# What is the physical/geometric meaning of spacelike, timelike and null

## Main Question or Discussion Point

What is the physical/geometric meaning of spacelike, timelike and null geodesics?

## Answers and Replies

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What is the physical/geometric meaning of spacelike, timelike and null geodesics?
All objects with mass move on timelike geodesics, massless objects move on null geodesics, and nothing can move on spacelike geodesics since that would mean moving faster than the speed of light.

nothing can move on spacelike geodesics since that would mean moving faster than the speed of light.
So what then is the practical relevance of spacelike geodesics to general relativity?

So what then is the practical relevance of spacelike geodesics to general relativity?
I can't think of any

Jonathan Scott
Gold Member

So what then is the practical relevance of spacelike geodesics to general relativity?
Locally, they are usually known as "straight lines".

If points on the geodesic are at the same time coordinate, they describe straight lines in the observer's own frame. If they are not at the same time coordinate, they describe straight lines in some other inertial frame.

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http://en.wikipedia.org/wiki/Geodesic_(general_relativity [Broken])

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So what then is the practical relevance of spacelike geodesics to general relativity?
They have no physical usage in any branch of physics but in the FTL theories which lean upon a premise that says there are particles that can be accelerated in such a way that their speed would be able to pass the speed of light! An example could be tachyons. The reason why such particles follow spacelike geodesics is that since they have tremendously ultra-higher speeds than $$c$$, so an interval in space is travelled by them in a very tiny interval of time, letting the line-element $$ds^2$$ be smaller than zero.

AB

Stingray