I know differentials (in a way of standard analysis) are not very rigorous in mathematics, there are a lot of amazing answers here on the topic.
But what about line element?
ds2=dx2+dy2+dz2
The way I think about this line element is being geometrically constructed from Pythagoras theorem as:
Δs2=Δx2+Δy2+Δz2
and then we assume that we can 'get' these quantities (Δx) to be infinitesimally small (as small as we like) and represent as dx instead, right?
Now then lets take line element on a sphere:
ds22=r2sin2(θ)dϕ2+r2dθ2
It is geometrically constructed again using Pythagoras theorem and assuming that sides of a 'triangle' are small:
Δs22≈(rsin(θ)Δϕ)2+(rΔθ)2
But this approximation never really becomes equality, the smaller the angles the better it works, but still never equality! People just replace Δ−>d and say ds and say it's differential.
I guess my question is this:
when we write something like
ds22=r2sin2(θ)dϕ2+r2dθ2
we actually have in mind that this quantity contains higher order terms, but they will vanish after we parametrise?
I think about parametrisation in a way:
ds22dt2=r2sin2(θ)dϕ2dt2+r2dθ2dt2
No comments:
Post a Comment