Then again, if the atmosphere clears up in a year or two, then they either are even more advanced than we are or they destroyed themselves and their planet healed itself.

We've two data points for the cleaning up of atmospheres after a sudden bout of pollution : the ozone hole we created in a few decades is steadily reducing and dispersing since the 1990 ban on producing CFCs ; that looks as if it'll be cleared up in a century or two (large, sulphate-rich volcanic eruptions not occurring, which may put it back by a few years or decades). Whether that was an externally detectable pollution event is more dubious - it was hard enough to detect from here.

The other datum is the decay of the PETM carbon dioide spike of 55 Myr ago. That took between 100,000 and 150,000 years to return to something resembling an equilibrium CO2 content in the atmosphere and reduce temperatures to something approaching their pre-PETM levels.

Combining the two, expect it to take 10s of thousands of years for a major pollution spike to "heal". If you look at it from the other end of the telescope, that's around 10 overturnings of the oceans (our largest and most massive environmental component).

We can almost create artificial gravity by finding a way to generate Higgs Bosons and attach them to matter.

Do you have a vaguely credible citation for that - an Arxiv paper, or a professor of physics describing a roadmap. I've never heard even a hint of anyone planning to do that. (Besides, for a long, long time, it'll be much easier to mimic gravity with centripetal acceleration of the floor.)

and we're almost done with fusion.

Well, give or take a decade or three. It does appear to be closer now than when I was an optimistic schoolkid hitch-hiking to university.

We already developed algae that can strip CO2 out of the air.

I'll grant you that. It means that when I stop drilling oil wells, I can start drilling wells to dump CO2 into. That's fine by me. (You do realise that we've got gigatonnes of CO2 that need to come out of the atmosphere and back into the ground before we can even start to consider the job done?)

I think you're being highly optimistic on a 20 year timescale. Maybe 20 years once we get the political will together and start to actually address the problem. 50 years being highly optimistic ; well over a century being realistic.

What kind of moron came up with that? Let's see, life was here for like 500 million years, for about 150 we've been ruining the atmosphere, and 100 years from now we'll have solved it.

OK. And now let's look at the real figures :

There has been life on the planet for approximately 3500 million years (definite fossils to 3.2 billion, more disputed going back to 3800 million).

The first major pollution event - the production of oxygen - started around 2600 million years ago, with oxygen becoming ubiquitous (if at 1/100th of current levels) by about 2300 million.

Multicellular life first left fossils (the Ediacara fauna) about 600 million years ago (what you think was the origin of life?).

Multicellular life came onto land about 420 million years ago.

For about 150 years we've been polluting the atmosphere significantly (NB : there is detectable pollution in the Greenland ice cores dating back to Roman times. If you consider lead dust from Britain under the Romans "significant".), and we're continuing to do it at an accelerating rate. Going on the previous occasion when this happened, it'll take around 100,000 to 150,000 years for the atmospheric perturbation to self-correct. At that scale, it doesn't really matter if we die this year, next year or 1000 years from now.

and 100 years from now we'll have solved it.

Can you cite a source for that? I've never heard that sort of claim, even from pot-smoking AGW-denying oilfield trash. (Actually, working in the oil field, I haven't met AGW-denying trash. We know fine and well what we're doing.)

MS Office is an obsolete dinosaur already. Light it's pyre and send it on its way.

Can't we just bury it in a hole in the ground (even if it leaves a small hill) and use the wood for something useful?

Why are you expecting them to stop? That's a huge waste of fuel.

Like I said, put them into a ship with big enough storage to drop off a colony-forming ship every 10 generations - let them do the deceleration, mine your consumables, and re-supply the mothership. If that's happening every 10-20 generations, then you've got a release valve for your society (something that we don't have at the moment, but designing a society with release valves is one of the influences you can have across the millennia). And if (again, racing certainty) some of your would-be colonists get freaked by leaving the mothership behind, then the colonists have a release valve as they're establishing their society since there will be a re-supply mission accelerating back to the mothership next generation.

you've confirmed there's a hospitable panet (gravitational lens telescopes are your friend)

Short of manipulating a large (planetary mass?) lump of neutronium (which I'm not sure can exist), we don't have even a vague direction for such an object. And if we had to do that, we might well find it easier to go there (or send robots and relay stations) than to build such a telescope.

would you be happy if our lives today were bound to the vision of some ancient Roman emperor?

Some people seem to want to bind themselves to the pronouncements of some Roman carpenter, of whose existence we're by no means confident and whose diktats are based another half-millennium further back when (putative) his ancestors were slaves. At least we're pretty confident in the existence of the Roman emperors, even if some of them were as mad as a box of badgers. (I'm actually planning a walk along Hadrian's Wall - after that, I can securely attest to the existence of a Wall, with at least legion-marks referring to Hadrian ; after which, disbelieving in his existence would be perverse. In a generation ship, the existence of the ship, and it's constructors, would be hard to ignore.)

Like I said, that's why you build your society with (ir-)regular break points. Whether you have the ship travelling on a loop, or just driving straight(-ish) on for the horizon ... well that might be something that you re-assess every millennium or so. It would be another break point. Maybe you build into the design so that every 10 dropped-off colony ships, you can fission your mother ship into two and then continue to grow each on their chosen routes. Each generation would still need to be making choices, but equally each generation would be subject to constraints (as we are) which were imposed on us by ancestors only a (relatively) small number of generations ago. If you're an American, then almost certainly one of your ancestors chose to travel half-way around the world less than ten generations ago ; if you're not an American, then almost certainly several of your ancestors chose to NOT travel half-way around the world less than ten generations ago. How do you feel about those choices, whichever way they went?

Assuming 20-30 years per generation

Big assumption. The pressure to use medical developments and technologies to extend life is strong. On the assumption that the mammal body plan can't be pushed beyond 200 years, why would you go around doing momentous things like breeding before your 80s? Remember that for most of human history it was reasonably common to co-exist with your grandchildren, but seeing great-grandchildren was pretty rare. I'm trying to think of a mammal (or bird ; I don't know about reptiles or elasmobranchs at all, to cover the disparity of the vertebrates) that does routinely see it's great-grand offspring. If you wanted to change the generation ship people into a new species, that might be one of the most effective ways to do it - change life spans.

Of course, I suppose after generations on a world-ship it's quite possible that not everyone would want to settle down.

I'd say that's a racing certainty. It's not a trope I've seen exercised much in SF (a notable exception being "Building Harlequin's Moon" by Larry Niven and Brenda Cooper. The necessity for a mutli-generational approach would tend to cramp things like character development (BHM spans a period IIRC of some 60,000 years, as the colony ship has to lay over to carry out repairs, and in the process need to, erm, build a moon. In orbit around "Harlequin." (Niven is Old School SF.)

There are interesting things to think about in such a situation and a mission. Including, particularly, how do you man a mission that is going to be profoundly multigenerational. How do you know you're going to be able to motivate the 79th generation after launch?

Those strong magnetic fields would, indeed, change the energies of electron orbitals (indeed, of proton orbitals inside complex nuclei too), but they'd do so in accord with the laws of physics. That would (probably ; IANA quantum mechanical chemist) change the laws of chemistry to be different to those that apply in lower magnetic fields (and lower field gradients too). However the underlying laws of physics will still be the same.

There's a very definite hierarchy of precision and strength of lawfulness in the sciences. If we accept economics as being a science (the dismal science), then it's "laws" are much looser than the laws of biology. (I was reading a paper last night on the laws of social evolution of non-breeding behaviour, couched in terms of probability of various outcomes, and the consequent effects on probably descendent count for each member of the population ; those laws were couched very much in economic terms, of calculating probabilities.) The laws of biology are much stricter ; egg plus egg does not make a fertilized egg ; 23 chromosomes plus 24 chromosomes makes for a pretty fucked-up organism, if it's viable at all ; oxygen metabolic enzymes plus sulphide (or hydrosulphide) ion makes for a broken or non-functional enzyme molecule. The laws of chemistry underlie the laws of biology and are considerably stricter ; in aqueous solution, silver ions plus chloride ions precipitates silver chloride if the solubility product of AgCl is exceeded (assuming no thiosulphate ion in solution) ; argon reacts with fewer elements than xenon, and forms less stable compounds ; silver chloride has the sodium chloride structure at NTP. The laws of chemistry themselves are founded on the laws of physics - those precipitations and crystal structures are basically the result of electrostatic interactions (as are the more subtle interactions of quantum mechanics in forming covalent bonds) ; when people talk about "unknown new laws of physics that will give us FTL travel, I invite them to jump out of a tall building and try to argue for an exemption from the laws of gravity.

In your example, the changes to the emergent laws of chemistry result from adherence to the more fundamental laws of physics.

If you can drag up a few string theorists, I can bring some mathematical philosophers ; we can throw them into a pit and let them fight it out to see if physics or maths is more fundamental to the universe. I'm not a great fan of either marshmallows, or popcorn, but I can bring a barbie and some great venison burgers.

SNAFU.

It also has an optional 'pull-down conversation mirror' that lets drivers check on kids without turning around."

A driver who even thinks about turning around to check on what the passengers are up to should lose their driving license until they've successfully re-passed their driving test.

That's why you strap them in. That's what you have other adults in the vehicle for. That's why you train the kids from before potty-training to not touch their seat belts on pain of straight back home and no fun for the rest of the day. That's why you train the kids over the same time scale to not distract the driver.

This is a technology which should not exist.

Memo from Self : Like I was going to do that? After the last time I worked on a Mac?

The laws of physics (and chemistry) are the same pretty much everywhere

Where, precisely, do we know that the laws of physics are different from those we see here? "pretty much everywhere" implies that there is somewhere that isn't included. Where is that?

even at small fractions of light speed, remain expansionistic, and avoid completely eradicating ourselves or transcending as a species we could colonize the whole friggin galaxy in only a few billion years.

Billion? A few tens of million years.

The galaxy is about 100,000 LY across. If we can get to 1% of c, then moving out to cover the galaxy would take (order of) 10 million years transit time. Since you're using generation ships, then while you're in flight you can be preparing a colonisation ship in the centuries between stellar encounters and drop the settlers off (and along with them, your political dissidents, mutants and space-sick passengers and other problems) ; if they think the star is settleable (does it have asteroids ; never mind the planets for the next x generations) then they stop, otherwise they do some quick (decades) mining for consumables and then depart to catch up with the mother ship.

I'd guess that "we" could colonise the galaxy in 100 Ma. Of course, by then, the species would certainly have changed, and probably fragmented into significantly different species. Certainly cultures would have changed drastically.

But it's all SF for the next number of generations.

Hmm, well I suppose if you were under hard acceleration it probably wouldn't be healthy for anything caught in the exhaust at close range,

In some SF universes that is codified as a "law" of warfare. e.g. "The Kzinti Lesson : a reaction drive is a weapon in proportion to it's efficiency as a drive."

A lot of those early mathematicians were a bit on the crazy side, having come to that realization and not having any of the framework for coping with the idea.

Well, they could have just invented a god of mathematics and had done with it. But they were pretty smart cookies, so they'd probably have noticed the stupidity of admitting a supernatural explanation of any sort into their attempts to understand the natural world.

Your analogy is wrong. You need to cut the top off the safe and then perform the rest of your experiment.

Air guns (they've never been called "sonic cannons" ; the author has been channelling early Hawkwind) are fired at a depth of 5~10m below water level, suspended from floats towed behind the survey boat. Normally there's a string of multiple hydrophones trailing along behind the air gun, held at a similar depth by tension between floats (pulling them up) and a hydroplane (underwater wing) pulling them down. Sometimes we lower a hydrophone (or several, for redundancy) into an existing well bore and lower it to the bottom, maybe as much as 7 or 8 km away from the surface, but we never lower air guns to that depth because they wouldn't work.