# Thread: expanding spacetime and relative speed

1. ## expanding spacetime and relative speed

I am wondering about something. We can determine our speed relative to the CMB by observing redshifting in one direction and blueshifting in the other, right? What about with space-time itself? I mean, if everything is moving away from everything else with approximately the same redshift per distance ratio for galaxies measured over the same distance minus small peculiar speeds, then if an observer were to travel at a very large speed to us, wouldn't galaxies at the same distances to that observer along the direction of travel, or direction of the original acceleration, and at the same distances perpendicular to that line of travel now appear more blueshifted in front of us than behind, so that we can approximately judge our speed relative to the expanding space-time?

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I'll copy something I just posted on another thread:

The key thing about relativity is that it's all relative. There is no standardized "ether" against which things moving in the cosmos are measured. Things move relative to other things, period. If there is a galaxy (Galaxy A) which is moving at .5 c relative to a cluster of galaxies (Cluster B) whose relative velocities to one another are less than .0001 c, one might assume that the .5 c galaxy is the speedy demon, but one would be wrong. It is just as relevant to say that Galaxy A is experiencing a half lightspeed flyby from Cluster B.
I've had that thought, too, grav, about using the CMB as an absolute measuring stick. The problem with using CMB Radiation is that it's not keyed to specific and known emission spectra. That is, we can't spot a shift relative to known spectra since there's no reference spectra against which to compare it. For all we know, the observable universe could be inside a larger bubble that's moving through a massive void in the entire universe at .1 c - and we'd never know it.

The second thing is that the CMBR isn't at the edge of the observable universe. Rather, it fills the universe, and is what's expected if a plasma cloud expanded into the size of the universe. It's not associated with any star, galaxy, or other object. Instead, it's the aftermath of the white-hot fog of hydrogen plasma existing after the Big Bang, but before the formation of stars and planets. These photons have been around ever since, growing fainter and less energetic.

Having said that, there is anisotropic (non-uniform) data which suggests our local group of galaxies is moving at 627 +/- 22 km/s relative to the reference frame of the CMB (also called the CMB rest frame) in the direction of galactic longitude l=276 deg, b=30 deg.

Thus, the final answer to your question is yes, it appears one side of the CMB appears a bit warmer than the opposite side, and that's been interpreted as a movement towards the blue-shifted direction.

However, this isn't necessarily the reason as to why it appears warmer on one side than the other. For all we know, the observable universe is the result of a relativistic expansion bubble and the observe CMBR anisotrop is merely the result the BB happening far away, along the line of our apparent movement.

3. Oops, sorry about that. I knew I shouldn't have started off with a reference to the CMB. I was just using that as an example, but that's not what I was asking about. What I am asking is similar, but actually has nothing to do with the CMB, but the expansion of space-time.

If all galaxies were to just follow along with the Hubble flow for the expansion of the universe, then observers in all galaxies should see the same redshifts for any other galaxies that lie at the same distance from the observer, isn't that right? The galaxies would also have a peculiar speed, but small compared to the redshift for large distances, so we will ignore that and just consider they are all just separating with the expansion of space-time. Now, if an observer in one of the galaxies were to quickly accelerate to some relative speed to his original frame, then wouldn't the galaxies in front of him become more blueshifted and the ones behind him more redshifted, while they were all redshifted the same before for the same distances? Couldn't one then tell from those shifts how fast they are travelling relative to space-time itself, since it is space-time that is expanding in order to produce the initial redshifts in the first place?

4. Originally Posted by grav
Couldn't one then tell from those shifts how fast they are travelling relative to space-time itself, since it is space-time that is expanding in order to produce the initial redshifts in the first place?
My amateur inclinations say you are right. If your travelers simply compare the spectrum shifts for known galaxies relative to the ones from back home, it should verify what they already know due to their known acceleration.

Of course, the spectra will vary with respect to time since as they get closer to the galaxies ahead, these galaxies will not be traveling as fast with the flow.

5. Originally Posted by grav
Now, if an observer in one of the galaxies were to quickly accelerate to some relative speed to his original frame, then wouldn't the galaxies in front of him become more blueshifted and the ones behind him more redshifted, while they were all redshifted the same before for the same distances? Couldn't one then tell from those shifts how fast they are travelling relative to space-time itself, since it is space-time that is expanding in order to produce the initial redshifts in the first place?
Personally, I don't think those are the kinds of questions you can really even answer with science. It is not the job of a scientific theory to say if you can, or cannot, "tell how fast they are traveling relative to spacetime itself". Spacetime itself is not an entity that would allow you to test that. Instead, the job of scientific theories is something quite specific: it is to answer questions like "if I do the following observation, what will I see, and how can I understand that result in terms of a unifying picture?" Note how that question has two parts: a part dealing with reality, the answer to the measured outcome, and a part dealing with our intellect, the language we use to organize and unify what we see. The former is testable directly and objectively (the prediction worked or it didn't), but the latter need be neither unique nor even objective (different people may prefer different stories about how to picture what happened). Questions that unflinchingly mix these two parts end up being pretty much meaningless, in my opinion.

In other words, to make the question one for science, one must reframe it in the form "what would we see if we maintained a large acceleration", and we know that answer (we'd see redshifts in one direction and blueshifts in the other). Then there could be a second question "what mathematical model will help us understand that result", and the answer is a coordinatization of spacetime. But we need not imagine that coordinatization is unique or physically real. To answer that part, you always must find a measurement that will come out A if spacetime is uniquely or physically real, and not A if it isn't, and then ask the question about the outcome of that experiment. I doubt you can imagine an experiment like that, so I suspect the question is not physically meaningful, it merely deals with convenience of various approaches to the first question.

I do think that one can view spacetime as a kind of physical field, but if things are really happening "relative to it", or if it is just a proxy for other things that are actually physical and actually do define the meaning of "relative motion", is unclear. For example, it seems perfectly possible to me that there is somewhere a very advanced civilization that does not use the concept of spacetime at all, except as a convenient approximation to the fundamental way they can think about things, and if that were true, what would you say is the answer to "do we really have a motion relative to an expanding spacetime"?

6. Originally Posted by George
My amateur inclinations say you are right. If your travelers simply compare the spectrum shifts for known galaxies relative to the ones from back home, it should verify what they already know due to their known acceleration.

Of course, the spectra will vary with respect to time since as they get closer to the galaxies ahead, these galaxies will not be traveling as fast with the flow.
I think this is what we've done. Given we've clocked our speed in regards to the CMBR we've effectively come up with our time dilation with regard to the CMBR.

Please someone correct me if I'm wrong here but along the axis of acceleration everything would be blue shifted both in front of us and behind. There is more blue shifting coming from the opposite direction of our travel but all light along this axis would still be blue shifted with regard to our rest frame.

I'm not sure what you are asking ultimately. Is it something like is the CMBR an absolute reference frame? Well anything can be regarded as the base reference frame. I think if you are at rest with regard to the CMBR and as far as you can get from the centre of all gravity wells then you could concider yourself to be experiencing time the fastest.

7. Originally Posted by WayneFrancis
Please someone correct me if I'm wrong here but along the axis of acceleration everything would be blue shifted both in front of us and behind. There is more blue shifting coming from the opposite direction of our travel but all light along this axis would still be blue shifted with regard to our rest frame.
No, if you could get in a rocket and accelerate to 0.5c, say, then the galaxies behind you would redshift. You'd be tacking a normal Doppler shift onto a cosmological redshift, moreso than you would be monkeying with the cosmological redshift itself. Of course, the actual story you tell to describe where the redshift comes from is not really unique, but this would be the conventional description.

I think what is confusing you is you are imagining there is such a thing as "our time dilation", and that's all that matters, such that if we go faster, then we are more time dilated so everything seems blueshifted. But we don't have any time dilation from our own point of view, our time is always perfectly normal, no matter how "fast we go". You are right that we would seem dilated to some galaxy behind us, but our distance would be increasing, so light waves from that galaxy would be taking longer and longer to get to us, and that redshifting effect (the Doppler effect) always wins over the dilation effect (which can be thought of as a correction that makes the light not as redshifted as you might otherwise think, but still redshifted by our motion).

8. Originally Posted by Ken G
To answer that part, you always must find a measurement that will come out A if spacetime is uniquely or physically real, and not A if it isn't, and then ask the question about the outcome of that experiment. I doubt you can imagine an experiment like that, so I suspect the question is not physically meaningful, it merely deals with convenience of various approaches to the first question.
Are you desiring imagination to get activated?

If we could go back in time and freeze space and time, then place synchronized clocks in each region of space that would always have the same isograv (gravitational potentials are the same for all the clocks within their regions to prevent unmatched time rates), then wait for 13.7 billion years to tick-off on the clock near the Milky Way, then what would the other clocks read? Reading the other clocks, of course, would take place by simply freezing the universe once again and reading them, so we avoid the "now" here vs. "now" there from over here complication. I am fairly sure that the clocks would all be the same, right?

I also see something similar for space with the Hubble flow, but I know I'm still a rookie on this. Einstein demonstrated that space and time are not separate absolutes, but that spacetime is an absolute. Too bad I don't understand what an absolute spacetime looks like. Nevertheless, it seems to me that it is helpful to see a somewhat default setting for spacetime based on the location and time on all those clocks we placed. The intelligent beings at those clocks would see the same redshifts per distances as we would. Further, the distances determine by all parties (ignoring peculiar motions) would be the maximum distances possible. Any relativistic action could only appear to shorten those distance, never lengthen them, assuming we travel along the shortest geodesic. Having such a distance maximum that all would agree upon seems argument for some sort of a default mode for spacetime, though it changes nothing about what we know of relativity. [Or at least what little I know of relativity. ] Is such a "defualt" view not unreasonable, or is it only helpful for pedagogical purposes?

Are my imaginations helpful, or am I only imagining that they are? Either way I win to some degree.
Last edited by George; 2009-Jun-29 at 04:07 PM. Reason: spell

9. Originally Posted by George
If we could go back in time and freeze space and time, then place synchronized clocks in each region of space that would always have the same isograv (gravitational potentials are the same for all the clocks within their regions to prevent unmatched time rates), then wait for 13.7 billion years to tick-off on the clock near the Milky Way, then what would the other clocks read?
If you could "freeze space and time", would you not be assuming what you are trying to show, that there "is such a thing" as space and time? It is fine to imagine technically impossible things, but by "freezing space and time", you may be imagining manifestly impossible things.

You can certainly imagine starting the "Big Bang" with clocks that all read zero, that are moving with the prevailing matter in the universe, but that may be an accident of how our Big Bang happened that is not connected to any laws of physics. In other words, you may be choosing a coordinatization that is connected to the initial conditions of a particular problem, rather than an innate aspect of the laws of physics.

To claim you can have motion relative to space and time itself, rather than just motion relative to an arbitrary coordinatization of space and time, you would need a more physical connection between the initial conditions and the laws of physics. Normally, there is not any connection like that, the only counterexample I can think of is Mach's principle, which comingles the initial conditions with the laws themselves in a kind of inextricable way. So in a Machian universe, I think one can imagine "motion relative to spacetime", but in a Machian universe, I would say that spacetime is functioning purely as a mathematical proxy for the matter and field distribution. So in that case the question becomes kind of moot, as spacetime is not then a separate entity from the matter and fields, it is just a language for talking about what the matter and fields are doing. Of course, "matter and fields" are just a language for talking about what experiments are doing, so it's language all the way down, and it really isn't clear at what point you can stop and say "this is a physical entity". Put differently, questions that are manifestly about precise language are often hard to put into precise language!

Einstein demonstrated that space and time are not separate absolutes, but that spacetime is an absolute.
I'm not sure I would say that spacetime is an absolute, first we need to define what an "absolute" is. One can certainly define what an invariant is, but invariants are measurable outcomes, and you can't measure "spacetime". You can predict what a clock will say, and imagine very long rulers, but spacetime is always a means to an end, not an end in and of itself. It is a mathematical milieu for making accurate predictions about experiments, what else can one want from it? The math has rules that allow us to identify invariants, does that make it "absolute"? Again I would not find it terribly surprising if a thousand years from now we don't use spacetime to predict experiments on the fringe of what is possible.

Having such a distance maximum that all would agree upon seems argument for some sort of a default mode for spacetime, though it changes nothing about what we know of relativity.
There are invariants, and spacetime can be used to compute them, but it seems to me the whole point of an invariant is that it is insensitive to what you are allowed do with spacetime. Mathematically, there is a class of transformations (in SR, the Lorentz transformations) that you can do to spacetime expressly because they don't change the invariants, and any resulting spacetime, or coordinatization of spacetime that is, is "as good" as any other. But this really gets into the deepest structure of relativity, which I am not expert on and would like to know more about.

10. Is space-time absolute? That's a good question, and my answer would be no, at least not in the way we generally consider "absolute".

I would say space-time is a theoretical/mathematical construct that allows to get the right answers (so far) by modelling and predicting what the "physical invariants" are.

Space-time is an invariant in the theory, but these "mathematical invariants" just connect the theory to the physical invariants that can be measured.

I know there are ways to get GR -- that is the exact same theory -- but using an entirely different type of geometry. Rather than "curvature", you're talking something else (and something really abstract).

For example this thing that the traditional GR structure calls "curvature of the manifold" just connected how free-fallers see other accelerate relative to them. No curvature, no relative acceleration. Now, come up with something "really different" that predicts the same thing, but uses something other than "curvature". Which is correct? Well, if neither predict any observable differences, then they are the same thing.

-Richard

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## Language in the search for truth

Originally Posted by Ken G
If you could "freeze space and time", would you not be assuming what you are trying to show, that there "is such a thing" as space and time? It is fine to imagine technically impossible things, but by "freezing space and time", you may be imagining manifestly impossible things.

You can certainly imagine starting the "Big Bang" with clocks that all read zero, that are moving with the prevailing matter in the universe, but that may be an accident of how our Big Bang happened that is not connected to any laws of physics. In other words, you may be choosing a coordinatization that is connected to the initial conditions of a particular problem, rather than an innate aspect of the laws of physics.

To claim you can have motion relative to space and time itself, rather than just motion relative to an arbitrary coordinatization of space and time, you would need a more physical connection between the initial conditions and the laws of physics. Normally, there is not any connection like that, the only counterexample I can think of is Mach's principle, which comingles the initial conditions with the laws themselves in a kind of inextricable way. So in a Machian universe, I think one can imagine "motion relative to spacetime", but in a Machian universe, I would say that spacetime is functioning purely as a mathematical proxy for the matter and field distribution. So in that case the question becomes kind of moot, as spacetime is not then a separate entity from the matter and fields, it is just a language for talking about what the matter and fields are doing. Of course, "matter and fields" are just a language for talking about what experiments are doing, so it's language all the way down, and it really isn't clear at what point you can stop and say "this is a physical entity". Put differently, questions that are manifestly about precise language are often hard to put into precise language!

I'm not sure I would say that spacetime is an absolute, first we need to define what an "absolute" is. One can certainly define what an invariant is, but invariants are measurable outcomes, and you can't measure "spacetime". You can predict what a clock will say, and imagine very long rulers, but spacetime is always a means to an end, not an end in and of itself. It is a mathematical milieu for making accurate predictions about experiments, what else can one want from it? The math has rules that allow us to identify invariants, does that make it "absolute"? Again I would not find it terribly surprising if a thousand years from now we don't use spacetime to predict experiments on the fringe of what is possible.

There are invariants, and spacetime can be used to compute them, but it seems to me the whole point of an invariant is that it is insensitive to what you are allowed do with spacetime. Mathematically, there is a class of transformations (in SR, the Lorentz transformations) that you can do to spacetime expressly because they don't change the invariants, and any resulting spacetime, or coordinatization of spacetime that is, is "as good" as any other. But this really gets into the deepest structure of relativity, which I am not expert on and would like to know more about.
Really enjoy your posts Ken G, you describe your concepts in a way that is erudite and
compelling. You struck an old chord with me on language and understanding.
Have you ever read Bertrand Russell? I did, many decades ago. Read his, ' An Enquiry
into the meaning of truth, 1940. And 'Human Knowledge', 1948.
The thing he says still with me today, 'in the search for truth, language is suspect'.
Hope you understand my response.
Nokton

12. Originally Posted by Ken G
If you could "freeze space and time", would you not be assuming what you are trying to show, that there "is such a thing" as space and time? It is fine to imagine technically impossible things, but by "freezing space and time", you may be imagining manifestly impossible things.
Wouldn't the isotropy of the CMBR suggest that such an idea, even if not "in principle", have merit? Isn't this consistent with the cosmological principle, I hope?

Reasking the clock view, is there any reason why distant galaxy beings would not determine a 13.7 billion year old universe based on their measurements of their CMBR and their redshift data for galaxies?

I'm not sure I would say that spacetime is an absolute, first we need to define what an "absolute" is. One can certainly define what an invariant is, but invariants are measurable outcomes, and you can't measure "spacetime". You can predict what a clock will say, and imagine very long rulers, but spacetime is always a means to an end, not an end in and of itself. It is a mathematical milieu for making accurate predictions about experiments, what else can one want from it? The math has rules that allow us to identify invariants, does that make it "absolute"? Again I would not find it terribly surprising if a thousand years from now we don't use spacetime to predict experiments on the fringe of what is possible.
I see I was being too general with the idea of an absoluted spacetime. I got that view from several general cosmology books that were trying to show in simple terms how Newton's absolute space separate from absolute time were erroneous given Einstein's discovery.

Mathematically, there is a class of transformations (in SR, the Lorentz transformations) that you can do to spacetime expressly because they don't change the invariants, and any resulting spacetime, or coordinatization of spacetime that is, is "as good" as any other.
No doubt the transformation equations do not reference to any absolute datum, else relative motion could produce two results -- one for the two parties and one for the value relative to the datum. The question seems to be whether or not we can use the datum idea for the Hubble Flow, which should be consistent with the cosmological principle. Since we can't determine accurately the distance to reshifted galaxies independent of their redshift, unless supernova measurements improve, then the CMBR dipole alone could be used and should be adequate to establish this datum (by compensating for the blue and red variance). Is this not how many other physicists see this? I don't think I am breaking any new ground here, unfortunately.
Last edited by George; 2009-Jun-29 at 06:08 PM. Reason: grammar

13. Originally Posted by nokton
Read his, ' An Enquiry
into the meaning of truth, 1940. And 'Human Knowledge', 1948.
The thing he says still with me today, 'in the search for truth, language is suspect'.
I believe I do undestand what you, and Russell, mean with that statement. It is the fundamental "Catch 22" of philosophy, and knowledge in general.

14. Originally Posted by George
Wouldn't the isotropy of the CMBR suggest that such an idea, even if not "in principle", have merit? Isn't this consistent with the cosmological principle, I hope?
Yes, but it sounds like a specific example, rather than a general law.
Reasking the clock view, is there any reason why distant galaxy beings would not determine a 13.7 billion year old universe based on their measurements of their CMBR and their redshift data for galaxies?
They'd get 13.7 billion years at the same, say, average density of galaxies that we now see. Again, that could just be saying our universe is uniform, as it happens. If it's uniform, everything we measure must come out the same.
Since we can't determine accurately the distance to reshifted galaxies independent of their redshift, unless supernova measurements improve, then the CMBR dipole alone could be used and should be adequate to establish this datum (by compensating for the blue and red variance). Is this not how many other physicists see this?
It's the standard convention, yes, but is it more than that?

15. Oops, I missed your nice post.

Originally Posted by publius
I would say space-time is a theoretical/mathematical construct that allows to get the right answers (so far) by modelling and predicting what the "physical invariants" are.
You and Ken have convinced me that such is the better way to look at it, though my comfort level is minimal in understanding such an invariance for spacetime.

Space-time is an invariant in the theory, but these "mathematical invariants" just connect the theory to the physical invariants that can be measured.
But is there a base level to space or space-time that exists where the CMBR dipole is nulled? Isn't this the at rest case for the fabric of the universe making it the last turtle on the bottom, at least for space?

I know there are ways to get GR -- that is the exact same theory -- but using an entirely different type of geometry. Rather than "curvature", you're talking something else (and something really abstract).
I kinda like the view of a 3D gradient using isograv lines that become compressed nearer to the mass. Is this better than the bowling ball on the trampoline approach? The density of the lines having a greater affect on the approaching object seems analogous to field lines say for magnetism. But I am no student of GR, just a floater, but am happy to be helpful for any pedagogical practices y'all wish to perform. If I can get it, most students will.

Originally Posted by Ken G
It's the standard convention, yes, but is it more than that?
I suspect so since it seems to be a rest state of some kind.

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Originally Posted by George
If we could go back in time and freeze space and time, then place synchronized clocks in each region of space that would always have the same isograv (gravitational potentials are the same for all the clocks within their regions to prevent unmatched time rates), then wait for 13.7 billion years to tick-off on the clock near the Milky Way, then what would the other clocks read? Reading the other clocks, of course, would take place by simply freezing the universe once again and reading them, so we avoid the "now" here vs. "now" there from over here complication. I am fairly sure that the clocks would all be the same, right?
In Wall Street, "time is money." In astrophysics, "time is distance" (among other things). Thus, if you had three clocks together, A, B, and C, at the same point in space, and after synchronizing them, moving A 1 AU distant, and C 1 AU distant in the opposite direction, A and C would still be synchronized with one another, but not with B. Just moving clocks interferes with their syncrhonization.

Having said that, there are ways to measure isogravs, as well as to sych clocks separated by a distance, as well as to check equally separated clocks after the passage of time.

17. Originally Posted by mugaliens
In Wall Street, "time is money." In astrophysics, "time is distance" (among other things). Thus, if you had three clocks together, A, B, and C, at the same point in space, and after synchronizing them, moving A 1 AU distant, and C 1 AU distant in the opposite direction, A and C would still be synchronized with one another, but not with B. Just moving clocks interferes with their syncrhonization.
Ah, but you had to accelerate them from their "rest space" in order to do that, right? Once you accelerate A & C, they will loose synchronization with A but by a known amount. Note that only the clock holder at A will see no CMBR dipole.

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## Language in the search for truth

Originally Posted by Ken G
I believe I do undestand what you, and Russell, mean with that statement. It is the fundamental "Catch 22" of philosophy, and knowledge in general.
Ken G, thanx your response, though I do not believe 'Catch 22' is involved
here, it is more, it is a matter of expressing concepts in a language that an open
mind can understand.
When I was seeking verification of my concept of the light from a pulsar forming
a spiral to an independant observer in space, and not like a lighthouse beacon
portrayed by astronomy books, was met with so many denials that light can never
bend in free space, it doesn't. But I was wrong because no one would do the math,
or worse still, accept the truth of the math.
Then got an e-mail from an old Prof telling me my concept was correct, and advising
me that the trained mind was a closed mind, and not be discouraged.
If I may, Ken G, would expand, not just language in the search truth, but concepts,
and a way of expressing them that like minds understand.
I feel you will understand my meaning.
Nokton.

19. Originally Posted by nokton
When I was seeking verification of my concept of the light from a pulsar forming
a spiral to an independant observer in space, and not like a lighthouse beacon
portrayed by astronomy books, was met with so many denials that light can never
bend in free space, it doesn't.
Anyone who doesn't see that the light of a pulsar will make a spiral pattern really is pretty ignorant about rotation (note it's still a lighthouse beacon because those would spiral too, just not very much). But it can be a subtle topic when you try to put it into language, that's true, because there is the difference between a "streamline" and a "streakline" that is relevant here. The light itself is moving in straight lines (the streamlines are straight) but the pattern it makes is a spiral (the streaklines curve). So we have straight and we have curved, it just depends on what we are really talking about, and that can be hard to be clear about.

The "Catch 22" I was referring to is indeed inescapable, unfortunately. It comes down to the fact that we must express our truths using language, but the definition of truth itself also has to be expressed in language. So our problem is not just that we have a clear idea what truth means but a hard time putting truths into words, our problem is that without words we do not even have a concept of truth itself. In other words, language is not a barrier to truth, language gave birth to truth. This means the limitations of language are not just an impediment to truth, they are fundamentally and inextricably linked to truth-- truth is not something different from language, so the limitations of language are not something different from the limitations of truth. The catch is that to tell if something is true or not it has to be translated into language, but the act of translation alters it so that we end up testing nothing but the truth of language itself. There just isn't any other kind of truth, though we'd like to imagine a truth that is independent of our efforts to understand it.

20. Originally Posted by nokton
When I was seeking verification of my concept of the light from a pulsar forming
a spiral to an independant observer in space, and not like a lighthouse beacon
portrayed by astronomy books, was met with so many denials that light can never
bend in free space, it doesn't. But I was wrong because no one would do the math,
or worse still, accept the truth of the math.

Nokton.
Very strange. This only takes a little thought to understand...What sort of "trained minds" decided not to draw a picture? I don't know any good physicist or astrophysicist who doesn't draw pictures and scratch down simple equations to get started.

21. Is it not identical to what we see of a stream of water that comes out of a garden hose that the holder is wiggling, or spinning round and round?

22. Originally Posted by George
Is it not identical to what we see of a stream of water that comes out of a garden hose that the holder is wiggling, or spinning round and round?
Yes, it is identical to that.

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## Confluence

Originally Posted by Ken G
Anyone who doesn't see that the light of a pulsar will make a spiral pattern really is pretty ignorant about rotation (note it's still a lighthouse beacon because those would spiral too, just not very much). But it can be a subtle topic when you try to put it into language, that's true, because there is the difference between a "streamline" and a "streakline" that is relevant here. The light itself is moving in straight lines (the streamlines are straight) but the pattern it makes is a spiral (the streaklines curve). So we have straight and we have curved, it just depends on what we are really talking about, and that can be hard to be clear about.

The "Catch 22" I was referring to is indeed inescapable, unfortunately. It comes down to the fact that we must express our truths using language, but the definition of truth itself also has to be expressed in language. So our problem is not just that we have a clear idea what truth means but a hard time putting truths into words, our problem is that without words we do not even have a concept of truth itself. In other words, language is not a barrier to truth, language gave birth to truth. This means the limitations of language are not just an impediment to truth, they are fundamentally and inextricably linked to truth-- truth is not something different from language, so the limitations of language are not something different from the limitations of truth. The catch is that to tell if something is true or not it has to be translated into language, but the act of translation alters it so that we end up testing nothing but the truth of language itself. There just isn't any other kind of truth, though we'd like to imagine a truth that is independent of our efforts to understand it.
Ken G, thankyou your response, but in my domain, ask you to stop the bus, and dwell
for a while on this concept. Truth is a concept, not a fact. It is a concept within the
mind of those who seek the truth. What is truth? It is the understanding of a beginning
and an end. You are young my friend, and I am old. But your heart in the right place.
So enjoy your posts, but never betray your true self, rather seek to to understand it.

Nokton

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## Time and the observer

Originally Posted by grav
I am wondering about something. We can determine our speed relative to the CMB by observing redshifting in one direction and blueshifting in the other, right? What about with space-time itself? I mean, if everything is moving away from everything else with approximately the same redshift per distance ratio for galaxies measured over the same distance minus small peculiar speeds, then if an observer were to travel at a very large speed to us, wouldn't galaxies at the same distances to that observer along the direction of travel, or direction of the original acceleration, and at the same distances perpendicular to that line of travel now appear more blueshifted in front of us than behind, so that we can approximately judge our speed relative to the expanding space-time?
Hi, red shifts and blue shifts are determined by the observers time frame,
When will anyone here know the truth? Speed is relative to the observer,
there is no contest here, just Albert and time.
Nokton.

25. Originally Posted by nokton
When I was seeking verification of my concept of the light from a pulsar forming a spiral to an independant observer in space, and not like a lighthouse beacon portrayed by astronomy books, was met with so many denials that light can neverbend in free space, it doesn't. But I was wrong because no one would do the math,
or worse still, accept the truth of the math.
Has this never been observed? Shouldn't there be nebulae that reflect these spiraling beams, like clouds reflecting those grand opening event spotlights used to attract us moths to its flames?

Back on topic...Is it appropriate, or just helpful for general cases, to consider the Hubble flow as a begining spacial state of some kind where movement began from here first? Using mugalien's example, were not the first ever motions of B & C those that moved relative to A, and, thus, observed a CMBR dipole (or whatever wavelength it was based on the eon they moved)?

26. Established Member
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665

## Concepts

Originally Posted by George
Has this never been observed? Shouldn't there be nebulae that reflect these spiraling beams, like clouds reflecting those grand opening event spotlights used to attract us moths to its flames?

Back on topic...Is it appropriate, or just helpful for general cases, to consider the Hubble flow as a begining spacial state of some kind where movement began from here first? Using mugalien's example, were not the first ever motions of B & C those that moved relative to A, and, thus, observed a CMBR dipole (or whatever wavelength it was based on the eon they moved)?
Hi George,
Of course it has never been observered, it is a coilition between mind and
intelligence. But picture this, you have a laser focused on one of two moons.
The distance between the two moons is one light year. You twist your laser to
the second moon in half a second, how does the light reach the second moon
without 'bending', it doesent. Lightspeed is fixed, a pulsar rotating at 30 times
per sec can never appear as a searchlight beam, it does not have the time.
Nokton

27. Originally Posted by grav
I am wondering about something. We can determine our speed relative to the CMB by observing redshifting in one direction and blueshifting in the other, right? What about with space-time itself? I mean, if everything is moving away from everything else with approximately the same redshift per distance ratio for galaxies measured over the same distance minus small peculiar speeds, then if an observer were to travel at a very large speed to us, wouldn't galaxies at the same distances to that observer along the direction of travel, or direction of the original acceleration, and at the same distances perpendicular to that line of travel now appear more blueshifted in front of us than behind, so that we can approximately judge our speed relative to the expanding space-time?
You can choose any reference frame that you wish, apply general relativity, and come up with correct predictions (within the intrinsic limits of the theory itself). If you choose to select a reference frame in which the cosmic background radiation is isotropic, then that is perfectly OK, and in fact is done for some purposes.

That does not make such a reference frame "absolute".

General relativity is quite democratic. There are no preferred reference frames from the perspective of the validity of the theory or the applicability of it. That is quite different from the case of Newtonian mechanics, in which only "inertial" reference frames are admissible.

A reference frame in which the CMB is isotropic may be preferreable from the point of view of convenience, for some specific applications. But it is not the only valid reference frame by a long shot. It is not "absolute" in any sense.

28. Originally Posted by nokton
Hi George,
Of course it has never been observered, it is a coilition between mind and
intelligence. But picture this, you have a laser focused on one of two moons.
The distance between the two moons is one light year. You twist your laser to
the second moon in half a second, how does the light reach the second moon
without 'bending', it doesent. Lightspeed is fixed, a pulsar rotating at 30 times
per sec can never appear as a searchlight beam, it does not have the time.
I would say photons become spread out between the moons, and there is no beam traveling faster than c between them.

I can envision a ring of light that would be formed by a pulsar beam reflecting off a nebula. Indeed, what else could it form? Perhaps even variability between opposite sides of the ring could be measured.

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Join Date
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Posts
665

## Relativity

Originally Posted by grav
I am wondering about something. We can determine our speed relative to the CMB by observing redshifting in one direction and blueshifting in the other, right? What about with space-time itself? I mean, if everything is moving away from everything else with approximately the same redshift per distance ratio for galaxies measured over the same distance minus small peculiar speeds, then if an observer were to travel at a very large speed to us, wouldn't galaxies at the same distances to that observer along the direction of travel, or direction of the original acceleration, and at the same distances perpendicular to that line of travel now appear more blueshifted in front of us than behind, so that we can approximately judge our speed relative to the expanding space-time?
Hi, if I may, galaxies are not expanding from each other per se, in fact the Andromeda
galaxy is on collision course with ours in the future.
It is unfortunate that we can never be an independant observer whilst locked in a
time frame, but, time frames cannot destroy a concept. That is what refreshes me.
Nokton

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