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No definite viewpoint for the accelerating traveler?

by Alain2.7183
Tags: accelerating, definite, traveler, viewpoint
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jbriggs444
#19
Feb21-13, 08:26 PM
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Quote Quote by PAllen View Post
Multiple mapping is prohibited for coordinates by definition. Do you really think it makes sense to say that NYC exploded at 3pm on my watch and also at 4PM on my watch, even though I only see it explode once, and no observation I can make is consistent with it being simultaneous to two points on my world line?
Yes, I do think it makes sense to say that NYC exploded at two distinct coordinates. And you are incorrect about this giving rise to inconsistencies. It's just a coordinate. It's not an observable.
PAllen
#20
Feb21-13, 09:03 PM
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Quote Quote by jbriggs444 View Post
Yes, I do think it makes sense to say that NYC exploded at two distinct coordinates. And you are incorrect about this giving rise to inconsistencies. It's just a coordinate. It's not an observable.
There are accepted definitions of coordinates. They label a point on a manifold (physically, an event) once. Thus, such a system is not a coordinate system.

Tell me, why would I want to use such a non-coordinate system reflecting a non-observables in a way the does lead to nonsense that has no observable or logical basis? Instead, I can use any valid coordinate system to compute any observable, and conceptually model reality in a consistent way.

[edit: Why absurd? I know that NYC blows up once; every possible observation I can make indicates it blows up once; I have a plethora of valid coordinates consistent with SR that model it blowing up once. Why should I choose a method that constructs mathematically invalid coordinates and models it as blowing up at two different times of my history? Really??]
jbriggs444
#21
Feb22-13, 08:33 AM
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Quote Quote by PAllen View Post
There are accepted definitions of coordinates. They label a point on a manifold (physically, an event) once. Thus, such a system is not a coordinate system.
Not all definitions of coordinates require that the things being labelled be manifolds. Not all "coordinate systems" on a manifold need be bijective, topology-preserving or even cartesian.

That said, I can certainly understand how you would want those properties to hold for useful coordinate systems. And I can understand that you might want to adopt terminology requiring this to be the case.

Tell me, why would I want to use such a non-coordinate system reflecting a non-observables in a way the does lead to nonsense that has no observable or logical basis? Instead, I can use any valid coordinate system to compute any observable, and conceptually model reality in a consistent way.
I can use polar coordinates to refer to the north pole without worrying overmuch about nonsense ensuing. Mind you I agree that using such coordinates to label the north pole is one thing. Using them to model physics at the north pole would be more difficult.

Topology is not my strong suit, but what I think you are saying is that you want a "mathematically valid" coordinate system to be one that embodies a homeomorphism between a manifold and cartesian n-space.

Any coordinate system which assigns multiple coordinates to the same point cannot (of course) be a homeomorphism because it fails to be a bijection.

[edit: Why absurd? I know that NYC blows up once; every possible observation I can make indicates it blows up once; I have a plethora of valid coordinates consistent with SR that model it blowing up once. Why should I choose a method that constructs mathematically invalid coordinates and models it as blowing up at two different times of my history? Really??]
The question you posed did not ask whether you should use a coordinate system that happens to have multiple coordinates for a single event. You asked whether I thought that it would make sense. I think that it does make sense. It's not an example that lends itself easily to a coordinate system that labels the same event twice, but one can contrive a labelling that does so.

Suppose that I am driving east when the NYC blows up. Let's say that it blows up at 2:30 pm EST. I glance at my clock and see that it reads 1:30 pm CST. But I am not paying careful attention and don't know whether I've crossed the time zone line yet.

I can label the NYC blow up at both 1:30 or 2:30 using "my personal time zone" coordinates. This does not entail that NYC blew up twice.
Nugatory
#22
Feb22-13, 08:42 AM
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Quote Quote by jbriggs444 View Post
I can label the NYC blow up at both 1:30 or 2:30 using "my personal time zone" coordinates.
Do you reset your watch as you're driving? If you do, you're changing from one coordinate system to another. If you don't, there's only one label you can assign to the blowup event, and that's the time on your watch when the blowup happens.
jbriggs444
#23
Feb22-13, 08:49 AM
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Quote Quote by Nugatory View Post
Do you reset your watch as you're driving? If you do, you're changing from one coordinate system to another. If you don't, there's only one label you can assign to the blowup event, and that's the time on your watch when the blowup happens.
I have one hour of coordinate values to play with. If I choose to use them to double-label events, that's my business, not yours.
PAllen
#24
Feb22-13, 09:00 AM
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Quote Quote by jbriggs444 View Post



The question you posed did not ask whether you should use a coordinate system that happens to have multiple coordinates for a single event. You asked whether I thought that it would make sense. I think that it does make sense. It's not an example that lends itself easily to a coordinate system that labels the same event twice, but one can contrive a labelling that does so.
Why do you think it makes sense to use a model that has a feature that is counter-factual to all observations, especially what there are a plethora of lmodels consistent with both observations and SR to choose from?

As for definitions, the following is one common definition of coordinates:

"In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element on a manifold such as Euclidean space"

To a mathematician, the pole in polar coordinates is not covered by the coordinate system. In fact, this feature, in the case of a sphere, is the quintessential example used to show that there are manifolds such that no single coordinate system (patch) can cover the whole object.
Nugatory
#25
Feb22-13, 09:10 AM
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Quote Quote by jbriggs444 View Post
I have one hour of coordinate values to play with. If I choose to use them to double-label events, that's my business, not yours.
As long as you don't call them "coordinates", sure. You can even call them "coordinates" if you want, but....

Q: If we call the tail a leg, how many legs does a horse have?
A: Four. Calling a tail a leg doesn't make it a leg.
WannabeNewton
#26
Feb22-13, 09:12 AM
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Quote Quote by jbriggs444 View Post
Yes, I do think it makes sense to say that NYC exploded at two distinct coordinates. And you are incorrect about this giving rise to inconsistencies. It's just a coordinate. It's not an observable.
You are being quite rude with your later posts to PAllen for no reason at all as he is correcting a very absurd statement you are making. A coordinate map is an n - tuple of coordinate functions on an open subset of the manifold representing space - time. You are claiming the coordinate map can take a point in this region and map it to two different coordinates (the events that characterize the space - time aspect of the manifold for a given family of observers). This is not even a problem of physics, you are going against the very definition of a function which is a basic set theoretical object.
DaleSpam
#27
Feb22-13, 09:31 AM
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Quote Quote by jbriggs444 View Post
Not all definitions of coordinates require that the things being labelled be manifolds. Not all "coordinate systems" on a manifold need be bijective
Those used in GR do. If you disagree, please provide a good reference supporting the use of non-bijective coordinates in GR.

See ch 2. (especially around p. 34-37): http://arxiv.org/abs/gr-qc/9712019
jbriggs444
#28
Feb22-13, 09:36 AM
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Quote Quote by PAllen View Post
Why do you think it makes sense to use a model that has a feature that is counter-factual to all observations, especially what there are a plethora of lmodels consistent with both observations and SR to choose from?
Regarded as a mere labelling of events, it is not counter-factual.

As for definitions, the following is one common definition of coordinates:

"In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element on a manifold such as Euclidean space"
As I read that definition, it requires that for every coordinate there is exactly one point. i.e. that the coordinate mapping be a function from coordinates to points, but not neccessarily either an injection (mapping to any given point by at most one coordinate) or a surjection (mapping to every point by at least one coordinate)

On the same page that you reference, one sees the following:

"Schemes for locating points in a given space by means of numerical quantities specified with respect to some frame of reference. These quantities are the coordinates of a point. To each set of coordinates there corresponds just one point in any coordinate system, but there are useful coordinate systems in which to a given point there may correspond more than one set of coordinates"

Emphasis mine.
WannabeNewton
#29
Feb22-13, 09:37 AM
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Quote Quote by jbriggs444 View Post
Not all definitions of coordinates require that the things being labelled be manifolds. Not all "coordinate systems" on a manifold need be bijective, topology-preserving...
Where exactly did you learn about coordinate charts and manifolds? You have all the wrong ideas about topological manifolds.
WannabeNewton
#30
Feb22-13, 09:41 AM
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Quote Quote by jbriggs444 View Post
Regarded as a mere labelling of events, it is not counter-factual.
It is utter nonsense.

Quote Quote by jbriggs444 View Post
As I read that definition, it requires that for every coordinate there is exactly one point. i.e. that the coordinate mapping be a function from coordinates to points, but not neccessarily either an injection (mapping to any given point by at most one coordinate) or a surjection (mapping to every point by at least one coordinate)
More nonsense. Let [itex]M[/itex] be a topological manifold. [itex]\forall p\in M[/itex] there exists a coordinate chart [itex](U,\varphi )[/itex] where [itex]U\subseteq M[/itex] is a neighborhood of [itex]p[/itex] and [itex]\varphi :U\rightarrow \mathbb{R}^{n}[/itex] is a homeomorphism.
DaleSpam
#31
Feb22-13, 09:41 AM
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Quote Quote by jbriggs444 View Post
On the same page that you reference, one sees the following:

"Schemes for locating points in a given space by means of numerical quantities specified with respect to some frame of reference. These quantities are the coordinates of a point. To each set of coordinates there corresponds just one point in any coordinate system, but there are useful coordinate systems in which to a given point there may correspond more than one set of coordinates"

Emphasis mine.
These are not valid coordinate charts in GR. Other disciplines may make use of such coordinates, but not GR.
jbriggs444
#32
Feb22-13, 10:05 AM
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Quote Quote by WannabeNewton View Post
You are being quite rude with your later posts to PAllen for no reason at all as he is correcting a very absurd statement you are making. A coordinate map is an n - tuple of coordinate functions on an open subset of the manifold representing space - time. You are claiming the coordinate map can take a point in this region and map it to two different coordinates (the events that characterize the space - time aspect of the manifold for a given family of observers). This is not even a problem of physics, you are going against the very definition of a function which is a basic set theoretical object.
Rather than continue this distraction, I will bow out, acknkowledging that I am at the very least using standard terminology incorrectly for purposes of this context.
WannabeNewton
#33
Feb22-13, 10:10 AM
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A function need not be injective mate but it cannot map a value in the domain to two values in the range which is what you are doing by assigning two different events to the same point on the manifold for a single observer.
Alain2.7183
#34
Mar3-13, 04:01 PM
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I've been looking at a lot of the posts that Mentz recommended to me in another thread:

Quote Quote by Mentz114 View Post

Quote Quote by Alain2.7183 View Post
What is CADO?
See this topic

http://www.physicsforums.com/showthread.php?t=490163
I also did a forum search on "CADO", and found some more recent posts, including this one:

http://www.physicsforums.com/showpos...&postcount=287

That post showed what someone who is going around and around in a circle, at constant speed, would say is the current age of some inertial person who is located some distance away from the circle. That post was interesting to me, because it is very similar to an example that I saw in a NOVA program called "the fabric of the cosmos". There, Brian Greene gave an example of someone on a planet in an extremely distant galaxy, who is riding a bicycle around and around in a small circle, and who says that for each of his loops, the time here on earth is swinging back and forth over centuries! Brian Greene got that result by using the spatial three-dimensional "simultaneous time slices" of the sequence of inertial frames that are momentarily co-moving with the bicycle rider. Brian also has essentially the same example in his book that has the same title as the NOVA show.

As far as I can recall, in both the TV show and in his book, Brian didn't seem to be presenting his method as "just one of several different possible answers" to the question of "What is the current age of the inertial person, according to the accelerating person?". My impression was that he seemed to present it as THE answer.

On my CADO forum search, I also found this link,

https://sites.google.com/site/cadoeq...eference-frame

that seems to give a pretty good summary of all the CADO stuff. The CADO equation explained in there gives the same result as Brian Greene's "momentarily co-moving inertial frame" method, but it's quicker and easier.
PAllen
#35
Mar3-13, 05:30 PM
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Quote Quote by Alain2.7183 View Post

As far as I can recall, in both the TV show and in his book, Brian didn't seem to be presenting his method as "just one of several different possible answers" to the question of "What is the current age of the inertial person, according to the accelerating person?". My impression was that he seemed to present it as THE answer.
This is Brian Greene in a pop-sci setting. Further, you are probably reading more into than Greene intended - I doubt he explicitly said this is the one valid way to look at things. A generic issue with pop-sci is that you select one simple, WOW point of view to describe about a more complex, multi-faceted situation. Also, if you want to argue by reference to authority, I could respond my noting that never in Einstein's life did he use such lines of simultaneity in either SR or GR.
DaleSpam
#36
Mar3-13, 06:35 PM
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Quote Quote by Alain2.7183 View Post
Brian didn't seem to be presenting his method as "just one of several different possible answers" to the question of "What is the current age of the inertial person, according to the accelerating person?". My impression was that he seemed to present it as THE answer.
THE answer to what question?

If the question is "how can you do physics in a non-inertial frame" then it is not even an answer let alone THE answer. If the question is "what is the naive non-inertial simultaneity convention which most novices tend to adopt and which is useless for any actual physics" then I would agree that it is probably THE answer.


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