Violation of simultaneity - real of frame-dependent perception?

[Saw]

This is one of the things I am trying to clear up. To say '... actually happen in a different order, according to the two sets of clocks...' - isn't it ultimately frame-dependent pereception? This is where one's 'reality' becomes another's 'perception' I think, leading to a non-definite description of reality itself - probably the same as saying there is no preferred frame which we can use to arbitrate between all frames.

Arindamsinha, since I initially had the same concerns as you, I think I can lend you a hand by explaining how I overcame them.

What Einstein and SR do, with this issue of relativity of simultaneity, is teaching a great lesson on how to build a concept:

- You must build it on the basis of empirical measurements. In our case, for an observer to state that a distant event is or not simultaneous with another, he must have a clock placed nearby.

- In a Newtonian universe you could use for this purpose a privileged method [you could carry out the synchronization with the aid of infinitely fast (instantaneous) signals or with some other method leading to the same outcome] and thus you’d get total consensus amongst observers about what is simultaneous and what is not.

- SR, instead, works upon the realization that such privileged method does not work and hence observers with different states of motion do make different measurements of simultaneity. So you have to live with that. Simultaneity is a relative concept.

- It is important you keep in mind how the synch operation is done under SR: typically (although other methods should lead to the same result), through the Einstein-Poincaré convention (A sets its clock to 0 and sends a light signal to a distant clock B, the latter receives it at time T, A records the time taken for the round trip as 2 s and hence clock B’s time T is fixed as 1s).

- Is that “real” or “perception”? It is a measurement. It is better than mere perception (which you make with your sensory organs, which are not equally calibrated for all human beings, whereas we suppose that different observers are equipped with identical clocks). And it is of course real, in the sense that each measurement is the result of an empirical operation, but that does not mean that observers are disagreeing about the same reality. We are talking here about different realities. The measurements are divergent because they project over different facts (observer A synched his clocks from a train, B did it from the platform), which circumstance does have a bearing on the outcome of the measurement operation.

- But what if observers want to say something about the same reality, for example, solve a problem: will the projectile I send now (event E) arrive in time to kill the villain before he slaughters the maid (event E’)? Well, that is the purpose of the concept. It would be dramatic if discrepancy persisted in this respect. Fortunately, it does not. All observers agree, also under SR, whether that feat is possible or not. If they say YES, they all agree that E happens before E’. If they say NO, one frame will say that E and E’ are simultaneous, another that E’ happens earlier, another that E is earlier but even the latter will admit that a projectile from E could not reach E’ in time, unless it traveled faster than light. But if faster than light travel is impossible for the purpose of synching clocks, it must also be so for real-life purposes. Thus the problem is solved.

To sum up, what SR has done with the concept of simultaneity is adapting it to reality: the way you measure it leads to discrepant measurements, but the latter contain sufficient information so as to solve causality problems, which is the reason why you invented the concept, after all.

 

[harrylin]

[..] - Is that “real” or “perception”? [..] it is of course real, in the sense that each measurement is the result of an empirical operation, but that does not mean that observers are disagreeing about the same reality. We are talking here about different realities. [..]

Perhaps you want to rephrase that? It is a single reality, in which different sets of clocks disagree with each other because the operators set them differently. And all operators agree that this is what they did. See my post #35 and the Wikipedia link.
It could be of course that you mean something else with "reality" than I do! :tongue:

 

[Chestermiller]

Yes indeed. Those sets of clocks which you both trust happen to disagree with each other, so you make it sound as if we should believe in some kind of "multiple worlds" interpretation. There is no need for that. See: http://en.wikipedia.org/wiki/One-way_speed_of_light

See the article above.

I didn't mean to imply that we believe in some kind of multiple worlds interpretation, and I agree that there is no need for that. If you got that impression, I am very sorry.

What I do believe is that we are talking about a sampling of empirical evidence from a set of hypothetical experiments (that would agree with actual experimental observations if we could perform the experiments accurately enough). These experimental results are telling us something about the fundamental geometry of 4D spacetime. Once we are at least tentatively willing to accept this experimental evidence as correct, the next question we should be asking ourselves is "what is it about the fundamental geometry and kinematics of spacetime that can possibly give rise to such unusual, unexpected, and counterintuitive effects?" Understanding the 4D geometry, in my judgement, is key to being able to understand SR at the gut level.

 

[Saw]

Perhaps you want to rephrase that? It is a single reality, in which different sets of clocks disagree with each other because the operators set them differently. And all operators agree that this is what they did. See my post #35 and the Wikipedia link.
It could be of course that you mean something else with "reality" than I do! :tongue:

No…, I think we agree, it is just a matter of expression. By “different realities” I simply meant that each synch operation is a different act, a different experiment. A measurement can be defined as an interaction with reality. Each observer uses the same procedure (Einstein convention) but does it from a different frame or state of motion and thus interacts with reality in a different manner. And, yes, of course, each operator agrees that the other should have obtained the result that he or she did. The measurement events are also events whose occurrence is a frame-invariant truth.

 

[harrylin]

I didn't mean to imply that we believe in some kind of multiple worlds interpretation, and I agree that there is no need for that. If you got that impression, I am very sorry.

What I do believe is that we are talking about a sampling of empirical evidence [..] These experimental results are telling us something about the fundamental geometry of 4D spacetime. Once we are at least tentatively willing to accept this experimental evidence as correct, the next question we should be asking ourselves is "what is it about the fundamental geometry and kinematics of spacetime that can possibly give rise to such unusual, unexpected, and counterintuitive effects?" Understanding the 4D geometry, in my judgement, is key to being able to understand SR at the gut level.

OK, now I get what you are aiming at. The block universe is of course one way to interpret the phenomena while other people make sense of it in other ways. We discussed that interpretation in two threads not so long ago, one of which, I see now, you started:
https://www.physicsforums.com/showthread.php?t=583606
https://www.physicsforums.com/showthread.php?t=595021
I think that my post #53 summarizes it well. (the last post there was barking up the wrong tree in the wrong forest)

No…, I think we agree, it is just a matter of expression. By “different realities” I simply meant that each synch operation is a different act, a different experiment. A measurement can be defined as an interaction with reality. Each observer uses the same procedure (Einstein convention) but does it from a different frame or state of motion and thus interacts with reality in a different manner. And, yes, of course, each operator agrees that the other should have obtained the result that he or she did. The measurement events are also events whose occurrence is a frame-invariant truth.

Good - that clarification could be helpful!

 

[nitsuj]

Yes indeed. Those sets of clocks which you both trust happen to disagree with each other, so you make it sound as if we should believe in some kind of "multiple worlds" interpretation. There is no need for that. See: http://en.wikipedia.org/wiki/One-way_speed_of_light

Woo hoo finally this is up for discussion, and with harrylin

I am of the school of thought that this is two physical separate physical realities. (let me know if "physical reality" is too ambiguous)

I would not even remotely call this "mutiple worlds" however I would go as far as saying multiple physical realities due to RoS.

With that said of course the "Holistic Reality" is c is invariant & this leads to RoS.

RoS is purely an "issue" for thoughtful observers, physics doesn't concern itself with metrics (specifically the comparative result) thoughtful observers do. In other words it's a comparative conundrum, not a physical one.

The is nothing remotely odd with there being two separate physical realities of the same events, as far as sticks & stones may be concerned.

It's the comparative result that's odd, thinking of these events simultaneously despite the events being separated by an interval. (and relative motion determines how that interval will be measured...) Note the light cone presentation, along with the labeling of "elsewhere", and what "elsewhere" physically means. Do you consider the "elsewhere" area to be a "reality"?

I'm going to read that article you had linked too, (again last time was some time ago). Is there a specific part you are referring to?

The "Ladder & Barn" paradox (or any other RoS "paradox") is a description of two separate physical realities imo.

 

[Chestermiller]

OK, now I get what you are aiming at. The block universe is of course one way to interpret the phenomena while other people make sense of it in other ways. We discussed that interpretation in two threads not so long ago, one of which, I see now, you started:
https://www.physicsforums.com/showthread.php?t=583606
https://www.physicsforums.com/showthread.php?t=595021
I think that my post #53 summarizes it well. (the last post there was barking up the wrong tree in the wrong forest)

Whoa Harry. I never mentioned the "B" word. For the record, when it comes to Block Universe, I am a devout agnostic.

If all the observers in a given frame of reference that is moving relative to mine are, according to their own reckoning, all of the same age, then when observers from my frame of reference view them directly (i.e., by being directly opposite them) at an agreed-upon time according the set of clocks synchronized in our frame of reference, my observers will see people of all different ages, varying in a systematic way in the direction of relative motion. One way of explaining this is by using the Block Universe model. But, I just doubt that this is the only possible explanation consistent with SR, although I haven't been able to think of other explanations. That, of course, could just be related to my own conceptualization limitations.

 

[harrylin]

Whoa Harry. I never mentioned the "B" word. For the record, when it comes to Block Universe, I am a devout agnostic.
[..] One way of explaining this is by using the Block Universe model. But, I just doubt that this is the only possible explanation consistent with SR, although I haven't been able to think of other explanations. That, of course, could just be related to my own conceptualization limitations.

OK - I'm still not sure what you had in mind otherwise, but I also gave the links because they might be of interest to arindamsinha.

 

[harrylin]

[..]
I am of the school of thought that this is two physical separate physical realities. (let me know if "physical reality" is too ambiguous)

Not sure if it is too ambiguous; in QM these are more often and recurring topics. Generally "reality" is thought to be not an issue with SR.

[..] I would go as far as saying multiple physical realities due to RoS.

With that said of course the "Holistic Reality" is c is invariant & this leads to RoS.

RoS is purely an "issue" for thoughtful observers, physics doesn't concern itself with metrics (specifically the comparative result) thoughtful observers do. In other words it's a comparative conundrum, not a physical one.

The is nothing remotely odd with there being two separate physical realities of the same events, as far as sticks & stones may be concerned.

It does sound as if you mean something else with "reality" than I do... This could become too philosophical.

It's the comparative result that's odd, thinking of these events simultaneously despite the events being separated by an interval. (and relative motion determines how that interval will be measured...) Note the light cone presentation, along with the labeling of "elsewhere", and what "elsewhere" physically means. Do you consider the "elsewhere" area to be a "reality"?

I'm going to read that article you had linked too, (again last time was some time ago). Is there a specific part you are referring to?

The "Ladder & Barn" paradox (or any other RoS "paradox") is a description of two separate physical realities imo.

This may need a separate thread... IMHO it's just different human perspectives of the same physical reality.
If you mean the one-way speed of light article, I merely aimed at the introduction. We also had very many detailed discussions on PF on one-way speed of light.

 

[nitsuj]

IMHO it's just different human perspectives of the same physical reality.

<- mostly 'cause I respect your opinion and look up to your understanding of SR...and it's different than mine.

 

[harrylin]

<- mostly 'cause I respect your opinion and look up to your understanding of SR...and it's different than mine.

Thanks for the thumbs up .
I checked back a little, and I now notice this post of you, of which the last sentence I overlooked or misunderstood:
https://www.physicsforums.com/showthread.php?p=4112638#post4112638
Maybe we should elaborate a little more, either here or there. At the time I thought that we essentially meant the same

 

[nitsuj]

Maybe we should elaborate a little more, either here or there. At the time I thought that we essentially meant the same

That sentence is much better said than my post above. The term mutually exclusive is perfect for describing "opposing" & symmetrical FoR's.

It maybe that I'm okay with saying both are physical realities & mutually exclusive.

 

[nitsuj]

post 667 (but not superstitious)

 

[harrylin]

That sentence is much better said than my post above. The term mutually exclusive is perfect for describing "opposing" & symmetrical FoR's.

It maybe that I'm okay with saying both are physical realities & mutually exclusive.

I'm still not sure how to interpret that ... Let's wait and see if the one who started the topic thinks that it is on-topic. If not, we will likely meet again at a discussion of a paradox, and then we can maybe solve any disagreement.

 

[arindamsinha]

Yes indeed. Those sets of clocks which you both trust happen to disagree with each other, so you make it sound as if we should believe in some kind of "multiple worlds" interpretation. There is no need for that. See: http://en.wikipedia.org/wiki/One-way_speed_of_light

Very interesting...

Arindamsinha, since I initially had the same concerns as you, I think I can lend you a hand by explaining how I overcame them...

To sum up, what SR has done with the concept of simultaneity is adapting it to reality: the way you measure it leads to discrepant measurements, but the latter contain sufficient information so as to solve causality problems, which is the reason why you invented the concept, after all.

Thanks. This makes sense.

I have a reasonable understanding of what you are saying. Intuition is not something that we can depend on, and have to go based on empirical experimental observations, and see what theory best fits them.

Perhaps you want to rephrase that? It is a single reality, in which different sets of clocks disagree with each other because the operators set them differently. And all operators agree that this is what they did. See my post #35 and the Wikipedia link.
It could be of course that you mean something else with "reality" than I do!

That is the thought where I began the thread. It could really be interpreted in different ways by different people, I guess.

I didn't mean to imply that we believe in some kind of multiple worlds interpretation, and I agree that there is no need for that...

Agreed, I think what we really are talking about is the same event as viewed by different frames of reference. The question becomes whether we should call it 'perception' or 'reality'. It appears that there may be little difference between the two (assuming we are talking about experimentally verifiable observations, not a 'mental state' or 'feeling').

... I am of the school of thought that this is two physical separate physical realities. (let me know if "physical reality" is too ambiguous)...

Agree with the general discussion in this part of the thread. I think one could call it 'different realities' and another 'different perceptions', and both could be equally right! (At least as far as SR theory is concerned)

... Let's wait and see if the one who started the topic thinks that it is on-topic...

I suppose it is still on-topic, because really the interpretation difference seems to be philosophical (as mentioned in a previous post) rather than what can be proven in black and white (within SR framework)...

I have now pretty much got the best answers I could hope for.

 

[bossman27]

Your explanation narrows this 0-1 type of situation to something a bit more continuous. What I am understanding from your explanation is that causality is sufficient, but not a necessary criterion for all frame to agree chronology. Instead, as long as there is a certain calculatable time-gap between the two events based on a frame at rest w.r.t. the crash locations (and velocities), all frames will agree on the chronology, even if there wasn't enough time for a message to necessarily pass between the two events (essentially enforcing causality).

Did I get this correct?

Sorry I haven't checked this thread in a few days so I didn't get back to you earlier. Even without reading all of the latest posts, I'm sure you have been getting good information and have gained some valuable insight.

That said, I am responding to this quote because it is not quite correct. It is correct that causality, as we usually mean it, is a sufficient but not a necessary condition for preserving the temporal order of events across all reference frames. It is not correct that the order would be preserved, "even if there wasn't enough time for a message to necessarily pass between the two events." In fact, the definition of a light-like interval is that light (a message) had exactly enough time to traverse the space between the two events between their occurences. That was actually the basis of the value (.018... seconds) that I calculated; if $\Delta t$ was exactly that value, we would have a light-like interval, and both spatial and temporal order would be preserved (though keep in mind the actual values calculated in different references frames would vary, we are just saying the sign of all of those values would agree). If $\Delta t$ was any greater than that value, then we have a time-like interval; here not only light but matter could also, in theory, traverse the distance in time to carry a message; in this case only temporal order is guaranteed to be preserved. (that is, the sign of $\Delta t'$ will be preserved)

The short version is: the requirement for the preservation of temporal order is exactly that light had at least enough time to make it from event A to event B. That is, the "calculable time gap" is the amount of time it takes to send a message (light) from point A to point B. We would say that even if it isn't really possible within the context of the situation (like two crashes A and B in NY and London), the laws of relativity cannot immediately rule out the possibility that A caused B, if light could have traveled between them in time. You could also think of the light as being in the form of a radio signal, if that makes the situation more plausible to you.

To reiterate one last time, when we're talking about determining the type of interval in SR, and hence the possibility of causality... "could event A have caused event B?" is equivalent to asking, "could a beam of light have made it from the occurrence of event A to the occurrence of event B?" (disregarding obstacles of course)

And again, I encourage you to read up or watch some youTube videos on Minkowski diagrams and light cones. I am very certain that this would really make everything I and others have described more visually intuitive for you.
Here we would say that an interval between A and B is:
1) Time-like if: event B is inside event A's future light cone.
2) Space-like if: event B is outside event A's future light cone.
3) Light-like if: event B lies on the edge of event A's future light cone.
-- This will all make sense when you see/draw the diagrams.

 

[arindamsinha]

OK, thanks for the elaboration.