View Full Version : True Time and True c
It seems to me...
If we are moving relative to CBR at 390 km/sec, that should have a minute effect on our atomic clocks that we use to measure c.
In other words, we should be experiencing a small amount of Time Dilation relative to CBR.
Also, at certain times in our rotation, our orbit and even our Sun's orbit around the Milky Way, we could even be accelerating relative to CBR, which would further skew our measurements of c, but I won't get into that here, because I think the 390 km/sec is an average anyway.
Using only our inertial velocity I figured this out:
For every 1 minute that we experience in our 390 km/sec bubble: 1.0000008461714416367280534757218 minutes pass for the CBR.
In 10 billion years, CBR's time will have elapsed more than ours by 84 years, 225 days, 9 hours, 53 minutes, and 8.7 seconds
That doesn't seem like much, because it isn't :) But to me, of all things to consider True Time, this is it. All other times must bow before True Time!
Now let's see what that does to the speed of light:
Earth time measurement of c is 299.792.458 km/sec
According to my calculations...
True c is 299,792.204 km/sec.
That means that in 100 light years, light actually travels 801,563,040 km less than we thought it did, and also making our measurement of 100 light years to be about 44 light minutes and 33.7 light seconds off.
Go ahead. Shoot! :)
russ_watters
Oct23-08, 12:06 AM
If we are moving relative to CBR at 390 km/sec, that should have a minute effect on our atomic clocks that we use to measure c.
In other words, we should be experiencing a small amount of Time Dilation relative to CBR.
Also, at certain times in our rotation, our orbit and even our Sun's orbit around the Milky Way, we could even be accelerating relative to CBR, which would further skew our measurements of c, but I won't get into that here, because I think the 390 km/sec is an average anyway.
Go ahead. Shoot! :) Special relativity states the following:
1. The laws of physics are the same for all inertial frames of reference.
2. The speed of light is constant.
So what you are saying does not match how the universe works. Time dilation isn't something that your clock shows, it is something that only shows up when you compare your clock to other clocks. And these clocks may be moving at any speed, so you can't fix your speed at one particular value. Except, of course, that you are always stationary wrt yourself. You could, of course, attach a frame of reference to the CMB and measure a time dilation against it, but that frame and that time dilation don't hold any special meaning. They are not any more or less valid than the time dilation experienced between us and a GPS satellite, for example.
And that also explains why the speed of light is constant for you. The same reasoning applies.
Special relativity states the following:
1. The laws of physics are the same for all inertial frames of reference.
2. The speed of light is constant.
You could, of course, attach a frame of reference to the CMB and measure a time dilation against it, but that frame and that time dilation don't hold any special meaning.
.
1. The CBR is the least inertial frame in the universe. Why should we not try to use it to find the true rate of time elapse? A satellite is in our frame. The dilation of a satellite's clock wouldn't matter because the satellite is moving with us relative to CBR.
2. The speed of light being constant is why it's possible to see that our measurements must be wrong. The speed of light is constant is a law. That our measurement of it must be correct is not. Time may be relative but distance is distance, and we must be experiencing a faster time elapse rate in our frame than CBR is in it's frame due to our velocity relative to it, which is as close to absolute as you can get. The least we can do is try to figure it out.
I'd like to add to this a little:
All of the 'True Time' numbers are taken as if we took an Earth clock and placed it in the CBR, where it would tick faster.
The CBR 'velocity of Earth' measurement was taken from a satellite and although the measurement would be a tiny bit different if taken from CBR, the difference should be minuscule.
By me saying that our measurements are "wrong" I mean that they are wrong compared to what we would measure from the CBR frame of reference, which is considered to be the most stationary reference point in the universe. It does not mean that our measurements are wrong relative to our own time. They are only wrong when using what I called "True Time". In other words, light travels at 299,792.458 km/ Earth sec.
But it travels 299,792.204 km/CBR sec. Realistically, it's still the same distance because our seconds are longer than those of the CBR, when using identical clocks.
I think that this idea is useful when one considers that time in the CBR frame is the fastest in the universe and that by CBR time all other time elapse rates can be measured and then compared to our own. With a few additional calculations to compensate for our time's small difference to CBR's, we can chart the "true" ages of celestial bodies by their own respective velocities relative to us and CBR. It seems to me that this could be useful considering the varyied velocities of objects in the universe. Put simply, objects' ages should be charted for their own frames' times as well as for ours.
Consider something that's moving much faster than us, where the time elapse rate is much slower than our own. To make an extreme example: we may think that a fast moving star lived 6 billion years when in it's own time it really only aged for 4 billion years.
I think it would be presumptuous and inaccurate of us to continue forever thinking that everything in the universe ages in Earth time.
Vanadium 50
Oct23-08, 03:38 AM
All of the 'True Time' numbers are taken as if we took an Earth clock and placed it in the CBR, where it would tick faster.
But it won't. It will tick slower.
I think it would be presumptuous and inaccurate of us to continue forever thinking that everything in the universe ages in Earth time.
SR does not say this.
russ_watters
Oct23-08, 05:41 AM
1. The CBR is the least inertial frame in the universe. What does that mean? That it is special and 'truly' at rest? No. Why should we not try to use it to find the true rate of time elapse? We can if we want, but we need to understand that that time dilation carries no special meaning. A satellite is in our frame. The dilation of a satellite's clock wouldn't matter because the satellite is moving with us relative to CBR. It isn't in our frame and it matters quite a bit when calculating positions using GPS. That's kinda the whole point here: the correct reference frame to use for "rest" depends on what you are doing with the calculations. 2. The speed of light being constant is why it's possible to see that our measurements must be wrong. The speed of light is constant is a law. That our measurement of it must be correct is not. No, that is not correct at all. If our measurements about things other than C were wrong, that would be a violation of the first postulate: that all laws of physics are the same regardless of inertial reference frame. If our measurements of C were wrong, that would be a violation of the second. Time may be relative but distance is distance... Also wrong, for the same reason. And note that distance being relative is not a new component in Relativity: it dates back to Galileo.
DaleSpam
Oct23-08, 07:59 AM
Out of curiosity, when you say that the time in the rest frame of the CMBR is "true" time, what do you mean by that? Is it "true" by definition or by experiment. If you are simply defining "true time" = "CMBR time" then it is only a semantic argument. Nobody disagrees with the idea that our clocks run differently than one at rest wrt the CMBR, we just disagree with the characterization of that as "true". If you are not simply defining it to be "true" then what experiment indicates that the CMBR time is more physically significant than any other time?
The CBR is the least inertial frame in the universe
All inertial frames are relative...how do you this one is least?? special relativity says you can't know...there is no absolute....
That means that in 100 light years, light actually travels 801,563,040 km less than we thought it did, and also making our measurement of 100 light years to be about 44 light minutes and 33.7 light seconds off.
In Newtonian physics there is a concept of uniform, universal time. Celestial bodies exert forces between each other at the same time. This permits determination of "true" parameters like distance, and of course, everybody agrees on time because its universal.
Not so in relativity where time and distance data depend on the observer, where ambiguities arise and cannot be resolved. There is no Lorentz frame that can be selected in order to define "true" parameters between astronomical bodies, for example to be used in computing gravitational interactions...frames of reference determine what we observe, not any absolute value of distance nor time.... there are no absolutes except the speed of light in appropriate frames of reference.
Also, at certain times in our rotation, our orbit and even our Sun's orbit around the Milky Way, we could even be accelerating relative to CBR,
It comes at us uniformly from all directions....space is isotropic...it does not vary over time...
SR does not say this.
I never said I was quoting SR.
1. The CBR is the least inertial frame in the universe.
What does that mean? That it is special and 'truly' at rest? No.
What I mean is that it has been determined to be the one thing in the universe that is moving the slowest and is everywhere so it makes a great reference for measuring velocity of objects.
Why should we not try to use it to find the true rate of time elapse?
We can if we want, but we need to understand that that time dilation carries no special meaning.
Are you sayng that time dilation doesn't physically happen? Are you suggesting that you would not come home having aged less than your twin brother after your long flight at high velocity?
A satellite is in our frame. The dilation of a satellite's clock wouldn't matter because the satellite is moving with us relative to CBR.
It isn't in our frame and it matters quite a bit when calculating positions using GPS. That's kinda the whole point here: the correct reference frame to use for "rest" depends on what you are doing with the calculations.
Don't be so difficult. You know what I mean. We don't measure the speed Earth is moving through the universe relative to a satellite. The satellite is moving with us.
2. The speed of light being constant is why it's possible to see that our measurements must be wrong. The speed of light is constant is a law. That our measurement of it must be correct is not.
No, that is not correct at all. If our measurements about things other than C were wrong, that would be a violation of the first postulate: that all laws of physics are the same regardless of inertial reference frame. If our measurements of C were wrong, that would be a violation of the second.
I already addressed this earlier when I explained my meaning of "wrong"
Time may be relative but distance is distance...
Also wrong, for the same reason. And note that distance being relative is not a new component in Relativity: it dates back to Galileo.
I know what you mean about distance being relative but apparently you don't understand anything I say. What I meant by "distance is distance" is that 390 km is real. If we really are moving at 390km/sec then our time must be different from something that ISN'T crossing that distance in that amount of time.
I may not spout Einstein quotes and complex formulas in every other post, but I am still capable of objective reasoning and your tone suggests that you have no patience for someone less educated than yourself. If you suspend your desire to shoot me down for a moment and try to understand the concept behind my 'unruly' and 'unschooly' method of presentation, you might actually see something you could contribute to with your knowlege, rather than a scratching post.
It comes at us uniformly from all directions....space is isotropic...it does not vary over time...
What I meant is that we may occasionally be experiencing acceleration on our way toward "Leo"
Out of curiosity, when you say that the time in the rest frame of the CMBR is "true" time, what do you mean by that? Is it "true" by definition or by experiment. If you are simply defining "true time" = "CMBR time" then it is only a semantic argument. Nobody disagrees with the idea that our clocks run differently than one at rest wrt the CMBR, we just disagree with the characterization of that as "true". If you are not simply defining it to be "true" then what experiment indicates that the CMBR time is more physically significant than any other time?
Saying 'true time" is just me having fun with the idea of a reference frame that is universal.
It is only my OPINION that CBR should be used for that purpose. I think that measurements taken relative to CBR would be as close to absolute as we could ever get. Anyone that considers CBR to be no better than anything else for such an application will certainly not agree with anything else I've said here and that's OK with me, but don't tell me I'm breaking laws, because you know that I'm not.
What it boils down to is I'm trying to use Relativity to find an 'absolute' and it ruffles feathers.
Somebody help me defend this idea!
You can't all be naysayers!
Saying 'true time" is just me having fun with the idea of a reference frame that is universal.
It is only my OPINION that CBR should be used for that purpose. I think that measurements taken relative to CBR would be as close to absolute as we could ever get. Anyone that considers CBR to be no better than anything else for such an application will certainly not agree with anything else I've said here and that's OK with me, but don't tell me I'm breaking laws, because you know that I'm not.
What it boils down to is I'm trying to use Relativity to find an 'absolute' and it ruffles feathers.
What you don't seem to understand is that when physicists say there is no preferred frame in relativity, they mean that the laws of physics work the same in all inertial frames--if you want to say that you find the CBR frame the most "natural" one to use in some vague aesthetic sense, that's fine, but the laws of physics won't work differently in that frame than in any other. If I put two physicists in windowless spacecraft moving inertially, and one ship is at rest in the CBR frame while the other is moving at 0.99c relative to it, if they both perform the same experiment inside their respective ships, they will both get the same results, there is nothing in the laws of physics governing any experiment they perform that will allow them to tell which ship they're on.
russ_watters
Oct23-08, 09:26 PM
1. The CBR is the least inertial frame in the universe.
What does that mean? That it is special and 'truly' at rest? No.
What I mean is that it has been determined to be the one thing in the universe that is moving the slowest... That's what I figured you meant and it is a pretty straightforward contradiction of Special Relativity. It is quite clearly wrong.
Right now, I can say I'm stationary and the CMB is moving relative to me. That's the entire point of the principle of relativity (the first postulate of Special Relativity). And what the CMB is doing relative to me has no implications for (for example) what a GPS satellite is doing relative to me. A GPS receiver can calculate it's position just fine without knowing it's speed relative to the CMB. Why should we not try to use it to find the true rate of time elapse?
We can if we want, but we need to understand that that time dilation carries no special meaning.
Are you sayng that time dilation doesn't physically happen? Are you suggesting that you would not come home having aged less than your twin brother after your long flight at high velocity? That isn't what he said. Having "no special meaning" means just that. It doesn't provide us with any information useful in other calculations. In fact, if you try to include it, you just add complexity while providing no additional value.
A satellite is in our frame. The dilation of a satellite's clock wouldn't matter because the satellite is moving with us relative to CBR. It isn't in our frame and it matters quite a bit when calculating positions using GPS. That's kinda the whole point here: the correct reference frame to use for "rest" depends on what you are doing with the calculations.
Don't be so difficult. You know what I mean. We don't measure the speed Earth is moving through the universe relative to a satellite. The satellite is moving with us. What you see as "being difficult" is me telling you the way it is. I know perfectly well what you mean and what you mean is straightforwardly wrong. In fact, that last sentence you wrote is an illustration of why! A velocity is the change in displacement between two objects, not three. There is no need to add a third reference to the calculation. It adds no value. The GPS system works just fine treating the earth as stationary because that's what the first postulate means. Time may be relative but distance is distance...
Also wrong, for the same reason. And note that distance being relative is not a new component in Relativity: it dates back to Galileo.
I know what you mean about distance being relative but apparently you don't understand anything I say. What I meant by "distance is distance" is that 390 km is real. If we really are moving at 390km/sec then our time must be different from something that ISN'T crossing that distance in that amount of time. Yeah, see that's just it - you don't understand it. That 390 km is 390 km to one observer but could be anything to another observer. Distance is observer dependent. I may not spout Einstein quotes and complex formulas in every other post, but I am still capable of objective reasoning and your tone suggests that you have no patience for someone less educated than yourself. If you suspend your desire to shoot me down for a moment and try to understand the concept behind my 'unruly' and 'unschooly' method of presentation, you might actually see something you could contribute to with your knowlege, rather than a scratching post. No. I am frustrated because you have no desire to learn. You didn't come here to learn about Relativity, you came to try out an idea that sounds good in your head and you are uninterested in hearing why it isn't correct. It is doubly-frustrating because this misunderstanding you have is very simple and you are simply unwilling to take the tiny effort to understand what those two simple postulates mean. You're trying to invent new physics when really all this is is a reading comprehension problem and you aren't willing to go back and reread what the theory says. You may be capable of objective reasoning, but you haven't demonstrated you are willing to use it.
russ_watters
Oct23-08, 09:35 PM
Let me tackle this one a little more specifically: Earth time measurement of c is 299.792.458 km/sec
According to my calculations...
True c is 299,792.204 km/sec. If a device that on earth measures the speed of light to be 299.792.458 km/sec is placed in the reference frame of the CMB, it will still measure the speed of light to be 299,792.204 km/sec. That's what it means to be "constant in all inertial reference frames".
DaleSpam
Oct23-08, 09:52 PM
Saying 'true time" is just me having fun with the idea of a reference frame that is universal.So then it is just semantics. There is nothing inherently wrong with that, but it is not particularly interesting either. The standard termniology works fine for me.
What you don't seem to understand is that when physicists say there is no preferred frame in relativity, they mean that the laws of physics work the same in all inertial frames--if you want to say that you find the CBR frame the most "natural" one to use in some vague aesthetic sense, that's fine, but the laws of physics won't work differently in that frame than in any other. If I put two physicists in windowless spacecraft moving inertially, and one ship is at rest in the CBR frame while the other is moving at 0.99c relative to it, if they both perform the same experiment inside their respective ships, they will both get the same results, there is nothing in the laws of physics governing any experiment they perform that will allow them to tell which ship they're on.
At it's core, this concept has been difficult for me to articulate. If I say that I'm trying to choose an absolute frame, I'll get another lesson in the basic idea behind Relativity. If I say that faster moving objects age more slowly than we do so we should measure their ages in their own times, I hear the same repetitive lessons in time dilation. Noone seems to understand this idea of mine. It's as if Relativity became a religion at some point and any hint at an absolute or 'true' measurement incites a lynch mob.
But I'll keep trying:
You see, I'm not really looking for the absolute. Just the next best thing to it. A surrogate, if you will...
In your "space craft" traveling inertially through space, everything in it is measured relative to it, right? Right.
Now visualize that space craft expanding to contain the entire solar system, the galaxy and finally the entire universe(don't bother with the expanding universe lesson here)
If the universe itself is the "space craft" then how do we measure everything in it? Just like before. BY IT. If you were given the task of finding out how to do this, what would you use? CMBR is what you would use, because it's everywhere. It IS the space craft, not just a tiny object in it.
I am also suggesting that we find a "universal" time by which to measure all other rates of time elapse (note that I did not say 'true' or 'absolute'), using the "space craft" as well.
Does anyone get this yet?
Saying 'true time" is just me having fun with the idea of a reference frame that is universal.
It is only my OPINION that CBR should be used for that purpose. I think that measurements taken relative to CBR would be as close to absolute as we could ever get. Anyone that considers CBR to be no better than anything else for such an application will certainly not agree with anything else I've said here and that's OK with me, but don't tell me I'm breaking laws, because you know that I'm not.
What it boils down to is I'm trying to use Relativity to find an 'absolute' and it ruffles feathers.
Somebody help me defend this idea!
You can't all be naysayers!In cosmology, using General Relativity, there are concepts of universal time which are prettly close to what you seem to be interested in here. If you look here (http://en.wikipedia.org/wiki/FLRW_metric) under the heading of "General Metric", you can see that two symbols are used there for time, \tau for local (proper) time and t for "universal time". Learning to use General Relativity will take a lot of work, but it is available for anyone who wants to learn it.
At it's core, this concept has been difficult for me to articulate. If I say that I'm trying to choose an absolute frame, I'll get another lesson in the basic idea behind Relativity.
Actually, from a strictly philosophical perspective there is nothing stopping you from believing in an absolute frame whose measurements of time/distance/simultaneity represent the "real truth"--relativity only says that there is nothing in the laws of physics to distinguish that frame from any other, so there can be no empirical reasons for believing there is anything special about that frame.
If I say that faster moving objects age more slowly than we do so we should measure their ages in their own times, I hear the same repetitive lessons in time dilation. Noone seems to understand this idea of mine. It's as if Relativity became a religion at some point and any hint at an absolute or 'true' measurement incites a lynch mob.
Philosophically, you are free to believe that objects moving relative to some metaphysically preferred frame are "really" aging more slowly. But again, if relativity is correct there will be no physical experiment that will allow you to empirically discern which frame is the absolute frame.
But I'll keep trying:
You see, I'm not really looking for the absolute. Just the next best thing to it. A surrogate, if you will...
In your "space craft" traveling inertially through space, everything in it is measured relative to it, right? Right.
Only if you wish to use the ship's rest frame. Nothing would stop an observer on the ship from measuring time and distance using a system of rulers and clocks that are in motion relative to the ship, it's purely a matter of convention that we define the "ship's frame" in terms of rulers and clocks at rest relative to the ship.
Now visualize that space craft expanding to contain the entire solar system, the galaxy and finally the entire universe(don't bother with the expanding universe lesson here)
If the universe itself is the "space craft" then how do we measure everything in it? Just like before. BY IT. If you were given the task of finding out how to do this, what would you use? CMBR is what you would use, because it's everywhere. It IS the space craft, not just a tiny object in it.
I am also suggesting that we find a "universal" time by which to measure all other rates of time elapse (note that I did not say 'true' or 'absolute'), using the "space craft" as well.
Does anyone get this yet?
If you want to define "universal time" in terms of the frame of the CMBR just as a matter of convention, the way time zones on Earth are a convention which everyone agrees to, that's fine. But there is nothing in the laws of physics themselves that should cause you to prefer to use this frame as opposed to any other, that's just an aesthetic decision on your part.
Here are some readings you may find useful.
Reference frames attached to the earth and the sun
Soffel et al, http://arxiv.org/abs/astro-ph/0303376
Effect of solar system motion through the CMB
(Skip the part on inflation)
Wright, http://www.astro.ucla.edu/~wright/cosmo_04.htm
Wright, http://www.astro.ucla.edu/~wright/CMB-DT.html
Estimate of local group motion through the CMB
Padmanabhan, http://books.google.com/books?id=-yvq5BEsFvcC&pg=PA349&lpg=PA349dq=thanu+padmanabhan+theoretical+astrophy sics+cosmic+microwave&source=web&ots=pBTKumDfaR&sig=pMRFvF1_LilUkr9LwycA_nd5Qy4&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA379,M1
Cosmic time and expansion of space for observers at rest in the CMB
Francis et al, http://arxiv.org/abs/0707.0380
Other useful assignments of time and space in cosmology
Tsagas et al, http://arxiv.org/abs/0705.4397
"There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe. But for doing any physics experiment, any other frame is as good as this one."
Scott, http://www.astro.ubc.ca/people/scott/faq_basic.html
Test of whether the CMB rest frame violates local Lorentz invariance (including assumptions about the locally measured speed of light). It mentions other tests, so you can look those up in here.
"the cosmic microwave background is chosen as the preferred reference frame."
Bell et al, http://arxiv.org/abs/astro-ph/9512100
Thank you all for your knowledge and insight on this. And thank you Aether and atyy for the links. I'll read them all. It's good to know that not everyone thinks this is completely harebrained.
In cosmology, using General Relativity, there are concepts of universal time which are prettly close to what you seem to be interested in here. If you look here (http://en.wikipedia.org/wiki/FLRW_metric) under the heading of "General Metric", you can see that two symbols are used there for time, \tau for local (proper) time and t for "universal time". Learning to use General Relativity will take a lot of work, but it is available for anyone who wants to learn it.
Note that although there is one coordinate system most commonly used in cosmology because it is most convenient (this coordinate system is the one that makes the density of matter/energy throughout space as close to homogeneous as possible at any single coordinate time), there is nothing in the laws of physics that causes this coordinate system to be preferred over others which might use different definitions of simultaneity (and might have different definitions of which clocks were ticking fastest). In GR, when the laws of physics are stated in tensor form, they work the same way in all coordinate systems, because of a principle known as "diffeomorphism invariance"--see this article (http://www.einstein-online.info/en/spotlights/background_independence/index.html) for a discussion (though it's also worth pointing out that any laws of physics can be made diffeomorphism invariant by putting them in tensor form, so this is sometimes seen as more a matter of mathematical formalism than an insight into physics--see my post #8 on this thread (http://www.physicsforums.com/showthread.php?t=220331)). Besides this, there's also the fact that in any small region of curved spacetime in GR, the laws of physics approach those of special relativity, with the coordinate system of a freely-falling observer in this region being equivalent to an inertial frame in SR, and just as in SR you have an infinite number of different locally inertial frames in this region where the laws of physics work the same even when stated in non-tensor form.
Note that although there is one coordinate system most commonly used in cosmology because it is most convenient...in any small region of curved spacetime in GR, the laws of physics approach those of special relativity...Okay.
...with the coordinate system of a freely-falling observer in this region being equivalent to an inertial frame in SR...As we zoom in on smaller and smaller regions of curved spacetime in GR, at what point exactly does the absolute simultaneity of coordinate systems that are convenient for use in cosmology become equivalent to the relative simultaneity of inertial frames? At the point where one stops using tensors?
DaleSpam
Oct25-08, 11:47 AM
As we zoom in on smaller and smaller regions of curved spacetime in GR, at what point exactly does the absolute simultaneity of coordinate systems that are convenient for use in cosmology become equivalent to the relative simultaneity of inertial frames?There are two answers to this question.
The most direct answer is that you can ignore the curvature as long as the region you are interested in is small enough that the curvature is undetectable with whatever experiment you are considering.
The more important answer is that there is no absolute simultaneity in GR. In fact, time coordinates in GR are even less physically significant than in SR since in SR they are at least determined by the Einstein synchronization procedure and in GR they are not necessarily determined by any physical procedure. Also, there are an infinite number of ways to parameterize any spacetime, all of which are physically indistinguishable and could have a different choice for the time coordinate or even no timelike coordinate at all.
RandallB
Oct25-08, 12:49 PM
…. CMBR is what you would use, because it's everywhere. It IS the space craft, not just a tiny object in it. Not sure using the CMBR as a spacecraft is the best analogy. I would rather use the SLS that generated the CBR as close enough to the single event of the Big Bang to be considered Simultaneous based on the SR Simultaneity rule that only local events can be truly Simultaneous.
It may lead to a similar result but for large scale measures I think it applies modern interpretations more directly.
The more important answer is that there is no absolute simultaneity in GR.There are perfectly valid coordinate systems in GR (and in SR) within which absolute simultaneity holds (conventionally).
In fact, time coordinates in GR are even less physically significant than in SR since in SR they are at least determined by the Einstein synchronization procedure and in GR they are not necessarily determined by any physical procedure.I have not claimed that there is more "physical signifiance" for one valid coordinate system over another.
Also, there are an infinite number of ways to parameterize any spacetime, all of which are physically indistinguishable and could have a different choice for the time coordinate or even no timelike coordinate at all.I know.
DaleSpam
Oct25-08, 12:56 PM
There are perfectly valid coordinate systems in GR (and in SR) within which absolute simultaneity holds (conventionally).
I have not claimed that there is more "physical signifiance" for one valid coordinate system over another. Then I don't know what you mean by "absolute" simultaneity. If it is not coordinate independent and has no physical significance then it sounds like another semantic argument.
As we zoom in on smaller and smaller regions of curved spacetime in GR, at what point exactly does the absolute simultaneity of coordinate systems that are convenient for use in cosmology become equivalent to the relative simultaneity of inertial frames? At the point where one stops using tensors?
As DaleSpam said, your use of the phrase "absolute" simultaneity is confusing, usually "absolute" means "the same in every coordinate system", yet you seem to allow different coordinate systems to have their own distinct "absolute simultaneity". Each coordinate system of course has its own well-defined notion of simultaneity, this is just as true of inertial coordinate systems in SR as it is of cosmological coordinate systems. According to the way you use the phrase "absolute simultaneity", can you imagine any coordinate system where it would not be true that "absolute simultaneity holds"? Doesn't every coordinate system by definition have a single yes-or-no answer to the question of whether two events have the same t-coordinate or not? The point is that in both flat SR spacetime and the curved spacetime found in cosmology you are free to pick different coordinate systems with different definitions of simultaneity, and there is nothing in the laws of physics to cause you to prefer one coordinate system over another.
Then I don't know what you mean by "absolute" simultaneity.I clearly said that I was talking about the "absolute simultaneity of coordinate systems that are convenient for use in cosmology".
If it is not coordinate independent and has no physical significance then it sounds like another semantic argument.Why don't you go back to the other thread where you were claiming that the isotropy of space has been firmly established on the basis of conservation of momentum, show an actual experiment where this has been done, and then accuse me there of making a semantic argument?
I clearly said that I was talking about the "absolute simultaneity of coordinate systems that are convenient for use in cosmology".
Again, this makes little sense if we interpret "absolute" to mean "coordinate-independent" as is normally done, so if you are using a different definition of "absolute", you need to spell out what that definition is. Why couldn't I just as easily talk about the "absolute simultaneity of the inertial coordinate system that is convenient for me to use in SR because it is the frame in which I am at rest"?
As DaleSpam said, your use of the phrase "absolute" simultaneity is confusing, usually "absolute" means "the same in every coordinate system", yet you seem to allow different coordinate systems to have their own distinct "absolute simultaneity". Each coordinate system of course has its own well-defined notion of simultaneity, this is just as true of inertial coordinate systems in SR as it is of cosmological coordinate systems. According to the way you use the phrase "absolute simultaneity", can you imagine any coordinate system where it would not be true that "absolute simultaneity holds"? Doesn't every coordinate system by definition have a single yes-or-no answer to the question of whether two events have the same t-coordinate or not?
Again, this makes little sense if we interpret "absolute" to mean "coordinate-independent" as is normally done, so if you are using a different definition of "absolute", you need to spell out what that definition is. Why couldn't I just as easily talk about the "absolute simultaneity of the inertial coordinate system that is convenient for me to use in SR because it is the frame in which I am at rest"?There is one notion of simultaneity in the FLRW metric, and if we zoom in on smaller and smaller regions of curved spacetime in GR until finally we make a transition to using SR spacetime (with or without inertial frames) then we are moving from one GR coordinate system that is convenient to use in cosmology to a set of SR coordinate systems that are convenient to use in a laboratory. Within this set of SR coordinate systems we could choose to have absolute simultaneity that is consistent with the cosmological coordinate system we are using, or we could choose to have relative simultaneity that is disconnected from our cosmological coordinate system.
The point is that in both flat SR spacetime and the curved spacetime found in cosmology you are free to pick different coordinate systems with different definitions of simultaneity, and there is nothing in the laws of physics to cause you to prefer one coordinate system over another.I agree, but you are the one who singled out inertial frames as being equivalent to the coordinate system of a freely falling observer in a small region of spacetime in GR. These are not the only valid coordinate systems for freely falling observers. They may be more convenient to use in a small region because they allow us to drop tensor notation, but is there any other reason that you singled them out?
DaleSpam
Oct25-08, 02:22 PM
I clearly said that I was talking about the "absolute simultaneity of coordinate systems that are convenient for use in cosmology".It is certainly not standard, but you are obviously free to define "absolute" to mean "convenient for use in cosmology". I have no objection to the claim that the time coordinate in the FLRW metric is convenient for use in cosmology.
Why don't you go back to the other thread ... and then accuse me there of making a semantic argument?What happened in some other thread is irrelevant to the question of whether or not your argument here in this thread is purely semantic.
There is one notion of simultaneity in the FLRW metric
Only if by "metric" you refer to the expression for the line element in a particular coordinate system, rather than to the curvature of spacetime (a geometrical notion which is expressed differently in different coordinate systems). For the same curved spacetime representing the same FLRW universe, you can come up with an infinite number of different coordinate systems with an infinite number of different definitions of simultaneity.
and if we zoom in on smaller and smaller regions of curved spacetime in GR until finally we make a transition to using SR spacetime (with or without inertial frames) then we are moving from one GR coordinate system that is convenient to use in cosmology to a set of SR coordinate systems that are convenient to use in a laboratory.
Why a "set" rather than a single SR coordinate system which is most convenient to use because it represents the laboratory's rest frame? You're really making an artificial distinction if you act like there's only one possible coordinate system that's "convenient" in cosmology but a set that are "convenient" in SR, this depends on an arbitrary aesthetic choice of what you find convenient.
Within this set of SR coordinate systems we could choose to have absolute simultaneity that is consistent with the cosmological coordinate system we are using
How could you "choose to have absolute simultaneity", exactly? Simply by throwing away every local inertial frame except for the one whose definition of simultaneity matched that of the cosmological coordinate system?
I agree, but you are the one who singled out inertial frames as being equivalent to the coordinate system of a freely falling observer in a small region of spacetime in GR. These are not the only valid coordinate systems for freely falling observers. They may be more convenient to use in a small region because they allow us to drop tensor notation, but is there any other reason that you singled them out?
You can "drop tensor notation" in non-inertial coordinate systems too. What makes inertial coordinate systems special is that when you do express the laws of physics in non-tensor form, you find that the laws turn out to obey the exact same equations when expressed in any inertial coordinate system, while they'd obey different equations in non-inertial coordinate systems. This is a symmetry that's built into the laws of physics, we can imagine different laws of physics where this wouldn't be true, in which case all inertial frames wouldn't be on the same footing physically in this way (so all the different inertial frames' definitions of simultaneity wouldn't be on the same footing either).
Only if by "metric" you refer to the expression for the line element in a particular coordinate system, rather than to the curvature of spacetime (a geometrical notion which is expressed differently in different coordinate systems).I am referring to the FLRW metric that I linked to in post #18 above which gives one definition of simultaneity (the hypersurface of homogeneity).
For the same curved spacetime representing the same FLRW universe, you can come up with an infinite number of different coordinate systems with an infinite number of different definitions of simultaneity.Okay, I didn't claim that this definition was unique, only that it was "one GR coordinate system that is convenient to use in cosmology".
Why a "set" rather than a single SR coordinate system which is most convenient to use because it represents the laboratory's rest frame?We can define an inertial frame \Sigma to represent the laboratory's rest frame, but then we would also need to define frames for objects in relative motion to \Sigma to do SR.
You're really making an artificial distinction if you act like there's only one possible coordinate system that's "convenient" in cosmology but a set that are "convenient" in SR, this depends on an arbitrary aesthetic choice of what you find convenient.I am not claiming that there is only one possible coordinate system that is convenient in cosmology, only giving an example of one that is convenient.
How could you "choose to have absolute simultaneity", exactly? Simply by throwing away every local inertial frame except for the one whose definition of simultaneity matched that of the cosmological coordinate system?No, I would first define a locally co-moving \Sigma inertial frame at any small co-moving region of the cosmological coordinate system, and then use Mansouri-Sexl (aka, LET, GGT, etc.) transforms instead of Lorentz transforms between SR frames in relative motion to \Sigma.
It is certainly not standard, but you are obviously free to define "absolute" to mean "convenient for use in cosmology". I have no objection to the claim that the time coordinate in the FLRW metric is convenient for use in cosmology.I have not defined "absolute" to mean "convenient for use in cosmology". My apologies if I used the term "absolute" in a confusing way above. All that I refer to is any given definition of simultaneity that is convenient for use in cosmology which is then heritable by a set of SR coordinate systems at any small region of spacetime within which absolute simultaneity holds by convention.
If it is not coordinate independent and has no physical significance then it sounds like another semantic argument...What happened in some other thread is irrelevant to the question of whether or not your argument here in this thread is purely semantic.Are you dismissing the way that I used the term "absolute" to describe the definition of simultaneity in the FLRW metric, or are you dismissing my entire attempt to show a spatially homogeneous formulation of SR?
DaleSpam
Oct26-08, 07:41 AM
My apologies if I used the term "absolute" in a confusing way above. All that I refer to is any given definition of simultaneity that is convenient for use in cosmology which is then heritable by a set of SR coordinate systems at any small region of spacetime within which absolute simultaneity holds by convention.Definition or convention, calling it "absolute" is still just semantics. The point is that you are free to label the FLRW coordinates as "absolute" and Idjot is free to label the CMBR as "true", but neither choice has any physical significance. It is simply a matter of personal preference.
Are you dismissing the way that I used the term "absolute" to describe the definition of simultaneity in the FLRW metricYes. Since you are not using the term "absolute" in a standard manner your argument is semantic. I am not going to attempt to convince you to use the standard definition of "absolute" in order to promote clarity, but I am going to clearly point out that your use of the term is non-standard.
Definition or convention, calling it "absolute" is still just semantics. The point is that you are free to label the FLRW coordinates as "absolute" and Idjot is free to label the CMBR as "true", but neither choice has any physical significance. It is simply a matter of personal preference.I don't intend to label the FLRW coordinates in a non-standard way, and have already withdrawn the term "absolute" in favor of "definition" with respect to the standard simultaneity of FLRW coordinates. I am now only using the term "absolute" in reference to the simultaneity convention of the non-standard (Mansouri-Sexl, LET, GGT, etc.) formulation of SR that I am using. I don't understand your continuing objection to how I am describing the FLRW coordinates.
Yes. Since you are not using the term "absolute" in a standard manner your argument is semantic. I am not going to attempt to convince you to use the standard definition of "absolute" in order to promote clarity, but I am going to clearly point out that your use of the term is non-standard.Since I am no longer using the term "absolute" in reference to simultaneity in FLRW coordinates, I do not understand your continuing objection. What you seem to be objecting to as a semantic argument was unintentional at first, and then withdrawn/corrected. I do not understand what your continuing objection is.
Not sure using the CMBR as a spacecraft is the best analogy. I would rather use the SLS that generated the CBR as close enough to the single event of the Big Bang to be considered Simultaneous based on the SR Simultaneity rule that only local events can be truly Simultaneous.
It may lead to a similar result but for large scale measures I think it applies modern interpretations more directly.
That sounds like a good idea. Yet I have no knowledge of SLS. In fact I don't even know what SLS stands for. Please elaborate if you don't mind.
Definition or convention, calling it "absolute" is still just semantics. The point is that you are free to label the FLRW coordinates as "absolute" and Idjot is free to label the CMBR as "true", but neither choice has any physical significance. It is simply a matter of personal preference.
The personal preference is in an aesthetic label, but what that label is intended to represent, in my case, is a reference point that cannot be replaced with just anything. It's been reiterated a few times already that there is nothing in GR that says any one frame should be preferred over any another. But for reference sake, wouldn't it be just plain easier to compare everything to one universal frame that everyone agrees is moving slower than anything else ever measured?
Humor me for a moment please...
Suppose the scientific community accepted CMBR or SLS(?) as the universal frame, calculating a universal time as I did at the beginning of this thread, possibly tweaked for accuracy. Then we compare all objects' velocities relative to this new universal frame. Then we find the time dilation of each object relative to the universal frame. Then we determine how many Earth years old the objects are but at their own rates of time elapse according to their dilation relative to the universal frame.
Anyone would have to agree that if there really was something truly at rest in the universe, that all moving objects would experience real time dilation, right? They, not the object at rest, would age more slowly, but at different rates according to velocity of course. But, with everything relative and nothing at rest, calculating time dilation would seem almost pointless.
Using a universal frame like CMBR would allow us to use time dilation for more than just aesthetics. It would allow us to use time dilation to determine the ages of celestial bodies more accurately. They are, after all, aging at their own respective rates with regard to their velocity, whether or not we have found the absolute. Just because GR fanatics refuse to say that one thing is moving faster than another without the word "relative" in the sentence, does not mean that one is not in fact moving faster than the other. It only means that GR tells us we have no way of knowing which is which. Using a universal frame like CMBR will allow us to reasonably decide which is which and that decision will lead to determinations that will at least be closer to determinations in an absolute than what we have now, which is basically nothing according to GR.
I agree that the word "true" has no physical significance. But in my opinion neither does "simultaneity". It's an unanswered question that's been accepted as an answer.
DaleSpam
Oct26-08, 06:22 PM
Since I am no longer using the term "absolute" in reference to simultaneity in FLRW coordinates, I do not understand your continuing objection.Sorry, I missed the retraction. I have no continuing objection, only continued posts on the original objection :smile:
Just because GR fanatics refuse to say that one thing is moving faster than another without the word "relative" in the sentence, does not mean that one is not in fact moving faster than the other.Don't go there Idjot. Scientists are not fanatics.
Suppose the scientific community accepted CMBR or SLS(?) as the universal frame, calculating a universal time as I did at the beginning of this thread, possibly tweaked for accuracy.SLS is "surface of last scattering" and that's what most of the photons of the CMBR last reflected off of, a few hundred thousand years after the big bang, so that's what we are actually looking at when we look at the CMBR. When you see something with your own eyes you are seeing photons that came from the Sun or from a light bulb usually, and they were reflected by some SLS...which is what you would say that you are looking at.
Sorry, I missed the retraction. I have no continuing objection, only continued posts on the original objection :smile:Understandable, thank-you.
DaleSpam
Oct26-08, 06:59 PM
The personal preference is in an aesthetic label, but what that label is intended to represent, in my case, is a reference point that cannot be replaced with just anything. Why not? All of the stuff you describe could be done just as well in any reference frame. Tell me, do you think that Greenwich Mean Time is in any sense "true" or "absolute"? Accepting the CMBR as a universal frame would be no more significant than accepting GMT as the coordinated universal time.
By the way, I believe that the scientific community already does something similar in the field of cosmology, which is the only field where this would be a useful convention.
Don't go there Idjot. Scientists are not fanatics.
Aether,
One's occupation does not decide whether or not they are fanatical about something. A scientist could be fanatical about killing insects around his home. The plumber next door may be fanatical about GR. There is nothing in GR that says a scientist cannot be a fanatic and there is nothing in GR that says a GR fanatic has to be a scientist. :)
Seriously though, no offense to anyone ever! This thread turned out to be more fun and interesting than I expected. But now I've given all of what little knowledge I had to offer beyond the idea itself and I think I'll leave it to the experts now and just read along to see what I can learn, before I offend any more scientists with my crudeness. Thanks again to everyone for all the input and advice.
russ_watters
Oct26-08, 11:14 PM
The plumber next door may be fanatical about GR. There is nothing in GR that says a scientist cannot be a fanatic and there is nothing in GR that says a GR fanatic has to be a scientist. :) You seem to have the attitude that since people can be flawed, science is probably flawed too. You really need to get onboard with how science works. Sure, a person can be fanatical, but real scientific theories do not have personalities. Whether the plumber next door is fanatical about GR or not has nothing to do with whether GR is correct. That attitude of yours is what generates attitude in the responses to your post.
Vanadium 50
Oct27-08, 06:51 AM
But for reference sake, wouldn't it be just plain easier to compare everything to one universal frame that everyone agrees is moving slower than anything else ever measured?
But, as has been pointed out before, it's not true that "everyone agrees [the CMBR frame] is moving slower than anything else ever measured".
Just because GR fanatics...
You are making an error. Calling people who do not make the same error "fanatics" is dangerously close to crackpottery.
DaleSpam
Oct27-08, 07:03 AM
Seriously though, no offense to anyone ever! No offense? I guess you were just using a non-standard definition of "fanatic" too. :rolleyes: One that doesn't lump scientists (or plumbers) with terrorists.
RandallB
Oct27-08, 10:10 AM
That sounds like a good idea. Yet I have no knowledge of SLS. In fact I don't even know what SLS stands for. Please elaborate if you don't mind. That sounds like a good idea. Yet I have no knowledge of SLS. In fact I don't even know what SLS stands for. Please elaborate if you don't mind.
When thinking of the SLS (surface of last scattering) as the source of the CMB; remember it is not a physical surface of something.
It is a matter of the universe as it aged from the Big Bang less than 0.4 G-Years became transparent basically everywhere.
[EDIT :Oops – off by a few decimal places. SLS is about 377,000 years after the Big Bang which is less than 0.0004 G-Years after the Big Bang.]
From the view of the Local Matter then from which we are made now the light at some distant radius from it could not be seen until it traveled that distance plus the distance we covered in moving away from it (Hubble expansion); That defines a set source distance or “surface” for what we see today as CBR.
CBR from a closer radius has already passed us by, and from a greater radius will not be seen by us until sometime in the future.
The point is:
from our view about 14 G-Years later, and the Big Bang being a single event, and all SLS light from all areas or radiuses starting out at less than 0.4 G-Years from the Big Bang - we can define SLS light as having an “Absolute” Simultaneous start time.
MeJennifer
Oct27-08, 10:56 AM
the Big Bang being a single event
Feel free to argue how a big bang singularity is an event in GR.
I do not think it is.
You seem to have the attitude that since people can be flawed, science is probably flawed too. You really need to get onboard with how science works. Sure, a person can be fanatical, but real scientific theories do not have personalities. Whether the plumber next door is fanatical about GR or not has nothing to do with whether GR is correct. That attitude of yours is what generates attitude in the responses to your post.
I agree. Scientific theories do not have personalities and I shouldn't confuse them with the people who sleep with them printed on their pajamas. Haha I'm sorry. I couldn't help myself there. I know what you mean. It was a bad choice of word.
.
You are making an error. Calling people who do not make the same error "fanatics" is dangerously close to crackpottery.
I agree. It did sound almost crackpottish.
No offense? I guess you were just using a non-standard definition of "fanatic" too. :rolleyes: One that doesn't lump scientists (or plumbers) with terrorists.
:rolleyes: , :sigh:, :burp:, :fart:
Yes. A non-standard definition and one I shouldn't have used.
Listen up! I admit it! I used the word "fanatic" in exaggeration!
In reality I only think it's a bit stubborn to deny the usefulness of using CMBR as a universal frame because of simultaneity. I really don't think it's fanatical and it was rude of me to say that.
But you could lighten up a little. Scientists aren't drama queens either (joke).
When thinking of the SLS (surface of last scattering) as the source of the CMB; remember it is not a physical surface of something.
It is a matter of the universe as it aged from the Big Bang less than 0.4 G-Years became transparent basically everywhere. From the view of the Local Matter then from which we are made now the light at some distant radius from it could not be seen until it traveled that distance plus the distance we covered in moving away from it (Hubble expansion); That defines a set source distance or “surface” for what we see today as CBR.
CBR from a closer radius has already passed us by, and from a greater radius will not be seen by us until sometime in the future.
The point is:
from our view about 14 G-Years later, and the Big Bang being a single event, and all SLS light from all areas or radiuses starting out at less than 0.4 G-Years from the Big Bang - we can define SLS light as having an “Absolute” Simultaneous start time.
Thank you!
While we're on the subject of nonstandard definitions, I can't help thinking of this guy's definition of SLS as the "surface of last screaming". :rofl: http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html
While we're on the subject of nonstandard definitions, I can't help thinking of this guy's definition of SLS as the "surface of last screaming". :rofl: http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_2.html
Thank you too!
RandallB
Oct27-08, 05:07 PM
Feel free to argue how a big bang singularity is an event in GR.
I do not think it is. Personally I don’t believe the idea of a singularity, naked or otherwise Big Bang or Black-Holes, although it is favored speculation.
Plus I don’t think anyone even speculates as to how to apply GR to whatever was there before or even a couple minutes after the Big Bang began.
But there is a wide agreement on much of what happened after the Big Bang as a single event.
For most the timing of when the universe became transparent is 370,000 to 380,000 years after the Big Bang whatever that event might have been.
Most of direct measure of the age of the universe as 13.7 Billion years, nearly 14 G-Years, comes from measuring the that SLS or CBR.
And on a scale of 14 G-Years compared to 0.0004 G-years the SLS or CBR source is effectively simultaneous with the Big Bang itself.
IMO all SLS observations should be considered as coming from simultaneous SLS source events.
I don’t think Astrophysics can make most of its claims about expansion and large distance measurements without taking the Big Bang and its aftermath within just a short Million years as being a single simultaneous event.
DaleSpam
Oct28-08, 07:00 AM
In reality I only think it's a bit stubborn to deny the usefulness of using CMBR as a universal framePlease, don't misunderstand me. I am not denying its usefulness (at least for cosmology) as a convention. However your thread was about it as "true" time, not just its usefulness.
Using the CMBR as a universal reference frame would be akin to the international date line, or the fact that red lights mean "stop", or right handed coordinate systems. All of those are very useful conventions, but nothing would be fundamentally different if we had chosen and agreed upon different conventions.
If you had started your posts simply talking about the usefulness of the CMBR frame nobody would have objected. That is not what I was denying.
RandallB
Oct28-08, 09:44 AM
Using the CMBR as a universal reference frame would be akin to the international date line, or the fact that red lights mean "stop", or right handed coordinate systems. All of those are very useful conventions, but nothing would be fundamentally different if we had chosen and agreed upon different conventions.
I have to disagree,
picking a place for the dateline is an arbitrary choice we make from our POV to establish a useful convention. What other arbitrary choice can we make to serve as an equivalent but different perspective than what we get from the CBR? IMO there is nothing arbitrary about the CBR, it is a fact presented to us by nature. And if we are to believe our interpretation of nature as our universe having a single beginning with what we define as the Big Bang and the CBR as coming from the resulting SLS, it gives us as far as I know the only know instance of simultaneous events we can see form our local view that is “outside the box” of the SR- simultaneity rule.
That does not automatically say it will resolve to something as “Absolute” or “Universal”. But IMO it rates more than just thinking it was some arbitrary reference choice that we somehow had a hand in picking.
DaleSpam
Oct28-08, 11:17 AM
I have to disagree,
picking a place for the dateline is an arbitrary choice we make from our POV to establish a useful convention. What other arbitrary choice can we make to serve as an equivalent but different perspective than what we get from the CBR? IMO there is nothing arbitrary about the CBR, it is a fact presented to us by nature. And if we are to believe our interpretation of nature as our universe having a single beginning with what we define as the Big Bang and the CBR as coming from the resulting SLS, it gives us as far as I know the only know instance of simultaneous events we can see form our local view that is “outside the box” of the SR- simultaneity rule.
That does not automatically say it will resolve to something as “Absolute” or “Universal”. But IMO it rates more than just thinking it was some arbitrary reference choice that we somehow had a hand in picking.Do you think that any of the laws of physics will be different in the CMBR frame than in any other frame? If not then it is an arbitrary convention.
Certainly, if you live near a mountain or some other prominent geographical feature it would be convenient to measure coordinates relative to that feature which is as you say "presented to us by nature". But that does not make any other choice less valid, nor does it make that choice any more fundamental. For the purposes of defining distances and times the CMBR is the cosomlogical equivalent of an easily identifiable landmark.
Defining the CMBR as "at rest" is every bit as arbitrary as defining the solar system, our local group, or some other object "presented to us by nature" to be at rest. Or as arbitrary as defining any of those things to be moving at some specific velocity.
RandallB
Oct28-08, 11:54 AM
Do you think that any of the laws of physics will be different in the CMBR frame than in any other frame? If not then it is an arbitrary convention.
Certainly, if you live near a mountain or some other prominent geographical feature it would be convenient to measure coordinates relative to that feature which is as you say "presented to us by nature". But that does not make any other choice less valid, nor does it make that choice any more fundamental. For the purposes of defining distances and times the CMBR is the cosomlogical equivalent of an easily identifiable landmark.
Defining the CMBR as "at rest" is every bit as arbitrary as defining the solar system, our local group, or some other object "presented to us by nature" to be at rest. Or as arbitrary as defining any of those things to be moving at some specific velocity. I do not think how we understand Physics and the Laws that define it is “Complete” (if they were we would have unification forces and QM & GR). So, yes I think CBR might yet be a part of changes some of what we think we know. What and how exactly some law may change is speculation at this level, it is just my opinion the CBR could contribute more to helping complete our understanding of physics.
I understand picking one mountain as a reference point over a second, third, or other mountain option certainly cannot be thought of as a fundamental reference point.
But certainly, Astrophysics seems to regard the CBR as fundamental, and I’ve yet to see anyone point out a second option for the fundamental reference CBR provides them. Thus I find no grounds to support claiming their using it as fundamental was in fact an arbitrary choice.
Unless you include a completely different interpretation of, or completely reject, how they interpret the Big Bang; I don’t see where you can justify these fundamental assumptions of Astrophysics as being arbitrary.
But, you are entitled to your opinion. We just have differences here.
DaleSpam
Oct28-08, 04:36 PM
Is GR not the fundamental theory accepted by the Astrophysics/Cosmology community? If so then you are incorrect to claim that they regard the reference frame of the CMBR as fundamental. According to GR the rest frame of the CMBR is no different than a reference frame where it is moving at some arbitrary velocity.
RandallB
Oct28-08, 07:27 PM
Is GR not the fundamental theory accepted by the Astrophysics/Cosmology community? If so then you are incorrect to claim that they regard the reference frame of the CMBR as fundamental. According to GR the rest frame of the CMBR is no different than a reference frame where it is moving at some arbitrary velocity.
I would not call GR exclusively “THE FUNDAMENTAL THEORY” of Astrophysics/Cosmology. Just one of the fundamental theories Astrophysics/Cosmology uses. Much of the conclusions drawn by them depends on the chemistry in space which is based on the Standard Model and therefore the fundamental theory of QM. Meaning it accepts working with two theories that are fundamentally incompatible.
Now a significant part of what the Astrophysics/Cosmology community tells us about the history of our reality, especially as it relates to the sequence of events following the Big Bang up to when stars first began to form, is fundamentally based on the Big Bang including how that unique point of view cannot be replace by any arbitrarily chosen frame of reference.
To the extent that means they may well be working with three fundamental theories (by adding Big Bang CMR as something new) each in some respect incompatible with the others that is OK by me.
IMO your opinion requires rejecting the proposed reactions, interactions and formation of fundamental particles like quarks proceeding to the formation neutrons, protons, etc as not plausible.
Although that theory of fundamental particle and force development may not by 100% on the mark and complete; I consider to be largely correct and take to be very plausible. Thus I cannot reject the Big Bang or the uniqueness of a fundamental SLS benchmark as used in Astrophysics/Cosmology.
That remains my opinion. To the extend you can minimize the value of the Big Bang and the early history implied by it we just have different opinions. I only ask you give mine sincere consideration before rejecting it. However, unless you have something additional to offer to support it, I cannot adopt yours.
I would not call GR exclusively “THE FUNDAMENTAL THEORY” of Astrophysics/Cosmology. Just one of the fundamental theories Astrophysics/Cosmology uses. Much of the conclusions drawn by them depends on the chemistry in space which is based on the Standard Model and therefore the fundamental theory of QM. Meaning it accepts working with two theories that are fundamentally incompatible.
Now a significant part of what the Astrophysics/Cosmology community tells us about the history of our reality, especially as it relates to the sequence of events following the Big Bang up to when stars first began to form, is fundamentally based on the Big Bang including how that unique point of view cannot be replace by any arbitrarily chosen frame of reference.
To the extent that means they may well be working with three fundamental theories (by adding Big Bang CMR as something new) each in some respect incompatible with the others that is OK by me.
IMO your opinion requires rejecting the proposed reactions, interactions and formation of fundamental particles like quarks proceeding to the formation neutrons, protons, etc as not plausible.
Although that theory of fundamental particle and force development may not by 100% on the mark and complete; I consider to be largely correct and take to be very plausible. Thus I cannot reject the Big Bang or the uniqueness of a fundamental SLS benchmark as used in Astrophysics/Cosmology.
That remains my opinion. To the extend you can minimize the value of the Big Bang and the early history implied by it we just have different opinions. I only ask you give mine sincere consideration before rejecting it. However, unless you have something additional to offer to support it, I cannot adopt yours.
Please keep arguing! I'm learning as you reply to him! Haha Thanks.
russ_watters
Oct28-08, 10:13 PM
But certainly, Astrophysics seems to regard the CBR as fundamental, and I’ve yet to see anyone point out a second option for the fundamental reference CBR provides them. Could you explain what you mean by "fundamental" in this context?
Heck, is the CMB even really a reference frame? It is a reference for zeroing a velocity, but it has no intrinsic xyz coordinate reference, unlike the prime meridian, which is one axis of a two-axis coordinate system and stationary.
And the date line may be completely arbitrary, but the equator is not. The equator (and the poles) couldn't be arbitrarily located anywhere else. Does that have implications for your usage of the word "fundamental"?
DaleSpam
Oct29-08, 05:09 AM
I would not call GR exclusively “THE FUNDAMENTAL THEORY” of Astrophysics/Cosmology. Just one of the fundamental theories Astrophysics/Cosmology uses. Much of the conclusions drawn by them depends on the chemistry in space which is based on the Standard Model and therefore the fundamental theory of QM. Meaning it accepts working with two theories that are fundamentally incompatible.The principle of relativity is an integral part of both theories (GR and the SM). So I don't understand how you can make the leap from GR and the SM are fundamentally incompatible to the conclusion that one of the things that they do agree on is wrong. Neither of these two fundamental theories of astrophysics/cosmology support your claim that the rest frame of the CMBR would be anything more than a convenience.
The topic of this thread is simply whether the rest frame of the CMBR represents a "true" time or if it would merely be a convenient convention. Accepting the premise that it is anything other than a convenience is a rejection of the principle of relativity, which is certainly not supported by the theories used by the astrophysics community.
IMO your opinion requires rejecting the proposed reactions, interactions and formation of fundamental particles like quarks proceeding to the formation neutrons, protons, etc as not plausible. I don't know how you conclude that I reject the Big Bang or any of the fundamental particles. You are way off-base here.
RandallB
Oct29-08, 10:09 AM
The principle of relativity is an integral part of both theories (GR and the SM). I don’t understand this sentence, it offers nothing to this debate. It seems to say SM is a relativity theory but still calls it one of “both theories”. SM is fundamentally a QM theory that to the best that a QM theory can, it tries to respect the principle of relativity. But I know of no one that considers SR/GR and SM/QM as compatible, they are two independent view with fundamentally incompatible ideas of how gravity works. If we are not on the same page here then there we are having an argument (seems to delight Idgot) and I am not interested in dealing with augmentative rhetoric so lets try to this a rational debate. … how you can make the leap from GR and the SM are fundamentally incompatible to the conclusion that one of the things that they do agree on is wrong. Neither of these two fundamental theories of astrophysics/cosmology support your claim that the rest frame of the CMBR would be anything more than a convenience. You are repeating my point here. As we both say nether SR/GR or SM/QM can directly support or define the idea that the Big Bang and nearby events to it can be taken as a single simultaneous event relative to our view 14 Billion years later. And yet Astrophysics/Cosmology takes as a fundamental truth that the SLS events are simultaneous much like as Russ puts it, the equator or pole locations have a fundamental truth about where they are located and cannot be arbitrarily positioned anywhere on a spinning globe. The topic of this thread is simply whether the rest frame of the CMBR represents a "true" time or if it would merely be a convenient convention. Accepting the premise that it is anything other than a convenience is a rejection of the principle of relativity, which is certainly not supported by the theories used by the astrophysics community.
I don't know how you conclude that I reject the Big Bang or any of the fundamental particles. You are way off-base here. I didn’t say your claim would lead to rejecting the existence of any fundamental particles, I said it would reject how the astrophysics community explains they were created.
I understand the OP, what we a debating is your assertion that the OP idea should not even be considered because the astrophysics community use of the SLS was an “arbitrary choice”. My point is that astrophysics/cosmology use of SLS is not arbitrary but recognizes it as a fundamental thing that allows them to build the early history of how fundamental particles were made.
Now if all we have left is an argument about which opinion is better than this discussion was done a few posts ago – we have different opinions and I have no interest in arguing about it, that’s non-productive.
But if you have a debatably point that you think can change my opinion I’m willing to continue as a debate, but not an argument. For that you need to answer my question from post 56 :
“What other arbitrary choice can we make to serve as an equivalent but different perspective than what we get from the CBR?”
Include with that how this alternate “mountain” choice can be used by astrophysics/cosmology to build a new description that is:
A) Still identical to the currently defined Big Bang history.
or
B) Builds an alternative to the Big Bang and the history of particle creation it defines that is considered by anyone to be viable.
Unless there are such addition choices the way astrophysics/cosmology uses CBR & SLS is “Fundamental” and that is the extent of my point.
How or IF that might impact on ideas about “Absolute” or “Universal” time is a topic for Journal Papers. But I cannot deny anyone in astrophysics the right to consider it.
My point is that astrophysics/cosmology use of SLS is not arbitrary but recognizes it as a fundamental thing that allows them to build the early history of how fundamental particles were made.The SLS is calculated using GR (FLRW metric + GR --> Friedmann equations, Friedmann equations + CMBR --> SLS). How would you know that there was an SLS without GR?
The SLS is calculated using GR (FLRW metric + GR --> Friedmann equations, Friedmann equations + CMBR --> SLS). How would you know that there was an SLS without GR?
1. You use a shovel to unearth a buried rock.
2. You use a wet saw to cut that rock into pieces.
3. You use a microscope to study the pieces.
Maybe GR was the shovel here.
I don’t understand this sentence, it offers nothing to this debate. It seems to say SM is a relativity theory but still calls it one of “both theories”. SM is fundamentally a QM theory that to the best that a QM theory can, it tries to respect the principle of relativity. But I know of no one that considers SR/GR and SM/QM as compatible
Everyone recognizes that SM/QM are fundamentally not compatible with GR (one reason is that in quantum theories, the Planck scale should be associated with large energy fluctuations which in GR would themselves contribute significantly to the curvature of spacetime on those scales, but existing quantum theories assume a fixed background spacetime--see here (http://64.233.169.104/search?q=cache:96vSRqNGD6AJ:hyperphysics.phy-astr.gsu.edu/Hbase/quantum/string.html+%22conflict+between+the+uncertainty+pr inciple+and+general+relativity%22&hl=en&ct=clnk&cd=1&gl=us) for a discussion). However, the Standard Model and quantum field theories in general are both compatible with SR (which does not assume energy curves spacetime) because they are Lorentz-invariant theories. So, an absolute definition of simultaneity is every bit as incompatible with the Standard Model as it is with SR.
And yet Astrophysics/Cosmology takes as a fundamental truth that the SLS events are simultaneous much like as Russ puts it
No it doesn't. Where did you get the idea that cosmologists could not analyze the surface of last scattering in a coordinate system where different points on the surface are non-simultaneous? I'm sure it is most convenient to use a coordinate system where all points on this surface are assigned the same time-coordinate, but there are no fundamental physical considerations that forbid you from using other coordinate systems.
RandallB
Oct29-08, 04:31 PM
The SLS is calculated using GR (FLRW metric + GR --> Friedmann equations, Friedmann equations + CMBR --> SLS). How would you know that there was an SLS without GR? I don’t see your point. - - Did you think I’d denied SR/GR was used by astrophysics?
I think I was clear that they use both SR/GR and SM/QM.
My contention is that they are successfully using not only those two theories while they are technically incompatible (yet to be unified on the gravity issue) with each other. And my main point is that astrophysics use of SLS is not an arbitrary selection of a reference point. But that they fundamentally recognize the Big Bang and the SLS as we observe it as effectively from our POV sourced from simultaneous events. And they depend upon using all three things without allowing any one of them to invalidate either of the other two.
I suppose I should acknowledge they did not need modern CBR observations to develop this mixed view of reality. The Hubble expansion model modified how they applied GR (with was approved of by Einstein) that lead to the development of the big Bang Bang idea before the SLS was indentified. Prior to the SLS and CRB they were using the distant star background in much the same way they now define SLS.
And again:
I already asked what arbitrary selection of a reference point differing from a fundamental use of all SLS points coming from simultaneous events, would allow astrophysics to define the early Timeline of Cosmology as they have.
No one has provided one yet.
So the rest is just argumentative rhetoric, that will not change my opinion.
It is a legit question so unless someone cares to address it seriously I choose not to continue this further – it can only turn to pointless arguments and IMO that is not the purpose of PF.
But that they fundamentally recognize the Big Bang and the SLS as we observe it as effectively from our POV sourced from simultaneous events. And they depend upon using all three things without allowing any one of them to invalidate either of the other two.
Totally wrong. Again, GR allows you to use any coordinate system and still get the same predictions about all empirical observations like how the CMBR will look from our perspective--nothing would stop you from using a coordinate system where different points on the surface of last scattering were non-simultaneous, it wouldn't have any effect on predictions about how the CMBR looks.
I don’t see your point. - - Did you think I’d denied SR/GR was used by astrophysics?Do you want to consider the SLS as being outside of the context of GR? How would you know that there was an SLS without GR?
And again:
I already asked what arbitrary selection of a reference point differing from a fundamental use of all SLS points coming from simultaneous events, would allow astrophysics to define the early Timeline of Cosmology as they have. No one has provided one yet.
The definition of simultaneity in the FLRW metric, which comes from assuming that the universe is spatially homogeneous, is the only justification that I know of for taking all SLS points to be simultaneous. Do you know of another justification? We could easily change the FLRW metric around, compute modified versions of the Friedmann equations, and then show the SLS as something that looks quite different; no problem. The real questions are "what are dark matter and dark energy?", which are both required to project the CMBR back onto a SLS.
So the rest is just argumentative rhetoric, that will not change my opinion.
It is a legit question so unless someone cares to address it seriously I choose not to continue this further – it can only turn to pointless arguments and IMO that is not the purpose of PF.Can you show how to find/detect/model/describe the SLS without first assuming that the universe is spatially homogeneous? It isn't just argumentative rhetoric to point out that the concept of an (symmetric) SLS is based mathematically on a certain definition of simultaneity, and that you're limited by that definition unless you can describe the SLS without making that assumption.
We could easily change the FLRW metric around, compute modified versions of the Friedmann equations, and then show the SLS as something that looks quite different; no problem. The real questions are "what are dark matter and dark energy?", which are both required to project the CMBR back onto a SLS.For example, what if we wanted to simply get rid of "dark energy" and/or "dark matter" by modifying the FLRW metric to re-define its standard simultaneity in such a way that the CMBR projects onto a simultaneous big bang at t=0, but not onto a spatially homogeneous universe at t=370,000 years? The SLS would no longer be spherically symmetric then, but dark matter and/or dark energy would be gone.
For example, what if we wanted to simply get rid of "dark energy" and/or "dark matter" by modifying the FLRW metric to re-define its standard simultaneity in such a way that the CMBR projects onto a simultaneous big bang at t=0, but not onto a spatially homogeneous universe at t=370,000 years? The SLS would no longer be spherically symmetric then, but dark matter and/or dark energy would be gone.
What do you mean when you say the SLS is "spherically symmetric"? In terms of the FLRW model of the universe the SLS occupies all of space at a particular moment (according to the definition of simultaneity in the standard coordinate system where each surface of simultaneity is spatially homogenous)...are you just talking about the visual sphere with the Earth at the center and the CMBR as the outer surface (and objects being older the farther you go from the center) which constitutes the "observable universe"? If so, aren't visual appearances independent of your choice of coordinate system, since they are a consequence of how the worldlines of photons intersect our own worldline?
Also, what did you mean earlier when you said dark matter and dark energy "are both required to project the CMBR back onto a SLS"? Dark matter and dark energy are needed to explain the particular rate of expansion at different times as measured in the pattern of redshifts of distant objects, but the original FLRW model was invented before the idea of dark matter or dark energy and the idea of a "surface of last scattering" was around then too, it only depends on the idea that there was a moment when the temperature of the universe became low enough that free electrons were captured by protons to form hydrogen atoms.
DaleSpam
Oct30-08, 08:23 PM
Hi RandallB,
Sorry about the delay in my response. I think we may be having a simple miscommunication here. I will answer your question below, but I wanted to put what I think is the source of the miscommunication at the top.
My intent in this thread is simple: to argue in favor of the principle of relativity. If the principle of relativity applies to the CMBR rest frame then its time coordinate would only be "true" or "universal" by convention. From your comments I cannot tell if you believe that the principle of relativity applies to the rest frame of the CMBR. If you do then we simply have a miscommunication somewhere, if you do not then we have a genuine disagreement.
So, do you accept the principle of relativity in general, and specifically do you believe that it applies to the CMBR rest frame?
as Russ puts it, the equator or pole locations have a fundamental truth about where they are located and cannot be arbitrarily positioned anywhere on a spinning globe. The axis of rotation of the earth is a physical feature of the earth, but the choice to use that axis as the z-axis of a spherical coordinate system is a completely arbitrary choice. We could just as well define lattitude and longitude by magnetic north rather than true north. In such a coordinate system expressions for the position of the sun in the sky would be more complicated, but expressions for the direction of compass needles would be simplified. Using each set of coordinates you would obtain the same predictions for the results of any physical experiment.
I didn’t say your claim would lead to rejecting the existence of any fundamental particles, I said it would reject how the astrophysics community explains they were created.That is quite possible. Standard explanations are generally in terms of the conventions which are accepted by that community and which aid communication and understanding within the community. That is not to say that the explanation is incorrect in any way or that the conventions are in any way unreasonable, but it is simply a recognition of the conventions for what they are.
“What other arbitrary choice can we make to serve as an equivalent but different perspective than what we get from the CBR?”
Include with that how this alternate “mountain” choice can be used by astrophysics/cosmology to build a new description that is:
A) Still identical to the currently defined Big Bang history.
or
B) Builds an alternative to the Big Bang and the history of particle creation it defines that is considered by anyone to be viable.
Unless there are such addition choices the way astrophysics/cosmology uses CBR & SLS is “Fundamental” and that is the extent of my point.Under the principle of relativity any other coordinate system would be just as valid, e.g. a coordinate system where the CMBR is moving at 600 km/s (and our local group is at rest). In order to ensure that the new coordinate system makes the same predictions, you can simply take the standard metric (FLRW?), do a change of variables so that our local group is at rest and the CMBR is moving, and thus obtain a new coordinate system and a new metric describing the same spacetime. This coordinate system would be every bit as valid a description of the universe as the conventional one, and they would agree on all observations, but in the new system the CMBR would not be at rest.
What do you mean when you say the SLS is "spherically symmetric"? In terms of the FLRW model of the universe the SLS occupies all of space at a particular moment (according to the definition of simultaneity in the standard coordinate system where each surface of simultaneity is spatially homogenous)...are you just talking about the visual sphere with the Earth at the center and the CMBR as the outer surface (and objects being older the farther you go from the center) which constitutes the "observable universe"? If so, aren't visual appearances independent of your choice of coordinate system, since they are a consequence of how the worldlines of photons intersect our own worldline?I mean that a hypersphere of homogeneity is spherically symmetric.
Also, what did you mean earlier when you said dark matter and dark energy "are both required to project the CMBR back onto a SLS"?To actually project the CMBR anisotropies back onto a SLS, you need to model the expansion history of the universe. The SLS is just one point on that timeline. Dark matter and dark energy are required to generate an expansion history for an FLRW/Friedmann universe that is consistent with the CMBR anisotropies.
Dark matter and dark energy are needed to explain the particular rate of expansion at different times as measured in the pattern of redshifts of distant objects, but the original FLRW model was invented before the idea of dark matter or dark energy and the idea of a "surface of last scattering" was around then too, it only depends on the idea that there was a moment when the temperature of the universe became low enough that free electrons were captured by protons to form hydrogen atoms.To actually find an expansion history for an FLRW/Friedmann universe that is consistent with the CMBR anisotropies, dark matter and dark energy is required. This is usually done using the Friedmann equations, and the dark matter and dark energy denity are just parameters added in addition to normal matter density.
If, for example, we were to modify the Friedmann equations as needed to generate the same expansion history without dark matter and/or dark energy, then maybe we could reverse the GR procedures that are used to get the Friedmann equations from the FLRW metric, and show a metric that is consistent with the CMBR anisotropies without dark energy and/or dark matter. At least that may serve as a good example of why the standard simultaneity of the FLRW metric and SLS are not fundamental physical things.
I mean that a hypersphere of homogeneity is spherically symmetric.
So you're specifically referring to a spatially finite universe with positive curvature? The evidence suggests the spatial curvature is very close to flat, although I think it's possible space could have very slight positive curvature, too small to be seen with current observations.
To actually project the CMBR anisotropies back onto a SLS, you need to model the expansion history of the universe. The SLS is just one point on that timeline. Dark matter and dark energy are required to generate an expansion history for an FLRW/Friedmann universe that is consistent with the CMBR anisotropies.
OK, that makes sense--I was confused because you didn't specify that you were talking about the observed CMBR anisotropies, as opposed to just the existence of the CMBR in general.
So you're specifically referring to a spatially finite universe with positive curvature?No, I am specifically referring to FLRW space at any given time.
The evidence suggests the spatial curvature is very close to flat, although I think it's possible space could have very slight positive curvature, too small to be seen with current observations.I do not mean to imply that there is spatial curvature necessarily. I am only trying to show an alternative to standard FLRW space at any given time.
No, I am specifically referring to FLRW space at any given time.
But then why did you refer to a hypersphere? In a spatially flat universe, using the standard choice of coordinate system a hypersurface of simultaneity would be be an infinite hyperplane (i.e. infinite 3D Euclidean space), and in a universe with negative curvature it'd be an infinite hyperbolic space. Either way, the SLS would fill all of space in a hypersurface of simultaneity, it wouldn't be a finite hypersphere.
But then why did you refer to a hypersphere? In a spatially flat universe, using the standard choice of coordinate system a hypersurface of simultaneity would be an infinite hyperplane (i.e. infinite 3D Euclidean space)...the SLS would fill all of space in a hypersurface of simultaneity, it wouldn't be a finite hypersphere.We can only see out to our own particle horizon which currently has a radius of about \eta_0 \cdot c = 46 Glyr. It is the difference between this distance and about t_0 \cdot c = 13.7 Glyr which is accounted for in the computation of expansion histories by means of inserting dark energy and dark matter into the Friedmann equations. This may imply the use of a slightly different choice, though still a standard choice, of coordinate system than "the" standard choice that you are thinking of. Nevertheless, what I am referring to as hyperspheres are spacelike slices of the observable universe which have a comoving radius of \eta \cdot c. These are not infinite hyperplanes, though I suppose that they might be considered as the observable piece of an infinite hyperplane.
...are you just talking about the visual sphere with the Earth at the center and the CMBR as the outer surface (and objects being older the farther you go from the center) which constitutes the "observable universe"?Yes, except that when we generate an expansion history it is well defined almost all the way down to t=0 (the SLS is at around t=370,000 years).
If so, aren't visual appearances independent of your choice of coordinate system, since they are a consequence of how the worldlines of photons intersect our own worldline?Yes, but how we parameterize the Friedmann equations in order to generate an expansion history that is consistent with visual appearances today, an SLS at t=370,000 years, nucleosynthesis at t<a few minutes, and a big bang at t=0 is not independent of our choice of coordinate system.
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