6 ms·

Historical speed of light measurements in southern California (2014)

41 points by dwohnitmok 7y ago

sixdimensional 7y ago
I’ve been to the top of Santiago Peak, one of the mountains mentioned in the article. Its got a handful of big line of site radio towers with microwave tx/rx on it [1]. These peaks would probably be a good candidate for laser-based radio transmission one day. I find that kind of ironic given the fact that Michelson tested with light between 1922 to 1931! [1] https://en.m.wikipedia.org/wiki/Santiago_Peak#Radio_communication_facilities https://en.m.wikipedia.org/wiki/Santiago_Peak#Radio_communic...
- Rebelgecko 7y ago
  FWIW, Facebook has gotten permits to construct some laser comm facilities on Mt Wilson
whatshisface 7y ago
>The proper terminology is of course “velocity of light”, but most non-physics people conventionally use “speed of light”, which I shall do so from this point forward. That's not right, velocity is a vector (a magnitude and a direction) and speed is a scalar (just a magnitude). The speed of light, c, is a scalar: in fact, one of the biggest things that Michelson showed is that it is the same in every direction! (If the speed of light was anisotropic then I think c would be a tensor). I don't think I've ever heard anybody say "the velocity of light." I wonder where that came from.
- pmiller2 7y ago
  > I don't think I've ever heard anybody say "the velocity of light." I wonder where that came from. Most likely, it’s a form of linguistic hypercorrection: https://en.wikipedia.org/wiki/Hypercorrection https://en.wikipedia.org/wiki/Hypercorrection
  - mc32 7y ago
    But descriptively it’s a hypercorrection in prescriptive grammar. It’s a-ok in descriptive grammar.
    - whatshisface 7y ago
      Let me tell you something about descriptive grammar, leaf swam differential either sugar and cup.
  - BurningFrog 7y ago
    Right, "velocity" is a more formal and scientific term, so it must be the more correct one.
- cygx 7y ago
  Fun fact: The speed of light is only constant by convention. There's a nice creationist paper where they try to twist physics into a pretzel in order to make it fit the biblical narrative, using a convention where the speed of infalling light rays is infinite, but the speed of outgoing ones only c/2. Of course, any such attempt will eventually fail due to invariants that are independent of convention, such as the age of the universe as measured by an observer following the Hubble flow. One can't help but be impressed at least a little bit by the ingenuity of it all, though... edit: cf https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light
  - burnte 7y ago
    "Fun fact: The speed of light is only constant by convention." Well, the speed of light through a non-vacuum isn't the same as any medium will slow it to some extent, but the speed of light in a vacuum is the definition of "c" and as such, that IS a constant and invariant.
    - cygx 7y ago
      Because we define the speed of light in terms of spatially homogeneous and isotropic inertial coordinates. We do that because we want "the statements of the natural laws [to be] as simple as possible" (Poincaré), but it's not the only way to do it...
      - 6gvONxR4sf7o 7y ago
        Sure the speed is a function of the metric you use, but physics also tells us what metric is useful.
        cygx 7y ago
        It's less about the metric but the choice of space/time decomposition.
        7y ago
- ChrisLomont 7y ago
  By saying speed of light, you make the assumption it is the same in any direction or reference frame. This was not known to be true, and is still the subject of some question, hence the more modern experiments measuring the speed in a fixed frame to compare to other frames. For example, what is the speed of a bullet? It depends on the observer (as does velocity, but velocity is a more accurate thing to measure for this reason). It so far is experimentally true that the velocity of light has the same magnitude (up to experimental error) in any frame, which was a tremendously crazy result at first, so much so that it forced mankind to give up the notion of constant flow of time, and it forces space and time to "rotate" into each other in a weird manner called space-time. So the velocity of light is the proper term to use in an experiment.
  - whatshisface 7y ago
    >For example, what is the speed of a bullet? It depends on the observer (as does velocity, but velocity is a more accurate thing to measure for this reason). That sentence doesn't make sense to me, velocity is even more sensitive to coordinate change than speed. If you rotate your coordinate system, all the velocities will change but speeds will stay the same. The phrase used in the article was "the velocity of light," which was known to be wrong even back then because light has a different velocity depending on which way it is going.
  - jayd16 7y ago
    But this proves the point that speed should be used. C is the theorized speed of light in all frames. The experimental velocities of light also exist but that shouldn't be what we call C.
  - twiceaday 7y ago
    This doesn't make any sense. If there is an oddity its in saying "THE speed of light." THE is what makes it singular and begs the question "in what direction?" The answer is in any direction. But that doesn't mean direction is not important. If somebody asks you for THE velocity that is a nonsense question. If somebody asks you for A velocity they are asking you for the speed in each direction. Even in 1D, where the units of velocity and speed are the same, these are still not the same concepts because velocity can be negative to signify the direction of "backwards" so there are two possible velocities of light even in 1D.
- tjchear 7y ago
  I checked out one of the links [0], which lead to an article presumably by the same author, with this accompanying picture of a plaque installed at the site of measurement on Mount Wilson [1]. It clearly says velocity of light - so that might have been where the author got it from. [0] https://www.otherhand.org/home-page/physics/historical-speed-of-light-measurements-in-southern-california/the-mount-wilson-station-1922-1928/ https://www.otherhand.org/home-page/physics/historical-speed... [1] https://www.otherhand.org/wp-content/uploads/2012/04/Mt-Wilson-plaque.jpg https://www.otherhand.org/wp-content/uploads/2012/04/Mt-Wils...
thunderrabbit 7y ago
> Que Cue
- hinkley 7y ago
  Que?
- JetSpiegel 7y ago
  Queue?
jihadjihad 7y ago
There is currently a post on the front page of HN about Michelson, for those interested: https://news.ycombinator.com/item?id=20962592 https://news.ycombinator.com/item?id=20962592
test31 7y ago
Ancient Indian Rishis had calculated this astonishingly accurately.

41 points by dwohnitmok 7y ago

I’ve been to the top of Santiago Peak, one of the mountains mentioned in the article. Its got a handful of big line of site radio towers with microwave tx/rx on it [1]. These peaks would probably be a good candidate for laser-based radio transmission one day. I find that kind of ironic given the fact that Michelson tested with light between 1922 to 1931! [1] https://en.m.wikipedia.org/wiki/Santiago_Peak#Radio_communication_facilities https://en.m.wikipedia.org/wiki/Santiago_Peak#Radio_communic...

Rebelgecko 7y ago
FWIW, Facebook has gotten permits to construct some laser comm facilities on Mt Wilson

whatshisface 7y ago

>The proper terminology is of course “velocity of light”, but most non-physics people conventionally use “speed of light”, which I shall do so from this point forward. That's not right, velocity is a vector (a magnitude and a direction) and speed is a scalar (just a magnitude). The speed of light, c, is a scalar: in fact, one of the biggest things that Michelson showed is that it is the same in every direction! (If the speed of light was anisotropic then I think c would be a tensor). I don't think I've ever heard anybody say "the velocity of light." I wonder where that came from.

pmiller2 7y ago
> I don't think I've ever heard anybody say "the velocity of light." I wonder where that came from. Most likely, it’s a form of linguistic hypercorrection: https://en.wikipedia.org/wiki/Hypercorrection https://en.wikipedia.org/wiki/Hypercorrection
- mc32 7y ago
  But descriptively it’s a hypercorrection in prescriptive grammar. It’s a-ok in descriptive grammar.
  - whatshisface 7y ago
    Let me tell you something about descriptive grammar, leaf swam differential either sugar and cup.
- BurningFrog 7y ago
  Right, "velocity" is a more formal and scientific term, so it must be the more correct one.
cygx 7y ago
Fun fact: The speed of light is only constant by convention. There's a nice creationist paper where they try to twist physics into a pretzel in order to make it fit the biblical narrative, using a convention where the speed of infalling light rays is infinite, but the speed of outgoing ones only c/2. Of course, any such attempt will eventually fail due to invariants that are independent of convention, such as the age of the universe as measured by an observer following the Hubble flow. One can't help but be impressed at least a little bit by the ingenuity of it all, though... edit: cf https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light
- burnte 7y ago
  "Fun fact: The speed of light is only constant by convention." Well, the speed of light through a non-vacuum isn't the same as any medium will slow it to some extent, but the speed of light in a vacuum is the definition of "c" and as such, that IS a constant and invariant.
  - cygx 7y ago
    Because we define the speed of light in terms of spatially homogeneous and isotropic inertial coordinates. We do that because we want "the statements of the natural laws [to be] as simple as possible" (Poincaré), but it's not the only way to do it...
    - 6gvONxR4sf7o 7y ago
      Sure the speed is a function of the metric you use, but physics also tells us what metric is useful.
      - cygx 7y ago
        It's less about the metric but the choice of space/time decomposition.
        7y ago
ChrisLomont 7y ago
By saying speed of light, you make the assumption it is the same in any direction or reference frame. This was not known to be true, and is still the subject of some question, hence the more modern experiments measuring the speed in a fixed frame to compare to other frames. For example, what is the speed of a bullet? It depends on the observer (as does velocity, but velocity is a more accurate thing to measure for this reason). It so far is experimentally true that the velocity of light has the same magnitude (up to experimental error) in any frame, which was a tremendously crazy result at first, so much so that it forced mankind to give up the notion of constant flow of time, and it forces space and time to "rotate" into each other in a weird manner called space-time. So the velocity of light is the proper term to use in an experiment.
- whatshisface 7y ago
  >For example, what is the speed of a bullet? It depends on the observer (as does velocity, but velocity is a more accurate thing to measure for this reason). That sentence doesn't make sense to me, velocity is even more sensitive to coordinate change than speed. If you rotate your coordinate system, all the velocities will change but speeds will stay the same. The phrase used in the article was "the velocity of light," which was known to be wrong even back then because light has a different velocity depending on which way it is going.
- jayd16 7y ago
  But this proves the point that speed should be used. C is the theorized speed of light in all frames. The experimental velocities of light also exist but that shouldn't be what we call C.
- twiceaday 7y ago
  This doesn't make any sense. If there is an oddity its in saying "THE speed of light." THE is what makes it singular and begs the question "in what direction?" The answer is in any direction. But that doesn't mean direction is not important. If somebody asks you for THE velocity that is a nonsense question. If somebody asks you for A velocity they are asking you for the speed in each direction. Even in 1D, where the units of velocity and speed are the same, these are still not the same concepts because velocity can be negative to signify the direction of "backwards" so there are two possible velocities of light even in 1D.
tjchear 7y ago
I checked out one of the links [0], which lead to an article presumably by the same author, with this accompanying picture of a plaque installed at the site of measurement on Mount Wilson [1]. It clearly says velocity of light - so that might have been where the author got it from. [0] https://www.otherhand.org/home-page/physics/historical-speed-of-light-measurements-in-southern-california/the-mount-wilson-station-1922-1928/ https://www.otherhand.org/home-page/physics/historical-speed... [1] https://www.otherhand.org/wp-content/uploads/2012/04/Mt-Wilson-plaque.jpg https://www.otherhand.org/wp-content/uploads/2012/04/Mt-Wils...

thunderrabbit 7y ago

> Que Cue

hinkley 7y ago
Que?
JetSpiegel 7y ago
Queue?

jihadjihad 7y ago

There is currently a post on the front page of HN about Michelson, for those interested: https://news.ycombinator.com/item?id=20962592 https://news.ycombinator.com/item?id=20962592

test31 7y ago

Ancient Indian Rishis had calculated this astonishingly accurately.