# V relative to black hole

1. Feb 13, 2008

### W3pcq

When determining an objects velocity in space, what do we use to relate it to? Sometimes I get the impression that we use the earth as the reference sometimes. Do we ever consider our velocity relative to the black whole at the center of the galaxy?

We know that gravitational fields and high speeds can slow time down. We have the immense gravitational forces of the black whole in contrast with the speeds relative to the black whole. If you are close, the gravity will be high, and the speed low. If you are far away, the speed will be greater, but the gravitational fields weaker. That would be a way of the balance of time. Maybe as you get far away enough to not orbit the black hole, but to just float off on your own, then the effects of speed are diminished, and the effects of gravity are diminished. This would cause time to move faster. Could this be the cause of expansion? This of coarse assumes v relative to the center of the galaxy is the factor pertaining to special relativity. Maybe a wak idea, let me know?

Last edited: Feb 13, 2008
2. Feb 13, 2008

### Staff: Mentor

Since we've never sent a spacecraft much outside the solar system, all velocities are calculated relative to some object in the solar system. The earth, the sun, another planet, etc. "The fixed stars" do provide a reference for rotation, though.

Also, expansion is observed only on scales larger than a galaxy.

3. Feb 13, 2008

### pixel01

Because the sun orbits the milkyway's center at a very small angular velocity (about 245million years period) so we can consider the solar system is an inertial frame when calculating motion of spacecrafts.

Last edited: Feb 13, 2008