Ken Fabian

That is fair, but you did suggest a magnetic sail scoop to collect Hydrogen. I think that faces the same problems achieving magnetic fields large enough and strong enough as Bussard 'scoops', with the added issue of magsail design being inherently different. Yes, I admit I am inclined to add more to my comments than specific responses, including to other possibilities and suggestions that may not have come from you. But I think I will refrain from further comments on this thread.

Theoretically we can't. Too few hydrogen atoms - hydrogen ions? - and magnetic fields lose strength rapidly with distance; the cross section of a magnetic field 'scoop' will be small. There are good reasons why Bussard ramjets are not considered feasible, no matter their ongoing popularity within science fiction. Popular much the way FTL is popular. Even apart from not yet having successfully developed useful controlled fusion without any mass restrictions a proton-proton type appears significantly more difficult than the types being attempted. The onboard capability to produce deuterium or tritium from such collected ions or onboard stores adds yet more dry mass. Produced from lithium within the reactor means carrying lithium in sufficient quantities. And I remain doubtful of the durability of hypothetical fusion or fission hardware - as I am for the durability of everything. As for passing through planetary atmospheres and scooping up gases? By a ship in orbit going around and around again, combining deceleration and refueling - at relatively low speeds - that seems vaguely possible enough to seem possible (but I seriously doubt that it is close to feasible) but faster than orbital speeds, let alone at interstellar velocities in a single pass, no. Like hitting a solid wall I would think. You would have to already have done almost all the deceleration.

A good thing taking a lot of work that should not have been necessary. Unfortunately there can be no assuming the science programs doing the fundamental observations and producing the underlying data will continue intact; clearly the current US Administration, including Congress continue to seek to hamstring climate science. But it appears there is serious intent, commitment and organisation to put politically partisan flunkies in charge of every US government agency and thoroughly vet their activities for political correctness. The extent that climate monitoring and climate science conducted elsewhere can replace the US contributions isn't clear - they have been the largest contributors and the gold standard for more than half a century.

As Australians are wont to say - tell 'em they're dreaming. As thought experiment it can be fun considering possibilities and exposing the impossibilities. As a serious goal to commit vast resources too... in an age where our overuse of Earth resources and carelessness are turning around to bite us? No. And perhaps the most essential thing for keeping such possibilities possible is a healthy, wealthy Earth society. I would be concerned for its mental health if interstellar expansion became an obsession. As threads grow longer the older posts get overlooked and a lot of what DanMP is suggesting has been discussed and I'm probably reiterating some of my own by saying - Saving humanity from extinction is to my mind one of the most unrealistic reasons for interstellar travel. We'll go to space and succeed there from the economic opportunities that require on-site human labour and without them succeeding in delivering more back than the investments they will whither. I am still struggling to see what those opportunities could be. If surviving extreme disasters is the overarching intent then artificial habitats closer to hand, beginning with bunkers before space colonies and space habitats before planets and within the solar system before other stars makes more sense - if you can really call it sense. (Switzerland has (had?) nuclear war bunkers for the entire population - using their abundance of tunnels - but I would find it objectionable for my nation's government to commit to them for only a select few.) Try building a bunker that can last a thousand years without outside resources and see how that works out. I'm more inclined to being 'philosophical' about species mortality; I don't like my impending mortality and putting up a good fight against it is in our nature but so is acceptance of the inevitability. Making the best of what we get and not spending the best of it chasing dreams of immortality is sane and reasonable too. And I think any worlds with existing life are far more likely to be uninhabitable than present as colonial opportunities - nothing makes more varieties of poisons, toxins, allegens than biochemistry and it is big stretch to assume compatibility, even aside from potential for predators and plagues; for most of Earth's living history humans would find Earth's atmosphere un-breathable. I would think if it were somehow possible to go there such living alien life and biochemistry, uncontaminated, is worth more to humanity for what we can learn from it - from an in-space habitat, deploying robotic sampling - than as something to displace and extinct on purpose or by accident with Earth life in pursuit of dreams of land ownership. I hope humanity can rise above such self indulgence.

Not really - societies and economies face some serious problems over the not entirely foreseeable future. If high levels of funding and support continue to flow to fundamental physics without discontinuities I expect more puzzles will get solved. I'm not entirely convinced that will be the case.

Not that he remains as the pre-eminent authority on evolution, but Darwin had some insightful things to say about human societies - not all I would consider correct but he never subscribed to the view that human evolution was driven by individual fitness alone. Somewhat ironic with the rise of Christo-Fascists that detest 'Darwinism' in the US (and to a lesser extent here in Australia) that he considered Euro-Christian societies the superior form. My own view is that we are getting more variation within the population due to our nurturing of the 'weak' but that should circumstances change and our technology dependency break down we will see a lot more natural selection. Yet I think it will be the survival and adaptations of societies that largely determine who does best and which physical traits are lost and which persist and become more common within subsequent populations.

I stand corrected that there are a lot of puzzles within physics that are unsolved. Yet I still think there will be a finite number of them and if not yet more answers than questions as long as we keep trying the proliferation of new question will diminish whilst those answered will increase. Identifying the puzzles is a big step to solving them. Like many truisms, I think it will prove incorrect that the more we learn, the more we realize how much we don't yet know. More specifically to the OP I'm not convinced solving the known unknowns is going to lead to FTL or other workarounds to the physics underpinning the time and resources issues for interstellar travel and I remain skeptical the set of unknown unknowns will reveal any. I would not be disappointed to be wrong - but possibly alarmed with it. I will add that I think that for some of these puzzles resolving one may provide resolution to others - that the number of solutions needed will be less than the number of questions.

A bit of cross posting - edit added in between. Einstein's insights - and not only Einstein's -have lasted a century of ever better observations and experiment. I do think we are getting closer to a true understanding of fundamentals that will last forever. Those fundamentals are not falling to new, better theories the way 'trusted' knowledge of the 1800's did.

@MigL You really expect a state of ignorant and incorrect to last forever? More than a century of more, better physics with high levels of confidence since the late 1800's - a lot of knowledge that has persisted and resisted all efforts to falsify.

Is it unreasonable to think that we are closing in on a complete understanding of fundamental physics? Yes, we have seen prior understandings shown as (sometimes profoundly) incorrect and overturned but is that happening that way now? There are some significant issues that present as yet inexplicable and unknown - dark matter as an example - but can we expect an emerging understanding of those missing parts to rewrite all we know or just refine and clarify whilst leaving a lot of what we think we know largely intact?

There is vast complexity between raw energy and minerals and advanced robots. AI seems to require a lot of hardware and a lot of energy - more suited to a centralized controlling 'mind' than for individual robots. We can speculate about how small AI might get by taking us into the realms of imaginary things. I can also imagine plasma refining that turns any raw material into component elements of high purity - but how hard would that be in practice? The reassembly after that into the materials and components seems possible, but how complex? With biology we start with replicating organisms of vast complexity - perhaps even more complex - that already disassembles raw materials and reassembles them into a vast array of materials and functional organs; they would be incredibly difficult to design and build if they were not pre-existing - but they are pre-existing. I would find it repugnant but behavior control implants in animals might manage to combine the replicating advantages of biology with technology. Might they be able to grow such 'organs' as a computer chip and a wifi tranceiver? I note that they also sometimes manufacture materials we tend to think of as inorganic, eg silicates. They may struggle to make native metals but may be able to extract and concentrate compounds incorporating them. I still struggle to see how a Mech civilisation would expand endlessly and move beyond the solar system. Or why it would want to. But I admit I struggle to see AI having any innate desires or goals; I think those must be incorporated into them, by humans. Whether, given the capability to rewrite their own fundamental programming, they would always keep those goals or abandon them as pointless is yet another unanswerable. Even the reward loops that give impetus to motivations like growth and expansion could be overwritten as unnecessarily burdensome - just write a feel-good subroutine, like putting an electrode into a brain's pleasure centre.

You think? Yes, space presents big challenges for biological life, but it does for machines also. Both depend on a broader supporting base - an ecosystem - but living things already replicate successfully, managing to derive their essential materials from unrefined inorganic materials and sunlight.

@TheVat "The advantage a machine has is ease of copying off clones of itself to provide backups." A novel use of the word 'ease'. And 'clones' Around here making a robot involves multiple specialist materials and component suppliers and a diverse range of factories, each in turn dependent on a fractally proliferating range of other service and equipment providers.

Michael Barnard at Cleantechnica did a critique of Dense Fluid Pumped Hydro that may or may not have addressed this specific version. A solution in search of a problem seems to summarise a lot of his article; yes there would be more suitable locations but there isn't actually a shortage of locations or water for pumped hydro, not when a transmission line can readily accommodate storage at a physical distance. And reducing the elevations needed doesn't reduce the quantities of fluid needed (which are still enormous) and the kinds of materials that can be mixed with water to increase the fluid density are not cheap and depending on what is used, bring other problems, including changed viscosity and increased wear and tear. And, inevitably, leaks. My own view is pumped hydro does have enormous potential for long, deep storage but the levels of planning and forethought and commitment to investment have been lacking, not suitable sites. Getting the approvals can be problematic and time consuming but not really for regulators that rate decarbonising as essential; governments have the power to push through critical projects, if they so choose. Until (too) recently the confidence that the solar and wind capacity to need pumped hydro would be built was largely absent - but batteries offer a short term, fast fix, with enough convenience and added benefits that investment that might have flowed to pumped hydro is going to batteries irrespective of lower overall costs per MWh. I think batteries have managed to take advantage of being less impeded by siting approvals process - and from being able to help defer transmission upgrades rather than requiring them. Dense Fluid Hydro startups may see that as an advantage applying to them but I don't think so; batteries will always be easier to site. Australia has a large pumped hydro project in construction, Snowy 2, somewhat remarkable for being committed to ahead of the solar and wind it is intended for; that project has not been going well (which should not be seen as typical) but the time scales as well us up front costs are still an issue for pumped hydro and the additional transmission capacity is a big cost. I note that Snowy 2 was committed to the same year as South Australia committed to the first 'large' battery installation (to widespread derision); the battery was in operation before the end of that year and in power delivery terms (MW) batteries now exceed what Snowy 2 will add when it does come online - and are catching up fast in total energy capacity term (MWh). LFP has displaced and exceeded NMC and very possible Sodium will displace and exceed LFP; can't rely on assumptions about batteries being unsuitable for long deep storage.

I think there is no decoupling the costs of such goals from the economics of the greater economy that is trying to do them. Economics is about how resources are managed and marshaled to do the things we want from available resources. And absence of positive economics of the intended goal seems as likely to lead to failure as resource shortfalls (I suppose it is a resource shortfall) and technological inadequacies. Yes but it seems very likely to add significantly to the dry mass/dead weight that has to be carried and accelerated. I had thought ion engines - the most efficient in terms of impulse for reaction mass ejected - would be used a lot more but the mass of hardware needed undercuts the gains.I doubt one of the favoured reaction mass types - Xenon - is going to be easily obtainable, so what gets used may be less than ideal. Such rockets must also include an energy source. Chemical rocket hardware can be low mass and disposable. I doubt nuclear will be like that. Do the "Orion" types (the "Orion Shall Rise" type) - riding on top of nuclear detonations - rely on the gradual vaporisation of the blast plate between the ship and the blasts to provide some of the thrust? I think that plate (whatever it is named) would be consumed one way or another. Apart from anything else long term self reliant survival within a closed system - something we can study experimentally here on Earth - has not been demonstrated. The slower the journey the longer that closed system must last. Sending robotic miners and factories ahead to make supplies and boost them to reach the main ship sounds hypothetically possible - with imaginary technologies much more advanced than we have. Then you would have to carry such equipment - more payload - and have big velocity differences for those supply tenders to overcome, over and over. It all starts sounding harder the more I think about it.