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Author Topic: Relative Simultaneity  (Read 24000 times)

PhysBang

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Re: Relative Simultaneity
« Reply #75 on: 27/02/2015 01:18:47 »
The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.

Arguably the most common measurement of velocity is that of the speed of a moving vehicle. This measurement is almost never done from the reference frame in which the vehicle is at rest, nor from the reference frame of some observer within the vehicle. Yet we do not think that these measurements are without merit.

When we speak of a reference frame (in the context of this discussion), we speak of a system of coordinates. I can use kilometers as my metric or I can use miles. I can use hours or I can use some other measurement of time. I can use one set of axes or I can use another. I can use Cartesian coordinates or polar or spherical...

So even considering those reference frames in which I am at rest, there are an infinite number of such frames.

PmbPhy

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Re: Relative Simultaneity
« Reply #76 on: 27/02/2015 01:58:16 »
Quote from: PhysBang
.. he would rather ignore the definition ...
Why are you saying that and why are you saying it in that way (i.e. quite rude)? You're not a mind reader you know. I didn't ignore "the" definition? In fact I used the actual correct definition that is defined and used by the relativity community. As for you claim, what you believe to be a definition doesn't merely become one just because you want it to be and then pronounce that you're correct you know.

Quote from: PhysBang
and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.
Hmmm. I didn't realize that you were that rude of a poster when someone corrects you or you disagree with a member. Since this doesn't seem to be something that's going to change I will no longer respond to any of your comments to come since I'm not in the mood for rude people/comments.

Goodbye!!

PhysBang

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Re: Relative Simultaneity
« Reply #77 on: 27/02/2015 02:13:52 »
Dude, if you are going to post a definition that you then ignore, then fine by me if you're going to ignore me too.

Ethos_

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Re: Relative Simultaneity
« Reply #78 on: 27/02/2015 03:00:02 »
When speaking about frames of reference, one needs to remember the difference between identifying local coordinates of space and time and referring to Einsteinian relativity. Einsteinian frames of reference apply to observational references between a moving observer and the phenomenon or phenomena under observation.

I think what Pete is referring to are Einsteinian frames of reference and not just reference frames associated with coordinates of local space and time.

The following text is directly from Wikipedia:

In Einsteinian relativity, reference frames are used to specify the relationship between a moving observer and the phenomenon or phenomena under observation. In this context, the phrase often becomes "observational frame of reference" (or "observational reference frame"), which implies that the observer is at rest in the frame, although not necessarily located at its origin. A relativistic reference frame includes (or implies) the coordinate time, which does not correspond across different frames moving relatively to each other.
« Last Edit: 27/02/2015 03:35:01 by Ethos_ »

jeffreyH

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Re: Relative Simultaneity
« Reply #79 on: 27/02/2015 03:11:23 »
The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.

Arguably the most common measurement of velocity is that of the speed of a moving vehicle. This measurement is almost never done from the reference frame in which the vehicle is at rest, nor from the reference frame of some observer within the vehicle. Yet we do not think that these measurements are without merit.

When we speak of a reference frame (in the context of this discussion), we speak of a system of coordinates. I can use kilometers as my metric or I can use miles. I can use hours or I can use some other measurement of time. I can use one set of axes or I can use another. I can use Cartesian coordinates or polar or spherical...

So even considering those reference frames in which I am at rest, there are an infinite number of such frames.

I have reviewed Pete's posts and never once saw him tie an observer to a reference frame.

PmbPhy

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Re: Relative Simultaneity
« Reply #80 on: 27/02/2015 04:26:32 »
Quote from: PhysBang
..., if you are going to post a definition that you then ignore,....
Wrong. False accusations and attempts at mind reading like these are why I'm ignoring you.
« Last Edit: 27/02/2015 04:29:46 by PmbPhy »

PhysBang

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Re: Relative Simultaneity
« Reply #81 on: 27/02/2015 13:11:10 »
PmbPhy is actually not using reference frames as special relativity used them, even though Einstein introduced much of the use of talking about observers that ended up being misleading. And even though the wikipedia article uses the phrase "observational frame of reference", it does not do so in the manner in which one identifies one and only one reference frame for an observer. The term is defined as a short-hand way to speak of a reference frame in which an observer is at rest (hopefully when this term is used both the observer and the frame are defined before using the term). The article also suggests that the term can be used to speak of "an entire family of coordinate systems", i.e., that one can identify a set of reference frames in which the observer identified is at rest.

As I said, and as the definition that PmbPhy pasted from wikipedia says, a reference frames is "a coordinate system used to represent and measure properties of objects, such as their position and orientation, at different moments of time." This is the sense in which reference frame is used in special relativity, which seems to be the context of this thread.

As such, we can, for any object--whether the object can make observations or not--identify an infinite number of reference frames for which that object is at rest, merely by arbitrary shifts along any axis or by rotation of axes, or by choice of alternate form of axes (e.g. Cartesian vs. polar).

Where I definitely part ways with the wikipedia article is in holding that one can describe observations only about the properties of one's own frame. This is simply not the case. One can make observations only of events that one can observe, but one can use those events to describe how those observations appear in any frame. Every day, millions of people determine their own speed as non-zero relative to a reference frame tangential to the surface of the Earth when they drive a car and they look at their spedometer. The core of special relativity (and GR as well) is that one can use what one observes to determine the series of events in any reference frame.

David Cooper

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Re: Relative Simultaneity
« Reply #82 on: 27/02/2015 17:55:23 »
This is just another row about language and meaning, so we're arguing at cross purposes as usual. I can now see why PhysBang says there is an infinite number of frames of reference in which the same object is at rest, but I've always thought of them all being the same frame (because the coordinate system imposed on it is arbitrary and not a physical reality). Both approaches are valid.

Instead of arguing about which usages of terms are right or wrong, it's better either to agree on specific definitions to be used within a particular conversation or to assume different definitions are being used by different individuals and always interpret them the way those people do when you are reading their words rather than misunderstanding them by imposing your own definition on what they're saying. That way, the conversation can focus on the actual meat of the issue instead of being diverted into pointless wars over which definitions are officially correct. There are too many people falling out over such trivial matters. Different people acquire their knowledge from different sources and end up using the same or similar terms while understanding them quite differently from each other. Ideally everyone would use the same definitions, but standard definitions don't appear to have been established strongly enough to be universal, and that means we all have to work hard to make sure we are in sync with each other before we write people off as stupid and start being rude to them. I'm not aiming this at any particular person, but at all of us, and myself included. We can all do better.

Ethos_

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Re: Relative Simultaneity
« Reply #83 on: 27/02/2015 18:37:59 »
We can all do better.
I agree completely Dave. These disagreements are becoming very tiresome and I suggest we all just take a deep breath and try to place our efforts toward congeniality instead of personal ego.

Bill S

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Re: Relative Simultaneity
« Reply #84 on: 27/02/2015 20:03:35 »
Pete, I owe you an apology!  In #65 you asked me a question which I have not answered.  I'll rectify that now.

In #53 you made the point that a frame of reference is not synonymous with a location.  You also linked to Wiki. It was following that link that led me to wonder:

If, “in physics, a frame of reference (or reference frame) may refer to a coordinate system……”, and if: “In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point….”; could the following argument be made.

1.  A frame of reference refers to a coordinate system.

2.  A coordinate system determines a position.

3.  A position is synonymous with a location.

4.  A frame of reference may, therefore, be synonymous with a location, at least in certain circumstances.

PmbPhy

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Re: Relative Simultaneity
« Reply #85 on: 27/02/2015 22:51:39 »
Quote from: Bill S
1.  A frame of reference refers to a coordinate system.

2.  A coordinate system determines a position.

3.  A position is synonymous with a location.

4.  A frame of reference may, therefore, be synonymous with a location, at least in certain circumstances.
No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".

jeffreyH

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Re: Relative Simultaneity
« Reply #86 on: 27/02/2015 23:09:08 »
You can think of the location of either an observer or an object being observed but not the frame, but only if they are at rest in the frame under consideration. I made the remark that a frame of reference can be thought of as an infinitesimal point in spacetime as any point adjacent to it could be different by some infinitesimal amount. Pete rightly jumped on that and corrected me. I was just showing how absurdly detailed you can be if pedantic enough. We are mainly considering either particles or macroscopic objects when using frames of reference. A moving object has no definite position over time.

PmbPhy

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Re: Relative Simultaneity
« Reply #87 on: 28/02/2015 00:43:04 »
Quote from: PmbPhy
There's no such frame as "The frame where the event occurred." An event exists independent of a frame of reference. For example: consider the event: "Fire cracker exploded."  There is no unique frame which can be said that this event occurred in that frame. The event occurred in "all" frames of reference, inertial or otherwise.
I'd like to clarify something. This comment is referring to globally inertial frames. Not frames of reference in GR where the spacetime is curved. When I said that there is no unique frame I had something specific in mind.

When one speaking of problems in special relativity (SR) the one is talking about spacetimes which are flat (i.e. zero spacetime curvature). So when someone is working an SR problem and mentions two or more inertial frames, i.e. S, S', S", etc. which may or may not be in relative motion then its assumed that they're speaking about the same region of spacetime, not two totally disconnected regions of spacetime.

I should have made that clear. But this is what I had in mind when I said what I did, especially since the context in which you were speaking indicated that those inertial frames were in the same spacetime.

PmbPhy

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Re: Relative Simultaneity
« Reply #88 on: 28/02/2015 00:49:08 »
Quote from: PhysBang
PmbPhy is actually not using reference frames as special relativity used them, ...
See? More nonsense/bogus claims. In the first case nowhere did I post anything regarding frames of reference which makes your claim true. Also regarding the definition of "frame of reference" I listed the Wikipedia site giving the definition since that encyclopedia since it has a high track record of being correct. I know what an inertial frame of reference is since I've been a relativist for 15 to 20 years and know what I'm talking about. Look in Exploring Black Holes by Edwin F. Taylor and John A. Wheeler and read the glossary of terms and you'll see that I wrote it, and it contains that term (which in that text is called "free-float frame"). So you have absolutely NO justification for making such an accusation.

Bill S

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Re: Relative Simultaneity
« Reply #89 on: 28/02/2015 01:39:27 »
Quote from: Pete
No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".

Thanks Pete, that's clearer.

PmbPhy

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Re: Relative Simultaneity
« Reply #90 on: 28/02/2015 02:33:48 »
Quote from: Pete
No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".

Thanks Pete, that's clearer.
Hi Bill,

I just found a definition that I really like. It's from A First Course in Special Relativity - Second Edition by Bernard Schutz, page 2.
Quote
It is important to realize that an 'observer' is in fact a huge information-gathering system, not simply one man with binoculars. In fact, we shall remove the human element entirely from our definition, and say that an inertial observer is simply a coordinate system for spacetime, which makes an observation simply by recording the location (x, y, z) and time (t) of an event. This coordinate system must satisfy the following three properties to be called inertial:

(1) The distance between point P1 (coordinates x1, y1, z1) and P2 (coordinates x2, y2, z2).

(2) The clocks that sit at every event point ticking off ticking off the time coordinate t are synchronized and all run at the same rate.

(3) The geometry of space at any constant time t is Euclidean.

Notice that this definition does not mention whether the observer accelerates or not. ... It will turn out that only an unaccelerated observer can keep his clocks synchronized.

PmbPhy

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Re: Relative Simultaneity
« Reply #91 on: 28/02/2015 03:17:07 »
Quote from: phyti39
To you, but are you speaking for everyone?
Not simply to me. This is how it is to the entire physics community. I've been a relativist for some 15-20 years and know how my colleagues think and what they mean when they speak or when I read their private communications to me or their publications.

Keep in mind that the context from what you're talking about means that you're talking about special relativity and in special relativity what I say is correct. And I've been tutoring physics long enough to know what is and what isn't confusing. Where did you get the impression that
Quote
A frame of ref. is merely a location common to a set of measurements...
is true or meaningful?

The term "location" is defined as
http://www.merriam-webster.com/dictionary/location
Quote
: a place or position

: a place outside a studio where a movie is filmed

: the act of finding where something or someone is : the act of locating something or someone
From the context of that post you used it as the first meaning, i.e. "a place or position". A coordinate system is a key part of the definition of a frame of reference. A coordinate system is not a place or position since for something to fit that definition it must be finite in size, which a coordinate system is not.

If you go on to study special relativity then part of that is the example of the twin paradox. Part of that example to have two planets which are many light years apart. In that example the two planets are said to be in the same frame of reference. Its implicit in the physics and the mathematics.

PhysBang

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Re: Relative Simultaneity
« Reply #92 on: 28/02/2015 14:10:33 »
Quote from: PhysBang
PmbPhy is actually not using reference frames as special relativity used them, ...
See? More nonsense/bogus claims. In the first case nowhere did I post anything regarding frames of reference which makes your claim true.
RiiiiIIIIiiiight.

So first you "stongly disagree" with the idea that there is not one single reference frame for every observer. Then you cling to this idea despite pasting a definition that indicates that this is false.

Quote
Also regarding the definition of "frame of reference" I listed the Wikipedia site giving the definition since that encyclopedia since it has a high track record of being correct.
Yes, but you didn't really use the definition you cut and pasted from that.

Quote
I know what an inertial frame of reference is since I've been a relativist for 15 to 20 years and know what I'm talking about. Look in Exploring Black Holes by Edwin F. Taylor and John A. Wheeler and read the glossary of terms and you'll see that I wrote it, and it contains that term (which in that text is called "free-float frame").
First, you can't say that a text contains a term and then say that it uses a different set of words for that term. Second, there are frames of reference and then there are a sub-class of all frames of reference where we identify that the object of which we are interested is freely falling or floating. So that is a significant difference in definition.

PhysBang

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Re: Relative Simultaneity
« Reply #93 on: 28/02/2015 14:11:58 »
Quote from: Pete
No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".

Thanks Pete, that's clearer.
Hi Bill,

I just found a definition that I really like. It's from A First Course in Special Relativity - Second Edition by Bernard Schutz, page 2.
Quote
It is important to realize that an 'observer' is in fact a huge information-gathering system, not simply one man with binoculars. In fact, we shall remove the human element entirely from our definition, and say that an inertial observer is simply a coordinate system for spacetime, which makes an observation simply by recording the location (x, y, z) and time (t) of an event. This coordinate system must satisfy the following three properties to be called inertial:

(1) The distance between point P1 (coordinates x1, y1, z1) and P2 (coordinates x2, y2, z2).

(2) The clocks that sit at every event point ticking off ticking off the time coordinate t are synchronized and all run at the same rate.

(3) The geometry of space at any constant time t is Euclidean.

Notice that this definition does not mention whether the observer accelerates or not. ... It will turn out that only an unaccelerated observer can keep his clocks synchronized.
This is exactly the point I was getting at. It is misleading to cling to talk of observers or claim that observers are wedded to reference frames; there is a much better way to think of them and use them.

phyti39

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Re: Relative Simultaneity
« Reply #94 on: 01/03/2015 19:05:27 »
PhysBang #75
Quote
The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.

A proxy such as a video device can be used to collect image data (space probe, telescope, etc.), but ultimately the data only has meaning to the human observer/s, who designed and implemented the experiment.
Einstein's definition of a 3-axis Cartesian coordinate system and a clock, as a system of measurement, works just as well today as in 1905, when he published it. His later examples of the train car and the embankment demonstrated that either location  served as a reliable reference frame, for measurements, but when there was relative motion, the observers would record different values for distance and time.

phyti39

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Re: Relative Simultaneity
« Reply #95 on: 01/03/2015 19:10:13 »
PmbPhy #91
Quote
Not simply to me. This is how it is to the entire physics community. I've been a relativist for some 15-20 years and know how my colleagues think and what they mean when they speak or when I read their private communications to me or their publications.

In reading books and papers by various authors involved in physics, I don't find a unanimous agreement about everything. Some consider time dilation and length contraction real physical effects, while others consider them as measurement effects, Einstein being one of the latter.

Quote
Keep in mind that the context from what you're talking about means that you're talking about special relativity and in special relativity what I say is correct. And I've been tutoring physics long enough to know what is and what isn't confusing.

It's good of you to educate those interested, in contrast to forums that can't or won't attempt to explain in terms they can understand.
"A frame of ref. is merely a location common to a set of measurements..."
Quote
is true or meaningful?

A coordinate system has an origin. The origin has a location relative to a "body of reference". There is a clock at the origin. These are the necessary elements for a reference frame. The array of rods and clocks is optional. If you are an anaut in a space capsule, in space, you have no array, but you can still make measurements using a laser and a watch. All your measurements are relative to your position, which you have assumed to be static. The "infinite extent" is superfluous, since no one can make measurements of that type. Even radar type measurements are only practical over distances that are "short" in astronomical terms. There are other means of determining distances.
I regularly consult a dictionary for word meaning and origin, and would recommend it as the primary reference for any field of knowledge. It would eliminate some of the arguing over semantics.

Quote
If you go on to study special relativity then part of that is the example of the twin paradox. Part of that example to have two planets which are many light years apart. In that example the two planets are said to be in the same frame of reference. Its implicit in the physics and the mathematics.

What makes you think I have not studied SR?

The "twin paradox" doesn't require two planets. It involves the accumulated time for each twin on separate paths between positions A and B. The example could be simplified to two clocks, in separate capsules, initially at rest, i.e. having the same velocity.

yor_on

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Re: Relative Simultaneity
« Reply #96 on: 01/03/2015 19:24:47 »
Not really Phyti, he defined different clocks and rulers as a result of local measurements, those comparing other frames of reference relative ones local clock and ruler. When it came to which 'frame of reference' that was more 'correct' he gave every frame a equal (locally defined) importance. He tried to avoid the discussion of whether you should blame it on 'illusionary effects', relative 'real effects'. It was just 'frame dependent', and that was it.

PhysBang

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Re: Relative Simultaneity
« Reply #97 on: 01/03/2015 22:36:27 »
PhysBang #75
Quote
The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.

A proxy such as a video device can be used to collect image data (space probe, telescope, etc.), but ultimately the data only has meaning to the human observer/s, who designed and implemented the experiment.
Einstein's definition of a 3-axis Cartesian coordinate system and a clock, as a system of measurement, works just as well today as in 1905, when he published it. His later examples of the train car and the embankment demonstrated that either location  served as a reliable reference frame, for measurements, but when there was relative motion, the observers would record different values for distance and time.
I agree that Einstein's 1905 work is pretty good. However, none of that work relies on the idea of an observer at the origin. It is far better to simply consider reference frames as the best, in principle, that can be done to assign coordinates to an event, whether or not there is an observer that is at the origin of that reference frame and whether or not there is an observer that is at rest in that reference frame. The only role for and observer in "On the electrodynamics of moving bodies" is to establish a best, in principle procedure that everyone agrees, given the starting assumptions, should allow for the synchronization of clocks at a distance--this gives the idea of synchronization at a distance something to mean for physics.

When Einstein writes of what an observer would measure, he does not actually write about what someone would see, he writes about what events would be simultaneous or synchronous in a given system of coordinates.

jeffreyH

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Re: Relative Simultaneity
« Reply #98 on: 01/03/2015 22:46:56 »
One of the problems for the layman in understanding physics is the level of abstraction. The absence of an observer is such an abstraction. Much of the mathematics of physics is opaque to the layman. This is not a bad thing as abstraction is sometimes a necessity in simplifying things. This forum however is not, to my knowledge, a place for mainly professional physicists. Sometimes, in order to demonstrate something of interest, the abstraction could be minimized.

JohnDuffield

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Re: Relative Simultaneity
« Reply #99 on: 02/03/2015 14:32:23 »
In reading books and papers by various authors involved in physics, I don't find a unanimous agreement about everything. Some consider time dilation and length contraction real physical effects, while others consider them as measurement effects, Einstein being one of the latter.
Well said phyti. Like PmbPhy I consider myself a "relativist", but I don't agree with everything he says. And I certainly don't agree with everything by Wheeler or Taylor or Schutz.

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Re: Relative Simultaneity
« Reply #99 on: 02/03/2015 14:32:23 »