
Historical speed of light measurements in southern California (2014) - dwohnitmok
https://www.otherhand.org/home-page/physics/historical-speed-of-light-measurements-in-southern-california/
======
whatshisface
> _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.

~~~
cygx
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
"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
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
Sure the speed is a function of the metric you use, but physics also tells us
what metric is useful.

~~~
cygx
It's less about the metric but the choice of space/time decomposition.

~~~
6gvONxR4sf7o
I'm not talking about a metric over space, but rather a metric over spacetime,
if that's what you mean. I would consider that to encode the space/time
decomposition. My differential geometry sucks, but I thought you could write
general relativity in a coordinate free manner that specifically doesn't rely
on strong coordinate choices like you're talking about.

~~~
cygx
_I thought you could write general relativity in a coordinate free manner that
specifically doesn 't rely on strong coordinate choices_

Indeed. But even before you start messing with the formalism, the metric alone
cannot be used to define a speed as 4-velocities are normalized. You need to
decompose your tangent space into timelike and spacelike subspaces so you can
have a _dx_ and a _dt_.

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sixdimensional
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_communic...](https://en.m.wikipedia.org/wiki/Santiago_Peak#Radio_communication_facilities)

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

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jihadjihad
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)

------
thunderrabbit
> Que

Cue

~~~
JetSpiegel
Queue?

