The conventionality thesis on the synchronisation of clocks separated by distance

I recently read the book “Concepts of Simultaneity” by Max Jammer[1] where Jammer presents the history of concepts of simultaneity. Primarily he starts with the Greek philosophers and works through to the modern age—with philosophers and physicists. The book outlines that in regards to the concept of simultaneity the most significant developments occurred in the past 110 years, starting with Albert Einstein when he published his famous paper on Special Relativity in 1905.

Time is intrinsically linked with simultaneity. Simultaneity involves how we might synchronise distant clocks with our own local clocks. This issue then has an enormous bearing on some significant philosophical questions. How do we measure the speed of light coming from the distant cosmos? How old is the universe? How do we know how old the universe is?

We accept as fact, even consider it as empirical fact, something that is actually not fact at all but conventional. I explain below. But an incorrectly held notion has led to the idea that we can definitively know how old the universe is. Since we know the speed of light is finite (not infinite) and the universe is enormously large, then it is concluded that it must have taken light billions of years to travel to Earth from the distant cosmos. From that it follows that both theist and atheist have incorrectly concluded that the biblical timeline cannot be correct. The atheists even use this as an argument against the existence of the Creator.

In classical (Newtonian) physics, up to several hundred years before Einstein and any measurement of the speed of light, a distant event was considered simultaneous with a local event if the local observer saw them occur at the same time, as measured by his local clock. This is because the speed of light was assumed to be infinite. There was no delay between the light leaving its source and its reception at the observer. But that all changed.

The speed of light was measured from the late 17th century into the 20th century many times, and by different methods and different observers. The results agreed at about 300,000 km/s and is designated by the letter c. This means that all signals that carry information in the universe travel at this limiting speed. However it was not until Einstein in 1905 that the full ramifications of the speed of light being a limiting speed began to be understood.

Einstein and Special Relativity

Starting with his famous 1905 paper on the theory of Special Relativity Einstein made the significant discovery that underlines the importance of the concept of simultaneity.

He had three principles (1) the relativity principle – where the physics is the same for all inertial[2] observers; (2) the principle of the constancy of the speed of light regardless of the uniform speed of the observer and (3) the transformation equations, which up until that time were the Galileian equations, which were used in classical Newtonian mechanics.

But Einstein pointed out that these three (1), (2) and (3) are incompatible. Experiments confirmed (1) but also confirmed that the equations of electrodynamics or optics are not invariant in transition from one inertial system to another under the transformation equations (3). Thus the relativity principle (1) was in conflict with (3). For example, a moving source should add its speed to the measured speed of light. If a laser light was seen coming from the back of a spaceship speeding away, at speed v, from an Earth observer the speed of the light beam coming from the back of the spaceship, according to (3), should be the speed c – v but a light beam shining out in front of the spaceship should be c + v, due to the added speed of the spaceship.

Einstein rejected (3) in favour of (2) and his Special Relativity theory was born. This meant a major revision of the concept of time and simultaneity, in regards to the equations that govern the time we measure on clocks.  It also meant different transformation equations were required. By using the Lorentz transforms Einstein was able to develop a consistent theory. From that theory strange effects like time dilation and length contraction were predicted and many experiments since have established them as experimental facts.

Einstein wrote:[3]

“We have to bear in mind that all our propositions involving time are always propositions about simultaneous events.” Albert Einstein, 1905.

How do we know events are simultaneous? If they occur in the same location they can easily be seen to be simultaneous by the fact they both occur at the same time, measured by the same clock. Only one clock is involved. But if the events are not both local, but one is distant, then only by sending a light signal from one to the other, can one ascertain their simultaneity.  This means there are two clocks, one distant and the other local.

To determine the time on the distant clock when the event occurred there one needs to synchronise the distant clock (A) with a local clock (B). If the speed of light was infinite there would be no problem, because what you see is what you get. But if the speed of light ONE WAY from clock A to clock B is not infinite, what is it? You would need to measure this one-way speed of light. If you knew what it is you could make an allowance for the difference in times showing on the distant clock A compared to the local clock B. So to synchronise these clocks, in order that you might know if a distant event is simultaneous with a local event, you need to know the one-way speed of light. But here is the problem. To measure the one-way speed of light you need synchronised clocks separated by a distance. That is you need to know the time of flight of the light from A to B and with the distance calculate the one-way speed.[4] It is totally circular, which Einstein recognised.

Einstein simply chose his method of clock synchronisation that most simplified his equations of special relativity—the standard clock synchronisation method,[5] which means the speed of light one way in any direction is the same (or isotropic) and equal to c. He had no empirical reason to choose that, but like most theorists, symmetry and simplicity are sufficient reason.

This means the choice of the one-way speed is conventional, and not an empirically measured fact. This is the conventionality thesis on the synchronisation of clocks separated by distance.

The conventionality thesis has been debated vigorously in the scientific literature, mostly amongst philosophers of physics, who have come up with proposals in an effort to refute it. But they have been met each time by other who have exposed their errors. Most often it has meant that somewhere in their proposal they have implicitly assumed the isotropic speed the light, hence the standard synchronisation method.

This debate has ensued most significantly since the 1950’s, yet no one has produced a method to either measure the one-way speed of light or to refute the conventionality thesis. Of course, by measuring the one-way speed of light one would refute the conventionality thesis.

We may conclude that the conventionality thesis is just that, the subject of a convention, a choice, and not something empirical. That means it is not discoverable by science. The universe cannot tell us the one-way speed of light and as a result it is impossible to synchronise distant clocks with local clocks apart by choice of a convention.

The ramifications of this are significant. And no choice of any particular clock synchrony convention, hence the one-way speed of light, can have any bearing on any physics. The physics is the same under any chosen synchrony convention.  Thus there can be no experiment that can refute the conventionality thesis.

What has this to do with biblical creation?

Just about everything! I believe it comes down to the same issues addressed by biblical creationists in regards to epistemology and the origin, not only of the Earth and solar system and life on Earth, but also of the whole universe.

How do we know what happened in the past? Can we directly see past events while they are occurring? No, we cannot! We live in the present. The past is gone forever. We cannot see dinosaurs allegedly evolving into birds. We cannot see anything living in the past. Dinosaur fossils are real enough but they are non-living stone. Though in some cases some biological material has been discovered, which makes you think that they could not be 65 million years old.

However my main point here is we have no access to the past. No scientific experiment can tell us how old the rocks or fossils are. No time machines exist! We cannot see the solar system and Earth while it is being formed. We can only know what happened from the history book God has given us.

I used to believe that it was different in the cosmos but the same goes for the stars and galaxies. I had incorrectly been assuming an isotropic, one-way speed of light, which meant the universe must be ten billion years old,[6] even though I believed the Earth/solar system was only about 6000 years old. I now see how mistaken I was.

We cannot know the one-way speed of the light from the distant cosmos, or from anywhere else. So we cannot know by scientific measurement how long light from the distant galaxies has taken to reach Earth. It is just not possible by scientific measurement.

Now this may surprise you. But since the one-way speed of light is conventional, it can be chosen as any value between ½c and infinity. The limits of this range are imposed by causality. The essential requirement though is that any round-trip value of the speed of light must average to c. This is the value that has always been measured. That is, it is only possible to measure the speed of light by reflecting it back from a mirror or by using another device which responds and sends the signal back. This is then a two-way speed measurement which is what all measurements have ever been (Fig. 1).

Figure 1: Typical method for the measurement of the round-trip speed of light.

Thus we can choose an infinite speed for light coming to Earth from the cosmos, i.e. one way. This means all clocks in the universe are chosen as showing the same time as Earth clocks.  So if only about 6000 years have passed for Earth clocks since Creation then the same is true for cosmic clocks. This is a matter of convention, and not a matter of empirical measurement.

The argument can be made that the language of the Bible supports the choice of such a convention.[7] We see everything in the universe NOW, not in the past. This claim is not refutable as it is not subject to empirical measurement.

The conventionality thesis has over one hundred years of support in the scientific literature.  It has been hotly debated but it has never been refuted. Even at the present time the debate continues with proposals to measure the one-way speed of light but none have successfully devised a method. In the conclusion of his book Max Jammer writes:1

“… the question of whether the thesis of the conventionality of the concept of distant simultaneity is correct has not yet reached a final or generally accepted satisfactory solution”. (p.300)

Maybe it will never be finally settled.[8] Possibly it is one of those mysteries of the universe that we are never going to get a definitive answer to. In the same way we will never be able to measure the one-way speed of light, as unsatisfactory as that may seem.

The scientific and philosophical arguments have not be in defence of the biblical age of the universe. They have been made solely on the basis of the physics; epistemology and ontology. However, based on our understanding of the scriptures, the universe is about 6000 years old. And biblical creationists are free to choose a convention that agrees with this biblical timescale. The conventionality thesis supports the young universe biblical creation interpretation of the history in Genesis.

References and notes 

[1] Jammer, M., Concepts of Simultaneity, Johns Hopkins University Press, 2006. The book is expensive so I would suggest you find it in a library if you plan on reading it.

[2] Not accelerating, i.e. moving at a constant speed relative to one another.

[3] Quoted on introductory page of Jammer, M., Concepts of Simultaneity, Johns Hopkins University Press, 2006.  

[4] The problem is even bigger because to know the distance you also need to know the time on the distant clock. Ultimately a distance measurement is a measurement of time on a pair of synchronised clocks at each end of the distance to be measured.

[5] Also called the Einstein Synchrony Convention (ESC)

[6] I sought for relativistic solutions that used time dilation to solve the problem.

[7] Genesis 1:14,15; Psalm 33:9; 2 Peter 3:7.

[8] The debate is even much more uncertain when it comes to general relativity, where gravity is added. It is even more uncertain in quantum mechanics where any concept of relative clock synchrony seems to be elusive.  Newtonian absolute simultaneity seems to be more compatible with that theory, which includes faster-than-light action at a distance from the collapse of the wavefunction with entangled pairs of quantum states.

Recommended Reading

16 thoughts on “The conventionality thesis on the synchronisation of clocks separated by distance

  1. Dr. Hartnett, this is genius! CMI needs to read this and hear this out. Honestly, after I read that email letter of Jason Lisle from which you pasted here at this forum, I found myself not being sympathetic to Humphreys’ Gravitational Time Dilation model. I appreciate his work, but your critiques were correct, there’s just not enough dilation for billions of years to have occur on cosmic clocks to give us the measured secular age when this model is tested. Maybe it works on farther galaxies, but it breaks down closer to us. I think the Anisotropic Synchrony Convention model does a better job, it’s simpler (I’m no physicist, I’m studying chemistry, so me saying it’s simpler it’s for you physicists’ benefits, lol) and consistent with the data. Perhaps Humphreys may have some points about GTD in the future, but ultimately, the ASC will quickly incorporate that as part of the model and thus the ASC, in the end, will be the best solution to the YEC starlight problem, or possibly even the secular’s starlight problem (though I’m not sure how’d they incorporate ASC to their horizon problem).

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    • I don’t agree. I sent my response to the article, but it hasn’t posted yet. Dr. Hartnett states the following:
      “The conventionality thesis has over one hundred years of support in the scientific literature. It has been hotly debated but it has never been refuted. Even at the present time the debate continues with proposals to measure the one-way speed of light but none have successfully devised a method. ”
      However, I think it is easily refutable with simple experimentation. If the one-way speed of light is infinite then radio communications would be nearly instantaneous at long distances. The sender of a transmission would send at infinite speed and the receiver responding would be replying back at infinite speed. The only delays would be in processing the signal both ways. I don’t think this can be refuted, because each transmission is a one-way trip and if both are infinite speed then there is no delay. However, we see that there are quite measurable delays in radio signal communication that cannot be accounted for by merely attributing them to the processing delays.
      I don’t see how this can be refuted.

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      • There is no suggestion anywhere that the one-way speed is infinite in both directions. The round trip average must always be the canonical speed c. I think you need to read more carefully what has already been written on this subject because what you have written here is a strawman.

        I have listed many recommended articles. Some cover this in great detail.

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      • John, no I have not written a strawman. But you failed to respond to the core argument, which is that the one-way speed of light can be known from experimentation to not be infinite. And the article suggests that this is possible. In fact, only if that is possible can the conclusions drawn here be valid. You are trying to skirt the obvious, which is that ASC cannot be correct and can indeed be experimentally disproven.

        If I misunderstand something here, I agree that might be possible, but that doesn’t change the fact that you can indeed come very close to measuring the one way speed of light by the method I suggested. Are you saying that I’m wrong about that? If so, please provide a refutation. If hat’s not really what ASC is all about then what is it all about? It can’t possibly solve the distant starlight problem if the one-way speed is not infinite or nearly infinite.

        I think it’s a bit dishonest to hide behind ASC as if it’s irrefutable and feel all cozy about that idea when, in fact, it is easily dismissed.

        Your article did, in fact, state the following:

        “Thus we can choose an infinite speed for light coming to Earth from the cosmos, i.e. one way.”

        And I argue that, no, you can’t just choose whatever you feel like. Because we can prove that the one-way speed is not infinite. That’s not a strawman.

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      • If you read the other articles you will discover this has been discussed at great length. No experiment has ever measured the one-way speed of light. This is in fact what the conventionality thesis is about. As mentioned in the above article physicists and philosophers of physics have for more than a century tried to refute the conventionality thesis by either proposing thought experiments or actual empirical measurements but all have been shown to be flawed. Usually the author implicitly assumes the isotropy of the speed of the one-way light which is the standard Einstein convention.

        I am not hiding from anything. At no time have I suggested an isotropic infinite one-way speed of light. All non-standard synchrony conventions means that the one-way speed is anisotropic. One particular choice the ASC stipulates an incoming infinite speed but that means on the outgoing, return journey, the speed must be at c/2. That means the average round-trip speed is c, which agrees with all measurements. The ASC is by definition a convention, which means that the one-way speed of light is chosen not measured. This is true for al standard and non-standard synchrony conventions.

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      • I too encountered this apparent paradox while mulling over the ramifications of the ASC. It presents what appears to be a paradox, but there may very well be an explanation for it within the ASC (much as the Twin Paradox disappears when GR is considered in addition to just SR). In that case, it would benefit everyone for the solution to be made widely known, so in that spirit of scientific investigation I will attempt to formulate the (apparent) problem as best I can for reply:

        To the best of my understanding (and the problem may very well lie here!), the ASC admits an anisotropy to the speed of light, but one which is observer-dependent. To an observer, all in-bound light travels infinitely fast, while all out-bound light travels at c/2, thus making the round-trip travel speed average out to c. The question then arises, what happens when you introduce a second observer, and they communicate with each other via light signals?

        Suppose the following gedanken experiment: we have Alice, standing at the origin of a coordinate system. Bob stands a distance D away in the positive-x direction. Alice, at time 0, sends a pulse of light towards Bob, who, upon receiving the signal, immediately responds by sending a signal back (we’ll assume the time it takes for Bob to react is negligible).

        Under the ESC, Alice emits a pulse at t0, the light takes a time D/c to travel to Bob, who sends a pulse back. The light takes another time D/c to return, and the total time taken is 2(D/c) as measured by Alice.

        Now…from my admittedly not-as-informed knowledge of the ESC, under its conditions, Alice emits a pulse of light at t0. However, Bob, being an observer, sees this instantly, because the light travels infinitely fast towards him. He then responds, and Alice, also being an observer, sees the light instantly as well due to it traveling infinitely fast towards her, for a total time taken of 0. This presents what appears (and I stress “appears”) to be a paradox, as the times measured would be different depending on the simultaneity convention used to describe the setup.

        Though while writing this up it occurs to me that perhaps the resolution, as in the Twin Paradox, exists in the switching of observer viewpoints. Were Bob to be replaced by a mirror, then under the ASC Alice would send out a light pulse that would travel at c/2 outwards and infinitely quickly back, giving a time of 2(D/c), same as the ESC.

        Perhaps the solutions lies in Bob simply having a different “now,” a conclusion already arrived at from SR: from Alice’s perspective the light travels outwards for a time 2(D/c) and she gets her answer back from Bob instantly at the end, while from Bob’s perspective he sends his reply instantly at the beginning but it takes a time 2(D/c) to get back to Alice. They’d disagree on when Bob sends his reply—for Alice, it’s at t=2(D/c), but for Bob it’s at t=0 (and I’ve just realized that assumes an observer-independent clock, something prohibited by relativity!)—but they’d both agree on when Alice receives it, at t=2(D/c). Hmm. Relativity incorporating an observer-dependent anisotropic speed of light really messes with one’s head; I’ll need to mull this over some more. Please let me know if that sounds like an acceptable resolution of the paradox, or if there’s something else I’m missing.

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      • Daniel, the resolution is that you must look at it from only one observer reference system. There is no universal reference system in Special Relativity. Each observer can choose whatever synchrony convention she likes, whether the standard convention (which Einstein chose) with isotropic one-way speed of light, or, any one of many non-standard anisotropic conventions, like the ASC. But you must consider only what the observer can measure. You must only use the equations of SR for the particular conventon you choose. You can’t mix them up. If you assume anything from the other observer perspective, while making measurements at your position, you are begging the question.

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  2. John, thanks for the reply. I reasoned myself into that position by writing out my argument in detail, but it’s good to get independent confirmation. Cheers.

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    • You stated:

      “One particular choice the ASC stipulates an incoming infinite speed but that means on the outgoing, return journey, the speed must be at c/2.”

      Again you stipulate that the incoming speed is infinite. And That can be disproven with the experiment I propose. Because we can measure the processing time of a radio signal and thus know that neither way is infinite. You still failed to address my experiment.

      And the whole point is that ASC cannot be a solution to the distant starlight problem. Yes, you are hiding behind a convention you feel is not disprovable. We can’t just choose what we feel is right so it fits our model. It has to agree with reality. I think most of the work of creation scientists is stellar and worthy or support, but this ASC is absolute nonsense.

      Please provide the refutation to my experiment, which would disprove the one-way speed of light being infinite. Since you continue to fail to do this, I can only conclude that you have no refutation. And please do not just say read all the literature about it. My experiment is super simple that even a child could understand it so the refutation should be quite easy to explain.

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      • Tom,

        Sending and receiving a radio signal is a two-way speed of light experiment. If the observer at A sends a radio signal to a receiver on Pluto, call him B, then A gets the returned signal back from B about 8 to 14 hours later depending on where Pluto is respect to the Earth. This is a two-way round-trip measurement. The average speed is c.

        To measure the one-way speed, say from B to A, Pluto to Earth, you would need to have synchronised clocks at A and B so that when B sends his signal (which can contain the information when he sent it) you will know how many seconds have elapsed since he sent it. But how can you synchronise clocks separated by a distance (i.e. not co-located)? To do so you have to sent a radio (or light) signal from A to B to synchronise clock B with clock A, so that B can send his radio signal back to A, and you get a measure of the elapsed time. But to do the synchronisation leg A to B you have to assume the one-way speed of light, otherwise B cannot know what time his clock was reading when A’s clock showed a certain time at its transmission. You now no longer have a one-way measurement B to A because you have assumed the one-way speed A to B.

        You wrote:

        If the one-way speed of light is infinite then radio communications would be nearly instantaneous at long distances. The sender of a transmission would send at infinite speed and the receiver responding would be replying back at infinite speed. The only delays would be in processing the signal both ways. I don’t think this can be refuted, because each transmission is a one-way trip and if both are infinite speed then there is no delay. However, we see that there are quite measurable delays in radio signal communication that cannot be accounted for by merely attributing them to the processing delays.
        I don’t see how this can be refuted.

        My example of sending and receiving a radio transmission signal between Earth and Pluto explains what you are arguing. You would be saying it proves the speeds are not infinite. But the non-standard synchrony convention we call ASC (anisotropic synchrony convention) stipulates that the incoming speed with respect to observers on Earth (A) is infinite and the outgoing signal with respect to observers on Earth (A) is c/2, a finite number and NOT infinite as you allege. Therefore the delay A to B and then back B to A is exactly as measured. We do not expected the signal to be instantaneously returned. The outgoing signal takes twice as long as under the assumption of the standard ESC convention, but is returned instantly. So the total round trip-time is the same as under the standard convention. This is the strawman I am referring to. No advocate of the ASC is saying the speed is infinite AND isotropic, but that it is anisotropic and the average speed over the round-trip is the speed c, as experimentally measured and as radio communications testify to.

        You cannot swap over to the reference frame of B on Pluto and say it is incoming infinite speed to him and then say the speed is infinite in both directions. That means you are imposing a universal reference frame on both observers. But they experience a different now, if you like to think of it that way. Only co-located observers experience the same now. This basic to Special Relativity. There is no universal frame of reference. Einstein developed his SR theory on that basis.

        Maybe you feel strongly that the speed of light is isotropic but it cannot be determined empirically to be the case. The physics of Einstein, which includes this problem of the synchronisation of clocks separated by a distance, is intrinsic to this issue.

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      • “Sending and receiving a radio signal is a two-way speed of light experiment. If the observer at A sends a radio signal to a receiver on Pluto, call him B, then A gets the returned signal back from B about 8 to 14 hours later depending on where Pluto is respect to the Earth. This is a two-way round-trip measurement. The average speed is c. ”

        No, that is not correct. You are missing one important point that I’ve made and you seem to keep missing. This radio experiment can prove that the one-way speed of light cannot be infinite even if it cannot precisely measure the one-way speed of light. Synchronizing clocks has nothing to do with it because you can prove this by only using observer A’s clock. Why? Because we know some very basic things that can be proven experimentally such as how long it takes to process an incoming signal and how long it takes to send an outgoing signal. So, knowing that, if the one-way speed of light was infinite then the only delay we would experience is the time to process incoming/outgoing signals. It’s just as simple as that.

        Does what I’m saying make more sense now? I really find it hard to believe that you are having trouble grasping this. In your example you express a delay of 8-14 hours. So, you have to believe that the processing of the incoming/outgoing signal takes that long in order to account for the delay. It’s not a two-way experiment because each signal, send and receiver, is only sending one way.

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      • Tom, I am stating an experimental fact that the time to get a transponded (returned) signal from a spacecraft near Pluto is 8 to 14 hours. That is a fact, that we cannot argue over. Though it is often stated as 4 to 7 hours to get a signal from Pluto but that assumes the one-way speed of light is finite, hence the standard synchrony convention is assumed. The delay in the returned signal is due to the non-infinite speed of light averaged over the return journey. If you believe that radio communications measure the one-way speed of light how is that done without synchronising clocks at A and B?

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      • John, are you serious? I believe I’ve explained it very well.

        “I am stating an experimental fact that the time to get a transponded (returned) signal from a spacecraft near Pluto is 8 to 14 hours. That is a fact, that we cannot argue over. ”
        Exactly, and that’s a very good point to consider when we think about this because it is the very reason we can prove that the one-way speed is not infinite as I will attempt to explain again.

        “The delay in the returned signal is due to the non-infinite speed of light averaged over the return journey.”
        I don’t think this is saying what you think. And you are not thinking this through clearly. The return signal is a one-way trip as it originates from Pluto in this case. It is not the same light that was sent to Pluto from observer A. So, what are you averaging this one-way speed over?

        “If you believe that radio communications measure the one-way speed of light how is that done without synchronising clocks at A and B?”
        I don’t just believe it. It is a simple fact based on experimental evidence. I’ll go through it step by step. First, we must know beforehand what the time is required to transmit an outgoing signal and what the time is to process an incoming signal. However, for sake of simplicity we can consider these negligible since we know they cannot be 8-14 hours, but rather milliseconds or even microseconds. Now, here is how it goes.

        1. Observer A transmits a signal to Observer B and records the time at which the signal was sent.
        2. Observer B receives the signal, processes it, and transmits his own signal to Observer A. The only delay being the time to process the incoming signal and the time to transmit the outgoing signal. Which, as we’ve said, can be considered negligible in this case.
        3. Observer A receives the signal from Observer B and records the time at which he receives it. Note that only Observer’s A clock was ever used. We don’t care what Observer B’s clock says. He doesn’t even have to have a clock.

        Both the signal from A to B and the signal from B to A are independent one-way signals. If the one-way speed of light was infinite then the time it takes for A to receive the transmission back from B would be nearly instantaneous. Since it is not and we know that it is a fact that the signal takes 8-14 hours, we can therefore say that the one-way speed of light cannot be infinite.

        Now, I don’t think I could say it any more clearly than that. Does it make sense now?

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      • Tom

        John, are you serious?

        Yes I am.

        I believe I’ve explained it very well.

        You have this time and you describe a two-way speed of light measurement.

        “I am stating an experimental fact that the time to get a transponded (returned) signal from a spacecraft near Pluto is 8 to 14 hours. That is a fact, that we cannot argue over. ”
        Exactly, and that’s a very good point to consider when we think about this because it is the very reason we can prove that the one-way speed is not infinite as I will attempt to explain again.

        So you agree with the experimental time delay measure to send and receive the returned signal. It involves both outward and inward travel times whatever they may be but we can only measure the round-trip time by our clock at A on Earth.

        “The delay in the returned signal is due to the non-infinite speed of light averaged over the return journey.”
        I don’t think this is saying what you think. And you are not thinking this through clearly. The return signal is a one-way trip as it originates from Pluto in this case.

        No way it is. It is the total time from sending the signal from Houston to receiving the signal back at Houston from the spacecraft near Pluto. It is not one-way but round-trip.

        It is not the same light that was sent to Pluto from observer A. So, what are you averaging this one-way speed over?

        It may not be the same photons but it could as well be as if we had a mirror on the spacecraft reflecting the signal back. But because we have only a clock at A, which you agree with later on, we can only measure the total time from sending to receiving.

        “If you believe that radio communications measure the one-way speed of light how is that done without synchronising clocks at A and B?”
        I don’t just believe it. It is a simple fact based on experimental evidence. I’ll go through it step by step. First, we must know beforehand what the time is required to transmit an outgoing signal

        Do I assume you mean the one-way travel time to Pluto (B) from Earth (A)? If you say you know beforehand, then that is just begging the question. No travel time one-way can be assumed else you are assuming the standard convention, or at least, you are assuming what you are trying to prove.

        and what the time is to process an incoming signal. However, for sake of simplicity we can consider these negligible since we know they cannot be 8-14 hours, but rather milliseconds or even microseconds.

        I agree assume the electronics delay is negligible but not the travel time from A to B.

        Now, here is how it goes.

        1. Observer A transmits a signal to Observer B and records the time at which the signal was sent.
        2. Observer B receives the signal, processes it, and transmits his own signal to Observer A. The only delay being the time to process the incoming signal and the time to transmit the outgoing signal. Which, as we’ve said, can be considered negligible in this case.

        How does B know what time it was sent? And by whose clock? But the time of travel for the outgoing signal cannot be assumed to be negligible. You cannot assume anything you don’t/can’t measure.

        3. Observer A receives the signal from Observer B and records the time at which he receives it. Note that only Observer’s A clock was ever used. We don’t care what Observer B’s clock says. He doesn’t even have to have a clock.

        Ok, I agree. We only need observer A’s clock. We have then the total time from when the signal was sent to when the signal was received. This is the two-way round-trip travel time of the signal, sent out and the signal received back. Round-trip not one-way.

        Both the signal from A to B and the signal from B to A are independent one-way signals.

        That is not correct. They are essentially the same signal, since the spacecraft B at Pluto transponds to the receiving of signal from A.

        If the one-way speed of light was infinite then the time it takes for A to receive the transmission back from B would be nearly instantaneous.

        That may well be the case, but we cannot measure that, nor can A know that. She only can know the total time from when she sent the outgoing signal as her clock A indicates.

        Since it is not and we know that it is a fact that the signal takes 8-14 hours, we can therefore say that the one-way speed of light cannot be infinite.

        Here is the strawman. When you say “since it is not” it is an assumption. A statement without empirical measurement. The one-way speed has never been measured. The returned round-trip signal takes 8-14 hours not the one-way signal. That is the only known fact here.

        Now, I don’t think I could say it any more clearly than that. Does it make sense now?

        Yes, what you are describing is a two-way round-trip speed of light measurement, which is all any radio communications ever gives us.
        Please read some of the Recommended Reading especial Update on the ASC model and the one-way speed of light.

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      • No, John, you are again wrong on a lot of points. I see what you are “trying” to say, but it’s not correct. I will refute your points now.

        “You have this time and you describe a two-way speed of light measurement.”
        Technically, this is true, but then again it is more technically two one-way speed of light measurements. And that’s the part you refuse to acknowledge. As I said before, you are hiding behind a convenient fact, but one that can be disproven if you will just broaden your thinking a bit.

        “So you agree with the experimental time delay measure to send and receive the returned signal. It involves both outward and inward travel times whatever they may be but we can only measure the round-trip time by our clock at A on Earth.”
        I agree that the round-trip time is an accurate, experimental fact. The 8-14 hours as you say.

        …………………………. (the rest deleted}

        [Editor: The rest of the comment was deleted as it adds nothing but goes around in circles. Tom agrees with the premise that it is a two-way measurement and that ends the debate.]

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      • Tom,

        You wrote:

        “You have this time and you describe a two-way speed of light measurement.”
        Technically, this is true, but then again it is more technically two one-way speed of light measurements. And that’s the part you refuse to acknowledge.

        This the whole point. You agree. It does not matter if it is two one-way signals. The only thing that can be measured is the round-trip time, say from Houston and back again to Houston. It is not two one-way measurements because the clocks at A and B are not synchronised. As soon as you assume they are synchronised you have assumed a certain synchrony convention and it is not any more a one-way speed of light measurement. This fact you cannot understand. This is the subject of the conventionality thesis, which has been debated by philosophers and physicists for more than 110 years. They were much better minds than us. And no one has ever come up with a scheme to measure the one-way speed of light without assuming a synchrony convention (which means you a priori assume a one-way speed of light).

        As I said before, you are hiding behind a convenient fact, but one that can be disproven if you will just broaden your thinking a bit.

        I am not hiding behind anything. Your “convenient fact” cannot be disproven. It is an empirical measured fact. One does not disprove such a fact but one tries to explain the theory in light of the fact.

        I do not like your accusative tone; I have overlooked it to this point, but I am now tired of it. This exchange is now ended. Read some of the Recommended Reading and/or the published papers on the subject.

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