c The Speed Of Light

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is it really a vacuum if there is light?

it may change very slowly over time but there's no evidence for that, as rigorous measurements were only done in the last years. At the moment it's believed to be constant

>>8840275
So there's no evidence? Not even stuff that would make more sense if maybe c changed over time?

Also another question, is listed in OP pic the only results we have measuring it?

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It's the universe that changes not the constant. Why else would we call it a constant?

>>8840325
What do you mean? Can you post some articles discussing this that we're sure c is constant and that the universe changes?

>>8840258
constant changing over time would imply that energy does not conserve, which is very unlikely with our current understanding of physics

>>8840329
I mean just set c equal to a constant and say it's true. It's just a natural unit after all.

How is stuff like this not hand wavey?
http://curious.astro.cornell.edu/about-us/101-the-universe/cosmology-and-the-big-bang/general-questions/571-did-the-speed-of-light-change-over-the-history-of-the-universe-intermediate
>The article in Nature talks about the possibility of the speed of light changing over the history of the universe. Where this stems from is observational evidence that the "fine structure constant" has changed determines the exact wavelength of fine structure lines in the spectra of atoms, and measurements of the spectra of quasars suggest that it many have decreased by 0.00072 +/- 0.00018 % over the past 6-10 billion years (ref in the Nature article).
>The authors of the paper argue that since the fine structure constant is equal to the charge on an electron squared divided by Planck's constant times c, then for the fine structure constant to change one of these must also change. They go on to provide an arguement as to why they think it much be the speed of light, and that's what caught the eyes of the popular press.
>However this article out today on the LANL preprint server argues why dimensionful constants (like c) cannot change, since we can just define them to be whatever we want by changing units. For example if we define units in which the speed of light and Planck's constant are both exactly equal to one, then the fine structure constant is just equal to the charge on an electron squared

this one talks about it changing over time.
https://www.sciencenews.org/article/speed-light-not-so-constant-after-all
>A new experiment reveals that focusing or manipulating the structure of light pulses reduces their speed, even in vacuum conditions.
>A paper reporting the research, posted online at arXiv.org and accepted for publication, describes hard experimental evidence that the speed of light, one of the most important constants in physics, should be thought of as a limit rather than an invariable rate for light zipping through a vacuum.

>>8840310
stuff makes more sense if c stays constant given our current understanting

no, a lot of these measurements are done regularly

>>8840355
But the speed of light depends on the definition of length and the definition of length depends on the speed of light. The definitions of which are very slippery they are arbitrarily defined, and we have changed the definitions of both over time. We have measured it in such a tiny small fraction of the history of the universe under conditions like one mile above sea level on the third planet in the backwaters of a spiral galaxy. Saying its always been constant or all of history of the universe billions of years is a very big claim.

>>8840258

>>8840370
THIS

Another way of looking at it is "If the universe is constantly expanding"

Does the SOL travel a fixed % distance of the universe or a fixed distance?

Aren't they the same thing with regards to planks constant?

https://en.wikipedia.org/wiki/Speed_of_light
>it is exact because the unit of length, the metre, is defined from this constant and the international standard for time.
Cant make this up. Okay so what if time changes over time, or over different conditions in the universe?

>In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.

>>8840381
You're right!

Also how far does acceleration due to gravity on an apple travel after it's released from a 10m tall tree?

Aren't they both the same thing with regards to the gravitational constant?

Meanwhile over the history of making these observations attempting to measure the speed of light we have not been using the same definitions of all of the units involved, every unit involved, has been changing and redefined. So has anyone back-adjusted historical data over time to compare and if so which that uncertainty? So is it possible to say that the natural units that our units measure haven't been subtly changing?

These definitions have been redefined so many times.
https://en.wikipedia.org/wiki/History_of_the_metre
https://en.wikipedia.org/wiki/MKS_system_of_units
https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units

https://en.wikipedia.org/wiki/Second#History_of_definition

>>8840258
I like to think that c is a variable, but then you have to change a whole set of physics inner workings (definitions) until you efectively make a second branch of physics just so that you can make afirmations that hold true only on your branch (like you can travel faster than light). Not worth the effort.

>>8840401
>Okay so what if time changes over time
interesting question. our current definition of a second is a certain interaction between cesium atoms. I suppose if the strong nuclear force was changing then our definition of a second could change

>>8840446
I see that its defined based on this
https://en.wikipedia.org/wiki/Second
>SI definition of second is "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom".
Is there a brainlet explanation of what this means?

It also depends on other things like the gravity surrounding the area of space it's in. If the measurement is taking place deeper in gravity wells then it's different than the intergalactic space in the middle of a super cluster of galaxies which is different than intergalactic space away from galaxy clusters.