quote:

*Originally posted by thebrain13*

yes you can say that distance is relative to how far(notice far is another term for distance)light from krypton-86 in vacuo travels given a specific duration of time. however lets say that light from krypton-86 takes eight minutes to reach the sun. There are no platinum bars in empty space to set the speed of krypton relative to, so why does it take that specific length of time to complete the trip. The answer is there are eight light minutes of space between the sun and the earth. you must traverse a billion light years to reach a distant galaxy, you must traverse eight light minutes to reach the sun. but what exactly is it that is so different about the sun and the distant galaxy, that makes light take more time?

It is the kind of question that can be taken at a great many levels.

At its simplest level, since distance is defined in terms of time and the speed of light, so that is all there is to it. Something is defined as being twice as far away simply because light (in a vacuum) takes twice as long to get there from here, or here from there.

Ofcourse, we have a lot of everyday prejudices about what distance should mean; but these are everyday ideas of distance rather than a technical definition of distance.

Firstly, distance really only has a common sense meaning within a common inertial reference frame (i.e. if two things are moving relative to each other at speeds close to the speed of light, then a lot of the common prejudices about distance no longer hold true – and the same is true of distance within a gravitational field). The most extreme consequence of this is distances measured within a black hole. Since light can never leave a black hole, then simply using the speed of light to measure distance would make the event horizon an infinite distance from the centre of a black hole. Yet, light can fall into a black hole, and thus the distance from the event horizon to the centre becomes different from the distance from the centre to the event horizon – yet common prejudice dictates that the distance from

*A* to

*B* should be the same as the distance from

*B* to

*A*. To a very much lesser extent, this is even true of the distance between the Sun and the Earth, since light will travel very slightly faster from the Earth to the Sun than from the Sun to the Earth.

All of the above relates to distance as it is defined by physicists. In everyday terms, distance is usually regarded more as a matter of how much can you fit between two points. In the simplest aspect, this goes back to the platinum/iridium standard metre, when you might ask how many standard metres can you fit between two points. On the other hand, it applies not only to standard metres, but to more everyday objects. For instance, you might measure a length of road by how many lamp posts separated by a standard distance may fit between two points in the road, or how many houses might you build along a stretch of road. You might measure a chain by how many links you would fit within the chain. In each case, the measure of distance is relative, but works by asking mow many of a smaller unit of distance/length you can fit into the larger unit, and then scale that up to how many of the larger units you would fit into yet an even larger unit, and so on and so on.

This does, as with the measure against the speed of light, only really work if the whole system has a common inertial frame (i.e. you cannot say how many links are in a chain if the links keep moving or the chain does not stay a constant length). In another example (maybe easier to visualise), you could measure the length of a stretch of road by how many motor cars you can fit on the road. This is simple if the cars are parked, but as the motor cars start to move, then the distance between the cars has to increase, and so the number of cars you can fit on the road reduces. Even worse, if the road is on a gradient, then the number of cars that you can fit travelling up the road may be different to the number of cars you can fit moving down the road, and so suddenly the distance from

*A* to

*B* is no longer the same (as measured in numbers of cars fitted) as the distance from

*B* to

*A*.

**G***eorge*