# What are the Lorentz Transformations between polar coordinates? Or can Lorentz Transformations be Non-Linear?

This question rises from the comments on @G Smith's answer's to this question https://physics.stackexchange.com/a/603032/113699

Precisely I was trying to understand the Lorentz Transformations between Spherical Polar Coordinates. The point is that I have read that Lorentz transformations have to be Linear but if for spherical polar coordinates they will be Non-Linear.

These answers here say that Interval preserving transformations/ Lorentz Transformations need to be linear Interval preserving transformations are linear in special relativity and Kleppner derivation of Lorentz transformation ( the answer by Selena Routley) and Why is this non-linear transformation not a Lorentz transformations? It does preserve $x^2 + y^2 + z^2 - c^2t^2 = x'^2 + y'^2 + z'^2 - c^2t'^2$ ( especially Knzhou's comment on A Hussain's answer where he also says we should do SR in cartesian coordinates and not polar coordinates).

But the answer by Void here Are Lorentz transformations linear transformations?

says that Lorentz Transformations should be Homogenous and not Linear.

So my question is-

Can be there Lorentz Transformations between spherical polar coordinates or cylindrical coordinates. If yes, then they will be Non-Linear. So can the Lorentz Transformations be Non-Linear too?

• @CosmasZachos Sorry, I didn't understand the clue. How is rotation to going help. I want to write the Lorentz Transformations for boosts in spherical coordinates. In the 1st link G Smith says it's possible and perfectly okay to write Lorentz Transformations for boosts in spherical or cylindrical coordinates. But then my query is that I have read in Caroll, that Lorentz Transformations are linear. If I will write Lorentz Transformations for boosts in spherical coordinates, the transformations would be non linear. Can Lorentz Transformations be Non Linear. Could you please elaborate the hint Dec 27, 2020 at 5:56
• You were given the explicit answer in the answers you quote. Dec 27, 2020 at 16:14
• @CosmasZachos yes. But those Lorentz transformations are not linear in the coordinates. Lorentz transformations need to be linear. The question I am asking is can Lorentz Transformations be NON LINEAR Are they allowed to be non linear... Dec 27, 2020 at 16:22
• ? A linear function of nonlinear functions can be nonlinear. Dec 27, 2020 at 16:39
• @CosmasZachos so it doesn't mean that Lorentz Transformation need to be always linear. Is that correct, because the 1st three linked questions have answers where the answered insists that Lorentz Transformation need to be Linear ( Caroll also says that)... What's the final thing... Can Lorentz Transformations be NoN linear. They don't need to be necessarily linear Dec 27, 2020 at 16:42

If you choose polar (or other nonlinear images of Cartesians), even in Minkowski spacetime we don't have this luxury anymore and the Lorentz transformation $$\Lambda:T_XM \to T_XM$$ only has meaning as the linear transformation between the common tangent spaces $$T_XM$$ to the manifold $$M$$ constructed by two two relatively moving observers at the point $$X$$ where they are momentarily collocated. A tangent vector $$A$$ with contra components $$A^j$$ as seen by observer $$\mathbf{A}$$ naturally has components $$B^k = \Lambda^k_{{}j} A^j$$. And, as usual, if we want to construct the components of $$\Lambda$$ in polars $$\bar{x}$$ from the Cartesian components $$x$$, we invoke $$\Lambda^k_{{}j}\mapsto \frac{\partial x^k}{\partial \bar{x}^\ell} \frac{\partial \bar{x}^m}{\partial x^j} \Lambda^\ell_{{}m}$$, where the expressions of the form $$\frac{\partial x^k}{\partial \bar{x}^\ell}$$ are calculated from the derivatives of the polar in terms of Cartensian co-ordinate transformation in the wonted way.
I'm pretty sure i have made exactly this mistake too in the past, and it arises from the ubiquitous use of Cartesians in special relativity, which in turn makes us forget that Lorentz transformations do not usually apply to global co-ordinates. Of course, when you wish to do this latter, you concatenate $$C\circ \Lambda \circ C^{-1}$$ where $$C$$ is the nonlinear polar to Cartesian co-ordinate conversion. This is a nonlinear process and a simplification of the more general, nonlinear process of co-ordinate transformations between charts in a general manifold.