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qtm912

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- Thread starter qtm912
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- #1

qtm912

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- #2

PeterDonis

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A null surface is a surface that has null tangent vectors; in other words, there are null vectors that lie completely within the surface. For example, the event horizon of a black hole is a null surface.

A null geodesic is just a geodesic whose tangent vector is null; for example, the worldline of a light ray traveling through a vacuum is a null geodesic. So there is indeed a close connection between null vectors and null geodesics.

- #3

qtm912

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- #4

WannabeNewton

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It means the vector field associated with the coordinate is null.

- #5

qtm912

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Ok many thanks to you both for clarifying.

- #6

WannabeNewton

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Note that for any tangent vector ##v \in T_p \Sigma##, ##v \perp \nabla r## implies that ##v## is itself null so this is equivalent to Peter's definition above.

- #7

qtm912

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Hi again, I am a bit unfamiliar with the notation here, how is g(del-r, del-r) defined?

- #8

Abbas Sherif

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That's the scalar product (the dot product) of the vector field del-r with itself.Hi again, I am a bit unfamiliar with the notation here, how is g(del-r, del-r) defined?

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