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Relativity of Simultaneity (Not Quite)

March 4 2011 at 12:12 PM
Jose Rodriguez 

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

Relativity of simultaneity

From Wikipedia, the free encyclopedia

[linked image]
Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.

The author here is writing as if the events actually occur in different order just because some observer or other is moving in relation to the events. The truth is that observers stationary to the events will observe events in different order. The order of events does not depend on motion, at all. It depends on the position of the observer in relation to the two or more events. In fact the observer at A will observe the event at A before the event at B, and the observer at B will observe the event at B before the event at A. So0 much BS here!

See link for the next picture.

Events A, B, and C occur in different order depending on the motion of the observer.

The author is misinforming again: The order of events does not depend on motion, it only depends upon the position of the observer with respect to the events, when the wave fronts from the events simultaneously meet the observer's eyes.


The white line represents a plane of simultaneity being moved from the past to the future.

In real life there is no plane of simultaneity. Each of us live in the present. Events close to us happen in the present. Events which we see being further away happened further into the past. Events happening now on the sun are 8 minutes into our future as far as light is concerned, further into the future as far as the effects of a CME. The 8 minute delay is not caused by motion, it is caused by distance from the Sun

In physics, the relativity of simultaneity is the concept that simultaneitywhether two events occur at the same timeis not absolute, but depends on the observer's reference frame.

[i]Wrong-O; If a moving observer and a stationary observer are at the event at the same time, they will both see the event happen at the same time![/i]


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.

Not true! If one is known to be located half way between two locations and sees that both events happen at the same time, they did absolutely happen at the same time!

Where an event occurs in a single placefor example, a car crashall observers will agree that both cars arrived at the point of impact at the same time. But where the events are separated in space, such as one car crash in London and another in New Delhi, the question of whether the events are simultaneous is relative: in some reference frames the two accidents may happen at the same time, in others (in a different state of motion relative to the events) the crash in London may occur first, and in still others the New Delhi crash may occur first.

Not true either, see above comment.

If we imagine one reference frame assigns precisely the same time to two events that are at different points in space, a reference frame that is moving relative to the first will generally assign different times to the two events. This is illustrated in the ladder paradox, a thought experiment which uses the example of a ladder moving at high speed through a garage.

In reality, the shrunken ladder/ garage is fiction. Einstein arrived at the idea of matter shrinking wrt observers in relative motion to objects by means of a Train Gedanken: For instants: the "measure the moving train gedanken:" As the front of the train passes the signalman at the far end of the platform, he simultaneously marks the platform and signals the rear signalman to mark the platform at the rear of the train. Einstein claims that due to the finite speed of light, the train will be measured shorter than it is, since the train moved while the signal traveled back to the rear signalman.

Now, stop and think: If the rear signalman sends the signal to the front of the train, while the train is moving, the same train will measure longer! Duh!


A mathematical form of the relativity of simultaneity ("local time") was introduced by Hendrik Lorentz in 1892, and physically interpreted (to first order in v/c) as the result of a synchronization using light signals by Henri Poincaré in 1900. However, both Lorentz and Poincaré based their conceptions on the aether as a preferred but undetectable frame of reference, and continued to distinguish between "true time" (in the aether) and "apparent" times for moving observers. It was Albert Einstein in 1905 who abandoned the (classical) aether and emphasized the significance of relativity of simultaneity to our understanding of space and time. He deduced the failure of absolute simultaneity from two stated assumptions:

1. The principle of relativitythe equivalence of inertial frames, such that the laws of physics apply equally in all inertial coordinate systems;

2. The constancy of the speed of light detected in empty space, [is](sic) independent of the relative motion of its source.

This second postulate is bogus! Most people acknowledge that light propagates away from a source in an expanding sphere, centered upon where the source was when the wavefront was emitted. All other observers in relative motion to the source, when the wave front was emitted, will see either aberration or Doppler shift. Do you need to consult Sherlock to see the fantasy?

 
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Anonymous

Re: Relativity of Simultaneity (Not Quite)

March 6 2011, 3:55 PM 

everyone has to sacrifice at the altar of stupidity from time to time

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

December 26 2014, 8:21 PM 

“Relativity of simultaneity” seems to be unacceptable. See two pictures, please.

Picture 1 : Many books show a picture of a passenger car. At the center of it, there is a light source and the flash reach the front and rear inner walls. Now, on the both walls, luminous bodies are fixed and these also flash when flashes reach. Both flashes are visible from the outside. The passenger car is moving in a tunnel. On the side wall of the tunnel, a long linear sensor is laid. To the sensor, both flashes will be at the same time.

Picture 2 : A passenger car is moving in a tunnel. The whole of outer side wall of this car is made of luminous plate and is sending plane waves of light (frequency is constant). Plane waves will reach side wall of the tunnel as plane.

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

December 29 2014, 8:08 PM 

This is a version of the picture 1 (shown in the above post : 26 Dec). Now, two light flashes emitted from the source (set at the center of the roof) reach the bottom end of the front and rear walls (of the passenger car). So, two traces and floor form an isosceles triangle. How does relativity of simultaneity explain ? And how about the length contraction of the passenger car ? It’s impossible to imagine.

 
 
Ufonaut99

Re: Relativity of Simultaneity (Not Quite)

December 30 2014, 10:06 AM 

Hi Jose & Nakayama,

.... And hoping everyone on the forum has had a wonderful Christmas, and wishing all a great New Year happy.gif

Hmmm, now about Relativity of Simultaneity ....
First, let's clarify what RoS is .... and what it is not !

Wiki: Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.

Jose: The author here is writing as if the events actually occur in different order just because some observer or other is moving in relation to the events. The truth is that observers stationary to the events will observe events in different order. The order of events does not depend on motion, at all. It depends on the position of the observer in relation to the two or more events. In fact the observer at A will observe the event at A before the event at B, and the observer at B will observe the event at B before the event at A.

Jose, you seem to be regarding RoS (and presumably SR) as literally only about what light rays an observer "sees". Not so; let's take an example to describe what's happening :

Say I happen to be with my telescope in my garden at 5pm one day, when both the moon and the sun are visible in opposite sides of the sky. Looking at the moon (1 light second away) I see an asteroid strike it. Some time later (actually at 5:07:59) I'm looking at the sun (8 light minutes away) when I see a solar flare erupt. So I "saw" the asteroid strike BEFORE the solar flare - but that is NOT what RoS is about.

Rather, I reckon that both the asteroid strike and the solar flare occurred at 4:59:59 - in other words, in my frame, they occurred at the same time - ie. they were "simultaneous". Note that bit "in my frame" - EVERYBODY in my frame will ascribe the same "time" value to both those events, and will therefore also regard them as "simultaneous" REGARDLESS of how far away they are from the sun or the moon. RoS is NOT just about "when the wave fronts from the events simultaneously meet the observer's eyes"; Rather, it is about spatially separate events being deemed "simultaneous" if their "t" coordinates are the same within a given frame.

by RoS, another frame may regard the asteroid strike and the solar flare as non-simultaneous (eg, the asteroid strike at 4:58:00 and the flare at 5:00:00 within that frame's coordinate system) - again, regardless of where any individual in that frame happens to be physically located when they receive the light from those events.

There is no inconsistency nor violation of Cause/Effect resulting from this (and, to ward off another common misconception, no frame ever regards anything in any other frame as going "backwards" in time); Each frame maintains a perfectly self-consistent view of every event and every law of physics within its coordinate system.
That means you cannot simply declare "RoS is not true" (or "both flashes will be at the same time." - RoS means they will not be in the sensor's frame). Since RoS is a direct consequence of the two postulates, such statements do not prove anything except your rejection of the postulates - but I think we already guessed that wink.gif

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

December 30 2014, 10:00 PM 

To Ufonaut99, Jose and all, "Have a Happy New Year !!"

”Relativity of simultaneity” seems to be unacceptable.

From a laser source set at the center of a passenger car, laser beams are sent to the front and rear walls. On the both walls, mirrors are fixed and reflect laser beams. Paths of four beams form rectangular lozenge. Yes, two reflected beams are crossing and interference fringe is visible. This interference fringe will be the same to all including the one who is moving in a relative motion.

 
 
Ufonaut99

Re: Relativity of Simultaneity (Not Quite)

December 31 2014, 6:58 PM 

Nakayama : This interference fringe will be the same to all including the one who is moving in a relative motion.

Correct. This is what we would expect from RoS and SR, given the First Postulate that they are based on.

The first postulate is summed up by (and dates all the way back to) "Galileo's Ship" - In a sealed cabin, all laws of physics still hold, so you cannot tell (ie. there is no experiment you could possibly perform) if you are "stationary" or "moving at uniform velocity". This means anybody who is "moving at uniform velocity" relative to somebody/something else can perfectly validly consider themselves as being "stationary" and the other as "moving".

So, given RoS and SR are fundamentally based on this, then naturally they predict that experiments between a "stationary" car and one in "relative motion" would produce the same results (ie. the fringe will be the same), since the one in "relative motion" IS "stationary" in their frame.

That sounds perfectly acceptable to me happy.gif

 
 
Jose Rodriguez

Re: Relativity of Simultaneity (Not Quite)

January 1 2015, 2:33 AM 

(UFO)This means anybody who is "moving at uniform velocity" relative to somebody/something else can perfectly validly consider themselves as being "stationary" and the other as "moving".


I have pointed out numerous time that the terms "stationary" and "moving" are ambiguous, confusing statements. We're discussing relativity, so just use the term "the relative distance is changing between reference frames," or between source and observers. If the distance is not changing, source and observer are in the same reference frame. Remember: unreflected Light travels away from the source. You cannot ignore the source in these discussions.

(UFO)Jose, you seem to be regarding RoS (and presumably SR) as literally only about what light rays an observer "sees". Not so; let's take an example to describe what's happening:


Sorry my friend, nothing (as far as we know) travels faster than the speed of light. So all of your lecturing is baseless fantasy. Light rays, or better yet, a strong short pulse of light, and the accompanying wave front is all that can really be physically investigated. You cannot admit that RoS exists within the the reference frame of the source, or sources, where distance is not changing among them and their observers. Sure, Einstein never mentioned that, but it would ruin his fantasy.

Understand this: The GPS system "space segment" satellites all broadcast the same identical series of bits simultaneously, wherever they are in their orbits. The end user, "rover" receives the bits at a given time "simultaneously" but the bits are no longer in order, because each "space segment" satellite is a different distance from the rover's location. The varying distances change the latency of the signal from each satellite. GPS does not need Einstein's RoS.

Have some more eggnog with me.

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

January 1 2015, 8:48 PM 

At the center of a passenger car, imagine letter L. Two light sources are set at the two ends of L. Is breakdown of simultaneity in two ways ? And when light is switched off, is simultaneity restored ?

To Ufonaut99,
Thank you so much for your replay. However, in the castle of relativity, a sign of any changes is not seen at all. Relativity will rule us hereafter.

 
 
Ufonaut99

Re: Relativity of Simultaneity (Not Quite)

January 2 2015, 1:37 AM 

Jose : Have some more eggnog with me.

Any time happy.gif Must admit, we hit the bubbles for 2015, though.

Jose : I have pointed out numerous time that the terms "stationary" and "moving" are ambiguous, confusing statements. We're discussing relativity, so just use the term "the relative distance is changing between reference frames," or between source and observers.

Well, it's been 400 years since Galileo came up with his Ship - people should have had enough time to get to grips with it by now !

Seriously, though, what is confusing or ambiguous about saying that a passenger sitting on a train considers himself "stationary", regardless of whether the train itself is moving or not? Everybody can relate to that common everyday situation. The trouble is, if you leave the term "stationary" out, then people will start factoring in the observer's "velocity" into the measurements he makes, and so treating the other frame as the "special" or "real" one - just as Nakayama did above (and, of course, Pentcho does on a regular basis, every time he posts about the Doppler Shift)

How about we compromise, and just say that an observer making a measurement always considers himself to be stationary.

Jose : Sorry my friend, nothing (as far as we know) travels faster than the speed of light. So all of your lecturing is baseless fantasy. Light rays, or better yet, a strong short pulse of light, and the accompanying wave front is all that can really be physically investigated.

And sorry myself, but ..... Errrr, where did anything I wrote above require anything faster-than-light ?

For example, I said I saw a solar flare at 5:07:59 (ie, that's when the wavefronts from the flare met my eyes), and so since the sun is 8 light minutes away, I said that the flare must have happened 8 minutes previously - ie. at 4:59:59. Yes, we can only physically investigate the light - but that doesn't rule out that I knew before yesterday (from previous investigations) that the sun was, and is, 8 light minutes away from me.

Nothing there (nor in RoS, nor SR) requiring Faster-Than-Light. I didn't know before the light-speed signal reached me at 5:07:59 that a flare had gone off, for example.

Jose : You cannot admit that RoS exists within the the reference frame of the source, or sources, where distance is not changing among them and their observers. Sure, Einstein never mentioned that, but it would ruin his fantasy.

I repeat what I said above, EVERYBODY in my frame will ascribe the same "time" value to both those events, and will therefore also regard them as "simultaneous" REGARDLESS of how far away they are from [the events]. RoS is NOT just about "when the wave fronts from the events simultaneously meet the observer's eyes"; Rather, it is about spatially separate events being deemed "simultaneous" if their "t" coordinates are the same within a given frame.

Different observers will "see" (ie. the wavefronts will hit their eyes) at different times. Doesn't matter - they will each measure the distances from themselves to the events and subtract light travel time - exactly as I described I did for the asteroid strike and the solar flare above. The end result of those observers doing that is that they will also determine that both the asteroid strike and the solar flare occurred at 4:59:59 within our frame - that's the result EVERY observer in the frame will determine, regardless of their physical location, or the order that the wavefronts hit their eyes.

 
 
Ufonaut99

Re: Relativity of Simultaneity (Not Quite)

January 2 2015, 1:37 AM 

Nakayama : At the center of a passenger car, imagine letter L. Two light sources are set at the two ends of L. Is breakdown of simultaneity in two ways ? And when light is switched off, is simultaneity restored ?

Sorry, I'm not sure what you mean about the L. Let's clarify : every event happens at one point in space and time - ie. at an (x,y,z,t) coordinate.
In RoS (and SR), "simultaneous" is defined as two events having the same value for their "t" coordinate (eg. they both happened at "4:59:59").
Whether there actually IS any light travelling (eg. if the light has been switched off) may stop me knowing about the event, but doesn't change the coordinate values (and so doesn't change whether they occurred at the same time).

Nakayama : To Ufonaut99,
Thank you so much for your replay. However, in the castle of relativity, a sign of any changes is not seen at all. Relativity will rule us hereafter.

You're welcome happy.gif There's always "signs of changes" in relativity - after all, science is an on-going investigative process, and naturally everybody is keen to push their own angle on experiments and edge cases (time has to tell whether any of those ultimately has merit).
Further, we know Relativity is NOT the "Final Absolute TRUTH". It is simply the most useful and accurate model we have at the moment for high-velocity/mass/acceleration/gravity scenarios.

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

January 2 2015, 10:36 PM 

In the passenger car, a suspended ceiling is put up. On the ceiling, there is a small hole (at just above the light source set at the center of the car). A light ray is passing through the hole and reach the roof above. How about this ray to an observer outside the car (in relative motion)? According to a picture of books, light ray cannot pass through (or, passed ray is different ray). Picture shown in many books will be wrong.

To Ufonaut99,
Very sorry for insufficient explanation !! About letter L. Vertical difference is only to separate light paths. Horizontal difference will cause difference of value (that of breakdown of simultaneity : in one car, two breakdowns ?). I can’t tell latter’s meaning. I want relativists to teach its meaning (also meaning of darkness).
P.S. It's a simple idea (no confidences at all).

 
 
nakayama

Withdrawal and Apology

January 4 2015, 12:56 AM 

To Ufonaut, To all,

Former my post on “letter L” was wrong. I withdraw and apology.

 
 
ufonaut99

ufonaut99

January 5 2015, 4:02 AM 

Hi Nakayama, No worries happy.gif

For the passenger car, remember that if an event (like light going through a hole in the ceiling - so something at a single (x,y,z,t) coordinate) is observed in one frame, then it will be observed in every frame.

So picture an observer standing in the middle of the platform, watching the car pass left-to-right in front him. It could be that the laser and ceiling-hole start are on his left when the laser fires from the floor, but the motion of the car means that both are on his right by the time the light gets to the hole in the ceiling. This means that although the light travels vertically when measured by the observer in the car, it travels a diagonal path when measured by the observer on the platform.

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

January 5 2015, 11:41 PM 

Allow me to replace the picture (posted on Jan 2) with a picture following. The former was rather not clear.

A new picture : In a passenger car, two rays are shown symmetrically. On the other hand, ray of light in a light clock is bent. So, a ray that heads upward is bent (in this car). Then, how about two rays emitted to the upper right and the upper left (45 degrees) ? Consistent explanation seems to be impossible.

I don’t write on this problem (today’s picture) furthermore. Genuine (pure) pictures on simultaneity are shown formerly (in my post).

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

January 5 2015, 11:59 PM 

To Ufonaut99,
Thank you for your reply.

In a picture on “Relativity of simultaneity”, two rays horizontal are symmetrical. So, a photon emitted (from the same source) just upward seems not to pass the hole. But it’s rather not clear. Today’s picture above will be better somewhat.

 
 
Jose Rodriguez

Re: Relativity of Simultaneity (Not Quite)

January 8 2015, 9:49 AM 

Jose(previously):"I have pointed out numerous time that the terms "stationary" and "moving" are ambiguous, confusing statements. We're discussing relativity, so just use the term "the relative distance is changing between reference frames," or between source and observers."

Ufonaut:"Well, it's been 400 years since Galileo came up with his Ship - people should have had enough time to get to grips with it by now!"

"Seriously, though, what is confusing or ambiguous about saying that a passenger sitting on a train considers himself "stationary", regardless of whether the train itself is moving or not? Everybody can relate to that common everyday situation. The trouble is, if you leave the term "stationary" out, then people will start factoring in the observer's "velocity" into the measurements he makes, and so treating the other frame as the "special" or "real" one - just as Nakayama did above (and, of course, Pentcho does on a regular basis, every time he posts about the Doppler Shift)"


I am saying that relativity concerns merely the change in distance between source and receptor. Pretty simple aye? What light does in between is what Einstein and you are speculating. Whether either source or receptor are "stationary" is irrelevant.

"How about we compromise, and just say that an observer making a measurement always considers himself to be stationary."
Whatever. Your method is more confusing.

Jose(previously): "Sorry my friend, nothing (as far as we know) travels faster than the speed of light. So all of your lecturing is baseless fantasy. Light rays, or better yet, a strong short pulse of light, and the accompanying wave front is all that can really be physically investigated."


Ufonaut:"And sorry myself, but . . . Errrr, where did anything I wrote above require anything faster-than-light?"
I was merely making that point. I am not saying that you made any statement.

Ufonaut::For example, I said I saw a solar flare at 5:07:59 (ie, that's when the wavefronts from the flare met my eyes), and so since the sun is 8 light minutes away, I said that the flare must have happened 8 minutes previously - ie. at 4:59:59. Yes, we can only physically investigate the light - but that doesn't rule out that I knew before yesterday (from previous investigations) that the sun was, and is, 8 light minutes away from me."


Actually you didn't investigate anything. Everyone assumes that because we find the speed of light to be about a foot per nanosecond here on Earth, that it is true between the Earth and the Sun. (I go along with the assumption too, but it is basically an assumption.) Since the actual distance between the Earth and the Sun is continually changing due to the eccentricity of Earth's orbit and other factors, the elapsed time of flight of wave fronts between the Sun and Earth is not constant.

Ufonaut:Nothing there (nor in RoS, nor SR) requiring Faster-Than-Light. I didn't know before the light-speed signal reached me at 5:07:59 that a flare had gone off, for example.


Let us not be pedantic. You know that I know this.

Jose(previously): "You cannot admit that RoS exists within the the reference frame of the source, or sources, where distance is not changing among them and their observers. Sure, Einstein never mentioned that, but it would ruin his fantasy."


Ufonaut:"I repeat what I said above, EVERYBODY in my frame will ascribe the same "time" value to both those events, and will therefore also regard them as "simultaneous" REGARDLESS of how far away they are from [the events]. RoS is NOT just about "when the wave fronts from the events simultaneously meet the observer's eyes"; Rather, it is about spatially separate events being deemed "simultaneous" if their "t" coordinates are the same within a given frame."


In real life, everything depends upon what is detected at a given place (wave fronts). Minuscule number of people have the equipment and technology to perform what you suggest. I am quite sure you don't. GPS uses absolute time to determine simultaneity, all your philosophizing aside.

Ufonaut:"Different observers will "see" (ie. the wavefronts will hit their eyes) at different times. Doesn't matter - they will each measure the distances from themselves to the events and subtract light travel time - exactly as I described I did for the asteroid strike and the solar flare above. The end result of those observers doing that is that they will also determine that both the asteroid strike and the solar flare occurred at 4:59:59 within our frame - that's the result EVERY observer in the frame will determine, regardless of their physical location, or the order that the wavefronts hit their eyes."


What I said was that for any observer, at a given instant, that observer receives wave fronts simultaneously that were emitted at innumerable instances in the past. This idea is totally separate from the idea that at some instant in the past, in separate places in the universe, unique wave fronts were emitted from said places simultaneously. Where relative distances between said places are changing, the said changes in distance can be eliminated from consideration by merely taking several successive still picture of the said situation. Astronomers do this all the time. See the link ://www.cbsnews.com/pictures/hubble-telescope-reveals-hd-images-of-pillars-of-creation/4/

Whether one is relating the time of day as given on a clock face or some other set of events, t0, t1, t2, tetc.are all durations of time from some initial point in time to said point in time. So . . . What is the meaning of time2 (squaring a duration of time)?

 
 
nakayama

Re: Relativity of Simultaneity (Not Quite)

January 9 2015, 11:16 PM 

From just above, plane waves of light are coming horizontally. At the front edge and rear edge each on the roof of a moving passenger car, a sensor and a light source is set. In response to a special change of plane waves, the two flash. Two flashes will be simultaneous also to an observer stands on the ground. “relativity of simultaneity” seems not to stand up.

At the center on the roof of the above passenger car, the third sensor and light source is set. In response to flashes of the front and rear sources, it flashes. To an observer stands on the ground, “constancy of light speed” seems not to stand up.

 
 
ufonaut99

ufonaut99

January 10 2015, 10:07 AM 

Ufonaut:"Different observers will "see" (ie. the wavefronts will hit their eyes) at different times. Doesn't matter - they will each measure the distances from themselves to the events and subtract light travel time - exactly as I described I did for the asteroid strike and the solar flare above. The end result of those observers doing that is that they will also determine that both the asteroid strike and the solar flare occurred at 4:59:59 within our frame - that's the result EVERY observer in the frame will determine, regardless of their physical location, or the order that the wavefronts hit their eyes."


Jose : What I said was that for any observer, at a given instant, that observer receives wave fronts simultaneously that were emitted at innumerable instances in the past. This idea is totally separate from the idea that at some instant in the past, in separate places in the universe, unique wave fronts were emitted from said places simultaneously.


EXACTLY (*) - YES !! Now we're getting somewhere happy.gif

The Relativity of Simultaneity that Einstein wrote as the foundation of SR is the second sentence in green above. Nobody in SR is remotely interested about the stuff in the red , except insofar as it guides us to the timing of when the events happened in the past (ie. to the Relativity of Simultaneity stated in the green)

So in the opening post, when you took the the Relativity of Simultaneity page from wiki, and attempted to rebut it based on your ideas from your first sentence in red, you were actually writing about something "totally separate from the idea" that the the Relativity of Simultaneity page was writing about. Actually, it does make the title that you chose for this thread "Relativity of Simultaneity (Not Quite)" really rather apt wink.gif

So, how about we start addressing Einstein's Relativity of Simultaneity, from your second sentence I've marked in green ? It begins, as so many do, with Einstein's train gedanken. (Which, I suppose, brings us back to the document you wanted to discuss )

You wake up sitting on a train. You glance out the window, and see me standing on the platform directly outside your window (as it happens, I'm also exactly in the middle of the platform) - but just at that moment, the wavefronts from lightening strikes at the front and rear of the platform reach our eyes.

So far as I'm concerned, the strikes were simultaneous. I've measured the platform to be exactly 600m long, and the fronts hit our eyes at exactly 3:00:00 (yes, we both happened to notice that our watches both happened to read exactly the same time as we were passing as well ).

So for me in the middle of the platform, I reckon the light from each strike took the same time to reach me, so both strikes must have happened at the same time (in the past)

How about you on the train? Since we're discussing the consistency of SR here, let's temporarily assume the postulates are both true - would you reckon the strikes happened at the same time in the past ? Or can you show some logical inconsistency with SR's position that, by the passenger on the train's reckoning, they did not?

(*) well, not quite "exactly", since the "simultaneous" from the first sentence in red is not "totally separate" from the Relativity of Simultaneity stated in the green, but is actually a "special case". As I stated above according to the Relativity of Simultaneity, two events are deemed "simultaneous" if their "t" coordinates are the same. Naturally, if two wavefronts hit an eye at the same time, then by definition both light-Rays hit at the same (x,y,z,t) coordinate, and so also by definition the "t" coordinates for each light ray will also be the same.

 
 
Jose Rodriguez

Re: Relativity of Simultaneity (Not Quite)

January 11 2015, 1:19 PM 

Ufonaut99, we have had this discussion a year or two ago. You didn't retain much from then.

First of all, let's just use unit dimensions. The foot and nanoseconds are much easier to follow, Ie: light nanoseconds. Thus a pulse of light ray or beam travels about a foot per nanosecond.

Secondly, The pulse must have a duration. A bolt of lightning of no duration doesn't happen. Real lightning bolts last several microseconds. So, during a real lightning strike, the leading edge of the expanding pulse will be several thousand feet from the strike by the time the strike is over.

If the bolt strikes a single spot on each end of the platform, these spots will become scorched lines on the side of the train. If the duration of the strike is 5 nanoseconds, at the train's stated velocity, each line will be 3.75 feet long at each end of the train . Likewise the duration of time I would be aligned* with you would be in the femtosecond range. Not much time to be setting clocks. Not to mention what the Doppler effect does to the the communication between you and me.

Each lightning strike has only one light sphere expanding from it, towards us. (A 5 nanosecond duration of the strike makes the shell 5 feet thick.) Each ray approaches you at the speed of light, one foot per nanosecond. If you are 100 feet from each end of the platform, you will see the beginning of the strikes (coming from opposite directions) 100 nanoseconds after they struck.

If you had synchronized clocks located at the points of the strikes, they would show the strikes started 100 nanoseconds before 3 o'clock,(When at 3 o'clock you saw the pulses arrive at your midway station,). At each strike point, the opposite lightning pulse would arrive 200 nanoseconds later, the opposite clock showing 100 nanoseconds before three.

Since the train is moving at 3/4 the speed of light, during the time (100 nanoseconds) the pulses take to reach you at 3 o'clock, the train has moved 75 feet (plus or minus the 3.75 foot length of the scorch marks). If I am at the mid point between the scorched lines on the train, I won't be directly opposite of you when you see the simultaneous lightning strikes*. I will be 75 feet past your position. On the other hand, if you meant that I was directly across from you when you saw the flashes simultaneously, I would have been 75 feet to the rear of the midpoint between scorch marks on the train. (In which case I would see the pulses more or less simultaneously just as you, disregarding Doppler shifts; the front pulse duration being shorter and the rear pulse longer duration.)

(If I am at the midpoint between scorch marks on the train), when you see the simultaneous strikes on the platform, I am closer to the position of the front strike on the platform, and I see it happen before the rear strike, because I am farther from the rear strike. Each end point on the platform sees the opposite strike later than their own. I see the front pulse before you do, and the rear pulse after you do.

*This is one of your fallacies: See the second asterisk in the seventh paragraph. I cannot be in two places at once.



 
 
 
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