Number of unknowns - Coordinate Transforms

  • Context: Graduate 
  • Thread starter Thread starter thehangedman
  • Start date Start date
  • Tags Tags
    Coordinate Unknowns
Click For Summary

Discussion Overview

The discussion centers on the number of unknowns involved in generic coordinate transformations in the context of general relativity. Participants explore the dimensionality of these transformations and whether they can be categorized into specific types such as boosts, rotations, and reflections.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests that there are 16 unknown functions of space-time for a generic coordinate transform, based on a 4x4 transformation matrix.
  • Another participant argues that there are actually an infinite number of unknowns, even in a 1D manifold, due to the infinite degrees of freedom in functions.
  • A later reply clarifies that the initial question was misphrased, indicating a focus on the number of functions in R^4, while still leaning towards the idea of 16 functions.
  • One participant proposes that there are only 4 unknown functions, suggesting a specific form for the transformation equations.
  • Another participant introduces the metric tensor gμv, noting it has 16 components but is symmetric, which reduces the number of independent components to 10.

Areas of Agreement / Disagreement

Participants express differing views on the number of unknowns, with some asserting 16, others proposing 4, and yet others stating there are infinite degrees of freedom. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants mention potential symmetries that might affect the number of independent components, but these aspects remain unclear and unresolved.

thehangedman
Messages
68
Reaction score
2
In general relativity, what are the total number of unknowns for a generic coordinate transform? Is it just 4 * 4 = 16? Is there a way to break those down into combinations of types, such as boosts, rotations, reflections (parity?), etc, or is it just left wide open from an interpretive standpoint? My feeling is the answer is in fact 16 unknown functions of space-time and that the actual interpretation can't really be broken out like we do in SR (Lorentz)...

Your help is greatly appreciated...
 
Physics news on Phys.org
It is an infinite number of unknowns. Even if you only had a 1D manifold, there are an infinite number of degrees of freedom.
 
DaleSpam said:
It is an infinite number of unknowns. Even if you only had a 1D manifold, there are an infinite number of degrees of freedom.

I made a mistake in my question. I know that functions have an unlimited number of degrees of freedom, I meant to ask how many functions are involved in a generic coordinate transform in R^4. My guess is 16, since there are two indexes in the transformation matrix, each running over 4 values, but wanted clarity since there might be symmetries that eliminate elements (though I'm guessing not).
 
I think it is only 4 unknown functions. You can always write it as:
x'_0=f_0(x_0,x_1,x_2,x_3)
x'_1=f_1(x_0,x_1,x_2,x_3)
x'_2=f_2(x_0,x_1,x_2,x_3)
x'_3=f_3(x_0,x_1,x_2,x_3)
 
May be you are talking about gμv which seems to have 16 components.But it is symmetric,which will eliminate 6 so there will be only 10 independent components.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 12 ·
Replies
12
Views
3K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 33 ·
2
Replies
33
Views
8K
  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 29 ·
Replies
29
Views
2K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 85 ·
3
Replies
85
Views
13K