Relation between coordinate time and proper time

[ash64449]

At first, I could not get your link to work. I was able to get to this page:

http://www.marxists.org/reference/archive/einstein/works/1910s/relative/index.htm

And from there to the chapter you referenced:

http://www.marxists.org/reference/archive/einstein/works/1910s/relative/ch10.htm

However, this chapter is not talking about observing or seeing a high speed object as length contracted, it's talking about an observer measuring the length of a high speed object, something that takes time for him to do and is based on assumptions that pinpoint the IRF in which he is making the measurements and doing the calculation.

I discussed this for the scenario of the observer and his mirror:

wait,,, so... You agree that length contraction is observed??

 

[ash64449]

Explain exactly how. You are still saying 'when the observer sees ...' which is imprecise ( to me in any case).

why do you say that 'seeing' is imprecise?
Look at the article that i linked to george,einstein was also used the method similar to what i said.
The reason why length contraction takes place is because light takes time to reach and as a result the word 'seeing' is very important..

 

[ash64449]

Mentz14, the reason why we cannot 'see' time dilation is because time is not something that we cannot 'see'. Instead we see time ticking faster and slower because of light and that is why relativistic doppler effect is seen..
Another reason why time dilation is not seen is because it not only involved proper time,coordinate time is too involved which we cannot in real situations cannot understand it because light has no role to play in coordinate time..

 

[Mentz114]

why do you say that 'seeing' is imprecise?
Look at the article that i linked to george,einstein was also used the method similar to what i said.
The reason why length contraction takes place is because light takes time to reach and as a result the word 'seeing' is very important..

It is 'when' that is imprecise ! What do you mean by 'when X sees Y' ? It has been stated many times on this forum that the LC phenomemnon is caused by the relativity of simultaneity - i.e. we cannot simultaneously perform two remote merasurements.

You have ignored the best part of my post ( the radar measurement).

I have to tell you that concentrating on LC and/or TD is not a good way to approach relativity. Stick to observables like radar and invariants like proper time. I've nothing more to say on this.

 

[Mentz114]

Mentz14, the reason why we cannot 'see' time dilation is because time is not something that we cannot 'see'. Instead we see time ticking faster and slower because of light and that is why relativistic doppler effect is seen..
Another reason why time dilation is not seen is because it not only involved proper time,coordinate time is too involved which we cannot in real situations cannot understand it because light has no role to play in coordinate time..

Sorry, I'm not involved in that discussion. Let's stick to 'seeing length contraction'.
(I've edited my post #34)

 

[ash64449]

You haven't understood my post. I said exactly what you said.

You said:LC phenomenon is caused by the relativity of simultaneity

And i said:The reason why length contraction takes place is because light takes time to reach and as a result the word 'seeing' is very important..

Both of them are same. You didn't understand that.

i.e. we cannot simultaneously perform two remote measurements.
.

Relativity of simultaneity doesn't mean the above statement. It means that events being simultaneous depends on reference frames.

 

[Dale]

YouYou said:LC phenomenon is caused by the relativity of simultaneity

And i said:The reason why length contraction takes place is because light takes time to reach and as a result the word 'seeing' is very important..

Both of them are same. You didn't understand that.

They are not both the same. The reason why length contraction takes place is because the speed of light is invariant, not because it is finite. If we had a universe where the speed of light were finite but there invariant speed was not then we would not have length contraction. Your statement is wrong.

IMO, Mentz114's statement is also wrong. Both length contraction and relativity of simultaneity are "caused" by the principle of relativity and the invariance of the speed of light. They don't cause each other.

 

[ash64449]

They are not both the same. The reason why length contraction takes place is because the speed of light is invariant, not because it is finite. If we had a universe where the speed of light were finite but there invariant speed was not then we would not have length contraction. Your statement is wrong.
.

DaleSpam,

see this thread: https://www.physicsforums.com/showthread.php?t=688843

Read the comment #8...

And You will find that Why speed of light is invariant even though light moves in a reference frame c+v and c-v relative to a observer by reading further...

So My statement holds true.

 

[ash64449]

IMO, Mentz114's statement is also wrong. Both length contraction and relativity of simultaneity are "caused" by the principle of relativity and the invariance of the speed of light. They don't cause each other.

Sorry DaleSpam,You are right in saying that Mentz114's statement is wrong.

Actually,i should say that they are closely related and not length contraction is caused by relativity of simultaneity..

I faced a similar problem like this when we first met and i made a similar assertion.. Now i understood it..

 

[Dale]

see this thread: https://www.physicsforums.com/showthread.php?t=688843

Read the comment #8...

And You will find that Why speed of light is invariant even though light moves in a reference frame c+v and c-v relative to a observer by reading further...

So My statement holds true.

The referenced post has nothing to do with the "cause" of length contraction. It simply assumes length contraction (and time dilation and relativity of simultaneity) in order to show a graph of a radar pulse.

References to random irrelevant comments is a bad habit of yours which you need to work on.

 

[ash64449]

The referenced post has nothing to do with the "cause" of length contraction. It simply assumes length contraction (and time dilation and relativity of simultaneity) in order to show a graph of a radar pulse.

Sorry,that is not what i was saying..in that thread you can see that to an observer,light moves c+v in one-way of the round trip and when it reflects back light moves c-v relative to observer. But one can only measure a full round trip and measuring full round trip will only come to the conclusion that light is invariant. See? Light speed is finite and even tough Light speed is invariant,because one can only measure full round trip.

I am extremely sorry if you felt that i linked that comment to say that length contraction is caused by relativity of simultaneity. I actually linked to show how light is invariant in different frames even though light travels finite speed and even if it moves c+v and C-v relative to observer too..


.

 

[ash64449]

And this "c+v" and "c-v" is same as telling that "Light takes time to reach" and that is what according to me Relativity Of simultaneity. I.e. I consider c+v and c-v as relativity of simultaneity.

 

[ash64449]

And i consider "hastening towards the beam of light" and "moving ahead of the beam of light" as light takes less time to reach and light takes more time to reach as it is said in this book:

http://www.marxists.org/reference/archive/einstein/works/1910s/relative/relativity.pdf.

Go to chapter Relativity of Simultaneity.

""he is hastening towards the
beam of light coming from B, whilst he is riding on ahead of the beam of light coming from
A. Hence the observer will see the beam of light emitted from B earlier than he will see that
emitted from A"".

and then he says ""Observers who take the railway train as their reference-body must therefore
come to the conclusion that the lightning flash B took place earlier than the lightning flash
A""

If you consider i have made a wrong conclusion,then help me correct it..

 

[Mentz114]

IMO, Mentz114's statement is also wrong. Both length contraction and relativity of simultaneity are "caused" by the principle of relativity and the invariance of the speed of light. They don't cause each other.

Can you be specific and say which of my several statements is wrong. I need to know to avoid the error in future.

[Edit]
I presume this is the one

It has been stated many times on this forum that the LC phenomenon is caused by the relativity of simultaneity - i.e. we cannot simultaneously perform two remote measurements.

I don't say that the oft quoted explanation is correct, merely that it was quoted many times (true).

Also, it is not possible to perform simultaneous remote measurements, is it ?

I apologise to the OP if I've confused the issue, but he does rather ignore most of what I posted.
I get impatient with people who think relativity is about LC and TD, and I suspect they have a hidden agenda.

 

[Dale]

I actually linked to show how light is invariant in different frames even though light travels finite speed and even if it moves c+v and C-v relative to observer too.

OK. That is also irrelevant to the topic. You stated "The reason why length contraction takes place is because light takes time to reach". That is false, and the fact that the closing speed is c+v or c-v is not relevant. There is no mention in the quote of anything which even remotely supports the claim that "length contraction takes place ... because light takes time to reach".

This posting of irrelevant and unresponsive quotes must stop.

 

[Dale]

Can you be specific and say which of my several statements is wrong. I need to know to avoid the error in future.

My apologies. I was reacting specifically the statement in ash64449's post which he attributed to you: "LC phenomenon is caused by the relativity of simultaneity". I didn't even check to see if the attribution was correct. The correct statement would be: "LC phenomenon and the relativity of simultaneity are both caused by the principle of relativity and the invariance of c"

I apologise to the OP if I've confused the issue, but he does rather ignore most of what I posted.
I get impatient with people who think relativity is about LC and TD, and I suspect they have a hidden agenda.

I have noted the same about the OP and agree with your impatience.

 

[ghwellsjr]

At first, I could not get your link to work. I was able to get to this page:

http://www.marxists.org/reference/archive/einstein/works/1910s/relative/index.htm

And from there to the chapter you referenced:

http://www.marxists.org/reference/archive/einstein/works/1910s/relative/ch10.htm

However, this chapter is not talking about observing or seeing a high speed object as length contracted, it's talking about an observer measuring the length of a high speed object, something that takes time for him to do and is based on assumptions that pinpoint the IRF in which he is making the measurements and doing the calculation.

I discussed this for the scenario of the observer and his mirror:

Thanks, I'm glad you liked them.

Now I want to take that same diagram that depicts the situation that adjacent described in his Opening Post (OP) and show you how it depicts the Length Contraction of the distance between the "person" in blue and the mirror in red which the "person" measured to be 6 feet with his ruler. There are a couple ways that other people, stationary in the IRF in which the "person" is moving can make this assessment. They both involve radar measurements. This is similar to the way a cop can clock you for speeding. It works by sending a light (or radar) pulse at an object and waiting for the return echo and then measuring how long the round trip took and dividing it by two and assuming that it took the same amount of time to get to the object as it took for the light to get back from the object. So we place the time of the measurement at the midpoint of the measurement and we consider the measurement of the distance to be how far the light traveled in the measured amount of time. By making successive measurements, we can establish a speed.

Here's the first spacetime diagram:

I have drawn the second observer as a black line at coordinate distance of 15 feet. At coordinate time of 1 nanoseconds, he happens to send out the first radar pulse in blue and a short time later he sends out the second radar pulse in green at 9 ns. He receives the first reflection at coordinate time of 13 ns and the second one at 15 ns. After doing the calculation I previously described, he calculates that the mirror was 6 feet away at time 7 ns and it was 3 feet away at 12 ns. The differences between these calculates establishes that the mirror is moving toward him at 3 feet in 5 ns. which is 0.6 feet per ns or just 0.6c.

So now, armed with the measurement of the mirror's speed of 0.6 feet/ns, he waits until the mirror reach him which happens at time 17 ns. Then he waits for the "person" to reach him which happens at time 25 ns. Since it took 8 ns for the object to pass him at 0.6 feet/ns, he concludes that its length is 0.6 times 8 or 4.8 feet, the same as the gamma process determined.

Now I want to show you another way. This involves measurements of both the "person" in blue and the mirror in red taken at the same "time":

First, the observer in black sends an orange radar pulse at time 4.2 ns and a second green one at time 9 ns. He receives the echoes at 15 ns and 19.8 ns. He concludes that the blue "person" is 7.8 feet away at time 12 ns and the red "mirrors" are 3 feet away at the same time leaving a difference of 4.8 feet.

All methods agree.

wait,,, so... You agree that length contraction is observed??

No, I never said that. What I said was that an observer can make some assumptions, take some measurements, do some calculations and from that determine the Length Contraction of a moving object as determined from his own rest frame. I showed you in the above two diagrams how the black observer can do this and I illustrated it in his own rest frame. Now I don't call that observing, do you?

Furthermore, you should not think that just because he is able to make some assumptions, take some measurements, do some calculations and come up with the same determination of the Length Contraction of a moving object as would be determined from his rest frame that he is actually determining a real Length Contraction because if he makes slightly different (but just as valid) assumptions, he can determine different Length Contractions.

The assumptions he is making is that the radar signal takes the same amount of time to propagate to an object as it takes for the echo to return to him, and that the time of the measurement applies at the midpoint in time between when he sent out the radar signal and when he received it. Are these valid assumptions? They are exactly the same assumptions that we use to define an IRF in SR, so it should be no surprise that they lead to the same determination of Length Contraction for a moving object in a particular IRF.

But rather than make some different assumptions, let me show you the same processes that the black observer makes but using the rest frame of the blue "person" and his red mirror (which we will assume are connected with a six-foot rod) and in which the black observer is now moving. We are going back to the original IRF from post #14 and you will note that the distance between the blue "person" and his red mirror is not contracted but is six feet.

Here is the first scenario where the black observer first measures the speed of the approaching (as far as he's concerned) rod and then times how long it takes for the rod to pass him:

Read the explanation above for the assumptions, measurements and calculation that the black observer performs to determine that the rod is Length Contracted to 4.8 feet, even though in this IRF it is not.

And for the second scenario where the black observer makes two different measurements but comes to the same conclusion:

I hope you can see that in neither IRF, can the black observer have any inkling what IRF is being used and therefore what the Length Contraction is. I hope you can also see that if the black observer had assumed that the radar signal took a different length of time to get to each target than it did to get back (something that cannot be known apart from an assumption or definition), he could have determined that the rod was not Length Contracted.

 

[ghwellsjr]

However, this chapter is not talking about observing or seeing a high speed object as length contracted, it's talking about an observer measuring the length of a high speed object, something that takes time for him to do and is based on assumptions that pinpoint the IRF in which he is making the measurements and doing the calculation.

I cannot understand.Let me explain how length contraction can be observed. You can correct in what i have said so that i can understand what i have missed.

Determine the length of the rod when it is at rest relative to a coordinate system. Now let the rod move relative to that system. Let the rod be 10 meters long. mark two points that are 10 meters apart. Let us name the first point as 'A' and second point as 'B'.when rod passes the point 'B',Note whether the other end of the rod is at point 'A'. If the other end is at point 'A',then rods do not get contracted(or we can say that length contraction is not observed). And if the other end is not in point 'A',instead the other end is in between those two points,then rods contract when they travel(or length contraction is observed)..

What is wrong with this experiment??

I totally agree that Time Dilation is not observed.. I cannot think of any thought experiment that can prove that...

What Einstein said to do in your referenced article is exactly what I described in post #14 after the second diagram. It's very easy to see on a spacetime diagram because the coordinates provide the means to know when the measurements of A and B are taken at the same time.

However, doing it in a real situation, as you requested and as Einstein described, is not easy. You basically would require a number of synchronized clocks all along the tracks that could record when each end of the rod reached them and then you would have to go back and examine the records to find two times on two different clocks that were the same and each indicated the passing of one end or the other of the rod.

Your method lacks any means to determine the same time at both events, but Mentz pointed this out already so I won't belabor the point.

 

[ash64449]

OK. That is also irrelevant to the topic. You stated "The reason why length contraction takes place is because light takes time to reach". That is false, and the fact that the closing speed is c+v or c-v is not relevant. There is no mention in the quote of anything which even remotely supports the claim that "length contraction takes place ... because light takes time to reach".

This posting of irrelevant and unresponsive quotes must stop.

i am sorry that i wrote ' length contraction takes place because light takes time to reach'.

 

[ash64449]

No, I never said that. What I said was that an observer can make some assumptions, take some measurements, do some calculations and from that determine the Length Contraction of a moving object as determined from his own rest frame. I showed you in the above two diagrams how the black observer can do this and I illustrated it in his own rest frame. Now I don't call that observing, do you?

Furthermore, you should not think that just because he is able to make some assumptions, take some measurements, do some calculations and come up with the same determination of the Length Contraction of a moving object as would be determined from his rest frame that he is actually determining a real Length Contraction because if he makes slightly different (but just as valid) assumptions, he can determine different Length Contractions.

The assumptions he is making is that the radar signal takes the same amount of time to propagate to an object as it takes for the echo to return to him, and that the time of the measurement applies at the midpoint in time between when he sent out the radar signal and when he received it. Are these valid assumptions? They are exactly the same assumptions that we use to define an IRF in SR, so it should be no surprise that they lead to the same determination of Length Contraction for a moving object in a particular IRF.

But rather than make some different assumptions, let me show you the same processes that the black observer makes but using the rest frame of the blue "person" and his red mirror (which we will assume are connected with a six-foot rod) and in which the black observer is now moving. We are going back to the original IRF from post #14 and you will note that the distance between the blue "person" and his red mirror is not contracted but is six feet.

Here is the first scenario where the black observer first measures the speed of the approaching (as far as he's concerned) rod and then times how long it takes for the rod to pass him:

https://www.physicsforums.com/attachment.php?attachmentid=58449&stc=1&d=1367619642

Read the explanation above for the assumptions, measurements and calculation that the black observer performs to determine that the rod is Length Contracted to 4.8 feet, even though in this IRF it is not.

And for the second scenario where the black observer makes two different measurements but comes to the same conclusion:

https://www.physicsforums.com/attachment.php?attachmentid=58450&stc=1&d=1367619642

I hope you can see that in neither IRF, can the black observer have any inkling what IRF is being used and therefore what the Length Contraction is. I hope you can also see that if the black observer had assumed that the radar signal took a different length of time to get to each target than it did to get back (something that cannot be known apart from an assumption or definition), he could have determined that the rod was not Length Contracted.

George,i cannot see the images that you posted in this post.

Anyway i understood the significance of this post-it says that you cannot conduct experiment by only observing.There always include assumptions and the fact that we cannot observe 'two' events at the same time. But please clarify a little bit by answering to the post that is present below to this one.

 

[ash64449]

However, doing it in a real situation, as you requested and as Einstein described, is not easy. You basically would require a number of synchronized clocks all along the tracks that could record when each end of the rod reached them and then you would have to go back and examine the records to find two times on two different clocks that were the same and each indicated the passing of one end or the other of the rod.

No,You don't need a number of synchronized clocks all along the tracks that could record when each end of the rod reached them

Your method lacks any means to determine the same time at both events, but Mentz pointed this out already so I won't belabor the point.

Yes.Now i understood the meaning of what Mentz was actually saying-You cannot measure at the same time,both events. Well,there is a way to measure both events at the same time:

Take a Snapshot.

Think that i have a detector in my hand,which immediately photographs the whole events taking place when the rod meets at point "A". Now whatever we see in that snapshot are the simultaneous events with the event of which one end of the rod meets at point A.

So,here we see that observing only 'one' event helps us to know all other events that are 'simultaneous' with other events.

What is wrong with this?

 

[Popper]

I don't know why you would express Proper Time in this way. It makes it sound like there is a single Proper Time between two events but as you correctly point out, it is measured by a clock which moves through both events, but what you didn't point out is that it is dependent on the path of that clock between those two events so two different clocks taking two different paths can end up with different accumulated times on them.

Because the OP asked what the difference was between proper time and coordinate time. He didn't ask what the properties of proper time are. I was only attempting to explain what the definition is and that definitions I stated is precisely correct. In my opinion just because I didn't say that there are many ways for a clock to move between two clocks doesn't mean that my answer was wrong or lacking.

But if you insist on being complete; the proper time between two events is the time measured by a clock (or person attached to the clock/wristwatch) which travels on a timelike worldline between the two events.

Also, when you are talking about coordinate time, you should not be connecting it with actual clocks.

I disagree. Coordinate time refers to actual clocks so it was very important to mention them.

 

[ghwellsjr]

But if you insist on being complete; the proper time between two events is the time measured by a clock (or person attached to the clock/wristwatch) which travels on a timelike worldline between the two events.

It's not a matter of being complete--it's a matter of adding so much extraneous stuff. Let me take your definition and purge it of what isn't needed:

the proper time [STRIKE]between two events[/STRIKE] is the time measured by a clock[STRIKE] (or person attached to the clock/wristwatch) which travels on a timelike worldline between the two events[/STRIKE].

Since a clock can only travel on a timelike worldline, why do you include that? And why do you want to limit it to two events? Every clock continuously measures out Proper Time.

Also, when you are talking about coordinate time, you should not be connecting it with actual clocks.

I disagree. Coordinate time refers to actual clocks so it was very important to mention them.

If Coordinate Time refers to actual clocks and Proper Time refers to actual clocks, then what's the difference? Remember, you said:

Because the OP asked what the difference was between proper time and coordinate time.

 

[ghwellsjr]

George,i cannot see the images that you posted in this post.

You need to be logged on in order to see images that are uploaded to the Physics Forums.

Anyway i understood the significance of this post-it says that you cannot conduct experiment by only observing.There always include assumptions and the fact that we cannot observe 'two' events at the same time. But please clarify a little bit by answering to the post that is present below to this one.

Please log on an study my post. It can teach you a lot. That's what you are badly in need of.

 

[ash64449]

You need to be logged on in order to see images that are uploaded to the Physics Forums.

Please log on an study my post. It can teach you a lot. That's what you are badly in need of.

George,i don't understand.Still i cannot find the images.

What do you mean i need to be logged on?

Shouldn't i need to log on to reply post? Do you mean that? Well,then i am logged on and still i cannot see them. The images that you posted on #47.I am not talking about the images in the quote.i am not seeing images that you posted..

 

[ash64449]

George,when i open the image in new tab,this message appears:

vBulletin Message

Invalid Attachment specified. If you followed a valid link, please notify the administrator

 

[ghwellsjr]

I re-uploaded them, even though I could see them. Can you see them now?

 

[ash64449]

I re-uploaded them, even though I could see them. Can you see them now?

YES! I can see them now.. I will read them very carefully and then reply

 

[ghwellsjr]

However, doing it in a real situation, as you requested and as Einstein described, is not easy. You basically would require a number of synchronized clocks all along the tracks that could record when each end of the rod reached them and then you would have to go back and examine the records to find two times on two different clocks that were the same and each indicated the passing of one end or the other of the rod.

No,You don't need a number of synchronized clocks all along the tracks that could record when each end of the rod reached them

Your method lacks any means to determine the same time at both events, but Mentz pointed this out already so I won't belabor the point.

Yes.Now i understood the meaning of what Mentz was actually saying-You cannot measure at the same time,both events. Well,there is a way to measure both events at the same time:

Take a Snapshot.

Think that i have a detector in my hand,which immediately photographs the whole events taking place when the rod meets at point "A". Now whatever we see in that snapshot are the simultaneous events with the event of which one end of the rod meets at point A.

So,here we see that observing only 'one' event helps us to know all other events that are 'simultaneous' with other events.

What is wrong with this?

Mentz already pointed out what is wrong with taking a snapshot when you said the same thing way back in post #29:

well,there is a way to do that. Take a camera,when the observer sees the rod reach at the points B,take the picture. In it i am sure that he can determine that.

I don't know why you think a camera would be any better than an observer's eyeballs. Both are subject to the light travel time from each event to the camera/eye and since it is different for the two events in question, they will not see both events at the same time, even though we specify that they must be in order to determine the correct length of the rod.

Are you aware that if you set up a row of clocks and synchronized them and then looked at them, they would all have different times on them? Each further clock would be for an earlier time. That's why taking a picture won't help.

 

[ash64449]

George,in shorter terms,you were trying to show that Black observer cannot know which IRF he is using so as a result he cannot know what the length contraction is?(#47)

Makes sense to me!