3 ms·

> In 1728, James Bradley made another estimate by observing stellar aberration.. Bradley measured this angle for starlight, and knowing the speed of the Earth a

> In 1728, James Bradley made another estimate by observing stellar aberration.. Bradley measured this angle for starlight, and knowing the speed of the Earth around the Sun, he found a value for the speed of light of 301,000 km/s. So technically, he almost got to the official number so far which is 299,792.458 km/s. What an achievement!

ndsipa_pomu 3y ago
That is incredible. It seems like a bit of a cheat that we no longer measure the speed of light and instead specify its exact value. I guess that means that the error margin is now exactly zero.
- mikewarot 3y ago
  Not measuring the speed of light on a continuing basis is huge mistake, in my opinion. How do we know it's not slowly varying due to some currently unknown mechanisms? The same could be true of gravity, etc. It seems the height of hubris to stop measuring and say we've certain that it never varies. This implies that the distance between the ends of a ruler are always the same distance apart (except for contraction/expansion due to temperature, barometric pressure, etc.) which might not be true, etc.
  - adrian_b 3y ago
    Nope. We continue to measure the speed of light every day, much more frequently than ever before, even in every mobile phone with GPS. Except that now this has been renamed as measuring the length by using interferometry or propagation time. Any change in the speed of light would be immediately seen as unexpected changes in lengths.
  - eesmith 3y ago
    Scientists measure the speed of light often. Including to test if the speed varies due to some currently unknown mechanisms. For example, there are varying speed of light cosmological models, and astronomers looking for any evidence that the speed of light was different in the early universe. I don't know the field at all, so I stuck likely search terms into Google Scholar and easily found papers like https://arxiv.org/pdf/2302.00867.pdf https://arxiv.org/pdf/2302.00867.pdf "Do current cosmological observations hint at the speed of light variability?" saying "We find null evidence for the speed of light variability hypothesis for most choices of priors and data-set combinations, except for a mild deviation from it (at ∼ 2σ confidence level) at z > 1 when the a-BAO data are included for some priors and reconstruction kernel cases. Still, we ascribe no statistical significance to this result for the incompleteness of this data set at such higher redshifts." People also measure the speed of light indirectly, every time someone measures the frequency of a light source, or an index of refraction, or the distance to the Moon (or GPS!), or any of the myriad of measurements connected to the speed of light. I'll add that my Google Scholar search terms ('"speed of light" early universe measured') pulled up a lot of crackpot-seeming papers, so if you go that route be extra careful.
    - mikewarot 3y ago
      GPS is all about the difference in arrival times of signals, so it wouldn't make a huge difference if there were a PPB/year difference in the speed of those signals. It likely wouldn't even be noticed by the DOD who runs the constellation. I think it might prove fruitful to keep on measuring against physical artifacts from time to time. Even a null result can be useful.
      - eesmith 3y ago
        The GPS signal includes ephemeris data about where the satellites are located. As Wikipedia puts it, "In addition to the PRN ranging codes, a receiver needs to know the time and position of each active satellite." Yes, if there's a very small change - and I don't know what the limits are - then it wouldn't be detectable. FWIW, 1ppm is about 1.2 cm at GPS orbital distances. My point is still that the speed of light is being measured all the time, not that every measurement is continually improving upon the accuracy of that measurement. To further complicate things, as far as we can tell the speed of gravity is the speed of light. Should the speed of light change, then either GPS orbits change (due to the change in gravity) or they are not at the same speed.
      - adrian_b 3y ago
        In the national metrology laboratories of most developed countries, lengths are measured by interferometry with extremely high precisions, very frequently. Even very small variations in the speed of light would be detected, because they would cause correlated variations in all measured lengths. In any case, the point is, as another commenter has already written, that every length measurement done with lasers or radars is a rebranded measurement of the speed of light and such measurements have become very frequent.
  - kolinko 3y ago
    As far as I understand, speed of light might as well be changing, but as long as it's tied to the speed of other forces, we would have no way of noticing. By the way, an amazing story of Ted Chiang, on the subject - Exhalation - about a race of machines fuelled by compressed air that one day discover their source of air is losing pressure. Linking to audio-book, since it's one of the best audiobooks I heard :) https://www.amazon.com/Exhalation-Ted-Chiang-audiobook/dp/B07PP9MXPL https://www.amazon.com/Exhalation-Ted-Chiang-audiobook/dp/B0...
printerdev 3y ago
Well - if you look at puranas, they had calculated speed of light long back - https://hinduism.stackexchange.com/questions/12075/the-speed-of-light-taittirya-brahmana-of-the-yajurveda https://hinduism.stackexchange.com/questions/12075/the-speed...

ndsipa_pomu 3y ago

That is incredible. It seems like a bit of a cheat that we no longer measure the speed of light and instead specify its exact value. I guess that means that the error margin is now exactly zero.

mikewarot 3y ago
Not measuring the speed of light on a continuing basis is huge mistake, in my opinion. How do we know it's not slowly varying due to some currently unknown mechanisms? The same could be true of gravity, etc. It seems the height of hubris to stop measuring and say we've certain that it never varies. This implies that the distance between the ends of a ruler are always the same distance apart (except for contraction/expansion due to temperature, barometric pressure, etc.) which might not be true, etc.
- adrian_b 3y ago
  Nope. We continue to measure the speed of light every day, much more frequently than ever before, even in every mobile phone with GPS. Except that now this has been renamed as measuring the length by using interferometry or propagation time. Any change in the speed of light would be immediately seen as unexpected changes in lengths.
- eesmith 3y ago
  Scientists measure the speed of light often. Including to test if the speed varies due to some currently unknown mechanisms. For example, there are varying speed of light cosmological models, and astronomers looking for any evidence that the speed of light was different in the early universe. I don't know the field at all, so I stuck likely search terms into Google Scholar and easily found papers like https://arxiv.org/pdf/2302.00867.pdf https://arxiv.org/pdf/2302.00867.pdf "Do current cosmological observations hint at the speed of light variability?" saying "We find null evidence for the speed of light variability hypothesis for most choices of priors and data-set combinations, except for a mild deviation from it (at ∼ 2σ confidence level) at z > 1 when the a-BAO data are included for some priors and reconstruction kernel cases. Still, we ascribe no statistical significance to this result for the incompleteness of this data set at such higher redshifts." People also measure the speed of light indirectly, every time someone measures the frequency of a light source, or an index of refraction, or the distance to the Moon (or GPS!), or any of the myriad of measurements connected to the speed of light. I'll add that my Google Scholar search terms ('"speed of light" early universe measured') pulled up a lot of crackpot-seeming papers, so if you go that route be extra careful.
  - mikewarot 3y ago
    GPS is all about the difference in arrival times of signals, so it wouldn't make a huge difference if there were a PPB/year difference in the speed of those signals. It likely wouldn't even be noticed by the DOD who runs the constellation. I think it might prove fruitful to keep on measuring against physical artifacts from time to time. Even a null result can be useful.
    - eesmith 3y ago
      The GPS signal includes ephemeris data about where the satellites are located. As Wikipedia puts it, "In addition to the PRN ranging codes, a receiver needs to know the time and position of each active satellite." Yes, if there's a very small change - and I don't know what the limits are - then it wouldn't be detectable. FWIW, 1ppm is about 1.2 cm at GPS orbital distances. My point is still that the speed of light is being measured all the time, not that every measurement is continually improving upon the accuracy of that measurement. To further complicate things, as far as we can tell the speed of gravity is the speed of light. Should the speed of light change, then either GPS orbits change (due to the change in gravity) or they are not at the same speed.
    - adrian_b 3y ago
      In the national metrology laboratories of most developed countries, lengths are measured by interferometry with extremely high precisions, very frequently. Even very small variations in the speed of light would be detected, because they would cause correlated variations in all measured lengths. In any case, the point is, as another commenter has already written, that every length measurement done with lasers or radars is a rebranded measurement of the speed of light and such measurements have become very frequent.
- kolinko 3y ago
  As far as I understand, speed of light might as well be changing, but as long as it's tied to the speed of other forces, we would have no way of noticing. By the way, an amazing story of Ted Chiang, on the subject - Exhalation - about a race of machines fuelled by compressed air that one day discover their source of air is losing pressure. Linking to audio-book, since it's one of the best audiobooks I heard :) https://www.amazon.com/Exhalation-Ted-Chiang-audiobook/dp/B07PP9MXPL https://www.amazon.com/Exhalation-Ted-Chiang-audiobook/dp/B0...

printerdev 3y ago

Well - if you look at puranas, they had calculated speed of light long back - https://hinduism.stackexchange.com/questions/12075/the-speed-of-light-taittirya-brahmana-of-the-yajurveda https://hinduism.stackexchange.com/questions/12075/the-speed...