So today we are looking at using small, artificially created black holes as a way of powering interstellar
spaceships and we started on that last week by discussing the properties of micro-black
holes and Hawking Radiation.
If you haven’t seen that video, unless you happen to be very familiar with how Hawking
Radiation works, you might want to click the video link on the screen and catch that first.
Like all the video links on this channel, doing so will just pause this video and open
that video in a new window.
But the thirty second summary of that video is that smaller black holes emit a lot of
energy, which we call Hawking Radiation, and the power released by a given black hole roughly
scales up inverse-square with its mass.
Meaning that if you have two black holes, one twice as heavy as the other, the bigger
one gives off only a quarter of the power the smaller ones does, and since they are
emitting this energy by evaporating their own mass till they run out, the bigger one
will live eight times long, since it emits only a quarter the energy and has twice as
much mass to fuel that emission.
Similarly a black of only a tenth of the mass will emit a hundred times the power, but with
only a tenth the mass will run of fuel a thousand times quicker.
I should note that this is just an approximation, not an exact calculation.
Since the black holes we can detect are so massive they don’t put out enough Hawking
Radiation to power a tiny LED light we couldn’t measure their Hawking Radiation if they were
even as close as our own Moon, let alone hundreds of light years away.
So we have only theoretical models and those don’t actually exactly follow the inverse-square
mass relationship I just mentioned anyway, and there are competing models.
So today I’ll specifically be using the values from Westmoreland and Crane’s 2009
paper originally discussing Black Hole Starships, since it is the one you will most likely hear
referenced if you decide to do some more personal research on this subject, and a link to that
paper is included in the video description below.
That said, the core concept for the ship then is pretty straight-forward.
You make a small black hole, one with a mass somewhere between an aircraft carrier and
a small fleet of oil supertankers.
Once you have a black hole of that size you have an object emitting a huge amount of power,
the ones we’ll be looking at today emit power somewhere between a percent or so of
what the Sun hits the Earth with to several times what the sun hits the Earth with.
That figure incidentally is usually given as a couple hundred petawatts, and a petawatt
is a million gigawatts, where most big nuclear reactors and hydroelectric dams produce about
a gigawatt, so Earth’s solar power supply is on an order of a hundred million times
larger than our biggest power plants produce and the black holes we’re looking at today
produce power comparable to that, millions to billions of times more powerful than our
largest power plants.
I’ve mentioned in the past that the concept of an ‘unarmed spaceship’ is an oxymoron,
that the sci-fi staple of an unarmed freighter getting attacked just isn’t plausible, and
this is another example of that.
The sheer power output of any interstellar spaceship is truly immense, and a petawatt
is the equivalent power output of 16 Hiroshima nuclear bombs going off every second.
The ships we’ll be talking about today operate anywhere between 1 petawatt to several thousand
petawatts.
So even the lowest powered of these ships, even if you could only effectively direct
1% of that output as a weapon, could blast a large city into rubble every few minutes,
while the higher power versions operating at highest efficiency could effectively wreak
havoc as though they had a machine gun that shot Hydrogen bombs.
That’s without even directly weaponizing a black hole either, which we’ll discuss
near the end of the video, that’s simple recognition that if you have that much power
you can do a lot of damage.
Now that power is omnidirectional in its output when emitted, same as a star, and you can
generate thrust by putting a mirror up on one side, so light flying out in the wrong
direction reflects back in the right direction and the radiation is no longer omnidirectional.
You can do even better with a parabolic dish.
That’s your simplest black hole drive, conceptually anyway, a black hole with an attached parabolic
dish.
You stick the rest of your ship on the other side of the dish, and it turns out black holes
in around the megaton range with attached ships of similar mass can pull off accelerations
and maximum speeds that can get you from one star to a neighboring one is less than a human
lifetime and let you wander around solar systems, even the deeper darker outer zones, in timelines
of months.
In the case of Black Hole Ships, the key figure, if things are running very efficiently, is
that you get one-gee of acceleration on a one megaton total mass ship for every 3000
petawatts of power you have.
As huge as the power output sounds, it still isn’t terribly impressive when it comes
to shoving things up to the speed of light.
The table I’m bringing up is an extraction of the calculated power outputs, in petawatts,
of various black holes by their mass in megatons that was discussed in Westmoreland and Crane’s
2009 paper.
I’ve gone ahead and added to that the Power to Mass ratio, as well as what the acceleration,
in gees, of a spaceship would be that had equal mass of black hole to the ship and its
cargo.
So a two megaton ship would be half black hole by mass and half ship and cargo.
Lastly I’ve added in a column for how long it would take for that ship to get to just
1% of light speed.
Which I picked strictly to be able to avoid relativistic effects, since its minimal at
that point.
At 1% of the speed of light your clocks will lose only a few seconds a day in terms of
time dilation and good-old fashioned Newtonian equations for velocity and kinetic energy
would only be inaccurate with high precision measurements.
Now that would imply an obvious preference, you want the lightest black hole since it
gives the best acceleration, and sure you don’t want eight-and-a-half gees, but just
as we could almost double that acceleration if we could strip the ship mass down to near
zero besides the black hole, we can slow it down by adding more mass.
The problem is, as I’ve mentioned, that small black holes don’t live long and the
smaller they are, the shorter their life.
So the last column is the approximate rounded lifetimes of the black holes as listed in
the original paper.
So unless you can find some way to refuel your black hole, by dumping more mass into
it for instance, these smallest black holes won’t last long enough to get you to your
destination.
As they put out energy they lose mass, which causes them to emit energy even faster, and
lose mass faster, until eventually they are so small and high powered that they essentially
explode.
So if your black isn’t massive enough to survive your trip you eventually need to jettison
it and now you have no power source to slow back down with when you reach your destination.
Which isn’t necessarily a problem, we discussed in the Interstellar Colonization video some
of the tricks you can use to slow a spaceship down without using fuel.
One of those is the Bussard Ramjet, a concept for a spaceship that ran by magnetically sucking
in interstellar hydrogen gas and ramming it down the axis of the ship to produce fusion
and thrust.
This concept turns out not to work because when we could run the calculations better
we found that all that gas, which is essentially stationary to interstellar space, would slow
the ship down more by being absorbed then it would produce.
Which was unfortunate but had the silver lining that even though you can’t accelerate with
it, you can use it to slow down for free.
So if you have a short-lived black hole accelerate your ship up to cruising speed you could then
slow down at your destination this way, and power yourself during the trip more conventionally
with a nuclear reactor, fusion if you have that, or otherwise classic fission, as life
support is only a tiny fraction of the energy budget for an interstellar trip at relativistic
speeds.
Of course another possible use of that magnetic ramscoop method might be to suck in matter
and jam it into your black hole to refuel it.
This doesn’t give you infinite acceleration, since eventually you will reach a speed where
even the near-total conversion of mass-to-energy by that black hole won’t match the lost
momentum of sucking in that relatively slow gas, but it gets you a very high speed and
lets you keep your black hole.
But refueling a black hole is easier said than done and the smaller the black hole,
the harder the refueling.
I mentioned in the last video that refueling a small black hole is much harder than making
one in the first place.
In that we suggested the best way to make one would probably be with tons of lasers
all pumping energy into the exact same place at the same time, a place much smaller than
the nucleus of an atom.
This concept is called a Kugelblitz black hole, Kugelblitz just being ‘ball lightning’
in German, since you a making a tiny little ball of light.
Light, being made of photons, doesn’t have a problem being squeezed together like normal
matter does so it’s easier to make black holes out of.
A kugelbltiz black hole is hard to do simply because it requires immense energy and precision.
If you try to do it with normal matter instead, like interstellar hydrogen, you are trying
to jam materials together to pressures and temperatures far beyond what is necessary
for fusion it’s very improbable we’d find a way to do that especially without spending
more energy than we put in.
Same problem, you can’t refuel a black hole on your ship with lasers since you’d burn
more energy up making those lasers than you’d get out of it.
And the smaller the black hole the harder it is to do, with normal matter, since you
are trying to squish that matter into an even smaller spot and fighting against even higher
power output resisting that matter input.
The analogy I used last time was that it was like trying to shove a beachball down the
nozzle of a firehose that’s turned on.
Making them, the kugelblitz way, is essentially the process of having a massive swarm of power
collectors that fire lasers with high precision at one spot at one moment, allowing you to
use a star as a black hole generator for spaceships.
That requires ludicrous levels of precision and vastly huge solar collectors but that
doesn’t appear to break any known laws of physics, refueling with random hydrogen probably
does, so midtrip refueling is probably not an option.
We also have the problem that black holes emit very high powered particles, like gamma
rays, making them very hard to reflect the power from, so you can’t just wrap a black
hole with a highly reflective material to bounce the emission back in to the black hole
to be reabsorbed.
At this time we lack any materials that acts as good mirrors to gamma radiation.
If we did have one it would make things a lot easier since you could create a throttle
that let you bounce some of the emitted energy back into the black hole, decreasing its net
power output and extending its life, when you wanted to do so.
Also without anything that can reflect gamma radiation you have to absorb all that gamma
radiation as it emerges from the black hole and let it heat up a material to just below
its melting temperature.
So you place a sphere, or hemisphere, around the black hole.
It glows red hot and emits normal light, which can be reflected by a parabolic dish.
I’ll refer to this as an absorption shell.
Unfortunately the more power you have, the bigger your absorption shell needs to be.
Tungsten, the element with the highest known melting temperature, about 3700 Kelvin, can
radiate about ten megawatts per square meter without melting.
Twenty since it can emit from both sides.
That still means that you need about 50 million square meters of the stuff for every petawatt
of power you want to absorb.
Now the good news is there are some new alloys with even higher melting points than Tungsten,
and blackbody radiation goes with the fourth power of temperature in Kelvin, so if we found
an alloy that had twice the melting point of Tungsten it could radiate 16 times as much
power without melting.
But even then you’d still need a few square kilometers of absorption shell to handle all
that energy.
Now these are massive ships weighing at least hundreds of thousands of tons if not millions
of tons, so you can get away with absorption shells that big, especially since the shell
needn’t be terribly thick.
However that raises yet another problem, and that is how you can keep the black hole tied
to the ship.
The black hole is emitting its energy omnidirectionally, so it’s not accelerating itself at all,
and your ship will just fly off leaving the black hole behind.
You can hardly attach a rope to the black hole since it is smaller than an atom and
will flat out shred anything it touches even if it didn’t melt it apart first.
Now a number of methods are possible, such as giving the black hole an electric charge
and binding it to the absorption shell that way.
The absorption shell can be leashed to the ship conventionally by some struts connecting
it to the parabolic dish.
That may or may not work, but to prove it is possible, the conceptually easiest is just
to use the black hole’s own gravity to hold on to the absorption shell while it’s radiation
pushes it away.
This usually known as a gravity tractor, and it’s a lot like the Statites or Shkadov
thruster we’ve discussed in the past.
Something hangs above a radiant object, pushed away by that radiation, but pulled on by its
gravity.
The hard part about doing this with a black hole, a small black hole, is that they don’t
actually have that much gravity but do have an awful lot of power output, so getting close
enough to the black hole to be gravitationally bound to it means you are sucking up even
more radiation.
Just as an example, for a one megaton black hole, the distance at which it pulls someone
with the same force as Earth pulls on you is about an inch.
Meaning you’d need your absorption shell, and overall the majority of your ships mass,
only an inch away to get one gravity of tug on the ship.
At that distance the power being absorbed would obviously melt any material but even
if it didn’t the radiation pressure would fling it away at more than one gee.
For bigger, longer-lived, lower-powered black holes that radiation pressure drops off a
lot and that gravity ramps up a lot, so you could use gravity to leash a bigger black
hole for use as a ship drive but there wouldn’t seem much point except maybe for intergalactic
travel, because it would simply take way too long to accelerate everything.
But I offer that just as a way of explaining how you can leash a black hole to a moving
object in a way that’s simple to understand and definitely works.
Ideally, if the technology emerges to reflect gamma rays, and we do have some tricks for
doing that which are improving, and if you can feed matter into a black hole, you could
set up some particle beams shoving matter into the black hole from behind, and giving
it forward momentum and refueling it, then have the gamma reflective material helping
cut down on your absorption shell size, or simply letting you discard the absorption
shell in favor of just the parabolic reflector dish able to reflect gamma rays.
That’s probably the key piece of technology to make such a system genuinely viable, we
might be able to do without it and still use the concept, but ability to make a material
that reflected gamma rays as cleanly as a normal mirror reflects visual light makes
this technology vastly less cumbersome.
If you also had the ability to beam hydrogen you picked up along the way into the black
hole, to refuel it and help push it to keep it in place its even easier.
With such a setup you would have a ship able to get pretty close to the speed of light,
do so in a reasonable period of time, and run indefinitely off the fuel just lying around
in the interstellar void.
Your maximum speed with such a setup wouldn’t be infinite, even ignoring the speed of light,
since you’d eventually reach a point where the matter you were sucking in was slowing
you down by the same amount as the power it produced would speed you up, but that would
be quite high.
If you don’t have those options you really need to use larger sized black holes unable
to produce accelerations of one-gee, but even if you did you’d probably never build a
ship that produced much more than one-gee of acceleration, that would get uncomfortable
for the crew so even if you had a very small and powerful black hole you could refuel and
contain you’d probably just have a much larger over all ship.
The values I gave on the table just assume the total ship and cargo not including the
black hole had the same mass as the black hole, to keep it mentally easy, but if you’ve
got a black hole that would produce 10 gees of acceleration on its own mass, you could
simply have a ship that weighed 10 times as much as the black hole, including the black
hole, so that it was 10% of the mass, pushing the ship at one-gee.
In such a setup the ship is basically a skyscraper with the black hole in the basement, rather
than having any rotating sections to provide artificial spin gravity.
That’s usually our ideal ship for people anyway, one able to accelerate at one gee.
Without the ability to refuel one and reflect gamma rays you can never have that, and frankly
I don’t think black hole powered ships could ever be viable without at least one of those
technologies.
Before we close out let’s talk about two other things.
First, the impact with SETI, and second the ability to weaponize these things.
We’ve talked a lot on this channel about the Fermi Paradox, the question of where all
the aliens are hanging out, and SETI, the Search for Extraterrestial Intelligence, is
the effort to answer that, either by finding them or showing they don’t exist.
One of the ways we do that is to listen for radio chatter, but the more advanced concepts
always involve trying to figure out what technologies they have and how we might see the byproducts
of those technologies.
For instance if the aliens are making Kugelblitz black holes you’d expect to see stars with
large solar collector swarms dwarfing planet in sheer area.
You’d also expect to be able to pick these things up from their gamma radiation or emitted
gravitational waves or gravitons if gravitons exist.
We don’t have detectors at this time hunting for such emissions but they would be one more
weapon in the arsenal for SETI hunting.
Speaking of weapons, there are few obvious ways to weaponize a black hole.
None of those involve just dumping a black hole onto a planet for it to eat the planet
up, I explained why that wouldn’t work in the previous video.
Your first and simplest one would just be to crash a black hole starship into the target.
A megaton of relativistic mass would unleash nearly as much power as a star emits in a
second when it hit, something akin to a billion h-bombs.
This isn’t as threatening as you might initially expect since if you saw the ship coming you
could vaporize it, and while the black hole would still be there it would just fly right
through the planet without doing much damage and continue to sail on till it evaporated.
Of course when these things do evaporate the unleash quite a lot of energy, somewhere around
10^24 joules in their last second of life.
And you can control their initial speed and direction when making them so if you can make
on the fly and aim it at the right place and time it will explode pretty impressively.
That’s not a covert and subtle weapon though, since they glow very brightly, especially
near the end of their life, but you couldn’t shoot one down either, at best if you could
measure its position and speed with great accuracy you might be able to hit it with
a beam, just like you were refueling it, and shove it off course.
If you can’t hit it off course that target is dead unless it can move out of the way
in time.
Of course a ship and probably even a bulky space station probably could see it in time
and move, and while these would seriously damage a planet’s surface they aren’t
anything like powerful enough to blow up a planet.
Excellent bunker buster though, since it will sail through anything unimpeded.
If you’ve got the gamma-ray reflective materials and can refuel the things, implying you could
knock one of course, then you can probably also make them on board a ship.
And you could make very small ones inside what would amount to a missile with a very
high acceleration and maneuverability and just cut off the fuel at a time to make it
explode when it arrived.
That would be fairly hard to detect since it would be small and emitting almost all
its detectable radiation behind it, where the target can’t see it well.
So if you’ve got the ability to reflect gamma rays and also feed raw matter into small
black holes a black hole missile would be pretty devastating, even if you vaporize the
missile, which would have enormous kinetic energy, if the timing on the fuel was done
right you’d still get that black hole explosion, and that would be very hard to avoid since
the things would be hard to see and highly maneuverable.
So black hole starships are probably limited to the land of science fiction for a while
but show real promise especially if we get a couple fairly plausible technologies down
the road.
Next time we’ll be looking at some stuff that’s more implausible when we look at
various concepts for Faster Than Light Travel and Communication.
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and this week we also have a poll again for the next subject we’ll be covering after
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