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The concept of a Von Neumann probe negates all that. The time factor is only a viable criticism if you insist on the experiment not being able to outlive the experimenter, in a short lived civilization like ours that is a reasonable complaint but in a civilization a Million years old where biological beings might have practical immortality or even be an intermittent part in or not be present at all in a million year old civilization. waiting a few thousand years for data might not be a problem at all especially when the data inflow involves millions of Probes sending back data as they gain knowledge of the system they were sent to. No reason to sit around and twiddle your thumbs waiting on data from a single probe when it comes in continuously from millions of probes. Such Probes would reproduce and head out for more stars to rinse and repeat.

How about simple practicality ? It makes absolutely no sense to have to wait a century or more get information returned. You are well aware of time dilation for a craft in regards to the at home time frame compared to flight time. You cannot randomly ignore involved factors. Great you can send signals back at c. Yeeha so what ? You still have to get to Earth and factor in the time differentials for time dilation. It would be more practical to build a solar system scale telescope and get data faster.

I didn't suggest aliens had to be close by, why do you think they need to be close by? I see no reason for new physics to attain star travel.

Really and is that particularly practical unless the aliens live on some nearby star system where it's close enough we could detect chemical and biological signatures via spectography or recieve their signals ? Not very practical in my opinion. Your still looking at years for signals. Yeah that would certainly require some new physics lol. However unlikely still has some plausibility.

To paraphrase Dr. Venkman, “I’d call that a big yes” as far as new physics goes Wouldn’t you want to know the planet is habitable before you head out? That requires data return. In any event, these are not consistent with the sightings we have. Those would be landings; the first one would likely be a large craft.

Home destroyed due to bad management or disaster(s)? propagation?

Roadside Picnic - Wikipedia

No, data can be transmitted at c, no need for a biological being to make the trip, quite possibly no need for biological beings at all during the trip or the exploration. Even we primitive humans see the need for Probes over biological beings in our exploration of just our solar system.

Adventure

Do you want any information resulting from any studies to return to home planet ? It doesn't matter if the occupants are biological or not in that regard. What would be the point of any visitation if you cannot return with any data or resources

Turn around time has no bearing on the possibility of star travel, it only relates to the biological individuals involved. There doesn't need to be biological beings involved in the transfer from one start to another.

If you want turnaround to home planet in any reasonable time frame it's unavoidable even then its too slow for cosmological distances

I have to agree, near light speed travel is problematic for many reasons... why does this negate star travel? What we need to do is justify thinking near c travel is necessary for star travel.

Muc Much like that. It's a feasible possibility one could gather resources as they go but it would have to be of sufficient size for any manufacturing of those resources. As well as population growth. One could use the Oort cloud to hide in as their are lots of objects in the Oort cloud that escapes detection.

Like a customised self-gravitating body ~100+ km diameter built from carefully redirected small asteroids to put it into a path of ejection from the solar system? Internal thermal energy may be a viable long term energy source (or nuclear). Deep subterranean accommodation caverns should give reasonable protection from small collisions. Not sure I'd pick the lifestyle choice myself...

Could we not just make the world a little bigger and carry on as normal? Dig a hole into the centre of the earth and let off a nuclear bomb ( or an anti gravity device ?) so as to increase the radius of the planet to a suitable degree. The surface would expand and there would be endless opportunities for new resource exploitation and enough lebensraum for all. Those who don't like the idea could just sign up with Elon Musk.

If we're sticking to known physics a feasible possibility being the Alcubierre drive it's feasible if one can solve the solutions for reduced energy requirements and eliminate any need for negative mass. Theoretically this would address the collision scenario as the spacetime bubble would cause deflection. However it also in turn generates a greater problem that the bubble may also cause gamma ray production. One study I'm familiar with showed that even at 50 percent c. That gamma ray production could wipe out life on the planet it's leaving and arriving at. However I've only ever come across the one paper on it. Other than that I can't think of any viable means that isn't a 1 way trip. Which really negates curious visitations. Colonization would be far more likely than visitation in a 1 way trip scenario. Though one other possibility is some species that lives strictly in space however that would require am extremely large craft with a huge infrastructure for resource production

Solar sails feel a force of P/c, so you need ~3 x 10^9 watts to accelerate a 1 kg payload at 1g. And a point source of light will drop off as 1/r^2 Solar sails are not really maneuverable - it will get you from point A to B, but if you want to do other than straight-line motion it gets tough. No stopping off to gather raw materials. Such efforts require additional infrastructure which adds mass.

Rubber science, for sure. Except light sails, laser stations, the modes with external push. Again, maybe why Von Neuman devices could be more feasible - one the size of a beer can, with light sail wings. The physics is known, but I don't know how far the engineering will go. We are talking civs with massively elongated time frames, a hypothetical entity at this point. (shrug)

Agree that all such socially structured causes have deeper roots. If we can't fix dopamine rushes, maybe we could try some form of socialism and combine it with Green ideas. The Scandinavian Model seems to go that direction. And consuming less has reached the status of a fad in some wealthy countries, though it's really hard to say how far that will go. People who embrace Marie Kondo or home minimalism or Tiny Houses may not always stick with that. A minimalism that made community sharing its focus (as the Japanese fellow spoke of) would probably need a near-miraculous resurgence of the Counterculture in the US. I.e. Americans would be more motivated by framing it in terms of less housework, more disposable income, fewer time payments...

Everyone suffers somewhat, because under extreme regimes the truth is a casualty. And telling the truth becomes hazardous to your health. And even if you're an old quiet guy who just putters in his garden and reads pulp novels, you're not an island - you experience the weight that lies on everyone around you, the constant threat to anyone waving their freak flag, books vanishing from library shelves and stores, and so on.

I think we have to limit ourselves to known physics. If you extrapolate based on something new being discovered, all bets are off. Energy? No problem - we find out that over-unity is possible! Warp drive! Stable wormholes! Impervium! Mithril!

Technical analysis of this kind is useful IF we have reason to assume that a civilization millennia ahead of us still uses impulse rocketry for starfaring. So that assumption deserves scrutiny as well. Getting whacked by an interstellar proton does seem like a potentially serious problem, especially von Neuman machines with nanoscale engineering. Imagine a civ has molecule scale memory and you are stored as a miniature thumb drive aboard and after a thousand parsecs you're full of flipped bits or whatever. Woops, there goes third grade...there goes first kiss... Asimov. One S. Yes, hyperspace was a handy sci-fi workaround. Plenty of sci-fi writers took that route - Niven, Herbert, Heinlein, Clarke, et al. Such a story device is called rubber science . https://en.wikipedia.org/wiki/Rubber_science

That understanding is correct. The elephant your missing is that their are two categories (primarily) for cosmic rays. Primary and secondary. Primary cosmic rays are those sent from stellar objects like the Sun Secondary cosmic rays are those particles produced by the Primary rays interactions with our atmosphere. Neutrons are part of the secondary group. Part of that process includes mesons decays to that further allows leptons to form. Here is an examination of some of the different processes at different atmospheric levels https://pdg.lbl.gov/2019/reviews/rpp2019-rev-cosmic-rays.pdf

Cyclic universe models is one possible cause for our universe coming into existence. It does nothing to address how the first universe developed but for this thread your not concerned about that. This however doesn't really follow. The conditions of the BB according to mainstream physics was in a thermal equilibrium state. The extreme high temperature wouldn't even allow atoms to form. The four forces were in a state of thermal equilibrium. Once electroweak symmetry breaking occurs the 4 fources could separate. Gravity Em, weak and strong force. Particles could then drop out of thermal equilibrium. That's simply one process where it would be impossible for past information ideas etc would be literally obliterated even assuming such things could even survive the initial collapse of the previous universe.