One of the more irritating problems about
space travel is that if you want to get up to decent speeds you need to burn huge amounts
of fuel. Not just the fuel to get the ship up to speed, but the fuel to get the rest
of the fuel up to speed. It is a losing battle, even if you have the sorts of super-fuels
like fusion or anti-matter. Way back at the end of the first year of the
channel I mentioned an alternative to it, and also one that can hypothetically be done
without any new science at all, even fusion, something I normally take for granted whenever
discussing anything involving travel to other solar systems.
We covered it rather hastily so I thought it was time to look at it in more detail.
The strange thing about a lot of the earliest episodes is that I often covered the most
practical options in those, in abridged form, then covered less practical options in more
detail in later episodes. It seemed a good time to correct that.
The funny thing is it doesn’t sound terribly practical, building a highway between stars,
and of course we’re not talking about a literal highway, that’s simply an analogy,
and in some ways a river might have been a more apt one since we’ll essentially be
talking about sailing between stars using actual sails on rivers of light.
I’ve noticed the idea of propelling spaceships with lasers has been getting some coverage
in the press of late and I should probably explain how that works and why it is an attractive
option first. The big down side of rockets is that you have to carry all your fuel with
you and that your final speed is strongly controlled by how fast your exhaust flies
out the back. If you want to go as fast as your exhaust
flies out, you need 63% of your ship to be fuel, and if you want to slow down at the
end you need to be 86% fuel. To get to double that speed and back down again you need to
be 98% fuel, and for triple 99.75% fuel. What that actual speed is depends on what
your fuel is. If it were just compressed gas that’s quite slow, like if you were an astronaut
who lost anchorage and started spinning away from your ship. You could vent your air tanks
in one direction and the rocket equation works just fine, your propellant is leaving at about
the speed of sound, about 300 meters per second. Of course you’re mostly not fuel, or air,
but if you were 63% fuel by mass you could get up to about the speed of sound, ignoring
obviously that there’s no sound in space. Our typical rocket fuels have exhaust velocities
10-20 times higher than that, meaning they could get to that same speed of sound with
the ship only being a small fraction fuel, about 5-10%. As you’d imagine you want the
highest exhaust velocity possible with a rocket because it’s pretty much impossible to get
to more than 4 or 5 times your exhaust velocity with any rocket, again getting to just triple
the speed and back down again required 99.75% fuel ratio, or 400 parts fuel for every 1
part of ship and cargo. It gets even worse to get to 4 or 5 times that speed.
So your ideal exhaust is photons or neutrinos or gravitons, things already moving at or
near the speed of light, which is a million time faster than sound. A rocket that shoots
light, or photons, out of the back is called a photon rocket, and you can build one at
home as easily as you can one that operates using baking soda and vinegar. You just need
a flashlight, and indeed if you turned a flashlight on and tossed it out an airlock it would begin
accelerating off… but very, very slowly. A flashlight with two D-Cell batteries in
it might be able to emit 90,000 Joules of light before dying, and as we know from Einstein’s
E=mc², 90,000 Joules is the equivalent of one trillionth of kilogram’s mass energy,
or one nanogram. So it’s great rocket fuel in terms of mass,
though not terribly practical. If I had a box made of perfect mirrors that let me dump
light in and it couldn’t escape until I wanted it too, then a box with a kilogram
of light in it, and yes photons do have mass in this context, which I then handed to an
astronaut who weighed 100 kilograms including his suit, could use that 1 kilogram box of
light to get up to about 1% of light speed. We can’t box light up like that, but if
we could it would be a great fuel, and that’s basically what makes anti-matter or Kugelblitz
black holes so ideal. However they still have to obey the rocket equation and of course
rely on science we don’t have yet, and maybe never will.
However we can actually make a better ship than those using just modern technology, and
we do that by chucking the rocket equation out the window and just not having the ship
carry fuel at all. Hit something with light and it reflects or
absorbs that light. Photons it absorbs transfer all their momentum to it, photons it reflects
transfer twice their momentum to it. This is how a solar sail works, but if you want
any real speed and mass you need to concentrate that light either with lenses or by using
a laser. Especially as you get further from the sun.
The force a laser exerts on a non-reflective object is just the power of the beam divided
by the speed of light, double that if it is reflective. So if I want to exert a force
of one gee, the equivalent of earth gravity, on a 1 kilogram object, or about 10 Newtons,
I need about 1.5 gigawatts. Which is about the power output of the Hoover Dam. That might
sound rather outrageous but when dealing with interstellar travel it will turn out to be
cheaper than the alternatives and it means nothing inside a fusion economy or solar powered
Dyson Swarm or Kardashev-2 economy. Thinking of it terms of hydrogen, and rounding
down to assume low efficiency of fusion and laser generation, a 1.5 gigawatt laser would
run through about a kilogram of hydrogen each day. Hydrogen again is the most abundant stuff
in the Universe, so that while this is in some ways and insanely wasteful use of energy
you could run a trillion of these lasers for a trillion years without even putting a noticeable
dent in Jupiter. And again we were assuming we were only getting about .1% of the mass
energy of that hydrogen turned into a laser beam.
This is the idea behind Starshot, you lug some solar panels and a laser up into space
and push a tiny probe with solar sails on it with that laser.
Here’s where we get problems though. First, it is not easy to keep a laser targeted on
an object when it starts getting far away. It needs to stay on course quite precisely
or the laser misses, and it is quite easy to get a tiny variation in speed. Your mirror
for instance does not have to bounce light straight backwards, even a miniscule change
of angle on that mirror will bounce the beam off at that angle and give you a little lateral
velocity. This is actually handy since it means ships can move other than just away
from the beam. But even a tiny drift of a millimeter a second means an hour later your
ship is 3.6 meters off to one side, and your laser flies past if your sails aren’t at
least that wide. If your laser is light-hours away it won’t even know its missing you
for hours and it will take just as long before the laser is back on target.
Second, lasers beams are not infinite thin cylinders, they do spread out.
For both those reasons the further you get from the laser, the bigger the sail you want
to have. So you want your sail as thin as possible, preferably less than a micrometer
thick, but we have a lot of problems going much thinner than that.
Third, you need some way to slow down. Now you can just use the laser to get up to speed
and fuel to slow down, which is handy, but still not ideal. Nonetheless you will always
want two things on any laser-sail ship. First, a very good GPS system, it helps to be able
to calculate your position quite exactly and tell the laser where you are and where you
will be whenever that message gets to them and the laser gets to you. Second, you always
want some fuel to maneuver with on board, because it means you can correct your position
if you lose lock with that laser. The reserve air on your ship might do the job.
Of course an obvious way to slow down presents itself, you have a laser on the other end
that does the job, pushing back against you. Needless to say, something has to go build
that first. Also, you probably have a maximum range you
can realistically keep pushing at, where it just isn’t viable to keep the laser on the
sail or expand the sail further, so you probably want spots along the way that can pick up
the job. Space Stations between stars with their own lasers who can push on the ship
to speed it up more or push back to slow it down.
It’s as simple as that. We’ll walkthrough a fairly realistic case of this in a moment
but that’s the basic idea, so let’s do a few notes before that example.
First is power supply. Near the Sun obviously solar power is handy, no clouds or nighttime
in space. It’s always noon. You can beam power directly out to your distant relay stations,
easier to keep a lock on something that isn’t really moving much and can be quite large
compared to a ship. But by preference you’d have local power generation, if you had fusion.
If you did you wouldn’t use this for interplanetary travel because even the crappiest fusion reactor
is going to let you let get very high interplanetary speeds with a lot less hassle than lasers
involve. Hydrogen, again, is the most abundant stuff in the Universe and if they’re offering
an exhaust velocity of hundreds or thousands of times what chemical rockets offer they’d
just tend to be easier to work with. Either way, getting the power out there is
a hassle but it’s a doable hassle. Second, laser beams diverge over distance
and the maximums allowed relate to wavelength, so blue light is better than red light, and
ultraviolet is better than blue, whereas microwaves are worse by far than any of those. I mention
this because you can use any type of photon you can reflect. Lower frequencies have other
advantages though, since you need fairly sturdy mirrors to reflect ultraviolet light whereas
you can reflect or absorb microwaves with a metal mesh. That’s why your typical microwave
door is plastic with a mesh in it. The holes in that mesh are smaller than the microwave
wavelength and keep it from getting through, thus allowing you to watch your food cook
without your eyeballs exploding, which tends to ruin dinner.
I should also not that you can bounce a beam back and forth several times and gain momentum
each bounce, so when you are near a laser you can get more push than you’d expect.
Of course beams tend not to stay together after many bounces and that’s hard to do
at a distance. You also don’t have to use light at all,
charged particles flying out of a particle accelerator and hitting a magnetic sail would
work, so too would neutrinos if someone were ever to figure out a way to make a substance
that could reflect them. You can also use that beam as your power source.
You just have solar panels on the back not just mirrors. Absorbing the light to use it
for power only gives you half the push reflecting does, but considering the beams need to have
the power of the Hoover Dam to push a kilogram as hard as gravity does, you obviously don’t
need most of that, so you sip in a little and reflect most.
Also as a note, while I’ll be ignoring relativity when talking about speeds, to save the headaches,
this still cannot be used to reach or exceed the speed of light. As you get up to those
speeds, not only do you have to worry about all the travel hazards we discussed a couple
weeks back in the Interstellar Travel Challenges episode, but your laser pushing on you is
going to start red-shifting, meaning it is getting weaker. You start getting up close
to light speed you’ll be wondering why your nice big 200 gigawatt blue laser now appears
to be a weak red 100 gigawatt laser, and if you keep going, why it is now a measly 1 megawatt
microwave beam. Lastly before we get to our fictional example,
I want to emphasize that this method is superior not because it saves energy. It does but who
cares? Again Hydrogen is super-abundant. It beats out chemical rockets in all respects
but beats out fusion and even anti-matter or black hole starships in terms of maximum
velocity. It lets you get faster on less energy, but it lets you get faster in general. It’s
maximum speed, on a long enough chain, like one running across an entire galaxy, is whatever
velocity that local chunk of space will allow before all the drag of the interstellar medium
finally cancels out your acceleration from your increasingly weaker and red-shifted beam.
But if you have relays all along the way helping to clear out dangerous debris and pushing
you along, 99% of light speed is doable with such a laser highway, even higher if for some
reason you need to slow down your onboard time enough to justify such a thing. Out in
the intergalactic void where things are even thinner and where trips would take million
of years, I could see justifying the power expense to push your speed up to ultra-relativistic
speeds, so the trip only took a thousand years of your time even though you still arrived
millions of years later. And yes you could scale this system up to stretch between galaxies.
Okay, onto our example. We’ve covered the science put I’ve been finding a hypothetical
fictional example often helps cement the idea, I’ll return us to the one we used for the
Life in a Space Colony Trilogy but we’re back home at Earth. It’s the year 2500 and
mankind has a couple dozen or so interstellar colonies and has settled thousands of asteroids
in our own system. The population in the solar system is a few trillion, and they’ve got
fusion power and are slowly adding rotating habitats to a Dyson swarm that so far is cobweb
thin, not blocking even 1% of 1% of the sun’s light.
Very little of the solar system’s GDP is going into interstellar flight, we’ll say
1% of 1%, the equivalent of 2 billion dollars a year from the current US economy. But they’re
so much bigger and higher tech than us that such an expenditure still means they are building
and equipping dozens of fusion-powered colony ships a few kilometers long and sending them
out every year. Those ships only travel at a maximum of 20% of light speed so even though
they’re spewing out massive arkships they’ve only got a couple dozen colonies that have
actually arrived and sent home ‘mission accomplished’ messages. Most are still en
route, the ones that arrived all date back to when you only built maybe one ship a year.
Of course some of those messages are saying ‘Hey, send more people’ and others are
saying ‘Hey, we’ve got some people who want to come home.’
So we go talk to the giant supercomputer named Deep Thought sprawling over Mountview, California
at the former Headquarters Quarters of Google and we ask it if Faster Than Light Travel
might be on the table. It says yes, it probably is, but it will need some time to think about
it. We decide to go get lunch while the God-machine figures it out and the TV reports as breaking
news that Deep Thought has gone into very deep thoughts and says it will be busy for
the next seven and a half million years because some unidentified morons asked it to solve
faster than light travel. So none of the other AI’s will talk to us
and the various cyborged up transhumans also tell us to take a hike, if they want to visit
another solar system they’ll just send a digital copy of their own brain by radio at
light speed. So we decide we’re going to have to do this using fairly basic technology.
At the dawn of the 26th century you don’t get to play with human-level intelligence
AI’s or independent self-replicating machines if you don’t get one of those to sign off
on it. Not since ‘the Incident’ that resulted in unhinged Grey Goo turning the Mars’s
Moon Phobos into quadrillions of paper clips. So we can’t send out any pencil-sized tubes
of self-replicating machines to impact into various interstellar rocks and turn them into
entirely automated laser platforms. We are going to have to build it all ourselves. And
our relays are going to have to be manned and when we talk to our engineering team they
tell us the real issue is making our relays close enough that they can clear out all the
space junk. If we want ships slamming along at 90% of light speed we need to have a big
corridor millions of kilometers wide that we are constantly clearing of anything big
enough to be visible to the naked eye. They also say they’ve got a great sail design
made out of graphene that’s a hundred kilometers across and can reflect our lasers at up to
10 megawatts per square meter without melting. And a whole sail like that will only mass
1000 tons, and they can target that easily a whole light week away. They say we can drop
fifty stations spread out over a light year and they can keep a laser on a sail like that
the whole way and keep those corridors clear of debris and space dust so that they can
do those speeds and that the corridor is wide enough that millions of ships could be passing
by at different speeds or in the opposite directions without collision concerns.
So we get them to design us a ship with some spare sail segments, a small fusion reactor
and some fuel for maneuvering, and all that and the main hull will cost us 3000 tons.
We are going to go with a nice 10,000 ton ship design, 10 million kilograms. We could
go bigger and slower, we could go smaller, with smaller sails, but we are building to
10 million kilograms, the mass of modern Ticonderoga naval cruiser, very small by this channels
standards. I think our interstellar arkship example, Unity, had shuttles bigger than that.
This time around our ship isn’t a big long cylinder or even a sharpened pencil, whose
front cone shape helps bounce debris away, but is an outright cone. We are going fast,
we want all the bounce we can get, and we are accelerating fast too, so we don’t really
need spin gravity, or at least if we do, we need a cone shape to let us merge that with
the acceleration-gravity the beam is providing. We don’t have any massive artificial habitats
that can’t be easily moved around. There’s some hydroponics and a few small gardens,
but this is a passenger ship, not some huge colonial arkship. We’re also high tech so
if the ship’s shape isn’t ideal halfway through the trip because we can’t accelerate
anymore it will just stretch itself into a cylinder. It’s the sort of ship where a
thousand people could enjoy modestly comfortable personal cabins comparable to modern luxury
cruise ships. You could pack more folks in but this is still a journey of many years
and living space isn’t really a big constraint, mass is, so it’s not about cramped cabins,
more that the furniture is light-weight stuff. Our spare sail and fuel are packed up front
acting as shielding for the trip along with all our spare air and water and supplies.
The neat thing about our relay is that if something happens to us we could adjust speeds
enough that some other ship behind us could come up and take us on board. Indeed the relays
might even be able to send us accelerated matter streams of oxygen or hydrogen if we
needed them. So how about those stations and their lasers?
First let’s talk power. If our 10 million kilogram ships are going to be getting pushed
along at one-gee, those lasers need to be 10 million times more powerful than the 1.5
gigawatt one we discussed for shoving a one-kilogram object. That’s 15 million gigawatts or 15
petawatts. So you know, the light hitting Earth from the sun is only a bit over ten
times that, so we’re talking about lasers that could provide noon-time sunlight to an
entire continent. They’re also burning through 10 million kilograms of hydrogen a day, using
our earlier figure. They could be getting that from home, and
endless chain of massive freighters coming in slower stretching back to Jupiter. But
interstellar space is lousy with giant icebergs so each station is probably built on one dragged
at lower speeds to be on the chain. Or it sends out swarms of smaller ships to mine
its local area, after all it has a whole light week, a volume much bigger than our solar
system, all to itself. There’s a huge amount of hydrogen gas floating around there and
it could just have collectors out past the corridor that sucked up hydrogen to be shot
to that relay. But a single tanker the size of modern oil
tanker could dock every few months to replenish their needs. You could slap speed limits on
your highway if fuel consumption got to be an issue but even one small gas giant could
fuel hundreds of these lines for longer than most stars live without being significantly
reduced in mass. You might even use a slower matter stream down the corridor to supply
fuel, you don’t just move ships with such a highway, you can move atomic matter via
particle accelerators and information too, since even modest lasers running between each
relay can allow huge bandwidth transmissions with little signal loss.
So fifty relays a light year, and we will say we’ve got a total of 2000 light years
worth of relays going to all our neighboring stars inside about 20 light years, there’s
a bit over a hundred of them, giving us a decently round number of 100,000 relays, with
a few hundred on each line on average. What are the relays like? Ships aren’t really
stopping at these places often. There’s a sweetspot between fusion engines and laser-sails
probably somewhere in between 10-20% of light speed, so you might have slower ships just
above that which stop from time to time, one-gee of acceleration or deceleration will let you
get up to or down from 10% of light speed in just over a month, or down from 20% in
about two and half months, so slower economy style ships, as it were, making slower trips
might make layovers at them. They also might be fairly busy local ports for mining activity.
In terms of size they can be quite huge if they want to be, again they’re running lasers
strong enough to light a continent so if they were only running them at night they could
be that big, lasering ships in their night cycle and using that juice during the day.
These are non-moving objects, so they don’t really need the kind of shielding the ships
do. They also aren’t going to be one big laser, more like an array of thousands of
them, and so they could be a small swarm or string of rotating habitats serving as a home
to a billion souls. Or they could be quite small, just a station of a few hundred for
maintenance and maybe using their excess power for lighting large forest preserves or such.
The other neat thing about a system like this is you can have shuttles. You need a big ship
to ferry normal humans around for a decade, and again cyborgs or AI’s can probably skip
the whole thing in favor of traveling as data so we might as well think normal humans. So
you need a big ship for the whole journey but not for jumping around. A single person
pod or small ship for maybe a dozen with a big sail could take a lot more than one-gee
of acceleration, so people could rendezvous with a ship or station by boarding such a
shuttle and getting pushed up to or down from speed. The equivalent of a life pod might
be some barebones life support with a big sail designed for the maximum acceleration
possible. As to constructing such a chain, you could
build the relays by normal means or by using existing lasers to get out the next location
and slow down more conventionally, saving a lot of fuel. Once built they allow near-light
speed travel from system to system. On a scifi note it actually allows piracy,
since folks could lurk in bases near the trail and come out and pirate ships and data. Those
outposts are devastatingly heavily armed with the megalaser but that’s only a good weapon
at relatively close ranges since people can intentionally jink their ships around to avoid
it, as we discussed in the Space warfare episode. Okay, we’ll leave off there for today. Those
are interstellar highways, a means of travel that requires no high technology but allow
the best performance of any interstellar option currently allowed under known physics, even
some that seriously bend it. I’d like to thank Stefan Blandin for making
some of the excellent animations for this episode. I’m always grateful whenever anyway
submits artwork for the channel to use but this kind of custom-tailored animation is
the sort of thing that can take hours and even days to make and I really appreciate
that. There are obviously no animations to use for something like this when even laser
sails are still a pretty new idea let alone some massive network like the one discussed
today. Next week we will be beginning our new series,
Upward Bound, where we will come home to Earth in modern times and look at various launch
assist systems that might let us get into space cheaply enough to start setting up the
kind of interplanetary infrastructure you need to have before you could ever contemplate
something like the Interstellar highway. Make sure to subscribe to the channel for
alerts when that and other episodes come out, and if you enjoyed this episode, please like
it and share it with others. Until next time, thanks for watching, and have a great week!
I'd suspect the ships will try on the laser track more than the lasers try to keep on the ships.
One limitation as dependent on aperture and wavelength is it spreading at least λ/D.(i remember a two.. maybe that was a radius..) Multiple laser sources that properly correspond the phases of their output can get an effective aperture to be something like the distance between them. Not sure if any complications can worsen it. How to get the phases correct could range from sending this information as data, to using a seed laser and delaying the appropriate time.
Spot size = distance⋅λ/D or rather distance=Spot size⋅D/λ for 1km, 500nm, for both 2⋅1012 m = 2⋅10-4 c⋅yr far, but many stations needed for those values. Of course 1km is absolute peanuts in terms of how far you can put multiple laser sources apart, and probably also kindah small for the sail itself.(depending size ship) In comparison 1AU=1.5⋅1011 m effective aperture gets a range of 30kc⋅yr.
About the clearing of space debris, what about the velocities of the dust? I am not sure what those would be, would suspect the larger the particle, the higher the typical velocity. Because the particles do have some friction with interstellar gas.(begging-the-quantitative question) Velocities could be as high as 100km/s though 1-10km/s seems more likely. So the frontal shield does not cover a cylinder, but a cone, tightening at (v/10km/s) if you want to have a shield a second of travel ahead cover everything, it has to be 10km wider or stuff will gets itself inbetween.(let along a light second which might be ten times as much) Frontal shields are fine, of course, going at 0.01c, 0.01s ahead, or +100m wide, is already 10-4 lightseconds=30km ahead, suspect such shields ahead are probably useful even if just a few meters ahead... Plus, the shield ahead still reduces the number of particles recardless.
Similarly for corridors, stuff would constantly float itself into it. A lightsecond away, it takes at least a second to see it and another to destroy it, so you'd need 20km extra space in which it gets destroyed. If stations are lighthours apart it is 7200km. (Detecting particles on these long ranges may be difficult aswel.) Again, even without this, it could really reduce particles along the route, but ships still have to deal with those remaining..
Edit: probably have to design for outliers too, can't have your ship destroyed by a single largish outlier..