[elorant]

Actually, the universe might be too young for life, and we're one of the first. The further you go back in time the more violent events like supernovas you'd find which would make the universe a very hostile environment for life.

[ncmncm]

A galaxy is really a pretty dangerous place for organic life.

One supermassive black hole merger in the same or nearby galaxy, a biggish black hole merger within 10,000 light years, a magnetar hiccup within 500 ly, or a workaday supernova 5 ly off might be enough to sterilize a whole solar system. There is no guessing how many times life has been wiped clean that way.

The sooner our descendants can get the hell out of the galaxy, the better.

[brabel]

I think we are in a relatively safe corner of the galaxy though... IIRC there's no cosmical existential threat we know of within the next billion years or so, and the kind of cataclism you're talking about would be the kind of thing we should already be able to predict millions of years in advance.

[ncmncm]

Sol orbits the galaxy independently of all other bodies. We don't know where most of the magnetars are, never mind their orbits, which are anyway unpredictable. (The nearest we know of is ~5kly? away. Or more, or less.) We don't know where any but two of the black holes are, although there must be a billion or more. Less than ten million years ago we flew into a dust cloud left over from a recent supernova.

We could have been in sterilization range of a magnetar dozens or hundreds of times since life evolved, so we're just lucky none erupted when we were. The core supermassive black hole blew somehow just 100Mya, fortunately pointed somewhere else.

The only safe-ish corner is very far from everything else, and off-axis from the central black hole. We are deep in the thick soup.

[Thiez]

And what exactly would our hypothetical descendents do there, in the middle of nowhere with no star, no planets, and no energy?

[jeremyjh]

There are stars that are far from everything else and off-axis. There are even stars that get ejected from the galaxy all-together.

[ncmncm]

Yes, one of those last would be a good choice, if we could get to one.

But I don't expect our distant descendants to be dependent on starlight. A nice Kuiper Belt and Oort Cloud are good to stay in for access to building material and fusion fuel, but the most valuable resource, otherwise, will be abundant free cold.

Planets, as such, and sunshine are for the extremest primitives. Nobody advanced would want to be closer to the sun than, say, Neptune.

[GoblinSlayer]

We are in the galactic habitable zone.

[ncmncm]

There is no such thing as a "galactic habitable zone" defined or meaningful.

There is a an obviously uninhabitable zone close to the core black hole, and uncountable similar zones elsewhere, but Sol could move into one of the latter at any time. And, distant events could wipe us out, anyplace else, with decreasing probability as the density of other stars in the immediate neighborhood decreases.

All we can say is that we have not entered any uninhabitable zones in the last billion years or so, i.e. five orbits, and not been blasted too badly from a distance, likewise. Milder events could wipe out everything but deep-sea microbes, which at this late date might not leave time to evolve intelligent life again.

Lots of galaxies have heavy star-forming going on in the rim, with increased risk of supernovas. I don't know if we know whether ours is among those. (I doubt we can tell. Such regions are identified by high UV flux, but UV is blocked by dust, which is plentiful.) But otherwise, farther from the core is generally safer.

[GoblinSlayer]

Habitable in a sense that we can stay away from the active star formation zones for a long time. Assuming those zones are deadly.

[ncmncm]

You can see active star formation zones with the naked eye from Earth's surface. Star formation is happening literally all around us.

Star formation zones are very far from the most deadly places in the galaxy, or even in our neighborhood.

[api]

I think this is likely.

Always remember that intergalactic travel is exponentially harder than interstellar travel and that is already brutally hard. We need only be among the first in our galaxy to explain the Fermi paradox.

[UncleOxidant]

I don't see how there could ever be intergalactic travel.

[api]

Theoretically if you could travel very close to the speed of light time dilation would make the trip fairly short. Of course it’s one way since it would take hundreds of thousands to millions of years for everyone else. If you came back you would be returning to a different geological epoch.

Of course the energy requirements are insane and would require some kind of physicists nightmare propulsion system like antimatter rockets. Lose reactor containment and you are instantly converted into plasma and gamma rays. If you hit something the size of a dust particle it will destroy your spacecraft.

A bit hard.

Near light speed is a requirement for intergalactic travel because even solid state machines will not last long enough to make it without time dilation to shorten the trip.

Interstellar travel could theoretically be achieved with 1960s tech. Look up Project Orion. It could even be done with chemical rockets if you could hibernate, greatly extend life span, build a generational ship, or be an AI and just turn yourself off for the trip.

[UncleOxidant]

> Theoretically if you could travel very close to the speed of light ... Of course the energy requirements are insane

Yeah, this is why I don't think we'll ever achieve this and I very much doubt other species have or will either. I don't think the incentives are there to expend this kind of energy (and capital) trying to get to another galaxy - especially since it would a very, very long time before anyone would know if the project succeeded (if it would ever be known at all)- there's no success feedback for a project like that. Interstellar travel will be difficult enough and the feedback time there would likely be at least a couple generations (longer depending on where the travel is to).

[Scarblac]

> Of course the energy requirements are insane and would require some kind of physicists nightmare propulsion system like antimatter rockets. Lose reactor containment and you are instantly converted into plasma and gamma rays. If you hit something the size of a dust particle it will destroy your spacecraft.

And, this one spacecraft needs to be able to slow down, have the telescopes and equipment needed to identify exoplanets and decide they are suitable, have the means to travel all the way there, and have everything aboard to start a complete new viable colony on some unknown planet. It probably needs to bring all the energy for the first few millenna in batteries. That must mean a huge spacecraft, and there also needs to be some way to land it all on the planet, hopefully with all the pieces close to each other. Et cetera.

The engine that got all that close to light speed is just a tiny piece of the puzzle.

[rewgs]

Ha! Funny, I've never considered it even though it's so incredibly obvious: the spacecraft needs to be able to slow down out of light speed. That might honestly be even harder than getting up to light speed, and that of course is almost certainly impossible.

Imagine the "runway" you would need. How many lightyears would it take just to slow down? And that's before you even consider how some sort of reverse propulsion mechanism would even begin to turn on without tearing the ship apart in a microsecond.

[hwc]

If you simply assume that your spacecraft uses its own engines to accelerate to cruising speed, then the Δv to decelerate is approximately equal to the Δv to accelerate.

Similarly, if the engines are operated at a constant acceleration, then the distance needed to decelerate is equal to the distance needed to accelerate.

Fuel(/reaction mass), on the other hand, us not equal. The fuel to be used to decelerate must also be accelerated first, meaning more fuel is used to accelerate than to decelerate.

[ncmncm]

Maybe count on somebody where you are going to help you slow down? They will see you coming a hundred thousand years ahead of time. They might decide it is safer to just park a planet in your way, though.

So for, what, two million years or five, the front of your spaceship is blasted with near-lightspeed intergalactic-medium molecules, plus trillions of dust particles and an unknown number of rocks.

"Erosion" is not an adequate word, but we don't have any that is.

[TheRealNGenius]

much akin to how cavemen couldn't fathom how they could possibly talk to another caveman on the other side of the earth.