How to resolve the contradiction in twin clocks?

[JM]

True.

Not true. You just quoted me as saying that "Proper Time is what any clock measures" so why would you say the time for this clock is not a proper time?

George: In post 102 you agreed, I think, with the definition that 'proper time is the elapsed time between two events as measured by a clock that passes through both events.' Are you now saying that a clock that does not pass through both events is also measuring proper time? JM

 

[Dale]

George: In post 102 you agreed, I think, with the definition that 'proper time is the elapsed time between two events as measured by a clock that passes through both events.' Are you now saying that a clock that does not pass through both events is also measuring proper time? JM

Yes, it is also measuring proper time, but along a different worldline. All clocks measure the proper time along their own worldline.

 

[ghwellsjr]

George: In post 102 you agreed, I think, with the definition that 'proper time is the elapsed time between two events as measured by a clock that passes through both events.' Are you now saying that a clock that does not pass through both events is also measuring proper time? JM

Yes, it's measuring the Proper Time between any two other events that it passes through. Clocks can only measure the time where they are, not somewhere else. It's kind of like saying that rulers can only measure lengths where they are, not somewhere else.

 

[JM]

Clocks can only measure the time where they are, not somewhere else.

Ah, but they do. Consider Einsteins watch, 1905,Part I,section1. In order to be useful a watch must be in synch with clocks at other locations. We synch with GMT in everyday life. So when the arrival of the train at the station (where Einstein is located) coincides with the hand of his watch pointing to 7, all the other watches/clocks also point to 7. So a person across town, who knows the schedule, and sees his clock point to 7 can conclude that the train has arrived.
In Einsteins theory the clocks of a given frame are synched by the exchange of light signals. So in my first example Post 103, when the clock at x' = -5 reads ct' = 10 then all the clocks of K' also read ct' = 10, including the one at x' = 0. For this specific example with these two events the clock at x' = 0 is not present at both, and is therefore not measuring Proper time, according to our agreed definition.
Other events can be specified as you suggest and as I did in my second example, but those other events don't change the analysis of my first example.
If there is a theory that doesn't allow for synch'ing of clocks then that theory is different from Einsteins 1905 theory.
JM

 

[Nugatory]

Clocks can only measure the time where they are, not somewhere else.

Ah, but they do. Consider Einsteins watch, 1905,Part I,section1. In order to be useful a watch must be in synch with clocks at other locations. We synch with GMT in everyday life. So when the arrival of the train at the station (where Einstein is located) coincides with the hand of his watch pointing to 7, all the other watches/clocks also point to 7. So a person across town, who knows the schedule, and sees his clock point to 7 can conclude that the train has arrived.

That's true, but I disagree that it follows that the clock at one location is measuring time at another location. I prefer to say that we're using the clock at one location to measure the proper time (precisely the proper time elapsed since some arbitrary zero event) at that location ("my clock says 7:00"), then using the Einstein synchronization process to map that value to coordinate time ("Hey guys, we all agree that it's 7:00 right now").

Much of the confusion here stems not from what the theory of special relativity is, but from how our language for discussing these concepts has evolved. When Einstein was writing in 1905 there was no distinction between coordinate and proper time as we understand the terms, so Einstein couldn't use them in his writing.

 

[ghwellsjr]

Ah, but they do. Consider Einsteins watch, 1905,Part I,section1. In order to be useful a watch must be in synch with clocks at other locations.

No, I can use my watch to measure how long I should brush my teeth, even if I'm on a fast moving train.

We synch with GMT in everyday life. So when the arrival of the train at the station (where Einstein is located) coincides with the hand of his watch pointing to 7, all the other watches/clocks also point to 7.

No, not my watch on a fast moving train (where I'm brushing my teeth).

So a person across town, who knows the schedule, and sees his clock point to 7 can conclude that the train has arrived.

But if that person is on another high speed train across town approaching the first train to make a transfer, he might be late by looking at his own watch.

In Einsteins theory the clocks of a given frame are synched by the exchange of light signals. So in my first example Post 103, when the clock at x' = -5 reads ct' = 10 then all the clocks of K' also read ct' = 10, including the one at x' = 0. For this specific example with these two events the clock at x' = 0 is not present at both, and is therefore not measuring Proper time, according to our agreed definition.

I think your problem is that you are equating Proper Time with a Proper Clock. If you had said, "the clock at x' = 0 is not present at both, and is therefore not a Proper Clock", then you'd be correct but as it stands, you are incorrect.

Other events can be specified as you suggest and as I did in my second example, but those other events don't change the analysis of my first example.
If there is a theory that doesn't allow for synch'ing of clocks then that theory is different from Einsteins 1905 theory.
JM

True, but I'm not aware of anyone proposing a theory that doesn't allow for synch'ing of clocks so I don't know why you would bring this up.

As I have told you before, you should quit thinking about a Proper Clock or its definition since it's not a commonly accepted term.

 

[pervect]

Ah, but they do. Consider Einsteins watch, 1905,Part I,section1. In order to be useful a watch must be in synch with clocks at other locations. We synch with GMT in everyday life. So when the arrival of the train at the station (where Einstein is located) coincides with the hand of his watch pointing to 7, all the other watches/clocks also point to 7. So a person across town, who knows the schedule, and sees his clock point to 7 can conclude that the train has arrived.

It appears to me that you've missed the point that simultaneity is relative. This is mentioned in for instance"Albert Einstein (1879–1955). Relativity: The Special and General Theory. 1920.", the section on "The Relativity of Simultaneity", which is online at http://www.bartleby.com/173/9.html.

Einstein's exposition is confusing to some readers, there are other expositions that may be clearer nowadays. The basic point is that there is *not* any universal way to synchronize clocks, according to Einstein.

You can find many other expositions online by looking up "The Relativity of Simultaneity" if Einstein's is too confusing. The biggest hurdle seems to be that Einstein doesn't motivate or provide a physical mechanism for making two lightning flashes occur "at the same time", he just presuposes that it occurred by chance.

Its not really necessary to consider the mechanism to make his point, but I've seen a lot of readers get tangled up over the issue.

 

[Dale]

Ah, but they do. Consider Einsteins watch, 1905,Part I,section1. In order to be useful a watch must be in synch with clocks at other locations. We synch with GMT in everyday life.

Only if you adopt a synchronization convention, as you point out here. If you have to use a synchronization convention then the measurement is no longer a measurement of proper time. Furthermore, the standard synchronization convention is frame variant, but measurements of proper time are frame invariant. Therefore it is clear that they are not the same.

You should pay attention to the comments by ghwellsjr. He has given you correct advice and information here. Your objections have been in error.

 

[ghwellsjr]

When Einstein was writing in 1905 there was no distinction between coordinate and proper time as we understand the terms, so Einstein couldn't use them in his writing.

I don't understand what you are saying here. Just because Einstein didn't use the terms Proper Time and Coordinate Time, he still talked about those two types of time as distinct from each other in section 4 of his 1905 paper with regard to the time on a moving clock compared to the time on the stationary clocks and he derived the formula for Proper Time, ?, as a function of the speed of the clock, v, and the Coordinate Time, t, (assuming that the clocks started out synchronized). He then proceeded to give an example of a constantly accelerating clock taking a circular path so it could not be construed as exhibiting Coordinate Time but rather Proper Time. I don't think Einstein saw the need to coin a special phrase like Proper Time for the time on a moving clock because it also applies to every clock, so why have a special name for it? But we do need the special name Coordinate Time because it applies where there are no clocks.

 

[Nugatory]

I don't understand what you are saying here. Just because Einstein didn't use the terms Proper Time and Coordinate Time, he still talked about those two types of time

I'm saying that Einstein didn't use those terms, we do, and this may be contributing to JM's confusion.

 

[ghwellsjr]

I'm saying that Einstein didn't use those terms, we do, and this may be contributing to JM's confusion.

Maybe, but I think his problem is the misuse of the term Proper Clock (thinking it is directly related to Proper Time).

 

[JM]

Yes, it's measuring the Proper Time between any two other events that it passes through.

George; Of course, when the events occur at the position of a clock, it measures proper time for those events, according to the definitions stated earlier, and the clock doesn't have to be a proper clock.
But post 103 identified two specific events, one occurring at x,ct = 0,0 and the other at x,ct = 5,10. And the question is 'does the clock at x' = 0 measure proper time for those two events.' My answer is no, because the clock at x' = 0 is not present at both of those events. Note that the idea of proper clocks isn't involved here.
Do you disagree?
JM

 

[ghwellsjr]

George; Of course, when the events occur at the position of a clock, it measures proper time for those events, according to the definitions stated earlier, and the clock doesn't have to be a proper clock.
But post 103 identified two specific events, one occurring at x,ct = 0,0 and the other at x,ct = 5,10. And the question is 'does the clock at x' = 0 measure proper time for those two events.' My answer is no, because the clock at x' = 0 is not present at both of those events. Note that the idea of proper clocks isn't involved here.
Do you disagree?
JM

No, I don't disagree but for a different reason.

Events don't have Proper Times. Clocks have Proper Times. Events have Coordinate Times. When you talk about two events, you can't just ask what is the Proper Time between them without specifying the path through spacetime of the clock that you have in mind which will be present at those two events.

Here is a spacetime diagram for K' as you specified it in post #103 (I'm using the speed of light to be one foot per nanosecond) along with a black Proper Clock:

Note the green clock at x'=0. Note the first green event at x',t'=0,0. Note the blue event at x',t'=-5,10.

Note that the Coordinate Time interval between those two events is 10 which is identical to the Proper Time interval on the green clock at x'=0 between the Coordinate Times of 0 and 10 (because this clock is stationary in this frame).

If you specify a Proper Clock (an inertial clock as I show in the diagram) to go between those two events, which is identical to specifying the Spacetime Interval between those two events, then the time interval is 8.66 which you can either calculate using the formula for the Spacetime Interval, √(Δt²-Δx²) = √(10²-5²) = √(100-25) = √75 = 8.66 (and you can do this from any frame), or you could actually have an inertial clock go between the two events and measure its Proper Time interval as depicted in the spacetime diagram.

But you can have a different non-inertial clock go between those two events and measure a different Proper Time as depicted in this spacetime diagram which measures 7:

This is no different than the issue with the so-called Twin Paradox where a non-inertial twin can have a shorter Proper Time between two events than an inertial twin. Here is another path for a clock which has an even shorter Proper Time interval beween the two events:

This Proper Time interval is just 2. You can make this interval as small as you like by having the clock travel close to the speed of light away from the first event and back to the second event.

I have tried to cover as much as I can on this subject so that it will answer all of your questions. Does it all make sense to you now?

 

[JM]

No, I don't disagree but for a different reason.

Events don't have Proper Times. Clocks have Proper Times. Events have Coordinate Times. When you talk about two events, you can't just ask what is the Proper Time between them without specifying the path through spacetime of the clock that you have in mind which will be present at those two events.

Here is a spacetime diagram for K' as you specified it in post #103 (I'm using the speed of light to be one foot per nanosecond) along with a black Proper Clock:

Note the green clock at x'=0. Note the first green event at x',t'=0,0. Note the blue event at x',t'=-5,10.

Note that the Coordinate Time interval between those two events is 10 which is identical to the Proper Time interval on the green clock at x'=0 between the Coordinate Times of 0 and 10 (because this clock is stationary in this frame).

If you specify a Proper Clock (an inertial clock as I show in the diagram) to go between those two events, which is identical to specifying the Spacetime Interval between those two events, then the time interval is 8.66 which you can either calculate using the formula for the Spacetime Interval, √(Δt²-Δx²) = √(10²-5²) = √(100-25) = √75 = 8.66 (and you can do this from any frame), or you could actually have an inertial clock go between the two events and measure its Proper Time interval as depicted in the spacetime diagram.

George; I applaud your presentation. It is clear and it applies to the points I posed. I appreciate that.
I note that there are two different meanings of 'Proper Time' used. The time of the green clock is called 'Proper time' even though this clock is not present at both of the events specified. This doesn't agree with the definitions cited earlier. Then you cite a clock going between the points and measuring Proper Time, in accordance with the definitons. I suppose it's OK, but isn't it confusing?
In the 1905 theory all the clocks of K' move at constant speed +v along lines parallel to the x axis. You allow clocks to move in many other directions. So your theory must be something different from his, mustn't it? What might a connection be?
Have you ever wondered where the 'Space-Time Interval' comes from? Did Minkowski pluck it from thin air, or what? And the term "interval'- isn't an interval zero only when two points coincide? Some things to think about.
Best Regards
JM

 

[Dale]

I note that there are two different meanings of 'Proper Time' used.
...
So your theory must be something different from his, mustn't it?

There are not two different definitions nor are there two different theories. You are asserting differences that simply don't exist. There is just one definition and one theory applied more generally than you are used to. Instead of asserting non-existent differences you would be better served to actually learn from the good material that has been presented.

In any case, this is all off topic, the OP is banned, and this thread is now closed. If you wish to discuss physics please do so in a new thread, but even if your comments about multiple definitions and multiple theories were correct (which they are not) they would be a semantic debate, not a physics debate, and the semantic debate is closed and should not be reopened.