Why measuring one-way trip impossible?

[ash64449]

well,i have heard this in many threads. That we can only measure the speed of light by noting down when light completes its full round trip. Now why cannot we measure speed of light by considering the half-way trip?

 

[PAllen]

well,i have heard this in many threads. That we can only measure the speed of light by noting down when light completes its full round trip. Now why cannot we measure speed of light by considering the half-way trip?

Because you have to synchronize clocks at two different places. Then, the choice of clock synch is what determines how two way speed gets divided into two one way speeds rather than some property of light. The Einstein clock synch is defined as the choice which makes the two one way speeds the same.

 

[ash64449]

Because you have to synchronize clocks at two different places. Then, the choice of clock synch is what determines how two way speed gets divided into two one way speeds rather than some property of light. The Einstein clock synch is defined as the choice which makes the two one way speeds the same.

let me know how the clocks are syncronised so that i can make sense to your statement.

 

[Bill_K]

Because you have to synchronize clocks at two different places. Then, the choice of clock synch is what determines how two way speed gets divided into two one way speeds rather than some property of light. The Einstein clock synch is defined as the choice which makes the two one way speeds the same.

In Romer's original experiment, the moons of Jupiter serve as a distant clock. When Jupiter and Earth are at their greatest distance apart, the light coming from Jupiter has an additional 2 AU to travel, and we observe a 16 minute delay. This seems to me to be a one-way measurement of c with no synchronization required.

 

[ghwellsjr]

let me know how the clocks are syncronised so that i can make sense to your statement.

You synchronize the clocks so that the one-way speed of light, as measured by those clocks, comes out to be c.

 

[ash64449]

You synchronize the clocks so that the one-way speed of light, as measured by those clocks, comes out to be c.

then should i again syncronise those two clocks again to measure the second return one-way trip to come out to be c?

 

[ghwellsjr]

In Romer's original experiment, the moons of Jupiter serve as a distant clock. When Jupiter and Earth are at their greatest distance apart, the light coming from Jupiter has an additional 2 AU to travel, and we observe a 16 minute delay. This seems to me to be a one-way measurement of c with no synchronization required.

That is a slow transport of a clock--the same as Einstein's synchronization, assuming that the clock has not accumulated any offset during its trip.

 

[ash64449]

In Romer's original experiment, the moons of Jupiter serve as a distant clock. When Jupiter and Earth are at their greatest distance apart, the light coming from Jupiter has an additional 2 AU to travel, and we observe a 16 minute delay. This seems to me to be a one-way measurement of c with no synchronization required.

how did scientists calculated the distance between jupter and earth?

 

[ash64449]

That is a slow transport of a clock--the same as Einstein's synchronization, assuming that the clock has not accumulated any offset during its trip.

what is Einstein's syncronization?

 

[Bill_K]

how did scientists calculated the distance between jupter and earth?

Well-known story: the size of the astronomical unit was determined in ancient times by Aristarchus of Samos (or Eratosthenes, or Hipparchos), by comparing the sun's observed position in Alexandria and in Syene.

 

[ash64449]

Well-known story: the size of the astronomical unit was determined in ancient times by Aristarchus of Samos, by comparing the sun's observed position in Alexandria and in Syene.

do you know any current method used by scientists to measure the distance between them?

 

[ash64449]

do you know any current method used by scientists to measure the distance between them?

no need to reply.. I think it is unimportant for this discussion.

 

[PeterDonis]

Well-known story: the size of the astronomical unit was determined in ancient times by Aristarchus of Samos (or Eratosthenes, or Hipparchos), by comparing the sun's observed position in Alexandria and in Syene.

I thought this was how the circumference of the Earth was first determined (by Eratosthenes), not the distance of the Sun from the Earth. IIRC the assumption was that the Sun's rays in Alexandria and Syene were parallel, implying an infinite (i.e., too large to measure) distance to the Sun; the Sun was exactly overhead at noon on the summer solstice in Syene, so its angle from the vertical (as measured by the shadow cast by a vertical stick) in Alexandria allowed the circumference of the Earth to be determined. (This also assumes that Alexandria is due north of Syene, which I believe it is to a good approximation.)

 

[Bill_K]

I thought this was how the circumference of the Earth was first determined (by Eratosthenes), not the distance of the Sun from the Earth. IIRC the assumption was that the Sun's rays in Alexandria and Syene were parallel, implying an infinite (i.e., too large to measure) distance to the Sun; the Sun was exactly overhead at noon on the summer solstice in Syene, so its angle from the vertical (as measured by the shadow cast by a vertical stick) in Alexandria allowed the circumference of the Earth to be determined. (This also assumes that Alexandria is due north of Syene, which I believe it is to a good approximation.)

We're talking about what is possible in principle. So first measure the circumference of the Earth with a ruler, and then the Alexandria-Syene experiment will in addition tell you the solar parallax.

 

[glappkaeft]

how did scientists calculated the distance between jupter and earth?

The relative distances of two planets and the sun can be computed using Kepler's laws (the reason for using the Astronomical Unit). Back in the day of Rømer's observations the best estimate of the AU was based on parallax measurements of Mars done just 4 years earlier by Richer and Cassini. Today radar ranging and space probe telemetry is used.

 

[ash64449]

let me imagine a train,i placed two clocks on extreme parts of train. I syncronized the clocks when i am in rest. Now i made the train move,in uniform motion. Why do clocks get unsyncronized?

 

[PeterDonis]

We're talking about what is possible in principle.

Well, you did say it was an actual well-known story, not just something possible in principle. I agree that if the circumference of the Earth is already known (as well as the Alexandria-Syene distance, of course), then the difference between the Sun's actual angle from vertical at Alexandria, and the angle expected if the Sun's rays were exactly parallel, gives a measurement of the Sun's parallax. I'm not sure measurements in ancient times would have been accurate enough to measure the difference, though.

 

[ash64449]

Well, you did say it was an actual well-known story, not just something possible in principle. I agree that if the circumference of the Earth is already known (as well as the Alexandria-Syene distance, of course), then the difference between the Sun's actual angle from vertical at Alexandria, and the angle expected if the Sun's rays were exactly parallel, gives a measurement of the Sun's parallax. I'm not sure measurements in ancient times would have been accurate enough to measure the difference, though.

please,don't get diverted from the discussion. Please answer to my doubts.

 

[Bill_K]

In Romer's original experiment, the moons of Jupiter serve as a distant clock. When Jupiter and Earth are at their greatest distance apart, the light coming from Jupiter has an additional 2 AU to travel, and we observe a 16 minute delay. This seems to me to be a one-way measurement of c with no synchronization required.

Given the value of the AU from Earth-based parallax measurements, why then doesn't the Romer experiment serve as a one-way determination of c?

 

[PeterDonis]

Given the value of the AU from Earth-based parallax measurements, why then doesn't the Romer experiment serve as a one-way determination of c?

By interpreting the time delay as a one-way measurement of the time it takes light to travel 2 AU, you are implicitly assuming that both time measurements are made with reference to a single frame. But they're not; the Earth is in different states of motion when the two time measurements are taken. So one has to correct at least one of the time measurements for the difference in frames; but applying that correction requires you to already know the speed of light.

 

[ghwellsjr]

Given the value of the AU from Earth-based parallax measurements, why then doesn't the Romer experiment serve as a one-way determination of c?

See the paragraph about Romer's experiment in the wikipedia article on the One-way Speed of Light.

 

[Fredrik]

One thing that doesn't get mentioned enough in these discussions is that you can measure the one-way speed of light both in a Minkowski spacetime and a Galilean (i.e. pre-relativistic) spacetime. You just can't do it without knowing in advance if your spacetime is Minkowski or Galilean. If you know this, then you also know how to synchronize clocks.

 

[ash64449]

One thing that doesn't get mentioned enough in these discussions is that you can measure the one-way speed of light both in a Minkowski spacetime and a Galilean (i.e. pre-relativistic) spacetime. You just can't do it without knowing in advance if your spacetime is Minkowski or Galilean. If you know this, then you also know how to synchronize clocks.

if i synchronize the two clocks in a train and if the train moves in an inertial system,will my clocks be unsynchronized?

 

[Fredrik]

if i synchronize the two clocks in a train and if the train moves in an inertial system,will my clocks be unsynchronized?

I'm not sure I understand the question. Are you asking if two clocks at opposite ends of a train car that has a constant velocity will stay in sync once you have synchronized them? The answer to that question is yes. But if the observer at rest in the train describes them as synchronized, and the train's velocity relative to the ground is non-zero, then an observer at rest on the ground would describe the clocks as showing different times, and always differing by the same amount.

 

[ghwellsjr]

if i synchronize the two clocks in a train and if the train moves in an inertial system,will my clocks be unsynchronized?

But if you are asking if you synchronize your clocks while the train is at rest in an inertial system and then the train accelerates to some speed in the inertial system, the clocks will become unsynchronized.

 

[ash64449]

See the paragraph about Romer's experiment in the wikipedia article on the One-way Speed of Light.

I have read your pointing out article from Wikipedia. From it,i understood that if we bring on the two clocks together and synchronize them and then take a clock at some speed and take it back again,we can see that the two clocks remain out of synchronization.

So the method of slow transport is used,so that clock that is moving remains synchronized.(well,there will be a slight amount of out of synchronization)

Well,based on the above facts,i have found out a way to measure the one-way speed of light!

This involves a series of experiments and some amount of calculation.

From the Wikipedia article,i found out that for example,think about two clocks together,synchronized and the amount by which clocks get un synchronized depends upon the velocity and for how much time that clock remained in that specific velocity.

We need to conduct this experiment:

We should take two identical clocks. they have to be synchronized. take for example a clock 30 meters apart and make that clock move for about 15 meters per second. now look at the amount by which the clocks were un synchronized. Now keeping the same distance,make the clock move at 30 meters per second. Now look by how much amount clocks remain un synchronized.

Conducting many times these experiment,we will find a pattern and a relationship between the amount of un synchronization and with the velocity that clock is moved.

When we have successfully completed this experiment, I will provide you friends a way we can measure one-way speed of light!

For this we need two persons and two clocks which are synchronized. make a person move for example a distance of 'c'. Now the person which has moved now knows how much or how much amount his clocks are unsynchronized. so he will reverse it and make his clock synchronized.

Now think a person has send out a light beam.he notes the time. the other person notes time when he receives the signal. when they rejoin,we will find the time interval and we know distance. so we can now measure speed. and this is off course measuring one-way speed of light.

Do you think this will work out?

 

[Nugatory]

We should take two identical clocks. they have to be synchronized. take for example a clock 30 meters apart and make that clock move for about 15 meters per second. now look at the amount by which the clocks were un synchronized. Now keeping the same distance,make the clock move at 30 meters per second. Now look by how much amount clocks remain un synchronized.

Conducting many times these experiment,we will find a pattern and a relationship between the amount of un synchronization and with the velocity that clock is moved.

How are you going to measure the amount of desynchronization?
EITHER:
1) You exchange information between the two clocks in which case you are, one way or another, assuming the constant speed of light or performing a two-way measurement; OR
2) You bring the clocks back together to compare them, in which case you're no longer measuring the desynchronization at the remote location, which is what you need for a one-way measurement.

 

[ash64449]

How are you going to measure the amount of desynchronization?
EITHER:
1) You exchange information between the two clocks in which case you are, one way or another, assuming the constant speed of light or performing a two-way measurement; OR
2) You bring the clocks back together to compare them, in which case you're no longer measuring the desynchronization at the remote location, which is what you need for a one-way measurement.

yes. We measure the amount of desynchronization from a round trip.just add the distance. Here total distance 60 m. Find out desyncronization. Repeat the experiment with different velocity but keeping the full round trip distance same. Here we found out the amount. So two persons know how much clocks would be desynchronized with any velocity.so even if that person moves some distance,he knows how much his clock will be out of synchroniztion without the need of them to join together.

I think this answer's your question.

Don't you think this method would work?

 

[DrGreg]

yes. We measure the amount of desynchronization from a round trip.just add the distance. Here total distance 60 m. Find out desyncronization. Repeat the experiment with different velocity but keeping the full round trip distance same. Here we found out the amount. So two persons know how much clocks would be desynchronized with any velocity.so even if that person moves some distance,he knows how much his clock will be out of synchroniztion without the need of them to join together.

I think this answer's your question.

Don't you think this method would work?

If you use the round trip you're using the 2-way speed of light. You want the desynchronisation for half the trip. You might assume its half the desynchronisation of the full trip, but that's an assumption, not a measurement.

 

[ash64449]

If you use the round trip you're using the 2-way speed of light. You want the desynchronisation for half the trip. You might assume its half the desynchronisation of the full trip, but that's an assumption, not a measurement.

yeah.you are right.
But we can make distance differ too.
How about reducing the distance to 15 meters?
Then full round-trip will have 30 meters.
Compare the earliar result that was done for total round trip distance of 60 meters(same velocity). Find if there is a pattern.

Can't this help? I mean we got desynchronization for 30 meters. Now conduct the first experiment of one-way trip.

Hope you have got the point.

 

[ghwellsjr]

See the paragraph about Romer's experiment in the wikipedia article on the One-way Speed of Light.

I have read your pointing out article from Wikipedia. From it,i understood that if we bring on the two clocks together and synchronize them and then take a clock at some speed and take it back again,we can see that the two clocks remain out of synchronization.

So the method of slow transport is used,so that clock that is moving remains synchronized.(well,there will be a slight amount of out of synchronization)

Well,based on the above facts,i have found out a way to measure the one-way speed of light!

This involves a series of experiments and some amount of calculation.

From the Wikipedia article,i found out that for example,think about two clocks together,synchronized and the amount by which clocks get un synchronized depends upon the velocity and for how much time that clock remained in that specific velocity.

We need to conduct this experiment:

We should take two identical clocks. they have to be synchronized. take for example a clock 30 meters apart and make that clock move for about 15 meters per second. now look at the amount by which the clocks were un synchronized. Now keeping the same distance,make the clock move at 30 meters per second. Now look by how much amount clocks remain un synchronized.

Conducting many times these experiment,we will find a pattern and a relationship between the amount of un synchronization and with the velocity that clock is moved.

When we have successfully completed this experiment, I will provide you friends a way we can measure one-way speed of light!

For this we need two persons and two clocks which are synchronized. make a person move for example a distance of 'c'. Now the person which has moved now knows how much or how much amount his clocks are unsynchronized. so he will reverse it and make his clock synchronized.

Now think a person has send out a light beam.he notes the time. the other person notes time when he receives the signal. when they rejoin,we will find the time interval and we know distance. so we can now measure speed. and this is off course measuring one-way speed of light.

Do you think this will work out?

No. You are making a big mistake. You are thinking that just because you can start with two synchronized clocks and slowly move one of them away and back, that the amount of de-synchronization that you can measure when they come back divided by two is a measure of the de-synchronization that the two clocks have when they are separated. The thing you are overlooking is that the two clocks are continuing to accumulate time and you cannot tell if the rates at which they accumulate time are the same when they separate. In other words, how can you tell if the clock as it moves away ticks a lot faster while it is moving, in other words, gains a lot of time, and then when it moves back it ticks a lot slower and loses most of that gain? You can't tell. You are just assuming that the motion is symmetrical and that it gains the same amount of time in both directions. So all your efforts don't prove a thing.

If you want to study this some more, read this post:

https://www.physicsforums.com/showpost.php?p=4259474&postcount=8

 

[ash64449]

i would like to ask questions to all of you.
I know that length contraction and time dilation cannot be observed. Can we observe relativity of simultaneity?

 

[ghwellsjr]

i would like to ask questions to all of you.
I know that length contraction and time dilation cannot be observed. Can we observe relativity of simultaneity?

No. It has the same issue since it is depends on the selected frame.

 

[ash64449]

No. It has the same issue since it is depends on the selected frame.

i don't know why i feel like this. But i will say. I feel that relativity of simultaneity is somewhat connected to one-way speed of light. Since we cannot define one-way speed of light,relativity of simultaneity is not observed. What do you think?

 

[ash64449]

No. It has the same issue since it is depends on the selected frame.

sorry,i didn't understand which same issue. Are you talking things about another thread which you explained why length contraction cannot be observed? Are you saying that there is some connection in that?
Sorry,if i asked previous question(RoS) here. If felt there was some connection to this thread.

 

[ghwellsjr]

No. It has the same issue since it is depends on the selected frame.

i don't know why i feel like this. But i will say. I feel that relativity of simultaneity is somewhat connected to one-way speed of light. Since we cannot define one-way speed of light,relativity of simultaneity is not observed. What do you think?

Relativity of simultaneity is connected with the one-way speed of light, because we cannot observe or measure the one-way speed of light. All we can do is define the one-way speed of light in each Inertial Reference Frame (IRF). And that definition as applied in each IRF is what leads to different coordinate times in each IRF which is what we mean by Relativity of Simultaneity.

 

[ghwellsjr]

sorry,i didn't understand which same issue. Are you talking things about another thread which you explained why length contraction cannot be observed? Are you saying that there is some connection in that?
Sorry,if i asked previous question(RoS) here. If felt there was some connection to this thread.

All coordinate dependent concepts are not observable by observers for the same reason. That includes Length Contraction, Time Dilation and Relativity of Simultaneity.

 

[ash64449]

Relativity of simultaneity is connected with the one-way speed of light, because we cannot observe or measure the one-way speed of light. All we can do is define the one-way speed of light in each Inertial Reference Frame (IRF). And that definition as applied in each IRF is what leads to different coordinate times in each IRF which is what we mean by Relativity of Simultaneity.

george, a request.
Can you elaborate or explian how one-way speed of light relates relativity of simultaneity? I haven't understood clearly..

 

[ash64449]

the thing i want to understand is how is one-way speed of light is defined in IRF and how that definiton relates RoS?


Edit: wait.. I think i know the answer! Light travels the same speed in one-way trip too! And i know people will say that light needs to travel more distance and less distance and all and there comes RoS. But doesn't light need to travel more distance same as c-v and light needs to travel less distance c+v?

 

[ghwellsjr]

george, a request.
Can you elaborate or explian how one-way speed of light relates relativity of simultaneity? I haven't understood clearly..

Did you read and study section 1 on the Definition of Simultaneity in Einstein's 1905 paper like I asked you to in another post? He explains it all very well there. Study it until it sinks in (to your brain). After it makes sense to you, go on and read the next section. OK? Read. Study. Think. Let it sink in. Read it again. Study it again. Think about it again. Over and over again until it makes perfect sense to you. It's very simple. No complicated math.

 

[bahamagreen]

Your link to Einstein's paper shows him presenting this equation:

2AB / (t'a - ta) = c

Just to be clear, that "AB" in the numerator as shown must not be strictly algebraic in that it does not mean "AxB" but must mean "absolute value of (B-A) or "absolute value of (A-B)" so that "AB" is treated as a length distance between A and B.

This is trivial, but if one pulls the equation aside to run some calculations one might miss how "AB" is meant to be assigned and erroneously multiply them together... and doing so, if either position A or B is at coordinate 0, then c=0, and otherwise, c varies with the separation between A and B because the speed of light takes on the dimension of (m^2)/s.

 

[Fredrik]

the thing i want to understand is how is one-way speed of light is defined in IRF and how that definiton relates RoS?

Suppose that the light is emitted at an event A, and received at an event B. Then we want to interpret

distance between A and B
________________________________

clock display at B - clock display at A​

as a measurement of the speed of light. But this doesn't really work if the times displayed by the clocks at A and B depend on how those clocks were brought to A and B.

The above explains what sort of thing we would like to interpret as a measurement of the speed of light. But you asked about how we would define the speed of light in an IRF. The answer is similar to the above. The IRF needs to be one in which both the emitter and reciever have velocity 0. Then we can define the speed of light in that IRF as the absolute value of

x(B) - x(A)
_________

t(B) - t(A)​

where x(A) and t(A) respectively denote the position coordinate and time coordinate that the IRF assigns to the event A. So the one-way speed is always well-defined in the theory, but the IRF in which the emitter and detector are both stationary is not (always) the same in SR as in pre-relativistic classical mechanics. So the numbers t(A),x(A),t(B),x(B) depend on the theory.

 

[ash64449]

i have read first two chapters of einstein's 1905 paper. And i am a little bit surprised because i found einstein connecting RoS with that of non-synchronous of clocks of moving rods.
He said that two ends of the rods A and B have "Atime" and "Btime" but to attach a common "time" to both A and B,we need to synchronize the two clocks and it is assumed that two clocks are only synchronized if
Tb-Ta = T'a - Tb.
So we can see in the moving rod that the above cannot be maintained.so clocks in moving rod are not synchronized when viewed from stationary frame.
Because the clocks are not synchronized,events which are measured by local time(at A and B) which are spatially separated cannot be seen as simultaneous events(these two clocks are synchronized in stationary system,so above conclusion by comparing them).
Is this correct?(to george)

 

[ghwellsjr]

i have read first two chapters of einstein's 1905 paper. And i am a little bit surprised because i found einstein connecting RoS with that of non-synchronous of clocks of moving rods.
He said that two ends of the rods A and B have "Atime" and "Btime" but to attach a common "time" to both A and B,we need to synchronize the two clocks and it is assumed that two clocks are only synchronized if
Tb-Ta = T'a - Tb.

Einstein didn't say "to attach a common 'time' to both A and B,we need to synchronize the two clocks". That would be like saying "to attach a common time to both A and B, we need to set them to the same time." He saying something entirely different. He is saying that a common time to both A and B, that is, a synchronization of the two clocks has no meaning unless and until we give it meaning. We need to define what "common time" means which is the same as saying we need to define what "synchronization between two clocks" means. In other words, we give meaning to "common time" or to "synchronization between two clocks". That's why the title of his first section is Definition of Simultaneity.

So we can see in the moving rod that the above cannot be maintained.so clocks in moving rod are not synchronized when viewed from stationary frame.
Because the clocks are not synchronized,events which are measured by local time(at A and B) which are spatially separated cannot be seen as simultaneous events(these two clocks are synchronized in stationary system,so above conclusion by comparing them).
Is this correct?(to george)

I'm not sure what you are asking.

Instead of using the term "local time" which is a hangover from the LET days, you should use the terms "Proper Time" if you mean the time on an actual real clock and you should use the term "Coordinate Time" if you mean the synchronized time in an IRF. Note that for purposes of establishing what is meant by Coordinate Time, Einstein uses "imaginary" clocks but we should eventually disconnect the concept of Coordinate Time from any real clocks, just as we disconnect the concept of Coordinate Space from any real measuring rods.

With that in mind, what you should conceptualize is that a rod with clocks at each end and in which the Proper Times on the clocks have been set to the Coordinate Times of the IRF in which the rod and clocks are at rest, will not be set to the Coordinate Time of an IRF moving with respect to the first IRF.