If position in the universe is relative, why don't binary stars fall together?

Question

As I understand it, objects in the universe cannot be assigned an absolute position in space. Their locations can only be described in relation to other objects.

If this is the case, the two stars forming a binary star, orbiting around their centre of mass, could be considered to be stationary (relative to each other).

Why then do the two stars not fall together due to the gravitational attraction between them?

Does the fact that they don't fall together prove that there is a kind of 3D "cosmic grid" on which all objects can be located?

Answer 1

If you pick a reference frame in which the binary stars are stationary and the REST OF THE UNIVERSE is rotating, then the stars of the rotating universe will create an additional gravitational field due to their increased energy and momentum. This gravitational field will be exactly that required to prevent the binary system from collapsing.

General Relavtivity works for all reference frames, including those that are rotating or accelerating.

Answer 2

They aren't stationary; their angular momentum keeps them moving in their orbits the same way that the Earth orbits the sun. In many cases the binary stars are actually quite a large distance away from each other. Over time, however, the orbits decay as the angular momentum is dissipated through gravity wave emission, and the binary stars do approach each other, very slowly and very gradually. This takes quite a long time, so the stars have normally aged to white dwarfs or neutron stars by the time it becomes significant. Astronomers are able to observe this happening using radio telescopes. Eventually, if the binary stars are large enough, their merger would create a black hole.

Answer 3

The first two answers are correct and good. The 3D "cosmic grid" you mention is called a frame of reference. You can choose any frame of reference and locate objects on it by their coordinates in that frame of reference. I can choose another and it's just as valid. Since objects in the universe are moving with respect to each other, the coordinates in any reference frame will be continuously changing. The one in the reference is used by astronomers to catalog the locations of celestial objects. You would need to add distance from earth to make it a complete 3D coordinate system. Since the major motion of celestial bodies is the expansion of the universe, the equatorial coordinates change very little compared to the distance. And none change significantly during a human lifetime.

Answer 4

Hi. The same gravity and orbital processes we seen here (the moon does not fall to the Earth) works in multiple star systems. They simply orbit a common center of gravity.