Time, Gentlemen, Please!

Just shy of a couple of years ago, I wrote of the non-linearity of time in our perception. I've just learned something that hitherto I had not even remotely considered, even though I was already aware that gravity distorts space and time. What I hadn't realised was that that distortion could be felt and measured on as small a scale as the Earth's immediate environs. The fact is that GPS satellites' clocks 'see' time differently to the way we do on the surface of the Earth, due to the lessening of Earth's gravitational pull in Middle Earth Orbit; so we have to pre-correct the data we get from them and convert the timebase to suit the passage of time at our level in order for the data to be useful to us. Middle Earth Orbit is between 2,000 and 36,000 km above us: not exactly astronomical in scale: basically within our very immediate neighbourhood; but even at such a short distance away from us, gravitational differences are sufficient to alter time itself in practical, not just in theoretical or philosophical terms. Making the leap from this realisation of the local effects of gravity on time - and therefore space - to the cosmological level, where the concentrations of, and unimaginably massive differences in gravity; such as within the realm of a black hole, is frankly beyond the ken of most of us. If just a few kilometres of distance can have a measurable effect on time and its perception, what must it be like at the Heart of Darkness that is a super-massive black hole, where the rule-book of 'normal' starts to tear itself apart in the undertow of gravitational waves so powerful that light itself is powerless to escape. Sobering thought...