- #1
Pencilvester
- 199
- 46
- TL;DR Summary
- I haven’t studied coordinate change law— what am I doing here that’s illegal?
I would guess there’s some subtlety in the relationship between basis vectors and coordinates that I’m ignoring, but I really have no idea.
$$ ds^2 = -dt^2 + d\tilde{x}^2 $$
$$ d\tilde{t} = dt / \sqrt{\tilde{x}} $$
$$ \downarrow $$
$$ ds^2 = -\tilde{x} ~ d\tilde{t}^2 + d\tilde{x}^2 $$
$$ dx = -d\tilde{x} ~ \Rightarrow ~ \frac{d\tilde{x}}{dx} = -1 ~ \Rightarrow ~ \tilde{x} = -x ~ (+ C) $$
$$ \downarrow $$
$$ ds^2 = x ~ d\tilde{t}^2 + dx^2 $$
$$ dy = \sqrt{x} ~ d\tilde{t} $$
$$ \downarrow $$
$$ ds^2 = dx^2 + dy^2 $$
$$ ds^2 = -dt^2 + d\tilde{x}^2 $$
$$ d\tilde{t} = dt / \sqrt{\tilde{x}} $$
$$ \downarrow $$
$$ ds^2 = -\tilde{x} ~ d\tilde{t}^2 + d\tilde{x}^2 $$
$$ dx = -d\tilde{x} ~ \Rightarrow ~ \frac{d\tilde{x}}{dx} = -1 ~ \Rightarrow ~ \tilde{x} = -x ~ (+ C) $$
$$ \downarrow $$
$$ ds^2 = x ~ d\tilde{t}^2 + dx^2 $$
$$ dy = \sqrt{x} ~ d\tilde{t} $$
$$ \downarrow $$
$$ ds^2 = dx^2 + dy^2 $$