What is space?

[yuanwei]

For understand general relativity, they tell me, there have flat space, add a sun, the flat space changed to curved space, so the earth move around. There has a question, if we admit mass, space , time is created, how could exist flat space? there have space-time and there have nothing?

[Cantab Morgan]

Newton offered his famous postulates about time and space, and they served well for hundreds of years. Einstein came along and removed the taste of philosophy from them, and appealed to experiment. "Time is what a clock measures." So I suppose the answer to your question is: "Space is what a ruler measures."

[atyy]

Yes, according to general relativity, spacetime that is flat everywhere has nothing in it. Special relativity in which spacetime is flat and has stuff in it only holds as an approximation over small regions of spacetime.

[yuanwei]

I think space-time is same with mass, so there have nothing means there have no space-time.

[DaleSpam]

That would be Mach Principle 7 (http://en.wikipedia.org/wiki/Mach_principle) in Bondi's classification. Personally I think it is a fairly useless principle, as are many of the Mach Principles. In this universe there is mass so any discussion of an empty universe is irrelevant to this one.