The whole universe is moving at a constant speed
through
spacetime at the speed of light.

When we're standing still (in space), all of that speed is
movement in time, so time runs at the speed of light,
which is (if you'll pardon
the tautology), one second per
second. If we move in space, since our total speed in
spacetime is constant, we must be moving less fast in
time, so time slows down. If we move at 99.999999999%
the speed of light in space, then clearly most of our
movement is in space and therefore very little of it is in
time - so time moves verrrrrry slowly indeed. If we
could
move at 100% the speed of light, then clearly since our
overall speed in spacetime is also 100% lightspeed, we
must be not moving at all in time - which is why photons
don't move at all through time.

So far so relativity.

The real question, though, is whether we can change our
_overall_ speed through spacetime. For this, we need
some kind of thrust orthogonal to spacetime. It's a bit
like
a frictionless train coasting along a curvy track - it will
maintain a constant speed regardless of whether the
track
curves east-west (space) or north-south (time). But if
you
throw in a hill or a valley (at right-angles to space-time),
the train will slow down or speed up; with a steep
enough
hill, it may even stop and run backwards.

So, how can we create thrust at right-angles to
spacetime?
Clearly, by using black holes. Stuff thrown into a black
hole exits spacetime and gets spat out at right angles to
it.

From this, it follows that throwing mass into a black hole
will change the universe's speed through spacetime. I
don't know whether it increases or decreases that speed,
but conservation of momentum (through spacetime-plus-
another-dimension) would suggest that it ought to slow
us
down.

So, ultimately, throwing lots of stuff into black holes
ought
to slow down the universe, which means we could all
have
longer weekends. Or, with a really powerful hoover to
suck stuff out of black holes, we could make the working
week zip by a little faster.

Yes, the effects would be largely local. Imagine
spacetime as a rubber sheet hurtling along at a given
speed (c). Firing stuff out of the sheet will result in
a local peak (or dip) in the sheet, and that dip will
only spread out at a finite speed.

While it may have seemed novel at the time of discovery,
the fact that all experiments measuring the speed of light
got the same result is a tautology. The speed of all
processes use as timing mechanisms are ultimately
governed by the speed of light, so if the measurement
device is in the same frame of reference as the light then
the measurement of light speed always comes out the
same.

That means that we have DEFINED the speed of light to
be constant. Then taking that definition of the speed of
light we have deduced that space is curved and that time
speeds up and slows down.

I propose that if we "simply" recalculate it all without a
fixed speed of light, we could create a self-consistent
system where space doesn't curve and there is one
universal time. Since the speed of light changes relative
to an object with gravity, the bending of light by stars is
simply diffraction. Time dilation is simply that the aging
process happens more slowly when light in reference
frames where light is traveling more slowly. This
recalculation would not change any conclusions, but
might greatly simplify our thought process. Kind of how
the heliocentric model simplified things. The Ptolemaic
model could accurately describe planetary motion, but
had to be much more conceptually complex. It assumed
the earth was stationary and traced the positions of the
planets swirling around it. We have defined that light
speed is constant and have twisted time and space to
make the math(s) come out right.

I've always worried that we can never experience anything but
really tiny black holes from our frame of reference.

My reasoning is that I am led to believe (because I read it in a book
and saw it on television) if you were to observe something falling in
to the hole from a stationary frame of reference, the object would
appear never to make it across the event horizon. If that's true,
once the black hole has formed, then from our point of view,
nothing can ever be seen to make the black hole any bigger. This
surely must mean that the universe if full of tiny little black holes
with lots of matter poised to fall in but doomed never to make it.

//the object would appear never to make it across the event horizon// If this is true, then assuming these black holes have been around for millions of years, and in that time, had the opportunity to consume a great many objects, they must get very crowded at the edges.

If black holes consume objects like people consume doughnuts, but are unable to lick their lips, they'd have great sugary accretions on their lips so massive that no new objects (doughnuts) would be able to get within chewing distance.

I think the answer is that (at least for spinning black-holes, which I think are the normal kind) the sugar gets compacted so tightly that it spews back out again in great cosmic jets of treacle that zap whole swathes of the universe with deadly x-rays. For more sedentary ones, the universe swathes the passing of the object in a helpfully redshifted curtain of mystery that makes it impossible for anyone to actually see what happens, and the objects just fade out of view/existence.

//if you were to observe something falling in to the
hole from a stationary frame of reference, the object
would appear never to make it across the event
horizon.//

No. If you were _inside_ a spaceship falling into a
black hole, you would never see yourself cross the
event horizon. But an outside observer would just
see you continuing to accelerate across it.

But, if you used your spaceship to skim the edge of the
black hole, orbiting it closely, you would experience
this happening at normal speed. However, distant
observers would see you orbiting really slowly. In your
spaceship, looking at these distant observers through a
telescope, you would see them quickly grow old and die.

No because under high acceleration/high gravity (same
thing) time passes more slowly relative to a "stationary
frame of reference". So, in the spaceship near the black
hole, one hour of time is experienced, but this is a
year for the distant observer. So the distant observer,
looking at your wristwatch through the spaceship window,
would see it ticking really slowly.

Light does escape from the space nearby black holes -
e.g. clouds of gas heat heated up as they are drawn
into black holes and glow and we see that light. There
is though a certain radius from a black hole beyond
which light can't escape. There's nothing magic about
black holes though - if the sun was replaced with a
black hole of the same mass, we'd continue to orbit it
and wouldn't notice any difference (although it would
be a bit dark), and would still see light being
emitted from, say, Mercury if someone was standing
there with a powerful torch.

Remote observers could, in theory, see your wrist watch from
a distance because you haven't crossed the event horizon -
yet. And from this remote point of view, you never will
because you're moving more and more slowly as you get
closer. So, how does the black hole grow any bigger from the
point of view of these remote observers?

Does this mean that there can never be observable large
black holes? And yet, physicists say there are lots.