# Measuring Speed of light One-Way **URL:** \ **Category:** Factual Questions\ **Created:** [November 24, 2024, 5:36pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723 "2024-11-24T17:36:12Z")\ **Posts on this page:** 15\ **Page:** 5
**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\ **Post date:** [November 27, 2024, 11:26pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/81 "2024-11-27T23:26:26Z")
How does the Michaelson Morely Experiment figure into this? [Michelson–Morley experiment - Wikipedia](https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment) ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [November 27, 2024, 11:51pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/82 "2024-11-27T23:51:59Z")
> [@Great\_Antibob](#): > > It more or less is null, at least for getting the absolute accuracy we need. That’s one of the more interesting things that’s come about over the last century. > > We can use a ‘known’ distance to get a two-way measurement of the speed of light. Naturally, this measurement will come with error bars because our actual knowledge of distance has a limited precision… OK, thanks for taking the time to explain it better. That’s appreciated. I’ve taken the illustration on my linked article: [![](https://i0.wp.com/liquidinstruments.com/wp-content/uploads/2023/03/Screenshot-2023-03-14-at-2.26.43-PM.png?resize=700%2C218&ssl=1) ](https://i0.wp.com/liquidinstruments.com/wp-content/uploads/2023/03/Screenshot-2023-03-14-at-2.26.43-PM.png?resize=700%2C218&ssl=1) And added a laser starter. The starter sends out two simultaneous beams that trigger both of the light source/photodetectors. Each one then measuring a one way trip. [![](https://s3.amazonaws.com/gs-geo-images/36c8745f-9e66-426e-ac8e-a649f4973311_l.jpg) ](https://s3.amazonaws.com/gs-geo-images/36c8745f-9e66-426e-ac8e-a649f4973311_l.jpg) ISTM, that if the distance from the light source/photodetector to the reflector in the linked article is confident enough to give us a speed of light that is “good enough”, then so should my ‘starter laser’ version be good enough to show that light does (or doesn’t!) travel at the same speed going “left to right” or vice versa. Though, this stuff being way above my pay grade, I’m thoroughly prepared to be shown the error of my ways. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [November 27, 2024, 11:58pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/83 "2024-11-27T23:58:16Z")
> [@Lucas\_Jackson](#): > > ETA: OK, instead of a pulsar, put a device on the moon that fires simultaneous lasers at two points on the earth 100 miles apart. You can’t have “simultaneous” events at non-identical places - there will always be observers who will see “A” before “B” _and_ observers who will see “B” before “A”. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [November 28, 2024, 12:39am UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/84 "2024-11-28T00:39:47Z")
> [@Andy\_L](#): > > You can’t have “simultaneous” events at non-identical places Of course you can. It’s just that points A and B can’t relay information of those events between themselves simultaneously, but in this case that’s not needed. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [November 28, 2024, 3:43am UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/85 "2024-11-28T03:43:38Z")
> [@EnolaStraight](#): > > How does the Michaelson Morely Experiment figure into this? It doesn’t directly. The Michaelson Morely Experiment wasn’t trying to measure the speed of light, it was simply trying to detect some anisotropy in the speed of light through space either in the direction of the Earth’s known motion or at angles away from it. Even with the limited size of the experimental apparatus, it would have revealed any departure from the known speed of light based on a Newtonian absolute reference frame– i.e., “the ether”. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [November 28, 2024, 9:36am UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/86 "2024-11-28T09:36:23Z")
> [@cmyk](#): > > Am I correct in thinking that the fundamental problem is there is no such thing as simultaneity in SR/GR? You are exactly correct. This is a fundamental problem is many arguments trying to create the sorts of experiments we see here. The idea that two events separated in space can be unequivocally simultaneous simply isn’t how the universe operates. The only way we can claim simultaneity is if there is a privileged observer who can determine it. But there isn’t one. Every argument that has the word _simultaneous_ in it is flawed. Arguments relying on distance measurements, say with a ruler or the like run into trouble because the forces holding the measuring stick together are mediated by carriers that also travel at the speed of causality, or by carriers that the speed of causality is folded into their speed. In principle a ruler will shrink or expand as you rotate it in a universe with anisotropic _c_. Where this gets us is probably eventually an argument about why, in a universe of space time, that it actually makes no sense to ask the question. The speed of causality is 1. Exactly. We travel through spacetime at one second per second. We travel through space by rotating our direction of travel, so some part of it is now through a space dimension. We could posit that spacetime had some weird properties whereby it wasn’t uniform, but squished more in one direction than another. But it would make no sense. Not that it isn’t possible, but that it makes no sense. Moreover, reversing the direction of travel still has us move at _c_ which is still 1. GR lets us warp spacetime in the sense that geodesics can be curved as viewed externally, but for an object traversing the geodesic, they are travelling along a straight line. And everywhere you are, _c_ is still 1. If we want to talk about light instead of causality, _c_ is still 1, but one would need to find a way of slowing light down in an anisotropic manner. We can of course trivially do that with say glass with a gradient of refractive index. Which is a thing. What can’t happen is that if we reverse the direction of travel is that the speed is different. ---
**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\ **Post date:** [November 28, 2024, 3:28pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/87 "2024-11-28T15:28:18Z")
It doesn’t directly. The Michaelson Morely Experiment wasn’t trying to measure the speed of light, it was simply trying to detect some anisotropy in the speed of light through space either in the direction of the Earth’s known motion or at angles away from it. Even with the limited size of the experimental apparatus, it would have revealed any departure from the known speed of light based on a Newtonian absolute reference frame– i.e., “the ether”. If not by relative motion ( yeah yeah yeah…no privileged reference frames and all that) then by what mechanism theoretically generate a one-way speed of light different than c? The Earth spins on its axis, orbits the sun, which orbits the supermassive black hole at the center of the galaxy, which is expanding away from other galaxies at a speed proportional to the Hubble Constant. If we _could_ accurately measure the “non-c” one-way speed of light…in three dimensions/orthogonal directions…could we plot an Absolute Frame of Reference? Pick a point of Full Stop, and from there, a vector of the speed and direction of the Observer? ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [November 28, 2024, 4:10pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/88 "2024-11-28T16:10:04Z")
Different observers will disagree about whether the events were simultaneous. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [November 28, 2024, 4:14pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/89 "2024-11-28T16:14:40Z")
> [@Lucas\_Jackson](#): > > If absolute distance can’t be determined, how do we know what the (assumed) speed of light is? For the theoretical this is not necessary to start with. What we do first is see if the speed of light is the same in both directions and for that we don’t need to know the difference, just that both times are the same. ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [November 28, 2024, 4:29pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/90 "2024-11-28T16:29:35Z")
> [@Lucas\_Jackson](#): > > OK, thought experiment. > > You have two experimenters (X and Y) and a pulsar. Imagine they are arraigned in triangle shape in space. The bottom two points of the triangle would be the experimenters, say 1000 miles apart, and the pulsar is the top point. The pulsar is 1 light year away and equidistant from both experimenters. > > Now, a third experimenter (Z) has a giant opaque shield covering the pulsar from the view of both the first two experimenters. The pulsar is average and pulses about 1 pulse per second. Z removes the shield and at the moment the 10th pulse reaches X and Y, the experiment begins. X fires a light beam the moment the 10th pulse reaches him. When the 10th pulse reaches Y he starts his timer and stops it when the light beam arrives at his measuring device. > > Then repeat the experiment, but this time Y fires the light and Z does the measuring (this would rule out any anisotropic deflection of light coming from the pulsar. > > Now clearly this is impractical, but as a thought experiment could it work? > > Or maybe, along the same lines, instead of a pulsar have a device that fires simultaneous lasers at the experimenters. @Lucas_Jackson proposes to make a one way measurement of the speed of light. Is that correct? > [@Saint\_Cad](#): > > > [@Lucas\_Jackson](#): > > > > If absolute distance can’t be determined, how do we know what the (assumed) speed of light is? > > For the theoretical this is not necessary to start with. What we do first is see if the speed of light is the same in both directions and for that we don’t need to know the difference, just that both times are the same. If we begin by assuming that light is anisotropic, why do the experiment? We’ve already assumed the result. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [November 28, 2024, 5:05pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/91 "2024-11-28T17:05:35Z")
> [@74westy](#): > > If we begin by assuming that light is anisotropic, Because we are testing that hypothesis. Scientific method and all. ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [November 28, 2024, 5:24pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/92 "2024-11-28T17:24:50Z")
I’m sorry. I’m totally lost. By: > [@Saint\_Cad](#): > > Because we are testing that hypothesis. do you mean we are testing whether light is anisotropic? Are we using Lucas\_Jackson’s pulsar experiment to do that? By: > [@Saint\_Cad](#): > > For the theoretical this is not necessary to start with. What we do first is see if the speed of light is the same in both directions and for that we don’t need to know the difference, just that both times are the same. which two times are we comparing? How are we measuring the times? What can we conclude if both times are the same? ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [November 28, 2024, 5:26pm UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/93 "2024-11-28T17:26:21Z")
> [@74westy](#): > > do you mean we are testing whether light is anisotropic? Yes. > [@74westy](#): > > Are we using Lucas\_Jackson’s pulsar experiment to do that? I don’t know > [@74westy](#): > > which two times are we comparing? Outward time to return time > [@74westy](#): > > How are we measuring the times? By clock > [@74westy](#): > > What can we conclude if both times are the same? We have no clue how to do that yet. ---
**Author:** ![caligulathegod](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/caligulathegod/32/9685_2.png) [@caligulathegod](https://boards.straightdope.com/u/caligulathegod)\ **Post date:** [November 29, 2024, 1:46am UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/94 "2024-11-29T01:46:48Z")
As I understand it, not being a physicist, the speed of light isn’t really so much a velocity but is the speed of causality. Why would that be broken for an anisotropic convention? Isotropic is an assumption, but if causality is limited to it, why would it be reasonable for light to behave differently by traveling infinitely to the observer? The only person I’ve ever heard try to use this in any other way than an interesting physics exercise was Young Earth Creationist Jason Lisle to get around the problem of stars being billions of light years way yet the universe being only 6000 years old. ---
**Author:** ![sbright33](https://avatars.discourse-cdn.com/v4/letter/s/8e8cbc/32.png) [@sbright33](https://boards.straightdope.com/u/sbright33)\ **Post date:** [November 29, 2024, 1:53am UTC](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723/95 "2024-11-29T01:53:51Z")
We postulate that it’s the same speed in all directions when the observer is within the inertial frame of reference. And different from an external point of view. Think of Einsteins Train. I believe it is possible to measure, from some reference view. [Previous page](https://boards.straightdope.com/t/measuring-speed-of-light-one-way/1010723.md?page=4)