What is exactly Space which physicists refer to? Does Space specifically refer to a place where there is no atoms? Is the atmosphere over the earth can be called as "Space". I read that Space could be curved around the earth, is that true? If true, what makes a place with no atoms or anything to get curved?
> What is exactly Space which physicists refer to?
> Is the atmosphere over the earth can be called as "Space".
First thing to know: like many words in English (actually, in any language), "space" can have more than one meaning. You make note of the second, more conventional meaning -- when something is outside our Earth's atmosphere, it is said to be in "outer space" or just "space." This has no connection to the meaning physicists use.
> Does Space specifically refer to a place where there is no atoms?
Even a neutron star, where the sub-atomic particles are crammed together as tight as they can get, occupies a certain amount of "space."
In its simplest, most basic understanding in physics, "space" is the separation between two objects. When a physicist notes that object 'A' is at a different point than object 'B', she is saying that there is "space" between the two objects. The more difference that exists in the location of the two objects, the more "space" there is between the two. It doesn't matter if the separation in location is a galactic void (ie, no galaxies) with a density of much less than one atom per cubic meter, or a neutron star with densities of 10^17 kilograms per cubic meter -- it's still "space."
This concept of physical space get a LOT more complicated as one investigates its various meanings -- more complicated than I can explain here. This video
is the best explanation I have found.
> I read that Space could be curved around the earth, is that true?
The best way I have found to understand the concept of "curved space" is to imagine if our Universe had only two dimensions instead of the three it does have. In other words, the entire Universe would be like a sheet of paper, and the dots on the sheet of paper would be like galaxies.
Now imagine that the paper gets slightly folded -- THAT'S what it would mean for space to be "curved" in a two-dimensional universe. Lines that would seem to be straight are, instead, curved.
I admit it's very hard to take a curved 2D universe and then imagine something the same in a 3D universe. But, if you can do advanced graduate-level math, it's not difficult. And, if you then apply that math to our 3D Universe, you find that certain observations are predicted. And, if you go further and DO the observations, you find that what you see is exactly as predicted.
> If true, what makes a place with no atoms or anything to get curved?
Albert Einstein was able to show that anything with mass would curve space. So, strangely enough, if there are LOTS of atoms near to each other, that something we call space is curved by those atoms. Again, this curvature by mass predicts certain things, like light coming from distant stars appearing to be bent when traveling near to our Sun. Scientists looked for this apparent bending during eclipses in 1919 and 1922, and found it exactly as Einstein predicted it.