STRACTWhy Do We Always Rescale the Null Direction in Conformal Compactification?

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SUMMARY

The discussion focuses on the necessity of rescaling the null direction in the context of conformal compactification, specifically referencing Minkowski spacetime and the Kruskal extension of Schwarzschild. Rescaling the null direction preserves the general form of the metric, allowing for a conformal compactification that maintains the directional integrity of spacetime. In contrast, rescaling the spatial (x) or temporal (t) coordinates does not preserve the metric's general form, although it may still be applicable if the rescaling factor is coordinate-independent.

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  • Understanding of Minkowski spacetime
  • Familiarity with the Kruskal extension of Schwarzschild
  • Knowledge of conformal compactification techniques
  • Basic principles of metric preservation in general relativity
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paweld
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Let's consider for example compactification of Minkowski spacetime or
Kruskal extension of Schwartzschild. They are quite similar because in both cases
we rescale the null direction.
I wonder why we always rescale the null direction, not simply x or t.
 
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paweld said:
Let's consider for example compactification of Minkowski spacetime or
Kruskal extension of Schwartzschild. They are quite similar because in both cases
we rescale the null direction.
I wonder why we always rescale the null direction, not simply x or t.

Re-scaling the null direction has this advantage that since one side is equal to zero, then the re-scaling factor won't change the direction but rather compacts (or maybe expands) the direction conformally. However, re-scaling x or t does not preserve the general form of metric though wouldn't change its nature, too if the re-scaling factor isn't coordinate-dependent! In the latter case (a factor being independent of coordinates), one can make use of the re-scaling of x or t as well!

AB
 

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