Relative Simultaneity

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Offline phyti39

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Re: Relative Simultaneity
« Reply #50 on: 21/02/2015 17:53:14 »
PmbPhy #23

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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.
The "cat falling into the manhole' occurred on earth. It did not occur on the moon, or Jupiter, or on a body orbiting Betelgeuse. All those objects are moving relative to earth, thus the "event" did not occur at any of those locations (in their frame of reference). The "event" can be perceived at those locations, after occurring on earth. Until detecting the images of the "event", a viewer at those locations will have no awareness of the "event".
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You're not speaking of a single event here but two events since its only between two events that a spatial distance or a temporal separation exists between them.
In the last post to Bill S, the cat and cover moving simultaneously can be considered as one composite event.
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Not at all. No physicist would ever say that the synchronization procedure outlined by Einstein was misleading in any way. And it's quite reasonable to assign a time to any event regardless of where its located. You may have made such an assertion but you never said anything to support it.
Let's see what the author of SR says.
On the Electrodynamics of Moving Bodies', Albert Einstein, 1905:
part 1, par. 1
"But it is not possible without further assumption to compare, in respect of time, an event at A with an event at B. We have so far defined only an ``A time'' and a ``B time.'' We have not defined a common ``time'' for A and B, for the latter cannot be defined at all unless we establish by definition that the ``time'' required by light to travel from A to B equals the ``time'' it requires to travel from B to A."

"The "time" of an event is that which is given simultaneously with the event by a stationary clock located at the place of the event, this clock being synchronous, and indeed synchronous for all time determinations, with a specified stationary clock."

Relativity, Crown Publishers, 1961, pg 23
"That light requires the same time from A to M as from B to M (M being the midpoint of A to B), is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own free will in order to arrive at a definition of simultaneity."

There is no known method of measuring the speed of light relative to a moving observer, and consequently knowing the time and position of a remote reflection. By definition it is calculated to be a constant.

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Offline PhysBang

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Re: Relative Simultaneity
« Reply #51 on: 21/02/2015 18:21:59 »
The "cat falling into the manhole' occurred on earth. It did not occur on the moon, or Jupiter, or on a body orbiting Betelgeuse. All those objects are moving relative to earth, thus the "event" did not occur at any of those locations (in their frame of reference). The "event" can be perceived at those locations, after occurring on earth. Until detecting the images of the "event", a viewer at those locations will have no awareness of the "event".
And all this is irrelevant to the fact that every event is in every frame of reference (that is well-formed). It's similar to saying that everything measured in meters also has a measurement in feet, even if nobody does the measurement.

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In the last post to Bill S, the cat and cover moving simultaneously can be considered as one composite event.
One could do that, but then one would be abandoning contemporary relativity theory. Thens that happen separated by spatial distance are separate events. They may be simulataneous in some frames, but cannot be simultaneous in all frames.

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Offline David Cooper

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Re: Relative Simultaneity
« Reply #52 on: 21/02/2015 19:01:02 »
There is no spatial distance between two items here, so the action where the cat falls through the manhole is simultaneous in all frames of reference. What will vary in different frames will be the delay between the light travelling from the cat to the man with the lever and the signal from the lever getting back to the manhole, and that delay will be shortest when measured in the frame of reference in which the street is stationary. All frames in which the street is moving will perceive events as being slowed down, but at no point would any observer think the cat can walk across the manhole without falling in.

The other case with the fast moving man and the grid involves such a high speed of travel that there's no way the man could fall into it even if it wasn't length contracted, though he could perhaps trip up on it.
« Last Edit: 21/02/2015 19:07:06 by David Cooper »

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #53 on: 21/02/2015 20:47:30 »
Quote from: phyti39
The "cat falling into the manhole' occurred on earth. It did not occur on the moon, or Jupiter, or on a body orbiting Betelgeuse. All those objects are moving relative to earth, thus the "event" did not occur at any of those locations (in their frame of reference). The "event" can be perceived at those locations, after occurring on earth. Until detecting the images of the "event", a viewer at those locations will have no awareness of the "event".
You made the mistake of thinking that a frame of reference is synonymous with a location. It isn't. A frame of reference typically has an infinite extent. So one doesn't speak of "Their frame of reference" as being the same as a location. That is contrary to what a frame of reference is and how its defined. Please read the definition of it at: http://en.wikipedia.org/wiki/Frame_of_reference

Quote from: phyti39
There is no known method of measuring the speed of light relative to a moving observer, and consequently knowing the time and position of a remote reflection. By definition it is calculated to be a constant.
Prove it.

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Offline Bill S

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Re: Relative Simultaneity
« Reply #54 on: 22/02/2015 13:39:41 »
Quote from: Pete
You made the mistake of thinking that a frame of reference is synonymous with a location. It isn't. A frame of reference typically has an infinite extent. So one doesn't speak of "Their frame of reference" as being the same as a location. That is contrary to what a frame of reference is and how its defined. Please read the definition of it at: http://en.wikipedia.org/wiki/Frame_of_reference

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in physics, a frame of reference (or reference frame) may refer to a coordinate system……

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In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point….

How does the position of a point differ from the location of a point?
There never was nothing.

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Offline phyti39

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Re: Relative Simultaneity
« Reply #55 on: 22/02/2015 20:43:28 »
Bill S #45

Wolfson article
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Your birth is a necessary cause of your reading these words.

The birth of Bob did not cause Bob to read any particular text. His birth allowed the possibility of Bob to get an education, acquiring the skill of reading. He then willfully chooses to read some online text.

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Had the first event not occurred, the second could not have occurred either.

Had Bob not won the lottery, he could not have spent the prize money.
Does this not read like a tautology?

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Had the Titanic not hit the iceberg, it would not have sunk.

All boats that hit icebergs, do not sink.
Some boats have multiple encounters with icebergs.
If the Titanic had been built to better standards, it may not have sunk.

The writer, in his attempt to form a chain of causality, concludes, there is only one outcome.

Contrary to his thinking:
The world is not totally deterministic. There are multiple possible outcomes for many events. If there weren't there would be no variety. Introduce human free will, and it becomes more random.

Following the Wolfson logic:
Vinnie gets drunk drinking too much beer. While driving home, he hits a pedestrian, who dies. In court, Vinnie's lawyer says: "If the brewery hadn't made and sold the beer, my client would not have gotten drunk drinking it. The brewery is to blame for the pedestrian death.

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #56 on: 23/02/2015 03:18:25 »
Quote from: Bill
How does the position of a point differ from the location of a point?
There is none. The terms are synonyms.

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Offline Bill S

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Re: Relative Simultaneity
« Reply #57 on: 23/02/2015 11:34:33 »
Phyti, whilst what you say is correct, I suspect you are reading more into Wolfson’s reasoning than he probably intended.

I think his point was that if Bob had not been born he would not have been reading those words, or any others.  Thus there is a causative relationship between the two events, but it does not oblige him to read any specific text.

The statement: “Had the Titanic not hit the iceberg, it would not have sunk” says nothing about there being a causal relationship between the Titanic hitting the iceberg and the sinking, or otherwise, of any other craft. 
There never was nothing.

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Offline Bill S

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Re: Relative Simultaneity
« Reply #58 on: 23/02/2015 14:28:47 »
Quote from: Pete
There is none. The terms are synonyms.

Following the reasoning in #54, does your answer not indicate that frame of reference and locating can be the same thing?
There never was nothing.

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Offline David Cooper

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Re: Relative Simultaneity
« Reply #59 on: 23/02/2015 18:09:07 »
The writer, in his attempt to form a chain of causality, concludes, there is only one outcome.

Contrary to his thinking:
The world is not totally deterministic. There are multiple possible outcomes for many events. If there weren't there would be no variety. Introduce human free will, and it becomes more random.

There is no free will, so it doesn't become more random. Furthermore, the only room for non-deterministic events is in the quantum realm, and even then it may have a fully deterministic mechanism lurking behind it which we cannot access, so there may only be one path which the future can follow. To assert that there isn't a single path is at least as wrong as to assert that there is - we currently don't know. The whole universe we see around us could be a simulation in which there is no room for anything non-deterministic to happen at all (at any level).

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Following the Wolfson logic:
Vinnie gets drunk drinking too much beer. While driving home, he hits a pedestrian, who dies. In court, Vinnie's lawyer says: "If the brewery hadn't made and sold the beer, my client would not have gotten drunk drinking it. The brewery is to blame for the pedestrian death.

No one is to blame for anything because no one has free will. However, we still calculate that by punishing people we can modify their future behaviour and give people better lives on average as a result, so "blame" really relates to those causes which involve calculation within thinking systems like the brain, and especially where rules have not been followed correctly.

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Offline phyti39

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Re: Relative Simultaneity
« Reply #60 on: 23/02/2015 20:18:49 »
PhysBang #51
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And all this is irrelevant to the fact that every event is in every frame of reference (that is well-formed). It's similar to saying that everything measured in meters also has a measurement in feet, even if nobody does the measurement.
An event E happens once, at a specific location, at a time noted by a local (at that location) clock. There can be many perceptions of E at locations distant from E. These perceptions consist of detecting images of E, and recording the time of detection on a local clock. If the distant observer knows the approximate distance to E, they can assign a local time to E. The perception of an event is not the same as the occurrence of the event. A photo of a person is not the same as the person. Seeing the event is another "figure of speech".
The system of rods and clocks is a fictional and overly simplistic configuration of a frame of reference, and is logistically impossible. The GPS system of clocks requires periodic corrections, and that is local. A frame of ref. is merely a location common to a set of measurements, just as coordinates to an origin. It's not magic, it's a system of measurement requiring a reference and a unit of measure or standard.
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One could do that, but then one would be abandoning contemporary relativity theory. Thens that happen separated by spatial distance are separate events. They may be simulataneous in some frames, but cannot be simultaneous in all frames.
Nothing is abandoned.
The example specifies the cat being over the hole when the cover drops away. The person activating the cover could be replaced with a photocell detector at the hole, and is irrelevant to the setup. The desired effect is similar to a gallows, where the criminal drops simultaneously with the trap door. The cat and cover drop simultaneously, and therefore are a single event. There is no space or time
difference to manipulate via motion of a passing observer. All who view the event perceive a single cat+cover motion.

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Offline phyti39

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Re: Relative Simultaneity
« Reply #61 on: 23/02/2015 20:23:43 »
PmbPhy #53
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You made the mistake of thinking that a frame of reference is synonymous with a location. It isn't. A frame of reference typically has an infinite extent. So one doesn't speak of "Their frame of reference" as being the same as a location. That is contrary to what a frame of reference is and how its defined. Please read the definition of it at: http://en.wikipedia.org/wiki/Frame_of_reference [nofollow]
Wikipedia 2006
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Relativity theory depends on "reference frames". A reference frame is a point in space at rest, or in uniform motion, from which a position can be measured along 3 spatial axes. In addition, a reference frame has a clock moving with the reference frame, allowing the measurement of the time of events.

"Frame of reference" existed millenia ago, and is not a development unique to  Relativity. The definition above may not agree with the one you saw, since it depends on who is editing the articles, and when you look. I'll just repeat the part posted to #51.
A frame of ref. is merely a location common to a set of measurements, just as coordinates to an origin. It's not magic, it's a system of measurement requiring a reference and a unit of measure or standard.
Spatial measurements are between two objects, which implies two locations.
From "Relativity", Crown Publishers, 1961,
A.E. describes "a system of coordinates rigidly attached to a body of reference".
Since points in space are abstract and inaccessible, an object is required for a measurement, eg. the molecules at each end of the rod.

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Prove it.
This will get a separate response.

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #62 on: 23/02/2015 22:49:38 »
Quote from: phyti39
Wikipedia 2006
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Relativity theory depends on "reference frames". A reference frame is a point in space at rest, or in uniform motion, from which a position can be measured along 3 spatial axes. In addition, a reference frame has a clock moving with the reference frame, allowing the measurement of the time of events.
You didn't provide a link to this statement. In any case it's wrong. What I'm telling you is a fact. It's what all mainstream physicists use as the definition of "frame of reference". Do yourself a service and contact any relativist that you can find (e.g. Sean Carroll, Edwin F. Taylor, Hans C. Ohanian, etc) and ask them what the definition of a frame of reference is, or read their textbooks.  In no sense of the term does the phrase "frame of reference" apply only to a finite region of space. That's all there is too it. You double check me by picking up ****Any Text in Special or General Relativity ****. Please stop trying to pass off this misinformation as fact.

You can't simple make a claim that what you stated was the definition of a frame of reference. That definition is already given, has been defined for over a hundred years in relativity, and will not change in the foreseeable future. You can't take what's on the internet as being factual. Wiki is reliable and the definition I linked to is correct. That's why I used it. But I know from experience that the countless books and journal articles out there that use the term use it as I defined it for you. I have about 15 relativity textbooks and they all define the term that way.

Quote from: phyti39
"Frame of reference" existed millenia ago, and is not a development unique to  Relativity.
So what? The topic of this thread is relativity, nothing else.

I'm not about to keep arguing about a well-known definition from relativity. It's a waste of everyone's time. Either you wish to learn or you don't.

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Offline yor_on

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Re: Relative Simultaneity
« Reply #63 on: 23/02/2015 23:08:26 »
A frame of reference includes everything being 'at rest' with you. So if you're on a spaceship in uniform motion then this 'rest frame' should include all objects in a infinite universe that could be defined as being 'at rest' with you, and in reality we can't state how many objects that might be. There is always a locality to the definition in that you need to define what you consider as your object of interest (frame of reference) defining this 'at rest' from, in this case we use the space ship. Using scaling, and especially QM, it becomes a lot more tricky. What is a 'electron cloud' 'at rest' with? The nucleus? And when is it 'at rest', if so? Before the outcome, in the outcome? Never? I think never but that's just my thought.
=

Everything becomes weird down there, doesn't it? Can a electron cloud be defined as being 'at rest' with the nucleus before a measurement? Actually, just as you can split an acceleration into infinitesimally small 'bits' and so find a 'flat space', or in this case 'instants' of motion, so you should be able to do with a whole universe, all included. If you do so, presuming it correct, then motion should cease to exist. and that is one of the weirdest arguments still :)

Easiest to see if you think of splitting a circle into those bits. As you go down in scale each bit will be 'straighter and straighter' measuring it. At some scale it should be a line, although as I think, still containing the 'property' of a circle, as we know where we started. Property's are really weird, and so is scaling. Another reason why I like Planck scale btw, at least it puts a limit to our measurements.
« Last Edit: 23/02/2015 23:50:42 by yor_on »
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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #64 on: 24/02/2015 02:06:35 »
Quote from: PhysBang
And all this is irrelevant to the fact that every event is in every frame of reference (that is well-formed).
PhysBang - Will you do me a favor and please explain this fact to phyti39? He has a crazy notion of what a frame of reference is. Thanks.

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #65 on: 24/02/2015 02:08:54 »
Quote from: Pete
There is none. The terms are synonyms.

Following the reasoning in #54, does your answer not indicate that frame of reference and locating can be the same thing?
I don't understand your line of reasoning. Will you please explain/clarify it for me? Thanks.

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #66 on: 24/02/2015 02:26:07 »
Quote from: phyti39
An event E happens once, at a specific location, at a time noted by a local (at that location) clock. There can be many perceptions of E at locations distant from E. These perceptions consist of detecting images of E, and recording the time of detection on a local clock. If the distant observer knows the approximate distance to E, they can assign a local time to E. The perception of an event is not the same as the occurrence of the event. A photo of a person is not the same as the person. Seeing the event is another "figure of speech".
You didn't have to say all of this. PhysBang is quite aware of all of this as is any physicist. I don't understand your need to state something so obvious, unless you thought that he was extremely ignorant on such an obvious fact? Please explain. Thank you.

Quote from: phyti39
The system of rods and clocks is a fictional and overly simplistic configuration of a frame of reference, and is logistically impossible.
Yep. We're all aware of that. A coordinate system is an artificially imposed grid that you place on a problem in order to make quantitative measurements. When one actually looks at experiments and how measurements are really made it becomes clear that there is a definite reality to the coordinate system. It's just too difficult to explain or put in a simple definition.

Quote from: phyti39
...A frame of ref. is merely a location common to a set of measurements, ...
Both very wrong and very confusing.

Quote from: phyti39
It's not magic, ...
Did you think that someone here actually though it was really magic?

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Offline phyti39

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Re: Relative Simultaneity
« Reply #67 on: 25/02/2015 19:11:36 »
PmbPhy #62
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You didn't provide a link to this statement. In any case it's wrong. What I'm telling you is a fact. It's what all mainstream physicists use as the definition of "frame of reference". Do yourself a service and contact any relativist that you can find (e.g. Sean Carroll, Edwin F. Taylor, Hans C. Ohanian, etc) and ask them what the definition of a frame of reference is, or read their textbooks.  In no sense of the term does the phrase "frame of reference" apply only to a finite region of space. That's all there is too it. You double check me by picking up ****Any Text in Special or General Relativity ****. Please stop trying to pass off this misinformation as fact.

The link was (Wikipedia-Special Relativity-section on reference frame) as edited in 2006. When compared to the Einstein definition, the condition of rigid body was not included. It is usually understood that the "editor" knew that "point" implies a physical object, else how would you find it. The extent of the"frame" wasn't mentioned in the Wiki article or in the book cited. How would you perform an experiment involving an object at an "infinite" distance? Einstein required the measurement system to be anchored/attached to a rigid body, eg. the train car or the embankment. An anaut floating in space, with a laser and a clock is a frame of reference, with an extent depending on the range of the laser.

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Offline phyti39

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Re: Relative Simultaneity
« Reply #68 on: 25/02/2015 19:37:48 »

You didn't have to say all of this. PhysBang is quite aware of all of this as is any physicist. I don't understand your need to state something so obvious, unless you thought that he was extremely ignorant on such an obvious fact? Please explain. Thank you.

Yep. We're all aware of that. A coordinate system is an artificially imposed grid that you place on a problem in order to make quantitative measurements. When one actually looks at experiments and how measurements are really made it becomes clear that there is a definite reality to the coordinate system. It's just too difficult to explain or put in a simple definition.

If we both know these things, then we agree on something.

Quote from: phyti39
...A frame of ref. is merely a location common to a set of measurements, ...
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Both very wrong and very confusing.
To you, but are you speaking for everyone?



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Offline JohnDuffield

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Re: Relative Simultaneity
« Reply #69 on: 25/02/2015 20:22:21 »
I'm like to think of myself as a guy who "roots for relativity". But I also think that when it comes to special relativity, a reference frame is little more than a state of motion. Look at the Wikipedia article, and state of motion is mentioned 9 times. For example you start off motionless with respect to the Earth, then you accelerate to some speed relative to the Earth, and you say you've changed your reference frame. But you haven't changed your reference frame like it's some library book. All you've really done is changed your speed.

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Offline PhysBang

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Re: Relative Simultaneity
« Reply #70 on: 25/02/2015 20:59:29 »
But I also think that when it comes to special relativity, a reference frame is little more than a state of motion.
It's clearly far different from a state of motion, since reference frames are the standard against which we determine motion.

It's like you said that numbers are little more than bananas.
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Look at the Wikipedia article, and state of motion is mentioned 9 times.
Yes, mostly to point out the difference between choice of reference frame and the state of motion of a given observer. That's an important thing not to miss.

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For example you start off motionless with respect to the Earth, then you accelerate to some speed relative to the Earth, and you say you've changed your reference frame.
No, you don't. You can say that you are currently co-moving with a different inertial frame of reference than the one you were in before. Or you can identify a system of coordinates in which you were at rest. Or you can identify a system of coordinates in which you moved along only one axis. There is a great deal of freedom of choice.

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But you haven't changed your reference frame like it's some library book. All you've really done is changed your speed.
If an object, observer or not, is accelerated, then it accelerates in every inertial reference frame. If one allows any system of coordinates, like general relativity does, then whether something moves or not is dependent on the system of coordinates (the reference frame) chosen. (Even in Newtonian physics, one can use reference frames that have a parallel acceleration relative to other frames without problem, so some accelerations there are dependent on the reference frame chosen.)

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #71 on: 26/02/2015 16:56:26 »
Quote from: PhysBang
No, you don't.
I strongly disagree. To change one's frame of reference from one inertial frame to another one, one simply changes which frame of reference they are at rest with respect to. That requires accelerating from one frame to another.

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Offline PhysBang

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Re: Relative Simultaneity
« Reply #72 on: 26/02/2015 17:11:44 »
Quote from: PhysBang
No, you don't.
I strongly disagree. To change one's frame of reference from one inertial frame to another one, one simply changes which frame of reference they are at rest with respect to. That requires accelerating from one frame to another.
I do not have a frame. I am currently at rest with respect to many different frames. I am free to use many different frames of references to make measurements and to refer to positions, speeds, and momenta.

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Offline David Cooper

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Re: Relative Simultaneity
« Reply #73 on: 26/02/2015 18:51:05 »
I do not have a frame. I am currently at rest with respect to many different frames. I am free to use many different frames of references to make measurements and to refer to positions, speeds, and momenta.

You are only at rest in one frame of reference. In every other frame you are regarded as moving. You can certainly use any frame of reference to calculate things, but all but one of them will apply numbers to you that claim you're moving.

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #74 on: 26/02/2015 18:55:48 »
Quote from: PhysBang
I do not have a frame. I am currently at rest with respect to many different frames.
I disagree here too. Your "frame" is the system in which you are at rest as are the devices and coordinate systems used to measure observables. The only sense that you can legitimately say that you're at rest with respect to many different frames is to distinguish frames that are rotated and translated with respect to each other. However in "your frame" you measure one and only one value of the kinetic energy, momentum, etc.

May I inquire as to where you got your definition of "frame of reference" from? This is essentially the one I'm using at the present time: http://en.wikipedia.org/wiki/Frame_of_reference
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In physics, a frame of reference (or reference frame) may refer to a coordinate system used to represent and measure properties of objects, such as their position and orientation, at different moments of time. It may also refer to a set of axes used for such representation. In a weaker sense, a reference frame does not specify coordinates, but only defines the same 3-dimensional space for all moments of time such that the frame can distinguish objects at rest from those that are moving.
The underline is mine. It refers to the important aspects of the frame of reference.

I'll get back later with a survey from a sampling of my mechanics and relativity text books.
« Last Edit: 26/02/2015 18:58:34 by PmbPhy »

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Offline 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.


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Offline 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!!

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Offline 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.

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Offline 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_ »
"The more things change, the more they remain the same."

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Offline 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.
Fixation on the Einstein papers is a good definition of OCD.

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Offline 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 »

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Offline 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.

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Offline 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.

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Offline 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.
"The more things change, the more they remain the same."

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Offline 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.
There never was nothing.

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Offline 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".

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Offline 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.
Fixation on the Einstein papers is a good definition of OCD.

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Offline 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.

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Offline 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.

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Offline 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.
There never was nothing.

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Offline 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.

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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.

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Offline 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.

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Offline 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.

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Offline 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.

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Offline 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.

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Offline 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.
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Offline 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.

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Offline 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.
Fixation on the Einstein papers is a good definition of OCD.

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Offline 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.