The great filter Part 2

A while ago I wrote about the “Great Filter,” or the reason why we don’t see aliens everywhere we look in the universe. Read about it here:

Last time, I argued that the great filter cannot be a totalitarian regime, is very unlikely to be either berserkers or environmental damage, and is somewhat unlikely to be nuclear war and/or pandemic. Which leaves us with two more filters, the starships are hard, or that civilizations aren’t interested in colonization.

Today, I’ll talk about whether starships are hard.

In order for something to be a filter, it needs to have the following characteristics.

1: It must prevent the colonization of the galaxy.

2: It needs to be stable (or long-lasting), it if effects a civilization in time period x, it must still do so in period x+1

3: It needs to be universal, and effect (nearly) all civilizations, regardless of biology or culture.

So does difficulties building starships meet all three categories? For item 1, definitely. For item 2, also definitely, the laws of physics governing space travel aren’t changing. If its hard to build a spaceship today, just waiting won’t make it easier. At first glance, it appears to be universal as well, as all civilizations are facing the same laws of phyics. However, there may be two reasons why this would be different. First, some species may be more able to survive on starships, for instance they may be smaller. Secondly, some civilizations may start out closer to colonizable star systems than others. However, even with these two, we can say that space ships hit the “nearly” universal tag.

So, having established that difficulty in building spaceships can lead to Fermi Paradox, we tackle the more interesting question which is, has it?

At first it seems obvious that it will. Assume that the highest speed of a starship is 10% the speed of light. Next, assume that the average colonizable target is 80 light years away. Simple math says that it will take 800 years, or roughly 23 human generations to complete the journey. So you have to have a starship big enough to house enough people to preserve genetic diversity over such a time period (we’re talking about hundreds at the bare minimum, probably more realistically in the thousands), and enough space to grow food, house recycling functions (for not just materials but water, air, etc), provide living quarters, and enough energy to run the whole thing. Furthermore, we’d need to transport all sorts of animals, fish, livestock etc, to populate the new world, in addition to feeding people along the way. Also, all the equipment needed to actually colonize the new world. We’re talking about a big spaceship, and enough energy to get that spaceship that fast is enormous, not to mention slowing it down when it gets to the target.

All this seems to lead us to the conclusion that yes, space colonization is very hard.

But there are a few things we can do to modify this. First, let’s assume that people aren’t busy living on board the spaceship, but instead in a state of suspended animation. Ditto for all the cattle, fish, plants, dogs, and whatever else we want to bring. Suddenly, the total power requirements goes down, a lot.  Also, since we don’t have to worry about storing or growing food, we can cut our speed down, say to about 2% the speed of light, making the journey take 4,000 years instead of 800, which means that civilization on earth may no longer exist, but if the new colony can become self sufficient and expand, then it will be able to colonize the galaxy.

One might object by saying that I’m making up a technology that we haven’t proven to exist. Furthermore, while suspended animation may be possible, it might not work for long periods of time (it may work for 50 years, but not for 4,000), or that it may still take a lot of energy to keep the suspendee alive. All of this may be true, but I would argue that of course we assume there must be some technology which doesn’t exist for us yet which could lead to space colonization after all I don’t think we’ve discovered everything. And while there may be difficulties in suspended animation, there is nothing that I know of in the laws of physics which would prevent it, unlike warp drives for instance.

There is however, a much easier way to transport people across the vastness of space than suspended animation. And that is to transport not fully grown humans, but fertilized eggs. While we certainly don’t have experience freezing embryos for thousands of years, there’s no reason to assume that storing them at near absolute zero temperatures wouldn’t work. Furthermore, in the coldness of space (2.7 Kelvin), you wouldn’t need to spend energy on refrigeration technology. Now, we’d need a way to take those embryos and develop them outside of the womb, and then raise/educate those children, which could either be done by a subset of humans (if suspended animation is feasible), or by robots.

The same holds true not just for humans, but for all manners of plant and animal life, we can take an entire genetic ecosystem worth of genetic material in a series of canisters no bigger than a large room. And there may be even more compact ways. Instead of storing embryos, we could potentially just store DNA sequences of organism, then “build” them when the starship reaches its destination. Whether this is feasible or not is up in the air, but it certainly seems possible to me.

Now, how big would a spaceship need to be in order to do this. How about 10,000,000 metric tons of starship? That’s big, about 15 times the mass of the largest ship every built (the Seawise Giant), but small compared to something like the fleet of oil tankers on planet earth right now (less than 10% of the mass of Ultra Large Crude Carrier, when loaded with petroleum). Now, can a ship that size hold all the things needed to colonize a planet? Truth is, I have no idea, but lets run with it for a second.

So if we have 10 million tons, that’s 10 billion kilograms. Doing some math (e = 1/2 mv^2), it will take about 3,000 years worth of current us electricity consumption to get the ship up to a speed of 1% the speed of light, which is a lot, but is it too much? If we reach the point of harvesting energy using space solar panels, it becomes a bit easier. We would require only a solar panel of 136 miles on each side, placed at 0.1 AU, to harvest that amount of energy over a year (assuming 22% efficiency). This is about one and a half Marylands worth of solar panels. This seems like a lot, but (according to this source: it is only about half the total surface area of highways in the US. In short, if we get to the point where we’re mining asteroids, we can do it. Storing that energy and then using it to power the ship are another matter, and while it seems hard, it doesn’t seem impossible. (Math allows us to proportionally change things easily! If you want to increase the speed of the ship, to .02 c, for instance, just double the length of the solar panel side. If you want to double the mass of the ship, use two years instead of one).

One final thing, which I’ll comment on because I spent a long time figuring this out, is how to slow the ship down. There are plenty of actual starship designs out there, including hyrdogen scoops and the like (Bussard Ramjets), but I thought, hey, why not use a parachute to slow a ship down. I did the math to determine how big a parachute you’d need to slow down such a ship, and it was one of my favorite problems to solve ever. Feel free to skip all the math, but here it is:

To solve it, we use the drag equation, f = 1/2 p * v^2 & a * c

Where F is force, P is fluid density, V is velocity, A is area of the parachute, and C is the drag coefficient. For reasons I won’t go into, we can say that C = 2, so the formula becomes

f = p*v^2*a, or
v’ = -p*v^2*a/m (m is mass)

I put the negative sign in because the acceleration will always be negative, ie the ship will always be slowing down. In order to translate this to a function of time, we use the initial value problem: (

Since -p*a/m is a constant, lets just call it k, that gives us

dv/dt = v^2 k


dv/v^2 = k* dt

Integrate both sides and you get

-1/v + C = k * t + B

subtract C from both sides

-1/v = k*t + B-C

We can call B-C a new term (D), then simply isolate v

-1/v = k*t + D

1/v = -k*t – D
v = 1/(-k*t – D), which means that the function is

v(t) = 1/(-k*t – D), we know everything except D, but we know what the starting speed is (.01 c, or 2,997,925 m/s)

v(0) = 1/(-k*0 – D) or

2,997,925 = 1/-D, which gives us a D of -0.0000003335640952.

so v(t) = 1/(-k*t + 0.0000003335640952)

k, if you remember, is -fluid density * area / mass. So for units of k*t we get (mass / length^3 * length^2 / mass * time), this reduces down to 1/length * time, or -1/velocity, which is great because that’s the unit we need, our units match.

Now, it would be absurd to use this to slow the spaceship down to zero (it would take forever), but we can use it to slow the starship to say the speed at which the earth revolves around the sun (30,000 m/s). Finally, lets say how long we want it to happen (over a period of 3,000 years, for instance), we get

v(3000 years) = 30,000 m/s

Since we never defined how big the parachute is, we can now solve for it, given our constraints above:

v = 1/(-(-P*A/M) *t + D))

Rearrange to isolate A:

A = M/pt * (1/v + D)

throw in the numbers we know: (I originally did the math based on a 824 thousand metric ton ship)

A = 824,000,000 (mass of ship) / 2.39E-21 (density of space in kg/m^3) * 94,672,800,000 (3000 years in seconds) * (1/30,000 (speed in m/s) -0.0000003335640952 (our constant D, in s/m)

I love this so much because it uses ridiculously large and small numbers (giant ships, giant sails, the density of outer space!!)

Anyway, we get a value of 120,123,349,601,661, square meters, which is pretty big, or 10,000,000 meters on one edge of the (square) sail, about 6,850 miles, which is a sail about the diameter of earth. A sail built from any substance would be prohibitive in terms of mass, but using an electromagnetic field wouldn’t.

All of this is to say that I think it would be possible to slow the ship down. Can we speed it up? Put it this way; 1% of the speed of light is only 200 times faster than space probes we’ve already built. Surely we could built something to go, if not that fast, 50 times faster than the Voyager 1 spacecraft.

All this, though, leads us to the easiest path of all, while humans may or may not ever be able to colonize the galaxy, surely self replicating robots could? We’ve already built robots which can function for years on other planets, building some sort of robot or collections of robots which could construct more versions of themselves on other planets makes not only the difficulties of getting there, but the difficulties of transporting humans there almost disappear.

Building a collection of robots to colonize the galaxy might not seem romantic or noble, and it may not even be wise; in fact we might say that it is a very bad idea. But that doesn’t matter for our purposes, all we need is the idea that a: it’s possible and b: that somebody somewhere decides to do it. If we have those two conditions, then its pretty much inevitable that we get a galaxy full of robots, which, based on our observations, doesn’t appear to be what we have.

Starships are hard to build, no question. But I don’t think they are so hard to become the great filter. If there are enough intelligent civilizations, one of them will build self replicating robots and conquer the galaxy.

Next up, do we lose our desire?

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