Is a Self-Building 'Seed' Ship the Future of Space Travel?

[dansmith170]

Just wanted to share what I thought was a cool science fiction idea: it's a "seed" spaceship that transports to a distant star system and then builds into a larger structure (spaceport or something) by producing electrons and protons from the energy of the star in that system, and then arranging those electrons and protons into the larger structure. Why a seedship? Because then you don't have to accelerate a ton of mass to a distant star system.. the mass builds itself once it's there.

 

[phinds]

builds into a larger structure (spaceport or something) by producing electrons and protons from the energy of the star in that system

And what mechanism would you use do that?

 

[dansmith170]

And what mechanism would you use do that?

I'm not sure about the protons, but the electrons would be made by multiphoton Breit-Wheeler pair production.

 

[phinds]

Uh huh. I may be overly pessimistic here but I'm reminded of the old saying that if we had some ham we could have ham and eggs provided that we had some eggs. And a pan to cook in. And a fire too cook on. And some utensils to eat with.

 

[Vanadium 50]

And what mechanism would you use do that?

Magic.

 

[jrmichler]

The concept in the OP has been around for some time. Search self-replicating machine for examples and the history of the idea. Here's one good site: https://en.wikipedia.org/wiki/Self-replicating_machine.

When the self-replicating machine is part of a space probe, it's a Von Neumann probe (search that also), and here's one good link: https://en.wikipedia.org/wiki/Self-replicating_ spacecraft .

And, of course, as stated above, there is magic.

 

[phinds]

Magic.

Apparently

 

[phinds]

The concept in the OP has been around for some time.

Yes, I'm aware of that but as far as I know, none of them posit creating electrons and protons (and no neutrons by the way!). They use mining of local resources that already exist in raw form.

 

[Melbourne Guy]

it's a "seed" spaceship that transports to a distant star system and then builds into a larger structure (spaceport or something) by producing electrons and protons from the energy of the star in that system

Presumably, any civilisation that is sufficiently advanced that they can create / send a seed ship, can find systems with lots of physical resources for their Von Neumann machines to harvest, because that's the most obvious, easy source of raw materials. But, if your idea is just to use sunlight, this old PF thread might provide some story pathways, @dansmith170:

create matter from energy​

In the context of writing a story, presumptive technology beats overly descriptive technology every time, and especially for items that are common. Few of us need to explain anything of our motive means when we say, "I'm just off to the shops," and good authors keep that in mind when their protagonist acts.

"I'm just exiting through this hinged, solid object barrier, self-ambulating along this water, aggregate, and Portland cement pathway, using a remote device to send a short range electromagnetic encrypted signal to another solid barrier, this one hinged upward, which opens upon receipt of the short range electromagnetic encrypted signal to reveal a small interior space in which a metal, glass, and plastic transport vehicle with rubberised wheels resides..."

OMG, it's like a Jules Verne novel and you get the picture, I'm sure

 

[Hornbein]

Just wanted to share what I thought was a cool science fiction idea: it's a "seed" spaceship that transports to a distant star system and then builds into a larger structure (spaceport or something) by producing electrons and protons from the energy of the star in that system, and then arranging those electrons and protons into the larger structure. Why a seedship? Because then you don't have to accelerate a ton of mass to a distant star system.. the mass builds itself once it's there.

I like this idea. It seems rather impractical, but much SF is like that.

 

[BillTre]

Sounds equivalent to a seed growing into a plant (thus the name I guess).
Based on the seed analogy, the instructions for construction should be contained in the "seed", as well as sufficient energy and material resources to initiate construction.
A plant would then be able to harvest available resources from its environment to greatly enlarge the initial construction to something much bigger, like a tree.

 

[DaveC426913]

OK, the OP has come across an idea that many greats have thought of. He's got the idea of using the resources of the new system to bootstrap a civilization. He just needed a push to find the last piece.

Rather than build it out of raw electrons and protons, you build it from raw materials found by mining the system's asteroids.

@dansmith170 : you're on the right track. Read up on von Neuman devices and you're all set.

 

[stefan r]

I believe Carl Sagan/Ann Druyan wrote about this in the book Comet. It has been a while but:

The minimum size is approximately a walnut. Walnuts have 2 meaty lobes that we sometimes eat. The payload of the ship would dedicate one meat lobe to data storage. The other lobe has a moving mechanism with the brain equivalent of a jumping spider. These are encased in a shell which really is a shell and the "husk" is an outer casing of plutonium 238 (or suitable radioisotope).

The final payload can take advantage of a solar sail. The "spider" hatchling may have access to the sail, tethers, the plutonium fuel, and the shell components.

The half walnut can store full genomes of all known species on Earth plus all books and journals ever written. The instruction set the "spider" uses for its own replication and assembly can be smaller than a few animal genomes. It assembles larger and more complex machines which are able to download the rest of the data at a later time.

The ship would have a considerable mass ratio. We might have million to one for cruising and million to one for braking. The 1 kilo "walnut" can cruise inside of the thousand-ton cruise vehicle carrying enough propellant to brake. During cruise the mission might carry thousands of "walnuts" since that only increases the cruise mass by 0.1%. The cruise vehicle could potentially carry machinery capable of fabricating "walnuts". This let's us launch with "only" a billion tons of rocket built in the solar system (rather 1 billion and 1 million tons with redundancy). The braking propellant can shield the important payload components during the trip.

All numbers highly rounded. Would need the delta-v, exhaust velocity and mission time in order to estimate an actual mass ratio.

 

[phinds]

Seed ships are quite an old concept, although the OP apparently did not know that. The point of this thread is that he is positing the creation of electrons and protons (and no neutrons) rather than using found raw material (which is a much more reasonable way to go)

 

[Melbourne Guy]

...by producing electrons and protons from the energy of the star in that system

@dansmith170, did you deliberately exclude neutrons or was that an overisight? I assumed you were assuming shorthand 'energy to matter', which would include neutrons, but perhaps you were not.

 

[dansmith170]

@dansmith170, did you deliberately exclude neutrons or was that an overisight? I assumed you were assuming shorthand 'energy to matter', which would include neutrons, but perhaps you were not.

Not including neutrons was an oversight, although I did, as you say, mean 'energy to matter' to mean all sorts of matter including neutrons and whatever would be necessary for the construction of a larger structure. So yeah, neutrons too

 

[jrmichler]

a "seed" spaceship that transports to a distant star system and then builds into a larger structure (spaceport or something) by producing electrons and protons from the energy of the star in that system, and then arranging those electrons and protons into the larger structure.

Since this is PF, I'll try to back this idea with some calculations based on the following assumptions:

Assumption 1: The seed ship can convert the energy from a star to mass with 100% efficiency.
Assumption 2: Laws of physics apply - no free energy.
Assumption 3: Start at 150E6 km from a star similar to our sun, so identical solar constant.
Assumption 4: Solar collector is $1 m^2$ and is 100% efficient.

Then:
Solar constant = 1370 $W/m^2$
Since $E = mc^2$, $m = E/c^2$
Then $1370 W / {300E6}^2 = 1.5*10^{-14} kg/sec$ of mass created
Multiply by seconds per year to get $4.7*10^{-7} kg/year$ of mass created

That's less than half of a milligram of mass created in one year. If some optimistic assumptions of conversion efficiency are added, the mass created per year would be a small fraction of that, say about 0.1 milligram per year.

On the other, how much usable mass could be converted to useful objects if that same amount of energy was used to drive a Von Neumann machine?

 

[BillTre]

On the other, how much usable mass could be converted to useful objects if that same amount of energy was used to drive a Von Neumann machine?

This is why plants are successful. They don't create mass, they modify chemistry.
Capturing solar wind particles might provide a source of mass.

Also, as a plant grows its ability to absorb energy and make new stuff will increase, possibly exponentially if done right.

On the other hand, 0.1 to 0.5 mg a year, what's the hurry unless degradation is faster.
Consider the length of the trip to get there.
Patience may be a virtue in that less has to be transported.

 

[Melbourne Guy]

Since this is PF, I'll try to back this idea with some calculations based on the following assumptions:

Thanks, @jrmichler, it is certainly a small amount of matter per year. Presumably, time spent harvesting energy to create the necessary amount of matter is largely irrelevant to a machine of this kind, but given how vacuum, cold, and radiation are harsh mistresses, the machine can't spend forever building up enough stock to actually create something either.

The OP is a sci-fi idea, so that doesn't practically matter, but it might make for some narrative tension, the battle to generate matter faster than it degrades! Or, use the 'deep time' concept, where dumb machines are out there, laboriously seeding the universe with the progeny of a civilisation that has long since gone extinct.

 

[dansmith170]

Since this is PF, I'll try to back this idea with some calculations based on the following assumptions:

Assumption 1: The seed ship can convert the energy from a star to mass with 100% efficiency.
Assumption 2: Laws of physics apply - no free energy.
Assumption 3: Start at 150E6 km from a star similar to our sun, so identical solar constant.
Assumption 4: Solar collector is $1 m^2$ and is 100% efficient.

Then:
Solar constant = 1370 $W/m^2$
Since $E = mc^2$, $m = E/c^2$
Then $1370 W / {300E6}^2 = 1.5*10^{-14} kg/sec$ of mass created
Multiply by seconds per year to get $4.7*10^{-7} kg/year$ of mass created

That's less than half of a milligram of mass created in one year. If some optimistic assumptions of conversion efficiency are added, the mass created per year would be a small fraction of that, say about 0.1 milligram per year.

On the other, how much usable mass could be converted to useful objects if that same amount of energy was used to drive a Von Neumann machine?

Sounds like we both agree that somehow utilizing the energy or mineral resources in the destination system is preferable to accelerating a larger structure to the destination. I think accelerating a larger structure would cost Gamma times more energy ( E=(gamma)(m)(c^2) ) than utilizing the energy or mineral resources at the destination.

It seems to me that the conversions of the solar energy are the issue and perhaps we may ask what would be most efficient: building protons and electrons (and neutrons), or using a Von Neumann machine?

Time is also a factor as building the matter will probably take a lot of time as you have pointed out.

I agree with you that the matter-builder may lose some efficiency in converting energy to matter. On the other hand, the Von Neumann machine would also be somewhat inefficient in that energy would have to be expended to access resources from nearby asteroids.

For Assumption 3, maybe we could push the craft much closer to the star so that it's 1/1000 AU from the star. It'll be in the corona at that point so the craft will need to be heat resistant. At that distance intensity becomes 1.37e9 W / (meter squared). We would want the craft closer to the star anyways if it's building something like a Dyson sphere.

Then, for Assumption 4, multiplying the solar panels by 100 gives 100 meters squared of solar panels to intake energy (increasing the solar panels doesn't really change anything, but it makes the math easier for me). That results in about 48 kg / year. We'll subtract some from that because we'll assume the conversion to matter is only 90% efficient and that some of the energy from the sun is used for cooling the craft and any other processes. At 40 kg / yr. matter creation will be 6 times the starting creation rate in 57.1 years (4 solar panels added the first year at 10 kg per panel). In another 40 years, the creation rate is 25 times the original creation rate. The creation rate continues at what I think is an exponential rate.

Perhaps a hybrid of a Von Neumann machine and a matter-builder might be optimal.

 

[dansmith170]

Sounds like we both agree that somehow utilizing the energy or mineral resources in the destination system is preferable to accelerating a larger structure to the destination. I think accelerating a larger structure would cost Gamma times more energy ( E=(gamma)(m)(c^2) ) than utilizing the energy or mineral resources at the destination.

It seems to me that the conversions of the solar energy are the issue and perhaps we may ask what would be most efficient: building protons and electrons (and neutrons), or using a Von Neumann machine?

Time is also a factor as building the matter will probably take a lot of time as you have pointed out.

I agree with you that the matter-builder may lose some efficiency in converting energy to matter. On the other hand, the Von Neumann machine would also be somewhat inefficient in that energy would have to be expended to access resources from nearby asteroids.

For Assumption 3, maybe we could push the craft much closer to the star so that it's 1/1000 AU from the star. It'll be in the corona at that point so the craft will need to be heat resistant. At that distance intensity becomes 1.37e9 W / (meter squared). We would want the craft closer to the star anyways if it's building something like a Dyson sphere.

Then, for Assumption 4, multiplying the solar panels by 100 gives 100 meters squared of solar panels to intake energy (increasing the solar panels doesn't really change anything, but it makes the math easier for me). That results in about 48 kg / year. We'll subtract some from that because we'll assume the conversion to matter is only 90% efficient and that some of the energy from the sun is used for cooling the craft and any other processes. At 40 kg / yr. matter creation will be 6 times the starting creation rate in 57.1 years (4 solar panels added the first year at 10 kg per panel). In another 40 years, the creation rate is 25 times the original creation rate. The creation rate continues at what I think is an exponential rate.

Perhaps a hybrid of a Von Neumann machine and a matter-builder might be optimal.

I've been rethinking the energy needed to transport an already built structure to a destination system versus transporting a seed ship and then building (or accessing resources) in the destination system. The energy needed to build in the destination system would be:

E1 = M(c^2) + 2m(c^2)(gamma - 1) - m(c^2) where M = the mass of the structure and m = the mass of the seedship and M = 1000m (probably an overestimate on the seedship's mass).

The energy needed to transport the entire structure after building it in the home system would be:

E2 = M(c^2) + (1000 * (2m(c^2)(gamma - 1)).

(I estimated M(C^2) as the energy needed to build in the home system based on KE = m(v^2) and a final structure that is a Dyson Sphere made of 1e10 solar panels as well as v = 3000 m/s to construct the Sphere in what may be a reasonable amount of time (about 60 - 100 years). (It now occurs to me that there may not be enough resources in the home system to build such a structure - similarly, there may not be enough resources in the destination system to build the structure using a Von Neumann machine)).

When gamma is essentially 1, E2 > E1. As gamma increases, even more energy is used for E2 than for E1. Regardless, more energy is always needed to send the entire structure than to use a seed ship.

(I'm not 100% sure if my equations and math are right).

 

[DaveC426913]

I dunno, to me this seems like pennywise but pound-foolish, or not seeing the forest for the trees or some other metaphor.

In the same breath, you are hypothesizing a technology so advanced that it can directly convert light into matter on an industrial scale, and yet you're getting caught up in the maths of the surface area of ... solar panels?

That seems like positing a fusion power economy while calculating the size of the flint rocks to bang together to make the sparks.

I'd say you're better to get off the fence. Either:
- you go with conceivable technology - such as von Neumann devices - and lightly apply some realistic math to figure out its efficiency,
or
- you go with fabulously-advanced matter conversion technology and handwave any logistical considerations. Otherwise, you're guessing at the needs of a technology that's got to be a thousand years in the future if it's a day.

For example: why bother with harvesting the paltry solar wind? We have 4th millennium technology. Just plunge the matter converter directly into the sun. It can withstand it - after all it has complete control over matter and energy.