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Author Topic: Relative Simultaneity  (Read 23999 times)

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.
 

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.
 

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 »
 

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.
 

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?
 

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.
 

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.
 

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. 
 

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?
 

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.
 

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.
 

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.
 

Offline PmbPhy

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Re: Relative Simultaneity
« Reply #62 on: 23/02/2015 22:49:38 »
Quote from: phyti39
Wikipedia 2006
Quote
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.
 

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 »
 

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.
 

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.
 

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?
 

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.
 

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, ...
Quote
Both very wrong and very confusing.
To you, but are you speaking for everyone?


 

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.
 

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.
Quote
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.

Quote
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.)
 

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.
 

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.
 

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.
 

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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