A problem with time dilation help?

[ghwellsjr]

All that is important is that the time taken for that information to get from the sensor to the clock is the same for both detections.

Yes, that is the all-important requirement. But why should the time be the same for both detections? Isn't sensor B farther away from the clock than sensor A?

EDIT: Just to add to that, the reason for cables is that I am only using one clock as I didn't know if using two clocks at each sensor would cause problems, as they would be separated by a distance.

Yes, having two clocks separated by a distance would cause problems. I'm sure you're aware of the so-called Twin Paradox where two clocks are colocated and have the same time on them. One of them is moved to another location and then brought back and the two clocks have different times on them. Moving clocks around is known to make them have different times on them.

If this is not an issue, I could do away with the cables and have two sensors with clocks in that were synchronised. Then just take the difference in readings from the clocks to establish elapsed time.

Well now you just nailed down the problem. How do you synchronize two clocks that are at different locations? That's exactly the same problem as measuring the one-way speed of light. Don't forget how this thread started out: someone traveling at 99%C. Two observers traveling with respect to one another will not agree on how to synchronize a pair of clocks.

 

[rede96]

Yes, that is the all-important requirement. But why should the time be the same for both detections?

Because I have designed the system that way.

The physical position in space of the clock relative to Sensor A and Sensor B can help, but is not critical. Although it makes sense to have the clock equidistant.

As you’ve said, what is important is the process for sending information from A to the clock takes exactly the same time as the process to send information from B to the clock. So I have made the assumption that both processes have been calibrated and will take the same time, which is straight forward enough to do.

Just to add, this type of error exists inherently in ANY type of measuring system, as it is not possible to detect anything 'instantaneously'.

There is a process of detection, then sending information from a detector through something that will transform the detection into a result we can interpret.

No information can travel greater than c, so there will always be a delay and hence a potential error in any system of two or more detections.

The process of detecting a light source reflected from a mirror is no different in that respect.

Yes, having two clocks separated by a distance would cause problems. I'm sure you're aware of the so-called Twin Paradox where two clocks are colocated and have the same time on them. One of them is moved to another location and then brought back and the two clocks have different times on them. Moving clocks around is known to make them have different times on them.

Yes, this was the problem I was thinking of so I left it out. Although I am sure that done correctly, the value of error could made insignificant compared the value of the measurement.

Well now you just nailed down the problem. How do you synchronize two clocks that are at different locations?

As I am using just one clock, this is no longer an issue. Problem Solved :)

 

[Rishavutkarsh]

hey i just thought a thing as of twin paradox first twin's (traveller) has his same twin get older when he returns,correct?
so that means clock of first twin is running slow correct?
now then the first twin must see his twin faster in time when he was traveling so second twin sees him slow does it mean that if we get close to speed of light outer world seems to go faster and for the outer world we seem to get slower ?

 

[ghwellsjr]

hey i just thought a thing as of twin paradox first twin's (traveller) has his same twin get older when he returns,correct?
so that means clock of first twin is running slow correct?
now then the first twin must see his twin faster in time when he was traveling so second twin sees him slow does it mean that if we get close to speed of light outer world seems to go faster and for the outer world we seem to get slower ?

I only brought up the Twin Paradox to illustrate that for two clocks that start out at the same place with the same time on them and one of them is moved to another place, there is no guarantee that the two clocks will still have the same time on them.

When any two clocks are in relative motion, they will keep different time. Each one will see, measure, and conclude that the other clock is ticking slower than itself. It's reciprocal, just like relative speed is. Whatever speed I see you traveling at is exactly the same speed you see me traveling at. Whatever slowed down rate I see your clock ticking at is exactly the same slowed down rate you see my clock ticking at. In Special Relativity, nobody moving at a constant speed ever sees any other clock ticking faster than their own, they're all always ticking slower.

 

[harrylin]

Because I have designed the system that way.

The physical position in space of the clock relative to Sensor A and Sensor B can help, but is not critical. Although it makes sense to have the clock equidistant.

As you’ve said, what is important is the process for sending information from A to the clock takes exactly the same time as the process to send information from B to the clock. So I have made the assumption that both processes have been calibrated and will take the same time, which is straight forward enough to do.

Just to add, this type of error exists inherently in ANY type of measuring system, as it is not possible to detect anything 'instantaneously'.
[..]

No information can travel greater than c, so there will always be a delay and hence a potential error in any system of two or more detections.

The process of detecting a light source reflected from a mirror is no different in that respect.

[..]
As I am using just one clock, this is no longer an issue. Problem Solved :)

Indeed, there is no problem to obtain a pure measurement of a round trip speed (using one reference clock and one reference ruler), either with light signals or cables.

Note that these are both electromagnetic signals, but according to relativity theory it doesn't matter what you use: you can always pretend that the one-way speeds wrt *your* choice of inertial reference system are the same in both directions.

And with equal success, you can choose another inertial system that is moving wrt the first, and pretend that the one-way speeds are the same in all directions wrt that system.

There have been almost endless discussions about exactly the same in other threads that also did not have that topic - for example from post 189 of:
https://www.physicsforums.com/showthread.php?t=461266&page=12

Cheers,
Harald

 

[rede96]

There have been almost endless discussions about exactly the same in other threads that also did not have that topic

Hi harrylin, thanks for the link and point taken. :)

 

[ghwellsjr]

Hi harrylin, thanks for the link and point taken. :)

Yes, harrylin, all your points are well taken and exactly right.

Let's go over them one by one.

Indeed, there is no problem to obtain a pure measurement of a round trip speed (using one reference clock and one reference ruler), either with light signals or cables.

This is what I have been saying. You can measure the round trip speed of light and it's no better or worse than cables.

Note that these are both electromagnetic signals, but according to relativity theory it doesn't matter what you use: you can always pretend that the one-way speeds wrt *your* choice of inertial reference system are the same in both directions.

Yes, note that it's an arbitrary choice to define the time intervals for each direction of the measurement to be equal.

And with equal success, you can choose another inertial system that is moving wrt the first, and pretend that the one-way speeds are the same in all directions wrt that system.

And if you make that arbitrary choice, the two time intervals for each direction of the measurement will not be equal.

Whichever choice you make, it is purely arbitrary, and it is not a measurement.

There have been almost endless discussions about exactly the same in other threads that also did not have that topic - for example from post 189 of:
https://www.physicsforums.com/showthread.php?t=461266&page=12

Yes, some people never understand this simple concept, no matter how hard we try to explain it to them.

Cheers,
Harald

Rede96, do you understand what harrylin is saying?

 

[rede96]

Yes, some people never understand this simple concept, no matter how hard we try to explain it to them.

Rede96, do you understand what harrylin is saying?

To be honest not fully no.(Sorry!) The one bit that is confusing me is where the 'round trip' comes from in my experiment.

If I measure a light source between two points, where is the round trip? Maybe if I get that, the rest will follow.

 

[harrylin]

To be honest not fully no.(Sorry!) The one bit that is confusing me is where the 'round trip' comes from in my experiment.

If I measure a light source between two points, where is the round trip? Maybe if I get that, the rest will follow.

I think that it has been said before, but Einstein answered it rather well in section I of his first 1905 paper, http://www.fourmilab.ch/etexts/einstein/specrel/www/ .

In summary, you need to synchronise your distant clock in order to do a one-way measurement; and according to SR, whatever method you use to do that, you'll always obtain the same result, based on your assumptions.

The common method is to use radio signals, and the standard assumption is to declare your system to be "in rest". Effectively what you do then, is to read the distant clock time instead of reflecting the signal with a mirror at that point. But first you have to set it to the "correct" time. If you try to be as precise as possible, then you will set that clock at <distance> times <roundtrip speed> later than the time of emission. That doesn't add any new information. It's merely you defining half of the round trip time as "one-way time", and then you "measure" the one-way speed of light value that you had defined yourself.

As you appeared to have already explained that yourself, so it's unclear to me what is not clear to you. Perhaps what is needed, is that you do a little exercise: calculate what you will obtain if you assume that your whole system is in motion. You should then verify that with that assumption, everything also works out although the one-way speed wrt you is different in different directions.

Regards,
Harald

 

[rede96]

In summary, you need to synchronise your distant clock in order to do a one-way measurement; and according to SR, whatever method you use to do that, you'll always obtain the same result, based on your assumptions.

But I am not using a 'distant clock'. I only have one clock. So I do not need to do any synchronisation.

What I have are two sensors that will detect a light source as it passes them. Each time a sensor is triggered, it sends a signal to my one clock, which acts as a lap timer, starting with the first signal and stopping with the second.

As long as I know how long it would take for the signal for each sensor to reach my clock, I can work out the elapsed time for the light beam to pass between my two sensors. No return trip.

Timing the duration of a light beam passing my two sensors is no different to me than timing a spaceship passing them for example.

The only difference with a spaceship is that I can get a different time depending on what direction the same ship is moving relative to me.

However, that cannot happen with light, as the speed of light is the same for all observers.

As you appeared to have already explained that yourself, so it's unclear to me what is not clear to you. Perhaps what is needed, is that you do a little exercise: calculate what you will obtain if you assume that your whole system is in motion. You should then verify that with that assumption, everything also works out although the one-way speed wrt you is different in different directions.

As the speed of light is the same for all inertial frames, I would say that I have to measure the speed of light to be c, no matter what direction I was moving in.

 

[ghwellsjr]

However, that cannot happen with light, as the speed of light is the same for all observers.

As the speed of light is the same for all inertial frames, I would say that I have to measure the speed of light to be c, no matter what direction I was moving in.

The speed of light is defined to be c in all directions (that means the one-way speed of light) in any inertial frame in Einstein's Special Relativity. Why don't you read his 1905 paper that harrylin linked to in post #10 and study especially articles 1 and 2.

 

[harrylin]

But I am not using a 'distant clock'. I only have one clock. So I do not need to do any synchronisation.
[..] As long as I know how long it would take for the signal for each sensor to reach my clock, I can work out the elapsed time for the light beam to pass between my two sensors. No return trip. [..]

OK, one last time - although it's hard to be even clearer than in post #10.

What you describe is a round trip of signals - a light signal to the far away sensor, plus another signal back to you. You cannot really know how long it takes for the signal of the far away sensor to reach your clock, because in order to measure that you need to know the same distant time that you are trying to establish (do you know the song "there's a hole in the bucket"?). It's exactly as the example in the other thread that I referred to.

And as I suggested, as so often with physics, it may be necessary to actually do the calculation yourself (with the assumption that your system is in motion), in order to really understand this. Did you?

PS. I had overlooked your last answer which may be the key to the misunderstanding:

As the speed of light is the same for all inertial frames, I would say that I have to measure the speed of light to be c [if I assume that my system is in motion], no matter what direction I was moving in.

You will immediately discover that you should measure the speed wrt your moving system to be (c-v) if you actually do the calculation: imagine yourself moving at velocity v wrt the inertial frame of your choice, which you assume to be in rest - similar to Einstein's discussion in section 2 (and 3) of his paper.

Success!

Harald

 

[rede96]

The speed of light is defined to be c in all directions (that means the one-way speed of light) in any inertial frame in Einstein's Special Relativity.

I thought that was what I said, but I must need to work on my terminology.

However, at least we agree on something. :0)

Why don't you read his 1905 paper that harrylin linked to in post #10 and study especially articles 1 and 2.

I have done(1 and 2 only), and nothing I read was different to my current understanding.

 

[rede96]

I think I understand that in essence, we are not disagreeing in principle with any aspect of relativity. But I can see where the confusion might be.

Firstly, my poor terminology and non standard /mathematical approach can lead to confusion both for myself and for others. So let me apologise for that.

Secondly, the issue in my opinion revolves around this statement:

What you describe is a round trip of signals - a light signal to the far away sensor, plus another signal back to you. You cannot really know how long it takes for the signal of the far away sensor to reach your clock, because in order to measure that you need to know the same distant time that you are trying to establish

So let me recap.

1) My goal is to test the hypothesis that it is not possible to measure the speed of light in a one-way direction. I took that literally to mean that the laws of physics do not allow any one-way measurement of the speed of light.

2) There are no other reference frames involved in my measurement, me, my clock and my two sensors are all at rest wrt to each other. So I don't have to worry about being 'in motion' because there is only my frame. There is no coordinate transformation to be done.

3) I do not have to worry about synchronising clocks. I have got around this issue in a different way. (Which I think is where the confusion is.)

In Einstein's paper cited above, section 1 basically says that in order to synchronise two clocks that are separated by a distance, then the time required by light to travel from A to B equals the time it requires to travel from B to A.

My understanding of this is that in order to synchronise two clocks separated by a distance, they must be at rest wrt to each other. He was using the speed of light to validate this. So if they are at rest wrt each other, then this satisfies the equation tB-tA = t’A- tB.

In my set up, I know that sensor A and sensor B are at rest wrt to each other because they are physically joined together. So tB-tA = t’A- tB would always have to be true.


So this statement:

You cannot really know how long it takes for the signal of the far away sensor to reach your clock, because in order to measure that you need to know the same distant time that you are trying to establish.

is not correct for my set up.

I do know the time it takes for each signal to get from sensor A and sensor B, as I have measured and calibrated it and this is now a set process. Just as whenever I flick my light switch, after a very short delay, my light comes on. I can measure the delay and more importantly, this delay will aways be the same, as will the time taken for the signals to reach my clock always be the same.

So I can now use this equation to calculate the speed of light traveling from sensor A to sensor B:

2AB/(t'-tA) = c

If the time taken for Signal A to reach my clock is tSA and the time for B is tSB, then I derive t1-tA by subtracting tSA from tSB and then subtracting this from the total duration my clock read.

So if my sensors are separated by a distance of 150 meters say, and t’-tA = 1 microsecond, then I would get c as 300,000,000 m/s (Assuming 300,000,000 for c.)

So I have measured the one-way speed of light.

 

[San K]

thanks but i already know this concept please see my next post and tell me that am wrong as i want to be proved wrong but i want to proved

hi Rishav,

seems like, somewhere in your understanding, you are mixing up frames of references.

you can draw all the 5 frames (or whatever the number of frames may be depending upon the scenario/example)

Frame 1: moving at .99c
Frame 2: photons coming towards you from right
Frame 3: photons coming towards you from left
Frame 4: Another stationery observer
Frame 5: Another moving observer ( at some fraction of c)

To do any analysis:

1. Within the same frame of reference

This is easy. The results within the same of reference are "easily/simply" consistent.

2. To compare across frames

Frames are not comparable "in a simple way", i.e. some calculations transformations have to be made to bring both the frames of reference on the same page (speed).

you have to adjust for factors (time/length/space dilation) via Lorentz transformations etc to make the frame of reference comparable i.e. same i.e. same speed etc.

Bottomline: you are somewhere jumping/swapping frames, in your analysis, without realizing it. All the five frames above will have different interpretation of simultaneity...i.e. even the events are happening at different times for each (5) frame of reference.

The only thing that will always be same across all frames of references is the speed of light (c).

 

[Rishavutkarsh]

I only brought up the Twin Paradox to illustrate that for two clocks that start out at the same place with the same time on them and one of them is moved to another place, there is no guarantee that the two clocks will still have the same time on them.

When any two clocks are in relative motion, they will keep different time. Each one will see, measure, and conclude that the other clock is ticking slower than itself. It's reciprocal, just like relative speed is. Whatever speed I see you traveling at is exactly the same speed you see me traveling at. Whatever slowed down rate I see your clock ticking at is exactly the same slowed down rate you see my clock ticking at. In Special Relativity, nobody moving at a constant speed ever sees any other clock ticking faster than their own, they're all always ticking slower.

oh thanks . u mean that if i observe your clock moving at 87% of mine then u see mine moving the same the same way . correct? but consider this twin paradox
first twin moves 1 minute- 1 year of his second twin so u mean that we can see the whole of 1 year's time of second twin during one minute without observing him fast? duh!

 

[harrylin]

[...]
So let me recap.

1) My goal is to test the hypothesis that it is not possible to measure the speed of light in a one-way direction. I took that literally to mean that the laws of physics do not allow any one-way measurement of the speed of light.

Sure you can measure it, in a certain way; the point of SR is that the speed of light wrt an object (e.g. you or your system) is not an "absolute" - it's just a convention, as you will know now (that is, if you indeed did the calculation in which you are moving).

2) There are no other reference frames involved in my measurement, me, my clock and my two sensors are all at rest wrt to each other. So I don't have to worry about being 'in motion' because there is only my frame. There is no coordinate transformation to be done.

Ah, but this has nothing to do with "worry", as you know it's about the necessary insight that you get from doing it. But you did not do it and to keep staring from one perspective - and because of that you still don't get it. As going on like this is a waste of time, I won't look at this discussion anymore.

[..] In Einstein's paper cited above, section 1 basically says that in order to synchronise two clocks that are separated by a distance, then the time required by light to travel from A to B equals the time it requires to travel from B to A.

My understanding of this is that in order to synchronise two clocks separated by a distance, they must be at rest wrt to each other.

That's not necessary, and often it's not the case (e.g GPS). However, the simple method that he presents is only suited for that case.

He was using the speed of light to validate this.

No, as I explained and ghwellsjr emphasised (in bold) in #57, that's completely wrong.
Einstein used that definition to set the one-way speed of light according to convention, because it cannot be determined by a pure measurement.
Perhaps he formulated it clearer in 1907:

We [...] assume that the clocks can be adjusted in such a way that
the propagation velocity of every light ray in vacuum - measured by
means of these clocks - becomes everywhere equal to a universal
constant c, provided that the coordinate system is not accelerated.

rede96:

So if they are at rest wrt each other, then this satisfies the equation tB-tA = t’A- tB.
In my set up, I know that sensor A and sensor B are at rest wrt to each other because they are physically joined together. So tB-tA = t’A- tB would always have to be true.

Instead, it's only true for the assumption that your system is in rest - as you would know by now, if you had just done the calculation for your system moving.

As Einstein put it in section 3 of his 1905 paper, for a similar set-up:

the ray moves relatively to the [moving] initial point of k, when measured in the stationary system, with the velocity c-v

rede96:

[..] I do know the time it takes for each signal to get from sensor A and sensor B, as I have measured and calibrated it and this is now a set process.

Again (last time): you merely measure your own assumption. Don't you know circular reasoning when you see it?

Good luck,
Harald

 

[Dale]

3) I do not have to worry about synchronising clocks. I have got around this issue in a different way.

This is not possible. harrylin is correct, you cannot get around the synchronization issue.

I do know the time it takes for each signal to get from sensor A and sensor B, as I have measured and calibrated it and this is now a set process.

How did you measure and calibrate it?

You have accomplished nothing with this setup. All you have done is transform a problem measuring the one-way speed of light in vacuum to a problem measuring the one-way speed of light in your wire. You still have to measure a one-way speed, which requires two synchronized clocks and a rod.

 

[rede96]

Instead, it's only true for the assumption that your system is in rest - as you would know by now, if you had just done the calculation for your system moving.

If I knew how to calculate a Lorentz transformation I would have done it, but I don't.

Again (last time): you merely measure your own assumption. Don't you know circular reasoning when you see it?

Obviously not, well not until about an hour ago anyway. I think the penny dropped with DaleSpam’s post

How did you measure and calibrate it?

I took that as meaning that at some point I would still need to synchronise two clocks to calibrate.

I am still not certain if the issue is just with synchronization or why the one way speed of light has to be different for different observers, however, as you rightly said enough is enough.

I had no idea just how many times this issue had come up until I had time to do a bit of reading today. No wonder you guys get frustrated.

Good luck

Thanks, I'll need it.

 

[Dale]

I took that as meaning that at some point I would still need to synchronise two clocks to calibrate.

Yes, exactly, and your choice of synchronization convention determines the one-way speed of light.

The point is that there is a class of theories where the two-way speed of light is c, but the one-way speed of light is not. If you work through the math it turns out that these experiments can agree with experimental data, but only disagree with the Einstein synchronization convention. So in this sense, your choice of synchronization convention determines the one-way speed of light, and vice versa.

 

[harrylin]

If I knew how to calculate a Lorentz transformation I would have done it, but I don't.

Sorry for that (and I could not help looking ); but if you choose only one reference system (in which you are moving), you would not need a Lorentz transformation at all to determine your speed relative to that of a light ray - only basic vector subtraction of velocities. Einstein could also not use the LT in his section 3, as he still had to derive it!

I think the penny dropped with DaleSpam’s post [...]
I took that as meaning that at some point I would still need to synchronise two clocks to calibrate.
I am still not certain if the issue is just with synchronization or why the one way speed of light has to be different for different observers, however, as you rightly said enough is enough.

Those two issues are interdependent, that was the point.

I had no idea just how many times this issue had come up until I had time to do a bit of reading today. No wonder you guys get frustrated.

Hehe perhaps it should be reworked into an elaborated FAQ.

But if after reading and thinking more about it you still keep questions, don't hesitate to start a new topic on it.

Cheers,
Harald

 

[Rishavutkarsh]

hi Rishav,

seems like, somewhere in your understanding, you are mixing up frames of references.

you can draw all the 5 frames (or whatever the number of frames may be depending upon the scenario/example)

Frame 1: moving at .99c
Frame 2: photons coming towards you from right
Frame 3: photons coming towards you from left
Frame 4: Another stationery observer
Frame 5: Another moving observer ( at some fraction of c)

To do any analysis:

1. Within the same frame of reference

This is easy. The results within the same of reference are "easily/simply" consistent.

2. To compare across frames

Frames are not comparable "in a simple way", i.e. some calculations transformations have to be made to bring both the frames of reference on the same page (speed).

you have to adjust for factors (time/length/space dilation) via Lorentz transformations etc to make the frame of reference comparable i.e. same i.e. same speed etc.

Bottomline: you are somewhere jumping/swapping frames, in your analysis, without realizing it. All the five frames above will have different interpretation of simultaneity...i.e. even the events are happening at different times for each (5) frame of reference.

The only thing that will always be same across all frames of references is the speed of light (c).

oh well at last can i really do it ? i am just 13 and things like this can they be done without integration . ie does lorentz transformations include integration ?

 

[Dale]

does lorentz transformations include integration ?

The Lorentz transform is just algebra:
http://en.wikipedia.org/wiki/Lorent...ormation_for_frames_in_standard_configuration
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/ltrans.html#c2

 

[Rishavutkarsh]

The Lorentz transform is just algebra:
http://en.wikipedia.org/wiki/Lorent...ormation_for_frames_in_standard_configuration
http://hyperphysics.phy-astr.gsu.ed...c2[/rl][/QUOTE] oh then that's good thanks

 

[Rishavutkarsh]

i have a way to measure speed of light one way

take a thick mirror and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!

 

[harrylin]

i have a way to measure speed of light one way

take a thick mirror and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!

You can of course "measure" its speed relative to the glass of your mirror, but you're wrong if you think that every reference system will agree with:

1. the time that it takes to go through the glass
2. the distance that the light covers in that time

- Look at the discussions above;
- Make a sketch of your set-up.

You cannot escape the issues that we discussed.
But if you could do it, then you'd disprove relativity and you'd get the Nobel prize for sure!

Harald

 

[HallsofIvy]

i have a way to measure speed of light one way

take a thick mirror and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!

How is this "one way"? You have measured the time it takes light to go from the front of the mirror to the back of the mirror and return to the front.

 

[Rishavutkarsh]

How is this "one way"? You have measured the time it takes light to go from the front of the mirror to the back of the mirror and return to the front.

oops i mean't a thick piece of glass not mirror sorry

 

[Rishavutkarsh]

i have a way to measure speed of light one way

take a thick glass and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!
this ain't wrong now right?

 

[Ryan_m_b]

i have a way to measure speed of light one way

take a thick glass and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!
this ain't wrong now right?

What do you mean when light strikes the mirror? And how exactly are you going to monitor that the light has struck anything?

The point is to measure the speed of light you need to measure it traveling from A to B. It doesn't matter if A to B is via a mirror i.e. shine light at a mirror and monitor it when it get's back or if is in one direction i.e light a torch pointing at a detector. However what you can't do is just have a detector and measure the speed of light, that tells you no more information about the speed of light than a on/off button tells you about the speed of the pressing finger.

 

[ghwellsjr]

i have a way to measure speed of light one way

take a thick mirror and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!

oops i mean't a thick piece of glass not mirror sorry

i have a way to measure speed of light one way

take a thick glass and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!
this ain't wrong now right?

You repeated the same scenario but changed the word "mirror" to "glass" one time but not the second time.

I have no idea what you are describing. I know you have a thick piece of glass but is there also a mirror somewhere? Is the light entering/striking the glass/mirror in the same location where it is exiting the glass/mirror?

Please start over and describe you experiment so someone could actually builld it from your description and not from some other ideas you have in your head and haven't written down. We can't read your mind.

It might help if before you submit your posts you showed them to someone else who is there with you and asked them if they make sense or at least proofread them yourself and ask the question, "Would this make sense to someone else?"

 

[Dale]

i have a way to measure speed of light one way

take a thick glass and we know that how much % of light is slowed down so when the light strikes the mirror start the timer and when the light exits stop the timer that is it we measured the speed of light one way!
this ain't wrong now right?

You are making the same mistake that rede96 made above. All you have done is transform a problem measuring the one-way speed of light in vacuum to a problem measuring the one-way speed of light in the glass. You still have to measure a one-way speed, which requires two synchronized clocks.

 

[rede96]

You are making the same mistake that rede96 made above. All you have done is transform a problem measuring the one-way speed of light in vacuum to a problem measuring the one-way speed of light in the glass. You still have to measure a one-way speed, which requires two synchronized clocks.

Rishavutkarsh, how I eventually came to understand is this:

Firstly, the reason it is not possible to time anything accurately between two points without having two synchronized clocks is that unless you can capture the time that the light (or anything else I guess) is at the start point instantaneously AND the time it is at the end point instantaneously, then you are in effect adding another 'trip' in getting the signal from whatever point back to your clock(s).

And to do that, you would need to know how long that signal would take to get from that point back to your clock, which you can't know because you can't measure that time without having two synchronized clocks again.

Secondly, in order to synchronize two clocks you have to pass a signal between them and so you would need to know how long that signal takes. And as above, you can't know how long that signal takes without having two synchronized clocks.

Now I don't know if that is 100% accurate but it helped me to understand the problem better.

The bigger question I am trying to understand now is why do we need to know the one way speed of light? I thought we already knew through experiment that light travels the same speed in all directions?

 

[ghwellsjr]

Rishavutkarsh, how I eventually came to understand is this:

Firstly, the reason it is not possible to time anything accurately between two points without having two synchronized clocks is that unless you can capture the time that the light (or anything else I guess) is at the start point instantaneously AND the time it is at the end point instantaneously, then you are in effect adding another 'trip' in getting the signal from whatever point back to your clock(s).

And to do that, you would need to know how long that signal would take to get from that point back to your clock, which you can't know because you can't measure that time without having two synchronized clocks again.

Secondly, in order to synchronize two clocks you have to pass a signal between them and so you would need to know how long that signal takes. And as above, you can't know how long that signal takes without having two synchronized clocks.

Now I don't know if that is 100% accurate but it helped me to understand the problem better.

It's not an issue of being able to "capture the time" accurately at any point; we are assuming that we have very fast and accurate sensors and clocks but they don't help when the thing you want to time is located at a distance from the clock.

The bigger question I am trying to understand now is why do we need to know the one way speed of light? I thought we already knew through experiment that light travels the same speed in all directions?

No, we don't know from experiment that light travels the same speed in all directions. That is Einstein's second postulate.

Here is the first definition of a postulate given by dictionary.com:

something taken as self-evident or assumed without proof as a basis for reasoning​

 

[rede96]

It's not an issue of being able to "capture the time" accurately at any point; we are assuming that we have very fast and accurate sensors and clocks but they don't help when the thing you want to time is located at a distance from the clock.

Yes I agree, what I was saying is that we need to capture it instantaneously, i.e. we don't send a signal from the detector to a clock, which was pointed out to me in an earlier post.

No, we don't know from experiment that light travels the same speed in all directions. That is Einstein's second postulate.

Didn't the Michelson interferometer type experiments demonstrate a maximum anisotropy for C?

 

[ghwellsjr]

MMX worked on the two-way speed of light.

 

[Rishavutkarsh]

You repeated the same scenario but changed the word "mirror" to "glass" one time but not the second time.

I have no idea what you are describing. I know you have a thick piece of glass but is there also a mirror somewhere? Is the light entering/striking the glass/mirror in the same location where it is exiting the glass/mirror?

Please start over and describe you experiment so someone could actually builld it from your description and not from some other ideas you have in your head and haven't written down. We can't read your mind.

It might help if before you submit your posts you showed them to someone else who is there with you and asked them if they make sense or at least proofread them yourself and ask the question, "Would this make sense to someone else?"

well consider this-
we have a thick piece of glass and a stopwatch , we know how much % of light is slowed in the glass so do this-
when the light enters the glass start the stop watch
when light exits stop it .
now we know -
time took by light in the glass
we of coarse know the length of the glass
and % of light slowed by the mirror

this ends now we calculated one way speed of light right?
am i still unclear?

 

[ghwellsjr]

OK, when the light first enters the glass, some of the light has to find its way to the location of the stopwatch in order to start it counting, correct? And when the light exits the glass on the other side, some of that light has to find its way by a different path to the same stopwatch in order to stop it counting, correct? So now you can see that two (or more) directions are involved in making this measurement, correct? In any case, if you trace all the light paths from when it first enters the glass, including the path to the stopwatch, through the glass and including the path where it exits the glass and goes to the stopwatch, it will trace out a roundtrip.

To see this more clearly, assume that the stopwatch is located right near where the light enters the glass so that the stopwatch starts immediately when the light enters the glass. Then in order to stop the count, you have to have the light come from the exit point outside the glass back to the stopwatch. The light goes first through the glass and then back through the air so it is a round trip that your stopwatch measures.

 

[TrickyDicky]

No, we don't know from experiment that light travels the same speed in all directions. That is Einstein's second postulate.

Here is the first definition of a postulate given by dictionary.com:

something taken as self-evident or assumed without proof as a basis for reasoning​

Yes, it is postulated in SR (assumed, not derived), but in GR needs not be postulated as Einstein realized that the existence of the constant c presupposed the presence of a system of absolute measuring rods and clocks (aka spacetime, the subject of curvature), that is his "synchronization convention" didn't come out of nowhere, and in this sense light traveling properties are derived from the properties (curvature, isotropicity...) of spacetime.

 

[harrylin]

Yes, it is postulated in SR (assumed, not derived), but in GR needs not be postulated as Einstein realized that the existence of the constant c presupposed the presence of a system of absolute measuring rods and clocks (aka spacetime, the subject of curvature), that is his "synchronization convention" didn't come out of nowhere, and in this sense light traveling properties are derived from the properties (curvature, isotropicity...) of spacetime.

Not exactly: if you carefully check his 1905 paper,
http://www.fourmilab.ch/etexts/einstein/specrel/www/ ,
then you will notice that a standard reference system is set up in such a way that the one-way speed becomes equal to the two-way speed of light due to the Poincare-Einstein synchronization convention. As Einstein emphasised, this so "by definition"; IOW, it's made that way by human convention. That definition makes the one-way speed of light equal to the two-way speed of light, which was assumed to be "a universal constant—the velocity of light in empty space".

He formulated is as follows in 1907:

"We [...] assume that the clocks [of a certain reference system] can be adjusted in such a way that the propagation velocity of every light ray in vacuum - measured by means of these clocks - becomes everywhere equal to a universal constant c, provided that the coordinate system is not accelerated."

Obviously one has the free choice to adjust the clocks differently, in which case the one-way speed of light becomes different from c; the light postulate isn't affected by such a choice as it only makes a claim as to what we will measure if we set the clocks as prescribed. For example, the speed of light wrt GPS receivers on Earth is c-v.

I won't elaborate on this subtle issue in this thread (it's off-topic); but if it pops up again, then I'll start a topic on it.

PS: The essential point, perhaps, is that in SR physical concepts such as "speed" are purely operationally defined; they have no ontological meaning.

 

[TrickyDicky]

Your disagreement must indeed be subtle as I fail to see where you think I'm incorrect.

The only part of my post about SR is where I agree that the second postulate is indeed a postulate, do you not think the second postulate is a postulate?

 

[harrylin]

Your disagreement must indeed be subtle as I fail to see where you think I'm incorrect.

The only part of my post about SR is where I agree that the second postulate is indeed a postulate, do you not think the second postulate is a postulate?

Sure a postulate is a postulate.
However, you agreed that a definition is a postulate.
My point was that a mere definition isn't a postulate.

A postulate may be open to falsification (as is the case with both SR's postulates); and to be precise it should relate to definitions (as is also the case with both SR's postulates). In contrast, a definition is a human choice that can't be falsified (although it can be poorly chosen).

Is that too subtle??

Cheers,
Harald

 

[TrickyDicky]

Sure a postulate is a postulate.
However, you agreed that a definition is a postulate.
My point was that a mere definition isn't a postulate.

Nope, I agreed with the definition of postulate, which is not the same as agreeing that a definition is a postulate. Hope this is not too subtle.

A postulate may be open to falsification (as is the case with both SR's postulates); and to be precise it should relate to definitions (as is also the case with both SR's postulates). In contrast, a definition is a human choice that can't be falsified (although it can be poorly chosen).

Is that too subtle??

Ugh, I'd say it is, but I think I can agree with this.

 

[harrylin]

Nope, I agreed with the definition of postulate, which is not the same as agreeing that a definition is a postulate. [..]

Evidently it's still not clear to you that you called a mere definition a postulate... Thus I'll have to start it as a topic. Anyway, it should become a FAQ.

Cheers,
Harald

 

[harrylin]

Evidently it's still not clear to you that you called a mere definition a postulate... Thus I'll have to start it as a topic. Anyway, it should become a FAQ.

Cheers,
Harald

PS: On second thought I do agree with you that the way it was formulated here does fall under the second postulate: there was another subtlety in formulation that I overlooked!
I now reiterated this topic in the new thread by rede:
https://www.physicsforums.com/showthread.php?t=518005&page=2

Harald