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. If you want to be alerted
when that video comes out, make sure to subscribe to the channel, and this week we also have
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that. As always, comments, questions, and video
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Thanks for watching, and have a great day!