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Question inspired by discussion under this answer: https://worldbuilding.stackexchange.com/a/70360/2071

Dear people of Proxima Centauri. Our planet is dying. Our suns are dying. As you all know. We found out that nearest stellar system just 4 lightyears away has G2V star. And not only that. We think that one of its planet could hold life. Most probably the third from the star, maybe even the fourth one.

We made attempts to send signals to that star, but no one is answering to our signals.

We built the ship which can get about 50 000 of our people to that system and we will aim for third planet, because our data give us 98% probability that such planet has sufficient environment to allow survival of our people.

The trip will be one way ticket. People on that ship will most probably be the only ones surviving from our race, so we hand-picked the best of the best.

Now, we have tough decission to make: Should we pack weapons?

So far we listed these pros and cons:

Cons

  • Every gram of equipment is calculated and carrying x kilograms of weapons is real premium price for the bucks, because you will not need it until arrival
  • We can make weapons to hunt animals from materials on that planet

pros

  • We do not know what awaits us
  • If such planet holds intelligent life, it can fight back with firearms

Assumptions you can make:

  • Aliens from Proxima Centauri think as "human-like" as possible
  • They can 3D print the weapons, but to make bullets is quite complicated, so they still need to pack at least the bullets
  • They have no solid proof that there is intelligent life on Earth (their destination)
  • Once they set to voyage they cannot turn back. It is one way travel all long
  • Rest of planet will continue observing the Earth and send message to the ship
  • Aliens are leaving their planet at same weaponry level as today Earth. They can pack anything you know exists on Earth and can be considered as weapon. But still, remember, weight is premium
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    $\begingroup$ What constitute a 'weapon' for this question? The assumptions seem to hint at firearms only (no heavy weapons such as space-to-ground nukes, jet-fighters etc. no small arms or non-lethal 'peace-keeping' weapons for the 50,000 to police themselves, no unconventional weapons) is this the intention? Specifically, do you want answers to assume that despite their 'human-like' character, they won't be needing weapons for internal security during the journey and after settling the planet? $\endgroup$ – G0BLiN Feb 12 '17 at 9:07
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    $\begingroup$ @G0BLiN added to question. Since I know that mass is premium I instantly considered that they will pack only small firearms (best weight to effect ratio). But they can pack any weapon that exists here on Earth $\endgroup$ – Pavel Janicek Feb 12 '17 at 9:18
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    $\begingroup$ If the probe can't even determine if the planet has a technological species you have to seriously question its ability to tell if the planet is habitable. $\endgroup$ – John Feb 12 '17 at 15:00
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    $\begingroup$ What sort of idiot just tumbles out of there advanced spaceship to live as a caveman. First put your ship in a parking orbit, then capture some asteroids and make an extension, weight is no longer at a premium. Build up your space industry, send probes down to the planet and maybe start teraforming it to be suitable to live on. Put a sunshade or mirror swarm in orbit to control temperature, genetically modify some plants to change the atmosphere, build a space elevator. THEN you land on the planet. $\endgroup$ – Donald Hobson Feb 12 '17 at 15:58
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    $\begingroup$ Weapons aren't for the aliens, it's for the other people! You think out of 50,000 people, there won't be a disagreement or one sociopath? $\endgroup$ – Chloe Feb 12 '17 at 21:37

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No. They don't need to pack weapons. The very spaceship they are travelling from Proxima Centauri is already absolutely packed with weapons because by itself it is a weapon.

Its propulsion system will be powerful, destructive, energetic and deadly than any ship-mounted weapon would be. Even its communication systems, for example, interstellar range laser systems or even plain old fashioned radio will be powerful to fry targets from orbit.

If your fifty thousand can 3D print weapons like firearms I am dubious about your proposition "to make bullets is quite complicated, so they still need to pack at least the bullets". Which even if this was fully the case there is no reason why they couldn't fabricate high-powered air rifles or crossbows. If they can use local materials on their arrival, there should be very little to prevent setting up their munitions factories. Bullets seem to a quite straight forward and well known manufacturing process, requiring no more than a light industry workshop for their production and manufacture.

If you are so concerns about a hostile reception on reaching that G5V star and its attendant planets your brave Promixans are heading for, then train the entire population in martial arts. They are your best and brightest, and presumably also chosen for physical stamina, strength and general fitness. All capable of Olympic athlete performance levels. Would seem easy enough to train the bulk of them to Marine Commando levels of unarmed combat. Besides they need something to do during the long years of their interstellar voyage.

In summary: there is no reason to pack armaments. The spaceship is their most formidable piece weapon in and of itself. Vaporizing cities and continents from orbit should be a walk in the park. If their 3D printing can fabricate firearms, and since it appears likely bullets shouldn't be a problem too, then they don't need to bring their own. If bullets are genuine problem, air guns and crossbows are a suitable. If all else fails, the ship's complement can be trained to high levels of unarmed combat.

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    $\begingroup$ +1. If you take any martial arts you soon learn that a weapon is anything you consider a weapon. You can use a stapler or pen to injure someone; you don't require a knife or gun. Any society that has the ability to manipulate large amounts of energy such as in a star-drive, a power plant, or even a high powered communication laser has the tools required to make powerful weapons. I ran across the OP's concept when reading Larry Niven's SF story about the first human-Kzinti contact/war. The humans didn't have 'weapons', but adapted their spaceship's ion drive as one. $\endgroup$ – Mark Ripley Feb 12 '17 at 11:43
  • $\begingroup$ and in addition to this if they need to do orbital bombardment grab a few asteroids and hurl them at the planet. $\endgroup$ – Anketam Feb 12 '17 at 12:14
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    $\begingroup$ @MarkRipley Certainly martials arts does teach that lesson. I was thinking more Wil McCarthy's Waister series where the drive-systems of spaceships are explicitly their weapons. I've always thought the Niven story was a fudge. Anyone who knows anything about space-drives will absolutely know how dangerous they are. The Kzinti drive was a gravity-polarizer. So they were ignorant of ion-drives. Still it should have been obvious. $\endgroup$ – a4android Feb 12 '17 at 12:17
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    $\begingroup$ Perhaps a slightly more appropriate touchpoint in classical SciFi would be The Moon is a Harsh Mistress, rather than the first man-Kzin war (since, in the fiction, the Kzinti are pretty dumb). $\endgroup$ – Tacroy Feb 13 '17 at 0:21
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    $\begingroup$ Note to self: don't get into a fight with @MarkRipley. $\endgroup$ – Möoz Feb 13 '17 at 3:31
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Yes, they should pack some form of weaponry. Besides protection from alien creatures and for hunting, there's also security. You're talking 50,000 people. That's a city. Some of those people are bound to be criminals. Or will eventually become criminals, at least. So there will certainly be some form of security apparatus to protect the population both during the trip and after arrival.

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  • $\begingroup$ They've picked the best--no criminals. That doesn't mean there won't be mental cases, though. $\endgroup$ – Loren Pechtel Feb 13 '17 at 3:20
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    $\begingroup$ @LorenPechtel Picking "the best" does not exclude criminals. It excludes only those dumb enough to get caught... $\endgroup$ – Michael Hampton Feb 13 '17 at 7:41
  • $\begingroup$ @MichaelHampton I think by the time we can launch an interstellar mission we will be able to reliably determine if someone is lying. $\endgroup$ – Loren Pechtel Feb 14 '17 at 0:39
  • $\begingroup$ @LorenPechtel - I dunno, I have a feeling traveling interstellar space is probably less complicated than people...after all, people will be trying to be tricky $\endgroup$ – Megha Mar 25 '17 at 8:13
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The other answers are great, so I'm going to fill in some misconceptions in the question.


SEND A SCOUT FIRST!

Building and launching a ship with 50,000 people on board, and their life-support, for a 4 light year journey where they need to slow down and land at the end is a massive undertaking. Doing it with little information about where you're going is a risk you do not need to take.

While that's going on, make some small, fast, one-way robotic scouts and send them out. It can make some observations, answer some basic questions, and send the intel back. The composition of the planet and its atmosphere. Is there life? Is it intelligent? Is it hostile? Do we need weapons? What's a Kardashian and how much delta-V do we need to keep up with them?

Even if the colony ship launches before the scout reaches Earth, the data from the scout still gives the colony ship, with the help of the people back on Alpha Centauri, years to better prepare for the landing.


We can make weapons to hunt animals from materials on that planet

You can hunt them, but you probably can't eat them. Meat on Earth is useful as food for humans because humans evolved from Earth. We share the same amino acids and vitamins as the rest of life on Earth.

In fact, we're so hooked into the rest of life on Earth that we're dependent on eating other life to get some of those amino acids. We're so interdependent that there's some things we cannot digest, we need bacteria in our guts to digest it for us.

A life form that evolved on Alpha Centauri, even if it's using DNA and amino acids, will have evolved to use a completely different set of amino acids and vitamins. Even if they fill the same roles (they probably don't), they'll be at least slightly different and their bodies will not be able to use them. At best they'll be indigestible. At worst they'll be poison. They'll also lack our gut bacteria to digest things for us.

This isn't just a problem for eating animals, its a problem for eating anything on Earth. And this applies to all life coming from Alpha Centauri. Earth life has evolved to make use of, or at least tolerate, the trace elements present on the Earth like Molybdenum and even Arsenic.

Aliens from Alpha Centauri will have its own mix of trace elements, and its own trace needs and levels it can tolerate. It won't be the same as ours.

Life has evolved to deal with a certain amount of trace minerals naturally present, but no more. More would be inefficient, and something better optimized to the environment would have an advantage. This is part of why industrial pollution of heavy metals is so toxic to everything on Earth, we've changed the mix of trace elements faster than evolution can adapt. People from Alpha Centauri will also have no time to adapt. Something in our mix of trace elements will likely be above their tolerance levels, and something they rely on will likely be too low.

Farming, hunting, and even breathing will slowly kill them and leave them deficient. They'll need to filter and supplement everything.


They can 3D print the weapons, but to make bullets is quite complicated, so they still need to pack at least the bullets

You have this backwards. Making bullets is easy, 3D printing a gun is hard.

Brass cartridges haven't changed much in over 100 years. Molding bullets is trivial. Making smokeless powder is also trivial for a space-faring civilization. Drawing a modern brass or steel cartridge is harder, but not difficult, and they can be reused and reloaded. At minimum, you can make paper cartridges. We figured this all out almost 150 years ago, and the Alpha Centauri folks can be safe to assume they'll find material on site to make bullets.

In contrast, a gun has to withstand the high pressures of the exploding powder. If you want it to be accurate, it has to be machined to high tolerances. If you want it to be semi-automatic, it requires fiddly little parts. If you want it to last, it has to be hardened in the right places.

We can 3D print a modern gun, but we only got this working in the last few years. And they'll will only last for a few hundred rounds, as opposed to tens of thousands for a normally manufactured gun. Improvements in 3D printing will fix this.

Point is: making bullets is easy and very well understood and we've been doing it for over a century. 3D printing a gun is hard, and they're not the best quality, and we only recently figured it out. If you can 3D print a gun, you can most definitely make bullets.


Since every additional gram of mass will add many, many times its weight in fuel, any "weapon" they bring with them will actually be a tool that can serve as a weapon in a pinch. If they need real weapons, they can 3D print them.

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    $\begingroup$ So, what you're saying is that they should probably aim their ship for the asteroid belt where they can spend a few decades gathering the resources and building the infrastructure needed to sterilize the third planet so they can reseed it with life compatibile with their own. $\endgroup$ – Perkins Feb 13 '17 at 3:09
  • $\begingroup$ @Perkins Sterilizing the planet won't solve the trace compound problem. What they should do is send a scout for data. Then on their trip they can genetically engineer organisms to convert Earth biological compounds to ones they can digest, develop filters so they can safely breathe the atmosphere, and know if they need to bring guns. $\endgroup$ – Schwern Feb 13 '17 at 3:19
  • $\begingroup$ If there are trace biological compounds, you didn't turn the thermostat on your sterilizer ray up hot enough. There's no reason to be in a hurry, if you can build an interstellar ship, you can build comfy habitat domes on the asteroids and Mars while you wait for the geoengineers to finish crafting paradise in a few dozen centuries. $\endgroup$ – Perkins Feb 13 '17 at 3:46
  • $\begingroup$ @Perkins I used "compounds" perhaps too generically. No amount of sterilization will deal with there being, for example, too much (or not enough) arsenic in the soil and air. As for waiting centuries under domes for terraforming (centariforming?), it fundamentally changes the scenario. Why drag people to Mars and build domes there? Leave them under domes on Alpha Centauri until the terraforming is done. This is the major plot hole in Interstellar: if you can build a sealed, self-sustaining habitat in space, you can do it much easier on even a dead planet where there are resources. $\endgroup$ – Schwern Feb 13 '17 at 3:53
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    $\begingroup$ "use a completely different set of amino acids " - disagree, glycine, and there are other reasons to believe in similarity. However, problem of eating anything on earth is real, and it very good point in general. $\endgroup$ – MolbOrg Feb 13 '17 at 6:18
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By weapons, are you simply considering only handheld weapons, or also stuff like artillery and tanks, or more importantly, ship weapons? Because you have no reason to assume you're the only sentients in space, and piracy should always be a concern.

EDIT: You shouldn't assume you'll only need weapons at the end point, unlike what the question seems to assume but on the journey itself.

Secondly, we do have ship based weapons, even with current technology--just pull 'em off aircraft and/or scale up. Yes, weight is a premium, but your payload only really counts once you get there. You can manufacture hand held weapons if you have 3D printers once you've landed, ship weapons need to be installed before liftoff.

In summary: You do need weapons, but on the outside of the ship

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    $\begingroup$ Hello, please consider posting this as a comment, because this does not actually provide any answer to question being asked. And also, please read the question, namely very last bullet point $\endgroup$ – Pavel Janicek Feb 12 '17 at 10:32
  • $\begingroup$ @PavelJanicek: The point is that you shouldn't assume you'll only need weapons at the end point, unlike what the question seems to assume but on the journey itself. Secondly, we do have ship based weapons, even with current technology--just pull 'em off aircraft and/or scale up. Yes, weight is a premium, but your payload only really counts once you get there. You can manufacture hand held weapons if you have 3D printers once you've landed, ship weapons need to be installed before liftoff. $\endgroup$ – nzaman Feb 12 '17 at 10:40
  • $\begingroup$ As this is a demand to clarify the question and not an answer, it would be better to post it as a comment. $\endgroup$ – PatJ Feb 12 '17 at 10:56
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Much of this depends upon how the settlers will arrive and live. Will the ship set down? Will they (one-way) drop lifeboats from orbit, with the hulk of the ship remaining a communications relay to the home world?

I'll assume that the ship says in orbit, and that it's a one-way trip to the planet surface. That is, once you go down, you have a new home, forever.

  1. Not everybody goes down at once. There will be no infrastructure to support a civilization. So the marines and combat surveyors go first, to secure the area and find the best spots for several towns.
  2. When the spots are selected, marines form a perimeter and the construction engineers begin building the basics of infrastructure - water treatment systems, sewage systems, basic housing, etc.
  3. At the same time as #2, farming communities are also set up. The ship cannot contain an infinite supply of food. More marines, more engineers. These are not intended to be as large as the towns and do not need the level of infrastructure that New Town needs. The first non-construction-brigade inhabitants here could be hunters.
  4. It would be years before the last person left the ship. Some will probably remain on the ship in order to service the communications systems. Perhaps the ship will have enough supplies to feed a few generations of engineers.

So what we have here are 50,000 people living in small towns surrounded by farming communities. There could be predatory indigenous animals or animals that eat the crops. These animals endanger the food supply. There are 50,000 human-like minds. This means crime. When resources are scarce, there will be organized crime, so it isn't just petty theft.

The settlers will need weapons. I would not say military weapons - no attack helicopters, tanks, or field pieces. But there will probably need to me police-force level of weaponry. I would expect people outside of town to be armed with firearms sufficient to ward off the predators.

EDIT - off topic, but it occurs to me the colony ship would not have lifeboats. It would be made of lifeboats. Like an earn of corn, or a magnolia pod, once the lifeboats had been dropped, only a spent skeleton of the ship would remain. Getting to the surface of the planet would be a matter of great nicety when the characteristics of that planet are not known when the ship leaves the homeworld. So there would have to be enough ship-board manufacturing to tweak the lifeboats to ensure their safe arrival to the planet's surface...

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Hunting on an alien planet is risky, you never know what type of creatures they might encounter. They would right as well, earth has some scary predators. Thus to make sure to protect themselves and hunt for food, they should pack some hunting weapons. If they believe there would be lots of forests, they would pack short ranged firearms, if they think earth contains open plains, they will go for long range ones. But in either case, the weapons will not be very well suited for vs. human combat. After all, we pack a big punch.

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  • $\begingroup$ Also, how would you know the biology is similar enough to actually eat anything which lives there? $\endgroup$ – Paŭlo Ebermann Feb 12 '17 at 19:02
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If they are mentally "similar" to humans, then they will require people authorized to use deadly force. It isn't clear to me why this question needs asking, perhaps some of your assumptions haven't been made explicit? Perhaps a question opposite yours would provide clarity: Give a single example of a (historical) situation in which 50,000 people have lived together for years without weapons.

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  • $\begingroup$ I guess "hand-picking the best" might include they don't attack each other. $\endgroup$ – Paŭlo Ebermann Feb 12 '17 at 19:03
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  • Should we pack military hardware (tanks, fighters, stealth bombers, artillery)?

No. 50,000 people represents a small fraction of their population, and with all else that needs to be done to setup a colony and get an industrial base going, few would be available to fight. A well-equipped but tiny army with no logistical base and no source of replacements would be unlikely to be effective enough and wouldn't last long. Even if they got to use it, it wouldn't save them so it would be a waste.

Further, if built at today's tech levels, their equipment would require extensive maintenance and technological support, but would be quite obsolete by the time they arrived. And of course, if they don't end up needing it, it would be a total waste.

  • Should we pack individual weapons (firearms, melee weapons)?

No. Many ocean-going ships have a machine shop stocked with source materials so that if the ship is damaged or breaks down in mid-ocean, they can fabricate parts for repairs. If a ship traveling to Spain has that, a ship traveling to another solar system most certainly would.

The ship would most likely need pipes capable of withstanding pressure, so would have supplies capable of being made into firearm barrels. There may be no real reason to bring lots of gunpowder, but it isn't necessary. They would need tanks holding pressurized gases for life-support, firefighting, etc. Those could be used to make high-pressure airguns.

Contrary to the assumptions, bullets are not complicated to make. Explorers and settlers in the wilderness of the Americas made their own using nothing more than equipment they could carry - a handheld bullet mold and some lead. Assuming weapons like shotguns or blunderbusses, the shot needn't be too precisely molded. (But the Proxima Centaurians certainly would be capable of making precise ammo.)

Another thing that might be useful would be HERF and EMP weapons, in case they have to face electronically-augmented bandits or something. But again, the ship will most certainly have a store of electronic components anyway.

So overall, no need to bring weapons, just be prepared to weaponize unused spare parts and/or cannibalize the ship upon arrival.

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No.

Disclaimer 1: I'm basing the answer on musings in this article, which months ago I found in answer to another question in this very stack

Disclaimer 2: for flavour I decided to write using made up alien units, in parenthesis I included same values in normal units.

Disclaimer 3: this answer is quite ugly, including terrible style in few places. I'll try to rework it later, I don't think I can do any better right now.


Reaction mass and reactor fuel needs to be brought on board, which means that even more of both needs to be expended during acceleration phase to make deceleration phase possible at all. This growth is in fact exponential. At destination, decelerating single pound (kilogram, pounds are obviously some kind of incomprehensible alien unit) of payload, using our most propellant efficient engines would require 2.6*10^bę (10^13 Joules) and 8000 pounds (22000 kilograms) of propellant, assuming ship travelled at velocity of 0.01d (~0.01=1%) of the speed of light, and trip took 3d0 spins (~420 years)

This in turn means that accelerating payload AND propellant would require 8000*8000=48000000pounds=48000gras (~22000*22000=484000000kg=484000 metric tons) of propellant. Every kilogram of payload requires 7 orders (8 orders) of magnitude as much propellant to be delivered. To accelerate that much propellant, we would need b*10^14ę (4*10^21 Joules) of energy. Remember: that's for single pound (kilogram) of payload! And without factoring in weight of engines or fuel tanks!

Saying that every nan (milligram) is at premium would be a gross understatement.

It's of utmost importance to save every bit of mass possible. In fact, that's exactly why during preliminary phase I was so adamantly insisting on frozen-embryos-and-artificial-uteri design.

You want to send 14000 (~50000) colonists. Assuming some absurd efficiencies in life support, including 100Ɯ (100%) reliable cryosleep, we are still speaking of 1b00 gras (5000 ton) craft at minimum, and that's assuming that sleep chambers weight nearly nothing. This bumps up total propellant requirements to 2.7*10^13 pounds (2*10^15 kilograms) and energy requirements to 6*10^1a (2*10^28 Joules), for reference, energy-wise that's equivalent of total energy emitted by star in targeted system over 3 breaks (roughly a minute).


Now that we are done with numbers, we need to think of feasibility of bringing weapons. Main question is: what for? If that planet is inhabited, then either inhabitants are so primitive that we can simply land in remote location, build infrastructure and weapons utilising on site resources, or they are advanced enough to be able to fight back, in which case we have no chance of success with 14000 (50000) of our own against resources and manpower of entire planet.

Considering that we have no reason to assume compatibility of local biochemistry with our own, we will have to build closed habitats anyway. For that we will have to utilise local resources. We will have to mine minerals, process them into construction materials and construct habitats. We will have to build infrastructure supporting mining and processing. Our ship will have to pack machines capable of building such infrastructure from whatever we find laying on surface. I can see no reason why after we are done assembling habitats manufactured by habitat factory, we couldn't build weapons factory or weapons factory factory. If aborigines are primitive, then as I said, we can land in remote location, build out infrastructure, and then go to war, if we find it necessary. If aborigines are advanced enough to be able to even detect our landing, then we will be quite literally at their mercy - before we discovered radio there was about billion (milliard, short scale is an obvious alien conspiracy) of us, we have no reason to assume indigenous people would be less numerous, thus if they poses any form of radar to detect our landing, or long-range radio communication to relay optical observation of our landing/landing site they could throw millions of soldiers at our outpost. This is not something we could endure without extensive fortifications, and without massive (both literally and figuratively) force multiplication.

Remote-controlled tank company with required logistics would weight more than entire colony ship and would be utterly useless if aliens are advanced enough - they can afford to win by attrition, taking out one tank for gras (thousands) of lives would still be in their favour if we are unable to replace the tank. They could probably afford to simply run us out of ammo. Small arms would be much lighter but too dangerous since any death on our side is a direct threat to the future of our species. Remote controlled weapons are the only choice, infantry as well as crewed vehicles and fortifications might render entire operation moot due to combat loses. Depending on their advancement level, we would have to bring not just tanks, but also helicopters, planes, artillery, and infantry-replacement robots, eclipsing weight of ship by at least order of magnitude, and that still might be for naught.

As such, I think, we could as well bring no weapons.

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    $\begingroup$ If the planet can have life, a handful of rifles or shotguns would be a minimum, I mean what if you land in the equivalent of a bear mating grounds. $\endgroup$ – John Feb 12 '17 at 14:57
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    $\begingroup$ I do not know, is it that necessary to try the answer look as alien message and use symbols and number bases and units no one knows. If you will it to be strange then use the system which is used only in 3 countries and that's enough. Most of the people do not get numbers and calculations so any calculation will be enough alienated for them. As a practical reason, even if one will decide to improve your formatting, because of the way you have chosen to write it, it will make their task impossible. Your answer your rules, but ... $\endgroup$ – MolbOrg Feb 12 '17 at 15:06
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    $\begingroup$ I think what you're really saying is that it isn't feasible to accelerate a massive ship to 1% the speed of light using reaction mass AKA chemical fuel. But there are other ways than a brute-force burn such as gravitational slingshotting to get moving, and a similar approach on the far end to slow down. One will need mass to maneuver, but not to accelerate or decelerate. I could see the colony ship spending 15 years in local orbit maneuvering, on both ends of the trip. This technique will not produce speeds anywhere close to .01c, I don't think. But it is economical. $\endgroup$ – Tony Ennis Feb 12 '17 at 18:19
  • $\begingroup$ Here is the solution: photonic propulsion. You just pack for deceleration and that won't need fuel to get out of gravity well. It will still need the total energy emitted by star in targeted system over 3 breaks. $\endgroup$ – Cem Kalyoncu Feb 12 '17 at 19:51
  • $\begingroup$ @TonyEnnis NO! I did not use chemical propulsion exhaust velocity in my calculations! I used exhaust velocity of "Dual-stage 4-grid electrostatic ion thruster" which has 2 orders of magnitude higher exhaust velocity than chemical rockets. Ion thrusters don't provide much thrust, so st this point I already have ran very fast and loose with realism. Using chemical rocket would increase propellant requirements for deceleration phase to (2*10^434 kg), for a total propellant required per (1kg) of payload being (4*10^868 kg). $\endgroup$ – M i ech Feb 13 '17 at 4:45
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Yes, but only as props. The comment about "intelligent life can fight back with firearms" means to me that weapons described are not hunting tools but war weapons: the sort of thing intelligent beings use to fight each other. You could bring a couple of weapons which were examples of your advanced tech and it might be possible to use these to bluff. There is no point in bringing enough that you could really turn your refugees into warriors and fight a pitched battle or attack an advanced civilization from orbit. A plan for a first interaction being war against complete unknowns on their home turf is crazy cocky. If I understand right, even Cortez sort of worked up to the conclusion that he could pull off such a thing and it was still crazy cocky.

Your ship is a refugee ship. Fleeing as refugees is a desperation move. Refugees do not show up on foreign shores armed for war. People who show up ready for war are invaders or have the potential to become such no matter what they claim. If intelligent inhabitants of the destination planets are at a tech level sufficiently lower than yours that you can awe them with your tech, fine. A guess that this might be the case is not unjustified. These refugees might conclude that a failure to hear a response to their call means residents of the target world are not adequately advanced enough to receive or reply to those signals. Even if intelligent, such as these will not be able to determine what the refugee ship is capable of when it arrives. Some weapons could be used for bluff and bluster to cow the primitives and get the refugees established.

If the inhabitants technologically would be a match or better in a fight, then hopefully they will realize that any weapons aboard the ship have been brought as part of a tech museum and do not pose a real threat. That means the weapons you bring should not be nuclear weapons or similar, which could make trouble even for an advanced opponent. Your hope in that situation is that this advanced civilization receives your refugees and puts you up as honored immigrant guests or allows your ship to serve as an internment camp, or enslaves you all.

Some things about this question make me think that maybe these refugees are actually not very advanced themselves: e.g. making bullets is complicated? They do seem ready to "tumble out and live as cavemen" as Hobson eloquently put it above in the comments. It is not an impossibility. Maybe these refugees were given a starfaring ship by some advanced race that felt sorry for them. That would be a sweet story and one I cannot recall ever seeing or reading. And now I want to read it - so if one exists please post it in comments!

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The original question seems a bit illogical and reminds me of many very old science fiction stories. In that particular type of old science fiction story aliens develop a civilization equal to 20th century Earth and continue to develop for centuries, millennia, or even millions of years more.

And then all of a sudden those super advanced aliens discover their world is doomed and in a panic they build interstellar space ships and invade the closest habitable planet, Earth.

But if extinction events only happen every few tens of millions of years on a planet like Earth, an extinction event is not likely to happen on an alien planet less than ten million years before the aliens develop a 20th century technology or less than ten million of years after the aliens develop a 20th century technology. The probability that an extinction event, let alone the total destruction of the planet, might happen when the aliens are only a few hundred years or a few thousand years more advanced than 2017 Earth is very, very, very small.

Thus the aliens are likely to be many thousands of years, maybe millions of years, more advanced than 2017 Earth when their home planet becomes doomed. And if those aliens have not already built many space colonies and space habitats all over their star system, and have not already sent many unmanned probes and manned expeditions to other star systems, they have certainly be sitting on their alien asses (if any) for all those thousands of years that they have had space travel.

In real life the aliens would probably have billions living on their home planet and trillions living in space habitats in their on star system, and should already have colonies in other star systems by the time their stars are dying (which is not due to happen for billions of years in the Alpha Centauri system).

So the aliens should simply build a relatively small number of new space habitats for the ones living on the home planet, attach star drives to all the space habitats, and set off for the star systems which already have colonies in them.

For a species which has probably been more advanced than 2017 Earth for many thousands or even millions of years, there shouldn't be any nonsense like sending only 50,000 people in a single space ship to an unexplored solar system. They should be able to send all the trillions of their people to star systems here they know they will get a reasonably kind reception negotiated by radio with the colonists already there.

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The most obvious reason of which I can think to take a weapon, within the context of your specified hypothetical, would be to use the weapon on myself in the case of contact with a hostile alien race (who I am assuming are far more technologically advanced than our own species), not so that I can use the weapon on them. That is, I would want the weapon as a final fail-safe against the possibility of torture or torture-via-study.

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Remember this is a targeted colonization attempt, and because they are unable to terra form their own planet instead of abandoning it their terra forming capabilities are limited. This restricts them to planets that they feel are reasonably close to their own home world. Since the people of Proxima Centauri choose Earth as a colonization spot because it supports life as they know it (or close to it) there is going to be a better than average chance that Earth contains life. They don't know what stage that life is technologically but they do know that life evolved on their home world under similar conditions. As such it is reasonable and prudent to consider how to deal with the native flora/fauna.

Nothing in nature cedes territory willingly for the simple fact that access to resources is crucial for life to sustain itself. If an organism cedes access to there resources their chances of survival are diminished. It is true that some organisms do a tactical retreat, but they never truly cede their territory permanently. The colonization of the people from Proxima Centauri will require any native flora/fauna to cede territory to some degree or another.

If the planet is devoid of life then weapons won't be needed to fight any native flora/fauna, but planets that can support life as the people from Proxima Centauri know it are valuable, since they are the minority both in their solar system and in this new solar system at the very least. That means it is worth defending a planet devoid of life from others who might not want to share. Similarly if the planet isn't devoid of life it is imperative that the people from Proxima Centauri are able to persuade the natives to share the planet.

Once the people from Proxima Centauri arrive at Earth they may not be able to communicate with the natives. This will complicate diplomacy, and depending on the tech level of the natives the people from Proxima Centauri might be vulnerable to varying degrees. If the natives are any kind of threat then investing in weapons will be good insurance. If the natives aren't any kind of threat the weapons still serve as insurance against other aliens. Keep in mind that you don't buy insurance to make you money you buy it to protect your money.

Since they are traveling to a known valuable planet with a better than average chance to hold and sustain life it is wise to have the capability to defend this planet. A smart defense plan will include a range of weapons that aren't going to render the planet uninhabitable, and allow focused attacks to minimize collateral damage. This allows you to retain the inherent value of a habitable planet, and reduce the fallout from any diplomatic failures. Furthermore it is imperative that the people from Proxima Centauri don't come in contact with the natives until the natives have been studied because the natives might be poisonous or otherwise hazardous to the people from Proxima Centauri at a biological/chemical level.

One big problem with "the drive is a weapon" is the same reason why we don't just carpet bomb insurgents with nukes. Yes the drive can be a weapon but it isn't going to be an appropriate or useful weapon except in the more dire of circumstances. Like nukes the drive is more useful as a deterrent. Also martial arts does fill a vital space in the defense/offense strategy, but there is a reason why commando's carry guns instead of just relying on their kung fu. Having a full range of intermediate options allows you to defend yourself, and engage in diplomacy from a position of strength without having to deal with the diplomatic problem of indiscriminate destruction.

Bringing a variety of weapons allows the commanders from Proxima Centauri to use the least amount of force that is appropriate, which is helpful diplomatically. For example if the people from Proxima Centauri come to Earth and find it inhabited as it is now they might choose to setup shop in the Australian outback since that is relatively empty. If the people from Proxima Centauri imposed a no fly zone around their base in the outback, and pushed all natives outside of their base, that would be less diplomatically damaging once two way communication is established than orbital bombardment of Australia. By consistently using a minimum of force to establish a peaceful colony there is a better chance for acceptance by the natives.

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Reframe the problem: what do your people need?

Rather than trying to decide whether you need to bring weapons for your people or not, look to the people you are bringing along. What sorts of people are they?

Weapons are tools. In the end, they are nothing but tools. They happen to be a particular category of tool, but in the end whether or not you bring a weapon should go under the exact same sort of scrutiny as whether or not you bring the mass spectrometer. Who can use it? If you have a bunch of chemists on board, ask them what tools they need for their trade. If they say their job benefits greatly from having a mass spectrometer on board, you bring it!

Likewise, if you're bringing a large number of individuals who are well trained in how to use firearms, and they say that they can do their job better with firearms, pack 'em in. Your job description called for it.

There will be a mass balance for everything. Presumably you already have some sense of how much mass you want to bring for particular tasks because you're bringing the people along with you. A mass spectrometer will be about 200-300 pounds of gear. Depending on how much ammo you choose to bring along, you can probably get 3-5 rifles to fit in that weight.

Another question to ask is what are the stakes? If this is the survival of your species, and you are willing to literally wage war to protect your species, you will want to bring real weapons. In the case of species survival, the idea of bringing nuclear weapons along is not unfounded. If this is just the first ship of many, and you plan on relaying information to the next ships, you probably don't need an arsenal.

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It's important to reason out the possible scenarios you expect your travelers to face on arrival - not in huge detail, but for a few factors. Things to consider:

  • Planet habitability scale (move right in, "proximaform," nothing's right but the gravity, not even the gravity's close enough)
  • Existing Life (none, slimes, complex organisms, etc.)
  • Existing Intelligent Life Advancement (primitive, similar, more advanced, no common framework)
  • Existing Intelligent Life Hostility (welcoming, cautious, xenophobic, hungry "It's a cookbook!")
  • Travelers outlook (refugees, supplicants, neighbors, invaders)

For almost all combinations of those scenarios anything beyond a few handheld firearms will be pretty useless and other items that could be brought instead would likely be more useful.

Even for scenarios with intelligent hostile natives firearms would likely be pretty useless - 50,000 travelers on a generation ship means most are not actually combatants anyway even for a highly militarized society, and the refugee nature means that losses due to combat are something to be avoided at all costs. For intelligent but cautiously welcoming natives, showing up heavily armed would probably be a red flag and likely to make them less welcoming.

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