Deep Space Shots... Remarkable

[NickThissen]

What he says is actually true though, well at least part of it. The galaxies aren't moving very fast at all, it's the space in between that's expanding, and making them appear to move faster than the speed of light. The expansion of the universe is not simply galaxies moving away from each other at some speed. It's actual space expanding, which is a very different, and very hard to understand concept.

[Code Doc]

What he says is actually true though, well at least part of it. The galaxies aren't moving very fast at all, it's the space in between that's expanding, and making them appear to move faster than the speed of light. The expansion of the universe is not simply galaxies moving away from each other at some speed. It's actual space expanding, which is a very different, and very hard to understand concept.

Consider this:
Suppose that object A is moving at the speed of light. Object B is moving at the speed of light in the opposite direction, exactly colinear.

A<---------.---------> B

That means that a person on object A is moving away from a person on object B at twice the speed of light (and vice versa). However, it would be impossible for any two objects to be moving away from each other at more than twice the speed of light because the speed of light sets the limit on how fast an object can travel relative to a fixed point.

Make sense?

[NickThissen]

Consider this:
Suppose that object A is moving at the speed of light. Object B is moving at the speed of light in the opposite direction, exactly colinear.

A<---------.---------> B

That means that a person on object A is moving away from a person on object B at twice the speed of light (and vice versa). However, it would be impossible for any two objects to be moving away from each other at more than twice the speed of light because the speed of light sets the limit on how fast an object can travel relative to a fixed point.

Make sense?

If we ignore the expansion of space for a moment, then the statement in bold is false. They cannot move away from each other at twice the speed of light. Velocities cannot simply be added to find the new velocity.
Suppose you are in a car moving at 50 mph. Then you shoot a bullet at 200 mph relative to the car. How fast is the bullet traveling relative to the ground? You might say 250 mph, but that's false. It's actually something like 249.99999 mph. This is due to special relativity, but as you can see, it makes only a tiny difference in 'every day' speeds. If you get to the speed of light though, it makes a huge difference.
Now let's suppose your car is moving at 0.9c and the bullet you fire at 0.95c. According to your reasoning (simply adding the velocities) we would have about 1.85c, which is not possible.
Applying special relativity however, we get something like 0.999895c (I just made this up, but it will be similar to that).

The underlined statement is also not quite correct. The speed of light is constant in every inertial reference frame. An inertial reference frame is one that is not accelerating. Even if you are moving at a speed of 0.9c and then fire a laser beam, the speed of the light in the laser will still be moving at c relative to you. You do not see any difference, even though you are moving so close to c yourself.

Anyway, this has nothing to do with galaxies apparently moving at speeds > c. That is due to the expansion of space in between us; the space carries the galaxies away. The further away they are, the faster they are moving.