# Light years-distance or light travel time?

1. Oct 6, 2006

### Chip Orr

If I use light years to describe large reception distances,say 10 billion,am I also stateing the light travel time? The way I figure it at these distances you would have to increase the light travel time useing the hubble constant to quantify the increase in space a photon would have to travel. Kind of like swiming against a current to reach a destination.Parasecs are what I should state as a distance modulus at these lengths,but that does'nt describe light travel time. So,when useing LYs are you stateing distance & light travel time or just distance? This has been bugging me for a while now!! Please help me out with this one. Thanks.

2. Oct 6, 2006

### mathman

Light year is distance, defined by distance a photon travels in one year. Don't worry about Hubble constant - it doesn't effect the definition.

3. Oct 6, 2006

### Chip Orr

At 10 billion LYs is the light travel time 10 billion years,or is it longer due to the expantion of space? Thanks ,Chip

4. Oct 6, 2006

### hellfire

Light always travels 10 billion light-years in 10 billion years. However, when you send a light ray and wait 10 billion years, it will be located farther away than 10 billion light-years from you on a surface of simultaneity (of constant time) in the universe, because the space between you and the light ray has expanded during the 10 billion years of travel.

The distance between you and the light ray can be expressed according to both definitions. The proper distance, for the distance on a surface of constant time (which is by the way the one that enters the Hubble law) and the light travel distance, for the distance traveled by light. You just have to make clear which one is being used. And, of course, you can express both in light-years or in parsec.

Last edited: Oct 6, 2006