A Question on One-Way Speed of Light

This question is for the @physicists, by Ron.

Does gravity wave triangulation, with optical confirmation, constitute a one-way measurement of c?

Some of the responses on this thread might be helpful: One and Two Way Speed of Light.

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It was viewing that thread which actually prompted the question. When gravity waves detectors detect a passing wave and localize the source in an area of sky, guidance is issued in hope of achieving multi-messenger observations of the event. I presume, perhaps incorrectly, that the triangulation is done using the time of detection of the gravity wave at each site.

The difficulty of measuring the one way speed of light, as I understand it, lies in synchronizing two clocks at a distance. If the timestamps by LIGO clocks are not synchronized, however, how is it possible to localize the source of the gravity wave at all? When optical confirmations are found, that is a very precise determination of direction of propagation to the gravity wave detector sites, and this could be cross checked against the event timestamps.


Welcome @RonSewell! Thank you for the question. The @physicists are the ones to make sense of this.

Short answer, no. Gravitational waves and light travel at the same speed - the same one-way speed, even, though for both you can only empirically measure the two-way speed. You can’t use one to measure the one-way speed of the other, any more than you can measure the one-way speed of light using light signals alone.


From the LIGO website:
“Gravitational waves have a finite speed and are expected to travel at the speed of light. This will induce a detection delay (up to about 10 milliseconds) between the two LIGO detectors. Using this delay and the delay between LIGO and its international partners will help pinpoint the sky location of the gravitational wave source.”

Obviously, LIGO has been successful in using the delta in arrival time to narrow the portion of sky to search for optical sources. In my mind, this seems to conflict with Jason Lisle’s suggestion that massless entities such as photons and gravitation waves propagate to an observer instantaneously, so there should be no delta. Given instantaneous travel, why wouldn’t both observatories, independently detecting the GW, then time stamp the signal to the same instant against their clocks, instead of registering a difference?

If this is not based on a one way GW speed, then it must be a two way speed. Could be. What I am missing is, where is the round trip in all this?

Good point. since gravity waves and light waves are the same speed then it must be that theb speed of both is irrelevant to eithers nature. I mmean light does not have a speed but only goes THIS FAST like gravity does for the same reason. its moving through a medium that controls the speed
So then one asks why say light moves at all. all one needs is a ENERGY moving through the medium that provokes a underlying LIGHT medium.
its no true light has a speed of light. its just as true to say light has the speed of gravity waves.
a little off thread but maybe not.

The return leg in the round trip is, effectively, hidden in the method by which the clocks at the different gravitational wave detector sites are synchronized.


But back to my original post, how is it, if the clocks are not synchronized, possible to triangulate the correct area of sky to observe? If they are synchronized, but there is a return leg hidden in the method, how so? I’m not sure how this is actually done, but in principle one could just synchronize each clock together at one location and slow transport to sites. True, we cannot be assured that they have remained in sync with each other, but they all register, in their local frames, some definite elapsed time. Once installed, they no longer need to communicate with each other in any way. A detection event occurs, and some time later - no on line fiber cable or satellite linkup between sites required - a triangulation is calculated from the time of flight as determined by the independently issued time stamps. If the time stamps are out of sync, the triangulation is wrong and useless. What is the point in even having the clocks if they cannot register time of flight?

A similar discussion could be applied to the recent picture of the Messier 87 black hole. Each of the eight radio observatory was equipped with atomic clocks specifically for the project. The time stamped petabytes of data was stored on hard drives, “live” communication between the Event Horizon Telescope sites was out of the question. The Antarctica site would not deliver its bank of drives for many weeks. The rendition of the picture depended crucially on the accuracy of the timestamps - no synchronization, no picture. The climate control in the room housing an atomic clock malfunctioned at one of the sites, which threatened the whole endeavor, and that received lots of attention.

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Is that true @PdotdQ? Is reconstruction this fragile? Or would clock errors have just produced a distorted image?

Not exactly a peer reviewed technical site, but this from sky and telescope.


the scientists use sophisticated computer algorithms to fill in the blanks and reconstruct the image. To make that possible, the team installed atomic clocks at each telescope, devices so dependable that they’ll only lose 1 second in 10 million years. These clocks mark the observations with exact time stamps. The researchers then fly the hard drives with these time-stamped data back to their supercomputers and combine, or correlate , the many sites’ observations, aligning them to within trillionths of a second. Only then can they start reconstructing what the black hole’s shadow looks like.

So, there’s two perspectives one can take here, and I’m finding it easiest to just give answers from both perspectives, distinguishing them, rather than trying to give an answer that is neutral between them.

A bit of a preamble, for anyone stumbling upon this thread and wondering what the heck this is about. (I think @RonSewell seems familiar with this already, but it helps me to explain it.)

It is conceptually easy to measure the two-way speed of light. Get a clock, a ruler, and a mirror. Stick the mirror at the end of the ruler. Send a light signal along the ruler at the exact point that you start your clock. Record the time on the clock when the light signal gets back to you after bouncing off the mirror. Now you have the time of travel and the total distance travelled (2x the length of the ruler) and you can calculate the average speed of the light signal over the round trip.

It is trickier to measure the one-way speed of light. For that, you need a ruler and a clock, and the trick is the clock (at one end of the ruler) has to start at the exact moment that the light signal leaves the other end of the ruler. Which means you actually need two clocks, synchronized but separated in space, so you know when the clock at the other end of the ruler reads zero in order to send the light signal.

Getting the synchronized clocks turns out to throw a wrench in any plans to measure the one-way speed without either already knowing it, or assuming that it is the same in both directions. There’s really two ways you can synchronize distant clocks: you can start with clocks in the same location, and separate them, or you can send light signals between them. Here’s the wrench: due to relativistic effects, moving clocks slow down, so separating synchronized clocks causes them to go out of sync, and you can only correct for this if you know the one-way speed of light. And the light signal synchronization procedure also requires knowledge of the one-way speed, to know precisely when in the interval between sending your signal out and receiving one back to set your clock to zero.

Now, to try to answer @RonSewell’s question. How is it then that we can use synchronized clocks to do things like triangulate GW signals, or reconstruct pictures of distant black holes? First is the “Minkowskian” perspective, which is the one you’ll find adopted by most physicists. In this perspective, the one-way speed of light is the same as the two-way speed of light by assumption, and it is the same in every inertial reference frame. Because the one-way speed is the same in every reference frame (by assumption), you can (in a sense) measure the one-way speed just by measuring the two-way speed, and it is easy to synchronize clocks using the above procedures.

This has the consequence, however, that simultaneity is relative: for any two events A and B sufficiently distant from each other and close together in time (far enough apart that light sent from where A happens, when A happens, cannot reach where B happens before B happens, and vice versa), there is no objective fact of the matter as to whether A happens before, after, or simultaneous with B. Which event happens first will depend on the state of motion of the observer; on the reference frame in which he carries out the synchronization procedure.

(For practical everyday purposes, on human timescales and distances, the speed of light is so high that the degree to which simultaneity is relative is entirely unnoticeable. But whether it is relative or not has philosophical implications, which is why I like to keep the other perspective in mind as well.)

But it also has the consequences of time dilation and length contraction, and these effects all mesh together in a way such that although simultaneity and time intervals and distances are relative, there are certain things which are invariant between reference frames, and moreover, even the relative quantities transform in a predictable way between reference frames. So there is no problem triangulating a GW signal and getting an answer that is accurate in a given reference frame and also knowing how the answers will change as you change reference frames.

The second perspective one might take is the “Lorentzian” one (or neo-Lorentzian, if you prefer). In it, there is one particular reference frame which is the absolute frame, the one that gives us the “correct” answers for measurements of simultaneity and time intervals and distances. The one-way speed of light is the same in all directions in this absolute frame. Effects due to the form of the laws of physics cause moving clocks to slow down, and moving rulers to contract, compared to ones that are stationary in the absolute frame. These effects turn out to make it so that the easy to measure two-way speed of light (using a clock, ruler, and mirror as above) give the same answer no matter the state of motion of the measurement apparatus.

But then there’s the wrench. In order to get the absolutely correct measurements, we need to know if we are moving relative to the absolute frame so we can correct for the time dilation and length contraction of our measurement instruments. But we can’t do that: to know our motion with respect to the absolute frame we’d need to measure the one-way speed of light, and there’s no non-circular way of doing so, as described above.

However - thankfully! - the situation is not so hopeless as it sounds. It turns out that even though we can’t know what our state of motion is with respect to the absolute frame, if we use our clocks and rulers and synchronize distant clocks as if we were not moving, the laws of physics we discover will have exactly the same form as they would in the absolute frame. And the form of those laws makes it so that, at least as far as the purposes of physics are concerned, we can pretend the Minkowskian perspective is correct after all, and eschew absolute simultaneity in favour of relative simultaneity. So again there is no problem triangulating a GW signal and getting an answer that is accurate in a given reference frame and also knowing how the answers will change as you change reference frames - with the added bit of mystery that one of the reference frames out there is the absolute one; we just don’t know what it is.


No, this stems from a common misunderstanding of the one-way speed of light problem - this is a confusing topic, and misunderstandings happens often, so there is no shame in it. The LIGO data is unaffected at all by our convention for the one-way speed of light.

Here is the misunderstanding: there is NO problem in synchronizing the LIGO clocks. We synchronized them to extreme precisions assuming the Einstein synchronicity convention, i.e. that the one way speed of light is c. We can synchronize them with any other conventions too, and also to the same extreme precision. This will not change the LIGO results at all, as the LIGO results depend only on the fact that the clocks are synchronized, not the particular synchronicity convention that we used to synchronize our clocks.


I can see I’m sailing into strong headwinds, without benefit of charts, compass, or seamanship.

While I have always had a general fascination with physics and cosmology, my immediate interest in the one way speed of light is prompted by the widespread adoption in the YEC community of Jason Lisle’s Anisotropic Synchrony Convention [ASC] model. It is transparent that the motivation which underlays Lisle’s development of the ASC model is to work around the enduring challenge to a YEC worldview posed by distant starlight. The intended takeaway is that light from the farthest reaches of the cosmos reaches earth instantly.

The distant starlight problem is special, and what makes it special is that it is accessible. For those who fled science class after grade nine, much scientific information they are exposed to sounds like “take our word for it”. The implication of distant starlight for the age of the universe, however, is something mom, dad, and the kids can all realize on their own. The relationship between speed, distance, and duration is ingrained in human experience. That can be a deal breaker for belief in a young earth. There is little point in defending a youthful planet under an ancient sky.

National Geographic, Nova, Scientific American, Discovery magazine, and countless books have all inundated their viewership and readership with phrases along the lines of “before civilization”, “at the time of the dinosaurs”, “billions of years ago”, based on the lightyears of distance of supernovae, galactic collisions, and other celestial events. The impression, and I do not believe I am exaggerating here, was that the speed of light was the speed of light. There were no proviso or discussion of one way measurement. Now what I am hearing is that this is a bit of a gloss over, an arbitrary assumption, a convention.

So let us choose our own adventure and assume Lisle’s ASC. Actually, let’s just focus on his central contention, that light and gravity travels at near infinite speed to an observer, (who can be anywhere). The earth did not revolve around the sun 130 million times between when two neutron stars merged and their detection by LIGO, nor were there any dinosaurs at the time of the event. The gravitational waves were generated and radiated at near infinite speed to wash over the earth instantaneously. We saw it the moment it happened. The near infinite velocity of the gravitational wave is compatible with the spacing of time stamps at the detectors.

Given that this is all good and allowed under physics, the rest is easy. Distribution of metallicity and quasars? Created that way. CMB? Who cares. Galactic smashups and streaming gas? Yup, created that way. No need to sweat the technicalities.

And by now, I suppose I have sunk my little boat.


I like your simplicity. But I do not follow exactly what you are arguing for and against. As you write, you seem to shift gears. So please spell it out for us: 1. 2. 3. what you are arguing for. For instance, do you like Lisle’s ASC or dislike it as an explanation for distant starlight in our vicinity?

Hypothetically, if we split an entangled pair of particles and sent one to the moon in a device that could be triggered by a laser pulse sent from Earth, wouldn’t we be able to measure the passage time with the particle left on earth?

Entanglement doesn’t work that way. (If it did, this discussion would have been finished several decades ago!)

This is because, it appears my initial tract was wrong. Maybe not even wrong.

When I read this from Jason Lisle:

the one-way speed of light cannot be measured without first stipulating it either explicitly or implicitly. In the same way that we cannot test whether the English system or the metric system is “correct,” so we cannot test the one-way speed of light. It is chosen as a matter of convention.
…my initial response was, there must be some sleight of hand going on here. This is relativity of relativity. I thought special relativity was about preserving the invariance of the speed of light by melding space and time together. Surely, there must be some way to measure the one way speed of light, if not directly, then at least by confirming the assumption of uniform speed by some outcome. The idea that the same photon propagating at infinite speed to an observer was departing at ½ c away from the source, even in the same inertial frame, had to be vulnerable to some empirical or theoretical refutation.

I soon found out that interferometers and other benchtop measurements did not do the trick. Neither did the eclipses of the moons of Jupiter, or astronomical aberration. Light echoes in space and gravitational lensing do not seem to trouble ASC, for reasons I still do not fully understand. However, I did think that the LIGO detections, with the gravitational wave being time stamped consistent with propagation at c, could only be interpreted as a one way measurement, particularly if the timestamps were authenticated by yielding useful data. Apparently not.

We now have a pretty extensive list of approaches that do not measure the one way speed of light, and none that do. It appears that nature may frustrate any attempt at such a measurement with the same tenacity with which she guards the wave function of quantum mechanics, and some deep principle will not be violated. If so, while the assumption that the one way and two way speed of light are identical might remain as simple, elegant and keeping with common sense, the long sought YEC workaround of the distant starlight problem at last has resolution. You are free to look up at Andromeda and not deny the evidence of your naked eyesight when you say to yourself, “the universe is young”.

Now it is possible to be nonchalant about that and point out the evidence for an ancient universe goes far beyond the distant starlight problem, and that Lisle’s cosmology may have other unintended consequences, but these discussions tend to be much more arcane in nature. If God can build an ASC universe, He can commission the thing successfully. YEC adherents will seize on a plausible infinite speed of light, and why not? In a context where the hubble deep field is perceived not as a window into the early universe but is Greenwich right now time, technical concerns will be just outsourced to YEC organizations such as AiG. The association of deep time and deep space for me always provided a contextual scaffolding, a starting point, for how it all fit together, and if the speed of incoming light really cannot be determined, I may have to reconstitute some of my thinking.

As much as I wish, lots of gotcha’s here. Observing collapses the wavefunction [Copenhagen], so you can’t just keep checking for a changed state.

Jason Lisle, the YEC astronomer who defends the One-Way Speed of Light guesses that God has built the cosmos since the beginning so that light travels at near instantaneous speed (simultaneously from every star) when it is traveling “toward the earth?” Such an earth-centered assumption seems to go way beyond what science can actually say, or even how science works when studying the way the cosmos works.

Lisleʼs Additional Guesses: God could have created galaxies in mid-collision, and created rings of expanding matter in mid-explosion (rings so large that playing them backward in time tells the tale of a stellar explosion that would have had to have occurred eons before the cosmos was created according to YECs, hence God has to create them in mid-explosion). Lisle offers such non-falsifiable guesses to defend his view that the cosmos is only thousands of years old. And one need not stop piling on such guesses, but might just as well continue guessing with equal abandon that maybe God created some species in the act of already going extinct [in mid-extinction], or created the earth with fossils of long dead [long exploded] species already buried in the earth that never took a breath in the first place.

Lisle also denies that any new stars are forming because Genesis 1 says that God made “the sun, moon, and the stars also,” and “set them in the firmament” on “Day four,” and God never mentions anything about new stars being created or formed after He created the heavens and the earth. So Jason denies that new stars continue to arise in various places throughout the cosmos. Yet evidence of new star formation is constantly being reported in astronomy journals. Astronomers also point out that the cosmos has yet to reach its maximum number of stars and planets.

The star-forming region, known as Monoceros R2, sits in a massive dark cloud thatʼs rich in molecules and dust. The scene is packed with massive, young stars.

The starburst galaxy NGC 1313 has a center that is very active with many star formation regions. A great number of supershell nebulae, that is, cocoon of gas inflated and etched by successive bursts of star formation, are visible (http://spacefellowship.com/news/art23454/picture-of-the-day-the-topsy-turvy-galaxy.html).

Lisle also continues (https://answersingenesis.org/answers/books/taking-back-astronomy/the-universe-confirms-the-bible/) to promote the view that the “circle of the earth” in Isaiah is a revelation from God of the earthʼs sphericity, when it is no such thing (https://benstanhope.blogspot.com/2015/10/isaiahs-circle-of-earth-bad-creationist.html) for instance, though many Evangelicals have already debunked such a claim after noting how “circle of the earth” referred to a flat earth in the ancient world.

For much more on YEC astronomy’s erroneous predictions and guesses: Scrivenings: Young-earth creationists, Henry Morris, Ken Ham, and Jason Lisle fail to provide insight into the cosmos. Lisle's answer to “The Distant Starlight Question” (Alternate Synchrony Convention) fails to add scientific weight to young-earth creationism