Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Robro on 01/09/2011 09:04:06
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What is the mechanism that allows gravity to latch onto a photon?
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What is the mechanism that allows gravity to latch onto a photon?
I don't believe it does. A photon travels in a straight line but follows a geodesic in curved space-time.
Gravity does not directly affect the course of a photon, neither does it cause a photon to gain nor loose energy.
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Rob - here is a nice simple explanation
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961102.html
As Mike says the photon follows a geodesic (which is the shortest distance between two points) this is not a straight line when spacetime itself is curved. Mass and energy distort the actual geometry of spacetime - it is this distortion that einstein proposed and formalised in GR to explain gravity.
Mike - a photon in deep space that is not energetic enough to interact with a certain electron may by the time it is at the earth's surface be energetic enough to interact. I know the mechanism is more complicated than the simple heuristic that many believe - but be that as it may, it is a fact that by any method of measurement the energy will have increased due to a wavelength blue shift. Please keep your more speculative ideas and explanation to New Theories and do not include them as a first answer to questions on the main Physics board
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What is the mechanism that allows gravity to latch onto a photon?
If the question is: "how can a gravitational force affect the motion of a massless body", you can see it this way: let an object fall on the ground (in the void). Does its accelerated motion depend on its mass?
What happens with an object which mass tends to zero? Does its acceleration change?
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Photons could be considered to have mass based on the principle of mass-energy equivalence.
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Photons could be considered to have mass based on the principle of mass-energy equivalence.
Photons are massless. If you think they have mass, explain how an object with non zero mass m can have finite energy moving at c, since its momentum p is:
p = m/sqrt[1 - (v/c)2]
Put v = c and tell me how much is p.
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They have no rest mass, but since they are never at rest...
This is a tricky one, because I've read in more than one place that objects with finite energy density should generate gravitional fields. I've seen suggestions that concentrating enough high-energy light inside of its own Schwarzchild radius should allow for the creation of a black hole. Light can also transfer momentum to other objects (solar sail technology).
Perhaps what I should have said is that light behaves as if it has mass.
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They have no rest mass, but since they are never at rest...
The term "rest mass" is not a good one, exactly for that reason.
The correct term is "invariant mass" which now is simply called "mass". "Invariant" means that it doesn't change with the frame of reference. A photon has zero invariant mass.
This is a tricky one, because I've read in more than one place that objects with finite energy density should generate gravitional fields. I've seen suggestions that concentrating enough high-energy light inside of its own Schwarzchild radius should allow for the creation of a black hole.
Any physical system which is *stationary* in a given frame of reference, has a mass which depends on its energy content (m = E/c2), so if the system is a fixed region of space in the void which is crossed by a light beam, then in that moment it acquires mass, even if the light beam hasn't.
This has been discussed several times in this forum.
Light can also transfer momentum to other objects (solar sail technology).
And this has nothing to do with light's mass. Even this has been discussed several times in this forum (this even recently).
Perhaps what I should have said is that light behaves as if it has mass.
Ok, but if you don't specify in which exact conditions (see up) it behaves "as if", it's not useful, on the contrary, it makes confusion.
Light has zero mass.
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Photons are massless. If you think they have mass, explain how an object with non zero mass m can have finite energy moving at c, since its momentum p is:
p = m/sqrt[1 - (v/c)2]
Put v = c and tell me how much is p.
That formula is derived with the assumption that v < c. The momentum for a photono is P = mc where the m is the inertial.
Here is a nice quote from Feynman
From the Feynman Lectures Vol -I page 7-11
Section entitled Gravitation and Relativity
One feature of this new law is quite easy to understand is this: In Einstein relativity theory, anything which has energy has mass -- mass in the sense that it is attracted gravitationaly. Even light, which has energy, has a "mass". When a light beam, which has energy in it, comes past the sun there is attraction on it by the sun.
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We have already discussed that and we didn't agree. About what Feynman writes, I have explained the concept in my previous post. You are one of the few left who still talk of relativistic mass.
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This is a tricky one, because I've read in more than one place that objects with finite energy density should generate gravitional fields. I've seen suggestions that concentrating enough high-energy light inside of its own Schwarzchild radius should allow for the creation of a black hole.
As I see it, light having no mass, can not generate (or directly interact with) a gravitational field. Therefore it does not have a Schwarzchild radius so could not create a black hole. Light can only 'appear' to interact with a black hole because the black hole curves space-time geodesics. However, it could be argued that the Schwarzchild radius of light is the same as that of the universe but in that case 'light' is not the sole defining character.
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We have already discussed that and we didn't agree. About what Feynman writes, I have explained the concept in my previous post. You are one of the few left who still talk of relativistic mass.
I posted what I did because the source of the confusion is the use of the term mass without actually nailing down its definition. Someone above thought that the photon doesn't interact with a gravitational field because "light has no mass" meaning that the proper mass of a photon is zero. Light is affected by a g-field because light has passive gravitational mass.
As far as who uses what - I won't be changing my ressponses to fit with what "most" people do. Only what I think is the best way to explain something. E.g. in particle physics most of them use mass to mean propr mass. However that is not the same in GR where a lot of people use mass to mean gravitational mass, just like the first post since it was about the interaction of photons and gravity.
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MikeS - Light can generate a gravitational field.
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MikeS - Light can generate a gravitational field.
We are discussing about photons. Can a photon generate a gravitational field?
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MikeS - Light can generate a gravitational field.
We are discussing about photons. Can a photon generate a gravitational field?
yes.
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MikeS - Light can generate a gravitational field.
We are discussing about photons. Can a photon generate a gravitational field?
yes.
Anything with energy/momentum should be able to do so, and a photon has both.
By the way, a lot of the mass in matter is caused by the binding energy in the nucleus, and to a far lesser extent by electromagnetic binding energies between atoms. All this binding energy can be described in terms of gluons/photons, which are massless.
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MikeS - Light can generate a gravitational field.
We are discussing about photons. Can a photon generate a gravitational field?
yes.
Anything with energy/momentum should be able to do so, and a photon has both.
By the way, a lot of the mass in matter is caused by the binding energy in the nucleus, and to a far lesser extent by electromagnetic binding energies between atoms. All this binding energy can be described in terms of gluons/photons, which are massless.
I think it is debatable whether a photon has momentum.
Is this a mainstream answer, as I understand the situation, where mass comes from is still open to question.
Why would anything with energy be able to generate a gravitational field?
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MikeS - Light can generate a gravitational field.
We are discussing about photons. Can a photon generate a gravitational field?
yes.
Anything with energy/momentum should be able to do so, and a photon has both.
By the way, a lot of the mass in matter is caused by the binding energy in the nucleus, and to a far lesser extent by electromagnetic binding energies between atoms. All this binding energy can be described in terms of gluons/photons, which are massless.
I think it is debatable whether a photon has momentum.
Is this a mainstream answer, as I understand the situation, where mass comes from is still open to question.
Why would anything with energy be able to generate a gravitational field?
It is not really debatable that photons have momentum - can be quantified as either p=E/c or p=hbark (where k is the wave vector).
The mass of fundamental particles is still up for debate - see the lhc's search for the higg's boson - but we know that much of the mass of composites from small (protons) to large (macro molecules) comes from the binding energy.
energy is equivalent to mass - we calculate the spacetime curvature with the stress energy tensor within einsteins field equations. you might just as well ask why mass is able ... physics models and explains at lower and more basic levels - but the eternal verities are left to philosophers and drunks
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MikeS - Light can generate a gravitational field.
We are discussing about photons. Can a photon generate a gravitational field?
yes.
energy is equivalent to mass - we calculate the spacetime curvature with the stress energy tensor within einsteins field equations. you might just as well ask why mass is able ... physics models and explains at lower and more basic levels - but the eternal verities are left to philosophers and drunks
The fact that energy is equivalent to mass is irelevant. It does not mean they can be thought of as being the same. In most ways they are the complete opposites of each other. Mass is heavy, photons are light.
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Someone above thought that the photon doesn't interact with a gravitational field because "light has no mass" meaning that the proper mass of a photon is zero. Light is affected by a g-field because light has passive gravitational mass.
So which is the correct interpretation and why?
A photon can interact with a gravitational field because it has 'passive' mass. Can you please explain the term 'passive' as it implies it has no effect?
A photon can not interact with the gravitational field. It only appears to because it follows the geodesics of curved space time as caused by gravity.
One is obviously wrong.
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A photon can interact with a gravitational field because it has 'passive' mass. Can you please explain the term 'passive' as it implies it has no effect?
Active gravitational mass refers to the mass that generates a gravitational field. Passive gravitational mass is the mass that gravity acts on.
A photon can not interact with the gravitational field.
Sure it does. What would make you believe otherwise? There is an example of a pulse of light generating a gravitational field. Why would you think that light can generate a grvitational field but photons wouldn't?
It only appears to because it follows the geodesics of curved space time as caused by gravity.
That is incorrect. Consider a uniform gravitational field. The field will deflect matter, including light. However a uniform gravitational field has zero spacetime curvature, yet there can be a non-gravitational field present with a suitable change in spacetime coordinates from an inertial frame.
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We are discussing about photons. Can a photon generate a gravitational field?
yes.
And which is the quantum theory of gravity which says this?
Hint: there still isn't any accepted quantum theory of gravity...
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I think it is debatable whether a photon has momentum.
No, it's not debatable, it's ordinary physics.
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And which is the quantum theory of gravity which says this?
Hint: there still isn't any accepted quantum theory of gravity...
First off all qu7antum theories of gravity must reduce to GR in any acceptable theory. What I've done above is to use the approximation that a photon is a point particle with zero proper mass. Clifford Will uses this same approximation in his book Was Einstein RightI recommend that you take the following advice from Exploring Black Holes, by Taylor and Wheeler if you read the acknowledgement you'll see this
Phillip Morrison made several suggestions and convinced us not to invoke that weird quantum particle, the photon, in a treatment of classical theory of relativity (except in some exercises).
You're asking me about a quantum notion in a classial theory. I do know that people I've talked to about this, and friends/acquantances who are experts in their field, think of photons just like I do. It can be show, with classical GR, that a g-field which has been created by a source consisting of beam of EM radiation (a "pencil of light" as they call it) deflects particles. A pulse of light can be thought of as a collection of photons and the result is an interaction of the photon and g-field. How could you accept that a beam of light is deflected but a single photon can't? This is all explained in such text books such as Tolman's book.
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Consider a uniform gravitational field. The field will deflect matter, including light. However a uniform gravitational field has zero spacetime curvature, yet there can be a non-gravitational field present with a suitable change in spacetime coordinates from an inertial frame.
Hmm... In Newtonian gravity, a uniform field won't deflect matter, since it has zero gradient. From what I know of GR, the influence of gravity can be computed from gradients of the field as well. A uniform field in GR should still have zero gradient and no influence, shouldn't it? Or am I missing something?
By the way, I agree with you that a photon should be a source of gravity. In addition to arguing that light/EM fields are energy and a source of gravity, you can also put light in a box, as I mentioned above. The box has an effective mass and should be a source of gravity. In all these cases light generates gravity because it has energy. Since light's energy comes in packets called photons...
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Hmm... In Newtonian gravity, a uniform field won't deflect matter, since it has zero gradient.
A uniform field has a non-zero gradient in Newtonian gravity. If the gravitational potential has phi = gz then F = -m*grad phi = -mg.
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Ah, got it. I was thinking gravitational potential.
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A photon can interact with a gravitational field because it has 'passive' mass. Can you please explain the term 'passive' as it implies it has no effect?
Active gravitational mass refers to the mass that generates a gravitational field. Passive gravitational mass is the mass that gravity acts on.
A photon can not interact with the gravitational field.
Sure it does. What would make you believe otherwise? There is an example of a pulse of light generating a gravitational field. Why would you think that light can generate a grvitational field but photons wouldn't?
It only appears to because it follows the geodesics of curved space time as caused by gravity.
That is incorrect. Consider a uniform gravitational field. The field will deflect matter, including light. However a uniform gravitational field has zero spacetime curvature, yet there can be a non-gravitational field present with a suitable change in spacetime coordinates from an inertial frame.
This is a circular argument.
If you are referring to to the experiment you mention in a later post where you say that a pencil of light gravitationally deflects particles. The only reference I have found to such an experiment is one dated 1931. I don't wish to knock the experiment but I would not have thought they had the technology back then of carrying out such an experiment and obtaining unambiguous results. I don't think that light can generate a gravitational field. If you have references to any modern papers I would appreciate links to them.
I thought this was part of General Relativity and generally accepted.
There is no such thing as a uniform gravitational field from any reference frame other than the free falling reference frame. If it were uniform there would be no gravity. I agree it would have zero spacetime curvature. The free falling reference frame only appears to have zero curvature (acceleration) exactly because it is free falling. From any other reference frame light is following a geodesic in curved space-time.
Gravity causes space-time to be non linear (curved). From the viewpoint of an observer free falling in curved space-time an object falling with them will appear to be stationary, likewise a beam of light will have a constant frequency.
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First off all quantum theories of gravity must reduce to GR in any acceptable theory. What I've done above is to use the approximation that a photon is a point particle with zero proper mass.
So you also have found a model for the photon! Fantastic!
You didn't say that all this is your New Theory, however...
You're asking me about a quantum notion in a classial theory. I do know that people I've talked to about this, and friends/acquantances who are experts in their field, think of photons just like I do.
In some cases you can treat a photon in that way, but not in all.
It can be show, with classical GR, that a g-field which has been created by a source consisting of beam of EM radiation (a "pencil of light" as they call it) deflects particles.
Yes, and there isn't even need of talking of photons, since in that case you have a region of space with non zero energy density and so Einstein's equation tells us that there must be curvature, but unfortunately this has nothing to do with a single photon creating curvature.
A pulse of light can be thought of as a collection of photons and the result is an interaction of the photon and g-field. How could you accept that a beam of light is deflected but a single photon can't?
Because the total effect is not the simple sum of the single effects. For example, even if a single photon has zero mass, a system of 2 photons can have non zero mass.
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"What is the mechanism that allows gravity to latch onto a photon?"
Think of a photon as being 'something', whatever that is. It is defined by it's momentum which is a effect from its 'speed' and 'energy containment'. If it is 'timeless' which anything existing at that speed, consisting of matter, would be to us then nothing inside this SpaceTime would exist for it. This is assuming Einsteins definition of its speed as being 'c'. But it is a photon, nothing really 'material' so maybe that definition has nothing to do with it.
But any which way, we have yet to observe 'aging' photons from our frames of reference. And if it, as we observe it, won't age then it can't expend 'energy'. That means that, assuming a propagation, the photon is to us 'frozen in time', its 'metabolism' the same from source to sink.
Gravity is weird, we can play with it as thinking of it as a 'density', or 'gradients'. Gradients is probably a better description from the photons 'point of view'. What gravity does is that it lends you 'energy' or take it away, depending on your motion relative that gravity's potential. So when a photon moves against the gravity potential it redshifts, to the observer losing energy, or if moving with the potential blue shifts, gaining energy. But those effects are observer dependent, defined by your relation relative the 'photon' we assume you to observe. You might be able to see it the same way as in classical physics? Two cars colliding, or a car colliding with a wall, will give you a different energy, here letting the 'gravity' at each point of that photons propagation represent some 'energy' relative you. Not a good example but I'll let it stay for now :)
The point is that this timeless photon can't lose 'energy', but if it were to work against gravity's potential it would have to, intrinsically. And the only way it can avoid losing that energy is to move in geodesics. Gravity could be seen as when you blend colors, you see this mix of swirling colors as you blend, that's a little like gravity's potential on a 2-d plane as it in a way is a mix defined by the matter/mass, 'energy', and 'motion' locally. But you could also see gravity as something static, existing throughout SpaceTime, as some logical 'number space' defining its 'strength', using 'times arrow' to constantly adjust the numbers describing each point. Or as 'holes' and 'dips', whatever gravity is it gives only one 'direction' from where you won't expend energy, and the photon always follow that 'geodesic straight line'.
It's a weird subject.
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There is one more thing. If we were to assume that 'gravity' could be represented by a collection of numbers, defining each point in SpaceTime, times arrow constantly redefining them. Would they then be observer dependent? I think they would.
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There is no such thing as a uniform gravitational field from any reference frame other than the free falling reference frame.
Sure there is. Here's a Newtonian example whose size is finite in extent - http://home.comcast.net/~peter.m.brown/gr/grav_cavity.htm
If it were uniform there would be no gravity.
That depends on how you look at it. Einstein viewed a gravitational field which resides as being defined by the metric tensor and Christoffel symbols.
Here are some references
[1] Principle of Equivalence, F. Rohrlich, Ann. Phys. 22, 169-191, (1963), page 173/
[2] Radiation from a Uniformly Accelerated Charge, David G. Boulware, Ann. Phys., 124, (1980), page174.
[3] Relativistic solutions to the falling body in a uniform gravitational field, Carl G. Adler, Robert W. Brehme, Am. J. Phys. 59 (3), March 1991.
[4] Gravitation, Charles. W. Misner, Kip S. Thorne, John Archibald Wheeler, (1973), sect 6.6.
[5] The uniformly accelerated reference frame, J. Dwayne Hamilton, Am. J. Phys., 46(1), Jan. 1978.
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So you also have found a model for the photon! Fantastic!
You didn't say that all this is your New Theory, however...
I don't know what you mean. I havent' found anything "new". Here's an interesting article - "The mass of a gas of massless photons," H. Kolbenstvedt, Am. J. Phys. 63 (1), January 1995
Yes, and there isn't even need of talking of photons, since in that case you have a region of space with non zero energy density and so Einstein's equation tells us that there must be curvature, but unfortunately this has nothing to do with a single photon creating curvature.
What makes you believe that?
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From the viewpoint of an observer free falling in curved space-time an object falling with them will appear to be stationary, likewise a beam of light will have a constant frequency.[/color]
Relativity states that that cannot happen. Also, measurements from different frames will yield measure different values
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There is no such thing as a uniform gravitational field from any reference frame other than the free falling reference frame.
Sure there is. Here's a Newtonian example whose size is finite in extent - http://home.comcast.net/~peter.m.brown/gr/grav_cavity.htm
Ok, point taken but it could be argued this is still a free falling reference frame, it just appears to be static but it is still moving (falling) through space-time regardless of apparent motion. Just the same as any gravitating body may appear to be stationary but is still moving through space-time.
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From the viewpoint of an observer free falling in curved space-time an object falling with them will appear to be stationary,[/color] likewise a beam of light will have a constant frequency.[/color]
Relativity states that that cannot happen. Also, measurements from different frames will yield measure different values
"From the viewpoint of an observer" meaning from 'their' reference frame. I agree that different reference frames will yield different results.
Why can't that happen? Can you explain please.
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I don't know what you mean. I havent' found anything "new". Here's an interesting article - "The mass of a gas of massless photons," H. Kolbenstvedt, Am. J. Phys. 63 (1), January 1995
I haven't read the article. What does it says, essentially?
Yes, and there isn't even need of talking of photons, since in that case you have a region of space with non zero energy density and so Einstein's equation tells us that there must be curvature, but unfortunately this has nothing to do with a single photon creating curvature.
What makes you believe that?
Curvature cannot be frame-dependent. A photon's energy instead is frame-dependent.
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[quotre]Curvature cannot be frame-dependent. A photon's energy instead s frame-dependent.
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It sure can, at least that is the way that Einstein always used it. You're using a different definition of gravity than Einstein did. Einstein defined the gravitational force by the Christoffel symboles and the metric tensor. You and many others associate the presence of a gravitational field with a non=vanishing Reimann tensor.
It follows from Einstein's definition of "gravitational field." The curvature is a tensor so althought its components change with the frame of reference so do observer's observations. E.g. consider a photon. In one frame it has a certain energy but in another frame moving relative to the first. This is why Einstein said, in his 1916 review paper on GR, that if you change frames of reference you can go from a frame with no g-field to one that does have a g-field.
Forget that paper. I was thinking of something else, i.e. a paper by Rindler. I'll get that reference later.
Pete
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This is why Einstein said, in his 1916 review paper on GR, that if you change frames of reference you can go from a frame with no g-field to one that does have a g-field.
Do you have a link (not in german, possibly [:)])?
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By Tilman Sauer; "The first comprehensive overview of the final version of the general theory of relativity was published by Einstein in 1916 after several expositions of preliminary versions and latest revisions of the theory in November 1915. A historical account of this review paper is given, of its prehistory, including a discussion of Einstein's collaboration with Marcel Grossmann, and of its immediate reception." (http://arxiv.org/pdf/physics/0405066v1)
And S. N. Bose: The Principle of Relativity.
Author: Einstein, Albert, 1879-1955; Minkowski, Hermann, 1864-1909 Subject: Relativity (Physics) Publisher: Calcutta University of Calcutta (http://www.archive.org/details/theprincipleofre00einsuoft)
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Do you have a link (not in german, possibly [:)])?
I found on on the internet. See http://www.alberteinstein.info/gallery/pdf/CP6Doc30_English_pp146-200.pdf
Read page 156, especially where he says We thus find the ocurance of a gravitational field connected with the space-time variability of the g_s.
Recall the article I referenced above, http://xxx.lanl.gov/ftp/physics/papers/0204/0204044.pdf. It has all the relevant statements made by Einstein on this topic, at least all I could find.
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By Tilman Sauer; "The first comprehensive overview of the final version of the general theory of relativity was published by Einstein in 1916 after several expositions of preliminary versions and latest revisions of the theory in November 1915. A historical account of this review paper is given, of its prehistory, including a discussion of Einstein's collaboration with Marcel Grossmann, and of its immediate reception." (http://arxiv.org/pdf/physics/0405066v1)
And S. N. Bose: The Principle of Relativity.
Author: Einstein, Albert, 1879-1955; Minkowski, Hermann, 1864-1909 Subject: Relativity (Physics) Publisher: Calcutta University of Calcutta (http://www.archive.org/details/theprincipleofre00einsuoft)
Thank you, yor_on.
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Do you have a link (not in german, possibly [:)])?
I found on on the internet. See http://www.alberteinstein.info/gallery/pdf/CP6Doc30_English_pp146-200.pdf
Read page 156, especially where he says We thus find the ocurance of a gravitational field connected with the space-time variability of the g_s.
Recall the article I referenced above, http://xxx.lanl.gov/ftp/physics/papers/0204/0204044.pdf. It has all the relevant statements made by Einstein on this topic, at least all I could find.
Thank you Pmb.