# Q & A: time and space

Q:
How is time related to the speed of light?

Surely when we see things that are light years away, this tells us that we are only "perceiving" them as being in the past.

Doesnt the fact that we can never observe things "now" really only tell us that we can never observe things "as they happen", not that time itself is a physical part of space? Or have I got this wrong?
- john gribben (age 39)
belfast
A:
You're right that if there were just a delay due to the finite speed of light, that would not in itself mix up time and space into one mathematical unity. That's how people thought the world was from about 1700 to about 1900. It turns out, however, that they are mixed up in the following sense: If you measure the time delay and the spatial distance between two events, you get different numbers depending on your state of motion. If you try to figure out what distance number another observer will get, you need to know both your time and distance numbers. So you can't separate the coordinates into independent space and time categories. You also can't just settle on using one observer's set of coordinates, because the laws of physics look the same to any observer, so there's no way to say one is more 'right'.

Mike W.

(published on 11/18/2007)