# Can gravatational force only influence objects with energy?

1. Jan 19, 2016

As shown

2. Jan 19, 2016

### Buzz Bloom

Regards,
Buzz

3. Jan 19, 2016

GR, sorry.

4. Jan 19, 2016

### phyzguy

All objects have energy. Can you name an "object" with no energy?

5. Jan 19, 2016

### Buzz Bloom

I confess I am not an expert, but my understanding about GR is that the concept of force is not used. A gravitating body, e.g. the sun, does not in GR exert a force on an object, e.g. a planet. Rather, it distorts space (and time) so the the paths follow geodesics in the curved space. One can imagine a "test particle" with infinitesimal mass-energy. Such a particle will (approximately) follow the same geodesic as a planet. Thus the mass-energy of the relatively low mass object under the gravitational influence of a massive body does not depend significantly on the mass of the object. A relatively large mass object, like a planet, also distorts space, so it will follow a slightly different path than a low mass object. This is consistent with the Newtonian motion relative to the center of gravity of two masses.

One can conceptually extrapolate from an infinitesimal mass to zero mass which exactly follows the geodesic in the distorted space due to the large mass, e.g., the sun. Any real object does not exactly follow this geodesic since it also distorts the space.

Regards,
Buzz

6. Jan 19, 2016

Ya i was well aware of the wordings but i cant think of any replacement.

7. Jan 19, 2016

Well articulated thanks.
Thanks for your well articulated response. Or should i word the question like this : Is it valid to prove that an object has mass solely because of the fact that its path can be bent or altered by gravitational fields?

8. Jan 19, 2016

### Buzz Bloom

i assume by "mass" you mean "mass-energy".

It seems to me that the answer to your question is "No." If one imagines the existence of an object with no mass-energy, then such an object would follow a path through bent space, and the path would be approximate the same as the path an object such as a neutrino or a photon with very little energy would follow.

Regards,
Buzz

9. Jan 19, 2016