To synchronize two clocks that are stationary relative to each other, we send a time stamped light (or radio) signal from the master clock to the secondary clock. Knowing the distance between the two clocks and assuming that light travels at exactly c between the two clocks, the propagation delay is calculated. This delay is then added to the time stamp at the moment that the signal is received and the secondary clock is set accordingly.
As an example, say we have two spaceships at rest relative to each other, separated by exactly one light second in free space. At time 12:00:00 the mother ships sends a light signal time stamped 12:00:00 to the sister ship. The sister ship receives the signal, adds 1 second to the time stamp and sets its clock to 12:00:01.
If we now want to measure the one way speed of light by sending a signal between the two ships, we must obviously get c, what else? My question: is there a way to measure the one-way speed of light without needing two clocks that were synchronized by assuming the speed of light to be c?
SL: Your Aerospace Watchdog
If our reporting has informed or inspired you, please consider making a donation. Every contribution, no matter the size, empowers us to continue delivering accurate, engaging, and trustworthy science and medical news. Independent journalism requires time, effort, and resources—your support ensures we can keep uncovering the stories that matter most to you.
Join us in making knowledge accessible and impactful. Thank you for standing with us!
Christopher:
I thought I had already pointed out that light does not define geodesics. Light travels along certain kinds of geodesics (called “null geodesics”, since they are geodesics such that along any finite “distance” separation zero proper time elapses).
All free moving test particles travel along their own geodesics (called “time-like” since their local spacetime intervals advance more in “time” than in “space”, using the local “metric” [one may also formulate this in terms of an inner product with any physical particle trajectory: think about asking whether some trajectory goes more North-South vs. East-West]).
As to whether the “explanation” of a “trained physicist” is any more “real” than what you appear to propose as that of “simple minded observers”, in actuality boils down to the efficacy of the total “story” (how well does the explanation hold up to all previous experiments and observations, as well as its predictive power for any possible future experiment[s]), and, if said story passes that test, whether it pases all subsequent attempts at falsification (testing against experiments designed to distinguish between possible explanations).
If an explanation predicts exactly the same measurements for all possible experiments/observations as another, then we typically refer to such an alternate explanation as an “interpretation”, since there is nothing falsifiable about it vs. the other. Then the question of which “interpretation” is “preferred” may be simply a matter of “taste”, unless there is something “simpler” about one vs. the other (Occam’s razor).
Such is the nature of the scientific method. One is always free to devise alternative “explanations”. The determination, then, is in comparison with observations and experiments, not simply persuasion (with the possible exception of “interpretations”, of course). Just recognize that so long as one is going up against an explanation that has passed all challenges (experiments and observations) to date is a high bar to hit. (Certainly not impossible, since we have yet to reconcile the arguably two most successful theories to date: General Relativity and Quantum Field Theory.)
Does this help?
David
EDIT: I’m sorry I didn’t read Burt’s good reply before writing this. :-}
Thank you Burt. I will think about this. Christopher
Hi Christopher, you asked:
“I think my problem largely stems from this. I thought that acceleration does not alter clock function and that an acceleration field is equivalent to a gravitational one.”
If you read chapter 4 of Relativity 4 Engineers carefully, you will find that clock rates depend on the “depth” of the gravitational well and gravitational acceleration on the “slope” of the gravitational well (at least for clocks static in the field). In more technical terms, clock rates depend on the intensity of the field and acceleration on the gradient of the field.
As an example, at Earth’s center the gravitational well is “deepest”, while there is zero “slope” in the field; hence clocks run the slowest there (compared to distant clocks), but there is no gravitational acceleration there.
“Is the stationary clock of your statement stationary because it has reached the top of its free fall fountain trajectory, or because it is held still without free fall by a table, with legs on the surface of the sun (patent pending) or legs on the floor of a suitably driven rocketship, that it is sitting on?”
It does not matter; for the static formulas to be valid, it must just not move relative to the gravitational field.
“Are the “slower … ‘tick'” and the “slower … run” of the local clock near the sun intended to refer only to an appearance to a distant observer due to the use of light signals, or is there some sense in which the “slower … ‘tick'” is physically real?”
The “slower … ‘tick'” is real, both from a distant observer’s point of view and also from a direct comparison point of view. If two atomic clocks are synchronized on the surface of Earth and one is then slowly taken to the top of a mountain, left there for some time and slowly brought down again, the surface clock will be behind the mountain clock. Such an experiment has been done.
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
Hi Burt.
You are very kind to help me here. Thank you. It will take me a little time to get Will’s book.
Following your pointer, I read on page 71 “It tells us how much slower stationary clocks near a gravitational source ‘tick’ than clocks far away from any such source.” On page 120 I read “The closer a clock is to a gravity generating mass, the slower it runs”.
I think my problem largely stems from this. I thought that acceleration does not alter clock function and that an acceleration field is equivalent to a gravitational one.
This seems to make for an inconsistency. Perhaps it doesn’t.
The official definition is that local time is what the local official clock reads. By official definition the atomic clock doesn’t have a “slower … ‘tick'” or a “slower … run”, at least locally, as far as I can see.
Is the stationary clock of your statement stationary because it has reached the top of its free fall fountain trajectory, or because it is held still without free fall by a table, with legs on the surface of the sun (patent pending) or legs on the floor of a suitably driven rocketship, that it is sitting on? Does that make a difference? As far as I can work out, it is the proper teaching that it makes no difference. Perhaps there is no such thing as an atomic clock that for relevant features ‘sits on a table’, because the electrically neutral atoms are always in free fall at the relevant time? (All that matters for this is where is the clock in relation to the observer and the sun, provided they are all three stationary with respect to one another. The local strength of the gravitational field makes no difference. Then it comes down to how light propagates between local clock and distant observer. And that is where I started this line of questions.)
Are the “slower … ‘tick'” and the “slower … run” of the local clock near the sun intended to refer only to an appearance to a distant observer due to the use of light signals, or is there some sense in which the “slower … ‘tick'” is physically real? Perhaps this question is meaningless or nonsensical? If it is meaningless or nonsensical, is there some reasonable question that it is touching on?
These questions are naive (and perhaps, of course I fear, even irritating to you), but for me, a mathematical formula has physical meaning only inasmuch as what I can find in it in a plain English rendition. Doubtless you have come across a million students with a similar problem. I greatly value your patience.
Sincerely,
Christopher
Hi Fred, you wrote:
“Perhaps the reason this alternate approach has not entered the classroom, even at the graduate level, is that it has no new physics, just a different formulation.”
That, plus the fact that it is not nearly as user-friendly as Einstein’s synchronization scheme. I agree that scientists should perhaps be made aware of this “conventionality” at undergrad level, so that they do not become too ‘dogmatic’ about the conventional approach.
Scruffy is certainly challenging the ‘dogma’!
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
PS: In a sense, the whole “speed of light is constant and isotropic in all inertial frames” thing is conventional today. As Stephen Hawking said (I think it is in “A Brief History of Time”, here paraphrased from memory): “Since we use light to measure distance, it is no wonder that we always measure light to travel at the same speed”.
Hi Christopher, you wrote:
“But sad to say I have not answered my own question, which was “Does this make sense for a simple minded observer?””
The best “simple” answer to what you asked I found in Clifford M. Will’s popular book “Was Einstein Right?“, chapter 6.
A slightly more technical description of the so-called Shapiro time delay can be read in Relativity 4 Engineers, section 8.2, page 118.
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
PS: Readers not owning the eBook can download the chapter free from: http://www.einsteins-theory-of-relativity-4engineers.com/tests-of-relativity.html
Thank you Burt for this helpful reply to my question about the one-way speed of light near a heavy thing.
Yes, I take the point that there are countless geodesics other than null ones. Thank you for pointing this out. Light indicates (a more fitting word than my too-loose ‘defines’) some but not all geodesics.
Yes, thank you for pointing out that the local observer near the sun will see a higher frequency. A distant observer can assess the wavelength of light near the sun through the effect of a diffraction grating placed in the light’s path near the sun. In order to infer its frequency, the distant observer has to have his view about length intervals, clock performance, and light speed, near the sun.
The local observer will know the numerical value of the frequency of the light, but will he further know that it has been “shifted”? If so, how will he know? Will he know it from local experiments?
For a trained physicist the question I asked in my previous post, about the simple minded observer, is nonsense, because the trained physicist knows that space-time is curved near the sun.
But sad to say I have not answered my own question, which was “Does this make sense for a simple minded observer?”
Regards,
Christopher
Burt (and SL and readers),
I don’t have the paper, and I surely was one who was taught the conventional approach.
Perhaps the reason this alternate approach has not entered the classroom, even at the graduate level, is that it has no new physics, just a different formulation.
However, I wish it had been included in my undergraduate course, even as an aside to point out another way of understanding that some students might prefer.
Being able to look at the universe (or in this case, spacetime) from multiple perspectives is conducive to innovation.
In this case, there are many innovative people around who seek out papers like the one you describe, Burt. The fact that none of them has produced innovative physics, despite their innovative descriptions of that physics, leaves me content with my perspective for now. (It may be an age thing. I haven’t grappled with these issues as an undergrad or grad student would in 40+ years.)
SL asks the kind of questions that gets those innovative juices flowing. It is reasonable to expect that very few of those questions will lead to new physics, but we would not be true to our science if we did not encourage him to keep asking.
Sometimes you catch a wild goose and get a banquet as a result. :)
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Hi SL, surely an interesting article that you referenced. Intrigued by that article, I found the following abstract from the European Journal of Physics: http://gita.grainger.uiuc.edu/iop/JournalArticle.asp?issn=0143-0807&volume=13&issue=4&artnum=004&lang=
Volume: 13, Issue: 4, Date: July 1992 , Page: 170.
“Although the importance of clock synchronization for relativity is discussed from time to time in the educational literature, the fact that different synchronization conventions imply different coordinizations of spacetime with ensuing changes of the form of possibly all coordinate-dependent quantities, has neither entered textbooks nor undergraduate physics education. As a consequence, there is a widespread belief among students that the familiar form of coordinate-dependent quantities like the measured velocity of light, the Lorentz transformation between two observers, ‘addition of velocities’, ‘time dilation’, ‘length contraction’, ‘E=mc^2 gamma’, which they assume under the standard clock synchronization, is relatively proper. In order to demonstrate that this is by no means so, the paper studies the consequences of a non-standard synchronization, and it is shown that drastic changes in the appearance of all these quantities are thus induced. For example, the phrases ‘moving clocks go slow’, and ‘simultaneity is relative’, which are usually considered as intrinsic features of relativity, turn out to be no longer true, whereas all coordinate-independent quantities remain of course indifferent to such a change in coordinization. Although Einstein’s standard convention of clock synchronization enjoys distinct advantages over the ‘everyday’ method, the message clearly conveyed is that in the teaching of elementary relativity much more stress should be laid on the intrinsic (coordinate-independent) features of spacetime.”
Unfortunately I do not have access to the full paper, but it seems to support your view to an extent. Maybe yourself or Fred can help out with a copy?
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
I haven’t exactly been feeling lonely here, but I have been trying to see what is making SL uncomfortable.
That’s important, because I always think that discomfort is an important first step to learning.
Anyway, I guess the latest posting shows where SL is uncomfortable, but it still doesn’t give me any discomfort.
Perhaps I should feel uncomfortable about not getting SL’s discomfort. :)
If there is indeed something new to discover here, he is in a better position than I to seize on it. However, if the theory is correct, I am not wasting my time on a question that will ultimately lead SL on a wild goose chase.
We never know for sure about that, do we?
For a book review about another area of physics that may be a thirty-year wild goose chase, click the link. As you will see, I’m not saying that it is surely a wild-goose chase, though the honking sounds you hear may not be a good sign.
Fun, as usual, SL!
Fred
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Fred wrote:
If Fred finds himself a little lonely in this argument, here is some consolation. I stumbled upon this link to the Stanford Encyclopedia of Philosophy that argues both sides:
http://www.science.uva.nl/~seop/entries/spacetime-convensimul/
It’s quite a long article, but here’s the closing paragraph:
My headline: “One Way Speed of Light a Convention?” boils down to the same issue as simultaneity.
SL: Your Aerospace Watchdog
Christopher asked: “But what about [isotropy of spacetime] near heavy objects, such as the sun?”
Spacetime near massive objects is Lorentz invariant only on infinitesimal (small) regions.
“Trained physicists know of light defining geodesics, …”
Light does not “define geodesics”; light moves along so-called “null-geodesics” in spacetime. All free-falling objects move along spacetime geodesics that depend on relative speeds.
I think you have answered your own questions more or less correctly in your final statement:
“Of course this story is nonsense for a trained physicist, who knows that light defines geodesics and just increases its frequency as apparent to a distant observer, because time and space are different near the sun so that, to the distant observer, ideal clocks, near the sun, seem to run slowly and ideal measuring rods, near the sun, seem shorter.”
Just be aware that, as I wrote above, light does nor define geodesics in the general sense of the word. Also, a local observer, near the Sun, will observe incoming light rays as blue shifted, so it is not true “… that light defines geodesics and just increases its frequency as apparent to a distant observer,…”
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
Scruffy’s (erroneous) “example”:
“It could e.g. be that light goes at c+v in the one direction and c-v in the opposite direction and your test will not pick it up.”
and Fred’s (correct) reply:
“That gets us back to the classic attempts to measure the differences in the speed of light parallel and perpendicular to the Earth’s motion, which, as you know, have consistently produced null results.”
does not solve their issue.
Fred is right that SL’s c+v and c-v does not give the correct total time that we measure for light. However, Fred’s reference to the two-way isotropy of the speed of light is not evidence of the one-way isotropy that SL puzzles about. SL is right that the measured one-way speed of light depends on your method of synchronization of clocks. It so happens that Einstein’s method of clock synchronization is the simplest and gives coherent results in all cases, making our lives simpler. It is however not the only method.
Consider a rocket, L meters long, with a nose clock and a tail clock that was Einstein-synchronized while the rocket was coasting inertially in free space. Now ignite the propulsion and accelerate the rocket by a given “delta_v” and then let it coast again. Do not resynchronize the nose and tail clocks again (this is an alternative synchronization scheme, which one could call ‘Galilean synchronization’).
SL and Fred now both verify that the clocks were not damaged by the acceleration and they agree that the clocks are now out of sync in their displayed time, as judged by Einstein’s method (they will find the nose clock to be delta_v times L/c^2 seconds ahead of the tail clock).
In this new inertial frame, using these two clocks, Fred and SL perform a one-way speed of light test from nose to tail and then also from tail to nose. What would they find? The ‘rearward speed’ of light will appear to be lower than c (c_r = c-delta_v) and the ‘forward speed’ will appear to be higher than c (c_f = c+delta_v)
I think this was SL’s point, which Fred somehow refuses to address. One-way speed measurements does depend on clock synchronization. You read the departure time of light from one clock and the arrival time from another clock. As I’ve shown in my “Re: One way to look at two ways” above, even if you can get away with one clock, there is still that “pesky distance (d), determined by (indirectly) using light, …”
Regards.
Burt Jordaan (www.Relativity-4-Engineers.com)
Burt has proposed ways to measure one-way light speed that might work well for a laboratory in free fall far from heavy objects, because we believe that far from heavy objects space is isotropic and homogeneous and Lorentz invariant.
But what about near heavy objects, such as the sun?
Near heavy objects, simple minded observers see light travel more slowly and in hyperbolic arcs.
Trained physicists know of light defining geodesics, with space-time near the sun being such that things seem to a distant observer to happen more quickly because ideal clocks seem to run slowly, and ideal measuring rods seem to shrink, near the sun. The trained physicist, sciens sub specie aeternitatis, tells us that talk of things happening and light propagating in a frame of cause and effect is nonsense, but simply that in its zero proper time the pulse defines a geodesic, which is eternal.
Simple minded observers may accept that ideal clocks are not affected by acceleration, and so they may infer that they are not affected by gravity.
The effects of gravity on a light pulse can be on its speed, or on its energy content, the latter by way of greater frequency and energy density at the same duration, or by way of longer duration at the same frequency and energy density.
As a light pulse comes from afar to pass near the rim of the sun, it gains kinetic energy at the expense of its gravitational potential energy. Loaded with the extra kinetic energy, the light pulse is heavier and is attracted more by the gravity of the sun.
Supposing it cannot go faster, it has to increase its frequency or its duration (more cycles per pulse) to carry the extra kinetic energy.
Not going faster, and not increasing its duration, it has to delay its leading edge to crimp up the waves to increase the frequency, becoming shorter in length. That makes its midpoint move more slowly on the way towards the sun.
Not going faster, and not increasing its frequency, but increasing its duration, it has to delay its trailing edge to fit in the extra cycles, again making its midpoint move more slowly.
The two are compatible: both the frequency and duration could increase, with the combined effect slowing the motion of the midpoint of the pulse.
The reverse happens on the way out from the sun on the back afar.
Does this make sense for a simple minded observer?
Of course this story is nonsense for a trained physicist, who knows that light defines geodesics and just increases its frequency as apparent to a distant observer, because time and space are different near the sun so that, to the distant observer, ideal clocks, near the sun, seem to run slowly and ideal measuring rods, near the sun, seem shorter.
Please put me right about this.
Christopher
Scruffy notes, in way of explanation:
It could e.g. be that light goes at c+v in the one direction and c-v in the opposite direction and your test will not pick it up.
In that example, note that the presumed travel time over a length d back and forth, d/(c+v) + d/(c-v), is not equal to the measured time, which is presumed to be 2d/c, and which I will call t0.
In your example
t = 2cd/(c2-v2) = t0(c2/[c2-v2])
or t0 times a value that depends on v.
That gets us back to the classic attempts to measure the differences in the speed of light parallel and perpendicular to the Earth’s motion, which, as you know, have consistently produced null results.
Adding in the consistency of measured values of c as compared with the expected value based on electromagnetic measurements and I don’t understand what you find bothersome about this.
Sorry, SL!
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
Can anyone name anything in the human world that is more sublime than a responsible, able and courageous scientist who eschews political convenience, economic expediency and the hoarding of endless wealth in order to speak out loudly and clearly for new and unexpected science, especially when the unforeseen, good evidence has great explanatory power and profound implications for the future of the family of humanity on Earth?
Steven Earl Salmony
AWAREness Campaign on The Human Population, established 2001
http://sustainabilitysoutheast.org/
Fred wrote:
You are missing the fact that all those tests were two-way or at least closed loop measurements. If you do not agree, please point us to a test that was not.
A two-way (closed loop) test uses a single clock and no information about the one-way speed of light comes out of the test, because you measure a round-trip time. It could e.g. be that light goes at c+v in the one direction and c-v in the opposite direction and your test will not pick it up. That’s what I was getting at right from the start of this thread…
SL: Your Aerospace Watchdog
PS: It does not matter if it is a zig-zag path or not. The light signal has to “backtrace” all its steps to reach the original clock (or interferometer) again
Now I’m really puzzled. I was describing (in terms of an admittedly foggy memory) an experiment that measured the speed of light directly by a time-of-flight method, distance/time. I think Burt was describing the same. Neither of us were presuming a speed of light in order to measure it.
There is a long history of such measurements as well as strong classical theoretical support. Maxwell’s equations will be 150 years old next year. The equations predicted the existence of electromagnetic waves with a speed determined by electrical and magnetic measurements.
Maxwell compared that predicted speed to the best direct measurements of the speed of light, and the results supported that e-m waves indeed had that speed to within margins of error in the various measurements.
Since then, similar measurements have refined the values of the electrical and magnetic constants as well as the speed of light measured by time of flight.
None of these measurements presume a particular value for the speed of light in advance, and everything still fits.
What am I missing, Scruffy?
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
It seems that nobody could come up with a good argument for a one-way speed of light test that does not rely on using the fixed speed of light as a presumption.
Burt mentioned cosmological observations that may support an independent verification of the one-way speed of light, but the details were not presented.
As far as local (Earth based) tests are concerned, I have not heard of one that sports a one-way measurement that is totally free from any reliance on the assumed constant speed of light.
So, after all, the fixed one-way speed of light may still be just a useful convention, determined by the Einstein method for the synchronization of clocks.
SL: Your Aerospace Watchdog
Hi SL, you asked: “Are you now saying that that is not a valid method of synchronizing clocks in an inertial frame?”
No, the “bullet method” is equivalent to Einstein’s light method, with the proviso that the bullets must be fired at the exact same speed as determined inside the inertial frame. For other readers, here’s the simple set-up:
The “cross” is floating in free space and the central gun fires four bullets with identical speeds simultaneously towards the four clocks, p,q,r,s. When the bullets reach the clocks, the clocks are all set to read the same time. In this inertial frame, the clocks are then all synchronized, but an observer moving relative to this frame will not agree that they are synchronized.
Readers can download the chapter from which this image is copied free from: http://www.einsteins-theory-of-relativity-4engineers.com/inertial-movement.html
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)