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Thread: Timey-wimey Wibbly-wobbly Stuff

  1. #61
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    Quote Originally Posted by Grimble View Post
    Let me thank you one and all for your confirmations of my understanding.
    ...
    Thank you once again for confirming my theories.
    No one has confirmed your understanding. Unless you wish to withdraw the descriptions you gave in posts #45 and #53?

    You appear to have misinterpreted what has been said here just as much as you misinterpret what is said by Einstein and Minkowski.

    Walking away from a debate claiming to have won when everyone else has shown you to be wrong is a very poor tactic.

  2. #62
    How can this still be going on?

    Grimble, your beliefs have not been confirmed, they have been torn apart and your errors clearly described to you. An inertial frame is not a non-moving frame, it is a non-accelerating frame. Spacetime is not a frame of reference. There is no rest frame with respect to spacetime. The concept of spacetime describes the relationships between frames of reference, not a fixed universal background against which they are measured.

    And in any case, the notion of absolute time is unambiguously demonstrated to be incompatible with relativity with a much simpler argument, the twin paradox. Once again, observers taking different paths through spacetime can rejoin each other after having experienced different amounts of time. Time dilation and Lorentz contraction are not some illusion of Doppler effects and lightspeed lag, they are real.

  3. #63
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    Quote Originally Posted by Grimble View Post
    ...

    Just as one seeing simultaneity means each must see simultaneity, the one seeing that it is the only one to see the simultaneity means each will come to the same conclusion. (2)

    Each will see it and deny that all others will. (1)

    The simultaneity exists in Spacetime and will be recognised by any observer stationary in Spacetime. That can happen in each and every frame of reference.

    ...
    (1) Is correct * and is the reason why (2) is incorrect.

    (Edit: * that is, may be correct for one of the observers.)

    If one of the observers notes that the two flashes reach him or her at the same time, that observer (as you've previously agreed) will note that the two flashes of light reached the other observer at different times.

    Now, a flash of light reaching an observer is a single event that either happened or didn't; one observer can not see the two flashes hit the other observer at different times while that observer sees them arrive at the same time.

    (
    To be very mechanical about it:
    Let's say one of the flashes is green and the other is purple.
    The green and purple light travels to observer X, and reflects in all directions.
    Some of the light reflecting off observer X travels to observer X's eyes.
    Some of the light reflecting off observer X travels to observer Y's eyes.
    For the green and purple light to reach either observers eyes at the same time, they must have reflected off observer X at the same time, so they must have hit X at the same time.
    So they can't disagree that the flashes hit an observer at the same time (if they did).
    As you've noted, if one observer receives the flashes at the same time, they'll note that the other observer didn't receive the flashes at the same time.
    That other observer can't have seen the flashes at the same time.
    )

    Your new claim that one observer can receive the flashes at the same time, but will (i) see the flashes received at different times by the other observer, but (ii) that other observer will themselves have seen the flashes at the same time - is contradictory.

    ((i) is correct, (ii) is contradictory).
    Last edited by pzkpfw; 2012-May-12 at 06:40 AM. Reason: Edit
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  4. #64
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    Quote Originally Posted by Grimble View Post
    Let me thank you one and all for your confirmations of my understanding.

    ...
    This I find a bit stunning. Do you not even agree that your understanding is at odds with common science?

    -------------------------

    http://en.wikipedia.org/wiki/Relativity_of_simultaneity

    My bolds:

    Quote Originally Posted by First part of wiki page
    In physics, the relativity of simultaneity is the concept that simultaneity–whether two events occur at the same time–is not absolute but depends on the observer's reference frame.

    According to the special theory of relativity, it is impossible to say in an absolute sense whether two events occur at the same time if those events are separated in space. Where an event occurs in a single place–for example, a car crash–all observers will agree that both cars arrived at the point of impact at the same time (1). But where the events are separated in space, such as one car crash in London and another in New York, the question of whether the events are simultaneous is relative: in some reference frames the two accidents may happen at the same time, in other frames (in a different state of motion relative to the events) the crash in London may occur first, and in still other frames the New York crash may occur first. If the two events are causally connected ("event A causes event B"), then the relativity of simultaneity preserves the causal order (i.e. "event A causes event B" in all frames of reference).
    (1) That's partly what I was trying to show (and rely on) with my green and purple lights example.

    -------------------------

    It's not just about what frames "see" of other frames; it's about the actuality of whether events were simultaneous or not, according to each frame.

    I really can't see how you interpret it all your way.
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  5. #65
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    Quote Originally Posted by pzkpfw View Post
    This I find a bit stunning.
    Only a bit?

  6. #66
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    Thank you, Grimble, for accepting you were wrong and climbing down from your position. I am glad you have decided that the mainstream interpretations are in fact the best ones at the moment. I am also glad you decided to read some more up to date background material.

  7. #67
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    Quote Originally Posted by Grimble View Post
    Special Relativity has no hidden mystery but is a plain straightforward piece of mechanics following Einstein's Postulates.
    There are no deep mysteries and certainly no counter intuitive phenomena.
    So you don't find the fact that multiple observers, all travelling at different velocities to eachother, still all measure the light beam to be passing them at the exact same speed, to be counter-intuitive ??


    Quote Originally Posted by Grimble View Post
    Everything happens just as it logically should and as Einstein and Minkowski described.
    I have a thorough understanding of your Special Relativity what it means and how it works.
    Sorry, but I'm telling you - No, you don't.

    If you did, you wouldn't have made that mistake in the definition of Frame of Reference. That is such a crucial concept, you'll never understand Relativity until you have a clear understanding of what that term means - and what it doesn't.

    Quote Originally Posted by Grimble View Post
    But it is interesting to see how confused you can be by a few simple questions.
    Nope, no confusion here. Perhaps you get that impression since there are many of us, all with our own style, presentation, assumptions and points we're emphasizing. However, the model is clear and consistent, and amply demonstrated with mental pictures of what's going on. What question do you think has not been fully answered?

    In contrast, in post #45, you agreed that when events are simultaneous on the embankment, they will not be simultaneous for an observer on the train - and when they're simultaneous for an observer on the train, they won't be for the observer on the embankment. EXCELLENT You then go way off by trying to jam the two together, with claims that "ALL frames MUST make the same judgement about simultaneity .... one seeing simultaneity means each must see simultaneity". This flatly contradicts the Embankment/Train simultaneity, and you offer us nothing but the blind statement of faith : "All frame MUST". No reason why, and no mental pictures demonstrating how this could work in the face of the agreed observations.

    Quote Originally Posted by Grimble View Post
    It is worth noting that you seem to be unable to read what is written without making it read what you have already decided it says.p
    Sorry, but No (again!). Remember you claimed that Einstein's "vice-versa" proved your point, before I pointed out he actually said " not simultaneous ... , and vice versa"?

    Quote Originally Posted by Grimble View Post
    I am fortunate in my background of finding faults in technical writing and computer programming to see where and how such confusion arises and to have found the true simplicity of Relativity.
    On the contrary, what you've done is taken a few terms, and taken your own cut at definitions of them (like for Frame of Reference), and mashed them together to get a version of Relativity that allows you to keep the familiar 3+1 classical coordinate system for all observers. And that's a shame, since you're shortchanging yourself. The 3+1 coordinate is a natural comfort zone, but you've got to be prepared to let that go, because that's not how the universe really runs. You've taken the trouble to learn this far, I assure it's so much more exciting if you push through.

  8. #68
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    Allow me to apologise for the way I have let my responses affect ( in the worst possible way) the tenor of this thread.
    I will allow my myself to become carried away and rationality and courtesy go out of the window.

    So I will be more careful and civil. - and please tell me if I err once more?

    Quote Originally Posted by RobA View Post
    So you don't find the fact that multiple observers, all travelling at different velocities to eachother, still all measure the light beam to be passing them at the exact same speed, to be counter-intuitive ??
    Are you saying that Einstein's second postulate - the constancy of the speed of light - is counter-intuitive? I do not see how that can be as it is a precept for the theory.

    If you did, you wouldn't have made that mistake in the definition of Frame of Reference. That is such a crucial concept, you'll never understand Relativity until you have a clear understanding of what that term means - and what it doesn't.
    Could you explain what my mistake is? What constitutes a Frame of Reference seems to be self evident, but that leaves me confused according to your understanding so could you explain?


    Nope, no confusion here. Perhaps you get that impression since there are many of us, all with our own style, presentation, assumptions and points we're emphasizing. However, the model is clear and consistent, and amply demonstrated with mental pictures of what's going on. What question do you think has not been fully answered?
    My original post. Where the statements I made were attacked as individuals complained about the way I asked them, but the overall question, no one seemed to address.

    In contrast, in post #45, you agreed that when events are simultaneous on the embankment, they will not be simultaneous for an observer on the train - and when they're simultaneous for an observer on the train, they won't be for the observer on the embankment. EXCELLENT
    "when events are simultaneous on the embankment, they will not be simultaneous for an observer on the train " - from the perspective of the embankment. and
    "when they're simultaneous for an observer on the train, they won't be for the observer on the embankment." - from the perspective of the passenger on the train
    You then go way off by trying to jam the two together, with claims that "ALL frames MUST make the same judgement about simultaneity .... one seeing simultaneity means each must see simultaneity". This flatly contradicts the Embankment/Train simultaneity, and you offer us nothing but the blind statement of faith : "All frame MUST". No reason why, and no mental pictures demonstrating how this could work in the face of the agreed observations.
    It seemed to me to be rather transparent - and I am sorry to have thought so - It is simple perspective - like two men observing one another from the opposite ends of a field: each will see the other as smaller, even though they are of the same size. In the case of relativity (Einstein's train/embankment) it is their relative velocity rather than their separation that gives rise to the effect.


    Sorry, but No (again!). Remember you claimed that Einstein's "vice-versa" proved your point, before I pointed out he actually said " not simultaneous ... , and vice versa"?
    No, sorry, if we use the complete quote: "Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity)." as here; the vice versa gives us: " Events which are simultaneous with reference to the train are not simultaneous with respect to the embankment, and vice versa (relativity of simultaneity). "


    On the contrary, what you've done is taken a few terms, and taken your own cut at definitions of them (like for Frame of Reference), and mashed them together to get a version of Relativity that allows you to keep the familiar 3+1 classical coordinate system for all observers. And that's a shame, since you're shortchanging yourself. The 3+1 coordinate is a natural comfort zone, but you've got to be prepared to let that go, because that's not how the universe really runs. You've taken the trouble to learn this far, I assure it's so much more exciting if you push through.[/QUOTE]

  9. #69
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    Quote Originally Posted by Grimble View Post
    Are you saying that Einstein's second postulate - the constancy of the speed of light - is counter-intuitive? I do not see how that can be as it is a precept for the theory.
    I'm sure it was quite counter-intuitive at the time; why wouldn't people expect the velocity of light to be relative to the speed of the source in the same way a bullet is.

    Could you explain what my mistake is? What constitutes a Frame of Reference seems to be self evident, but that leaves me confused according to your understanding so could you explain?
    It might be more useful for you to explain what your definition of a frame of reference is because (a) you have made some confusing statements in this regard and (b) this is your ATM thread.

    No, sorry, if we use the complete quote ...
    This sort of "arguing by quotation" is irrelevant as the full argument and mathematics presented by Einstein (and tested and understood by many others since) proves your "understanding" is incorrect.
    Last edited by Strange; 2012-May-16 at 12:26 PM. Reason: fixed quote sources

  10. #70
    Quote Originally Posted by Grimble View Post
    Are you saying that Einstein's second postulate - the constancy of the speed of light - is counter-intuitive? I do not see how that can be as it is a precept for the theory.
    In what way is it intuitive? Its observed speed is independent of the motion of the observer and emitter. Nothing else in our experience has such a property. That it is an axiom that the theory is based on is quite irrelevant, and that you think it is intuitive makes me wonder if you actually understand what it means. It's not constant in some universal rest frame, constant with respect to space-time. It's constant with respect to every inertial observer, regardless of their motion with respect to the emitter or other inertial observers.


    Quote Originally Posted by Grimble View Post
    Could you explain what my mistake is? What constitutes a Frame of Reference seems to be self evident, but that leaves me confused according to your understanding so could you explain?
    Your basic mistake is that you appear to insist on some special rest frame that all others are measured against, with absolute distance and time, universal simultaneity, and so on. In post 34, you equated space-time itself with a frame of reference. This is simply wrong.


    Quote Originally Posted by Grimble View Post
    It seemed to me to be rather transparent - and I am sorry to have thought so - It is simple perspective - like two men observing one another from the opposite ends of a field: each will see the other as smaller, even though they are of the same size. In the case of relativity (Einstein's train/embankment) it is their relative velocity rather than their separation that gives rise to the effect.
    It's not an observational illusion or matter of perspective. Once again, look at the twin paradox. Two observers start out at the same spacetime coordinates, one departs on their own separate path through spacetime and then returns. By your reasoning, they must have experienced the same amount of time. By the predictions of Relativity, the twin that left and later returned experienced less time. Actual measurements show that the predictions of relativity are the correct ones.

  11. #71
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    Quote Originally Posted by Strange View Post
    I'm sure it was quite counter-intuitive at the time; why wouldn't people expect the velocity of light to be relative to the speed of the source in the same way a bullet is.
    Again we have a different understanding of 'counter-intuitive'.
    To me, I felt it means something that goes against the 'common knowledge/common belief base of science - including any extra postulates that are essential to the new theory.

    It might be more useful for you to explain what your definition of a frame of reference is because (a) you have made some confusing statements in this regard and (b) this is your ATM thread.
    I take as a basic 'building block' of my view of the Universe, Einstein's description of a System of Coordinates for fixing the relationship of each point in space with the rest of space using the 3 coordinates, x,y,z
    Further I accept Minkowski's addition of Time to make 4-dimensional space-time, where each Event in space-time can be defined by 4 coordinates, x,y,z,t.

    I can then take this then as a single and unifying definition of each and every, space-point and time-point that together make up an isotropic and homogeneous Space-time. Where the separation of any two events will have the same space-time interval in all Frames.

    The problem that this gives us is that we have no definition for those coordinates. No reference point against which to relate them, no definition, and no way to determine whether they are moving.

    This is dealt with by defining Frames of Reference. Which enables us to set coordinates relative to a specific point in space time and thereby making that point our one fixed point in Space-time.

    Each Frame of reference is another view of the same Space-time, only relative to a different fixed point in Space-time.

    So Space-time is not a Frame of Reference as it has no fixed origin. Yet each and every Frame of Reference is that same Space-time given coordinates.

    (I don't know what you think of this as it is a subjective view, arrived at by logic rather than Mathematics; but none-the-less valid for all that)

    This sort of "arguing by quotation" is irrelevant as the full argument and mathematics presented by Einstein (and tested and understood by many others since) proves your "understanding" is incorrect.
    Agreed it is all interpretation.

  12. #72
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    Quote Originally Posted by cjameshuff View Post
    In what way is it intuitive? Its observed speed is independent of the motion of the observer and emitter. Nothing else in our experience has such a property.
    What about the speed of sound in air? Sonic booms, etc.
    That it is an axiom that the theory is based on is quite irrelevant, and that you think it is intuitive makes me wonder if you actually understand what it means. It's not constant in some universal rest frame, constant with respect to space-time. It's constant with respect to every inertial observer, regardless of their motion with respect to the emitter or other inertial observers.
    Or is it that it is the fastest that can be measured?
    Your basic mistake is that you appear to insist on some special rest frame that all others are measured against, with absolute distance and time, universal simultaneity, and so on. In post 34, you equated space-time itself with a frame of reference. This is simply wrong.
    Oh, the difficulty of expressing an idea.
    I believe that the Universe exists. That for the purposes of relating to its physical form we can do what Einstein describes in Chapter II & paragraphs 1-3 and super impose coordinate systems upon it. That is what Frames of Reference are: coordinates laid upon space-time based upon different origins. They are different views of the same thing, different perspectives; this is another(?) reason why no Frame can be 'preferred' - that is have simpler rules than any other. If this understanding that I arrive at through logic is flawed, will you please explain why, rather than saying 'This is simply wrong'.
    It's not an observational illusion or matter of perspective. Once again, look at the twin paradox. Two observers start out at the same spacetime coordinates, one departs on their own separate path through spacetime and then returns. By your reasoning, they must have experienced the same amount of time. By the predictions of Relativity, the twin that left and later returned experienced less time. Actual measurements show that the predictions of relativity are the correct ones.
    It just seems to me that this last, emboldened statement proves itself, but doesn't disprove my thoughts.
    "they must have experienced the same amount of time." does not conflict with "Actual measurements show that the predictions of relativity are the correct ones." for it is the measurements that are changed not the time itself.

  13. #73
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    Quote Originally Posted by Grimble View Post
    Again we have a different understanding of 'counter-intuitive'.
    To me, I felt it means something that goes against the 'common knowledge/common belief base of science - including any extra postulates that are essential to the new theory.
    That is a really odd definition of counter-intuitive. To me it simply means something which is counter to one's innate/naive (in short, non-scientific) sense of how things should be.

    In case I was wrong, which is frequently the case, I checked with good old Wikipedia:
    Quote Originally Posted by Wikipedia
    A counterintuitive proposition is one that does not seem likely to be true when assessed using intuition or gut feelings. Scientifically discovered, objective truths are often called counterintuitive when intuition, emotions, and other cognitive processes outside of deductive rationality interpret them to be wrong.
    http://en.wikipedia.org/wiki/Counterintuitive

    So the fact that light from a moving source behaves differently than a bullet from a moving source is counter-intuitive.

    The resulting proof of the relativity of simultaneity is also counter-intuitive. Which, I assume, is why you have a mental block against understanding it.

    This is dealt with by defining Frames of Reference. Which enables us to set coordinates relative to a specific point in space time and thereby making that point our one fixed point in Space-time.

    Each Frame of reference is another view of the same Space-time, only relative to a different fixed point in Space-time.
    This is a bit vague but is basically correct but incomplete.

    It is pretty simple to demonstrate that the laws of physics are not changed if we just translate (change the origin) or rotate our frame of reference. This has been known for a long time.

    However, the interesting questions occur when we consider two frames of reference in relative motion. Then it becomes slightly more complicated (and counter-intuitive). This seems to be where your intuition is failing you. This is why you need to work through the mathematics.

    So Space-time is not a Frame of Reference as it has no fixed origin.
    Again, true but incomplete. Not just no fixed origin but no concept of "stationary" other than relative.

    Yet each and every Frame of Reference is that same Space-time given coordinates.
    When two frames of reference are in relative motion that is no longer true; the coordinates are no longer the same. (Actually, that is true even for rotation.)

    Agreed it is all interpretation.
    No. It isn't interpretation. It is mathematics. And mathematics (unlike science) can prove things to be right or wrong. That is we know your "interpretation" is wrong.

  14. #74
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    Quote Originally Posted by Grimble View Post
    Or is it that it is the fastest that can be measured?
    No. It is an absolute, mathematical, limit. We can measure faster. You may recall the recent furore when some chaps thought they had measured neutrinos travelling faster than light. Now it turns out that the measurement was in error, but there was no reason in principle why the measurement could not have been right. IF neutrinos did travel slightly faster than light (or twice the speed of light) then we could measure it (to a remarkable degree of accuracy).

  15. #75
    Quote Originally Posted by Grimble View Post
    Again we have a different understanding of 'counter-intuitive'.
    To me, I felt it means something that goes against the 'common knowledge/common belief base of science - including any extra postulates that are essential to the new theory.
    That is just not what the term means. Counter-intuitive means just what it says, "against intuition". It has nothing whatsoever to do with accepted science.


    Quote Originally Posted by Grimble View Post
    That is what Frames of Reference are: coordinates laid upon space-time based upon different origins. They are different views of the same thing, different perspectives; this is another(?) reason why no Frame can be 'preferred' - that is have simpler rules than any other. If this understanding that I arrive at through logic is flawed, will you please explain why, rather than saying 'This is simply wrong'.
    As Strange said, a frame of reference is not just a choice of origin, frames can have relative motion as well.


    Quote Originally Posted by Grimble View Post
    What about the speed of sound in air? Sonic booms, etc.
    A property of the medium the sound is traveling through, constant only when measured relative to that medium. Nothing like light.


    Quote Originally Posted by Grimble View Post
    Or is it that it is the fastest that can be measured?
    No. The postulate is merely that it is constant to all inertial observers. Its properties as a maximum velocity are a prediction of relativity, and as Strange points out we can measure faster speeds and commonly do so for things not restricted by the speed of light such as phase velocity.


    Quote Originally Posted by Grimble View Post
    It just seems to me that this last, emboldened statement proves itself, but doesn't disprove my thoughts.
    It directly contradicts your claims.


    Quote Originally Posted by Grimble View Post
    "they must have experienced the same amount of time." does not conflict with "Actual measurements show that the predictions of relativity are the correct ones." for it is the measurements that are changed not the time itself.
    ...and you're back to assuming some preferred universal rest frame which "true time" is measured against.

    Time is that quantity measured by time-measuring instruments. Math and experiments all say that it depends on reference frame and has no global, absolute value, and if relativistic travel were common so would human experience. Your dislike of the concept is quite irrelevant, and your definition of time as something with no connection to any measurable quantity is in fact quite useless.

  16. #76
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Grimble View Post
    On the contrary, what you've done is taken a few terms, and taken your own cut at definitions of them (like for Frame of Reference), and mashed them together to get a version of Relativity that allows you to keep the familiar 3+1 classical coordinate system for all observers. And that's a shame, since you're shortchanging yourself. The 3+1 coordinate is a natural comfort zone, but you've got to be prepared to let that go, because that's not how the universe really runs. You've taken the trouble to learn this far, I assure it's so much more exciting if you push through.
    This sort of "arguing by quotation" is irrelevant as the full argument and mathematics presented by Einstein (and tested and understood by many others since) proves your "understanding" is incorrect.
    Errr, "Grimble's" quote above was actually my post #67 to him. Is there anything I'm doing wrong?

  17. #77
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    I apologise too, if anything I wrote was harsh - good to stay friends (even in ATM ! )

    Quote Originally Posted by Grimble View Post
    Are you saying that Einstein's second postulate - the constancy of the speed of light - is counter-intuitive? I do not see how that can be as it is a precept for the theory.
    I've always taken Strange's definition, as something that is counter to "common-sense". Since Relativity is built on something that is counter-intuitive, it must have counter-intuitive features as a consequence (like time-dilation, length-contraction - and yes, even simultaneity issues)

    Quote Originally Posted by Grimble View Post
    Could you explain what my mistake is? What constitutes a Frame of Reference seems to be self evident, but that leaves me confused according to your understanding so could you explain?
    Let's take an example. Alice, Bob and Charlie are all in a room at 3pm. Alice and Bob are standing still talking to eachother. Now Alice (whose watch reads 2:55) likes Cartesian coordinates measured in centimetres from the bottom SouthWest corner of the room. Bob (whose watch reads 2:57) reckons everything in Polar coordinates from the lightbulb in the middle of the ceiling. Charlie (watch at 2:53) is sitting on top of a ramp, so he's using Cartesian coordinates with origin at the top corner of the ramp, and with the XY plane (and the Z axis) tilted at 30 degrees compared to Alice's. Oh, and he measures everything in inches.

    So, 3 totally different coordinate systems describing the room. Each has a unique set of (3 numbers plus a time) to map every point - every Event - in the room.

    Would I be right in guessing you would say that these are 3 Frames of Reference? In fact, there is just one. As I said in #59, "Multiple objects not moving relative to eachother do, by definition, share the same Frame of Reference". Basically, you can assume that all observers stationary relative to each other, agree a fixed coordinate system between them before the start of any experiment.

    Of course, it's also entirely possible that I have guessed your position totally wrongly - in which case feel free to set me straight forthwith

    Quote Originally Posted by Grimble View Post
    My original post. Where the statements I made were attacked as individuals complained about the way I asked them, but the overall question, no one seemed to address.
    The trouble is, the OP is written based on certain assumptions - it reads like "Space is Space, and Time is Time". We're questioning those assumptions (eg. "One man's space is another man's time"), and hoping to work you through them. To take a stab at a few of them :
    - Is time absolute? No.
    - Time is often described as another dimension but is this any more than a mathematical device for making calculations? Yes. eg. the Twin paradox.

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    Quote Originally Posted by RobA View Post
    Errr, "Grimble's" quote above was actually my post #67 to him.
    Sorry, should have looked more carefully - although the point was not the content of the quote so much as the argument-by-quotation strategy beloved of those who think you can change the meaning of a theory by changing the interpretation of the words describing the theory, forgetting that the math is what counts.

    Is there anything I'm doing wrong?
    Don't think so - one of the [QUOTE] tags seems to have got lost along the way...

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    Quote Originally Posted by Strange View Post
    That is a really odd definition of counter-intuitive. To me it simply means something which is counter to one's innate/naive (in short, non-scientific) sense of how things should be.

    In case I was wrong, which is frequently the case, I checked with good old Wikipedia:

    http://en.wikipedia.org/wiki/Counterintuitive
    Indeed. I can see that I am over complicating it. That is a much more realistic interpretation. Thank you.

    So the fact that light from a moving source behaves differently than a bullet from a moving source is counter-intuitive.
    Well, yes it could be, but it depends rather on how one thinks of it.

    Take, for example, a jet fighter approaching the speed of sound. Bullets fired at that speed would leave the gun at the same speed, relative to that gun as they would were they fired when the aircraft was stationary. The sound of the engine is very different for it travels at the same speed, relative to the point of emission, regardless of the speed of the aircraft. i.e. one is relative to the aircraft and the other relative to the position. I see a better analogy with sound than with the bullet. (obviously I am not saying that the effects are the same only that it is a better analogy.)

    The resulting proof of the relativity of simultaneity is also counter-intuitive. Which, I assume, is why you have a mental block against understanding it.
    As far as I can see the only difference in our understanding of the relativity of simultaneity is about where it is observed from.

    In Einstein's Train, the lightning is simultaneous from the embankment because M is a fixed point on the Embankment.
    From the embankment it can be seen that it is not simultaneous from the train, because point M' is moving relative to the embankment.
    Now, my problem is that if we take away the Frames of Reference, that is those particular sets of coordinates that we are using, what is left? (And I am aware that some will now throw their hands up in horror and say that if one takes away the Frames of Reference we are left with no coordinates, with nothing to relate anything to and probably loads of other complaints too, but please, humour me?)
    From my reasoning we are left with three points in spacetime A,B and P the point where the light pulses meet. They are not fixed, but nor are they moving, because such concepts are meaningless without coordinates to relate to.
    We do know however, that the pulses of light will meet at point P and that AP equals BP.
    If we now define a Frame of Reference, any Frame of Reference, points A,B and P will be stationary points in that Frame. And that any observer remaining at Point P will measure A and B as simultaneous.
    Within the Embankment's Frame of Reference it is point M that maps Point P whilst point M' is moving and does not measure simultaneity.
    All, as far as I can see agreeing exactly with what everyone tells me.
    But I want to take a step further, not trying to rewrite relativity, not to contradict any existing understanding but merely to take a step further along this same path of reasoning.
    That is to say that when we use the train's Frame of Reference it is point M', that, as Einstein pointed out is at the same location as Point M at the time of the lightning strikes. And that is at Point P in spacetime as we have just seen.
    Yet in the Train's Frame of Reference it is Point M' that is stationary and measures simultaneity, while Point M is moving and doesn't.

    As I said this is not denying the Relativity of Simultaneity, but only looking a little deeper and refining what it means.

    It all works exactly as you say, but it does so for each and every Frame of Reference.

    So can you tell me what is wrong, explicitly with that scenario?

    This is a bit vague but is basically correct but incomplete.

    It is pretty simple to demonstrate that the laws of physics are not changed if we just translate (change the origin) or rotate our frame of reference. This has been known for a long time.

    However, the interesting questions occur when we consider two frames of reference in relative motion. Then it becomes slightly more complicated (and counter-intuitive). This seems to be where your intuition is failing you. This is why you need to work through the mathematics.
    I am sorry but you lose me there, how is relative motion counter intuitive? It seems straightforwar and logical to me. Can you explain what you mean?

    Again, true but incomplete. Not just no fixed origin but no concept of "stationary" other than relative.
    So Space-time is not a Frame of Reference as it has no origin. - better?

    Yet each and every Frame of Reference is that same Space-time given coordinates.
    When two frames of reference are in relative motion that is no longer true; the coordinates are no longer the same. (Actually, that is true even for rotation.)
    No, agreed, of course they will not be the same. The coordinates will be particular to the Frame of Reference.



    No. It isn't interpretation. It is mathematics. And mathematics (unlike science) can prove things to be right or wrong. That is we know your "interpretation" is wrong.[/QUOTE]


    Quote Originally Posted by cjameshuff View Post
    As Strange said, a frame of reference is not just a choice of origin, frames can have relative motion as well.
    Yes, relative to other Frames, yet not relative to Spacetime for which there can be no concept of movement as nothing is fixed to move relative to?

    A property of the medium the sound is traveling through, constant only when measured relative to that medium. Nothing like light.
    We were discussing light. And you said “In what way is it intuitive? Its observed speed is independent of the motion of the observer and emitter. Nothing else in our experience has such a property. “

    And I was pointing out that the speed of sound is independent of the motion of the observer and emitter. Which works as an analogy. Why it is like that is irrelevant to the observation.

    I find nothing counter intuitive about the concept that light is emitted at a point in space and travels to another point a fixed distance away in a specific time. And that these are events and cannot, therefore, have motion.

    Maybe my brain and intuition is different from yours?

    No. The postulate is merely that it is constant to all inertial observers. Its properties as a maximum velocity are a prediction of relativity, and as Strange points out we can measure faster speeds and commonly do so for things not restricted by the speed of light such as phase velocity.
    I'm sorry once again, it was my phrasing. I was wondering whether the limit was the maximum speed for light can be measured. Nothing to do with measuring the speed of anything else.

    It directly contradicts your claims.
    You said: “By your reasoning, they must have experienced the same amount of time.*By the predictions of Relativity, the twin that left and later returned experienced less time. Actual measurements show that the predictions of relativity are the correct ones. .”
    But that is not true is it? Relativity predicts that less time, is measured to have passed for the twin that left and later returned by the twin that remained.
    I am not aware that the twin paradox has ever been measured?



    ...and you're back to assuming some preferred universal rest frame which "true time" is measured against.
    I'm not sure where this comes from. What I am assuming is that the Universe exists and can be described by Minkowski's 4 dimensional Spacetime. i.e. that every point in space can be given a three parameter space location and that every time point can be given a single time parameter. Every combination of time-point and space-point makes a unique event.

    Time is that quantity measured by time-measuring instruments.
    is it really self defining?
    If I have three clocks in my room and they run at different rates does that mean that they are experiencing time passing at different rates?

  20. #80
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    Quote Originally Posted by Grimble View Post
    In Einstein's Train, the lightning is simultaneous from the embankment because M is a fixed point on the Embankment.
    Well, that's not really the reason (it is because of the way the example is specified) but never mind.

    From the embankment it can be seen that it is not simultaneous from the train, because point M' is moving relative to the embankment.
    Similarly, as you fail to understand, it will not be simultaneous to the observer on the train.

    Now, my problem is that if we take away the Frames of Reference, that is those particular sets of coordinates that we are using, what is left? (And I am aware that some will now throw their hands up in horror and say that if one takes away the Frames of Reference we are left with no coordinates, with nothing to relate anything to and probably loads of other complaints too, but please, humour me?)
    That is just meaningless. You might as well say "let's take away the length of the train".

    From my reasoning we are left with three points in spacetime A,B and P the point where the light pulses meet. They are not fixed, but nor are they moving, because such concepts are meaningless without coordinates to relate to.
    We do know however, that the pulses of light will meet at point P and that AP equals BP.
    And this is where you go wrong: assigning absolute positions in spacetime to A, B and P.

    If they are not fixed (i.e. you have no way of specifying their locations) how can you say that "the pulses of light will meet at point P and that AP equals BP"? You can only do that with reference to a set of coordinates. These coordinate will differ depending on the frame of reference and their relative state of motion.

    If we now define a Frame of Reference, any Frame of Reference, points A,B and P will be stationary points in that Frame. And that any observer remaining at Point P will measure A and B as simultaneous.
    Nonsense. They will only be stationary in the frame of reference that is defined to be stationary with respect to A, B and P (e.g. the platform). From a relatively moving frame of reference (e.g. the train) they will (obviously) not be stationary.

    All, as far as I can see agreeing exactly with what everyone tells me.
    Nope.

    But I want to take a step further, not trying to rewrite relativity, not to contradict any existing understanding but merely to take a step further along this same path of reasoning.
    Well, as you have already wandered into illogical nonsense feel free...

    As I said this is not denying the Relativity of Simultaneity, but only looking a little deeper and refining what it means.
    No it just totally misunderstanding what it means.

    It all works exactly as you say, but it does so for each and every Frame of Reference.
    Again, no.

    So can you tell me what is wrong, explicitly with that scenario?
    Your utter lack of understanding of what a frame of reference is and your (implicit) belief that spacetime forms some sort of absolute backdrop.

    I am sorry but you lose me there, how is relative motion counter intuitive? It seems straightforwar and logical to me. Can you explain what you mean?
    I mean the consequences of taking into account frames of reference in relative motion are counter-intuitive. As you repeatedly demonstrate.

    every point in space can be given a three parameter space location and that every time point can be given a single time parameter.
    They can - for a given frame of reference. For a different frame of reference those space and time coordinates will be different. There is no universal set of spacetime coordinates.

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    Quote Originally Posted by Grimble View Post
    I'm sorry once again, it was my phrasing. I was wondering whether the limit was the maximum speed for light can be measured. Nothing to do with measuring the speed of anything else.
    Sorry, just noticed this. Exactly the same technique that was used for measuring the speed of neutrinos could (in principle) be used for measuring the speed of photons.

    On the one hand, you would have the challenge of building a suitably long evacuated tunnel. On the other hand, photons are much easier to detect.

    But there are plenty of other ways of measuring the speed of light. None of which are limited in the way you suggest - otherwise there would be no point doing it.

  22. #82
    Quote Originally Posted by Grimble View Post
    Take, for example, a jet fighter approaching the speed of sound. Bullets fired at that speed would leave the gun at the same speed, relative to that gun as they would were they fired when the aircraft was stationary. The sound of the engine is very different for it travels at the same speed, relative to the point of emission, regardless of the speed of the aircraft. i.e. one is relative to the aircraft and the other relative to the position. I see a better analogy with sound than with the bullet. (obviously I am not saying that the effects are the same only that it is a better analogy.)
    The target the bullets are hitting certainly sees their relative velocity to depend on the jet fighter's velocity. And both see the relative speed of sound in different directions to vary depending on their airspeed.


    Quote Originally Posted by Grimble View Post
    As far as I can see the only difference in our understanding of the relativity of simultaneity is about where it is observed from.
    That is simply nonsense.


    Quote Originally Posted by Grimble View Post
    We were discussing light. And you said “In what way is it intuitive? Its observed speed is independent of the motion of the observer and emitter. Nothing else in our experience has such a property. “

    And I was pointing out that the speed of sound is independent of the motion of the observer and emitter. Which works as an analogy. Why it is like that is irrelevant to the observation.
    The speed of sound is not independent of the motion of the observer or emitter. Your statement is only true in the rest frame of the medium. The speed of light is independent of motion in all inertial frames.


    Quote Originally Posted by Grimble View Post
    I find nothing counter intuitive about the concept that light is emitted at a point in space and travels to another point a fixed distance away in a specific time. And that these are events and cannot, therefore, have motion.
    That is not what generally goes against people's intuition. It is the fact that this is so for all inertial frames that generally causes people trouble. They instinctively desire a fixed rest frame, absolute time and space, universal simultaneity, etc.


    Quote Originally Posted by Grimble View Post
    Maybe my brain and intuition is different from yours?
    You do seem a bit more stuck on the idea of a fixed universal reference frame.


    Quote Originally Posted by Grimble View Post
    I'm sorry once again, it was my phrasing. I was wondering whether the limit was the maximum speed for light can be measured. Nothing to do with measuring the speed of anything else.
    That sentence does not make sense.


    Quote Originally Posted by Grimble View Post
    You said: “By your reasoning, they must have experienced the same amount of time.*By the predictions of Relativity, the twin that left and later returned experienced less time. Actual measurements show that the predictions of relativity are the correct ones. .”
    But that is not true is it? Relativity predicts that less time, is measured to have passed for the twin that left and later returned by the twin that remained.
    I am not aware that the twin paradox has ever been measured?
    That is not at all what relativity predicts. It doesn't even make any sense, the stationary twin can't measure time for the moving twin. The moving twin actually experiences less time.

    And yes, it's been measured, many times and in many ways since the early-mid 1900s. Time dilation effects must be accounted for whenever dealing with high precision atomic clocks at different locations on Earth or in orbit. GPS satellites demonstrate the effect constantly. Many experiments and pieces of scientific instrumentation must take time dilation and length contraction into account. We can even measure the difference in gravitational time dilation from lifting an atomic clock a few meters in Earth's surface gravitational field. It's not some abstract idea that has no observational justification, it's reality.


    Quote Originally Posted by Grimble View Post
    I'm not sure where this comes from. What I am assuming is that the Universe exists and can be described by Minkowski's 4 dimensional Spacetime. i.e. that every point in space can be given a three parameter space location and that every time point can be given a single time parameter. Every combination of time-point and space-point makes a unique event.
    Measured from some specific reference frame. There is nothing special about those coordinates, and events with the same time coordinates in one frame may have different time coordinates measured from another frame.


    Quote Originally Posted by Grimble View Post
    is it really self defining?
    If I have three clocks in my room and they run at different rates does that mean that they are experiencing time passing at different rates?
    Your clocks are essentially in the same frame. No, your clocks are merely inaccurate. Time dilation is not inaccurate clocks.

  23. #83
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    Quote Originally Posted by Grimble View Post
    Take, for example, a jet fighter approaching the speed of sound. ... The sound of the engine is very different for it travels at the same speed, relative to the point of emission, regardless of the speed of the aircraft.
    The trouble is that the sound analogy also breaks down very quickly. For one thing, it directly implies an absolute medium (or frame) that the signal is traveling in. For another, it's just not accurate - Take a plane flying at 9/10 the speed of sound. It's sound is traveling at 1/0 speed of sound ahead of it. However, a train traveling at 0.9 c past an embankment has it's light still traveling at a full c ahead of it.

    Rather than trying to find an analogy, it's better to contrast instead. A red car and a white car leave a house traveling at 90kph and 100kph. The red car sees the white car traveling at 10kph relative than it. Sure enough, after 1 hour, the white car is 100km from the house, and 10km from the red car.
    A red train and a light beam pass an embankment. The train is traveling at 0.9c. However, the train sees the light beam still traveling at c relative to it. After 1 hour, the light beam is 1 light hour ahead of the embankment, and 1 light hour ahead of the train (even though the train has been moving in that time as well). Now THAT's counter-intuitive.

    The resolution of this seeming paradox is tied up with time dilation and length contraction - and yes, simultaneity as well.

    Quote Originally Posted by Grimble View Post
    As far as I can see the only difference in our understanding of the relativity of simultaneity is about where it is observed from.
    No, it's a lot deeper than that. Our difference is that you insist that all observers, regards of their motion, will all agree that two events are simultaneous. This implies that there is a global, shared time axis that they all share - and that constitutes a preferred frame.

    Quote Originally Posted by Grimble View Post
    From my reasoning we are left with three points in spacetime A,B and P the point where the light pulses meet. They are not fixed, but nor are they moving, because such concepts are meaningless without coordinates to relate to.
    We do know however, that the pulses of light will meet at point P
    OK, I can run with that. A, B and P are unique events in spacetime - effectively they are points. As such, you cannot assign any spacetime coordinates to them - either space coordinates or time coordinates - since that would imply an absolute frame. This means that you CAN'T say from the outset that A and B happened at the same time.

    Quote Originally Posted by Grimble View Post
    and that AP equals BP. If we now define a Frame of Reference, any Frame of Reference, points A,B and P will be stationary points in that Frame.
    NO. You're being led astray by your implicit assumption of a background set of coordinates again.

    Think back to the train example, with the scorchmarks. The space coordinates where A and B happened are marked BY THE SCORCHMARKS. So each observer has a stationary set of space coordinates where A and B happened, but they're not - they CAN'T be - stationary in space in any absolute sense. The embankment observer thinks A and B happened at the embankment, stationary to him. The train observer thinks A and B happened at the marks on the train - stationary to him. THEY'RE BOTH RIGHT!

    Before we go round on simultaneity again, we really must get the foundations right and clear - especially the meaning of "Frame of Reference". I don't think you realise that you are using the term differently to the rest of us. I have addressed this in two separate posts, but you haven't responded to these points, so can I ask you again:
    Quote Originally Posted by RobA View Post
    Alice, Bob and Charlie are all in a room at 3pm. Alice and Bob are standing still talking to eachother. Now Alice (whose watch reads 2:55) likes Cartesian coordinates measured in centimetres from the bottom SouthWest corner of the room. Bob (whose watch reads 2:57) reckons everything in Polar coordinates from the lightbulb in the middle of the ceiling. Charlie (watch at 2:53) is sitting on top of a ramp, so he's using Cartesian coordinates with origin at the top corner of the ramp, and with the XY plane (and the Z axis) tilted at 30 degrees compared to Alice's. Oh, and he measures everything in inches.

    So, 3 totally different coordinate systems describing the room. Each has a unique set of (3 numbers plus a time) to map every point - every Event - in the room.

    Would I be right in guessing you would say that these are 3 Frames of Reference?

  24. #84
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    Quote Originally Posted by Strange View Post
    That is just meaningless. You might as well say "let's take away the length of the train".
    I am sorry but I do not understand your difficulty! It is quite simple.
    A Frame of reference is a set of coordinates that maps spacetime. At its simplest, no more than that.
    Or is that wrong also, if so will you please explain how and why?
    Space time exists, in-so-much that it is a description of the matter of the universe and how it is related in space and time. If you don't like the idea that it is fixed in any way, why not call it Wibbley-wobbley Timey-wimey stuff and have done with it.

    So, a frame of reference defines a particular view of spacetime.
    Spacetime's existence does not depend on frames of reference, which are no more than man made theoretical constructs.


    And this is where you go wrong: assigning absolute positions in spacetime to A, B and P.

    If they are not fixed (i.e. you have no way of specifying their locations) how can you say that "the pulses of light will meet at point P and that AP equals BP"? You can only do that with reference to a set of coordinates.
    No, no, no. All I am assigning are relative positions. Each is only defined by its relationship to the others. So I can say exactly that.

    These coordinate will differ depending on the frame of reference and their relative state of motion.
    Yes, yes yes, I think you are beginning to understand what I am saying. For each and every frame of reference that one defines those points will have a particular set of coordinates.

    the fact that they are different coordinates in each frame of reference does not imply that their relative positions change. Does it? If so how?

    Nonsense. They will only be stationary in the frame of reference that is defined to be stationary with respect to A, B and P (e.g. the platform). From a relatively moving frame of reference (e.g. the train) they will (obviously) not be stationary.
    I'm sorry but you have have me there. Please explain how when you define a set of coordinates, a reference frame, in which every point in space, is, of course, fixed according to its coordinates, one can have a point move????

    It is what is at that point that can move to other points, the point cannot move.

    In the embankments frame, M remains at point P and M' moves from it, in the trains frame, M' remains at P while M moves. This is because they have different coordinates that are moving relative to one another.

    Can you not envisage this, it is very straightforward and simple to a visual thinker. Simple geometry.



    Quote Originally Posted by cjameshuff View Post
    The target the bullets are hitting certainly sees their relative velocity to depend on the jet fighter's velocity.
    Precisely.
    And both see the relative speed of sound in different directions to vary depending on their airspeed.
    But not upon the speed of the Aircraft emitting the sound. It can, after all, travel faster than the sound can.




    That is simply nonsense.
    Explain. Have you understood what I am saying or have you merely dismissed it out of hand? (Which is of no help to anyone.)

    The speed of sound is not independent of the motion of the observer or emitter. Your statement is only true in the rest frame of the medium. The speed of light is independent of motion in all inertial frames.
    So? It was an analogy to show there are better analogies that a bullet.

    You do seem a bit more stuck on the idea of a fixed universal reference frame.
    No, I do not see see any universal reference frame. A Reference Frame of any kind is something added. It is a way of defining coordinates, relative to a set point. Spacetime exists, the universe exists, whether we define reference frames or not.

    That is not at all what relativity predicts. It doesn't even make any sense, the stationary twin can't measure time for the moving twin. The moving twin actually experiences less time.
    Is that just the time for the acceleration? For , by definition, the time that passes in inertial frames of reference, is the same. That is one of my problems, you cannot just ignore the postulates when making your calculations.

    And yes, it's been measured, many times and in many ways since the early-mid 1900s. Time dilation effects must be accounted for whenever dealing with high precision atomic clocks at different locations on Earth or in orbit. GPS satellites demonstrate the effect constantly. Many experiments and pieces of scientific instrumentation must take time dilation and length contraction into account. We can even measure the difference in gravitational time dilation from lifting an atomic clock a few meters in Earth's surface gravitational field. It's not some abstract idea that has no observational justification, it's reality.
    I have no problem whatsoever with time dilation etc. those calculations work wonderfully well, we all know that. But it is the measurements that change according to the conditions they are measured under. It is, as has been well established right from the start that it is the observed clock that slows.
    It slows for the stationary observer, which is why we talk of the clock having different readings depending on where it is measured from.
    Some one moving very fast relative to a clock reads the time differently, he does not, cannot affect, the way that time passes for a clock that he has no connection to. Or if there are two moving observers, is their slowing of the clock cumulative?

    Measured from some specific reference frame. There is nothing special about those coordinates, and events with the same time coordinates in one frame may have different time coordinates measured from another frame.
    Exactly! They will have different coordinates from different frames.

    Your clocks are essentially in the same frame. No, your clocks are merely inaccurate. Time dilation is not inaccurate clocks.
    So please explain “Time is that quantity measured by time-measuring instruments. ”
    Define what you mean by time, because that definition is meaningless!

  25. #85
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    Quote Originally Posted by Grimble View Post
    In Einstein's Train, the lightning is simultaneous from the embankment because M is a fixed point on the Embankment.
    From the embankment it can be seen that it is not simultaneous from the train, because point M' is moving relative to the embankment.
    I think a lot of it boils down to this simple statement. Where you are incorrect in your interpretation.

    It's more like this:

    In Einstein's Train, if the lightning is simultaneous from the embankment where M is a fixed point on the Embankment.
    From the embankment it can be seen that it is not simultaneous from the train, because point M' is moving relative to the embankment.
    And conversely:

    In Einstein's Train, if the lightning is simultaneous from the train where M' is a fixed point on the Train.
    From the train it can be seen that it is not simultaneous from the embankment, because point M is moving relative to the train.
    You are missing the if. It's quite clear from the paper you reference, that the lightening is not simultaneous in both frames. It's certainly not just about how each frame sees the other.

    (This results in your "fixed universal reference frame". You demand that if the events are simultaneous in one frame they must be simultaneous in all frames. Completely missing the point of the relativity of simultaneity. If it was all just about observations of one frame from another, it wouldn't be anywhere near so interesting.)


    You've not yet answered how one observer could see the two flashes hit the other observer at different times, while that observer him or herself sees them at the same time.
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    From : http://www.bartleby.com/173/9.html

    Quote Originally Posted by Einstein
    Are two events (e.g. the two strokes of lightning A and B) which are simultaneous with reference to the railway embankment also simultaneous relatively to the train? We shall show directly that the answer must be in the negative.
    Note: it's not just about the perception of the train, from the embankment. Just simply, is it also simultaneous there? No!

    Quote Originally Posted by Einstein
    When we say that the lightning strokes A and B are simultaneous with respect to the embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the mid-point M of the length A —> B of the embankment. ...
    Note: setting up a situation where the events do happen to be simultaneous with respect to the train. To try to argue this means simultaneous in all frames, in some absolute sense, misses the entire point of the paper.

    Quote Originally Posted by Einstein
    ... (skipping the detail about how the train is moving with respect to the embankment) ... Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result: ...
    Note: that train observer sees the lights at different times, so he must consider the flashes non-simultaneous. It's not just about the embankment observers observation of the train observer! That observer at M' is seeing the flashes at different times, and will conclude they were not simultaneous.

    Note: "Observers who take the railway train as their reference-body" means the person on the train. This extends to observers in different frames (i.e. moving with respect to each other).

    Quote Originally Posted by Einstein
    ... Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, ...
    Note: how much clearer can this be? Frames in relative motion get their own reality for deciding whether events are simultaneous.

    Quote Originally Posted by Einstein
    ... and vice versa (relativity of simultaneity). ...
    Note: there's no preferred reference frame. That train is just as valid as the embankment. So the train observer might see flashes (different ones than those before, of course) at the same time and consider them simultaneous. If so, then the embankment observer can't see those flashes at the same time, and won't consider them simultaneous.

    Quote Originally Posted by Einstein
    ... Every reference-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.
    Note: again, how could this be clearer? "Every reference-body (co-ordinate system) has its own particular time".
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    Using guns and bullets instead of light:

    Code:
    Tracks           Train ->
    
    ============[A      N      B]===============
    
    ----------------------------------------
    Embankment   C      M      D
    N is on a train moving right. A and B are guns pointing at her.
    M is on the embankment and is "still". C and D are guns pointing at him.
    The guns are set to fire as they pass each other.

    Assuming classical physics, a little while later:

    Code:
    =================[A......N......B]======
    
    ----------------------------------------
                 C......M......D
    Both N and M are hit at the same time by their bullets.

    In N's opinion, the bullets from A and B both travelled six spaces to her.
    In M's opinion, the bullets from C and D both travelled six spaces to him.

    But what does M think happened to N?

    Code:
                  ...........N.
    
    ----------------------------------------
                 C......M......D
    M will think the bullet from A travelled eleven spaces to hit N, and the bullet from N travelled 1 space. Because from the point of view of M, the speed of the train has added to the speed of the bullet from A, and has been subtracted from the speed of the bullet from B.

    So M and N can both agree that the firing of the guns was simultaneous.

    But, this only "works" because (in classical physics) the speed of the bullets can vary according to the observer.

    This does not apply to light. All observers must agree that light moves at c. e.g. A spaceship moving at speed 0.5c shooting a laser ahead of it, will not cause anyone to measure a beam of light going 1.5c

    When we use lightening flashes instead of guns and bullets, observer N and M must both measure the flashes travelling at c; so the situation seen above {where both agree the bullets hit each other at the same time and can both agree that the firing of the guns was simulataneous} simply can't apply. The two observers M and N won't both be hit by the flashes at the same; they won't both think the flashes were simultaneous.
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    Quote Originally Posted by pzkpfw View Post
    When we use lightening flashes instead of guns and bullets, observer N and M must both measure the flashes travelling at c; so the situation seen above {where both agree the bullets hit each other at the same time and can both agree that the firing of the guns was simulataneous} simply can't apply. The two observers M and N won't both be hit by the flashes at the same; they won't both think the flashes were simultaneous.
    No, absolutely, precisely, each will think their flashes were simultaneous and that the other's weren't! You are agreeing exactly with what I am seeing!

    One has to take into account that each is seeing the light travel with the speed of light in their rest frame

    So N will see the light from a and B, travelling at c in her frame reach her simultaneously, as will M see the light from C and D in his frame.
    N will also see the light from C and D move at C but M will appear to move toward C while M sees the light from A and B move at C but N will be moving toward B.

  29. #89
    Join Date
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    1,361
    Grimble, I'll make this simple; experiments and observation show that there is no absolute time. Our maths and practical application such as GPS depend upon that fact.

    Do you acknowledge that space and time are intertwined, not separate?

    If you do, then you should understand that there is no combination of separated points in spacetime that can all be called now in time, and that is what you are trying to do.
    "There are powers in this universe beyond anything you know. There is much you have to learn. Go to your homes. Go and give thought to the mysteries of the universe. I will leave you now, in peace." --Galaxy Being

  30. #90
    Join Date
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    415
    Quote Originally Posted by pzkpfw View Post
    From : http://www.bartleby.com/173/9.html



    Note: it's not just about the perception of the train, from the embankment. Just simply, is it also simultaneous there? No!



    Note: setting up a situation where the events do happen to be simultaneous with respect to the train. To try to argue this means simultaneous in all frames, in some absolute sense, misses the entire point of the paper.



    Note: that train observer sees the lights at different times, so he must consider the flashes non-simultaneous. It's not just about the embankment observers observation of the train observer! That observer at M' is seeing the flashes at different times, and will conclude they were not simultaneous.

    Note: "Observers who take the railway train as their reference-body" means the person on the train. This extends to observers in different frames (i.e. moving with respect to each other).



    Note: how much clearer can this be? Frames in relative motion get their own reality for deciding whether events are simultaneous.



    Note: there's no preferred reference frame. That train is just as valid as the embankment. So the train observer might see flashes (different ones than those before, of course) at the same time and consider them simultaneous. If so, then the embankment observer can't see those flashes at the same time, and won't consider them simultaneous.



    Note: again, how could this be clearer? "Every reference-body (co-ordinate system) has its own particular time".
    May I refer you to a blog that makes sense to me?http://fromtheretohereandonward.blog...-sideways.html

    It has a very erudite reading of just what Einstein wrote, considered in full without taking partial quotes to remove their context.
    Last edited by Grimble; 2012-May-19 at 08:06 PM. Reason: added last sentence

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