# Yet another idea for measuring the one way speed of light

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CompSci
Drive two 15m long optical fibers extended in opposite directions with a 1GHz pulse generator. The recievers at the end of each fiber are now syncronised sources of 1GHz pulses. Connect them to pulse counters, one of which provides a signal whenever a pulse train is present.

Connect this "clock on" signal to a laser aimed at the second station. When the pulse train appears, this laser will turn on. The beam is directed at a beam splitter. Adjust the beam splitter so that the intensity of the light beams hitting a detector located 1cm from the the splitter and a detector located 30.01m away at the second station are approximately equal.

These detectors are connected to the corresponding pulse counter's "count disable" input. When the pulse generator is turned on, both counters count pulses. When the laser turns on, the counter next to the laser stops counting almost immediatly. The second counter should stop counting when the laser signal has traversed the 30.01m separation. Build a 10ns delay into the firing circuit of the laser so that the counter close to the laser will show 10 counts. My expectation is that the distant counter will show 110 counts.

Since 100 pulses indicate a transit time of 100ns for a 1GHz clock, the measured velocity of c will be: 30m / 100*10^-9s, giving a value of c=3.0*10^8ms.

Is this a valid determination of the one-way speed of light?

No. Clock synchronisation is a convention, so any one-way measurement contains a choice. Or else relativity is fatally wrong - please see the experimental evidence for special relativity sticky thread at the top of the forum for reasons to believe it is not.
The recievers at the end of each fiber are now syncronised sources of 1GHz pulses.
Only if you assume that the speed of light is the same in both directions - there's the choice you made.

CompSci
Actually, I'm interested in disproving the "Anisotropic Synchrony Convention" proposed by Dr. Jason Lisle. According to that convention, the speed of light is infinite towards the observer, and c/2 for light traveling away from the observer. Since all of the detectors are observers, according to ACS, all the signals propagate at infinite velocity, and both counters will show 10 counts. If they show the results I expect to see, would this refute his convention?

No. It's a convention, not something open to proof or disproof. It's simply a rather odd choice of coordinate transform applied to the usual coordinates used in relativity. It propagates messiness through the maths, but will lead to the same predictions. I assume he has a reason to accept the pain.

Don't rely on intuition to analyse it. The concept of (three-)velocity isn't the same using these coordinates as it is in regular Einstein coordinates, which is why your "synchronised" emitters wouldn't be considered synchronised in Lisle's scheme.

Gold Member
not something open to proof or disproof.

Not sure if the below is a fair representation, but if it is, I am taken aback - are those statements really valid with an appropriate co-ordinate transform? Seems flatly contradictory to saying that c is the same for all observers in all reference frames.

"Light arriving from a distance source travels at a different speed than light traveling from a local source. Likewise the light reflecting off a surface may travel at a different speed than light arriving from an emitting source."

edit - added quotation marks to be clear about what is the snip from the linked article.

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CompSci
Thanks for your replies. Would you please direct me to a good resource on the differences in the concepts of velocity between these two conventions?

Light arriving from a distance source travels at a different speed than light traveling from a local source. Likewise the light reflecting off a surface may travel at a different speed than light arriving from an emitting source.
That's a more complex model than the one I'd understood from CompSci's post. You can certainly have leftward-travelling light doing infinite speed and rightward-travelling light doing c/2; that's a trivial coordinate transform as I said.

I'm not sure I've understood exactly what Lisle is claiming in that article. I'll have a look later. However - either it's a coordinate transform away from relativity or it's nonsense.

Grinkle
CompSci
@Grinkle That's the paper I'm referring to. Everything I've read, including what Ibix said about generating the same predictions, make it a valid, if cumbersome convention. Hopefully the differences in the concepts of velocity shed some light.

CompSci
From Dr. Lisle's paper, linked by Grinkle:

"Under ASC, the speed of light as a function of direction relative to the observer (θ) is given by cθ = c/(1-cos(θ)), where θ = 0 indicates the direction directly toward the observer."

Perhaps this is more than a convention? And it explains my question in Post #3. All the detectors in the experiment outlined in Post #1 are "observers", and therefore theta is necessarily zero for them. Therefore, under ASC, c is infinite for all observers in this experiment, and both counters will accumulate 10 counts. If they don't, then the quote in this post is invalid.

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Ok - the article linked by Grinkle starts reasonably, but goes off the deep end immediately after the quote from Einstein and is silly thereafter. I rather suspect the link will get deleted as inappropriate to PF.

I still don't understand the exact model being proposed. I think they're just using some curvilinear coordinates to satisfy their theology. Again it's not wrong to do so (I think defining hyperbolic planes of simultaneity instead of flat ones would match their rather vague description), just difficult to use. I suspect it gets more problematic as you get to cosmological distances and you can't use SR, but again only in the sense that the maths is nastier.

The point about three velocity is that it is ##dx/dt##, with ## dx## and ##dt## being small changes of coordinates. So if you change to a different definition of your time coordinate (which is all a synchronisation change is, really) then you differentiate by a different thing. Conceptually, it's no more complex than the idea that I can regard myself as at rest with respect to a coordinate system anchored to my sofa, or doing 30mph with respect to a car passing by on the street. In that example I'm changing the definition of the x coordinate (in Newtonian physics) or both x and t coordinates (in relativistic physics), and velocity means something different to the car driver and to me. If you pick complicated coordinates then you find velocities with respect to that coordinate system are complicated - the maths you need is basically the chain rule.

From Dr. Lisle"s paper, linked by Grinkle:

"Under ASC, the speed of light as a function of direction relative to the observer (θ) is given by cθ = c/(1-cos(θ)), where θ = 0 indicates the direction directly toward the observer."
That quote doesn't come from the article linked by @Grinkle.

Grinkle
Gold Member
I rather suspect the link will get deleted as inappropriate to PF.

Ibix
CompSci
Sorry, I assumed he was linking the same article I read. Here's the link to the article containing my quote:

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There's another link that's going to get deleted...

But yes, Lisle just seems to be adopting a complicated coordinate system because it suits his views. It's an extremely clunky one for most purposes, and appears to select one point in an isotropic universe as special for no physical reason. Choosing one point as special is where all the coordinate anisotropy comes from, including the angular dependence of speed relative to this coordinate system.

Have you heard of the block universe? The idea is that spacetime is a four-dimensional block. The Einstein synchronisation convention imagines slicing the block into flat planes and calling each one "all of space at the same time". Different frames are just a case of slicing the block into planes at different angles. Lisle is slicing spacetime into a stack of cones instead of a stack of planes. So there's no experiment to "prove him wrong" because all slicings are equally a human convenience and equally valid (as long as they don't cross). As long as you remember the knock on effects on the representations of vectors and other physical quantities, you'll end up with the same predictions for your experiments.

The invariance of physics under such coordinate transforms is a fundamental part of relativity.

Mentor
I'm interested in disproving the "Anisotropic Synchrony Convention" proposed by Dr. Jason Lisle

First you're going to have to provide an actual peer-reviewed paper as a reference. If you can find one, PM me. Unless and until that is done, this thread is closed as discussion needs to be based on a valid reference.

Mentor
There's another link that's going to get deleted

Yep.