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I'm still surprised that Pi isn't considered a universal physical constant, given Buffon's needle depends on (IIUC) the physical curvature of nonrelativistic sp

by dekhn 2y ago

I'm still surprised that Pi isn't considered a universal physical constant, given Buffon's needle depends on (IIUC) the physical curvature of nonrelativistic space.

IHLayman 2y ago
Is it because pi isn’t measured, but calculated? The wikipedia article (https://en.m.wikipedia.org/wiki/Physical_constant https://en.m.wikipedia.org/wiki/Physical_constant) makes a distinction between a mathematical constant and a physical constant, stating that the latter cannot be calculated but instead needed to be measured experimentally… Pi could be measured experimentally, but it has an exact definition and can be calculated outside of any experiment.
illini1 2y ago
Buffon’s needle assumes a flat space, while a non-Euclidean space or geometry would affect the probability leading to a different value other than pi. You can treat the space around us as Euclidean, but that isn’t true for every part of the universe.
- not2b 2y ago
  Even in a non-Euclidean space with positive or negative curvature, the limit of the ratio of the circumference to the diameter of a circle as the diameter goes to zero is pi.
mitthrowaway2 2y ago
Up until 2019, it sort of was, via the magnetic constant mu_0 = 4 pi x 10^-7 H/m exactly. Unfortunately the 2019 revision of SI redefined it.
ants_everywhere 2y ago
If you run Buffon's needle a lot you come to the conclusion that we live in a locally Euclidean space. That's fine and good. We also live in what you might consider a "locally Newtonian" world, but when things get very big or very small, the Newtonian approximation breaks down. The ratio of a circle's circumference to its diameter (or equivalently the sum of triangle angles) has the same problem. If you want general relativity to work, then we need to live in a curved spacetime. Depending on whether that spacetime is positively or negatively curved, the angles of a very large triangle may add up to more than or less than pi radians.

by dekhn 2y ago

I'm still surprised that Pi isn't considered a universal physical constant, given Buffon's needle depends on (IIUC) the physical curvature of nonrelativistic space.

IHLayman 2y ago

Is it because pi isn’t measured, but calculated? The wikipedia article (https://en.m.wikipedia.org/wiki/Physical_constant https://en.m.wikipedia.org/wiki/Physical_constant) makes a distinction between a mathematical constant and a physical constant, stating that the latter cannot be calculated but instead needed to be measured experimentally… Pi could be measured experimentally, but it has an exact definition and can be calculated outside of any experiment.

illini1 2y ago

Buffon’s needle assumes a flat space, while a non-Euclidean space or geometry would affect the probability leading to a different value other than pi. You can treat the space around us as Euclidean, but that isn’t true for every part of the universe.

not2b 2y ago
Even in a non-Euclidean space with positive or negative curvature, the limit of the ratio of the circumference to the diameter of a circle as the diameter goes to zero is pi.

mitthrowaway2 2y ago

Up until 2019, it sort of was, via the magnetic constant mu_0 = 4 pi x 10^-7 H/m exactly. Unfortunately the 2019 revision of SI redefined it.

ants_everywhere 2y ago

If you run Buffon's needle a lot you come to the conclusion that we live in a locally Euclidean space. That's fine and good. We also live in what you might consider a "locally Newtonian" world, but when things get very big or very small, the Newtonian approximation breaks down. The ratio of a circle's circumference to its diameter (or equivalently the sum of triangle angles) has the same problem. If you want general relativity to work, then we need to live in a curved spacetime. Depending on whether that spacetime is positively or negatively curved, the angles of a very large triangle may add up to more than or less than pi radians.