Light shone in a train bouncing off mirrors

jefswat

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

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

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

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

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

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.

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

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”) travelled 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

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

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

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

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

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

JesseM

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,

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 finally agree with an established theory

Saw

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.

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

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