Light shone in a train bouncing off mirrors

[DrGreg]

There is no "relativity of simultaneity" because the concept "simultaneity" only has meaning for events in their own inertial frame

Perhaps you could enlighten us with your definition of "the inertial frame of an event"?

Do you mean "the rest frame of an event"?

Events do not persist over an interval of time, so it's meaningless to talk about a rest frame, or the motion of an event. You can measure an event in any frame you like and there's no reason to prefer one frame over another.

 

[altonhare]

Perhaps you could enlighten us with your definition of "the inertial frame of an event"?

Do you mean "the rest frame of an event"?

Events do not persist over an interval of time, so it's meaningless to talk about a rest frame, or the motion of an event. You can measure an event in any frame you like and there's no reason to prefer one frame over another.

Loose language on my part. Two entities A and B are on a path that will intersect two other entities C and D. The question of the simultaneity of AC and BD only has meaning in a frame E in which the velocity of A and B relative to E are equal and the velocity of C and D relative to E are equal. Of course if AC and BD are local then they will be simultaneous in all frames, but for spatially separated events simultaneity only has meaning in a particular frame.

Spatially local events will always be agreed upon, a handshake is of course always simultaneous no matter how you look at it. Of course rel always gets this result, also. It is irrational to describe what physically happened as depending on one's perspective, the "relativity of simultaneity" just means two observer's are watching from different perspectives and must account for the shifts in their measurement apparatus. The correct answer is calculated in frame E. It makes no practical difference, but the conclusion that events can have two opposite characteristics (X and non X) violates basic logic and demands a more rigorous definition of X. When you arrive at a contradiction (this is both X and non X) it demands not that you change reality or accept contradiction, but that you change your premise, where your premise is "to be characterized as X means..."

The derivation of the Lorentzian wavelength broadening from classical EM indicates that the fundamental nature of entities is to engage in cyclical processes (such as the "orbit" or the "oscillation" of an electron) faster or slower depending on the circumstances. This is unsurprising. It would be more shocking to find that every entity behaves exactly the same. Objectively I see no reason to draw conclusions about time travel or "traveling into the future". This is reification of both time and future. I can travel into a house or a box, but not into distance, injustice, or time.

 

[JesseM]

I think you may understand better if I give you my version of the boxcar gedanken experiment. If it doesn't suffice perhaps I'll try something more formal.

This one is called "Matheinste in the hot seat". In the middle of the box car is an electric chair in which we place you. On either side of the chair, equidistant, are two lightning rods connected to a circuit on the back of your chair. A team of qualified electrical engineers designed, built, and tested this circuit to allow lethal current to flow into the occupant if it receives a voltage simultaneously from both sides.

You're being too vague here. Does the circuit only activate the chair if the energy from the two bolts arrives at the center of the train simultaneously (a local event that all frames will agree on), or does it only activate if the bolts hit either end of the train simultaneously in some frame (a judgement about simultaneity that different frames disagree on)? If the former, than you can form a perfectly good account of why the electric chair turned on from the perspective of the people on the embankment who said the strikes were non-simultaneous. Keep in mind that electricity doesn't actually move at the speed of light, so the electricity from each bolt will move at different speeds through the wires from the ends of the train to the middle in the embankment observers' frame. You might consider replacing the device at the center with a photosensitive device which detects light either end, and if it receives light from both sides simultaneously it turns on the electric chair (which in this case has a separate power source). In this case, the people on the embankment will just say that the light from the bolt at the front of the train had a shorter distance to travel to reach the photosensitive device, since the device in the middle was moving towards the position on the tracks where the front of the train was when the bolt hit it, while the device will move away from the position on the tracks where the back of the train was when the bolt hit it. Thus, the bolt can hit the back end before it hits the front end and still the light from both ends will hit the photosensitive device at the center simultaneously.

 

[altonhare]

Jesse, it just functions like a basic integrated circuit. A voltage on one side indicates a "1" logical argument and no voltage indicates a "0".

 

[JesseM]

Jesse, it just functions like a basic integrated circuit. A voltage on one side indicates a "1" logical argument and no voltage indicates a "0".

As I said, all frames will agree that the signals from the two strikes--whether electrical signals (which travel slower than light and thus have different speeds in different frames) or light signals--reach the device at the center of the train simultaneously, even though some frames think the lightning didn't strike both ends simultaneously. There is nothing paradoxical or strange about this, so your discussion of people on the embankment being puzzled as to why the chair was activated doesn't make any sense.

 

[jefswat]

So if we alter the experiment a bit and place a series of light bulbs along the path of the wires from the lightning rods to the chair the observers on the ground will see the bulbs lighting up slower on the side they saw lighting strike first (the electricity appears to be moving slower to them). So when we give the observers a way to see the electricity (like light bulbs) they will agree that the electricity from both wires hit the chair at the same time right?

 

[JesseM]

So if we alter the experiment a bit and place a series of light bulbs along the path of the wires from the lightning rods to the chair the observers on the ground will see the bulbs lighting up slower on the side they saw lighting strike first (the electricity appears to be moving slower to them). So when we give the observers a way to see the electricity (like light bulbs) they will agree that the electricity from both wires hit the chair at the same time right?

That's right. Keep in mind that from the perspective of observers on the ground, each bulb is actually moving away from the position on the track where the lightning hit the back end of the train and towards the position where the lightning hit the front end, so for the bulbs between the back end and the middle, the distance between the positions where each bulb lights up is greater than the distance between the positions where the bulbs between the front end and the middle light up. This is why, even if you removed the electrical wires and just gave each bulb a solar panel that would cause it to light up when the light from the lightning strike reached it, observers on the ground would still say the time between bulbs lighting up was greater for bulbs between the back end and the middle than it was for bulbs between the front end and the middle, without this being inconsistent with the idea that the light from both strikes moved at exactly c in the frame of observers on the ground.

 

[ZikZak]

but the conclusion that events can have two opposite characteristics (X and non X) violates basic logic and demands a more rigorous definition of X. When you arrive at a contradiction (this is both X and non X) it demands not that you change reality or accept contradiction, but that you change your premise, where your premise is "to be characterized as X means..."

To be characterized as "in motion" means that a body's position changes with time. Observer S on the embankment observes that the train's position changes with time, and thus the train is "in motion." Observer T in the train observes that the train's position does not change with time, and thus the train is "non in motion."

Observer S thus concludes X and observer T concludes non-X. Do you claim that this is a contradiction? Are we now required to change the definition of motion? What should the new definition be? Which of the observers conducts the "real" measurement, and which needs to "adjust" for his equipment, and how do we know?

 

[altonhare]

All observers will agree on if the signal reaches the chair from each side simultaneously because it's a local event. The point of the exercise is to illustrate that "simultaneous" is only meaningful (non contradictory) for local events in any frame and spatially separate events in a specific frame.

 

[altonhare]

To be characterized as "in motion" means that a body's position changes with time.

Incorrect. Motion and change are synonyms. Time is essentially motion+observer. This is circular and fails to actually define motion.

Motion: Two or more locations of an entity.
Location: The set of distances from an entity to every other entity.

Now we can use these terms consistently and meaningfully. If an entity was at two locations it moved by definition whether a particular observer saw it move or not.

Observer S on the embankment observes that the train's position changes with time, and thus the train is "in motion." Observer T in the train observes that the train's position does not change with time, and thus the train is "non in motion."

The only reason T would draw this conclusion is if T assumes s/he is the only other entity in the universe besides the train. This is an unjustified and immensely self-centered stance.

Observer S thus concludes X and observer T concludes non-X. Do you claim that this is a contradiction?

I'm glad you asked. Yes it does. It indicates that the definition of "motion" being used is wrong. If S and T come to different conclusions the only scientific, i.e. rational, explanation is that they were working from different premises and with limited information. Here, they are both working from the premise/assumption that the train is the only other entity in the universe and that they are motionless. The latter is justifiable, the former is not.

Are we now required to change the definition of motion? What should the new definition be?

Yes, yes we do. It should be the one I suggested.

Which of the observers conducts the "real" measurement, and which needs to "adjust" for his equipment, and how do we know?

For some reason people don't seem to understand anymore that every measurement in the history of mankind has been "relative". Rel doesn't change this, it's just the culmination of everything we know about how matter behaves classically and thus allows for the best measurements possible.

Nobody has ever measured an absolute length, distance, distance-traveled, "time", velocity, etc. This is because all knowledge is contextual. If you're concerned about questions of "real" and such I suggest you develop a sound philosophy. The philosophical foundation I have found most tenable and relevant to my life, experiences, and logic expressly states that all knowledge is contextual, i.e. based on premises and/or assumptions. We cannot investigate anything without first assuming something about its nature. When investigations bring us to a contradiction we need to go back and check our premises, at least one must be wrong.

Although we cannot measure it, at any instant we can imagine every entity in the U is at some distance from every other. This is its location. With two locations, we can conceptualize motion.

 

[Saw]

Keep in mind that electricity doesn't actually move at the speed of light, so the electricity from each bolt will move at different speeds through the wires from the ends of the train to the middle in the embankment observers' frame.

all frames will agree that the signals from the two strikes--whether electrical signals (which travel slower than light and thus have different speeds in different frames) or light signals--reach the device at the center of the train simultaneously, even though some frames think the lightning didn't strike both ends simultaneously.

So it doesn’t matter whether the signals that travel from the edges of the rods are light signals or electrical signals o (I suppose) even mechanical signals (steel balls, like I proposed in another post), right?

In the train frame:

- All types of signals depart simultaneously and traverse equal paths at equal speeds (it just happens that light signals travel faster).
- So they reach the target simultaneously.

In the embankment frame:

- In all cases, the signal from the back departs earlier but it also has a longer path to travel (since its target is racing away), while the signal from the front departs later but it also has to traverse a shorter path (since its target is heading towards it).

- In all cases, I suppose that the speed of the signal is calculated by applying the relativistic formula for the addition of velocities.

- For light, this leaves the speed of the signal as still c.

- For other types of signals, the signal from the back travels faster than the signal from the front, but the signal from the back travels less fast than it would if we had applied the Galilean addition formula (which compensates for the fact that it departs earlier) and the signal from the front travels less slowly than it would with the Galilean formula (which compensates for the fact that it departs later).

- Thus all types of signals would arrive at the centre simultaneously = same prediction as in the train frame.

Is this right?

 

[JesseM]

All observers will agree on if the signal reaches the chair from each side simultaneously because it's a local event. The point of the exercise is to illustrate that "simultaneous" is only meaningful (non contradictory) for local events and spatially separate events in a specific frame.

But you said that people in the trial would not be able to account for why the chair turned on:

At the trial tomorrow I plead the 5th, as is my right. Some of your closest family, though they love you dearly, cannot put an innocent man behind bars. They either plead the 5th or state that the two bolts did not appear simultaneous. The physicists present their calculations, based on measurements using the most advanced and precise technology, calculated using the prevailing theory of relativity, showing that the bolts were most definitely not simultaneous.

The electrical engineers, having had no idea what their circuit was used for, are nevertheless relieved at the testimonies. They present their circuit to the jury and state that it would only deliver a lethal shock if the strikes were simultaneous. They state that it had been subjected to numerous tests and the odds that some kind of fluke caused an unexpected outcome are billions to one.

The defense concludes that it could not have been their circuit, Alton's hand, or Dr. Mad's lightning that killed poor Matheinste.

The jury does not deliberate long, the evidence is overwhelming. They declare me and Dr. Mad innocent and the consciences of the electrical engineers are eased. What did kill poor Matheinste? Who knows.

This doesn't make any sense, since if anyone at the trial understands physics they can show that because of the speed of the signals and the movement of the device at the center, then even though the strikes were non-simultaneous, they were timed just right so that the signals would reach the device simultaneously. So why did the jury conclude that you and Dr. Mad were innocent? Are you just assuming that the jury isn't capable of basic physics calculations, and none of the physicists who testified bothered to correct them?

You also seem to be using this thought-experiment to support the idea that the strikes really were simultaneous in some objective sense. Why? 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.

 

[JesseM]

So it doesn’t matter whether the signals that travel from the edges of the rods are light signals or electrical signals o (I suppose) even mechanical signals (steel balls, like I proposed in another post), right?

In the train frame:

- All types of signals depart simultaneously and traverse equal paths at equal speeds (it just happens that light signals travel faster).
- So they reach the target simultaneously.

If we have two identical wires at rest in the train frame then the signals should move along both at equal speed in the train frame, and likewise for light signals, although there's no reason we couldn't build some kind of transmitters which sent signals at different speeds in the train frame.

In the embankment frame:

- In all cases, the signal from the back departs earlier but it also has a longer path to travel (since its target is racing away), while the signal from the front departs later but it also has to traverse a shorter path (since its target is heading towards it).

Yes.

- In all cases, I suppose that the speed of the signal is calculated by applying the relativistic formula for the addition of velocities.

You could do it that way, but strictly speaking it should not be necessary to think about what the speeds would be in the train frame and then transform into the ground frame, you can always just calculate things using the laws of physics in the ground frame without thinking about any other frames. In the case of wires, I suppose you'd have to use electromagnetic laws to calculate the speed of electrical signals moving in different directions in wires which are themselves moving in a particular direction.

- For light, this leaves the speed of the signal as still c.

- For other types of signals, the signal from the back travels faster than the signal from the front, but the signal from the back travels less fast than it would if we had applied the Galilean addition formula (which compensates for the fact that it departs earlier) and the signal from the front travels less slowly than it would with the Galilean formula (which compensates for the fact that it departs later).

- Thus all types of signals would arrive at the centre simultaneously = same prediction as in the train frame.

Yes, both these are right.

 

[ZikZak]

For some reason people don't seem to understand anymore that every measurement in the history of mankind has been "relative". Rel doesn't change this, it's just the culmination of everything we know about how matter behaves classically and thus allows for the best measurements possible.

Strange statements from someone who claims that there is absolute simultaneity.

 

[matheinste]

Hello all.

Question. Given certain provisos.

1. Two events are not colocated and are viewed from frames in relative inertial motion.
2. The direction of relative motion is not perpendicular to the line joining these events.
3. The clocks in each frame are synchronised using the Einstein procedure.

then if observers in one these frames regards these events as simultaneous then an observer in the other frame will not. The observer's positions in these frames are irrelevant.

Very loosely put, simultaneity is relative.

I, and all authors i have read, say that it is relative. Altonhare says it is not. If it is not will someone show me why.

Matheinste

 

[jefswat]

Incorrect. Motion and change are synonyms. Time is essentially motion+observer. This is circular and fails to actually define motion.

Motion: Two or more locations of an entity.
Location: The set of distances from an entity to every other entity.

Now we can use these terms consistently and meaningfully. If an entity was at two locations it moved by definition whether a particular observer saw it move or not.

I don't understand how your definition is any different from the previous. When you say the object was at two different locations then its position necessarily changed.

 

[jefswat]

To be characterized as "in motion" means that a body's position changes with time. Observer S on the embankment observes that the train's position changes with time, and thus the train is "in motion." Observer T in the train observes that the train's position does not change with time, and thus the train is "non in motion."

Observer S thus concludes X and observer T concludes non-X. Do you claim that this is a contradiction? Are we now required to change the definition of motion? What should the new definition be? Which of the observers conducts the "real" measurement, and which needs to "adjust" for his equipment, and how do we know?

Modified:
To be characterized as "in motion" means that a body's position changes with time relative to the observer. Observer S on the embankment observes that the train's position changes with time, and thus the train is "in motion" relative to him.Observer T in the train observes that the train's position does not change with time, and thus the train is "non in motion" relative to him

I see no way to refute this

 

[jefswat]

All observers will agree on if the signal reaches the chair from each side simultaneously because it's a local event. The point of the exercise is to illustrate that "simultaneous" is only meaningful (non contradictory) for local events and spatially separate events in a specific frame.

I agree with the first part. I still can't help but thinking you think the definitions of simultaneous and relatively simultaneous are the same and they are not.

Simulatneous- They happened at the same time(essentially)(only applies in special cases)

Relative Simultaneity-the concept that simultaneity is not absolute, but dependent on the observe(applies to all cases though in some cases it simplifies down to basic simultaniety)

Sorry I can't speel today

 

[altonhare]

Strange statements from someone who claims that there is absolute simultaneity.

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

I don't understand how your definition is any different from the previous. When you say the object was at two different locations then its position necessarily changed.

The definition I gave does not invoke circularity. Motion=change. When you define motion as change you are saying nothing.

Modified:
To be characterized as "in motion" means that a body's position changes with time relative to the observer. Observer S on the embankment observes that the train's position changes with time, and thus the train is "in motion" relative to him.Observer T in the train observes that the train's position does not change with time, and thus the train is "non in motion" relative to him

I see no way to refute this

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.

I agree with the first part. I still can't help but thinking you think the definitions of simultaneous and relatively simultaneous are the same and they are not.

Simulatneous- They happened at the same time(essentially)(only applies in special cases)

Relative Simultaneity-the concept that simultaneity is not absolute, but dependent on the observe(applies to all cases though in some cases it simplifies down to basic simultaniety)

Sorry I can't speel today

Observers should never disagree on the qualitative features of what happened, although they may disagree on the quantitative aspects if they are measuring using different reference standards (different rulers, clocks, "frames").

What this means is that we can talk about what happened or didn't happen, this is a qualitative true/false binary kind of situation. Did it move, or not? Were they simultaneous, or not? Or we can talk about how fast, how long, etc. This is a continuous quantitative situation.

I argue that if, based on our definition of X, one can state that something or some event was both X and not X then this is a contradiction and demands a non contradictory redefinition of X. In the case of motion I have resolved this issue in the way I have defined it. In the case of simultaneity this is resolved by defining simultaneity as either A) Local or B) In a specific frame (frame E in my example).

 

[Saw]

if anyone at the trial understands physics they can show that because of the speed of the signals and the movement of the device at the center, then even though the strikes were non-simultaneous, they were timed just right so that the signals would reach the device simultaneously. So why did the jury conclude that you and Dr. Mad were innocent? Are you just assuming that the jury isn't capable of basic physics calculations, and none of the physicists who testified bothered to correct them?

You also seem to be using this thought-experiment to support the idea that the strikes really were simultaneous in some objective sense. Why? 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.

I fully agree, but still think that althonhare has some valid points.

As I said in an earlier post, relative simultaneity is relative and absolute simultaneity is absolute and each concept has its own purpose and hence should have its own niche in language. The problems (the paradoxes) only arise if you use one for the purpose of the other or vice versa. But please don’t quote this clumsy paragraph in helpless isolation. I’ll try to explain myself.

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. 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.

However, and here I loosely follow Einstein himself, for practical reasons, we may have to leave aside the chimera of measuring absolute simultaneity and content ourselves with relative simultaneity. The practical reasons are the fact that our measurements are inherently relative, since they are made from a certain position and state of motion, with physical instruments affected by a physical environment and so on. In principle, one should not discard that, in spite of all that, those measurements yield homogeneous effects, at least in some respects, since there often arises the helpful phenomenon of “compensation of effects”. In fact, you would not be able to apply transformations between different relative values if you couldn’t rely on some common or homogeneous ground (absolute spacetime in SR?).

In particular, the specific measurement of simultaneity, as of today, with our current measurement technology, yields relative values. Thus the simultaneity measurements carried out in the thought-experiment from the train and from the embankment gave off frame-dependent values. Does it mean that they are not equally trustable? Yes, they are! For their purpose, they are! If you combine the RS with TD and LC, you get a coherent system where all observers make the right predictions. Hence, as long as you do not ask them to do a different thing, we cannot prefer one measurement of simultaneity over the other, we cannot say that one was wrong and the other was right: in fact, both types of measurements helped the respective measurers to predict adequately the single event, the simultaneous arrival of the two light signals at the centre of the chair. Each served the purpose for which it had been made, so it was right… in that sense, for that purpose! Of course, if someone comes to you and says that, just because in her frame one event happened earlier, your grandmother was killed before your father was born, you dismiss her immediately, because the purpose of a relative measurement of simultaneity is not to predict nonsense.

Thus the two concepts of simultaneity can live together peacefully, like good brothers, each serving its own purpose in life. This paradigm should protect us against two types of mistakes:

a) The mistake of some critiques of SR = thinking that one of the two relative measurements must be absolute for the wrong reason, just because it serves its purpose. I am sorry, but both measurements serve their purpose, the one from the train and the one from the embankment. As JesseM points out, if one thinks (for discussion purposes!) that there is an aether, a synch operation carried out with the Einstein convention at rest in the aether frame would yield a measurement of absolute simultaneity, in the above sense. But it might perfectly happen that the frame at rest in the aether is the embankment and then your criterion, althonhare (really simultaneous is what is simultaneous in the “local” frame), is not valid. Events do not belong to any frame in particular, they take place in all frames. In your trial, what makes you guilty is not the fact that the bolts are absolutely simultaneous in the train (most probably they are not), but the fact that the device had been designed so as to kill if it captured relative simultaneity as measured on the train or, if you prefer, a certain relative non-simultaneity as measured from the embankment.

b) The mistake of some SR defenders = But I am too tired now and probably little prepared for that…

Criticism for this part is welcome.

 

[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.