
Black Hole Propulsion as Technosignature - elorant
https://www.centauri-dreams.org/2019/03/06/black-hole-propulsion-as-technosignature/
======
eckmLJE
For a captivating exploration of these same ideas in contemporary fiction, try
Liu Cixin's The Three Body Problem. As a tip, I enjoyed it much better having
not even read the blurb on the back of the book. It spoils events that don't
truly unfold until far into the story.

~~~
ohaideredevs
The Three Body Problem is officially the most overrated book on Hacker News. I
am trying to understand why. It has no soul, the "science" is garbage, not a
single character is memorable, it's way, way too long for what it is.

~~~
levthedev
It's a sci-fi book with an impressive sense of scale. Every hundred pages or
so the story zooms out; from one man's life, to a country's struggle, to a
global calamity, to an interplanetary and then galactic crisis, and finally to
even higher dimensions.

It does this while threading characters from each stage of "zoom" into future
stages, creating for a lot of interesting cross-over plots and sub-plots.

As other commentator's have said, it also wove together Chinese culture with
science fiction and history with drama very fluidly.

It was, for me personally, a very "fun" read. Kept me excited at each chapter,
and I genuinely didn't know what to expect next. I highly recommend to any
sci-fi fans out there, even those like me, who initially didn't think they
could stomach around 1,000 pages of a translated-from-Chinese sci-fi novel.

~~~
arkh
> It's a sci-fi book with an impressive sense of scale

Which is not really new. The three books could be condensed a lot. It felt
like reading the Dune prequels and sequels: useless details not advancing any
plot just to make the book bigger.

It may come from the difference between western and Chinese ways to tell a
story. Or the translator job. But they were snooze-fest for 300 pages then 10
pages of something happening then back to boring.

------
LastZactionHero
I love Centauri Dreams. It's hopeful, makes me feel perspective, and realize
how small and primitive we are.

We fret every day about consumption, waste, and energy, and here are some
people discussing methods of moving planet-sized objects. It's a kind of
optimism that's so rare it almost feels outdated.

~~~
theothermkn
> We fret every day about consumption, waste, and energy,...

I'm afraid we have to keep fretting. In the Numberphile video about the number
of particles in the Universe
([https://youtu.be/lpj0E0a0mlU?t=378](https://youtu.be/lpj0E0a0mlU?t=378)),
they take an interesting diversion to ask "How long until every particle in
the Universe is _inside_ a human being?" In other words, when will we reach
the ultimate limit to growth? Well, at the current annualized population
growth, it's less than 9000 years! And that scenario involves every human
being at that future time floating around naked in the vacuum of space. There
is no food to eat, water to drink, nor air to breathe left over.

Practically, this means a couple of things that are less than obvious. First,
the best case is that we will reach a point where the growth in volume of the
space we occupy will exceed the speed of light. In other words, "long" before
9000 years from now, we will either establish some sort of equilibrium, or we
will run up against a hard Malthusian limit. We will absolutely be concerned
about "consumption, waste, and energy" in a way that is far more focused than
it is now. (The only economic models that are permitted to work are zero-
growth models, practically speaking.) Second, to give you an idea of how soon
this is coming, the Universe is _much_ larger than 9000 light-years across.
That is to say, even if we could manage the resource usage of our 1.011%
growth rate optimally, we could never actually fill the Universe in that time
frame, as it would take about 15 billion years or so if we were all flinging
ourselves outward at very near the speed of light. (n retrospect, these two
are restatements of the same idea.)

Finally, and most dismally, the analogous calculation for the Solar System (an
infinitely more reasonable calculation to perform) must be abysmally worse and
monumentally more urgent. To give you an idea, the current doubling time of
human population is something like 60 years. That means, for example, that if
we were to anticipate that we'd run out of meaningful room on Earth in 60
years' time, we'd have to start working on our "second home" now, to have it
ready. Once it was ready, we'd have to be working on two more. Then four, and
so on. Good luck with that!

(Note: After we have consumed the Earth and converted it to habitats, we will
have to consume Jupiter in a similar way in about 500 years. This gets out of
hand very quickly.)

EDIT: El-oh-el at the few people who's fee-fees were hurt enough by this back-
of-the-envelope analysis of growth rates and resource use to downvote it. Keep
fighting the good fight, no matter how lonely!

~~~
twodave
Of course, "current annualized population growth" is a hilariously bad
assumption to make for the next 9,000 years. I would expect population
controls long before having our "second home" prepared that would limit the
rate of our population growth to the speed we can realistically expand.

I also don't think humanity will last this long, but as an academic problem
it's fun to think about.

~~~
antt
>Of course, "current annualized population growth" is a hilariously bad
assumption to make for the next 9,000 years. I would expect population
controls long before having our "second home" prepared that would limit the
rate of our population growth to the speed we can realistically expand.

Funny I would have expected that to happen when we started consuming more than
the primary production of the planet:
[https://www.nationalobserver.com/2017/08/09/opinion/heres-
ho...](https://www.nationalobserver.com/2017/08/09/opinion/heres-how-many-
earths-we-need-meet-our-populations-demand)

We didn't.

~~~
twodave
This is not a planet-scale phenomenon, though. Any student of geography would
know that as nations become more mature their birth rates stabilize to more or
less match the death rate. It is the less-developed countries that would
suffer first in a world where our planet can no longer support our population,
in which case I would expect that either they would go to war with each other
over whatever resources can be taken from each other (and also reducing their
populations in the process) or else face starvation (again, reducing
population). Either way there's a large difference between starvation and
saturation. The human race is not endangered by the fact that we'll soon
saturate our planet, and those that do starve as a result would simply join a
long line of people who have already met the same fate.

------
Footkerchief
Although this article doesn't make it clear, the underlying concepts (like the
light slingshot / "halo drive") are explored in more detail in a previous
article:

[https://www.centauri-dreams.org/2019/03/05/investigating-
the...](https://www.centauri-dreams.org/2019/03/05/investigating-the-halo-
drive/)

------
simonh
The problem with firing lasers at a black hole light years away is beam
spread. Using the biggest best telescopes we currently have, at our admittedly
miserable technology level, the tightest beams we can project on as close a
target as the moon are kilometres across. Even if we were to create a mirror
as wide as a planet to create a beam, it would be many, many times that wide
by the time it reached the binary system. It would be further diverged by the
slingshot round the black hole (or neutron star), and diverge further again on
it’s way back to the spacecraft. I don't see any way such a system could be
made practical.

~~~
arisAlexis
why is our technology miserable? do you know of a better benchmark out there?
if not I would assume this is the most advanced technology <ever>

~~~
simonh
This is literally a discussion about detecting interstellar civilisations
using black holes for near light speed travel, and similar technologies we
might develop in the future. I am posting in that context.

~~~
arisAlexis
me too since we developed a framework to think about ways to detect more
advanced civilizations than us that may not exist. My context is that we are
very advanced. But you seem convinced that we are miserable and I guess our
difference is pessimistic vs optimistic rather than objective.

~~~
simonh
At every point in human history, current Human technology has been the best
technology ever developed. It's a completely useless perspective because it
always gives you the same answer under every circumstance that has ever
existed.

The discussion we are having now gives an eternal objective reference point -
the technology required to exploit the gravitational energy of binary black
holes. How advanced would you say our current technology is, compared to that
reference point?

~~~
arisAlexis
I don't find this fruitful. Compared to infinite possible intelligence that
creatures could conceivably reach either here or other worlds or machines,
Einstein was extremely stupid. What good of a convo does it make?

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Rooster61
Interesting concept. Why couldn't we do this with matter?

Instead of firing light off at black holes, why not fire some material around
a gravity well and allow it to strike the spacecraft in a way that it A)
imparts the inertia it has gathered from whatever body you shot it around and
B) is captured to be fired again at a new angle according to your new
position?

I know there are a ton of logistics to wade through with this, but it should
be at least hypothetically possible. It's the same concept as in the article,
save for the fact that we are now firing matter around objects we actually can
reach in our lifetimes rather than light around black holes that would take an
exceedingly long time to propagate.

~~~
lodi
You're proposing to fire some material around a planet in such a way that it
comes back to a mothership with more momentum than it started with. Now
imagine that instead of going "there and back", you find a trajectory that
sends it from planet A, to planet B, then back to the mothership. You're
coming back even faster now! Why stop at two planets? Find a trajectory so you
can do planet A, planet B, planet C, perhaps planet A again... and so on.

Now just delete the mothership from this picture; the 'material' _is_ the
spacecraft itself.

This is a gravity assist, and by stacking enough of them you can basically
pick up as much extra delta-V as you want (at least until you reach Solar
escape velocity).

[https://en.wikipedia.org/wiki/Interplanetary_Transport_Netwo...](https://en.wikipedia.org/wiki/Interplanetary_Transport_Network)

The main drawback is that it can take decades of flying a convoluted series of
gravity assists before you get to where you need to go, and that's all within
one system.

(The two-body idea you proposed has the same problem. The mothership has to
sit there and wait while the matter flies back and forth on multi-year long
orbits. Also now you have to figure out how to 'catch' a hypervelocity
projectile...)

~~~
Rooster61
This idea was to try to get AROUND some of the limitations of a gravity
assist. For instance, a gravity assist around a large body might require a
solar powered spacecraft to be in the shadow of the object for quite some
time, rendering the solar panels useless and requiring a radio blackout.
Assuming that the method of firing off the projectile and catching it can move
gimballed from the rest of the craft, you could allow the ship as a whole to
resume the same attitude/trajectory, save for the acceleration of the
returning projectiles.

Another thought is that if the projectile were small and aerodynamic enough,
you could dip closer into a body's gravity well than the spacecraft could due
to atmospheric drag, grabbing more momentum than the spacecraft itself could.
The lower periapsis might yeild more delta-v due to the more pronounced Oberth
effect than the spacecraft could provide. That's all assuming you don't go TOO
far into the atmosphere with the projectile.

~~~
jacobush
Or in the case of slingshot around a star, your projectile might be able to
take more of a beating than the ship. Something like tungsten balls or
something.

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JohnJamesRambo
>Remember the methodology: A spacecraft emits a beam of energy at a black hole
that is moving towards it, choosing the angles so that the beam returns to the
spacecraft (along the so-called ‘boomerang geodesic’). With the beam making
the gravitational flyby rather than the spacecraft, the vehicle can
nonetheless exploit the kinetic energy of the black hole for acceleration.

> Light acts the same way, but light cannot return faster than the speed of
> light. Instead, in gaining momentum from the black hole, the light
> blueshifts.

How do you get propulsion from blueshifted light?

~~~
mrfusion
I can see how you gain energy but you’re also putting yourself deeper into a
gravitational well that you’d have to expend energy to get out of.

Wouldn’t it be a wash?

~~~
ceejayoz
We do this fairly regularly with space probes already.

[https://en.wikipedia.org/wiki/Gravity_assist](https://en.wikipedia.org/wiki/Gravity_assist)

You steal energy from the object you're slingshotting off. The black holes are
slowing (infinitesimally) down every time you do it, gifting their energy to
the accelerated object.

Hence the article talking about potentially detecting this sort of behavior by
monitoring for prematurely merging and overly eccentric binary black holes.

------
ConcernedCoder
So if I shoot a beam of, let's say, a movie or live feed of myself typing this
comment around a black hole some 50 light years away precisely enough to have
it land back here on earth 100 years in the future, would an observer see back
into the past 100 years? ...in the same thread if I observe precisely enough a
position near a black hole some N light years away could I see what was
happening on the earth N*2 years in the past?

~~~
saagarjha
Yes, but there's an easier way: use a mirror. Depending on how far you place
it, you're looking at yourself in the past.

------
mudil
A question for a physicist from a layman. All objects in the universe are
connected to each other through gravity, no matter how distant. So what
happens to the energy or mass of an object (or total energy/mass) in the
expanding universe? As space inflates, the distances are greater between
objects, so where the excess of gravitational radiation went to? Can that
account for the dark energy?

~~~
sandworm101
Gravity is not radiation. The expansion of the universe does work against
gravity but there are limits. Gravity decreases with distance. So if the
universe is expanding fast enough (it is) it will overcome gravity. Imagine
setting a bomb off between two asteroids. Use a small bomb and they move apart
but eventually drift back. Gravity wins. But use a big enough bomb and they
both hit escape velocity. Gravity looses. The objects are moving away from
each other fast enough that the force of gravity falls away so fast that it
will never turn them around let alone bring them back together.

Dark energy is the concept that while the universe is expanding, gravity wins
/ everything is at escape velocity, it shouldn't be expanding faster. Dark
energy is the fact that the two asteroids separated by the bomb (the big bang)
aren't just at escape velocity, something is causing them to accelerate.

~~~
aeorgnoieang
> Gravity is not radiation.

'Gravitational radiation' is, apparently, a thing; gravity waves basically.
It's not _electromagnetic_ radiation tho, which is what I'm guessing what you
meant.

I don't think it's crazy to think that gravity is propagated at the speed of
light but that we can't, and likely won't ever be able to, measure any gravity
waves smaller than those generated by extremely large-gravity systems.

------
edem
Why not have an actual black hole on the ship and use the Hawking radiation to
gain energy? More
[here]([https://www.youtube.com/watch?v=EzZGPCyrpSU&feature=youtu.be...](https://www.youtube.com/watch?v=EzZGPCyrpSU&feature=youtu.be&t=461)).

------
JumpCrisscross
> _No fuel is spent aboard the spacecraft which, using stored energy from the
> beam, continues to accelerate up to terminal velocity_

Wouldn't fuel need to be expended on the ship to generate the beam? Or is the
beam reflected back to the black hole for re-energisation?

~~~
MuncleUscles
You reabsorb the beam which now has even more energy because it's been blue-
shifted, which means you can actually charge your batteries as well as
accelerate.

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dana321
If you think the title of this is pure porn, check out those images!

------
edem
Related: [Black Hole Bomb]([https://www.youtube.com/watch?v=ulCdoCfw-
bY](https://www.youtube.com/watch?v=ulCdoCfw-bY)).

