# Velocity and recessive velocity

1. Sep 5, 2005

### dracobook

What is the difference between velocity and recessive velocity. I read that with velocity, an object may not past the speed of light; but with recessive velocity, an object may past the speed of light.

2. Sep 6, 2005

### James R

There's no difference. "Recessive velocity" or "velocity of recession" just refers to the velocity of some object moving away from the observer. The opposite would be "approaching velocity" or "velocity of approach", for example.

The speed of light is a limit on the speed of an object in space, whether an object is coming towards you or going away. However, maybe you are thinking of the expansion of the universe, in which space itself expands, carrying objects such as galaxies along with it. The expansion of space, as opposed to the movement of objects in space, is not limited by the speed of light.

3. Sep 8, 2005

### dracobook

thanks James R. Yeah I was thinking of the expansion. I also have another problem. Well, according to relativity (I think) the speed of light is the same for all reference points. So, imagine if you willl, that you are a particle of light. You are travelling in a straight line from point A to B and another particle of light is travelling parallel and in the same direction as you. According to relativity, wouldn't the second particle of light be traveling ~3*10^8 meters per second relative to you? Wouldn't this happen if the object was traveling parallel yet in the opposite direction as well? ..:( I don't think I understand relativity very well..

4. Sep 10, 2005

### James R

You can't really look at things from the point of view of a particle of light. From that particle's perspective, the size of the universe would be zero, for a start.

If you consider your scenario from the point of view of a person travelling at 99.9999% the speed of light, then you would indeed see the light particle travelling the same direction as you as going at 3*10^8 metres per second, as usual.

5. Sep 10, 2005

### Sojourner01

At this point might I clarify that the crux of this is that all velocity is relative. There is no such thing as absolute velocity according to Relativity and every situation must be considered by the relative velocities of objects with reference to one another. In this sense, recession velocity would be the velocity with which an object moves away from the observer's specific point in space. Recession in this context just means 'moving away from'.

Hope that helps.