# Sources of gravitational fields

1. Feb 27, 2004

### lamp post

positive charges are sources of electric field and -ve charges are sink. what are sources and sink of gravitational fields?

2. Feb 27, 2004

### Staff: Mentor

Putting aside general relativity, I would say that mass is the "source" of the gravitational field. There is no "sink".

3. Feb 27, 2004

### turin

It's not that simple.

Poisson's equation for electrostatics (non-relativistic limit):
div E = +&rho;charge

Poisson's equation for gravity (non-relativistic limit):
div g = -&rho;mass

Therefore, these are two fundamentally different types of fields. The electric one sources with respect to like particles and sinks WRT unlike particles. The gravitational one sinks WRT everything.

4. Feb 28, 2004

### pmb_phy

Mass is the source of the gravitational field

5. Feb 28, 2004

### karanrustagi

source and sink of gravitational field is the center of mass itself

6. Feb 28, 2004

### Nereid

Staff Emeritus
What about anti-matter? Does a neutral anti-hydrogen atom (a hydrogen anti-atom? I mean a positron and anti-proton) fall to the floor of the chamber it's created in (somewhere in the CERN facilities, for example), or rise toward the ceiling (assuming other forces on the atom are balanced)?

To what extent has the gravitational interaction between matter and anti-matter been observed? What is the maximum deviation from theory consistent with the best experimental data? (or, how well do observations match predictions from theory?)

7. Feb 28, 2004

### Andre

Nereid,

I believe that the equivalence of accelleration and gravity forces (the elevator tale) is part of the general relativity theory. Anti-matter should behave the same to acceleration forces as normal matter, hence it is predicted that anti-matter is also reacting normally on gravity.

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/antimatterFall.html [Broken]

Last edited by a moderator: May 1, 2017
8. Feb 28, 2004

### pmb_phy

Antimatter is just matter. The name is misleading. E.g. Each particle has an antiparticle where antipartilc is defined as
a particle that has the same mass as another particle but has opposite values for its other properties; interaction of a particle and its antiparticle results in annihilation and the production of radiant energy.

It can't be said that one is matter and the other matter. It's really a matter of which one is discovered first

9. Feb 28, 2004

### Nereid

Staff Emeritus
What experimental data is there on the gravitational interaction of anti-matter?

IIRC, quite a few of the other properties of anti-matter have been tested experimentally, but I don't recall seeing anything on observations of gravity.

10. Feb 29, 2004

### pmb_phy

That isn't really a meaningful question. E.g. suppose there are two particles A and B and the two form an anti-particle pair. By this it is mean that A is the anti-particle of B and B is the anti-partilce of A. Each has a well defined mass and it is that which is the source of gravity and it is that which responds to gravity. It is therefore not meaningul to call one matter and the other anti-matter. It's only an historical quirk that the particles have those names. Had the positron been discovered before he electron then it would be the electron which would have been called "anti-matter" and not not the anti-electron - aka positron. The electron might then a have been called the anti-positron.

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/antimatterFall.html [Broken]

http://info.web.cern.ch/info/Press/PressReleases/Releases1996/PR01.96EAntiHydrogen.html

Last edited by a moderator: May 1, 2017
11. Feb 29, 2004

### Nereid

Staff Emeritus
Thanks pmb and Andre.

I really liked the opening para in the first link you posted:

"In theory, antimatter dropped over the surface of the Earth should fall down. However, the issue has never been successfully experimentally tested. The theoretical grounds for expecting antimatter to fall down are very strong, so virtually all physicists expect antimatter to fall down -- however, some physicists believe that antimatter might fall down with a different acceleration than that of ordinary matter. Since this has never been experimentally tested, it's important to keep an open mind."

A sentence or three in the third para of the second are also good:

"If the behaviour of anti-hydrogen differed even in the tiniest detail from that of ordinary hydrogen, physicists would have to rethink or abandon many of the established ideas on the symmetry between matter and antimatter. Newton's historic work on gravity was supposedly prompted by watching an apple fall to earth, but would an "anti-apple" fall in the same way? It is believed that antimatter "works" under gravity in the same way as matter, but if nature has chosen otherwise, we must find out how and why."

No matter how solid we believe a theory is, we should always be looking for ways to test it further.

[Edit: added Andre (I hadn't realised that the Andre's and pmb's first link were the same ]

Last edited: Feb 29, 2004