[delta_p_delta_x]

> The relative jump in difficulty from interstellar to intergalactic is much smaller than from interplanetary to interstellar.

Interesting way to put it... This doesn't seem that accurate. With sufficiently advanced technology, many of which we already possess, we could expect to propel a minute spacecraft to a considerable fraction of the speed of light, and reach nearby stars possibly within the end of the century. Reaching the other end of the galaxy is a massive undertaking. It's a logarithmic scale at every step of the way.

Pluto is about 38 AU from Earth. Proxima Centauri is about 6.3 × 10^4 AU away (or about 4.24 ly), and that's roughly a 2 × 10^3 multiplication. The Milky Way is about 50000 ly in radius, and the Andromeda Galaxy is about 3 × 10^6 ly away. Going from interplanetary distances to interstellar, and thence to intergalactic, involves at least a 10^5 factor (give or take) at each step.

[mkl]

If you can get to a star 100 light years away, you can get to Andromeda. It doesn't require going faster, just waiting longer.

[michaelmrose]

I feel like waiting longer in some sense may itself represent a substantial increase in difficulty in terms of creating something which remains stable for tens of thousands of years.

On the other hand who knows with zero samples how stable societies are thousands of years beyond our present level of development.

[mkl]

Yes, that's why I said 100 light years rather than 4.3. Maybe it's still too low, but I think there are targets within the Milky Way that would require solving pretty much all the problems of getting to Andromeda.

[ASalazarMX]

Imagine doing that, and being greeted with

ALL THESE WORLDS ARE YOURS EXCEPT ANDROMEDA

[skissane]

I guess the question is… we know what our current propulsion technology is capable of… given a million years of further technological development, where will our technology be?

The idea that, given a million years of further technological development, intergalactic travel might actually be feasible, isn’t really that implausible. Far from certain, but far from implausible either.

And that’s the thing-a million years is a technological eternity, a rounding error in estimates of time to colonise the galaxy/the local group/the observable universe.

[mrguyorama]

Any form of propulsion that obeys Newton has hard limits to it's space travel potential. Even spitting out single particles at near the speed of light for the most efficient way to generate thrust per unit of expelled mass still constrains you to the tyranny of the rocket equation, which puts hard physical limits on you.

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

The rocket equation also underestimates any craft that gets over a fraction of C

Currently, we have no evidence that reactionless propulsion is physically possible and one existing would directly contradict the conservation of momentum.

"technological development" isn't a magic word or force of reality. "Technological development" is the pay off of immense engineering investment and discovering new phenomenon, but every axis you can possibly put effort into engineering and optimizing has a finite limit at some point, and there are finite new phenomenons to discover.

The entire past 100 and some years of technological development has been basically down to mastering the electromagnetic force. But, we've basically used up the novelty that was there, and there is no new second electromagnetic force to discover. In fact, the nuclear force was also discovered and tapped out relatively quickly.

A great example of this is the elements. All evidence points to the outcome that the elements we can build stuff out of right now are the only elements you will ever be able to build anything out of. All artificial elements, even ones that are relatively "stable", have half lifes that preclude building stuff out of them, and there is no evidence that it is possible to modulate the rate that an unstable atom decays. So no "exotic" elements that could magically power space ships or anything will exist.

Intergalactic travel of humans is implausible unless you get into pretty radical transhumanism, or assume it's possible to perfectly maintain a biological human forever somehow, including brain functionality.

Brain uploads are another thing that people don't seem to recognize are radically more difficult and close to impossible. "Scanning" a brain is treated as an engineering problem, but it might not be. Every sensor relies on a physical interaction, most of them based on electromagnetic energy. How do you make an electron or photon or something interact in a measurable way with a cell deep inside someone's brain without that particle interacting with all the identical matter in the way or cutting open and taking apart that brain? Well, thanks to the mastery of the electromagnetic force, we have MRIs which kind of do in fact do that. But even if we had a magic MRI machine for example with infinite resolution (yet another thing that has fundamental limits), that would only let you look at molecules with with hydrogen, so you wouldn't be able to survey, say, the ion content of brain cells directly. If you are not aware, ion gradients are fairly important in human cell behavior.

Nevermind that scanning and uploading someone's brain, if it were possible, does not transfer the original conscious experience to the computer. A new copy may go on in a digital world but you still die.

[glenstein]

Lots of great points here, but I think there's a bit more cause for optimism. For one, generation ships I think are the long-term project for space travel that successfully gets humans somewhere. No easy feat by any means in terms of time, engineering, and risk, but not running up against a wall of physical impossibility.

And nuclear physics is still a wide open frontier. We don't yet have fusion, and there's a lot we don't yet know about quark and gluon plasma and nuclear behavior on astrophysical scales. And if we're talking about technological possibilities against time scales of forever, there's lots of interesting electromagnetic possibilities in the context of superconductivity and metamaterials that we haven't yet exploited and I'm probably not even beginning to do justice to it in its totality as an open ended frontier full of fertile (e.g. vacuum polarization is a poorly understood frontier that might turn out to have interestingly exploitable properties).

You did a great job outlining some devastatingly serious physical limits but I think, again against the timeline of forever, you may be perhaps underselling the possibilities of important and newly exploitable properties of electromagnetism and the nuclear force being brought into application.

[mr_toad]

The distance to andromeda is only about 20 times the width of our galaxy. And there are dwarf galaxies than are much closer.