# Where do the lines come from?

1. Sep 12, 2008

### chhitiz

if we assume the existence of gravitational field lines to be true, it implies that all massive bodies are spheres with field lines converging into them from all directions. this would be true for all planets, stars, even blackholes. so, where do these lines come from?

2. Sep 12, 2008

### atyy

From the bodies. This is why we say that mass is the source of gravity.

3. Sep 12, 2008

### chhitiz

but those lines converge into the bodies. what i am asking is where do they emerge from?

4. Sep 13, 2008

### granpa

good point. they have to end somewhere.

5. Sep 13, 2008

### Defennder

I think you're taking the concept of field lines too literally. The lines aren't real, they're just the path a point mass would take if it were placed along a field line, neglecting the gravitational attraction the point mass would have on the matter configuration from which the field lines terminates.

6. Sep 13, 2008

### granpa

I think you are taking his question too literally. the lines are mathematical artifacts but they still have to end somewhere.

7. Sep 13, 2008

### granpa

flux lines are not just the path a particle would take. as I'm sure you know, they also encode the strength of the field at each position. they can do this because the field follows an inverse square law (in 3 dimensions).

8. Sep 13, 2008

### granpa

I guess if the flux lines from a planet are thought of as the slope of a scalar field then eventually they must come back together again on the opposite side of the universe.

9. Sep 13, 2008

### Defennder

What does "opposite side of the universe" mean?

10. Sep 13, 2008

### granpa

as in universe=surface of hypersphere.

11. Sep 13, 2008

### chhitiz

what is a hypersphere? reminds me of the model in a book where universe was the surface of an ever expanding balloon

12. Sep 13, 2008

### Staff: Mentor

Field lines all come from the same source, wether you are talking about gravitational, electrical, or magnetic field lines. That source is your pen (or other graphing implement).

13. Sep 13, 2008

### ZapperZ

Staff Emeritus
Defennder is correct. One is taking the notion of "field lines" too literally here. One should always look at the formulation. Look at Coulomb's Law, for example, for a point charge. Can you see "lines" in that formulation? Or what about surfaces? Do you imagine the equipotential surfaces when you look at Coulomb's Law? No? Then why lines? After all, in a Hamiltonian, it is the potential energy form that is of more importance than forces.

What we end up doing in trying to conceptualize the direction of the force on a test charge in such a field is to sketch out such forces (or electric field). This is simply a "scheme" to help us tackle a problem or a visualization. It has nothing to do with ANY physical "lines".

Thus, the question on where "lines" ends really needs to be clarified as how a particular field geometry can be described at a boundary, meaning that this is more of a boundary condition issue than anything else.

Zz.

14. Sep 13, 2008

### chhitiz

what i meant was, if magnetic field lines originate from a positive pole, is it possible for matter to have 'emerging' gravitational field lines?

15. Sep 13, 2008

### granpa

I think you mean electric field lines. magnetic field lines are circles around the charges.

there are of course no actual lines. the lines are there just to help us visualize the vector field. if gravity didnt follow an inverse square law then flux lines wouldnt work. we would have to use something else. (actually inverse square works only in 3 dimensions. in 6 dimensions you would need an inverse 5th power law).

I have no idea what you mean by 'emerging'.

flux lines will work for any field that follows an inverse square law in 3 dimensions. that includes gravity.

that assumes an infinite perfectly flat universe. what happens when the universe inst infinite or flat I dont know.

16. Sep 14, 2008

### Defennder

If I'm not wrong he's asking where does he start drawing say a gravitational field line from the tail arrow to the head. Where do the lines emerge from? I think DaleSpam answered that question already. I don't think there's any place in the universe you could situate 2 masses without them having any gravitational attraction between them, so if you wanted to, you should start drawing at the universe's boundaries.

Unless of course there is such a thing as "negative mass" and I don't mean the semiconductor physics concept.