# Outer bands of galaxies speed

1. Oct 7, 2011

I am wondering about the speed of the outer bands of galaxies and if they are moving at or faster than the speed of light?

And then the thought of the speed of space expansion and and the body of the galaxy moving and the rotation of planets around the stars(solar system) and the spin of the planets themselves. If these were all to move in the same direction at the same time, how fast would that be (if you were on the planet)? Got to be moving faster than the speed of light.

Or does the proximity of mass negate these?

Particles in the accelerator at just below the speed of light only adding the rotation of the earth would qualify, right?

2. Oct 7, 2011

### DaveC426913

what? No. They're moving quite slow indeed. Our galaxy takes more than 230 million years to complete one rotation.[/QUOTE]

No. Nowhere near.

Also, speeds are only meaningful when relative to point of reference. From the PoR of a rogue planet speeding through intergalactic space at .9c, our galaxy is indeed moving at .9c.

3. Oct 7, 2011

Thank you for the response!

Due to the size of the galaxy and the distance covered on the outer bands I wasnt sure.

I was thinking about the speed of everything included like the ride at Disney with the tea cups or at the carnival. The tea cup spins and it is on a disc that spins and it is on an even larger disc that spins and when you reach the outside of all three you are really moving!

4. Oct 7, 2011

### DaveC426913

I know.

The simplest model for this is a rocket flying along at .9c that fires a missile at .2c.

Does the missile travel at 1.1c? No.

Velocity at relativistic speeds is not linear, it uses what's called the Lorentz transform. The addition of two velocities will always result in a value less than c.

The missile's velocity (from an external point of view) will be somewhere between .9c and c (not inclusive).

5. Oct 8, 2011

So would a person who is running at 15mph on a conveyor belt that is moving at 15mph only be traveling at 15mph or 30mph?

6. Oct 8, 2011

### DaveC426913

From what frame of reference?

Obviously, from the FoR of someone stationary wrt the road on which a pickup carrying the treadmiill is driving - but it is important that we define what FoR we are using.

If we use the correct Lorentz transform then the speed of the walker would be a vanishingly small smidge less than 30mph.

7. Oct 8, 2011

I guess for this I am thinking about space and the point being a stationary position in space (allowing for the expansion) and watching a galaxy go by and rotating. If I was to measure the distance covered by a planet orbiting a star on tip of an outer band of a galaxy from that stationary position would that speed exceed the speed of light?

8. Oct 8, 2011

### phinds

Not even close. I doubt it would be any significant portion of c.

9. Oct 8, 2011

### DaveC426913

No but let's grant that you do manage to find the right combination of snowball thrown from a moving train on a planet orbiting clockwise while going around its clockwise.

You will not ever measure the other object as moving faster than c. Your measurement will always be a fraction of c. This is how velocity addition works in our relativistic world.

10. Oct 9, 2011