Light shone in a train bouncing off mirrors

[ZikZak]

Unjustified and unwarranted assumption. Never have I stated that "there is absolute simultaneity".

No, you've only been arguing in its favour for 6 pages. You've only been arguing that people on an embankment will be stunned with surprise when an electric chair is activated by nonsimultaneous bolts.

 

[altonhare]

althonhare (really simultaneous is what is simultaneous in the “local” frame), is not valid.

I argue that it is valid because it makes no sense to say "X is both Y and non Y" i.e. that "AC and BD are simultaneous and not simultaneous". We might disagree on quantity but never quality.

I'll say this in anticipation of future comments. I said that it is impossible for an observer to conclude an entity is motionless. I justified this by saying that O would have to assume that A is the only other entity in the universe. In fact, it is impossible for O to conclude that A is motionless period. Imagine observer O is watching A. The only way to conclude that A is motionless is to write down A's location L1 and it's "time" T1, then repeat with L2 and T2. Now one claims that O can measure L1=L2 and T2>T1 and this "proves" A is motionless. Wrong! The only way for the statement: T2>T1 to be true is for A to have moved relative to the clock! If the clock emitted a photon then A's location is now different relative to the photon. If an arm moved then now A's location is different relative to the arm. L2 != L1 unless T1=T2.

Therefore the only sets of logic statements with physical significance are:

L1=L2 ; T1=T2

L1!=L2 ; T1<T2

And the statements corresponding to motion and motionless:

L1!=L2 ; T1<T2 corresponds to motion

L1=L2 ; T1<T2 corresponding to motionless, which is demonstrably nonphysical

 

[altonhare]

No, you've only been arguing in its favour for 6 pages. You've only been arguing that people on an embankment will be stunned with surprise when an electric chair is activated by nonsimultaneous bolts.

You're digging yourself in a hole. There is not a single incident of me making the claim you accused me of. Just admit you made a mistake and move on.

 

[matheinste]

Quote from #60 by altonhare:-
-----Here's the deal. There is no "relativity of simultaneity" because the concept "simultaneity" only has meaning for events in their own inertial frame, i.e. in the rest frame of the train in this instance. The observers on the embankment are simply wrong if they actually conclude that the events "were not simultaneous". It doesn't matter if they tack on "in our reference frame" because such a conclusion is worthless and irrelevant to the question of *were the events actually simultaneous?*. Obviously they actually were, because Mathe is dead, so simultaneity is NOT relative. If Mathe continues to argue that simultaneity is relative he'll be happy to strap into the chair as long as nobody on the train is watching the event, and lots are on the embankment to watch the bolts be "non simultaneous"! ------

Matheinste

 

[JesseM]

Quote from #60 by altonhare:-
-----Here's the deal. There is no "relativity of simultaneity" because the concept "simultaneity" only has meaning for events in their own inertial frame, i.e. in the rest frame of the train in this instance. The observers on the embankment are simply wrong if they actually conclude that the events "were not simultaneous". It doesn't matter if they tack on "in our reference frame" because such a conclusion is worthless and irrelevant to the question of *were the events actually simultaneous?*. Obviously they actually were, because Mathe is dead, so simultaneity is NOT relative. If Mathe continues to argue that simultaneity is relative he'll be happy to strap into the chair as long as nobody on the train is watching the event, and lots are on the embankment to watch the bolts be "non simultaneous"! ------

Matheinste

Yeah, that was the quote I thought of too. Altonhare, can you explain how to interpret the sentence "obviously they actually were, because Mathe is dead, so simultaneity is NOT relative" in a way that doesn't imply there is an absolute truth about whether the bolts were simultaneous?

 

[altonhare]

Yeah, that was the quote I thought of too. Altonhare, can you explain how to interpret the sentence "obviously they actually were, because Mathe is dead, so simultaneity is NOT relative" in a way that doesn't imply there is an absolute truth about whether the bolts were simultaneous?

Yes, it requires one to redefine what is meant by "simultaneous", as I've described.

 

[JesseM]

Yes, it requires one to redefine what is meant by "simultaneous", as I've described.

Described in what post exactly? Can you give (or quote from a previous post) the specific definition of "simultaneous" that would allow us to make sense of the claim that the lightning strikes were "really" simultaneous even if they weren't simultaneous according to the definition used in the ground frame?

 

[jefswat]

If this reasoning is sound,

Wrong. Motion means two or more locations of an entity where location is the set of distances from the entity to every other entity in the universe. This is the objective, scientific, and consistent definition.

Again the only reason for T to conclude that the train is motionless is if s/he thinks the train is the only other entity in the universe. At best T can only conclude that the train is not moving relative to him/her, but s/he cannot conclude that it is actually motionless.

Then the following should be true:

My eraser is motionless if there is only one set of distances from every other entity in the universe.

As near as I can tell that is what your definition implies. If that isn't what you imply then give us a clearer definition and an example like mine with the eraser.

At best T can only conclude that the train is not moving relative to him/her, but s/he cannot conclude that it is actually motionless.

You finally agree with an established theory

 

[Saw]

Yes, it requires one to redefine what is meant by "simultaneous"

You are perfectly right, in a sense. I think your opponents are not recognizing that there is a good part of truth in your words. See my post #90 for a discussion on when one must play with the concept of absolute simultaneity and when with relative simultaneity. It depends on the purpose. So, in certain contexts, when the purpose so requires, one must switch to the concept of absolute simultaneity. In this you are right.

as I've described.

No, because your redefinition gives prevalence to the “local” version, regardless the purpose. Again, see my post #90. Kindly, you are reacting to the critiques that contest your truths, but not to the ones that attack your fallacies…

 

[Saw]

Actually, when I talk about "absolute simultaneity" as a legitimate concept, what I mean is simply: an ideal measurement that hits at the UNIQUENESS OF REALITY (what has happened or must happen) "AT ONE SHOT". As this is difficult or maybe impossible to achieve in practice (the aether doesn't exist or, if it exists, our measurement instruments do not reveal our motion through the aether), SR achieves the same goal through an indirect route. But all paths lead to Rome. In the end, SR also has a unique prediction about what will happen in the future. But not about what is happening now at two distant places “in the absolute sense”, which is an ambition that SR, by definition, since it has adopted the concept of relative simultaneity, has waived.

The teaching is hence double =

1) You can still have in mind the concept of absolute simultaneity, if you wish, for discussion purposes, but you have to be aware that most real measurements of simultaneity are relative. So, if you wish to hit directly at the idea of a unique reality, do so, speak clearly.

2) Conversely, you may freely use the concept of relative simultaneity, which is the measurement you will most probably face in real life, as long as this does not lead you to think that two realities may co-exist!

I think that the example chosen by althonhare illustrates the first mistake. The mistake was made by his legislator. He looked for precise words to punish althonhare’s announced behaviour. He got advice from the electrical engineers and was told that the machine forcefully kills if the two bolts are “simultaneous”. He wrote so: “whoever activates this machine in a manner so as to generate simultaneous bolts, will be condemned to death”. Althonhare carries out his show. A cunning lawyer defending him argues that the bolts are not simultaneous in the ground frame. If he is skilful enough and dazzles the judge and the jury, althonhare will be declared innocent. That would not be a fair resolution, in my opinion, but it might be a realistic scenario, because the law was not well drafted. The legislator used bad English, ambivalent words. When he wrote “so as to generate simultaneous bolts”, in fact what he was willing to say is “so as to generate bolts that are simultaneous in the train frame” or rather, for completeness, “bolts timed in a manner that they will meet at the centre of the chair”. Or more simple: “bolts that meet at the centre of the chair”. He had the concept of absolute simultaneity (UNIQUE REALITY = murder) in mind, but didn’t write so and thus left room for manoeuvre to the cunning lawyer.

However, let us not forget about the other risk. I would like to propose another example that illustrates this, taken from a famous science book, where the author (wrongly) assumes that someone must be condemned, according to ground observers, and acquitted, according to the train observers. He phrases his claim in a clever manner, so as to suggest that the discrepancy is at the same time unsolvable and conforming to orthodox SR…

But only if althonhare consents to it. You are leading the thread and I would not like to contaminate your debate with a parallel discussion, especially if I only discuss with myself! Althonhare, would you consent? Otherwise I would initiate another thread, some day.

 

[matheinste]

Hello saw.

Is it not possible to ignore altonhares little scenario and just address the physics. Let me define absolute simultaneity thus. Absolute simultaneity means that two spatially separated events regarded as simultaneous by an observer at rest in an inertial frame will also be regarded as simultaneous by ALL observers at rest in ANY other inertial frames moving relative to the first. In this sense absolute simultaneity does not exist. It does exist in some subsets of these frames e.g. observers at rest in a frame. In the definition of absolute simultaneity the words ANY and ALL are the ones that differentaiate absolute from relative.

Provisos: Simultaneity of events to a single observer means that if the observer is positioned midway between events then the observer would see them at the same time. Here the same time obviously means at the same time at the same place i.e. at the observer so there is no contention here. Allowing for transit times of light the observer can be anywhere in this frame and by calculation infer simultaneity.

The clocks in the relatively moving frames are synchronised in their own frames by the Einstein procedure.

Matheinste

 

[JesseM]

Absolute simultaneity of two events is an idea, an intellectual construction, an invention of the mind. Nobody can forbid me to imagine and define that concept as such: an abstract notion, valid for discussion purposes. It means the following: if two events are “absolutely simultaneously”, that entails that they both have “happened” and hence it is logically impossible that one of them is prevented from happening.

Well, an event in 1879 and an event in 1732 both have "happened" from our perspective--would you call them "absolutely simultaneous"?

For example, if a witness from event 1 (located in a frame where that event has happened “earlier”) traveled towards the location of event 2 faster than the speed of light (that is impossible, but even if it were possible), she would not be able to avoid that event 2 happens, because it has already happened and it has not happened in isolation, it has immediately created a myriad of interactions with its surroundings (remember the butterfly effect) that cannot be blurred out, at least in this universe (leaving aside the funny idea of parallel universes). Thus this concept plays a useful role. For example, you don’t follow the threads where people talk about tricky ways to overcome the speed limit, time-travel and kill your dear grandmother before she gives birth to your father. This saves you a lot of time to study relativity.

The problem with this is that if information or people could travel faster than light, then information or people could be sent back in time, arbitrarily far into the past, if relativity is correct. The only way out of this would be to introduce a preferred frame for FTL travel, such that in the preferred frame things could travel FTL but not backwards in time. But of course, any physically preferred frame would invalidate relativity.

 

[Saw]

Well, an event in 1879 and an event in 1732 both have "happened" from our perspective--would you call them "absolutely simultaneous"?

Yes, I was thinking aloud... Maybe this is a better wording:

Two distant events have been “absolutely simultaneous” if an observer who has just watched one of them can do nothing to affect the other (eg: prevent it from happening) even if she reacted instantaneously and disposed of a means to instantaneously affect what happens at distant places, no matter how far away they are. And vice versa, of course.

Anyhow, for sure you catch the idea and could express it better than myself. I assume, as noted, that in practical terms measuring absolute simultaneity may be impossible. My point is only that the idea is thinkable. The fact that a concept is immeasurable doesn’t mean that we have to rule it out of our speech. You know, concepts are like software. You may have the best PC, but if you delete half of your software, it becomes a crippled machine. The same applies to your mind: you may rule out practical possibilities, but not mental tools, unless you wish to limit your thinking capacity.

Einstein was right in warning that if absolute simultaneity is immeasurable, we have to play with relative simultaneity. But if he meant that only the latter exists, he was wrong. None of them exist. They are just abstract concepts, the products of human mind and we can conventionally frame them the way we want, as long as that serves the purpose of comprehending what does exist: the real actors of the universe (matter particles and photons or wave-particles or whatever they are) and their interactions.

Imagine that it were true that lack of absolute simultaneity, as defined above, had practical consequences, under the appropriate circumstances. If so, whoever has ruled out that possibility would refuse to consider it. That would be a mistake, because we don’t really know… We would only know if we believed that there is either a divine decree forbidding it or that there is an unbreakable logic, inherent to the nature of the universe, in the sense that the principle of relativity works flawlessly, under ALL possible circumstances. In that case, we would have to adapt all our measurement instruments and physical/mathematical concepts and formulas, so that they work in view of that paradigm. But are you sure that you have done completely so, without missing any tiny detail...?

As to the rest (FLT), I have to study your posts. I think there was a thread about it…

 

[matheinste]

Hello saw.

Quote:-
---Two distant events have been “absolutely simultaneous” if an observer who has just watched one of them can do nothing to affect the other (eg: prevent it from happening) even if she reacted instantaneously and disposed of a means to instantaneously affect what happens at distant places, no matter how far away they are. ----

For "distant events" i am not quite certain as to what you mean and do not wish to assume anything. For timelike or lightlike separated events there is no simultaneity as they can be causally connected and so are in a before/after relationship. For spacelike separated events, which are by their nature not causally connected, there is no real time order (this is getting into philosophy and i am not totally sure that i express myself accurately)and we have to define, "at the same time", or simultaneous, by convention.The usual convention "seems" the most natural and also often simplifies the mathematics of the situation. But however we define it, if we all use the same convention, it is relative. For two relatively moving observers, events that are simultaneous for one of them using one conventional definition, can also be simultaneous for the other using a suitably chosen alternative definition, but this is a case we do not normally need to consider and may only serve to confuse this discussion. But this is still not even on the way to absolute simultaneity. Relative simultaneity is a conseqence of the constancy of light speed between frames.

I suspect you already worked out all these things and this is probably your line of thinking anyway.

Matheinste.

 

[JesseM]

Two distant events have been “absolutely simultaneous” if an observer who has just watched one of them can do nothing to affect the other (eg: prevent it from happening) even if she reacted instantaneously and disposed of a means to instantaneously affect what happens at distant places, no matter how far away they are. And vice versa, of course.

But here you seem to be begging the question by assuming that "instantaneously affect what happens at distant places" has any well-defined meaning. In SR an effect which traveled instantaneously in one frame would travel FTL but non-instantaneously in others and backwards in time in still others.

Imagine that it were true that lack of absolute simultaneity, as defined above, had practical consequences, under the appropriate circumstances.

Well, that would necessarily mean the laws of physics were not Lorentz-symmetric, and thus relativity was wrong. Of course this is not impossible, but you have to be clear on the logical incompatibility between relativity and any notion of absolute simultaneity with real physical effects.

As to the rest (FLT), I have to study your posts. I think there was a thread about it…

There have been a few, see here or here for example.

 

[altonhare]

"Simultaneous"

My point here is that observers should never disagree on qualitative, binary descriptions of what happened. To illustrate what I'm talking about here are a few such:

"A hit B"

"A moved faster than B"

"A is longer than B"

"A hit C as B hit D"

as opposed to quantitative, continuous descriptions:

"A is moving at X relative to B"

"A is Y times longer than B"

etc.

I argue that statements of the former kind are general logic statements of the form "X is Y" or "X is not Y" and that an event/entity/etc. cannot be observed to possesses some quality while also possessing the diametric opposite of that quality. If they do this is a basic logical contradiction and indicates a faulty premise on the part of the observer(s).

In general this is upheld, but in the "relativity of simultaneity" it is not. One observer says events AC and BD were not simultaneous, another says they were. This is logically unacceptable. This tells me that there is something special about either the "local frame" E that I described earlier or that there is some other "special" frame.

Then the following should be true:

My eraser is motionless if there is only one set of distances from every other entity in the universe.

As near as I can tell that is what your definition implies. If that isn't what you imply then give us a clearer definition and an example like mine with the eraser.

You do not misunderstand. If you have a single set of distances from your eraser to every other entity in the universe the eraser is motionless by definition.

No, because your redefinition gives prevalence to the “local” version, regardless the purpose.

I argue that it is illogical for two observers to ascribe diametrically opposed qualities to any observation. They may disagree on quantity/degree, but not on quality. If they disagree on quality they must check the presuppositions of their measurements/observations, at least one is ill-conceived. In all other areas we do not allow diametrically opposed qualitative descriptions. If I conclude that the metal brick is bigger than the cement brick and you conclude the opposite, we do not ascribe it to the relativity of lengths of bricks. We find out what was faulty about our method/assumptions. In this case I measured in the summer and you in the winter, perhaps. In another case I say the train is motionless and you say it isn't. As I've pointed out, the conclusion that X is motionless is impossible. I can only say it's motionless relative to me, and you will agree with this no matter what frame you're in because it is a qualitative, binary statement of logic.

This tells me that, for spatially separated events, there is either something special about the local frame or some other "special" frame. I do not know what it is and there is not an immediate practical result of this argument. As of now it is purely an argument of logic.

But only if althonhare consents to it. You are leading the thread and I would not like to contaminate your debate with a parallel discussion, especially if I only discuss with myself! Althonhare, would you consent? Otherwise I would initiate another thread, some day.

Lay it on us. Maybe someone will learn something by pointing out the author's fallacy or others will learn something by seeing it pointed out. The story may be fun also.

Let me define absolute simultaneity thus. Absolute simultaneity means that two spatially separated events regarded as simultaneous by an observer at rest in an inertial frame will also be regarded as simultaneous by ALL observers at rest in ANY other inertial frames moving relative to the first. In this sense absolute simultaneity does not exist. It does exist in some subsets of these frames e.g. observers at rest in a frame. In the definition of absolute simultaneity the words ANY and ALL are the ones that differentaiate absolute from relative.

I argue that, although the definition(s) you present seem reasonable, because they lead to a logical contradiction we conclude they are unacceptable.

 

[matheinste]

Quote:-
---I argue that, although the definition(s) you present seem reasonable, because they lead to a logical contradiction we conclude they are unacceptable. -----

Yes, the definition of absolute simultaneity does lead to logical contradictions in SR. That is exactly the point i was making.

Quote:-
----One observer says events AC and BD were not simultaneous, another says they were. This is logically unacceptable. ----

It is completely logical within SR. In fact it is fundamental. No one has to accept SR but if you do accept the light axiom of SR then you MUST accept the relativity of simultaneity. This is not just an opinion. The relativity of simultaneity follows as a logical consequence of the speed of light being the same in all inertial frames.

Quote:-
----You do not misunderstand. If you have a single set of distances from your eraser to every other entity in the universe the eraser is motionless by definition.----

Every "entity" at any point in time has a single set of distances from any and all other "entities". So by the above definition everything is always at rest. However my concern is with simultaneity. you can philosophise as much as you like but given an a set of axioms such as in SR you cannot argue against a consequence which follows logcally from these axioms. If you do not like the axioms then, fair enough, just say so. That is perfectly acceptable.

Matheinste

 

[JesseM]

"Simultaneous"

My point here is that observers should never disagree on qualitative, binary descriptions of what happened. To illustrate what I'm talking about here are a few such:

"A hit B"

"A moved faster than B"

"A is longer than B"

"A hit C as B hit D"

But the last three don't make sense except in the context of a particular reference frame, since different frames can disagree on which of two objects move faster, which of two objects is longer, and whether two spatially separated collisions happened at the same moment.

I argue that statements of the former kind are general logic statements of the form "X is Y" or "X is not Y" and that an event/entity/etc. cannot be observed to possesses some quality while also possessing the diametric opposite of that quality. If they do this is a basic logical contradiction and indicates a faulty premise on the part of the observer(s).

It's only a logical contradiction if you neglect to include the context of what reference frame you're talking about, which is always necessary for any physical claims which don't concern purely local events in SR. For example, there is nothing contradictory about the claims "X is simultaneous with Y in frame A" and "X is not simultaneous with Y in frame B".

In general this is upheld, but in the "relativity of simultaneity" it is not. One observer says events AC and BD were not simultaneous, another says they were. This is logically unacceptable.

No, neither observer claims they were or weren't simultaneous in any absolute sense, they both agree the events were simultaneous in one frame and non-simultaneous in another. This is no more logically contradictory than the notion that object A can have a larger x-coordinate than object B in a coordinate system with the origin at one position and the axes oriented at a particular angle but object B can have a larger x-coordinate than object A in a different coordinate system with the origin at a different position and the axes oriented differently (a situation which can be true in Newtonian physics, and even in ordinary algebraic geometry).

This tells me that there is something special about either the "local frame" E that I described earlier or that there is some other "special" frame.

So now you are back to saying there is such a thing as absolute simultaneity? Why? And if so, do you at least admit that there's no reason to think that the preferred frame needs to be the one where the lightning strikes were simultaneous, since as I said before in post #82:

Even if one believes in some sort of Lorentz ether theory where one frame's measurements are objectively correct and all other frames are distorted by the fact that their rulers are objectively shrunk and their clocks objectively slowed down and objectively out-of-sync, it is still perfectly possible that the observers on the ground were the ones at rest in this preferred frame while the train was moving relative to it, and thus the strikes really did happen at different times in an objective sense.

You do not misunderstand. If you have a single set of distances from your eraser to every other entity in the universe the eraser is motionless by definition.

So if even a single object in the universe has a changing distance from the eraser, the eraser is not motionless? Does this mean the only way for any object to be motionless is if every single object in the universe is at rest relative to every other object?

I argue that it is illogical for two observers to ascribe diametrically opposed qualities to any observation.

Why? If the qualities are coordinate-dependent ones, why is it problematic that there could be disagreements on which object has a greater value of the quality depending on which coordinate system is used? Again, even in simple plane geometry different coordinate systems can disagree on which of two points on the plane has a larger x-coordinate. Likewise, in one coordinate system two objects may share the same x-coordinate, while in another coordinate system they may have two different x-coordinates...how is this fundamentally different than the idea that in one frame two events may share the same t-coordinate (i.e. they are simultaneous in that frame) while in another frame the same events may have different t-coordinates (i.e. they are non-simultaneous in that frame)?

 

[jefswat]

So if even a single object in the universe has a changing distance from the eraser, the eraser is not motionless? Does this mean the only way for any object to be motionless is if every single object in the universe is at rest relative to every other object?

You have beaten me to the punch. While there may be nothing logically wrong with altonhare's definition of motion, it has no predictive power, explain's less than SR, and is utterly useless. Based on Quantum Mechanics and the Heisenberg (sp?) uncertianty principle we don't even know where any single particle may be, let alone everyone in the universe. Further more unless altonhare has a very good refidgerator that no one has ever seen before I don't think the word motionless has any interesting meaning within his framework (a pity, I am rather fond of the word). Further more, I challenge altonhare to explain, with his theory, how my eraser appears motionless to me without using relativity or even the word or concept of relativity. It can not be done which leads us back to Einstein and our friend relativity.

 

[Saw]

Lay it on us. Maybe someone will learn something by pointing out the author's fallacy or others will learn something by seeing it pointed out. The story may be fun also.

Hope so. The story is substantially equivalent to Althonhare's, but it has two advantages: (i) the author's interpretation, which is wrong, is based on the "multiple reality” approach that we contest and it clashes with a sound, less “fantastic” understanding of SR and (ii) its display gives occasion to discuss some issues that are not purely conceptual, but might have relevant practical consequences.

It is the well known story introduced by Brian Greene in the Fabric of the Cosmos. I loved the book and very much respect the author, but he seems to be wrong in this point.

Let me explain it with some adaptations:

There is a duel on a train, between duellers that we will call Back and Front, situated at the tail and the tip of the car, respectively. For their duel, they employ laser guns, of identical construction.

There are two referees: Althonhare, on the train, and myself, Saw, on the ground. The signal for the commencement of the duel is given at the precise instant when Althon and myself are lined up. Of course, it is impossible that the two referees occupy the same position in space and so the perception of this alignment would require light traveling some distance from one to the other. But we stipulate that the distance is so small that its consequences are negligible. We all agree that the alignment (which is a single event) is simultaneous in both frames.

In order to give the duellers the signal for shooting, a pile of gunpowder, set midway between them (i.e., where Althon, on the car, and myself, on the ground, are standing at that instant), explodes and thus sends flashes of light in both directions.

Was the duel fair?

Each referee has a different opinion:

* Althon is certain that light from the flare reaches the duelers simultaneously, so he raises the green flag and declares it a fair draw.

[More technically: In the train frame, the light pulses travel equal paths in their respective directions and do it at the same speed. So the two distant events (= arrival of the signals to shoot) are SIMULTANEOUS.)

* According to the author, I wildly squeal foul play, claiming that Back got the light signal from the explosion before Front did. I explain that, because the train was moving forward, Back was heading toward the light while Front was moving away from it. This means that the light did not have to travel quite as far to reach Back, since he moved closer to it; moreover, the light had to travel farther to reach Front, since he moved away from it. Since the speed of light, moving left or right from anyone’s perspective, is constant, I am supposed to claim that it took the light longer to reach Front, since it had to travel farther, rendering the duel unfair.

[Idem: In the ground frame, light towards Back travels a shorter path, since its target is heading towards it, while light towards Front travels a longer path, since its target is escaping away. As both pulses travel at the same speed, the one that hits Back arrives earlier than the one that hits Front. The two distant events are NOT SIMULTANEOUS.]

Who is right? The matter is not trivial, because the judge who has made a mistake will be sanctioned by the competent supervision body and he may not be able to exercise his profession any more.

Fortunately (?), Einstein comes to the rescue of both with a salomonic opinion:

“Einstein’s unexpected answer is that they both are (…) they simply have different perspectives on the same sequence of events. The shocking thing that Einstein revealed is that their different perspectives yield different but equally valid claims of what events happen at the same time. Of course, at everyday speeds like that of the train, the disparity is small – Saw claims that Front got the light less than a trillionth of a second after Back- but were the train moving faster, near light speed, the time difference would be substantial” (literal quotation, I just changed the names).

But I am a prudent referee. I try to do my job properly. I know that the words of the law (you shall raise a green flag if the duel is “fair”) have a practical purpose and I must interpret them in the light of this practical purpose. The duel is “fair” if the two duelers are given equal opportunities of hitting each other, in practical terms. So my opinion that “Back got the signal earlier” is only relevant for the matter to be judged if it gives Back a real advantage (or disadvantage!).

To this effect, I consider the following scenarios:

(a) Back sees the signal earlier and fires earlier as well. Can his laser pulse hit Front before the latter receives the signal?

(b) If not, they have both received the signals and shot their laser pulses. The latter will also take some time to reach their targets. During this time interval, theoretically, any dueler could try the usual trick: fire and stand aside, before being wounded. There is very little time for this deed, especially if we talk about laser guns, but we can imagine the distance arbitrary long for this purpose. Is this time interval longer for Back then for Front or vice versa?

(c) If not, we imagine Back badly wounded, kneeling on the floor of the car, but brave enough to fire a second shot. Can it reach Front before the latter has the opportunity to fire his own second shot…?

You can think of other scenarios if you wish. The more, the better. That is the point of the exercise.

Then I call a group of experts on SR and ask their answers to the practical questions. If the answers, as I expect, are negative (there is no breach of the principle of equal opportunities), then I do not “wildly squeal foul play” as Brian Greene suggests. Instead, I gently and gallantly raise a green flag, in agreement with Althonhare’s opinion. Consequently, I keep my job, anyhow, but at the same time with a clear conscience, because I’ve done a good job. Brian, in turn, keeps being a wonderful scientist but does not promote any more bold philosophical interpretations of SR.

 

[matheinste]

Can't we stick to the physics. All this stuff is quite unecessary and takes up time. The physics is very simple. I personally don't have the time to play out theatrical scenarios and so will take no more part in discussing them. My time would be better spent enhancing the limited knowledge i already have. Altonhare is incorrect as far as SR is concerned. Almost everyone agrees on that. If we cannot argue the physics sensibly without stories and getting sidetracked then count me out.

Matheinste.

 

[Saw]

Can't we stick to the physics. All this stuff is quite unecessary and takes up time. The physics is very simple.

Mathe, there are two levels at which all this (and if you take the time to study it, you will agree) may be highly relevant to physics in general and to the physics of the case in particular:

(i) To physics: There's a widespread opinion that physics has nothing to do with common sense and logic. "The universe is as it is and full stop", is the motto. In the realm of SR this view is unfortunately even more habitual. I think we do a great favour to SR, which is a beautifully true theory, if we reconcile it with common sense. Not with old common prejudices that were unjustified, as logic has proved, but with common sense truths that have succesfully resisted a most rigorous logical test.

(ii) To the phsyics: It is my humble opinion that, for example, if the duellers had used conventional guns, instead of laser guns, the solution is more complicated. In principle, SR gives the same solution for both cases: laser and conventional guns. But I have some doubts I would like to share. Maybe JesseM and others fully and convincingly dismiss my concerns, but in the meantime we will have had an enlightening discussion.

So, no need to move us to the metaphysical realm, for the time being…

 

[matheinste]

Hello w
saw.

Quote:-
----(i) To physics: There's a widespread opinion that physics has nothing to do with common sense and logic. In the realm of SR this view is unfortunately even more habitual. I think we do a great favour to SR, which is a beautifully true theory, if we reconcile it with logic.----

Physics in SR often goes against common sense in the sense that it is often counter intuitive. This is at the heart of the problem for beginners like myself. We must not hold on to our preconceived ideas when we swithc to SR. Physics has everything to do with logic. If you do not accept logical reasoning then anything can be accepted as true. Of course, on the other hand you must, having accepted a set of axioms, which of course in themselves may or not be true, you must accept the logical consequences of those axioms.

As to the scenarios used to illustrate the question of simultaneity, the normal train and embankment observer eaxample is simple and adequate for its purpose. Why introduce more complex examples to illustrate a consequence of the axioms of SR. It is a teaching aid. More complicated examples which are of the nature of a puzzle may be a useful exercise for the more competent student, as finding a resolution to the posed scenario can at that stage be a test of a thorough understanding of the principles involved. But in this case we are arguing the basics and if someone cannot understand and accept the basic principles how can they work out a resolution to the more puzzling examples.

Matheinste.

 

[JesseM]

Hope so. The story is substantially equivalent to Althonhare's, but it has two advantages: (i) the author's interpretation, which is wrong, is based on the "multiple reality” approach that we contest and it clashes with a sound, less “fantastic” understanding of SR and (ii) its display gives occasion to discuss some issues that are not purely conceptual, but might have relevant practical consequences.

It is the well known story introduced by Brian Greene in the Fabric of the Cosmos. I loved the book and very much respect the author, but he seems to be wrong in this point.

Let me explain it with some adaptations:

There is a duel on a train, between duellers that we will call Back and Front, situated at the tail and the tip of the car, respectively. For their duel, they employ laser guns, of identical construction.

There are two referees: Althonhare, on the train, and myself, Saw, on the ground. The signal for the commencement of the duel is given at the precise instant when Althon and myself are lined up. Of course, it is impossible that the two referees occupy the same position in space and so the perception of this alignment would require light traveling some distance from one to the other. But we stipulate that the distance is so small that its consequences are negligible. We all agree that the alignment (which is a single event) is simultaneous in both frames.

In order to give the duellers the signal for shooting, a pile of gunpowder, set midway between them (i.e., where Althon, on the car, and myself, on the ground, are standing at that instant), explodes and thus sends flashes of light in both directions.

Was the duel fair?

Each referee has a different opinion:

* Althon is certain that light from the flare reaches the duelers simultaneously, so he raises the green flag and declares it a fair draw.

[More technically: In the train frame, the light pulses travel equal paths in their respective directions and do it at the same speed. So the two distant events (= arrival of the signals to shoot) are SIMULTANEOUS.)

* According to the author, I wildly squeal foul play, claiming that Back got the light signal from the explosion before Front did. I explain that, because the train was moving forward, Back was heading toward the light while Front was moving away from it. This means that the light did not have to travel quite as far to reach Back, since he moved closer to it; moreover, the light had to travel farther to reach Front, since he moved away from it. Since the speed of light, moving left or right from anyone’s perspective, is constant, I am supposed to claim that it took the light longer to reach Front, since it had to travel farther, rendering the duel unfair.

[Idem: In the ground frame, light towards Back travels a shorter path, since its target is heading towards it, while light towards Front travels a longer path, since its target is escaping away. As both pulses travel at the same speed, the one that hits Back arrives earlier than the one that hits Front. The two distant events are NOT SIMULTANEOUS.]

Who is right? The matter is not trivial, because the judge who has made a mistake will be sanctioned by the competent supervision body and he may not be able to exercise his profession any more.

Fortunately (?), Einstein comes to the rescue of both with a salomonic opinion:

“Einstein’s unexpected answer is that they both are (…) they simply have different perspectives on the same sequence of events. The shocking thing that Einstein revealed is that their different perspectives yield different but equally valid claims of what events happen at the same time. Of course, at everyday speeds like that of the train, the disparity is small – Saw claims that Front got the light less than a trillionth of a second after Back- but were the train moving faster, near light speed, the time difference would be substantial” (literal quotation, I just changed the names).

But I am a prudent referee. I try to do my job properly. I know that the words of the law (you shall raise a green flag if the duel is “fair”) have a practical purpose and I must interpret them in the light of this practical purpose. The duel is “fair” if the two duelers are given equal opportunities of hitting each other, in practical terms. So my opinion that “Back got the signal earlier” is only relevant for the matter to be judged if it gives Back a real advantage (or disadvantage!).

To this effect, I consider the following scenarios:

(a) Back sees the signal earlier and fires earlier as well. Can his laser pulse hit Front before the latter receives the signal?

No. If two events are simultaneous in any frame, that must mean there is a spacelike separation between the two events, meaning neither event lies in the other event's future light cone.

(b) If not, they have both received the signals and shot their laser pulses. The latter will also take some time to reach their targets. During this time interval, theoretically, any dueler could try the usual trick: fire and stand aside, before being wounded. There is very little time for this deed, especially if we talk about laser guns, but we can imagine the distance arbitrary long for this purpose. Is this time interval longer for Back then for Front or vice versa?

Same time for both, since they're both at rest in the train frame, and they both fire at the same moment in this frame.

You can think of other scenarios if you wish. The more, the better. That is the point of the exercise.

Well, you could imagine a modified scenario where Front was at rest on the ground along with the referee, at just the right position so he happened to be right next to the front of the train when the light from the gunpowder explosion reached it. In this case, although Back and Front fire simultaneously in the train rest frame, in this frame Front is moving towards Back (and his clock is slowed down by time dilation), so Front has less time to step out of the way of Back's laser beam.

Then I call a group of experts on SR and ask their answers to the practical questions. If the answers, as I expect, are negative (there is no breach of the principle of equal opportunities), then I do not “wildly squeal foul play” as Brian Greene suggests. Instead, I gently and gallantly raise a green flag, in agreement with Althonhare’s opinion. Consequently, I keep my job, anyhow, but at the same time with a clear conscience, because I’ve done a good job. Brian, in turn, keeps being a wonderful scientist but does not promote any more bold philosophical interpretations of SR.

I think in Greene's scenario the point of the referees was just to judge whether both fired their lasers at the same time, not to judge whether the scenario was fair overall--it was really just a thought-experiment to illustrate the relativity of simultaneity, after all.

 

[Saw]

Well, you could imagine a modified scenario where Front was at rest on the ground along with the referee, at just the right position so he happened to be right next to the front of the train when the light from the gunpowder explosion reached it. In this case, although Back and Front fire simultaneously in the train rest frame, in this frame Front is moving towards Back (and his clock is slowed down by time dilation), so Front has less time to step out of the way of Back's laser beam.

That's another thought-experiment. I was asking about practical reasons why the duel might be fair for one frame and unfair for the other frame, in the original thought-experiment. The new configuration you mention is interesting but it is not "analogous". It raises a totally different discussion, which we could discuss somewhere else.

I think in Greene's scenario the point of the referees was just to judge whether both fired their lasers at the same time, not to judge whether the scenario was fair overall--it was really just a thought-experiment to illustrate the relativity of simultaneity, after all.

To go step by step, do you agree that the game was fair and that I should have raised the green flag?

 

[JesseM]

That's another thought-experiment. I was asking about practical reasons why the duel might be fair for one frame and unfair for the other frame, in the original thought-experiment. The new configuration you mention is interesting but it is not "analogous". It raises a totally different discussion, which we could discuss somewhere else.

OK, I misunderstood what you meant by "you can think of other scenarios if you wish". I wouldn't say that considering the same physical situation in a different frame is a new "scenario", it's just a different perspective on the same scenario.

To go step by step, do you agree that the game was fair and that I should have raised the green flag?

I agree the game was fair, but if your job was just to judge if the two guns were fired simultaneously in your frame, then you should not have raised the green flag because they weren't. You seem to be overthinking this, Greene really was just trying to explain about simultaneity, he wasn't making any more subtle point about the game being "fair" in one frame and "unfair" in another.

 

[Saw]

OK, I misunderstood what you meant by "you can think of other scenarios if you wish". I wouldn't say that considering the same physical situation in a different frame is a new "scenario", it's just a different perspective on the same scenario.

As Dalespam usually observes, conventional language (English or Spanish) is tricky. I say "scenario" having one meaning in mind and you read having a different meaning in mind. I have consulted my dictionary and both meanings could be acceptable. In every day language, my view (just a different plot in the same stage) might be the rule. But in specialised scientific language you admit that a different scenario may include a change of physical situation. And, by the way, in my own language, “escenario” means “stage”!

I say this because it “illustrates” very well the problems we face in our discussion. We have to be patient with one another because language will constantly play tricks on us.

But it is team work and it may be rewarding, because it may help us to better understand what our formulas and diagrams mean. Mathematics is a wonderful tool. You put concepts among its wheels, you switch on the machinery and the automat produces by itself amazing new concepts you had never thought of. However, mathematical language is not immune to the same problem: you have to understand very well the concepts with which you feed the automat. Otherwise, it may produce wrong predictions.

I agree the game was fair, but if your job was just to judge if the two guns were fired simultaneously in your frame, then you should not have raised the green flag because they weren't.

My job, as a judge, can never be just to judge if the two guns were fired simultaneously “in my frame”. My job is to make justice and I only do that if I declare the duel fair. Let us imagine that the legislator has read a little about SR, but has misunderstood it. So he has written “you shall raise the green flag if the two guns are fired simultaneously in your frame, but only in your frame, my friend”. Wouldn’t you agree that this law is defective, because it doesn’t understand how the universe works, as taught by physics? Wouldn’t you be forced, as a physic, to stand up and demand that it is amended?

You seem to be overthinking this, Greene really was just trying to explain about simultaneity, he wasn't making any more subtle point about the game being "fair" in one frame and "unfair" in another.

Yes, I overthink everything… But I don’t want to personalize on anybody and less on Brian Greene, whose writings are delicious.

If he was just describing how relativity of simultaneity is measured, without making any more subtle point, that is perfect. So we agree that the duel was fair in both frames, without forgetting what this means: this illustrates that the “discrepancy” on measurements, the relativity of simultaneity, does not entail any discrepancy at all on what may happen or may not. The film of life (reality) will be the same anyhow. I know you agree with that. But let me rhetorically ask “right?”, so that you can say “so what?”.

 

[Saw]

Physics has everything to do with logic. If you do not accept logical reasoning then anything can be accepted as true.

Hello, mathe. I fully agree.

Of course, on the other hand you must, having accepted a set of axioms, which of course in themselves may or not be true, you must accept the logical consequences of those axioms.

That is the question. You can do a better thing. You may try to grasp a better understanding of the axioms, so as to refine their meaning. This will ensure that, when you put them into the blind automat (math, geometry or logical reasoning with English), the outcome is a beautiful baby instead of a monster. That is what we are trying to do here. Although I admit it is sometimes a painful job, because words are devils!

 

[JesseM]

My job, as a judge, can never be just to judge if the two guns were fired simultaneously “in my frame”. My job is to make justice and I only do that if I declare the duel fair. Let us imagine that the legislator has read a little about SR, but has misunderstood it. So he has written “you shall raise the green flag if the two guns are fired simultaneously in your frame, but only in your frame, my friend”. Wouldn’t you agree that this law is defective, because it doesn’t understand how the universe works, as taught by physics? Wouldn’t you be forced, as a physic, to stand up and demand that it is amended?

Yeah, I agree that the law would be no good, since what's really important for the game to be "fair" is that they both have an equal amount of time on their own clocks (proper time) between firing their own gun and being hit by (or dodging) the other guy's laser.

Yes, I overthink everything… But I don’t want to personalize on anybody and less on Brian Greene, whose writings are delicious.

If he was just describing how relativity of simultaneity is measured, without making any more subtle point, that is perfect. So we agree that the duel was fair in both frames, without forgetting what this means: this illustrates that the “discrepancy” on measurements, the relativity of simultaneity, does not entail any discrepancy at all on what may happen or may not. The film of life (reality) will be the same anyhow. I know you agree with that. But let me rhetorically ask “right?”, so that you can say “so what?”.

Right, no disagreement here, and the fact that there's no disagreement about coordinate-independent facts like the proper time between two events on an observer's worldline is a key thing to understand when thinking about these kinds of thought-experiments.

 

[atyy]

Wow, relativistic jurisprudence!