how the gravitational field behaves when it heats up is still an unsolved problem.

 the gravitational field, as we saw in the first lesson, is space itself, in effect space-time. therefore, when heat is diffused to the gravitational field, time and space themselves must vibrate.
 

but we still don't know how to describe this well. we don't have the equations to describe the thermal variations of a hot space-time. what is a vibrating time?
 

such issues lead us to the heart of the problem of time. what exactly is the flow of time?
 

 for two different people "here" points to two different places. so too, does "now".
 
but no one would dream of saying things "here' exist, whereas things that are not "here" do not exist.

 so then why do we say that things that are "now" exist, and that everything else doesn't?
 

is the present something that is objective in the world, that "flows", and that makes things "exist", one after the other, or is it only subjective, like "here"?