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# Use black holes to slow down time

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So. Black holes, eh?

Let's begin at the beginning.

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.

 — MaxwellBuchanan, May 10 2016

Falling into a black hole http://www.bbc.co.u...ole-would-clone-you
The BBC's explanation [DenholmRicshaw, May 11 2016]

http://math.ucr.edu...kHoles/fall_in.html [hippo, May 11 2016]

http://www.phys.vt....blackholes.html#q11 [hippo, May 11 2016]

the black hole itself is only ever an approximation http://physics.stac...an-outside-observer
about a third of the way down [DenholmRicshaw, May 11 2016]

If turning back time were possible https://www.youtube...watch?v=AldUzWWq8iQ
[bungston, May 17 2016]

Marked for depletion https://iopscience....2041-8205/823/2/L25
speculatively known to exist [4whom, May 25 2016]

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Today is already dragging - are you working with prototypes again?
 — normzone, May 10 2016

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.
 — MaxwellBuchanan, May 10 2016

That probably explains why, as I've got older and heavier, I've become more optimistic.
 — MaxwellBuchanan, May 10 2016

 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.
 — scad mientist, May 10 2016

 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.
 — DenholmRicshaw, May 11 2016

 //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.
 — zen_tom, May 11 2016

Exactly - it's a worry
 — DenholmRicshaw, May 11 2016

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.
 — zen_tom, May 11 2016

 //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.
 — MaxwellBuchanan, May 11 2016

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.
 — hippo, May 11 2016

Shirley it's the other way around? The distant observers would see you moving at normal speed.
 — MaxwellBuchanan, May 11 2016

 It's still worrying - see the link for a description of what happens. I quite like this bit...

 //You remain plastered there, motionless, stretched across the surface of the horizon.//

Sounds like there's a bar.
 — DenholmRicshaw, May 11 2016

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.
 — hippo, May 11 2016

 Except they wouldn't see it, because the light wouldn't escape...

My head is now full.
 — MaxwellBuchanan, May 11 2016

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.
 — hippo, May 11 2016

 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.
 — DenholmRicshaw, May 11 2016

 — hippo, May 11 2016

 Thanks [hippo] - great links.

 Having read round for a bit I found a cosmologist who was quoted as saying (see new link)

 "...the black hole itself is only ever an approximation."

So I suppose I should worry a bit less because it means the theories aren't finished yet.
 — DenholmRicshaw, May 11 2016

 //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?//

Aren't you assuming that all large black holes are formed from small ones which grew?
 — Wrongfellow, May 17 2016