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Could we harness energy from black holes?:
A remarkable prediction of Einstein's theory of general relativity -- the theory that connects space, time and gravity -- is that rotating black holes have enormous amounts of energy available to be tapped.
[...] [Now] physicists Luca Comisso of Columbia University and Felipe Asenjo of the Universidad Adolfo Ibáñez in Chile have found a new way to extract energy from black holes by breaking and rejoining magnetic field lines near the event horizon, the point at which nothing, not even light, can escape a black hole's gravitational pull.
"Black holes are commonly surrounded by a hot 'soup' of plasma particles that carry a magnetic field," said Comisso. "Our theory shows that when magnetic field lines disconnect and reconnect in just the right way, they can accelerate plasma particles to negative energies, and large amounts of black hole energy can be extracted."
The U.S. National Science Foundation-funded research results could allow astronomers to better estimate the spin of black holes and possibly discover a source of energy for the needs of an advanced civilization, Comisso said.
[...] "Thousands or millions of years from now, humanity might be able to survive around a black hole without harnessing energy from stars," Comisso said. "It is essentially a technological problem. If we look at the physics, there is nothing that prevents it."
Journal Reference:
Luca Comisso, Felipe A. Asenjo. Magnetic reconnection as a mechanism for energy extraction from rotating black holes, Physical Review D (DOI: 10.1103/PhysRevD.103.023014)
(Score: 5, Interesting) by Immerman on Sunday January 24 2021, @02:47PM (5 children)
Yeah, I'm not sure what they're talking about either, but possibly it's related to the way gravitational potential energy is normally measured from infinity, which is the only common reference point shared by all contexts. E.g. at infinite distance, with no gravitational interaction, we say there's zero gravitational potential energy between objects, while at any real distance the potential energy is negative, perfectly canceling the kinetic energy of an object traveling at escape velocity.
And yeah, for an Alcubierre drive you need negative absolute energy, because you need to bend space in the opposite direction as normal mass: to invoke the common (though deeply flawed) "stretched rubber sheet of spacetime" analogy of gravity, you're trying to actually stretch the sheet upwards, so that things fall away from the source, rather than downwards so that they fall towards it. And no clever arrangement of "downward-stretching" (positive) mass energy can generate a net "upwards stretching" effect.
At least, you do if you want to travel at FTL speeds. There was a recent paper analyzing the concept at sub-light speeds, and apparently no exotic matter is needed when you're not trying to travel faster than causality. Of course, without FTL you lose the original motive, but you still have an inertialess, reactionless drive combined with a tunable time dilation field - so it could still be incredibly useful while becoming theoretically possible to build using only normal matter.
(Score: 2) by choose another one on Sunday January 24 2021, @03:55PM (4 children)
Does it still completely destroy the destination star system on arrival?
(Score: 2) by Immerman on Sunday January 24 2021, @08:59PM (3 children)
I can't imagine any way even an FTL Alcubierre drive could accomplish that - unless maybe you were to vaporize the phenomenal mass-energy of your warp field/engine itself in a tight beam, which would be kind of ridiculous. You don't vaporize an airplane upon landing, you just reconfigure it a bit to let you out.
(Score: 2) by choose another one on Monday January 25 2021, @04:22PM (2 children)
It's old news, but I haven't heard that they solved it: https://www.extremetech.com/extreme/140635-the-downside-of-warp-drives-annihilating-whole-star-systems-when-you-arrive [extremetech.com]
(Score: 0) by Anonymous Coward on Monday January 25 2021, @06:06PM
1) point the destructive cone at a direction away from your destination.
2) Do stuff to untrap the high energy particles before they build up to unsafe levels - e.g. go to sublight speeds.
Bonus points if you successfully create another universe on your arrival.
(Score: 2) by Immerman on Monday January 25 2021, @06:13PM
Oh, right, I do remember that now. I believe later analysis suggested that most particles and radiation wouldn't be trapped within the field, instead passing through it to cook the passengers.
But assuming the field is intense enough to create an event horizon, (though that would likely have issues with stopping) - you're talking about trapping roughly the quantity of mass-energy that occupied the full volume of the projected flight path at one single instant. Even assuming all the matter was torn apart and converted to energy, I don't think you're anywhere near the amount of energy necessary to destroy a star or probably even a planet - though you probably want to watch out for spaceships and stations, and might do some serious ecosystem damage if you blasted a life-bearing world.
Let's see, some numbers:
For convenience, lets say the warp field is about 18m across (1000m^2 cross section) Which is probably HUGE since as I recall a warp-bubble is TARDIS-like, with the size of the interior of the bubble being independent from the size of the exterior, and the energy requirements are based primarily on the external size.
So, lets say we fly to the nearest star, sweeping out a straight cylinder of 4light years * 1000m^2 = 10^19m^3.
Our sun is within the Local Bubble, an areas of hot interstellar medium with a density of about 1 atom per cm^3, with the nearest rim about 200 light years away - so I'll use that as the basic density. 1 (hydrogen) atom/cc = 1million atoms per m/3. Divided by Avogadros number to convert to mass = 1.7e-18 g/m^3
So the total mass swept up while flying between stars = ~17kg = 1.6e18 J of energy
Compared to the 1.7e17W of solar power constantly hitting the Earth, it would be about 10 seconds worth of solar energy. Released as a short enough burst that completely hits the planet that could probably do a lot of damage, but it's not going to vaporize the thing. Much less a star: the energy amounts to only a few picoseconds of our sun's energy output.
Or, let's boil water with it: it takes 2257J to boil one cc of water (without heating it), so 1.6e18J is enough to boil a cube of 100C water that's 891m on a side. Nothing to sneeze at, but not all that much in the grand scheme of things.