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

1. Mar 3, 2010

### Ranku

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

2. Mar 3, 2010

### Amanheis

Re: 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.

3. Mar 3, 2010

### Ranku

Re: Geodesics

So what then is the practical relevance of spacelike geodesics to general relativity?

4. Mar 3, 2010

### Amanheis

Re: Geodesics

I can't think of any

5. Mar 3, 2010

### Jonathan Scott

Re: Geodesics

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.

Last edited: Mar 3, 2010
6. Mar 3, 2010

### Naty1

Re: Geodesics

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

Last edited by a moderator: May 4, 2017
7. Mar 3, 2010

### Altabeh

Re: Geodesics

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

8. Mar 3, 2010

### Stingray

Re: Geodesics

Spacelike geodesics are commonly used in defining distances between two timelike worldlines (such as particle trajectories), constructing hypersurfaces "of constant time," etc.

9. Mar 4, 2010

### Ranku

Re: Geodesics

Thank you all...