People often talk about colonizing the galaxy, but today we are going to ask just
how far away humanity can stretch its reach.
So today we are wrapping up the third year of Science & Futurism with Isaac Arthur, and
I am your aforementioned host, Isaac Arthur.
It's been quite a year, and a long trip from our first year which only had 15 episodes,
not the weekly setup we started part way into year 2.
We closed that first year out by discussing Interstellar Colonization and I mentioned
near the end that you didn't have to stop at the Galaxy's edge.
Two years and a hundred episodes later, it seems appropriate to continue that topic,
and to reflect a bit on some of the concepts we've discussed since then and how they
impact today's subject.
We have discussed many times how you might travel to another solar system and colonize
it, if you were constrained by the speed of light.
Indeed we tend to assume moving at only a fraction of that speed.
To do that, especially with classic humans rather than some robotic probe or seed ship,
requires massive vessels that are almost miniature planets themselves, able to contain everything
you need to start up an ecosystem at your destination and keep thousands of people alive
either during the flight or in some sort of stasis to be awoken on arrival.
We saw that was possible, maybe even with modern science and technology, that you could
send out ships for century-long journeys.
What's interesting is that in most fiction, where they often have Faster Than Light or
FTL travel methods able to move someone across a whole galaxy in maybe moments or maybe a
few years, almost none of those sprawling galactic empires ever seems to settle other
galaxies.
That does makes sense when you have a galaxy already full of other intelligent life forms,
since you can assume other galaxies will have their own too and not welcome colonists from
outside, and it is a long trip just to say hello.
However we see it even in fiction where humanity has the whole galaxy to itself and no special
reason to think neighboring galaxies will have existing civilizations.
When that's the case, it makes a lot less sense.
If you've got a spaceship able to cross the whole galaxy in a year, crossing to another
galaxy should not be a problem.
Distances between galaxies don't scale up like distances between planets or stars.
Stars are typically hundreds of thousands of times further away from our Sun than other
planets are from Earth, and the distance to the Moon, still the only world a human has
set foot on, is about a hundred millionth the distance to the nearest star.
Alternatively galaxies are a lot closer together, relatively speaking.
The Magellanic Cloud Dwarf Galaxies are closer to some stars in our galaxy than they are
to stars on the other side of the galaxy from them, and even Andromeda, the nearest large
galaxy to ours, is only about 20 times further away than the galaxy is wide.
So there's no reason why, if you thought the neighboring galaxy was empty of civilizations,
you couldn't make that trip if you've got spaceships that can cross the galaxy in
a year, because they can get to Andromeda in 20.
That would barely count as a generational ark ship, something we can almost do now,
and should be child's play for most galactic civilizations we know from fiction.
The other big thing to keep in mind is that the space between us and other galaxies is
not empty.
If we view galaxies as continents, with intergalactic space as the ocean, there are plenty of little
islands to use as waypoints.
There are a lot of stars in between, and galaxies don't have firmly defined edges either.
Also, stars are often ejected from the galaxy, much like how planets can get ejected from
a solar system.
We aren't sure how many of these stars there are yet, I've seen estimates as high as
half of stars being intergalactic, but there's decent confidence of it being 10% of stellar
mass or higher.
Equal or lesser populations than galaxies, it's still spread over a much larger volume,
so these stars are much farther apart, light centuries not light years, but they make potential
waypoints on a trip.
Decent ones too, because while most stars on the outskirts of a galaxy have low metallicity
- and so probably not a lot of rocky material nearby - often these ejected ones were tossed
out by passing near the central black hole of our galaxy and are higher in metallicity.
Add to that, while an ejection of a planet from a system or a star from a galaxy often
strips it of its satellites, it also often does not, and the closer the satellite is
to its parent, the less likely it will be ejected.
So the rockier inner planets of a system are more likely to be retained.
That means these waypoints could have plenty of raw materials to use to refuel and repair,
and potentially have planets to settle on.
You don't necessarily have to go sundiving to capture fuel and raw materials on some
Icarus-like plunge into the star to pick up material, like we saw from the spaceship Destiny
in Stargate: Universe, one of the few scifi franchises to seriously tackle intergalactic
travel and timelines.
You can do stuff like that too, as we've discussed in the Starlifting episode and will
look at more next week in Colonizing the Sun.
Today we don't care about that though for three reasons.
First, as mentioned we have discussed before how it can be done if you need to, second,
odds are many of the stars will host planets which you can mine more conventionally.
But third, you normally don't stop on interstellar voyages to refuel.
Oh, in fiction you often do, they tend to have FTL systems that are non-inertial, a
warp drive that requires constant power input to maintain its speed rather than just coasting
along, or wormholes or gates or hyperspace jumps with maximum ranges that leave you stopped
relative to the local area, rather than needing to burn a ton of fuel to slow down and then
more to speed back up when you're done.
Normally in interstellar space you head to your destination without stopping, because
doing so costs you time and gains you nothing.
And while I always say it would be nice to have FTL, it doesn't really look like it
is in the cards, nor do we really know the logistics involved if it was, since they are
different for every hypothetical drive system.
So we always try to look at the future assuming no new physics and see if we can tackle a
problem anyway.
Normally you wouldn't want to stop a ship en route to another galaxy, or so we'd assume,
since it will tend to involve a not-quite straight path between various intergalactic
stars and that wouldn't seem to make sense, but we'll be giving that a second look today.
We also do have an existing precedent for stopping an interstellar spaceship.
In the Life in a Space Colony series, we examined a ship called Unity, a large interstellar
vessel kilometers long carrying hundreds of thousands of passengers.
After they arrived at their destination they realized that they did not really need to
stop and stay there.
They had all the manufacturing ability needed to take raw materials and build anything in
the ship's structure or colonizing inventory.
This was up to and including the colonists themselves, since they were making journeys
of many decades and could easily replenish their colonist pool simply by keeping a decent
portion of them on the ship to breed more colonists for the next stop.
We gave them both life extension and the ability to freeze people and thaw them out.
Although both technologies were handy for growing the colonists' numbers, they weren't
truly necessary since people weren't dying off and could continue to have children and
maintain a crew with the same goals and traditions.
So this ship, Unity, decided it could transform itself from a regular interstellar arkship
with one destination in mind into what we called a Gardener Ship, one that stops at
a system, builds a colony, picks up new raw materials and fuels, and heads off to a new
destination.
During the flight, they would breed up their numbers again, and work on turning all those
raw materials into colonial gear or replacement parts and supplies for the ship.
We ended up revisiting the crew in the episode Interstellar Travel Challenges to upgrade
how fast they could go and talk about all the problems one can encounter moving through
space that fast.
We also visited them again in the Dead Aliens episode but I consider that non-canon to their
tale, which we'll pick up again today because it's handy to have a narrative framing device.
So our gardener ship Unity has been slowly working its way out to the galactic rim, as
have various sister ships, and indeed every so often the ship divides itself up like an
amoeba.
They can make every part the ship needs so they can make a new twin ship and do upgrades
as new science comes in from home.
However, we will still limit them to the 20% of light speed we gave them in our last visit.
We will also ignore that the ship, which first went to the Tau Ceti than Epsilon Eridani,
was headed in the wrong direction for Andromeda, so they've kind of cork-screwed around.
Handily Andromeda is in the direction of the region of the galactic edge closest to us,
so we don't have to cross the whole galaxy to get there.
It's not quite the fastest route the galactic edge, which would lie more in the direction
of Orion, and we need to head more toward Perseus to aim for Andromeda, but it is fairly
close and a lot better than crossing the whole galactic disc.
That's also true of both Magellanic Clouds, we're closer to them than most of the galaxy
is.
That's worth mentioning because 'intergalactic' is a bit relative.
Andromeda is the nearest big galaxy to us, but the Magellanic clouds aren't much further
from us than the furthest parts of this galaxy, and they are no longer the closest known dwarf
galaxies.
The Sagittarius Dwarf Spheroidal Galaxy is considerably closer, just 70,000 light years
from Earth, and the Canis Major Dwarf Galaxy, whose status as a galaxy is still debated,
is only 25,000 light years away.
In all 4 cases, colonizing them is not really any harder than colonizing the more distant
parts of our own galaxy, and there are plenty of stars in between to use as way points.
I will be ignoring them today though beyond pointing out that they would tend to be settled
along with the rest of the galaxy, though in many cases you will need to cross some
areas fairly devoid of stars and pick your path accordingly.
But it also means our ship Unity has arrived at the galactic rim a long time later, on
the path they took it would be at least 30,000 light years and they've only been going
20% of light speed, not to mention stopping for at least a few years once or twice a century
to set up a colony.
So we last saw them sometime around the 26th century AD, centuries ahead of us in the twenty-first
century, but it is now closer to the twenty-first hundredth century.
They are fifty times further ahead in history than the pyramid builders are back in history.
They are parked at a last lonely star near the galactic rim, the Terminus System, and
the captain is deciding if they dare jump farther off and head for Andromeda or abandon
the mission, finally stay at a planet unlike the many hundreds she's colonized and left
behind.
Truth be told, she's been planning this for millennia, captain of one of humanity's
first interstellar colony ships, even if it's been rebuilt and subdivided dozens of times.
They could turn around, they could get back to Earth a good deal faster with the laser
highways between stars many worlds have been creating as they got bigger.
They could settle here or turn perpendicular and help colonize the galactic rim.
Indeed they could do all of the above.
Spending decades to build new ships, one to head off on each direction of the rim, one
to head off to Earth for those wanting to see home again, and one to head off to Andromeda.
The science officer points this out and that they probably want a much bigger ship or fleet
of ships to do the job.
He also points out that individuals don't actually have to choose, it's the year 200,000
AD, none from the original crew are entirely human anymore, and copying their minds onto
some clone bodies or androids isn't too hard.
They've done that before, for a crew member or colonist who wanted to travel on but also
wanted to settle down, folks who had a spouse or kids who wanted to stay and they couldn't
decide if they wanted to stay or go, so they did both, making a copy of themselves.
Or when the ship subdivided, building a twin to head off at a different angle to colonize
other systems.
They have some crew members who have done that many times, same as they have others
who sleep most of the journey.
This is the original ship, for a given value of original, that headed out from Earth 200,000
years ago, and the original captain, for a given value of original, who piloted it out,
and the original science officer, for a given value of original, who has been nitpicking
her plans since Unity was on the drawing board.
But the ship can't make the journey on its own, so the science officer says.
This ship is immense, bigger than when it left Earth and you could have crammed a major
metropolis into that one.
But to do this right, they are going to want a whole fleet and they need to build that
here at Terminus.
Now we say Terminus is the last star at this edge of the galaxy but that's not entirely
true.
It's actually an extragalactic system sent on its way many millions of years ago and
just now getting out of the galaxy.
In fact with a little bit of nudging, it could be aimed to reach Andromeda.
They could colonize it and just wait.
Andromeda is, after all, set to merge with the Milky Way galaxy in a few billion years
and they could get this whole system to arrive there a good deal sooner than that.
There is a highly advanced technology called a Shkadov Thruster, whose design is actually
a very simplistic one.
It calls for trillions of cheap mirrors to be placed around a star so that they bounce
all its light in one direction, providing thrust and allowing you turn a whole solar
system into an interstellar spaceship.
You can also boost that speed by using the solar wind of the star as propellant, or modifying
starlifting technology to create a giant plasma drive.
Normally, this is no way to cross between stars.
They take half of forever to get up to speed, but while it takes millions of years for them
to get up to even modest interstellar velocities, it also takes millions of years to travel
between galaxies.
At their current cruising speed of 20% of light speed, they will need ten million years
to get to Andromeda.
It's been fifty times as long since this ship was built as between then and when the
pyramids were built, and it will be fifty times as long as their entire past journey
to get to Andromeda this way.
They could turn the entire Terminus System into one huge spaceship - technologically
speaking it's simplistic - and head off to Andromeda that way.
However it will take longer, many tens of millions of years at least.
Potentially they could use a miniaturised version of this, turning a gas giant into
a giant fusion-driven ship, but fundamentally this is just a super-sized version of the
ship they already have.
The captain mulls both option over but rejects them.
She has been dreaming of being the first to set foot in another galaxy for thousands of
years, and that's the kind of commitment that borders on the obsessive.
Tell me how we do it fastest, that's what she want to know, and why a fleet, why not
a bigger ship?
Why even a bigger ship?
The science officer says he's very dubious - even with all their technology that can
fabricate any part they need to replace - of being able to cruise the entire intergalactic
void for ten million years without breaking down.
That's not why he wants other ships though, not as spares.
He wants to make stops along the way, and using astronomical data gathered from some
of their colonies a few light centuries back, which are now civilizations a couple of thousand
years old with lots of giant telescopes, they found a modestly straight path with stars
never more than a thousand light years apart.
The captain stops him though, and asks why not a straight path, instead of using Terminus
as a giant spaceship with all those mirrors, why not use a variant of that, turn it into
a giant laser to push the ship to near light speed so they can make the journey in a fifth
of the time.
He shakes his head at that, that's part of the idea in truth but too simplistic.
Even at that speed it's a 2 million year journey, and even if they hug the speed of
light to get time dilation, so less time passes for them and the ship, and even with the intergalactic
void being far thinner so they get less drag and collisions to damage or slow the ship,
going that long with no resupply is a dubious proposition.
But more to the point, they can't slow down when they arrive.
The ship's defense officer objects to that, if collisions slow the ship, why not let space
drag slow them down?
When they approach the galaxy they can expand out a thin sail to get smacked into and slow
the ship.
Even a magnetic field that will deflect ionized particles off and exchange momentum with them.
The faster you're going the more each particle slows you, so once you get down to a modest
speed you can use the ship's regular engine to finish slowing down.
The science officer agrees it is viable, but not ideal, such a slowdown still takes huge
amount of time and distance and would leave them a huge distance from any possible help
and needing to ration out their supplies on the whole journey with not much margin for
error, and on something totally untested.
It could work, they could build a massive laser array and use it like a giant cannon
to hurl themselves out of the galaxy at near light speed then slow down by using the local
interstellar medium to break them to more normal speeds.
But he hasn't lived 200,000 years by throwing dice and he doesn't want to leave it so
people have to do the same thing in the future.
The science officer wants to build a big intergalactic bridge, using the Laser Highway system, and
he wants to colonize each star system along the way.
By doing that they will always be in some sort of range of civilization and future ships
will be able to cross at near light speeds safely, with each bridge star from Terminus
running a laser relay to push ships faster or slow them down.
Done this way people can always go home, or send follow up missions that will be able
to catch up to them, and reinforce them.
Done the other way it's a bit like burning your ships when you reach a new shore.
What they'll do is build a fleet.
Once everyone is up to speed, they'll accelerate a bit more, more than they could normally
slow down from.
Then they'll transfer some extra fuel from each ship to one ship, which they'll all
then push on with their prow-mounted lasers, speeding it up a bit, and it's got fuel
to slow down from a slightly higher velocity.
When it arrives at the next target system, it will get to work building lasers to slow
them all down, though they've already slowed a bit pushing that vanguard ship.
If the Vanguard fails in its job they can cannibalize fuel from the various ships, transfer
people over, and jettison some mass, to ensure they can still slow down at the destination
system.
They'll have many stopovers to practice this and get it down right, and after thousands
of years of doing century long trips, they're confident they can keep the ships running
through their normal methods for the few thousand years most of the intergalactic jumps will
take.
Better yet, with each system along the way colonized and with a pushing laser there,
they can arrange for critical resupply if something goes wrong.
They can just wait at the next system or slow down as much as they can and go mostly on
ice till rescue arrives, even if that take thousands of years.
Thousands of years sounds like an eternity, but it's nothing compared to millions and
that your best time for rescue if you aren't doing stopovers.
You're also sailing through mostly empty intergalactic void if something goes wrong
here, and probably slower than before, not plowing through an uncharted galaxy at relativistic
speeds.
It's very iffy if a frozen body could be revived after millions of years too, thousands
would probably leave something to work with in terms of brain structure and is a lot more
realistic for a digital copy of a mind to repair off of.
And you only have to go that route if you're getting critical failure across the fleet.
Most of the time if something goes wrong you could transfer to another ship and cannibalize
some for fuel or material to repair and slow down.
When you're done, even though it takes around ten million years, you've been busy the
whole time founding new colonies that can still talk to each other and send ships through
at very relativistic speeds, enough that the journey will seem shorter to them from time
dilation.
Those ships only need enough extra fuel and supplies to course correct to the next relay
point if something goes wrong there, and it will be a whole colonized system.
Subsequent intergalactic trips will occur at nearly light speed, including follow up
colonization missions.
What's more, if it turns out that the galaxy you're arriving at is occupied, and ten
million years is a long enough time frame that something might have evolved from chimpanzee
to interstellar civilization in that time, you do have a way to go home.
So Unity sets off from the Terminus system toward the Andromeda Galaxy, and their first
waypoint along the way, and assuming nothing goes wrong, it will take them a bit less than
ten million years to arrive, using only fusion and lasers to do it, technologies probably
fully developed by the end of the 21st century.
Okay, let's consider some other scenarios.
First, we mentioned that you could mobilize an entire solar system as one giant space
ark, the straight Shkadov Thruster route is the technologically easiest approach but via
starlifting you can accelerate a lot faster by firing helium out as a propellant, siphoning
out heavier elements in the star for construction, and recycling hydrogen back down for the star
to eventually burn perhaps.
This same technique can be used galaxy-wide, and we'll come back to that in a moment,
but it's worth remembering that galaxies are not static and that Andromeda is headed
toward us.
That's actually quite rare, virtually every galaxy is moving away from us and the further
away they are, the faster they are moving.
Some folks ask how far off we could colonize the Universe, if it was empty, and the answer
is that your absolute maximum depends on how fast your ships can travel versus what the
redshift of the target galaxy is.
That's about 20 kilometers per second for every million light years of distance, or
20,000 kilometers at 1 billion light years, about 7% of light speed.
Our ships were averaging about 20% of light speed, so it could catch up to a galaxy moving
just under 20% of light speed some 3 billion light years away, though it would take longer
than the Universe is old to arrive with such a slow relative speed, just a little faster
toward its destination than that destination is moving away.
Fortunately the closer you get the slower it will be moving away, but unless you want
to take half of eternity to get to your destination you don't aim for anything moving away at
much more than maybe half your maximum speed.
This is part of why when asked I usually say a billion light years is about as far as humans
can colonize without FTL systems.
That's a lot of living space too, many thousands of galaxies.
However we do have other drive options, that laser slingshot and magnetic sail slowdown
method from earlier would probably work, it's not ideal for interstellar travel to neighboring
stars but it ought to work over much larger distances, even well short of intergalactic
ones.
And you could do it at every waypoint.
It only takes a year at one-gee of thrust to get to near light speed and the same to
go back down, so you can get away with stopping between stars as waypoints without losing
much time, at least when those distances are much larger than the normal interstellar scale.
Slowing down without fuel by collision or magnetic braking is all about how fast you
are going, how thick the interstellar or intergalactic dust and gas are, and how big your braking
sail is, but think centuries not minutes, even for an immense sail.
That aside, and ignoring our example of Unity today, it would probably be how I would do
it.
We also have options for black hole drives or anti-matter drives or maybe quark fusion
or some other new concept, which might allow higher travel speeds and expand that radius
we might colonize a lot.
However, I said that was part of the reason I usually put it at a billion light years.
There are three others.
First, it does take time to colonize places and most colonists will want to go the nearest
and easiest empty place, so even if you've got ships that could make speed to get to
some place four billion light years away, and do it fast enough to arrive before it
ran out of stars to colonize, you'd presumably want to stop along the way.
Second, if you aim to the edge of your speed, you could arrive when there are no stars left,
but more importantly if you are arriving somewhere 2 billion years from now, I'd find it very
hard to believe that you'd be arriving at an unoccupied place.
The older the Universe gets, the more likely life is to develop and get technology.
Worlds that already have life have more time to evolve, most stars live a lot longer than
ours does, and the metallicity of new stars rises, meaning more rocky planets in general.
I'm on the extreme skeptic end in terms of the Fermi Paradox, in that I doubt any
civilizations have arisen within a billion light years of us, see the Dyson Dilemma 2.0
episode or various Fermi Paradox episodes for explanations of that reasoning, but I'd
have a very hard time believing you could arrive at a galaxy 3 billion years away from
us, and maybe 10 billion years ahead in time, and find it was still absent of intelligent
life.
Third, the Universe is expanding, and only a handful of galaxies are near enough to us
to stick with us as that happens.
However, just as you can move a star you can move a galaxy, you just build those Shkadov
Thrusters around every star and let gravity tractor it with you.
And most of the galaxies within a billion light years of us are going slow enough you
could slow them down this way to stay bound to us.
If you can't, then any of those colonies are destined to be forever gone.
You won't ever be able to talk to them again at some point.
Of course you might not care, and anyone sent on billion year long quests are going to have
more time to have diverged from you than humans have with oak trees, though that problem already
exists at the Interstellar level and is probably beyond manageable even at the galactic scale
already.
That's why we discussed alternatives like the light year wide Birch Planets from the
Mega Earths episode.
Don't overlook that option though, as huge as a galaxy is and as long as billions of
years is, you can move one, there's no tricky physics involved, it's just an application
of brute force on a galactic level, astronomical timelines and energy needs, but you have both.
We have no idea where the closest civilization to us is, might be within a few hundred light
years, might be none in the whole Observable Universe, and with all the time it takes light
to reach us and for our ships to arrive there, especially in intergalactic terms, what we
can see now doesn't necessarily mean much compared to when you arrive.
A galaxy a billion light years away might be absent technological civilizations a billion
years ago, when that light left, but would it still be a few billion years from now,
when you arrived?
We can't know, but for my part an unused and dead solar system is one we should always
claim if there's nobody around there or nearby asserting their own claim.
And I would expect an alien civilization to do the same, not because they or we are aggressive
and hostile, but because a random space rock around a lifeless star just has less inherent
value than a tree or cat or a dog or a person or even an inanimate statue someone has carved.
And while some might claim otherwise, every breath they take puts the lie to that claim
that they think their life is no more valuable.
I don't know many people who say otherwise and I don't believe the few who do genuinely
believe that, so when someone asks why colonize other places, be it other planets or other
stars or even other galaxies, I always feel it's the wrong question.
Not why would you, if you could, but why wouldn't you?
I don't know if humanity is destined to colonize other galaxies, I'd never support
doing so if someone else already lived there, but should it turn out that intelligent life
is that rare I think we should, and as we've seen today it is on the table, even if we
never figure out how to make warp drives or wormholes.
As we head into 2018, two generations after we last set foot on the moon, I think it does
help to remember that the sky is not the limit and that we have potential new frontiers for
billions of years to come.
And we are going to keep on exploring them next year.
Until next time, thanks for watching, and have a great year!
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