5 ms·

How is the speed of light measured? (1997)

19 points by redbell 3y ago

redbell 3y ago
> 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.
    - 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...
baxtr 3y ago
There is a cool video by Veritasium about the specifics of how the speed of light is actually measured (1). I wasn’t aware that only the two way speed of light was measured so far, not the one way. Which is probably very irrelevant but still quite interesting. (1) https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k
- ndsipa_pomu 3y ago
  Came here to see if someone had mentioned this. It's interesting because it's just unbelievable that light would go at different speeds in different directions, but there's no way (that we can think of) to verify it. It's like the structure of space-time is conspiring against being able to measure one-way light speed.
  - gumby 3y ago
    Sure: the Michelson-Morley interferometry experiment: https://en.wikipedia.org/wiki/Michelson–Morley_experiment https://en.wikipedia.org/wiki/Michelson–Morley_experiment
    - ndsipa_pomu 3y ago
      Doesn't that rely on synchronising the detector with the emitter though? That's the issue that makes one-way measurements effectively a round trip measurement. Edit: looks to me like mirrors are used and it is comparing two-way speeds in different directions.
    - mikewarot 3y ago
      If you look at the folded light path used in the Michelson–Morley interferometer (figure 5), you'll see that the X and Y distances traveled on both paths add to the exact same value. Thus, it would be completely insensitive to changes in the speed of light in the X or Y directions, no matter how it were rotated.
      - gumby 3y ago
berbec 3y ago
The current page seems to show the same content http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLigh...
_8j50 3y ago
Ok, ok, this question is bugging me: could it be possible that the speed of light is just the maximum speed of anything that can be measured in a relative context? Is there proof that something couldn't go faster than light and slow down later on where then it's speed can be measured again? Given the whole quantum "spooky" action at distsnces where travel is only possible when going faster than light, I thought this was a curious question. If true, could all the weird quantum observations be explained by forces and effects that are takinng place at speeds faster than what observers that are not excited at similar speeds can't measure?
- adrian_b 3y ago
  I do not believe that there is a way to show that speeds greater than the speed of light are impossible due to some properties of measurement. On the other hand, there exists a line of reasoning that perhaps this speed limit is not some kind of intrinsic property of space and time, but only of the electromagnetic interactions. We do not have any other way to accelerate a body to very high speeds, except by using forces that ultimately are based on electromagnetic interactions (this includes the pressure of a hot gas that creates a high speed rocket jet). There is a possibility to describe the apparent mass increase of a fast-moving body, which limits the acceleration, as a consequence of the reduced effectiveness of the electromagnetic interactions when the relative speed of the interacting bodies approaches the speed of light. Nobody has constructed yet a complete theory along this line of thought, and even if such a theory existed, it is likely that it could not have any practical consequences. Within such a theory, the only way to exceed the speed of light would be to use a yet undiscovered kind of interaction, which is very unlikely to exist. Even if there is no unified theory yet, there is no doubt that both the strong and weak nuclear interactions and the electromagnetic interactions are just 3 forms of a unique kind of interaction. In the case of the strong interaction, this is proven by the fact that both the electric charges and the color charges of all quarks and leptons can be seen as projections of a unique quantized 3-dimensional charge vector (the 3 dimensions are in the space of charges, they do not have anything to do with the dimensions of space-time, exactly like the 3-dimensional vector that describes the brightness, saturation and hue of a colored light has nothing to do with the space-time), whose values sum to zero for each family of quarks and leptons, separately for the particles and for the antiparticles. So the only known interaction that could be distinct from the electromagnetic is the gravitational, but there are theories where the gravitation would arise as a high order effect of the electromagnetic interaction. Moreover, even if there are contradictory results about the speed of gravitation, the more recent results seem to be consistent with the speed of the gravitational waves being the same as the speed of light.

19 points by redbell 3y ago

> 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.
  - 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...

baxtr 3y ago

There is a cool video by Veritasium about the specifics of how the speed of light is actually measured (1). I wasn’t aware that only the two way speed of light was measured so far, not the one way. Which is probably very irrelevant but still quite interesting. (1) https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k

ndsipa_pomu 3y ago
Came here to see if someone had mentioned this. It's interesting because it's just unbelievable that light would go at different speeds in different directions, but there's no way (that we can think of) to verify it. It's like the structure of space-time is conspiring against being able to measure one-way light speed.
- gumby 3y ago
  Sure: the Michelson-Morley interferometry experiment: https://en.wikipedia.org/wiki/Michelson–Morley_experiment https://en.wikipedia.org/wiki/Michelson–Morley_experiment
  - ndsipa_pomu 3y ago
    Doesn't that rely on synchronising the detector with the emitter though? That's the issue that makes one-way measurements effectively a round trip measurement. Edit: looks to me like mirrors are used and it is comparing two-way speeds in different directions.
  - mikewarot 3y ago
    If you look at the folded light path used in the Michelson–Morley interferometer (figure 5), you'll see that the X and Y distances traveled on both paths add to the exact same value. Thus, it would be completely insensitive to changes in the speed of light in the X or Y directions, no matter how it were rotated.
    - gumby 3y ago

berbec 3y ago

The current page seems to show the same content http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLigh...

_8j50 3y ago

Ok, ok, this question is bugging me: could it be possible that the speed of light is just the maximum speed of anything that can be measured in a relative context? Is there proof that something couldn't go faster than light and slow down later on where then it's speed can be measured again? Given the whole quantum "spooky" action at distsnces where travel is only possible when going faster than light, I thought this was a curious question. If true, could all the weird quantum observations be explained by forces and effects that are takinng place at speeds faster than what observers that are not excited at similar speeds can't measure?

adrian_b 3y ago
I do not believe that there is a way to show that speeds greater than the speed of light are impossible due to some properties of measurement. On the other hand, there exists a line of reasoning that perhaps this speed limit is not some kind of intrinsic property of space and time, but only of the electromagnetic interactions. We do not have any other way to accelerate a body to very high speeds, except by using forces that ultimately are based on electromagnetic interactions (this includes the pressure of a hot gas that creates a high speed rocket jet). There is a possibility to describe the apparent mass increase of a fast-moving body, which limits the acceleration, as a consequence of the reduced effectiveness of the electromagnetic interactions when the relative speed of the interacting bodies approaches the speed of light. Nobody has constructed yet a complete theory along this line of thought, and even if such a theory existed, it is likely that it could not have any practical consequences. Within such a theory, the only way to exceed the speed of light would be to use a yet undiscovered kind of interaction, which is very unlikely to exist. Even if there is no unified theory yet, there is no doubt that both the strong and weak nuclear interactions and the electromagnetic interactions are just 3 forms of a unique kind of interaction. In the case of the strong interaction, this is proven by the fact that both the electric charges and the color charges of all quarks and leptons can be seen as projections of a unique quantized 3-dimensional charge vector (the 3 dimensions are in the space of charges, they do not have anything to do with the dimensions of space-time, exactly like the 3-dimensional vector that describes the brightness, saturation and hue of a colored light has nothing to do with the space-time), whose values sum to zero for each family of quarks and leptons, separately for the particles and for the antiparticles. So the only known interaction that could be distinct from the electromagnetic is the gravitational, but there are theories where the gravitation would arise as a high order effect of the electromagnetic interaction. Moreover, even if there are contradictory results about the speed of gravitation, the more recent results seem to be consistent with the speed of the gravitational waves being the same as the speed of light.