Timeline for Why is the candela a base unit of the SI?
Current License: CC BY-SA 3.0
7 events
| when toggle format | what | by | license | comment | |
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| Mar 17 at 12:28 | review | Suggested edits | |||
| Mar 17 at 19:19 | |||||
| Jan 19, 2016 at 22:51 | comment | added | Emilio Pisanty | The point is that measures of the brightness of a light source are split completely down the middle: one set which does not require access to humans to measure (i.e. using appropriate bolometers), and one set which does. Same for kinematic measures which require access to a standard mass versus the ones which don't. | |
| Jan 19, 2016 at 22:49 | comment | added | Emilio Pisanty | Yes. In the meantime, we'll just keep calling "luminous intensity as perceived by humans" simply "luminous intensity". The perceived brightness is as much an independent physical dimension from radiant intensity as length is independent from time: both pairs can be related given access to a special object (resp. light and humans) which we assume to be constant for the duration of the standard. If you're OK with taking down length as an independent physical dimension then that's fine with me, but I gotta wonder why you're debating the SI base units at all. | |
| Jan 19, 2016 at 22:43 | comment | added | David Hammen | @EmilioPisanty -- This is not a new physical dimension. The issue of how many physical dimensions exist is highly debated. Maybe zero, maybe one, maybe three, but not seven. The candela and units derived from it exist because people are important from a perspective of commerce and industry (which are the ultimate drivers of the standards). From the perspective of physics, there's nothing special going on here. Technically advanced beings who live on a planet orbiting a red dwarf will have a very similar concept of the speed of light but a very different concept of luminosity. | |
| Jan 19, 2016 at 22:25 | comment | added | Emilio Pisanty | I disagree. If commercial and industrial standards were the only thing at play, the candela would be very welcome as a unit accepted for use with SI units like the hectare or the AU, with no need to grant it base unit status. The reason it's a base unit is that luminous intensity is brightness as perceived by the human eye, and is this that introduces a new physical dimension and the corresponding need for a base unit. | |
| May 10, 2015 at 20:48 | comment | added | Sebastian Riese | Theoretical physicist in education here, and believe me: SI is far, far, far better than US customary units. And SI will improve vastly with the new definitions planned in the next years (all units will be defined by fixing universal constants). Although I have to admit I use atomic units, geometric units or Gauss-units where appropriate. The only thing that really annoys me about SI, is that temperature has its own base unit (instead of making the Boltzmann factor merely an energy unit conversion factor). | |
| May 10, 2015 at 16:13 | history | answered | David Hammen | CC BY-SA 3.0 |