bibcode 2007Sci...315...74F, DOI:10.1126/science.1135459, PMID 17204644

    T
    he result makes it obvious that G has not changed in the last 300 years or so at least, and obviously the measurement is also independent of where it is performed - if you were to move that apparatus to the moon, or anywhere else, you'd still get the same result.
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    And what is your point, exactly ? These are all different experiments, with different ways of measuring G. Obviously these will not all agree, and thus cannot be used to check for constancy of G.
    Your original assertion was that the G is not constant - in order to test this, one must run the same measurement setup over a long time, and see whether there are any variations in G. This has been done using the Lunar Laser Ranging Experiment ( LLRE ), which has now been running continuously for more than 40 years :

    Lunar Laser Ranging experiment - Wikipedia, the free encyclopedia

    The result ? There has been no change in G, up to a measurement accuracy of 1 part in 10^11.
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    You can easily verify it by yourself.
    Take a block of aluminum about 500g, weighing it.
    Heating it up to more than 300 degrees Celsius, weigh it again.

    It will become a little less weight about 0.1g.
    I am only interested in machine,sorry.
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    Quote Originally Posted by Emdrive View Post
    You can easily verify it by yourself.
    Take a block of aluminum about 500g, weighing it.
    Heating it up to more than 300 degrees Celsius, weigh it again.

    It will become a little less weight about 0.1g.
    What is this supposed to verify?

    If you have done this experiment and got that result, then there is something very wrong with either your method or your instruments. If anything, the block will become heavier (by an immeasurably small amount: E=mc2).

    I am guessing that any apparent change in weight is due to driving off moisture from the surface of the aluminium and/or the heat affecting your weighing scales.

    I am only interested in machine,sorry.
    What?
    pmb likes this.
    ei incumbit probatio qui dicit, non qui negat
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    As that page starts with the statement "Mass varies with its absolute velocity" I think we can safely ignore any conclusions drawn by the crackpot who wrote it.
    ei incumbit probatio qui dicit, non qui negat
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    Once Mr. fengjinsong reported when temperature rised from 15 to 300 ℃
    the weight of a block changed from 30.1201g to 30.0572g.
    The variation is -0.0629g.
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    Quote Originally Posted by Emdrive View Post
    Once Mr. fengjinsong reported when temperature rised from 15 to 300 ℃
    the weight of a block changed from 30.1201g to 30.0572g.
    The variation is -0.0629g.
    See my previous reply. Just adding more decimal places doesn't make the claim any more plausible.

    I guess any apparent change in weight is due to driving off moisture from the surface of the aluminium and/or the heat affecting the weighing scales. Without more details of the experimental method, it is impossible to be sure.

    And who is Feng Jin Song ?
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    They have tried to test 5 kinds of materials:
    52.3103g Brass, 一41.6mg;
    93.2593g iron、一20.6mg;
    81.0716g stainless steel、一5.8mg;
    30.1201g aluminum、一62.9mg;
    43.2777g ceramics , 一20.5mg.
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    In any case, that g stands for grams, not gravity.
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    Quote Originally Posted by Emdrive View Post
    You can easily verify it by yourself.
    Take a block of aluminum about 500g, weighing it.
    Heating it up to more than 300 degrees Celsius, weigh it again.

    It will become a little less weight about 0.1g.
    I am only interested in machine,sorry.
    The temperature of a sample has nothing whatsoever to do with the value of G. I have shown you already that G has been measured to be constant over a period of 40+ years.
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    Maybe you are right but current theories ignore another important point:
    The stars are losing their mass every moment and moving fast and people can only receive informations lagging behind the fact.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Emdrive View Post
    You can easily verify it by yourself.
    Take a block of aluminum about 500g, weighing it.
    Heating it up to more than 300 degrees Celsius, weigh it again.

    It will become a little less weight about 0.1g.
    I am only interested in machine,sorry.
    The temperature of a sample has nothing whatsoever to do with the value of G. I have shown you already that G has been measured to be constant over a period of 40+ years.
    Why are you talking about G? From what I've seen nobody talked about the gravitational constant. The only g that appears is when someone writes the amount of of mass and uses "g" to signify the units of "gram".
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    Quote Originally Posted by pmb View Post
    Why are you talking about G? From what I've seen nobody talked about the gravitational constant.
    See post 28, where it was asserted that G is not constant. That was the starting point of this discussion.
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    Quote Originally Posted by Emdrive View Post
    Maybe you are right but current theories ignore another important point:
    The stars are losing their mass every moment and moving fast and people can only receive informations lagging behind the fact.
    I don't know what you mean by this. We already know that the speed of light is finite, therefore what we observe around us is obviously "lagging behind". That has been a known fact for a very long time.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by pmb View Post
    Why are you talking about G? From what I've seen nobody talked about the gravitational constant.
    See post 28, where it was asserted that G is not constant. That was the starting point of this discussion.
    Thanks. I'm confused. what does this have to do with recent posts?
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    Quote Originally Posted by pmb View Post
    Thanks. I'm confused. what does this have to do with recent posts?
    That's precisely what I am wondering myself...
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    Quote Originally Posted by Markus Hanke View Post
    [QUOTE
    I don't know what you mean by this. We already know that the speed of light is finite, therefore what we observe around us is obviously "lagging behind". That has been a known fact for a very long time.
    Not really.
    The Olbers' paradox is a good example .It's just a mistakes .
    Let me use several parameters :
    L means the distance between earth and a star; T means the life of a star ;
    Being alive does not mean living forever! C is light velocity;
    When L>T*C ,we can not judge whether there is a star or not just according to brightness or not.
    Take an any bit X of the sky,
    If it's bright ,this maybe has two possibilities:
    1, now bit X has a star;
    2, once bit X had a star but now it's dead and the last beam of light is still on the way speading to the earth.
    If it's black, this maybe has two possibilities too:
    1, now bit X hasn't a star;
    2, now bit X has a star in deed but the first light will arrive at the earth many many years later!
    I am not sure if I have stated clearly for I seldom use English in my real life.
    I want to say: when we judge the universe is limited or infinite ,Olbers' paradox is not a perfect evidence.
    I prefer to believe the universe is infinite!
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    Quote Originally Posted by Emdrive View Post
    I want to say: when we judge the universe is limited or infinite ,Olbers' paradox is not a perfect evidence.
    I prefer to believe the universe is infinite!
    You are seriously confusing me. My last post was about the speed of light, and had nothing to do with wether the universe is finite or not...???
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    I like to start out saying I'm in the middle of writing a book on why both Newton and Einstein were in fact wrong and gave us junk science. I will wait to here some responses.

    Of topic fringe comments removed. - KALSTER
    Last edited by KALSTER; June 18th, 2012 at 03:47 AM.
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    Quote Originally Posted by Thomas Dutkiewicz View Post
    I will wait to here some responses.
    Rather make a thread in the New Hypothesis section and prepare to discuss your idea at length. If you are not prepared to do so, you have made a mistake coming to a discussion forum. Please don't post your own ideas in threads that has no call for it. Thanks.

    Welcome.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Ghrasp View Post

    How ? With a torsionbalance on the moon the total weight on the fibre (gravity with the moon) is less and the balance would torque easier towards M2's.
    The way G is meassured (this is still done classic) it would show an increase for it,s value when g decreases. Fz would also decrease.

    Maybe Fz . r^2 / M1.M2.G = 1 (thus constant) would be closer but not G.

    As a sidenote : Newton never tought of a torsionbalance as a situation that relates to this equation in a logic consitent manner.

    The classic horizontal component force I see in the balance between m1,s and M2's is more like cohesion in a horizontal plane. Similar as surface tension, gluon effect maybe made vissible on a macroscale.

    A modern gravity effect maybe can be seen in a deflection for the vertical angle to normal (from this gluon effect between m1 and m2's ?). But it sure (to me) has too much forces (at least two) mixed up to make it fundamental.
    I am really not sure what you are on about. Nowadays the measurement of G is done using precision atom interferometers - the latest measurement was done as recently as 2007 :

    ^ J. B. Fixler; G. T. Foster; J. M. McGuirk; M. A. Kasevich (2007-01-05), "Atom Interferometer Measurement of the Newtonian Constant of Gravity", Science 315 (5808): 7477, Bibcode 2007Sci...315...74F, DOI:10.1126/science.1135459, PMID 17204644

    T
    he result makes it obvious that G has not changed in the last 300 years or so at least, and obviously the measurement is also independent of where it is performed - if you were to move that apparatus to the moon, or anywhere else, you'd still get the same result.
    What shows there is not another type of force involved ? If this force responds to a change of distance like this it keeps the same problem ; It,s not proven - from all fysics forces - what force this is.
    Last edited by Ghrasp; June 18th, 2012 at 04:44 AM.
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    I'm also confused.
    The reality is many people don't understand how important it is: The gravity of the transmission speed is the speed of light but not instantaneous.

    This will produce a great trouble. Please look at the picture following:
    Fa>Fb and they are not in the same line any longer.
    Thus this the dignity of Newton's third law has to face a challenge!

    gravity.jpg
    Last edited by Emdrive; June 18th, 2012 at 05:07 AM.
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    Quote Originally Posted by Emdrive View Post
    I'm also confused.
    The reality is many people don't understand how important it is: The gravity of the transmission speed is the speed of light but not instantaneous.

    This will produce a great trouble. Please look at the picture following:
    Fa>Fb and they are not in the same line any longer.
    Thus this the dignity of Newton's third law has to face a challenge!

    gravity.jpg
    A change in gravity propagates at the speed of light, but two bodies in constant motion relative to each other act on each other as if the force is instant. In other other words, the earth orbits the gravitational centre where it is now, not 8 minutes ago. It is a consequence of relativity. So, while Newton's equations are perfectly valid solutions to relativity at low speeds, they don't cater for the whole picture. Relativity does.
    Last edited by KALSTER; June 18th, 2012 at 01:19 PM.
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    Quote Originally Posted by Ghrasp View Post
    What shows there is not another type of force involved ?
    No, it's just gravity.
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    Quote Originally Posted by KALSTER View Post
    A [i]change[i/] in gravity propagates at the speed of light, but two bodies in constant motion relative to each other act on each other as if the force is instant. In other other words, the earth orbits the gravitational centre where it is now, not 8 minutes ago. It is a consequence of relativity. So, while Newton's equations are perfectly valid solutions to relativity at low speeds, they don't cater for the whole picture. Relativity does.
    Precisely, that is why I said in post 30 that General Relativity needs to be used, and not just Newton's law.
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    And if anyone actually wants to know how gravity in GR avoids the problems Newton had with aberration, look no further than:

    [gr-qc/9909087] Aberration and the Speed of Gravity
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    There have been a lot of posts in this thread, but it seems that no one has addressed ravimi's question. Or am I misinterpreting it? From his follow up post #3:

    Quote Originally Posted by ravimi
    Thank you for commenting, I like the thoroughness of your answer for question #1. Also, I'm not sure if there might have been an understanding for question 2 due to lack of explanation on my behalf but what I meant to ask was: does gravity affect everything, containing mass, the same way (not referring to how much mass there is [which definitely has an effect], but the existence of mass itself in an object). I guess in a way I am asking if there are different types of mass... like there are different types of cake. Maybe I'm thinking to much into it haha. Let me know if you understand and have an answer or if I'm just rambling.
    emphesis mine

    This seems to be a question relating to the equivalence principle. Example for passive gravitational mass would be: Will a lead ball and a copper ball of equal inertial mass both accelerate at the same rate in a gravitational filed? Example for active gravitational mass would be: Will a lead ball and a copper ball of equal inertial mass both produce the same strength gravitational field?
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    There is only one kind of mass. A feater or a lead ball have the same effect if they have the same mass.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Ghrasp View Post
    What shows there is not another type of force involved ?
    No, it's just gravity.
    Keep in mind that Newton never tought of this experiment 髍 a torsionbalance in relation to his equation or a falling apple. That,s a historic fact.

    Another possibillity for explaining this interferometer (and torsionbalance) is that it doesn,t show a force between things but a force (or energy) acting from surrounding.

    The surrounding won,t be vacuum (technical impossible).

    When two ships come side by side (no wind) the water (the surrounding in this case) in between the ships looses some energy compared to the more open water at other sides of the ships.
    The ships behave as if it has a pressure difference to both sides for each ship. This drives them to bump into each other.

    This accellerates both ships to where they bump against each other without a real force acting between the ships.
    Just as I can,t see this as corresponding to a falling apple (as a phenomenon) I don,t see the correspondance between the falling apple and a torsionbalance or this interferometer.

    A correspondance between the ships and this interferometer is more obvious to me ; the pB block influences surrounding so the surrounding acts a-symmetric.
    Last edited by Ghrasp; June 19th, 2012 at 02:46 AM.
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    You are talking about push gravity. Push gravity doesn't work as a result of shadows for one. There are other reasons also.

    PS: I don't know why you would think two ships next to each other would move closer due to water pressure. I don't see your logic there.
    Last edited by KALSTER; June 19th, 2012 at 05:21 AM.
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    Quote Originally Posted by Ghrasp View Post
    Keep in mind that Newton never tought of this experiment 髍 a torsionbalance in relation to his equation or a falling apple. That,s a historic fact.
    What does it matter what Newton thought or didn't think?
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    Quote Originally Posted by KALSTER View Post
    PS: I don't know why you would think two ships next to each other would move closer due to water pressure. I don't see you logic there.
    That's essentially the Casimir effect, Kalster. See this article for a mention. I'm not sure what it's got to do with gravity, the thread seems a little confused.
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    I don't think there is any Casimir effect between ships. It manifests between two closely placed uncharged plates in a vacuum.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Quote Originally Posted by Ghrasp View Post
    the pB block influences surrounding so the surrounding acts a-symmetric.
    That would the curvature of spacetime caused by mass (or energy) as described by general relativity.
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    Quote Originally Posted by KALSTER View Post
    I don't think there is any Casimir effect between ships. It manifests between two closely placed uncharged plates in a vacuum.
    Read the article:

    "Although the Casimir effect is deeply rooted in the quantum theory of electrodynamics, there are analogous effects in classical physics. A striking example was discussed in 1836, in P. C. Causs閑抯 L扐lbum du Marin (The Album of the Mariner)5. Causs閑 reported a mysteriously strong attractive force that can arise between two ships floating side by side a force that can lead to disastrous consequences (Fig. 1). A physical explanation for this force was, however, offered only recently, by Boersma 6, who suggested that it originates in the radiation pressure of water waves acting differently on the opposite sides of the ships. His argument goes as follows..."

    But also see Popular physics myth is all at sea, a Nature article which rebuts the above. I don't know what's true, but this is what Ghrasp was referring to.
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    Note the very important word analogous
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    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by KALSTER View Post
    I don't think there is any Casimir effect between ships. It manifests between two closely placed uncharged plates in a vacuum.
    Read the article:

    "Although the Casimir effect is deeply rooted in the quantum theory of electrodynamics, there are analogous effects in classical physics. A striking example was discussed in 1836, in P. C. Causs閑’s L’Album du Marin (The Album of the Mariner)5. Causs閑 reported a mysteriously strong attractive force that can arise between two ships floating side by side — a force that can lead to disastrous consequences (Fig. 1). A physical explanation for this force was, however, offered only recently, by Boersma 6, who suggested that it originates in the radiation pressure of water waves acting differently on the opposite sides of the ships. His argument goes as follows..."

    But also see Popular physics myth is all at sea, a Nature article which rebuts the above. I don't know what's true, but this is what Ghrasp was referring to.
    Ah, ok thanks. Very interesting. Looks like your second link debunks Boersma pretty clearly though.

    Quote Originally Posted by MeteorWayne
    Note the very important word analogous
    Yep.
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    A far more likely cause for the ships being attracted (if it really happens) is the Venturi Effect.
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    Quote Originally Posted by MeteorWayne View Post
    A far more likely cause for the ships being attracted (if it really happens) is the Venturi Effect.
    Looks good! There has to be a current though afaik.
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    Quote Originally Posted by Ghrasp View Post
    Another possibillity for explaining this interferometer (and torsionbalance) is that it doesn,t show a force between things but a force (or energy) acting from surrounding.
    Please not that push gravity crap again !! So sick of it
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    Quote Originally Posted by Markus Hanke View Post
    Please not that push gravity crap again !! So sick of it
    What is push gravity?
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    Quote Originally Posted by pmb View Post
    What is push gravity?
    It made no sense 300 years ago, but assorted crackpots still bring it up: Le Sage's theory of gravitation - Wikipedia, the free encyclopedia

    That and the aether. I just feel sorry for phlogiston - why does no one want to resurrect that?
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    Quote Originally Posted by MeteorWayne View Post
    There is only one kind of mass. A feater or a lead ball have the same effect if they have the same mass.
    I think you mean there is only one type of mass as far as gravity is concerned? That is the postulate of the equivalence principle. Or put another way, gravity does not distinguish between different compositions of mass. So to answer the op question, if this is the correct interpretation, gravity affects all mass equally regardless of it's type (composition). This has been tested many many times to a very high degree of precision. However, for the case of active gravitational mass it's an entirely different story. The testing history for this aspect of the equivalence principle is very poor.
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    Quote Originally Posted by pmb View Post
    What is push gravity?
    You don't want to know. Really, you don't. It's just too embarrassing.
    We had a phase here on this forum not too long ago where everything was suddenly about push gravity. After a while it just stops being entertaining.
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    Quote Originally Posted by Strange View Post
    ...That and the aether...
    Bad news Strange, take a look on the arXiv for papers with aether in the title. Also check out Einstein's Leyden Address: "Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether". The aether is a dirty word in some eyes, but it's just "the vacuum", or space. And take a look at the abstract for Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime.
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    Quote Originally Posted by Farsight View Post
    Bad news Strange, take a look on the arXiv for papers with aether in the title. Also check out Einstein's Leyden Address: "Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether". The aether is a dirty word in some eyes, but it's just "the vacuum", or space. And take a look at the abstract for Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime.
    I'm not sure what you mean by "bad news".

    But, of course, most (all?) of those are using the word "aether" as a metaphor (for space-time, vacuum, em fields, etc.) rather than the "luminiferous aether" that used to be thought necessary as the medium for light.

    Sadly, assorted cranks (a) don't seem to understand the concept of metaphor and (b) think that "Einstein said" is scientific evidence.
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    The major problem with an aether is that it would provide a preferred reference frame, which doesn't exist in relativity.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Quote Originally Posted by Kalster
    The major problem with an aether is that it would provide a preferred reference frame, which doesn't exist in relativity.
    Einstein referred to space as the aether of general relativity, that's good enough for me. And the CMBR reference frame is a fact of life.

    Quote Originally Posted by Strange View Post
    ...Sadly, assorted cranks (a) don't seem to understand the concept of metaphor and (b) think that "Einstein said" is scientific evidence.
    Don't you call me a crank. Einstein said what he said. Was he a crank too? There are serious physicists and cosmologists out there who are quite happy to use the word aether, go look at some of the papers. And next time lift your head out of your pop-science ignorance before you start sneering at something you don't understand.
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    Quote Originally Posted by Farsight View Post
    Don't you call me a crank. Einstein said what he said. Was he a crank too? There are serious physicists and cosmologists out there who are quite happy to use the word aether, go look at some of the papers. And next time lift your head out of your pop-science ignorance before you start sneering at something you don't understand.
    I didn't call you a crank. Of course people are happy to use the word aether. I don't have a problem with that. People also use the word quintessence; that doesn't mean they want to go back to the idea of everything being made of Earth, Air, Fire and water.

    Are you going to start defending the Olde and Ancient Mystery of the Luminiferous Aether and How it Was Not Detected by the Famous Experiments Conducted by Messrs Lorentz and Fitzgerald?

    And the CMBR reference frame is a fact of life.
    So is the Earth Centered Inertial frame of reference and an infinite number of others. None of them are preferred or absolute. You choose whichever is most appropriate for the problem.
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    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by Kalster
    The major problem with an aether is that it would provide a preferred reference frame, which doesn't exist in relativity.
    Einstein referred to space as the aether of general relativity, that's good enough for me. And the CMBR reference frame is a fact of life.
    Yes Einstein talked about an aether, but not in the usual sense, certainly NOT one that would provide a preferred reference frame. Relativity is not compatible with one that can.

    The CMBR is a reference frame like any other. It is just convenient because of it's scope.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    You can easily verify it by yourself.
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    Quote Originally Posted by Michel2012 View Post
    You can easily verify it by yourself.
    Verify what? How?
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    Quote Originally Posted by Strange View Post
    I didn't call you a crank. Of course people are happy to use the word aether. I don't have a problem with that. People also use the word quintessence; that doesn't mean they want to go back to the idea of everything being made of Earth, Air, Fire and water.
    Good. Please accept my apologies for jumping down your throat.

    Quote Originally Posted by Strange View Post
    Are you going to start defending the Olde and Ancient Mystery of the Luminiferous Aether and How it Was Not Detected by the Famous Experiments Conducted by Messrs Lorentz and Fitzgerald?
    No. I stick pretty close to Einstein when I talk about relativity and gravity.

    Quote Originally Posted by Strange View Post
    So is the Earth Centered Inertial frame of reference and an infinite number of others. None of them are preferred or absolute. You choose whichever is most appropriate for the problem.
    The difference is that everybody in the universe can gauge their motion through the universe by looking at the CMBR. It isn't an absolute reference frame in the sense that you can sit in a black box and tell that you're moving, but the universe is as absolute as it gets. It postdates relativity of course.

    Quote Originally Posted by Kalster
    Yes Einstein talked about an aether, but not in the usual sense, certainly NOT one that would provide a preferred reference frame. Relativity is not compatible with one that can.
    Agreed. You can't gauge your motion in space alone, "in your box" as it were. Motion is relative. Whilst "the idea of motion may not be applied to it", he did talk about an aether in the sense that space was not empty.

    Quote Originally Posted by Kalster
    The CMBR is a reference frame like any other. It is just convenient because of it's scope.
    OK. I view the very wide scope as something that makes it more interesting than other reference frames.
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    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by Kalster
    Yes Einstein talked about an aether, but not in the usual sense, certainly NOT one that would provide a preferred reference frame. Relativity is not compatible with one that can.
    Agreed. You can't gauge your motion in space alone, "in your box" as it were. Motion is relative. Whilst "the idea of motion may not be applied to it", he did talk about an aether in the sense that space was not empty..
    Not the idea I get. He did not attribute anything mechanical to it as was the case with Luminiferous aether theories, which he abandoned in 1905. It's physical nature was purely in the form of geometry as a description of gravity. Not a simple void, but one with curvature.

    Luminiferous aether - Wikipedia, the free encyclopedia
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    Quote Originally Posted by Farsight View Post
    Please accept my apologies for jumping down your throat.
    No problem.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by pmb View Post
    What is push gravity?
    You don't want to know. Really, you don't. It's just too embarrassing.
    We had a phase here on this forum not too long ago where everything was suddenly about push gravity. After a while it just stops being entertaining.
    Yeah. I'm begining to get the same feeling.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Ghrasp View Post
    the pB block influences surrounding so the surrounding acts a-symmetric.
    That would the curvature of spacetime caused by mass (or energy) as described by general relativity.
    I wouldn,t use the word cause referring to the block or it,s mass (allthough it is related). I tend to see it more as caused by changing the position of the block doing these experiments two times. One time with the block at far distance for reference, other time placed at close distance.


    Question is does the pb block act with a distinct force and thereby increase gravity local (with a different angle for a resultant). Or does it 髇ly angle the existing g for direkion without increasing g quantitywise (local). That can give an illusion of a seperate force coming in from the block because it refers back to the former situation and direction for g before the placement.


    Simultaneous it would need something as a seesaw balance hanging from a ceiling (as the balances common in Newton,s days but then refined).
    Two blocks A and B first perfectly balanced on a seesaw balance.
    Another block C (first at large distance) transported and placed beside the balance nearby A ;
    If it,s not a change quantitywise the balance would adapt anglewise to it,s own pivot.

    When the pb block acts with a distinct Fz as distanceforce this force wouldn,t change the balance (Fz direction runs through A, B and the pivotpoint for the see-saw part of the balance). In this case the whole balance would be pulled sideways but A and B still at same height (but then a little higher)
    And this is strange for Newton because this force would act horizontal on A and B..How can it increase the gravitational energy for A and B while working horizontal ?

    With this atom interferometer the trajektory is also just compared in one dimension. It tells nothing about the question if the block can be considered to act with a force to accellerate an atom more or less (the Pb block after being placed does some work then on the atoms) or does it just influence the trajektory in an angular way.
    If a trajektory curves the projektion to a straight dimensionline is shorter but if before the block was placed the trajektory was more curved then the pb block could also straighten it more compared to what it was. The projektion to the dimensionline would seem to be longer then after placement of the pb.
    Last edited by Ghrasp; June 20th, 2012 at 05:46 PM.
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    Quote Originally Posted by KALSTER View Post
    Not the idea I get. He did not attribute anything mechanical to it as was the case with Luminiferous aether theories, which he abandoned in 1905. It's physical nature was purely in the form of geometry as a description of gravity. Not a simple void, but one with curvature.
    I don't get the same impression. No problem with "the idea of motion cannot be applied to it", it doesn't blow like the wind. But the impression I get is one of inhomogeneous space with curvature being a by-product. This is the passage that caught my eye:

    "Mach抯 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 揺mpty 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. But therewith the conception of the ether has again acquired an intelligible content, although this content differs widely from that of the ether of the mechanical undulatory theory of light".

    If you took an equatorial slice through the middle of the Earth extending out into the surrounding space, then placed light clocks at various locations and plotted the readings, you end up with a plot that's like the "upturned hat" on the wikipedia gravitational potential page. The curvature you can see in the plot denotes tidal force or Riemann curvature, whilst the slope denotes the local strength of the "force" of gravity. Send a light beam across it and it curves because of the gradient, even when you restrict yourself to a small region where the Riemann curvature is undetectable. The latter is however the defining feature of a real gravitational field because without it your plot stays flat and level and never acquires any gradient. Einstein used the word special instead of real in Relativity : the Special and General Theory. Here it is, near the end of section 20:

    "This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the earth (in its entirety) vanishes".
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    Quote Originally Posted by Farsight View Post
    Einstein referred to space as the aether of general relativity, that's good enough for me. And the CMBR reference frame is a fact of life.
    Spacetime is the "ether" of general relativity. It takes on properties (curvature) based on its contents. That's what Einstein was referring to.

    The CMBR reference frame is very handy for cosmology, but only when using the assumptions of the cosmological principle - homogeneity and isotropy.

    Being at rest in relation to the CMBR reference frame has no other use. There is nothing else "preferred" about it.

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    Not spacetime, space. Space is the "ether" of general relativity. Read what Einstein said carefully: 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. But therewith the conception of the ether has again acquired an intelligible content, although this content differs widely from that of the ether of the mechanical undulatory theory of light". See this from Dr AG Polnarev's Mathematical Aspects of Cosmology course for the ten functions, which include energy density along with pressure and shear stress.

    I'm pretty sure Einstein was describing a gravitational field as inhomogeneous non-isotropic space. Hence Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime makes sense. Which means that those assumptions of the cosmological principle are saying there's no gravitational field present!
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    Quote Originally Posted by Farsight View Post
    Not spacetime, space. Space is the "ether" of general relativity. Read what Einstein said carefully: 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. But therewith the conception of the ether has again acquired an intelligible content, although this content differs widely from that of the ether of the mechanical undulatory theory of light". See this from Dr AG Polnarev's Mathematical Aspects of Cosmology course for the ten functions, which include energy density along with pressure and shear stress.
    Thank you for that pdf file on the EFE and the stress-energy tensor showing the ten functions that are used to describe curved space-time.

    Quote Originally Posted by A. Polnarev
    In order words, the EFEs describe in self-consistent way the distribution and motion of matter and fields in curved space-time, while the geometry of the space-time itself is determined trough the EFEs by the distribution and the motion of matter and fields.
    ...

    Quote Originally Posted by Farsight View Post
    I'm pretty sure Einstein was describing a gravitational field as inhomogeneous non-isotropic space.
    Yup, that's curved space-time. There is a gravity gradient so the space cannot be homogeneous.

    Quote Originally Posted by Farsight View Post
    Which means that those assumptions of the cosmological principle are saying there's no gravitational field present!
    Well, that's gradient, not field, but I know what you mean.

    Not only is Einstein's ether space-time, but more accurately it is the curvature of space-time. Consider the results when there is no curvature.
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    No it isn't. The guy said what he said. He said space, not curved spacetime.

    Re the result when there is no curvature, see what I said in post #93. Without the Riemann curvature you can't get off the flat and level and thus you have no gradient in gravitational potential and no gravitational field and light doesn't curve. But the Riemann curvature is only there because the inhomogeneity of space diminishes in line with the inverse square rule.

    By the by, if you're subject to constant acceleration through space you're emulating the gradient in gravitational potential, and you see light curving. But you aren't emulating the Riemann curvature.
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    Quote Originally Posted by Farsight View Post
    No it isn't. The guy said what he said. He said space, not curved spacetime.

    Re the result when there is no curvature, see what I said in post #93. Without the Riemann curvature you can't get off the flat and level and thus you have no gradient in gravitational potential and no gravitational field and light doesn't curve. But the Riemann curvature is only there because the inhomogeneity of space diminishes in line with the inverse square rule.

    By the by, if you're subject to constant acceleration through space you're emulating the gradient in gravitational potential, and you see light curving. But you aren't emulating the Riemann curvature.
    Not sure what you are even talking about.
    Besides, the inverse square rule only holds as an approximation for weak fields; the full EFE, being a non-linear system of equations, has no simple solution in the general case.
    You also seem to be forgetting that the EFE is a relation between rank-2, 4x4 tensors, and thus yields a 4x4 tensor as solution. Such a tensor describes an object in a 4-dimensional Riemannian manifold, so time is clearly part of it. The EFE is not limited to the three spatial dimensions only. It is irrelevant what anyone says, the maths are quite clear. A theory of gravity in only three dimension would look very different.
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    Quote Originally Posted by Farsight View Post
    No it isn't. The guy said what he said. He said space, not curved spacetime.
    Then you cannot be reading the same pdf article you posted. I quoted what the guy said. He said what he said. Here is what he said, once again:

    In order words, the EFEs describe in self-consistent way the distribution and motion of matter and fields in curved space-time, while the geometry of the space-time itself is determined trough the EFEs by the distribution and the motion of matter and fields.
    That is from the article you posted to back up your position. Unfortunately, it doesn't.

    It is obvious that the "ether" of General Relativity is space-time. This subject comes up quite often and there seem to be a lot of misconceptions about it. Please stop helping to spread those misconceptions.
    Last edited by SpeedFreek; June 23rd, 2012 at 08:54 PM.
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    Einstein said space, not Polnarev. So when it comes to misconceptions and what's obvious, there's something of a problem. Have a read of the quantum vacuum section of the wiki "aether theories" page and see this quote from Robert B Loughlin:

    "It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed..."

    This conversation started when Strange said aether and crackpot in the same post, and I referred to Einstein's Leyden Address and arXiv. There's one misconception in that people think aether is some crackpot old nonsense that Einstein did away with. Then there's another misconception that confuses spacetime with space.

    Quote Originally Posted by Markus Hanke
    Not sure what you are even talking about. Besides, the inverse square rule only holds as an approximation for weak fields; the full EFE, being a non-linear system of equations, has no simple solution in the general case. You also seem to be forgetting that the EFE is a relation between rank-2, 4x4 tensors, and thus yields a 4x4 tensor as solution. Such a tensor describes an object in a 4-dimensional Riemannian manifold, so time is clearly part of it. The EFE is not limited to the three spatial dimensions only. It is irrelevant what anyone says, the maths are quite clear. A theory of gravity in only three dimension would look very different.
    I'm talking about the distinction between space and spacetime. Einstein gave the equations of motion and referred to inhomogeneous space, but nowadays it just doesn't feature, and people suggest light curves because it moves through curved spacetime. And yet there is no motion through spacetime because it's a static "all times" depiction. For a simple illustration of what I mean by that, I throw a red ball across the room and you film it. Then you develop the film and cut it into individual frames and form them into a stack, a block as it were. There's a red streak through the block of film which equates to the worldline of the ball. But the ball isn't moving through the block. There's some misconceptions here, and I think it's worth teasing them out.
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