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Thread: A universal freeze frame

  1. #61
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    Quote Originally Posted by Bogie View Post
    Thank you for that interesting and well informed recap. Why do you know all that?
    Welcome. Why do I know it? I read a book that led me to another book then to another, you know how it goes. I have a fondness for the QG crowd as I almost always barrack for the underdogs.

    Quote Originally Posted by Bogie View Post
    Did you by any chance think about the question in my last post (#58) about the preferred rest frame?
    The "co-moving" frame of the Big Bang, is, as you say, defined as having the microwave background the same temperature in all directions. It is a convenient frame for doing cosmology calculations within and has been called a "de facto rest frame" but that is all it is. The thing about reference frames is that you cannot extend them globally when gravity is around. The co-moving BB frame is really a collection of frames, one for each point in the universe, which are stitched together to form a global chart (I think that is the proper word). Each of those frames are not preferred rest frames.

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    Quote Originally Posted by loglo View Post
    Welcome. Why do I know it? I read a book that led me to another book then to another, you know how it goes. I have a fondness for the QG crowd as I almost always barrack for the underdogs.
    I do know how that goes.
    The "co-moving" frame of the Big Bang, is, as you say, defined as having the microwave background the same temperature in all directions. It is a convenient frame for doing cosmology calculations within and has been called a "de facto rest frame" but that is all it is. The thing about reference frames is that you cannot extend them globally when gravity is around. The co-moving BB frame is really a collection of frames, one for each point in the universe, which are stitched together to form a global chart (I think that is the proper word). Each of those frames are not preferred rest frames.
    I see what you mean if you mean that since every frame in the "global chart" has nothing special in regard to the microwave background, each being indistinguishable from any other.

    To follow up on that thought, since co-variance was a big thing in developing GR and I think that means that the equations apply to any inertial frames, isn't it theoretically possible to determine for each inertial frame, even in a gravitational field of curved space, its relative motion to that Big Bang "co-moving" frame?

  3. #63
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    Quote Originally Posted by loglo View Post
    a global chart (I think that is the proper word)
    Slightly more correct would be an atlas.

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    Of course caveman1917 was right in post #54 too .

    I found the following link, which is no big deal because it has always been just a "Google" away, but reading it this time and with some added perspective from responses here and from recent reading of more layman focused material on SR and GR, including Isaacson's, Einstein: His Life and Universe, it answers many of the questions I have posed on this thread. Take a look if you feel so inclined and point out any misconceptions I have drawn from it in the paragraphs below, or tell me if you think it is pretty good. http://en.wikipedia.org/wiki/Speed_of_gravity

    Simplified to the best of my layman ability, SR says that an event that is observed from two inertial reference frames in linear motion relative to each other can be precisely transformed from one frame to the other using Lorentz transformations which are based on equations that divide all measures by a common denominator, the speed of light (light as the common denominator is based on the axiom or premise that the speed of light is invariant, i.e. it travels at c in all frames). The results of transformations allow for the derivation of the concepts of length contraction and time dilation which become meaningful when the relative velocity between frames is "relativistic", i.e. close to the speed of light.

    SR brought with it the concept that the aether was superfluous, meaning that physics involving light, electric fields, or magnetic fields does not need to be complicated by the concept of an aether. Light self propagates at the speed of light via transverse oscillating electric and magnetic fields through an aether-less vacuum, and there is no need for an absolute frame of reference to describe the electromotive force or induction.

    The Wiki link says, "However, in the case of two gravitoelectrically interacting particle ensembles, such as two planets or stars moving at constant velocity with respect to each other, each body feels a force which is directed at the instantaneous position of the other body, without a speed-of-light delay". Therefore in GR, the speed of gravity is instantaneous just as caveman1917 said in post #54, the speed of gravity waves and/or gravitons is at the speed of light, and the speed of light is invariant at c.

    The link goes on to say that, "The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c. Within the theory of special relativity, the constant c is not exclusively about light; instead it is the highest possible speed for any physical interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space.[2] This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of "light" is also the speed of gravitational waves and any mass-less particle. Such particles include the gluon (carrier of the strong force), the photons that light waves consist of, and the theoretical gravitons which make up the associated field particles of gravity (a theory of the graviton requires a theory of quantum gravity, however)".

  5. #65
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    Now I have a question that comes to mind from that last post and the Wiki quotes I used:
    Quote Originally Posted by Wiki
    … in the case of two gravitoelectrically interacting particle ensembles, such as two planets or stars moving at constant velocity with respect to each other, each body feels a force which is directed at the instantaneous position of the other body, without a speed-of-light delay.
    Quote Originally Posted by Wiki
    The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c. Within the theory of special relativity, the constant c is not exclusively about light; instead it is the highest possible speed for any physical interaction in nature.
    It is safe to assume that two distant objects, let’s say one light year apart, will have been there and would have been influenced by each other for much longer than one light year, giving the gravitational waves enough time to cover the distance separating them with the time delay. The question concerns whether that gravitational influence being felt and being reacted to at the instant of the freeze frame was the instantaneous effect or gravitational wave effect that travels at the speed of light. How do the objects know the difference so that they can react to the "instantaneous position of the other body" instead of to the slower gravitational waves?

    Since the objects at the freeze frame instant are not in the same positions that they were back when the gravitational waves were emitted by each back one year ago, if one of the objects was knocked off course would the other object respond instantaneously or on a time delay of one year?

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    Quote Originally Posted by Bogie View Post
    Now I have a question that comes to mind from that last post and the Wiki quotes I used:It is safe to assume that two distant objects, let’s say one light year apart, will have been there and would have been influenced by each other for much longer than one light year, giving the gravitational waves enough time to cover the distance separating them with the time delay. The question concerns whether that gravitational influence being felt and being reacted to at the instant of the freeze frame was the instantaneous effect or gravitational wave effect that travels at the speed of light. How do the objects know the difference so that they can react to the "instantaneous position of the other body" instead of to the slower gravitational waves?
    Gravitational waves and gravitational influence are different beasts.

    Gravitational influence is otherwise known as the curvature of space-time, and can be described as an effect of geometry - the mass of the Sun curves the space-time around it, and the Earth follows that curvature. This curvature does not need to "propagate" as such. The Earth orbits the place the Sun is "now" (approximately), not where it was 8 minutes ago.

    Mass tells space how to curve, and space tells mass how to move.

    General Relativity tells us that in the case of weak fields (like our Solar System), where any changes in accelerations between bodies are very small, the curvature of space-time "telegraphs" the instantaneous positions of those bodies to each other (so the Earth already knows where the Sun was 8 minutes ago, and the curvature points the Earth towards where the Sun will be "now", if the Sun hasn't suddenly accelerated in the meantime). So, in the absence of any sudden changes in acceleration (known as "jerk"), the curvature of space-time neatly extrapolates (approximately) the instantaneous position of the bodies in question to each other.

    But, if there is any change in the acceleration of a body ("jerk"), that extrapolation then "misses", and the result is the propagation of gravitational waves across the curvature, at c.

    It might sound strange that gravitational influence works as if it is instantaneous, but if the information is already contained in the system as to what all the bodies involved are doing, and none of them changes what they are doing, it kind of makes sense. Only when something changes what it is doing are gravitational waves produced, as a result of the "miss" in the extrapolation.

    Sorry if I haven't explained this very well.

  7. #67
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    Quote Originally Posted by speedfreek View Post
    Gravitational waves and gravitational influence are different beasts.

    Gravitational influence is otherwise known as the curvature of space-time, and can be described as an effect of geometry - the mass of the Sun curves the space-time around it, and the Earth follows that curvature. This curvature does not need to "propagate" as such. The Earth orbits the place the Sun is "now" (approximately), not where it was 8 minutes ago.

    Mass tells space how to curve, and space tells mass how to move.

    General Relativity tells us that in the case of weak fields (like our Solar System), where any changes in accelerations between bodies are very small, the curvature of space-time "telegraphs" the instantaneous positions of those bodies to each other (so the Earth already knows where the Sun was 8 minutes ago, and the curvature points the Earth towards where the Sun will be "now", if the Sun hasn't suddenly accelerated in the meantime). So, in the absence of any sudden changes in acceleration (known as "jerk"), the curvature of space-time neatly extrapolates (approximately) the instantaneous position of the bodies in question to each other.

    But, if there is any change in the acceleration of a body ("jerk"), that extrapolation then "misses", and the result is the propagation of gravitational waves across the curvature, at c.

    It might sound strange that gravitational influence works as if it is instantaneous, but if the information is already contained in the system as to what all the bodies involved are doing, and none of them changes what they are doing, it kind of makes sense. Only when something changes what it is doing are gravitational waves produced, as a result of the "miss" in the extrapolation.

    Sorry if I haven't explained this very well.
    You have explained it at least so I think I understand.

    One of the freeze frame questions to which I did not fully grasp the mainstream answer was, do objects follow curved paths because of the time delay of gravity, or do they react instantaneously to the real position of the other body and follow the geodesic of curved spacetime in response?

    I think you are providing the answer. Objects (theoretically) emit gravitational waves, and it can be said that objects also curve spacetime based on their matter/energy content. I am assuming the combination of those two statements is mainstream thinking. On the basis of those two components of gravity, the instantaneous curvature and the time delayed waves, the path that two objects take relative to each other is 1) determined by the instantaneous curvature that establishes the geodesics and 2) the gravity waves that enable the objects to follow along those geodesics.

    If so, then we could think of gravitational waves as the initiation of the path determined earlier by the geodesics of curved spacetime instead of a backup system that only comes into play when there is acceleration (“jerk”) of one of the bodies.

    That brings up the question of the relative acceleration of one of the objects. Is it correct to think that when a jerk occurs, the new curvature is instantaneously felt by the other object, and there is a time delay before the other object follows the new geodesic imposed by instantaneous curvature?

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    Quote Originally Posted by Bogie View Post
    That brings up the question of the relative acceleration of one of the objects. Is it correct to think that when a jerk occurs, the new curvature is instantaneously felt by the other object, and there is a time delay before the other object follows the new geodesic imposed by instantaneous curvature?
    The way I understand it is that when a jerk occurs, the effects of that jerk propagate at c and thus there is a "miss" in the extrapolation of the instantaneous position - in effect the gravity "misses" and is radiated as gravitational waves. Only when a body is in uniform motion (or uniform acceleration) does the curvature point towards its instantaneous position.

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    Quote Originally Posted by speedfreek View Post
    The way I understand it is that when a jerk occurs, the effects of that jerk propagate at c and thus there is a "miss" in the extrapolation of the instantaneous position - in effect the gravity "misses" and is radiated as gravitational waves. Only when a body is in uniform motion (or uniform acceleration) does the curvature point towards its instantaneous position.
    That seems right. And while I am thinking about what you understand about the “miss” being the stimulus for the occurrence of the gravitation waves being utilized by each object for course corrections, obviously both objects feel the miss. I think it would be reasonable to expect that both objects would respond with correcting gravitational waves.

    I don’t think you are saying that there would be any difference in the emission mechanics between the gravitational waves emitted while the objects were in constant un-accelerated (uniform) relative motion and the waves emitted when there is a miss due to acceleration. In my words, are saying that as you understand it, when there is a “miss”, that event could be reduced to quantum effects that determine the effect of the corrective waves if we knew the quantum mechanics of it all?

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    I think had better just shut up and let someone like caveman1917 explain it more thoroughly, lest I just confuse things further.

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    Quote Originally Posted by speedfreek View Post
    I think had better just shut up and let someone like caveman1917 explain it more thoroughly, lest I just confuse things further.
    OK, but I see no indication that you haven't been entirely clear and on target with your responses so far, and I have come along in my understanding as a result, so thanks.

    Feel free to put your views in anytime because even if they are not the "latest and greatest" possible responses, they are greatly appreciated considering my remarkable level of misconception, lol.

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    Quote Originally Posted by speedfreek View Post
    I think had better just shut up and let someone like caveman1917 explain it more thoroughly, lest I just confuse things further.
    No you're doing great
    Though perhaps, from a pedagogical perspective, it would be better to start with the related but easier phenomenom in electromagnetism? The force on a charge being directed towards the instantaneous position of a uniformly moving charge, and where an accelerating charge radiates.

    Bogie, something that may have added to the confusion might be your thinking with gravitons. While it's true that gravitons should move at c, gravity isn't supposed to be mediated by gravitons but by virtual gravitons, where entirely different rules apply. This quickly gets very complicated when you need a better explanation than the almost non-explanation "virtual particles don't have to obey the speed of c limit". In any case, gravitons aren't part of our current mainstream theories, they are expected to play a role in quantum gravity, but we're not there yet. I think it would be better to just forget about them at this stage.

  13. #73
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    Quote Originally Posted by caveman1917 View Post
    No you're doing great
    Though perhaps, from a pedagogical perspective, it would be better to start with the related but easier phenomenom in electromagnetism? The force on a charge being directed towards the instantaneous position of a uniformly moving charge, and where an accelerating charge radiates.
    Perhaps so, if you say so, but I not sure I'm ready for that, lol.
    Bogie, something that may have added to the confusion might be your thinking with gravitons. While it's true that gravitons should move at c, gravity isn't supposed to be mediated by gravitons but by virtual gravitons, where entirely different rules apply. This quickly gets very complicated when you need a better explanation than the almost non-explanation "virtual particles don't have to obey the speed of c limit". In any case, gravitons aren't part of our current mainstream theories, they are expected to play a role in quantum gravity, but we're not there yet. I think it would be better to just forget about them at this stage.
    I don't know where I got the idea that gravitons were the missing particle in the standard particle model. Virtual gravitons it is. I'll keep a look out for something on that and will try to keep gravitons in their proper place.

    Also, as you may be able to tell from my earlier question about the CMBR being a universal rest frame, I thought that might allow us to determine which of two objects in relative motion is at rest (or is closer to at rest, lol). That notion, or misconception as the case may be, was stimulated in my mind by the presence of microwave energy coming and going in all directions at the speed of light as determined by the almost invariant temperature gradient we observe in every direction. I should ask here since I have this thread going, is the CMBR considered EM wave energy in the form of thermal energy or is there a more appropriate characterazation of it?

    Why wouldn't such a background allow us to determine the real “rest” frame between two frames where objects are moving at relativistic velocities to each other. Maybe I am projecting too much measurability into situations that are generally only thought experiments?

    In addition, in the same post, I pointed out that for the most part, the redshift observed in the light spectrum of distant galaxies carries a similar indication, i.e. any motion at a relativistic velocity would allow us to determine which of two objects in relative motion is at rest relative to the generally consistent raw redshift data. In my thinking, if there was a slow down in aging for example, wouldn't we know which frame the slow down would occur?

    Now, after the recent explanations of the wave energy content of space containing EM and possibly gravity waves, I wonder if it is thought that the theoretical gravitational waves or virtual gravitons even, contain or carry energy? I assume so.

    Also, could any such gravitational energy be said to traverse space and therefore be a component of the wave energy background (not electromagnetic waves in the case of gravitational waves of course) along with the CMBR’s long wavelength electromagnetic waves (thermal energy)? Assuming that is the case, then my question is this; does the space (all space) that contains CMBR also contain gravitational energy?

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    Quote Originally Posted by Bogie View Post
    I don't know where I got the idea that gravitons were the missing particle in the standard particle model. Virtual gravitons it is. I'll keep a look out for something on that and will try to keep gravitons in their proper place.
    No gravitons are that particle, however forces are carried not by the particles themselves but by their virtual counterparts. Just like photons are associated with electromagnetism, however it is virtual photons that mediate that force, not photons themselves.

    Also, as you may be able to tell from my earlier question about the CMBR being a universal rest frame, I thought that might allow us to determine which of two objects in relative motion is at rest (or is closer to at rest, lol). That notion, or misconception as the case may be, was stimulated in my mind by the presence of microwave energy coming and going in all directions at the speed of light as determined by the almost invariant temperature gradient we observe in every direction. I should ask here since I have this thread going, is the CMBR considered EM wave energy in the form of thermal energy or is there a more appropriate characterazation of it?
    You can use that, however it's still just a convention. A handy convention, but a convention nevertheless. We might by convention say that all objects at rest wrt earth are "really" at rest, however that doesn't change the physics.

    In addition, in the same post, I pointed out that for the most part, the redshift observed in the light spectrum of distant galaxies carries a similar indication, i.e. any motion at a relativistic velocity would allow us to determine which of two objects in relative motion is at rest relative to the generally consistent raw redshift data. In my thinking, if there was a slow down in aging for example, wouldn't we know which frame the slow down would occur?
    Other than by convention we wouldn't know.

    Now, after the recent explanations of the wave energy content of space containing EM and possibly gravity waves, I wonder if it is thought that the theoretical gravitational waves or virtual gravitons even, contain or carry energy? I assume so.
    Yes, both electromagnetic and gravitational waves carry energy.

    Also, could any such gravitational energy be said to traverse space and therefore be a component of the wave energy background (not electromagnetic waves in the case of gravitational waves of course) along with the CMBR’s long wavelength electromagnetic waves (thermal energy)? Assuming that is the case, then my question is this; does the space (all space) that contains CMBR also contain gravitational energy?
    All space containing CMBR contains gravitational waves, but not because the two are related somehow, they both just happen to permeate space.

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    Say we have two observers in motion relative to each other - Alice and Bob.

    Alice measures the CMB to be the same temperature in all directions, so we can conclude that Alice is in the CMB rest frame.

    Bob measures a directional anisotropy in the temperature of the CMB, so we can conclude that Bob is not in the CMB rest frame.

    Alice and Bob approach and pass each other, synchronising their clocks at closest approach. Alice calculates Bob to be time-dilated in relation to herself. Bob calculates Alice to be time-dilated in relation to himself. The relationship is symmetrical - each calculates the others clock to be "running slower" than their own, by the same amount.

    How can we determine who is "really" at rest here? What experiment might either observer perform to ascertain their "true" state of motion? What action can either observer take to establish if their calculations are correct, or not? How can we say who is "really" time-dilated?

    You might try the same experiment, but this time the observers are at rest in relation to each other, but sitting at different gravitational potentials. We could find the CMB rest frame in our Solar system if we liked, by accelerating in a certain direction (although maintaining it might be difficult!). But ultimately, this means that the CMB rest frame has no direct correlation to gravitational potential. You can be in the CMB rest frame and still be gravitationally redshifted in relation to another observer who is also in the CMB rest frame.
    Last edited by speedfreek; 2012-Apr-01 at 12:28 PM. Reason: clarification at end of last paragraph

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    I wish I had a device to stop time. Bank vault here I come...

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    Quote Originally Posted by caveman1917 View Post
    No gravitons are that particle, however forces are carried not by the particles themselves but by their virtual counterparts. Just like photons are associated with electromagnetism, however it is virtual photons that mediate that force, not photons themselves.
    I have to say that I knew about particles and force carries but didn’t know the carriers were virtual particles. That actually gives me a clue that should help me understand better next time I go to the Particle Adventure site.
    You can use that, however it's still just a convention. A handy convention, but a convention nevertheless. We might by convention say that all objects at rest wrt earth are "really" at rest, however that doesn't change the physics.
    If the entire universe can be considered causally connected, and if we can go by the observable part of the universe being filled with CMBR with the special characteristic of a relativistic motion sensor in all directions, then do we consider it mainstream thinking to consider the as yet unobservable portion of the causally connected big bang universe to be filled with the same CMBR that would display the same motion detector?

    If so then there is a much better case for a universal rest frame based on the CMBR than on the Earth only.
    Other than by convention we wouldn't know.
    The same thinking applies doesn't it? If the entire universe can be considered causally connected, and if we can go by the observable part of the universe being filled with galaxies and galaxy groups all moving away from each other such that the redshift has the special characteristic of being a relativistic motion sensor in all directions, then do we consider it mainstream thinking to consider the as yet unobservable portion of the causally connected big bang universe to be filled with galaxies that display the same observable redshift characteristics in all directions?

    Wouldn’t that support the idea of a universal rest frame too?
    Yes, both electromagnetic and gravitational waves carry energy.

    All space containing CMBR contains gravitational waves, but not because the two are related somehow, they both just happen to permeate space.
    Then isn't it alright to say the all space is filled with wave energy, whether it is EM or gravity?

    Another question, if gravity is associated with a field of its own, like EM has its electric and magnetic fields, then would we be able to say that all space also contains "field" of one kind or another; actually of both?

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    Quote Originally Posted by speedfreek View Post
    Say we have two observers in motion relative to each other - Alice and Bob.

    Alice measures the CMB to be the same temperature in all directions, so we can conclude that Alice is in the CMB rest frame.

    Bob measures a directional anisotropy in the temperature of the CMB, so we can conclude that Bob is not in the CMB rest frame.

    Alice and Bob approach and pass each other, synchronising their clocks at closest approach. Alice calculates Bob to be time-dilated in relation to herself. Bob calculates Alice to be time-dilated in relation to himself. The relationship is symmetrical - each calculates the others clock to be "running slower" than their own, by the same amount.

    How can we determine who is "really" at rest here? What experiment might either observer perform to ascertain their "true" state of motion? What action can either observer take to establish if their calculations are correct, or not? How can we say who is "really" time-dilated?

    You might try the same experiment, but this time the observers are at rest in relation to each other, but sitting at different gravitational potentials. We could find the CMB rest frame in our Solar system if we liked, by accelerating in a certain direction (although maintaining it might be difficult!). But ultimately, this means that the CMB rest frame has no direct correlation to gravitational potential. You can be in the CMB rest frame and still be gravitationally red shifted in relation to another observer who is also in the CMB rest frame.
    I think that the first example is purely Special Relativity at work and both Alice and Bob can rightfully consider themselves at rest. When they consider themselves at rest, both would consider that the other was aging slower. My question is, if Alice was at rest relative to the CMBR, then at the end of the day, wouldn’t she have aged more than Bob, i.e. Alice’s observations were correct in both SR and GR, while Bob’s observations of Alice were correct in SR by not in the reality of conditions under the influence of both rocket acceleration and gravity relative to such a motion detector like the CMBR? If she was the one who was at rest then she would have aged faster than Bob if he was being accelerated by the rocket.

    In regard to the two being at different heights, say Alice at the top of a mountain and Bob at the bottom, would the difference in acceleration due to gravity be too tiny to cause a noticeable aging effect since the difference in gravitational potential is not relativistic?

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    Quote Originally Posted by publiusr View Post
    I wish I had a device to stop time. Bank vault here I come...
    OK, we can work out a deal for you to use mine anytime you like. Will you send me rental fee (in advance)?

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    Quote Originally Posted by Bogie View Post
    I think that the first example is purely Special Relativity at work and both Alice and Bob can rightfully consider themselves at rest. When they consider themselves at rest, both would consider that the other was aging slower. My question is, if Alice was at rest relative to the CMBR, then at the end of the day, wouldn’t she have aged more than Bob, i.e. Alice’s observations were correct in both SR and GR, while Bob’s observations of Alice were correct in SR by not in the reality of conditions under the influence of both rocket acceleration and gravity relative to such a motion detector like the CMBR? If she was the one who was at rest then she would have aged faster than Bob if he was being accelerated by the rocket.
    The answer is no, Alice would not age more due to being in the CMBR rest frame. The CMBR rest frame has no direct correlation with gravity - you can be in the CMBR rest frame and be subject to higher gravity than someone not in the CMBR rest frame, it all depends where you are in the universe. You can be in the CMBR rest frame and age less than someone else, in other words.

    All the CMBR rest frame is, is a frame where the CMBR temperature is the same in all directions. If you move away from Earth at around 600 km/s in a certain direction, you are in the CMBR rest frame. You will still be in the gravitational field of the Solar system, however.

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    Quote Originally Posted by speedfreek View Post
    The answer is no, Alice would not age more due to being in the CMBR rest frame. The CMBR rest frame has no direct correlation with gravity - you can be in the CMBR rest frame and be subject to higher gravity than someone not in the CMBR rest frame, it all depends where you are in the universe. You can be in the CMBR rest frame and age less than someone else, in other words.
    Here’s my thinking on the aging of Alice and Bob. Bob is accelerated at relativistic velocity and so he ages slower. Alice is not accelerated at relativistic velocity and so she ages normally. Therefore she ages more than Bob.
    All the CMBR rest frame is, is a frame where the CMBR temperature is the same in all directions.
    It is true that the CMBR rest frame is identified by the characteristic that the background temperature is the same in all directions, but isn’t it also a way to tell whether it is Alice or if it is Bob who is being accelerated physically?
    If you move away from Earth at around 600 km/s in a certain direction, you are in the CMBR rest frame. You will still be in the gravitational field of the Solar system, however.
    In the Alice/Bob example gravitational acceleration is not a significant factor; almost immeasurable. The main question is, does relativistic acceleration slow down clocks and the aging process? You are correct to point out that clocks slow down in gravitational fields as well as when jet propelled; however in our example the gravitational time dilation is not a significant factor while the relativistic acceleration of Bob relative to Alice who stays at rest is.

    I was basing my question on the fact that acceleration cannot be distinguished from being in a gravitational field. They are equivalent. There are various degrees of gravitational acceleration but they are not considered relativistic in the Alice/Bob example and the minuscule slowing of aging that would ensue is not what I was referring to.

    What led me to the conclusion that Bob was the only one being accelerated relativistically was:
    Quote Originally Posted by Speedfreek
    Alice measures the CMB to be the same temperature in all directions, so we can conclude that Alice is in the CMB rest frame.

    Bob measures a directional anisotropy in the temperature of the CMB, so we can conclude that Bob is not in the CMB rest frame.
    If Alice can measure the same ~2.7 K temperature in all directions that puts her at rest in my thinking.

    Bob’s acceleration has the same effect as if he was being accelerated by gravity but in order for gravity to be accelerating him at a relativistic velocity he would have to be approaching a black hole wouldn't he?

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    Quote Originally Posted by Bogie View Post
    Here’s my thinking on the aging of Alice and Bob. Bob is accelerated at relativistic velocity and so he ages slower. Alice is not accelerated at relativistic velocity and so she ages normally. Therefore she ages more than Bob.
    How did you determine this? All we know is that Alice and Bob are in relative motion, and that Alice measures the CMBR to be the same temperature in all directions.

    For all we know, Alice might have accelerated at relativistic velocity before she found herself in the CMBR rest frame. For all we know, Bob may never have accelerated at all. This is pure SR, and without any knowledge of the history of their accelerations, all we can say is that the time-dilation between them is symmetrical and there is no experiment that can prove otherwise.

    Quote Originally Posted by Bogie View Post
    It is true that the CMBR rest frame is identified by the characteristic that the background temperature is the same in all directions, but isn’t it also a way to tell whether it is Alice or if it is Bob who is being accelerated physically?
    No, not at all. Bob might have been born on a planet that is in a frame close to the CMBR rest frame. Alice might have had to accelerate for years before she found herself in the CMBR rest frame.

    Quote Originally Posted by Bogie View Post
    In the Alice/Bob example gravitational acceleration is not a significant factor; almost immeasurable. The main question is, does relativistic acceleration slow down clocks and the aging process? You are correct to point out that clocks slow down in gravitational fields as well as when jet propelled; however in our example the gravitational time dilation is not a significant factor while the relativistic acceleration of Bob relative to Alice who stays at rest is.
    But I never mentioned any acceleration in the scenario I presented. All we know is that Alice and Bob are in motion relative to each other, and that Alice is in the CMBR rest frame. If they both started in the same frame, a frame in motion relative to the CMBR rest frame, but Alice then accelerated in order to put herself in the CMBR rest frame, then it is Bob who can consider himself at rest and that Alice is ageing at a slower rate.

    Quote Originally Posted by Bogie View Post
    What led me to the conclusion that Bob was the only one being accelerated relativistically was:
    If Alice can measure the same ~2.7 K temperature in all directions that puts her at rest in my thinking.
    She is only at rest in relation to the expansion of the universe, i.e. she has no peculiar motion relative to the CMBR rest frame. She can still be moving relativistically in relation to anything else.

    Quote Originally Posted by Bogie View Post
    Bob’s acceleration has the same effect as if he was being accelerated by gravity but in order for gravity to be accelerating him at a relativistic velocity he would have to be approaching a black hole wouldn't he?
    But who says that Bob accelerated in order to be in motion in relation to Alice? All we know is that there is relative motion between them, and that Alice is in the CMBR rest frame... she might have accelerated more than Bob, in order to be in that frame.

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    Quote Originally Posted by speedfreek View Post
    How did you determine this? All we know is that Alice and Bob are in relative motion, and that Alice measures the CMBR to be the same temperature in all directions.

    For all we know, Alice might have accelerated at relativistic velocity before she found herself in the CMBR rest frame. For all we know, Bob may never have accelerated at all. This is pure SR, and without any knowledge of the history of their accelerations, all we can say is that the time-dilation between them is symmetrical and there is no experiment that can prove otherwise.
    Agreed.

    No, not at all. Bob might have been born on a planet that is in a frame close to the CMBR rest frame. Alice might have had to accelerate for years before she found herself in the CMBR rest frame.


    But I never mentioned any acceleration in the scenario I presented. All we know is that Alice and Bob are in motion relative to each other, and that Alice is in the CMBR rest frame. If they both started in the same frame, a frame in motion relative to the CMBR rest frame, but Alice then accelerated in order to put herself in the CMBR rest frame, then it is Bob who can consider himself at rest and that Alice is aging at a slower rate.


    She is only at rest in relation to the expansion of the universe, i.e. she has no peculiar motion relative to the CMBR rest frame. She can still be moving relativistically in relation to anything else.


    But who says that Bob accelerated in order to be in motion in relation to Alice? All we know is that there is relative motion between them, and that Alice is in the CMBR rest frame... she might have accelerated more than Bob, in order to be in that frame.
    That is true, there are possibilities that could exist to make it unclear as to who was accelerated and who wasn't. I’m going to try to think up circumstances that can establish some mutual start point and some form of relativistic acceleration to apply to Bob. Then the operative question will be, given those circumstances, which will age more than the other, not under SR, but under GR.

    My question is, does anyone get what I am asking? Acceleration and gravity are equivalent, I think. I grant you the point that even though the CMBR and the redshift data can establish an interesting convention as to the existence of a so called rest frame, it isn’t something that has been seen as remarkable or useful in physics. But I am having a hard time, given my layman misconceptions , understanding why there aren't any circumstances, even in terms of a thought experiment, where this “convention” can be put to use to make a point that we might be able to use it to determine which person would age faster or slowing under a particular scenario.

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    I think I get what you are asking.

    You are looking for the frame of reference where time passes as fast as possible in the universe. A frame that is definitely not time-dilated in relation to any other frame in the universe.

    The CMBR rest frame is not the frame you seek. The frame you seek is the frame that is furthest away from any other gravitational influence in the whole universe. If it exists, this frame would probably be at one single location in the whole universe! And it might be in motion in relation to the CMBR rest frame.

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    Quote Originally Posted by speedfreek View Post
    I get what you are asking.

    You are looking for the frame of reference where time passes as fast as possible in the universe. A frame that is definitely not time-dilated in relation to any other frame in the universe.

    The CMBR rest frame is not the frame you seek. The frame you seek is the frame that is furthest away from any other gravitational influence in the whole universe. If it exists, this frame would probably be at one single location in the whole universe! And it might be in motion in relation to the CMBR rest frame.
    You mean like somewhere out there in the vicinity of Newton's bucket?

    No, that is not what I mean. I'll work up a thought experiment with Alice and Bob starting from Newton's bucket as a landmark though, out in the middle of nowhere, to take gravity out of the equation for all intents and purposes so we can concentrate on the effect of acceleration on aging and use the consistent background temperature to determine which is in the acceleration mode. Alice will turn the bucket over and sit on it, lol, while Bob jets off at 99% of the speed of light for twenty years. While Alice sits there she takes her measurements of the CMB temperature and it is ~2.7 K in all directions and Bob measures the background temperature as being very hot in the direction of motion (I have no idea how hot but at 99% of the speed of light I bet it is about a million degrees, lol) and almost absolute zero behind him.

    Bob jets around as he accelerates and winds up back at the bucket twenty years later.

    Gravity is still present but all objects are so distant as to make gravity an insignificant factor in the aging process for both Alice and Bob.

    Which one has aged the most during the experiment? What is the explanation for your answer?
    Last edited by Bogie; 2012-Apr-04 at 12:23 PM. Reason: spelling

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    I guess it is obvious that Bob ages slower than Alice. It is not a mystery that acceleration slows the aging process just like it slows clocks. My question was about whether we could use the consistency of the CMBR and/or the redshift data to determine which object or person was being accelerated, and thus which one would age slower. I set up the Alice/Bob scenario to show how that could be done. No sense beating that horse to death, lol.

    Since I came across Newton's bucket out there, I decided to examine it and the surrounding space to see if I could determine if and how the water has inertia. It is a given in this example that the bucket is in otherwise empty space; empty to the extent that there is no matter, only space, except for the bucket and the water (and the rope).

    I think Mach would say that if there were no distant stars and planets against whose positions the water is rotating relative to, then the water wouldn't creep up the side. He thought that gravity was instantaneous and so the inertia comes from the distant surrounding celestial objects which cause the water to creep up.

    I hear that Einstein came to the opinion that even if there were no distant objects whose gravity was felt instantaneously, there would still be inertia from the very nature of space. The bucket and the water would be sufficient to curve spacetime and establish a reference against which the inertia would show up as indicated by the water creeping up the side of the bucket.

    My question is, in the empty spacetime where it is only the presence of the bucket and water curving space, are there also gravity waves carrying energy away from the bucket of water? Would the bucket and water eventually be disbursed out into space in the form of gravitational energy? Could we expect the bucket to eventually disappear completely?

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    Quote Originally Posted by Bogie View Post
    Bob jets around as he accelerates and winds up back at the bucket twenty years later.

    Gravity is still present but all objects are so distant as to make gravity an insignificant factor in the aging process for both Alice and Bob.

    Which one has aged the most during the experiment? What is the explanation for your answer?
    The lack of gravity in the environment does not matter; it's the acceleration (equivalent to gravity) of the one jetting around that causes the (persistent) time dilation.


    Quote Originally Posted by Bogie View Post
    My question is, in the empty spacetime where it is only the presence of the bucket and water curving space, are there also gravity waves carrying energy away from the bucket of water? Would the bucket and water eventually be disbursed out into space in the form of gravitational energy?
    WRT modeling gravity it's either curved spacetime or gravity waves (or gravity particles), not both.

    Just to be sure there's no confusion: So-called "gravitational waves" that some observatories try to detect is something else. Analogy: if gravity is water, then "gravitational waves" are waves on the surface of the water; it is not the stuff that the water (gravity) is made of.

    Could we expect the bucket to eventually disappear completely?
    I'm not entirely sure but i think not. (Apparently gravity is not energy that results from the conversion of mass into energy.)

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    Quote Originally Posted by noncryptic View Post
    The lack of gravity in the environment does not matter; it's the acceleration (equivalent to gravity) of the one jetting around that causes the (persistent) time dilation.
    We agree then.
    WRT modeling gravity it's either curved spacetime or gravity waves (or gravity particles), not both.
    One of the early questions in the thread concerned whether or not it was mainstream to consider both QM and GR as mainstream theories. I concluded that when discussing mainstream ideas (say ideas about gravity) it was permissible to talk about gravity in both theories on the basis that the unification of the theories may require a quantum theory of gravity or the discovery of GR gravity waves and an explanation of how they apply in both the macro and quantum realms. Later it was pointed out that GR predicted gravity waves and QM predicted a particle, the graviton, and a force carrier associated with the particle, virtual gravitons. In addition, when we discussed the concept of instantaneous gravity vs. the time delay of gravity, it was brought up that the curvature of space is felt instantaneously and that when there is a “jerk” in the motion of an object due to acceleration by an intervening force, that change in momentum was communicated at the speed of light, presumably either by the gravity waves that GR predicts, or by the gravitational field generated by the gravitons and virtual gravitons.

    So my comment is that you seem to be correct, that with regard to modeling, gravity is modeled in GR as the curvature of space felt instantly across distance coupled with gravity waves conveying time delayed information about changes in forces applied to the co-moving objects, and gravity is modeled in QM as the result of particles and forces playing out in a gravitation field (described in my poor layman terms, lol).

    In addition, both QM and GR feature the recognition of wave-particle duality and it is around that feature that a unifying theory of gravity may well draw from both GR and QM (in my naive understanding). Please correct this wording and insert any clarifications you think would be helpful for me to better understand and state the mainstream ideas.
    Just to be sure there's no confusion: So-called "gravitational waves" that some observatories try to detect is something else. Analogy: if gravity is water, then "gravitational waves" are waves on the surface of the water; it is not the stuff that the water (gravity) is made of.
    Let me ask you a question about gravity wave detection. Are the existing gravitational observatories you mention looking specifically for GR gravity waves?
    I'm not entirely sure but i think not. (Apparently gravity is not energy that results from the conversion of mass into energy.)
    That brings up the question then, if there are gravitational waves emitted from objects, whether to communicate changes in applied force to one of the objects, or to “fuel” the graviton and the potential gravitational energy of the QM field, doesn’t the conservation of energy require there to be that kind of a conversion?

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    Quote Originally Posted by Bogie View Post
    gravity is modeled in GR as the curvature of space felt instantly across distance
    coupled with gravity waves conveying time delayed information about changes in forces
    The only thing that can be felt wrt curvature of spacetime is local, not "across a distance" even though the source of the curvature is at a distance. Distance comes into play when there is a change in the curvature: a change in the curvature propagates as a gravitational wave (like a wave on the surface of water - not a fundamental force carrier). That wave propagates at the speed of light so the change is not felt instantly. Ie if the sun would disappear we'd notice the change of gravity and light 8 minutes later.


    gravity is modeled in QM as the result of particles
    As far as i know that is speculative; as of yet gravity is not an established aspect of quantum theory.

    I suspect there may be confusion caused by the fact that often in science (at least in popular publications about science) the term "theory" is use both for well established theories (thoroughly tested and not falsified, such as Relativity and QM, QED and QCD) -and- for 'theories' that are scientific ideas that have not yet been thoroughly tested. The latter should more properly be called hypothesis, and any "theory of Quantum gravity" is one of those (String theory is another).



    Are the existing gravitational observatories you mention looking specifically for GR gravity waves?
    Yes. They are looking for wave-like changes in the (our local) curvature of spacetime that according to GR emanate from sources that undergo strong acceleration (ie close binary stars, colliding black holes). They are not looking for the fundamental carrier (particle or wave) of the force of gravity.

    http://en.wikipedia.org/wiki/Gravitational_wave

    Specific operational gravitational wave detectors
    http://en.wikipedia.org/wiki/Gravita...wave_detectors

    That brings up the question then, if there are gravitational waves emitted from objects, whether to communicate changes in applied force to one of the objects, or to “fuel” the graviton and the potential gravitational energy of the QM field, doesn’t the conservation of energy require there to be that kind of a conversion?
    There's probably a simple answer but i have the same question.

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    Quote Originally Posted by Bogie View Post
    My question is, in the empty spacetime where it is only the presence of the bucket and water curving space, are there also gravity waves carrying energy away from the bucket of water?
    No, there are not. As we previously discussed, gravitational waves are caused by "jerk". In an otherwise empty universe, what could possibly cause the bucket of water to "jerk"?

    From the link in noncryptic's above post:

    In general terms, gravitational waves are radiated by objects whose motion involves acceleration, provided that the motion is not perfectly spherically symmetric (like an expanding or contracting sphere) or cylindrically symmetric (like a spinning disk or sphere)
    .

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