Talk:Q173227
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Autodescription — physical constant (Q173227)
description: physical quantity that is generally believed to be both universal in nature and constant in time
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Are physical constants measured values?[edit]
@Infovarius: In my opinion, subclass of measured value should be removed again. For example speed of light in vacuum is a physical constant. You can measure the constant when meter is defined as some fraction of the earth's circumference and second as some fraction of a year, however today it is 299792458 m/s per definition, measuring something else means redefining your time unit to be different from SI second. If there is an instance of a physical constant that can be regarded as not being an instance of a measured value, then physical constant should not be subclass of measured value.--Debenben (talk) 18:21, 13 November 2019 (UTC)
- Speed of light is an exception and almost unique in this. And even it can be marked as measured value (in the past, or may be in some other system of units?). --Infovarius (talk) 21:40, 14 November 2019 (UTC)
- @Infovarius: You could take the view that speed of light not a physical constant anymore since 1960 when meter was redefined. It is just a definition like the constant 60 s/min that defines the unit minute in terms of second. SI defines minute and second to have the same dimension which allows you to write 1 = 60 s/min explicitly, whereas you are only allowed to write 1 = 299792458 m/s if you use natural units where the dimension of meter and second is the same.
- Since the redefinition of SI in 2019 the speed of light is no longer an exception: Planck constant, elementary charge, Boltzmann constant and Avogadro number have also defined values now, such that all SI units are now defined by defining the value of what used to be a measured physical constant, see w:en:International_System_of_Units#2019. The convention which units are called SI base units and have a different dimension is kept for historic reasons / convenience only.
- There are only few physical constants like the fine-structure constant that are not defined but measured and generally believed to be constant. Most physical constants like the cosmological constant are generally believed to change slowly over time like the solar constant. Those constants have in common that there is (or was) no established theory to assign an exact mathematical value or explain their current measured value.
- I am not really sure how to best reflect this in terms of wikidata properties, however I would favor not stating anything over stating something that can be regarded as wrong.--Debenben (talk) 16:49, 15 November 2019 (UTC)
- Then I suppose "speed of light-pre1960" and "speed of light-post1960" differs (they have even different expression in m/s). I can imagine 2 ways or reflecting this: either to add statements here with some qualifiers and lower rank, or create another item for pre1960 definition. The same for Planck constant and other examples. --Infovarius (talk) 16:48, 19 November 2019 (UTC)
- @Infovarius: I guess you could say the speed of light differs, but so does everything else because the formal way of how to measure lengths in SI changed. There are countless other examples e.g. Boltzman constant and thus temperature before and after 2019. It can even be the other way around: Before 2019 the magnetic constant had a defined value, now it is measured. My point is the statement that physical constants are subclass of a measured value is wrong, it directly contradicts the other subclass statement that states that physical constant would be a subclass of a mathematic constant (which by definition cannot be measured). I believe the correct way of handling this in wikidata would be to remove both subclass statements from this item about physical constants. Then speed of light can be an instance of a physical constant without qualifier, an instance of a measured value until 1960 and an instance of a mathematical constant from 1960 until today.--Debenben (talk) 21:06, 29 November 2019 (UTC)
- Sounds good, thanks. --Infovarius (talk) 16:10, 2 December 2019 (UTC)
- @Infovarius: I guess you could say the speed of light differs, but so does everything else because the formal way of how to measure lengths in SI changed. There are countless other examples e.g. Boltzman constant and thus temperature before and after 2019. It can even be the other way around: Before 2019 the magnetic constant had a defined value, now it is measured. My point is the statement that physical constants are subclass of a measured value is wrong, it directly contradicts the other subclass statement that states that physical constant would be a subclass of a mathematic constant (which by definition cannot be measured). I believe the correct way of handling this in wikidata would be to remove both subclass statements from this item about physical constants. Then speed of light can be an instance of a physical constant without qualifier, an instance of a measured value until 1960 and an instance of a mathematical constant from 1960 until today.--Debenben (talk) 21:06, 29 November 2019 (UTC)
- Then I suppose "speed of light-pre1960" and "speed of light-post1960" differs (they have even different expression in m/s). I can imagine 2 ways or reflecting this: either to add statements here with some qualifiers and lower rank, or create another item for pre1960 definition. The same for Planck constant and other examples. --Infovarius (talk) 16:48, 19 November 2019 (UTC)