[guest39538]

2 video cameras that are synchronised in their start.  0t

One recording the ground at point (A) and one much further away at point (B) recording the area of (A)

We then just need lightning to hit the ground, Recorder (A) for example records the strike hitting the ground at 1 min 20.s

Recorder (B) records the strike a short amount of time later.

Compare differences work out the speed of light over distance x


cam.jpg (22.19 kB . 985x507 - viewed 6478 times)

Time line (A) shows 1 minutes 20.s the lightning strikes the ground

Time line (B) shows 1 minutes 20.1s the lightning strikes the ground

So in this example the light travelled a 1000m in 0.1s because camera (B) was 1000m away exact.

ok?

[Bored chemist]

"2 video cameras that are synchronised in their start.  0t"
That's where the problems start.
But, if we can assume that local gravity is small enough that it has little effect and that the cameras are synchronised while next to each other then moved apart sufficiently slowly that relativistic effects are small and that you bring the cmaeras together again slowly to compare the recordings. then yes.

You can use that setup to measure the speed of light.

However, one way of "synchronising" the videos would be to wait for a lightning strike and say  that, since it clearly was only 1 event it must happen at the same time for both cameras.
Then you get into problems.

[guest39538]

"2 video cameras that are synchronised in their start.  0t"
That's where the problems start.
But, if we can assume that local gravity is small enough that it has little effect and that the cameras are synchronised while next to each other then moved apart sufficiently slowly that relativistic effects are small and that you bring the cmaeras together again slowly to compare the recordings. then yes.

You can use that setup to measure the speed of light.

However, one way of "synchronising" the videos would be to wait for a lightning strike and say  that, since it clearly was only 1 event it must happen at the same time for both cameras.
Then you get into problems.

The synchronisation is a lot easier than you think, the cameras can already be recording in situate position before we synchronise them, the only synchronisation needed on the videos is the timelines, 1 reset button  tarring both recorders to 0 to synchronise recordings.

[Bored chemist]

The synchronisation is a lot easier than you think, the cameras can already be recording in situate position before we synchronise them, the only synchronisation needed on the videos is the timelines, 1 reset button  tarring both recorders to 0 to synchronise recordings.

No they can't.
Simultaneity is only defined locally. You have to take account of how long it takes for the signal saying you pressed the "zero button" to get to the two videos.

Since you seem to be trying to set up a thread about relativity, I think you might start by learning about it

[guest39538]

The synchronisation is a lot easier than you think, the cameras can already be recording in situate position before we synchronise them, the only synchronisation needed on the videos is the timelines, 1 reset button  tarring both recorders to 0 to synchronise recordings.

No they can't.
Simultaneity is only defined locally. You have to take account of how long it takes for the signal saying you pressed the "zero button" to get to the two videos.

Since you seem to be trying to set up a thread about relativity, I think you might start by learning about it

This is a thread about measuring the speed of light if not testing simultaneity.

[Bored chemist]

The synchronisation is a lot easier than you think, the cameras can already be recording in situate position before we synchronise them, the only synchronisation needed on the videos is the timelines, 1 reset button  tarring both recorders to 0 to synchronise recordings.

No they can't.
Simultaneity is only defined locally. You have to take account of how long it takes for the signal saying you pressed the "zero button" to get to the two videos.

Since you seem to be trying to set up a thread about relativity, I think you might start by learning about it

This is a thread about measuring the speed of light if not testing simultaneity.

They are strongly related.

[guest39538]

The synchronisation is a lot easier than you think, the cameras can already be recording in situate position before we synchronise them, the only synchronisation needed on the videos is the timelines, 1 reset button  tarring both recorders to 0 to synchronise recordings.

No they can't.
Simultaneity is only defined locally. You have to take account of how long it takes for the signal saying you pressed the "zero button" to get to the two videos.

Since you seem to be trying to set up a thread about relativity, I think you might start by learning about it

This is a thread about measuring the speed of light if not testing simultaneity.

They are strongly related.

Well I suppose so.   My mate just shown me something, the great pyramids earthly co-ordinates the same as the speed of light in numbers?

[dutch]

What is this experiment trying to show?

It's NOT showing that you can measure the one-way speed of light independent of two spatially separated clocks.This has never been done.

https://en.wikipedia.org/wiki/One-way_speed_of_light
https://en.wikipedia.org/wiki/Einstein_synchronisation

Einstein synchronization (or Poincaré–Einstein synchronization) is a convention for synchronizing clocks

Most attempts to negate the conventionality of this synchronization are considered refuted

But, if we can assume that local gravity is small enough that it has little effect and that the cameras are synchronized while next to each other then moved apart sufficiently slowly that relativistic effects are small and that you bring the cameras together again slowly to compare the recordings. then yes.

But from who's perspective is the relativistic effect small?

To an observer's moving at 90% the speed of light past one clock then the other relative to the spatially separated clocks the simultaneity difference is rather extreme. Slow clock transport in one direction would be vastly different than slow clock transport in the other direction according to this observer. The clocks are out of sync according to this observer. However, this moving frame of reference is equally valid and stands on equal footing as another inertial reference frame. Einstein showed that slow clock transport and fast clock synchronization (sync clocks with light) were identical. Einstein and most physicists (who actually study up on this subject) know that "Einstein Clock Synchronization" is a convention. A very useful convention but a convention nevertheless.

[guest39538]

What is this experiment trying to show?

It's NOT showing that you can measure the one-way speed of light independent of two spatially separated clocks.

Yeah , I have a vague idea how to do it.

[guest39538]

I have thought some more, if we had the two cameras recording the one location live and two corresponding monitors of each camera, we should be able to observe a flash of lightning on one screen before the second screen . .....Then if some how we can measure the time difference in screens, we should be able to calculate a speed.


cc.jpg (21.85 kB . 1152x648 - viewed 6149 times)


Maybe 3 cameras for triangulation.

[puppypower]

If two references were synchronized that means there should be no observed time difference. The easiest way to show this is to take a still photo of both references at the same instant of time. The still photo will stop time and only show position in space. From the still photo one can see how they were synchronized for that instant of time.

[Bored chemist]

But from who's perspective is the relativistic effect small?

For the sake of discussion, mine.
I'm sat round near one of the cameras. and both cameras are on the ground. All of us are stationary wrt one another.
I took a while, and I delivered the cameras to their locations three million meters apart by bicycle. They certainly never exceeded 30 m/s i.e. C/10,000,000 .
Most of the time they were travelling at about  a millionth of C
And I found a "flat" bit of the Earth to use as my baseline. There were no hills on my journey.
So the relativistic effects on the clocks due to their changes in velocity and altitude (thus gravity) were small.
If you are not content that they are small enough we can, in principle, repeat the experiment with cameras delivered by snails or even continental drift.

"To an observer's moving at 90% the speed of light past one clock"
OK, so we won't ask him, we will ask me.
I set off a flash bulb near one of the cameras. and the cameras capture that flash.
Then I go and collect the cameras and I develop the films.
They are slightly odd cameras (since this is just a thought experiment). They use old fashioned film and they run at a million frames per second.
I count the frames in each film until I reach the frame with the flash in it (never-mind how long a real flash takes- this is a thought experiment).
There are lots of frames but the important thing is the difference in frame number.
I find that the camera near the flash records it on frame number  n and the one far away records it on frame number n+m.
So it takes m millionths of a second for the flash to travel 3 million metres from the first camera to the second camera.

Now, I know from other experiments - Fizeau and so on, that the speed of light is about 300 million metres per second.
So I expect it to take about a hundredth of a second to make the journey.
Each frame takes a microsecond, so I should find that m is about 10,000

What value of m do you expect to find?

[dutch]

For the sake of discussion, mine.
I'm sat round near one of the cameras. and both cameras are on the ground. All of us are stationary wrt one another.
I took a while, and I delivered the cameras to their locations three million meters apart by bicycle. They certainly never exceeded 30 m/s i.e. C/10,000,000 .
Most of the time they were travelling at about  a millionth of C
And I found a "flat" bit of the Earth to use as my baseline. There were no hills on my journey.
So the relativistic effects on the clocks due to their changes in velocity and altitude (thus gravity) were small.
If you are not content that they are small enough we can, in principle, repeat the experiment with cameras delivered by snails or even continental drift.

"To an observer's moving at 90% the speed of light past one clock"
OK, so we won't ask him, we will ask me.
I set off a flash bulb near one of the cameras. and the cameras capture that flash.
Then I go and collect the cameras and I develop the films.
They are slightly odd cameras (since this is just a thought experiment). They use old fashioned film and they run at a million frames per second.
I count the frames in each film until I reach the frame with the flash in it (never-mind how long a real flash takes- this is a thought experiment).
There are lots of frames but the important thing is the difference in frame number.
I find that the camera near the flash records it on frame number  n and the one far away records it on frame number n+m.
So it takes m millionths of a second for the flash to travel 3 million metres from the first camera to the second camera.

Now, I know from other experiments - Fizeau and so on, that the speed of light is about 300 million metres per second.
So I expect it to take about a hundredth of a second to make the journey.
Each frame takes a microsecond, so I should find that m is about 10,000

What value of m do you expect to find?

None of this is measuring the one-way speed of light independent of two spatially separated clocks. Measuring the one-way speed of light by ANY experiment ever done or probably could be done doesn't measure the one-way speed of light without using a synchronization convention. There has never been any way around this. You're assuming clocks are running at the same rate, the clocks are synchronized, and the one-way speed of light is the same in both directions. You can always assume these to be true from your point of view... because you can choose any valid point of view. However, you can ask any inertial reference frame their point of view because they are all equally valid. From other reference frame's points of view your assumptions aren't true. These other inertial reference frames can produce the exact same physics and you could also use their synchronization to produce the exact same predictions. They could easily predict what you'd see in your reference frame when using their synchronization. They can predict vastly different relative speeds for light in different directions relative to your reference frame because they are assuming their reference frame's synchronization (how great depends on their velocity relative to you).

OK, so we won't ask him, we will ask me.

You could run all your math off of the other inertial reference frame. You could even join a religion where you believe the other inertial reference frame is the "right" one and thus use it and only it. You assume the Einstein Synchronization (some people assume it like its a religion) because you can but not because it was ever proven as the only synchronization (in fact there are infinitely many). Why did you assume you're particular synchronization? I don't think you understand how many assumptions you're making. Did you read the following below? It's a very strong statement.

Most attempts to negate the conventionality of this synchronization are considered refuted

You could say the two-way speed of light has been measured. You could even say information moves slower or at the vacuum speed of light in all experiments ever done. As proven by Einstein you could say slow clock transport and fast clock transport are identical (you assume the opposite for some reason. Your assumption is wrong; Transport an actual clock slowly from point a to b using the Lorentz Transformation. Use a light clock for simplicity to visualize your error)

However, the one-way speed of light is only measured based on an assumed synchronization. Regardless of how one chooses to view relativity the one-way speed of light is a lot more subtle than most think.

I have thought some more, if we had the two cameras recording the one location live and two corresponding monitors of each camera, we should be able to observe a flash of lightning on one screen before the second screen . .....Then if some how we can measure the time difference in screens, we should be able to calculate a speed.

If you could do it then you may have something but that's a rather big if. People have been saying they've measured the one-way speed of light independent of a clock synchronization convention for over 100 years but they never have. Amusingly, they attempt to measure this value to defend Einstein's Relativity unnecessarily because it was Einstein who showed that you couldn't.

[Bored chemist]

In doing science by thought experiments it's not  exactly rare to start with the simplest possible case as a foundation.That's pretty much the case I have considered.
You don't seem to have included an answer to my question.
What value do you expect me to get for m?

After that we can consider the more complicated cases like spacemen who believe in unicorns if that's what we want to, but simply not answering a question isn't helpful

Incidentally, it's not so much that I assume that slow transport doesn't dilate time.
It's that I know that people who know more about t than I do assumed that fast transport dilated time more.
Are you saying that teh people who did this experiment
https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment
should have used a rowing boat, rather than a jet plane?

Incidentally, I'd like to introduce a slight refinement to the thought experiment.
Imagine that , in the field of view of each camera is a clock counting (local) microseconds.
That will number the frames for me and stop me needing to count them.
With that amendment we can talk about what time things are recorded as happening from the point of view of each camera.

[dutch]

In doing science by thought experiments it's not  exactly rare to start with the simplest possible case as a foundation.That's pretty much the case I have considered.
You don't seem to have included an answer to my question.
What value do you expect me to get for m?

The answer to your question is straight out of the Lorentz Transformation and I did answer it. See below:

They could easily predict what you'd see in your reference frame when using their synchronization.

In fact you could use their synchronization to synchronize your clocks (simply use the Lorentz Transformation). In practice it's often easier to assume Einstein Synchronization but it's still a convention.

After that we can consider the more complicated cases like spacemen who believe in unicorns if that's what we want to, but simply not answering a question isn't helpful

Relativity allows us to use any valid reference frame we wish. This has nothing to do with unicorns or spacemen. It has to do with hard physics. I've done quite complicated particle physics problems based solely off of the lab reference frame. I've also done them off of other particular reference frames as it suited the problem (often vastly different than the lab frame). Regardless of what reference fame I choose the observations are the same. The relative speed of light outside of my choice of reference frame (and thus my chosen synchronization) could be vastly different than c and different in different directions. I don't have to choose isotropy for my own reference frame to get the right answer. However, I always have the choice to choose isotropy.

Incidentally, it's not so much that I assume that slow transport doesn't dilate time.
It's that I know that people who know more about t than I do assumed that fast transport dilated time more.
Are you saying that teh people who did this experiment
https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment
should have used a rowing boat, rather than a jet plane?

No, but in terms of spatially separated clock synchronization it doesn't matter if you transport the clocks slow or fast. Do you want me to do the math? I don't care about the round trip time dilation as this is a related but separate effect (Twins Paradox). The issue at hand is the synchronization of the clocks for t = 0 ( the γ v x / c² term and not the time dependent γ t term). It's the change in simultaneity from point a to b not how fast clocks run relative to another reference frame as time ticks forward.

The Lorentz Time Transformation is below:

t' = γ ( t - v x / c²)

Incidentally, I'd like to introduce a slight refinement to the thought experiment.
Imagine that , in the field of view of each camera is a clock counting (local) microseconds.
That will number the frames for me and stop me needing to count them.
With that amendment we can talk about what time things are recorded as happening from the point of view of each camera.

You still can't measure the one-way speed of light independent of a clock synchronization. This is a subtle but very important point coming out of Relativity.

[Bored chemist]

There has never been any way around this. You're assuming clocks are running at the same rate, the clocks are synchronized, and the one-way speed of light is the same in both directions.

Yes, I know I'm making those assumptions (and some others).
Under what circumstances are they invalid?

[dutch]

Yes, I know I'm making those assumptions (and some others).
Under what circumstances are they invalid?

Where did I say they were invalid? Why are you responding with this? Are you reading what I wrote? You can choose many different synchronizations this doesn't make any of them invalid. You can choose the Einstein Synchronization Convention. However, there is a reason it's called a convention...

Other conventions wouldn't have the one-way speed of light isotropic and we can equally use those conventions. We don't know what simultaneous is (this would require an instantaneous signal) so we can't measure the one-way speed of light without assuming a convention. The problem is we can get any number of an infinite number of answers depending on what convention we use.

[guest39538]

If you could do it then you may have something but that's a rather big if. People have been saying they've measured the one-way speed of light independent of a clock synchronization convention for over 100 years but they never have. Amusingly, they attempt to measure this value to defend Einstein's Relativity unnecessarily because it was Einstein who showed that you couldn't.

I do not think measuring the one way speed of light seems that difficult of a thought. I know I could certainly measure the one way speed of light to observe if we have the correct speed of light at the moment of 299792458 m/s.  That is a relative simple challenge. 
I am sure we could then adjust the experiment in my thoughts of above to measure a true speed if 299792458 m/s was incorrect.
To test the present speed of light one way we can simply use a strobe set to flash once per second.   We can then have a detector (radiometer) a set distance away to detect the light.

For example :  If the detector was 149896229 m away from the strobe that was flashing 1 flash/second, the detector would detect 2 flashes/second if c is 299792468 m/s

We can calculate the above down to a usable distance then with a bit of ''magic'' , work out the new speed.


added- something like that anyway, I am going fishing in the morning for 4 days so will think about it under the stars and clear up my own thoughts.

[David Cooper]

To test the present speed of light one way we can simply use a strobe set to flash once per second.   We can then have a detector (radiometer) a set distance away to detect the light.

How are you going to time it? Are you going to use one clock or two?

If you use one clock and place it by the strobe unit, how long does it take for the information to get back from the detector to the clock once the light has reached the detector? That signal returns at the speed of light, so you're actually timing the two-way speed of light (i.e. a round trip).

If you use two clocks and have one by the strobe and the other by the detector, you need to synchronise the two clocks, and how do you synchronise them? Any viable method of synchronisation that you use will automatically lead to you measuring the two-way speed of light instead of the one-way speed of light.

[Bored chemist]

Yes, I know I'm making those assumptions (and some others).
Under what circumstances are they invalid?

Where did I say they were invalid?

You didn't.
But, unless there are circumstances where they are invalid then it doesn't matter that I made them.
Pointing out that I'm making an assumption is only worthwhile if that assumption is, or might be, wrong.

So, when would the assumptions I made be wrong?
Also, if they are right from my point of view, the I can measure the one-way speed of light from my point of view.
And, as far as I can tell, it's going to be pretty much the same as the two way speed.
And as far as I understand it, that's the same for everyone.
And there's nothing special about me.
Anyone anywhere could do a similar experiment and get the same answer: in the limit, the measured 1 way speed of light-per this experiment- is C