why do we need to enclose a system to hence study it? "It is useful to enclose in a box the system we want to study, be it the electromagnetic field or quantum particle. Unfortunately, there is no known material out of which we can construct a box to contain the gravitational field" -Antony Zee (Einstein Gravity in a Nutshell)
After reading this, I asked myself, why do we need to enclose systems in a box to understand their properties? what's so special about bounded space that isn't available in any infinite space, that make us able to study any system/field?
 A: Modern scientific theory is founded on the principle that experiments are repeatable and reproducible.  This means that an experimenter can repeat the experiment multiple times with the same results, and that other experimenters can reproduce the results.  Theories which do not meet these criteria are deemed non-scientific (or at the very least, "less scientific").
Repeatability and reproducability are impossible unless you can quantify the initial conditions for your experiment.  If the system is not "enclosed in a box," then it becomes virtually impossible to define these initial conditions, and repeatability and reproducability fly out the window.
In practice, we have many tools at our disposal to beat down the uncertainty about the initial conditions.  We can measure things, we can cool them down, we can put them in a faraday cage.  We can do all sorts of things which decrease the uncertainty about the initial conditions.  Antony Zee is pointing out that we don't have very many tools like this to explore gravity.
Of course, we do find ways.  Consider the work of LIGO, which effectively uses a very large box containing lots of black holes, and listens for the "loudest" events they can: two black holes merging, on the assumption that the signal to noise ratio will be good enough to pick up what we want to hear =)
