# Question on simultaneous events.

Originally posted by gnome
Did you learn all this at the Ralph Cramden Institute of Physics, or is this your own discovery? You really have a profound misunderstanding of the concept of special relativity (or any relativity, for that matter). For example, the idea of a 4-dimensional universe has clearly escaped you entirely. Even worse, you seem to be suffering from a rather shaky concept of the meaning of "dependent variable", you can't seem to distinguish between the relative motions of frames of reference themselves and the motions of objects within those frames, and you appear to be laboring under the misconception that all motion must be described in relation to some set of UNIVERSAL COORDINATE AXES, aligned with the ether and with its origin presumably identified by the location of the GREAT PUMPKIN.

In short, you really ought to learn to crawl before you try to fly.
No, you are the one who thinks only 1 spatial dimension is necessary to describe the movement of objects in 3 dimensional space. LOL. I guess the fact has escaped you for all these years that objects take up volume and things move in the universe in other than parallel directions. You are the one using some special coordinate scheme that magically aligns arbitrarily to every other object in space, and that completely ignores or removes all other existing velocities between frames except those in that very special x dimension.

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Dearest Eyesaw,

Obvious you don't know very well wat SR is about.
I sugest that you first learn SR (you don't have to believed, but studying it), and then say whats wrong with SR.

In SR we always speak about objects who doesnt accelerate. So forget sattelites , falling balls etc.

Second we always measure in our own frame of reference.
When a object is moving, we measure time dilitation for that object.
In the reference frame of the object there is no time dilitation to maeasure.

posted by lky said:
Am I correct to conclude that the 2 light bulbs are indeed turned on simultaneously, as if viewed by a stationary observer, since that the speed of light is constant to all observers, irregardless of their motion?

Doc AI in response to Icky

Everyone agrees that the light from each bulb arrived at the ship at the same time. But they disagree on whether the lights were switched on at the same time.

The fact that the two flashes reach the midpoint at the same time is evidence that they were turned on simultaneously according to observers in the rest-frame of the light bulbs. Observers in the ship will disagree that the lights were turned on at the same time. Observers in the ship will conclude that light B must have turned on first, since it is moving towards the ship.
Geistkiesel to Icky and Doc AI

If the moving platform merely detects the position of two pulses of light no assumptions about when the pulses left the sources can be made. If the stationary and moving observers know of the experiment then the detection of the pulses at the same place removes any ambiguity of when they were turned on. Only if the stationary observer triggers the pulses on does the experimental question have any relevanceThe mere fact of measuring the point two light pulses meet is insufficient to determine the respective positions of the pulse sources or when the sources emitted the pulses.

Let us assume the stationary and moving platform detectors are located within an x-ray wavelength from each other when measuring the arriving pulses. Here, all must agree that the simultaneous arrival of two light pulses measured by two light detectors were spatially equivalent and only if the stationary observer triggered the pulses simultaneously is there a thread to any discussion o 'relativity theory'. If moving observers then conclude the pulses were turned on at different times the basis of that conclusion is faulty. The mere fact of motion of a detector does not insert any ambiguity into the equation. There is simply a point in space where the pulses meet, where thestationary detector is located and whee th moving platform detector is located. Motion is not an issue. If the moving observers know the pulses were turned on by the staionary observer all must agree that the point the pulses meet is the midpoint of the sources.

Icky said:
Or would the motion of the ship have any effect on this simultaneity?

To which Doc AI responds:

Simultaneity is relative to the observer's frame. Observers at rest with the bulbs and those in the ship will disagree on what is simultaneous.

Geistkiesel to Icky and Doc AI

Doc AI cannot be correct.

Nobody is counting time. The measurement is purely a spatial determination ofwhere the light pulses meet. Here the pulses and all detectors are spatially equivalent. Watches aren’t relevant. Unless the participants know of the experiment do we have a quesion of simultaneity.

Icky said:
Or I am incorrect to assume that the midpoint of my journey means light has to travel the same distance for both the cases of points A and B?

The light from each bulb is only seen to have traveled the same distance according to the observers at rest with the bulbs. Folks in the ship disagree.
Geiskiesel to Icky and Doc AI

Neither the stationary observer or the moving platform observer can make any assumptions of when the lights were turned on. Two colliding pulses do not provide sufficient histories of their respective emissions. We can eliminate the stationary observer by saying she sent light pulses to A and B simultaneously from her midpoint position which triggered the pulses leaving A and B at the same time.

Now if her little sister in the moving platform knows the outbound pulse to A triggered the light that subsequently arrived from A (she could have measurd the wave front passing her ship to A behind her) and then she subsequently detects the light triggered from A and B at the same point and time she must conclude that she is also at the midpoint of the sources even with respect to her moving frome, otherwise all motion is ambiguous. Any calculation that disagrees with the measured event is theoretically faulty.

If the little sister on the space ship knows of the experiment, knows the lights are equal distant from the stationary observer, knows about the triggering pulses arriving at A and B, then the simultaneous positions of moving and stationary platform detectors and light pulses unambiguously assures the moving platform observer that her measurement was also at the midpoint of the lights at the instant of detection.

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Doc Al
Mentor
geistkiesel said:
Geistkiesel to Icky and Doc AI

If the moving platform merely detects the position of two pulses of light no assumptions about when the pulses left the sources can be made.
No need to assume anything: when the pulses were emitted can be deduced.
Only if the stationary observer triggers the pulses on does the experimental question have any relevanceThe mere fact of measuring the point two light pulses meet is insufficient to determine the respective positions of the pulse sources or when the sources emitted the pulses.
Nonsense. Remember that everyone knows where A and B are located. And that everyone agrees that the pulses arrive simultaneously at the midpoint. Both observers have all the information needed to calculate when the pulses were emitted.

Let us assume the stationary and moving platform detectors are located within an x-ray wavelength from each other when measuring the arriving pulses. Here, all must agree that the simultaneous arrival of two light pulses measured by two light detectors were spatially equivalent and only if the stationary observer triggered the pulses simultaneously is there a thread to any discussion o 'relativity theory'.
Again, the stationary observer (at the midpoint) doesn't have to "trigger" anything. (A and B have clocks, you know. )
If moving observers then conclude the pulses were turned on at different times the basis of that conclusion is faulty.
Wrong again. When the pulses were turned on can be deduced and is frame dependent.
The mere fact of motion of a detector does not insert any ambiguity into the equation. There is simply a point in space where the pulses meet, where thestationary detector is located and whee th moving platform detector is located. Motion is not an issue. If the moving observers know the pulses were turned on by the staionary observer all must agree that the point the pulses meet is the midpoint of the sources.
The fact that the observers are at the midpoint between the sources is known from the start. They don't deduce that from the fact that the light arrives simultaneously.
Doc AI cannot be correct.
Perish the thought.
Nobody is counting time. The measurement is purely a spatial determination ofwhere the light pulses meet.
They aren't "measuring" location--they both know they are at the midpoint; they are detecting that the light arrived simultaneously. Time is very relevant.
Here the pulses and all detectors are spatially equivalent. Watches aren’t relevant. Unless the participants know of the experiment do we have a quesion of simultaneity.
I have no idea what you mean by "spatially equivalent". And the issue is when the signals were emitted, which they both can deduce from their knowledge of how light works.
Neither the stationary observer or the moving platform observer can make any assumptions of when the lights were turned on.
They don't have to assume anything.
Two colliding pulses do not provide sufficient histories of their respective emissions.
Sure they do: We know where they started and how fast they move.
We can eliminate the stationary observer by saying she sent light pulses to A and B simultaneously from her midpoint position which triggered the pulses leaving A and B at the same time.
Whether the stationary observer triggers the pulses by sending a signal to A and B--or not--is irrelevant.

Now if her little sister in the moving platform knows the outbound pulse to A triggered the light that subsequently arrived from A (she could have measurd the wave front passing her ship to A behind her) and then she subsequently detects the light triggered from A and B at the same point and time she must conclude that she is also at the midpoint of the sources even with respect to her moving frome, otherwise all motion is ambiguous. Any calculation that disagrees with the measured event is theoretically faulty.
Again you seem confused about the assumptions of the problem: Everyone knows and agrees that: both observers are at the midpoint and that the light arrives there simultaneously. That's all anyone needs to know. If someone tries to "calculate" travel times with other assumptions, they will get nonsense.
If the little sister on the space ship knows of the experiment, knows the lights are equal distant from the stationary observer, knows about the triggering pulses arriving at A and B, then the simultaneous positions of moving and stationary platform detectors and light pulses unambiguously assures the moving platform observer that her measurement was also at the midpoint of the lights at the instant of detection.
Again the moving observer doesn't have to know anything except that the light was emitted by A and B, which are equidistant from her. You have come full circle. Was there a point you wanted to make?

Realize that the moving observer, if she knows anything about how light works, will insist that according to her the signals were emitted at different times. And if (as you insist, but is irrelevant) the stationary observer triggered the light emissions at A and B by sending her own signal to A and B, realize that the moving sister will disagree that those signals arrived at A and B simultaneously. There is no way around it: Simultaneity is frame dependent. And that is the point of this exercise.

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Doc Al said:
All observers will agree that the light arrives simultaneously at the ship. So, the observer on the ship would see the the two light beams arrive simultaneously. This does not mean that observers would agree that the lights were turned on simultaneously; that is a deduction, not a direct observation.

See my comments above. No one "sees" the lights turned on simultaneously. But observers at rest with A-B will insist that they were turned on simultaneously. Observers in the ship will not.
I was under the impression that the problem started out with evryone knowing that M was the midpoint between A and B and that the pulses met at M simultaneously. O, knowing he is at the midpoint and seeing the pulses meet there at the same time deduces the lights were turned on at the same time. If O' also knows he is at the midpoint and that the pulses are detected simultaneously with O, there can be no relativity significance to the problem. The mere fact that O' is moving at the instant the lights were turned on does not mean the O' conclude that B must be turned on first in order that O' see them meet at the midpoint.

In a hypothetical automobile race. A and B are equidistant from the finish line heading toward each other at the same speed. The automobiles meet at the finish line, at the same time, obviously. When we rewind the race and insert a slower automobile, O', moving in the same direction as A at such a speed and distance from the finish line that all three meet at the finish line at the same time, what theory is there that insists O' must conclude that B cheated on A and started out earlier in order for all of them to meet at the same place at the same time?

The point of all this, as was originally asserted, regards the question of, "would O' think the lights started out at different times?"

Answer: No, and further, this is not an SR problem.

Janus
Staff Emeritus
Gold Member
geistkiesel said:
I was under the impression that the problem started out with evryone knowing that M was the midpoint between A and B and that the pulses met at M simultaneously. O, knowing he is at the midpoint and seeing the pulses meet there at the same time deduces the lights were turned on at the same time. If O' also knows he is at the midpoint and that the pulses are detected simultaneously with O, there can be no relativity significance to the problem. The mere fact that O' is moving at the instant the lights were turned on does not mean the O' conclude that B must be turned on first in order that O' see them meet at the midpoint.

In a hypothetical automobile race. A and B are equidistant from the finish line heading toward each other at the same speed. The automobiles meet at the finish line, at the same time, obviously. When we rewind the race and insert a slower automobile, O', moving in the same direction as A at such a speed and distance from the finish line that all three meet at the finish line at the same time, what theory is there that insists O' must conclude that B cheated on A and started out earlier in order for all of them to meet at the same place at the same time?
In this case, the addition of velocities therom:

$$w=\frac{u+v}{1+\frac{uv}{c^{2}}}$$

In which case w would be the relative velocity of O' to A or B when u is the velocity of the finish line with respect to O' as measured by O' and v is the relative velocity of A or B to to the finish line with from the respect of the finsh line as measured by the someone at the finish line.

The result of which will show that according to O', A and B will not have the same velocities with respect to the finish line, And thus, since they start an equal distance from the starting line, one must have started earlier in order for them both to reach the finish line at the same time as O'.

Of course at normal every day speeds, this difference is so small it is not noticeable, it isn't until the velocities involved reach a good fraction of c that it becomes measureable.

Doc Al
Mentor

geistkiesel said:
I was under the impression that the problem started out with evryone knowing that M was the midpoint between A and B and that the pulses met at M simultaneously. O, knowing he is at the midpoint and seeing the pulses meet there at the same time deduces the lights were turned on at the same time.
Right.
If O' also knows he is at the midpoint and that the pulses are detected simultaneously with O, there can be no relativity significance to the problem. The mere fact that O' is moving at the instant the lights were turned on does not mean the O' conclude that B must be turned on first in order that O' see them meet at the midpoint.
Sure it does, if you think about it. Did you actually read the previous posts in this thread? We've discussed this in excruciating detail. Ask yourself this: In O's (moving) reference frame, where were A and B when they switched on their lights? Were they equidistant from O' at that time? (We all agree that they are equidistant from O' at the moment that O' passes the midpoint--but so what? We need to know where A and B were when they flashed their lights.)

In a hypothetical automobile race. A and B are equidistant from the finish line heading toward each other at the same speed. The automobiles meet at the finish line, at the same time, obviously. When we rewind the race and insert a slower automobile, O', moving in the same direction as A at such a speed and distance from the finish line that all three meet at the finish line at the same time, what theory is there that insists O' must conclude that B cheated on A and started out earlier in order for all of them to meet at the same place at the same time?
Of course, for ordinary cars moving at ordinary speeds, nothing special happens. But get those cars moving at light speed (or an appreciable fraction of light speed) and things are quite different. The theory that describes how lengths, times, and simultaneity changes from one fast moving frame to another is called Special Relativity.

The point of all this, as was originally asserted, regards the question of, "would O' think the lights started out at different times?"

Answer: No, and further, this is not an SR problem.
Please take the time to study the previous posts in this thread. Not only is this an SR problem, it is a canonical SR problem!

Everybody knows the lights wee turned on at the same time . . .

Geistkiesel said:

Only if the stationary observer triggers the pulses on does the experimental question have any relevance.The mere fact of measuring the point [the] two light pulses meet is insufficient to determine the respective positions of the pulse sources or when the sources emitted the pulses.

Doc AI responds
Nonsense. Remember that everyone knows where A and B are located. And that everyone agrees that the pulses arrive simultaneously at the midpoint. Both observers have all the information needed to calculate when the pulses were emitted.

Geistkiesel to Doc AI said:
OK, you win a point. I had the parameters skewed. Everybody knows that the meeting of light pulses and the observer O' is simultaneous. O knows he is at the midpoint and knowing the speed of light he determines when the light was turned on. ["when" the lights were turned on is not the issue, only that they were turned on simultaneously is within the limits of the problem] O' also knows he is at the midpoint of the sources, but because the O' speed is slower than c, he must have been moving to M before the pulses left A and/or B. If the speed of O' were too slow, then the light from A would pass him and later he would see the oncoming light from B. Similarly if the 0' speed were too fast O' would cross the midpoint and then see the light from B before he saw the light from A and would then know he had passed the midpoint of A and B, because only by simultaneous meeting of the pulses at M can O' know he is at the midpoint, unless there is a litle flag there, which is within the parametric limitations of he problem.

If O' were moving at just the correct speed, such as the problem states, then the simultaneous meeting of pulses, O and 0', would tell O' that he had intercepted the pulses at M, which is given [he can verify this by seeing the midpoint flag]. The problem states that everybody knows where A and B are and that all met at M, the half way point at the same instant. In the too slow and too fast cases O' would therefore know if he were short of M or had passed M, which would all be suppored by O' seeing the flag at M..

The problem clearly stated that O' was at the midpoint of A and B when the pulses met. Without any relativity calculations by O', the fact that he knew he was in the midpoint in the O frame, he knows that the lights were turned on simulataneously in the O frame. Now, if O' makes any calculation that informs him the lights were turned on at different times in the O' frame then the O' theory used in making this calculation is flawed.

The calculations that are discussed in this thread all have O' moving at some speed less than C. If O' is moving at 1% of c or 99% of c the given limitations of the problem change nothing. I have struggled to determine what calculations O' can make that would change his observation and knowledge that all pulses and detectors were in a simultaneous configuration from beginning to end.This is because everybody knew the light left A and B at the same time and met at M at the same time.

In maintaining his mathematical instincts intact O' must offer a hearty "ho ho" to O, "we disagree on the math, but we agree on the physics."

Doc AI, please tell us what are the mechanics of the theory, SR I presume, that dictates O' must conclude the lights were turned on at different times?

Doc Al
Mentor

geistkiesel said:
The problem clearly stated that O' was at the midpoint of A and B when the pulses met. Without any relativity calculations by O', the fact that he knew he was in the midpoint in the O frame, he knows that the lights were turned on simulataneously in the O frame.
Right. Everyone agrees that the lights were turned on simultaneously as observed in the O frame.
Now, if O' makes any calculation that informs him the lights were turned on at different times in the O' frame then the O' theory used in making this calculation is flawed.
Nonsense. O' knows what he observes and he knows how A and B are moving and he knows how light behaves. He knows that with respect to his own frame the lights must have been turned on at different times. This is no big deal to O', since he understands that time, length, and simultaneity are relative to the frame making the observations.
... I have struggled to determine what calculations O' can make that would change his observation and knowledge that all pulses and detectors were in a simultaneous configuration from beginning to end.
O' does not directly "observe" that A and B were turned on simultaneously. (In fact, no one does!) In fact, based on what he knows about how the world works, he would vehemently disagree that the lights were turned on at the same time! The light from A and the light from B travel very different distances to get to O'--so they must start out at different times according to O's clocks. Otherwise things just don't make sense. (The light from A and the light from B only travel the same distance in Os frame.)
This is because everybody knew the light left A and B at the same time and met at M at the same time.
Not everybody! Only the frame in which A and B are at rest (the O frame) did the light leave A and B at the same time.

In maintaining his mathematical instincts intact O' must offer a hearty "ho ho" to O, "we disagree on the math, but we agree on the physics."
No. O', being sophisticated and wise, would say "We agree on the math and the physics, but since we are in relative motion we disagree on the times that the lights were turned on." You will, I hope, forgive me for assuming the validity of special relativity. After all, this is the Relativity forum.
Doc Al, please tell us what are the mechanics of the theory, SR I presume, that dictates O' must conclude the lights were turned on at different times?
I'm not about to give a class in relativity. There are many good books and web sites where you could learn the basics. The math of special relativity is easy. The hard part is believing it. We believe it for many reasons, not the least of which is that its consequences have been experimentally verified over and over again.

In a nutshell, start with this fact: Light always moves with the same speed (c) with respect to any observer, no matter what the observer's speed (relative to some other observer). Wrap yourself around that strange fact. If you consistently apply that fact you can deduce the consequences of special relativity: moving clocks are observed to slow down, moving sticks are observed to be shorter, and clocks that are in synch (in their own frame) are observed to be out of synch.

Doc Al said:
All observers will agree that the light arrives simultaneously at the ship. So, the observer on the ship would see the the two light beams arrive simultaneously. This does not mean that observers would agree that the lights were turned on simultaneously; that is a deduction, not a direct observation.

See my comments above. No one "sees" the lights turned on simultaneously. But observers at rest with A-B will insist that they were turned on simultaneously. Observers in the ship will not.
Geistkiesel responds to Doc Ai thus:
If the ship is at the midpoint of the sources of light, there is one spot in the universe where the pulses first meet and this is at the midpoin M. It seems Doc AI finds disfavor with the word "deduction", well, so be it. There is no way that the observer on the ship can manipulate reality and have the light pulses start out at different times and meet at the mid point. This is a physical impossibility, but mathematics can manipulate the best of experimental results. If the ship's crew "disagrees" with the stationary observers we must concede to them their very deep and inalienable right to be in error.

What does Doc AI replace the word "deduction" with? And why does he want to discard what someone deduces? I was under the impression this is what scientists do in their proffesion. Doc AI has mentioned a number of times that the people on the ship willl "not agree" the lights were turned on at the same time and his argument isn't through the principals of physics, rather it is something else that I am unable to properly catagorize. I have some deep doubts regarding the literal reality as expressed by SR, but using SR as it is understood will not change the reality of this particlular situation.

Everybody knows what the answer is at the instant the lights meet at M.

As it was pointed out to me in no uncetain terms (by Doc AI no less) the parameters of the problem are that everybody knows the light meets at the midpoint. If someone wants to manipulate numbers to obfuscate the unambiguous experimental result to dscard a distastful "deduction", this I can understand when coming from the dogmatcally inclined.

Who was it who remarked: "The enemies of truth. Convictions are more dangerous enemies of truth than lies."

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Doc Al
Mentor
Please learn the correct use of the "quote" feature--it's hard to respond to your comments, since you are quoting yourself.

geistkiesel said:
If the ship is at the midpoint of the sources of light, there is one spot in the universe where the pulses first meet and this is at the midpoin M.
Right!
It seems Doc Al finds disfavor with the word "deduction", well, so be it.
What makes you say that? I do have a problem with incorrect "deduction" based upon mistaken preconceptions.

There is no way that the observer on the ship can manipulate reality and have the light pulses start out at different times and meet at the mid point.
Ah... and what makes you say they started out at the same time? Did you check A's watch and B's watch when they switched on the light? Are you sure they were synchronized? Do their clocks keep correct time? You merely argue in a circle by assuming that arriving simultaneously at the midpoint implies that they left at the same time. (It does imply that--but only in the rest frame.) Remember nobody is able to directly observe the light leaving A and B at the same time: A and B are far apart!

Of course, knowing how light works, an observer O at the midpoint can deduce (correctly) that A and B emitted their light pulses at the same time--according to their own clocks. But moving observers (like O') can make their own valid deductions--and they get a different answer! (You don't have anything against making deductions... Do you? )
This is a physical impossibility...
And planes will never fly!
What does Doc Al replace the word "deduction" with? And why does he want to discard what someone deduces?
But if you "deduce" something based on false premises? ... Remember: garbage in, garbage out.

I was under the impression this is what scientists do in their proffesion. Doc Al has mentioned a number of times that the people on the ship willl "not agree" the lights were turned on at the same time and his argument isn't through the principals of physics, rather it is something else that I am unable to properly catagorize. I have some deep doubts regarding the literal reality as expressed by SR, but using SR as it is understood will not change the reality of this particlular situation.
Clearly you have your own ideas about how the world works. (I've seen your website.) If you'd like to discuss them, the place to do it is Theory Development.

Everybody knows what the answer is at the instant the lights meet at M.
Everyone knew the earth was flat, also.

As it was pointed out to me in no uncetain terms (by Doc AI no less) the parameters of the problem are that everybody knows the light meets at the midpoint. If someone wants to manipulate numbers to obfuscate the unambiguous experimental result to dscard a distastful "deduction", this I can understand when coming from the dogmatcally inclined.
I think we're getting to the root of it. You have made up your mind that "relativity is wrong" (gee... where have I heard that before?). And anyone who disagrees with you is dogmatic? Hmmm....

You seem to think--by some stretch of logic--that simultaneous arrival somehow implies simultaneous emission in every frame. But that deduction assumes things that just aren't true: Like an incorrect understanding of the speed of light, for one.

Who was it who remarked: "The enemies of truth. Convictions are more dangerous enemies of truth than lies."
My sentiments exactly!

Who was it who said: "But the fact that some geniuses were laughed at does not imply that all who are laughed at are geniuses. They laughed at Columbus, they laughed at Fulton, they laughed at the Wright brothers. But they also laughed at Bozo the Clown."

Janus
Staff Emeritus
Gold Member
geistkiesel said:
Geistkiesel responds to Doc Ai thus:
If the ship is at the midpoint of the sources of light, there is one spot in the universe where the pulses first meet and this is at the midpoin M. It seems Doc AI finds disfavor with the word "deduction", well, so be it. There is no way that the observer on the ship can manipulate reality and have the light pulses start out at different times and meet at the mid point. This is a physical impossibility, but mathematics can manipulate the best of experimental results. If the ship's crew "disagrees" with the stationary observers we must concede to them their very deep and inalienable right to be in error.

What does Doc AI replace the word "deduction" with? And why does he want to discard what someone deduces? I was under the impression this is what scientists do in their proffesion. Doc AI has mentioned a number of times that the people on the ship willl "not agree" the lights were turned on at the same time and his argument isn't through the principals of physics, rather it is something else that I am unable to properly catagorize. I have some deep doubts regarding the literal reality as expressed by SR, but using SR as it is understood will not change the reality of this particlular situation.

Everybody knows what the answer is at the instant the lights meet at M.

As it was pointed out to me in no uncetain terms (by Doc AI no less) the parameters of the problem are that everybody knows the light meets at the midpoint. If someone wants to manipulate numbers to obfuscate the unambiguous experimental result to dscard a distastful "deduction", this I can understand when coming from the dogmatcally inclined.

Who was it who remarked: "The enemies of truth. Convictions are more dangerous enemies of truth than lies."

Maybe a visual aid will help.

First, it is important to remember that the speed of light has the same value for all observers regardless of their relative motion. This comes from the fact that the speed of light is dependant on just two parameters of free space. Since these parameters don't change with motion, neither does the measured speed of light.

Thus we have the following animations showing two lightning strikes along a train track and are observed by two observers, one standing along the track and one on a moving railway car. The strikes occur equidistant form the first observer and the flash arrives at the same time as the car observer passes him (Both observers see the flashes at the same instant.)

The first animation shows what happens according to the embankment observer.

http://home.teleport.com/~parvey/train1 [Broken]

As you can see the strikes must happen at the same instant in order for both observers to see them at the same time.

The second animation shows what happens according to the car observer.

http://home.teleport.com/~parvey/train2 [Broken]

Again the flashes must reach both observers at the same time. Again, each flash expands outward from the point of emission at the speed of light relative to the observer. The center of this expansion does not move with respect to this observer. (if it did, then different points on the expansion wavefront would have different velocities with respect to the car observer, and this would conflict with the fact that the light has to have a constant velocity with respect to any observer.

Due to this, it is easy to see that according to the car observer, the lightning strikes cannot take place at the same time. One strike must occur later than the other in order to have the flashes arrive at the proper time and place. (such that both observers see both flashes at the same time.)

This is known as the "Relativity of Simultaneity"

"Simultaneous" is relative. Events that are simultaneous in one frame are not in another that is moving with respect to the first. Nor is it correct to say that the events as according to the embankment observer are "true" and those according to the car observer aren't. Both versions are equally valid.

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SOmetimetimes the world just turns out silly, simultaneoulsy silly.

Quote:
Originally Posted by Geistkiesel said:
I was under the impression that the problem started out with everyone knowing that M was the midpoint between A and B and that the pulses met at M simultaneously. O, knowing he is at the midpoint and seeing the pulses meet there at the same time deduces the lights were turned on at the same time.
Which DocAI responds:[i"]Right"

Quote.
Originall posted by Geistkiesel said:
If O' also knows he is at the midpoint and that the pulses are detected simultaneously with O, there can be no relativity significance to the problem. The mere fact that O' is moving at the instant the lights were turned on does not mean the O' conclude that B must be turned on first in order that O' see them meet at the midpoint.
Originallly quoted by DocAI said:
Sure it does, if you think about it. Did you actually read the previous posts in this thread? We've discussed this in excruciating detail. Ask yourself this: In O's (moving) reference frame, where were A and B when they switched on their lights? Were they equidistant from O' at that time? (We all agree that they are equidistant from O' at the moment that O' passes the midpoint--but so what? We need to know where A and B were when they flashed their lights)

Oh, I get it. O' gets to impose his universe on O and move A and B around until they dovetail with DocAI's contrived perception of reality.

If O' also knows he is at the midpoint and that the pulses are detected simultaneously with O, there can be no relativity significance to the problem. There is no two measurements of one event from a distance where one is moving the pther stationary. The mere fact that O' is moving at the instant the lights were turned on does not mean the O' conclude that B must be turned on first in order that O' see them meet at the midpoint.This is single point measuremdent experiment that does not involve dialting clocks or shrinkng measuring rods. Remember we are only looking at a single measurement.

Whatare you doing here DocAI? I detect no scientific an analytic
effort on you part. I see that your position has nothing to do with SR, flashing lights, reason, truth and scieitific integrity. I see a level of implied SR theoretical affect in this experiment as is applicable to a farmer milking his cow.The farmerand his caow can care less.

You chastise me with an insult that I hadn't done my homework yet you consciously twist words, your own in this case, to fit your personal and egotistical contrived agenda.

I tell you this is what I see, observe and of what I take notice.

Quite frankly, I've had just about enough of your silliness, but I guess were stuck with each other,, so of this I have no complaints.[/QUOTE]

Doc Al
Mentor
geistkiesel said:
Oh, I get it. O' gets to impose his universe on O and move A and B around until they dovetail with DocAI's contrived perception of reality.
You don't get it at all. Both O and O' are perfectly entitled to their different measurements of when the lights were turned on. They don't contradict each other, since they are in different frames. What you amusingly call my "contrived perception of reality" is what everyone else just calls "physics".

If O' also knows he is at the midpoint and that the pulses are detected simultaneously with O, there can be no relativity significance to the problem.
Not until they wish to determine when the pulses were sent. If all they care about is: "Did the pulses arrive simultaneously?", then everyone agrees that they did!

There is no two measurements of one event from a distance where one is moving the pther stationary.
As long as all you care about is what happens at that single event in spacetime--the midpoint at the moment the two pulses arrive--you are correct. But I know you wish to deduce more than that!

The mere fact that O' is moving at the instant the lights were turned on does not mean the O' conclude that B must be turned on first in order that O' see them meet at the midpoint.This is single point measuremdent experiment that does not involve dialting clocks or shrinkng measuring rods. Remember we are only looking at a single measurement.
As soon as you want to draw conclusions about about when the pulses were emitted, you are talking about observations of things happening at multiple points in time and space! This involves clocks and measuring rods, and understanding how they work. It is no longer a "single point measurement". SR demands that moving frames get different answers.

Whatare you doing here DocAI? I detect no scientific an analytic
effort on you part. I see that your position has nothing to do with SR, flashing lights, reason, truth and scieitific integrity. I see a level of implied SR theoretical affect in this experiment as is applicable to a farmer milking his cow.The farmerand his caow can care less.
Well... if you had the courtesy to actually read the previous posts, you will find this problem fully analyzed according to standard SR. (And Janus has prepared some excellent animations illustrating how light behaves according to both observers.)

But you seem to prefer arguing for your "common sense" convictions. But your arguments are nothing more than repeating "it obviously can't be that way" with a few personal insults tossed in for good measure.
You chastise me with an insult that I hadn't done my homework yet you consciously twist words, your own in this case, to fit your personal and egotistical contrived agenda.
Nice attempt at a reversal. You obvious didn't do your homework if you can seriously persist in arguing that this problem has nothing to do with relativity. Give me a break.
Quite frankly, I've had just about enough of your silliness, but I guess were stuck with each other,, so of this I have no complaints.
Take care then. When you are ready to talk physics, come on back.

Taps are playing for erstwhile relativity theorists . . .

Doc Al said:
You don't get it at all. Both O and O' are perfectly entitled to their different measurements of when the lights were turned on. They don't contradict each other, since they are in different frames. What you amusingly call my "contrived perception of reality" is what everyone else just calls "physics".

Not until they wish to determine when the pulses were sent. If all they care about is: "Did the pulses arrive simultaneously?", then everyone agrees that they did!

As long as all you care about is what happens at that single event in spacetime--the midpoint at the moment the two pulses arrive--you are correct. But I know you wish to deduce more than that!

As soon as you want to draw conclusions about about when the pulses were emitted, you are talking about observations of things happening at multiple points in time and space! This involves clocks and measuring rods, and understanding how they work. It is no longer a "single point measurement". SR demands that moving frames get different answers.

Well... if you had the courtesy to actually read the previous posts, you will find this problem fully analyzed according to standard SR. (And Janus has prepared some excellent animations illustrating how light behaves according to both observers.)

But you seem to prefer arguing for your "common sense" convictions. But your arguments are nothing more than repeating "it obviously can't be that way" with a few personal insults tossed in for good measure.

Nice attempt at a reversal. You obvious didn't do your homework if you can seriously persist in arguing that this problem has nothing to do with relativity. Give me a break.

Take care then. When you are ready to talk physics, come on back.
Okay, Doc AI, here's your break.

The simplest way to solve the problem is to start with O' moving to M at some velocity v(O'). O' must know he is moving otherwise the problem shifts to O who has the same problem. However, we can verify that O' knows he is moving by a measure of the red/blue shifts in the recorded light pulses at M. If this were all, then we can only say that the surface of the radiated wave fronts, A and B, of any arbitrarily located sources must be equidistant from M at some time in the past. This is so as the O' clocks are stablized at a rate determined by their velocity wrt time in the O'. The time for the light to reach M is the same for both A and B, even though the sources may be anywhere as long as their wave fronts are equidistant from the eventual meeting point at M. The time for the wave frionts located at t/c from A-M and B-M is the same as the O' frame is the same for both wave fronts.

DocAI is partially correct in insisting that light dfrom B must have been turned on before the light from A. The light from B can be turned on at any time before the light from A is turned on as long as the wave front from B is located at t/c at the time the light from A was pulsed on. We must only determine the time t' when the wave front (or other physical source at A) was at t'/c.

O' can trigger a delayed pulse time, t = 0 for A as he passes by. A can then send pulses calibrating B as long as the delay time from t = zero allows the calibration signal from A to B plus the time for the pulses to arrive at M is sufficiently long. So O' dutifully waits until the signal from A and B arrive at the same time. In O' time O' can then calculate A and B distances from M. Without a physical source at B the requirment for the wave front is as determined above, yet the measured time from passing A to signal arrival detemines the distance for both A and B.

Or O can measure the O' relative velocity wrt M and share the information with O' for all locations to be calculated when M is reached. Clearly, in the O frame M is the midpoint of the A-B line. Likewise, in the O' frame the distances are equivalent. If we take the zero point in time at A then t'(d') = t(d) and all clocks can be calibrated. Even though the output from the clock on O' says t' =d'/c seconds and that t =d/c seconds we know t' < t in absolute second counts, but once determining that t' = d/.8c the clock differences are easily calibrated. As long as there is sufficient amount of time O can send a steady signal of dots measured in dt = 1 second in the staionary frame. O' receives the dots, calculatess the time difference in th O' frame, hence relative velocities may be determined.

Let us take the time zero point (OO') at A when O' passes by. Then we all have to agree that the zero time in both frames is equivalent. Likewise, the stop times when the pulses (delayed) reach M are equivalent and simultaneous. Therefore the distances A-M and B-M in both frames are equivalent notwithstanding that the clock times measuring the distances need calibrating which can be accompliched as described. The distance in both are the same but the clock differences leaves the illusion the distances are different.

Do they play 'Taps" when a cherished theory asks "for whom the bells toll and discovers that it was answered "for thee"?
"
So this sounds like some relativity legs just got cut off at the knees, doesn't it. Like Richard Nixon said " . . . a million dollars in bribe money could be raised, but that would be wrong", that opting only for a "break", by one anyway, we will realize that the "cutting iff at the knees" 'would be wrong', excessive and beyond the request of an erstwhile relativity theorist.

So we will politely retain a semblance of RT by recognizing that there is a measurable difference in the two systems, that is the old SR/GR system and the new SR/GR system.

We all know that an electron will radiate EM quanta during acceleration and at constant velocity the electron radiation ceases, yet the electron's energy is proportional to 1/2 mv2 wrt the lab frame. The electron is in a higher energy state than before it was accelerated. All moving mass wrt the lab frame has some increase in relativistic mass and at sufficiently high velocities the mass energy difference can be measured as a measuremnt of the 2(pi)hf of the electron.

So very briefly, why do clocks slow down at elevated velocities? Because the masses constituting the the clocks have all increased in energy to a level that the intake of subsequent accleratinmg phonons cannot be processed with the same efficiency as at lower velocities. Likewise, the masses of the clock do not sit in isolation from each other. Any and all inter-mass energy exchange coupling efficiencies are effectivley lowered.

Linear velocity increases, meaning increase in velocity is linear with the increase in energy intake, are sacrificed for the sheer purpose of increasing the vibration rate of the particle. Velocity is a measure of the current relative energy difference of the acclerated mass and the zero lab frame mass. The increase in vibiration lowers the ability of the particle to store energy as iincreases in velocity relaqtive to lower velocities.

Some wanted a break, so there you got it. Relativity phenomena is measured by the relative acceleration of mass wrt to zero velocity wrt lab frame.

An example:
Mossbauer measurements where gamma radiation input into a stationary mass target can show recoiless gamma radiation from the target when the velocity of the gamma particle source is a few centimers/second. A dv = 0 (for a properly chosen gamma source) there is no recoil experienced by the target particle, or said another way, there is a complete energy exchange efficiency of gamma and target. The gamma source is slightly high, then the target recoils, or the gamma source is low , the target recoils. In either case the gamma-target source velocity differences are, energetically speaking, incoherent.

The effect is a measure of the relative energy difference of gamma particles accelerated with the added mass source velocity many orders of magnitude less than the natural frequency of the test gamma radiation particles at rest in the lab frame. (It takes at least 4 biggrins to constitute gloating.)