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Scientists have just released details on their discovery of inter-galactic baryon material - "Dark Matter" that turns out to be regular matter, except that it is 'dark' - dispersed throughout the inter-galactic spaces and therefore not hot enough to detect. Sort of like it is all black-body radiation at very low temperatures.

Scientists think they've spotted some of the universe's 'missing' matter and it could be a lot

It could be a considerable amount of material.

Previously, it was assumed that inter-galactic space was empty, devoid of any usable resources.

My question is, 'By what means could this baryonic material be collected by a mega-huge multi-generational ship that has been sent on a one-way mission to spread humanity throughout the universe, in sufficient quantities to replenish supplies?'

Consider that the recent collision between two neutron stars, that produced the most recently-detected gravitational waves, probably created elements as dense as gold and platinum. Very useful elements to go after.

Scientists discover neutron star collisions produce gold, platinum and other elements

It would have to be collected very slowly, atom by atom, maybe even baryon by baryon.

Further referenes to describe the scope, are Half the universe’s missing matter has just been finally found and MYSTERY OF THE UNIVERSE’S MISSING MATTER FINALLY SOLVED BY SCIENTISTS Which explain that this matter is concentrated in filaments between galaxies, up to six times greater density than normal. These filaments or strands would form the path that such an intergalactic ship would follow, like mining a mineral seam on Earth. And since they are filaments stretched between galaxies, they could easily be material pulled FROM the galaxies, in a 'bridge' between them. All bets as to the elements in them are off, if this is so.

As for dark matter being something different that regular matter, all bets are also off.

Strangely familiar: Is dark matter normal stuff in disguise? from August 2015

IT’S matter, but not as we know it. In July, an unexpected visitor appeared at CERN’s Large Hadron Collider near Geneva, Switzerland. Dubbed the pentaquark, this peculiar particle represents a fundamentally new way to aggregate the basic building blocks of matter. Although not forbidden by our current understanding of how stuff comes together, it had never been conclusively spotted before.

He even argues that ordinary matter in extraordinary formations could solve one of the greatest cosmological mysteries of our time – dark matter.

Ideas about physics that are based on information and theories that are more than ten years old are, well, ancient history.

Contact with those aboard the ship would probably be lost by anyone in our galaxy, and they would be multi-generations before they arrived at their destination, but our 'seeds would be cast', randomly 'blowing in the inter-galactic wind'.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monica Cellio
    Nov 1, 2017 at 3:25

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Any sort of ion trap. But let's look at it differently.

You are overestimating the benefits of a sustainable spaceship.

The intergalactic medium has 1 particle per cubic metre. According to wikipedia, 0.1% of these are heavy elements for the interstellar. Let's say that's true for the intergalactic for which I didn't find a value quickly. Let's be super optimistic and say all of them are useful and can be collected. You know what, let's make it even simpler. Let's assume all of them are iron. Now let's ask a very simple question: How much iron will be harvested on a journey from the milky way to the andromeda galaxy?

That's not the longest journey one can imagine, but it's pretty long. The distance is 2.5 million lightyears. That means 0.001 * 1/m³ * 2.365 * 10^22 m (that's your distance) = 2.365 * 10^19 / m². That's such a high number, I can't do much with it. So what is that in kg iron?

55.85 * 1.661 * 10**(−27) * 2.365 * 10^19 kg/m² = 2.19*10^(-6) kg/m².

That means in order for your spaceship to collect 1 kg iron in its journey, it needs a collector with a surface of about 455801 m². That means you need to build an iron collector that weighs less than 1 kg per 455801 m² surface area. Whatever the difference in weight between collector and collection rate and whatever you lose by having a giant, fragile apparatus out there in space is what you gain as net yield.

I don't know how big your ship is supposed to be, but I'd assume in almost any case it would be better to just bring the extra insert some heavy element before your journey starts. You can stop on some planet once you reach a new galaxy every million of years. Please be aware that you will not get only iron and of course a lot of elements you do not need (there is no way you'll end up with the ideal element distribution) and that you will manage to collect less in practice. A highly advanced, intergalactic civilization should also be able to just make whatever they please from hydrogen/helium, so there is little sense in collecting those special elements you are after.

Also consider: Since you need to build such a massive ship for collecting stuff being worth it, losing some atoms to the interstellar medium will hardly be noticed. The bigger your ship, the less loss per cubic metre (unless you are building a giant flat something), the less need for a collector.

But of course it is possible, see the ion trap. It's just not practical.

Please let me know if I made a mistake in my calculations.

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  • $\begingroup$ The article you link to is about the interstellar medium, the question is about intergalactic medium. (I think, I could have misunderstood.) $\endgroup$
    – Ville Niemi
    Oct 27, 2017 at 17:42
  • $\begingroup$ You are right. But I'd guess it's about the same $\endgroup$
    – Raditz_35
    Oct 27, 2017 at 18:02
  • $\begingroup$ The Wikipedia article has now become completely obsolete with this new discovery. Googling this will do no good. This is something that has not even been published. But it does not eliminate the relevance of the question. The details on the discovery are not necessary. If it makes it easier, assume the particles. But how do you harvest them? If google had an answer to this, I would not be asking the question. $\endgroup$
    – Justin Thyme
    Oct 27, 2017 at 19:57
  • $\begingroup$ The ship is 10 km. in diameter,as previously stated. $\endgroup$
    – Justin Thyme
    Oct 27, 2017 at 20:00
  • $\begingroup$ .I am well aware the collector has to cover, not just cubic kilometers of space, but cubic megameters of space. Fragility is not an issue. A spider web is fragile,and doesn't take much material, but it still captures rather large insects. And this just has to collect atoms. $\endgroup$
    – Justin Thyme
    Oct 27, 2017 at 20:06
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I think that a huge multigenerational star ship would be unlikely to show any interest in attempting to collect interstellar gold and platinum. What would they do with them? In fact any such material would likely pose a significant hazard if present in any kind of particulate form due to the huge kinetic energy involved in an impact with a fast moving star ship.

That said other materials in interstellar space such as hydrogen might be useful. In the nineteen sixties Robert W. Bussard suggested that very tenuous clouds of interstellar hydrogen might be swept up by a vast electromagnetic scoop and used to fuel a fusion rocket. But the practicality of this so called Bussard Ramjet is debatable.

If you’re really set on trying to extract gold and platinum from interstellar space rather than from the much easier option of extracting it from a large metallic asteroid, then some form of modified Bussard ram jet might be of use.

Note it would not be possible to collect gold or platinum “baryon by baryon” in the same sense that you could not build an adult human by accumulating cells one at a time.

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  • $\begingroup$ They would use it for electrical systems and solid state devices. Every computer has gold in it. Highly conductive, extremely stable, and virtually completely non corrosive. Our technology would not function without it. $\endgroup$
    – Justin Thyme
    Oct 27, 2017 at 19:03
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    $\begingroup$ True gold can be a useful material; they would take as much as they needed, but no more. Given the situation why would they want to collect gold or platinum from the interstellar medium? $\endgroup$
    – Slarty
    Oct 27, 2017 at 23:20
  • $\begingroup$ Because in a trip lasting thousands of years, eventually the gold is no longer recoverable from high tech devices. $\endgroup$
    – Justin Thyme
    Oct 28, 2017 at 3:34
  • $\begingroup$ Ultimately however diffuse the gold is aboard ship it would always be far more concentrated than the gold in space and much easier to extract. But if the engineering problem is the need to find highly conductive and oxidation resistant materials for making microchips then there are a range of materials that could be used instead of gold. They might not be quite as good as gold but they would be adequate and they would be used because they would be so much more readily accessible. news.softpedia.com/news/… $\endgroup$
    – Slarty
    Oct 28, 2017 at 7:25
  • $\begingroup$ It would seem to me that you are just transferring the problem - eventually THOSE alloys will become scarce as well. But even recovering an ounce of gold in 100 years is STILL one ounce more that you didn't have. Obviously, in order to recover this intergalactic space stuff, the method would have to incorporate dome kind of 'concentrator'. Say, a funnel that brings it all to one spot. $\endgroup$
    – Justin Thyme
    Oct 28, 2017 at 15:52
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Dark matter has nothing to do with baryonic material aka conventional matter.

Anyways to answer your question,

Same ways all the previous space mineral harvesting go

  • mine the asteroids to get your materials

  • use bucket collectors to catch gas/dust as you pass through clouds

  • you could use a magnetic field to attract some particles

edit: I also wanted to point out that intergalactic matter would be incredibly sparse and most likely hydrogen

[hydrogen is the most abundant element generated from the BB, the heavier elements are believed to be generated mostly from stellar activities which happen 99% in galaxies so what would most likely be between galaxies is unused hydrogen]

Ps: technically the OP did not ask how to collect dark matter

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  • $\begingroup$ It's not dark matter, it is regular matter that is dark. The regular 'stuff' that is in the periodic table. Gold, platinum, iron, all the regular 'stuff'. But you would want to scoop it from an area of space much larger than the ship, to get enough. Say, several cubic megakilometers of space. $\endgroup$
    – Justin Thyme
    Oct 27, 2017 at 15:59
  • $\begingroup$ I get that, hence my answer, though your question at first glance seems to ask how to collect dark matter. Anyways, the dust is impractical to actively collect hence why I suggest passive options like the magnetic field and bucket collectors. In these areas there would still be asteroid fields which can be actively mined. $\endgroup$
    – anon
    Oct 27, 2017 at 16:03
  • $\begingroup$ Sorry for the confusion, my comment was not for you particularly, it was a generic comment that addressed your last line. $\endgroup$
    – Justin Thyme
    Oct 27, 2017 at 16:10
  • $\begingroup$ And mining the asteroid bit I have already considered and built in to the story. Whenever they come close (or even hit) an asteroid, there is a celebration of good fortune. The problem is in detecting asteroids in inter-galactic space and then getting to them while traveling at point 8 cee. You pretty much have to wait for one to hit you. The space ship is about ten km. across. $\endgroup$
    – Justin Thyme
    Oct 27, 2017 at 16:32
  • $\begingroup$ @JustinThyme That's what make intergalactic travel even harder than interstellar travel. Its akin to crossing a desert, you could waste energy trying to paw at the sand looking for water or you could use all your energy to get to the nearest oasis and then paw at the sand for water. $\endgroup$
    – anon
    Oct 27, 2017 at 17:02
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Picking up materials while coasting through intergalactic space at 0.8c is simply not cost effective. Collecting the material will cause a significant drag (and catching particles with a difference in speed of 0.8c will be non-trivial).

Continued propulsion is absolutely needed.

For every ounce of material you collect, you will need to expel enough mass at sufficient velocity to offset it.

Assuming an miraculously excellent means of propulsion with an exhaust speed of 20k m/s, the exhaust speed will be about 4% of 0.8. You will need to expel 25 ounces of propellant for every ounce of anything else you pick up. Stocking up on raw materials at the start of the mission will be much more efficient.

*I based my answer on additional information given by the author in comments on other answers.

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