[candiddevmike]

Aside from unanswerable questions (has the universe started to fill it's container? Is a simulation property nearing "1"?), does this make long distance space travel feasible again? I thought there was something around the universe is expanding too fast to visit places like Alpha Centuri (and preventing visitors to us).

[Sharlin]

Edit: A big brain fart, ignore the retracted part below. Colonizing the universe is of course impossible in 100My, barring FTL. What the paper I referred to [1] says is that colonizing the Milky Way may take less than that, and if you can do that, spreading to the rest of the observable universe is fairly easy, very relatively speaking.

<retracted> According to some calculations, it should in principle be possible to colonize the entire observable universe in less than a hundred million years. It's much too fast for the expansion to affect except marginally.</retracted>

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

Anyway, as others said, mere intragalactic (and intra-Local Group) travel is not affected by expansion in any way whatsoever.

[1] https://www.sciencedirect.com/science/article/abs/pii/S00945..., PDF at https://www.aleph.se/papers/Spamming%20the%20universe.pdf

[hn_acc1]

I found someone saying colonize the Milky Way Galaxy in ~90m years? Is that what you meant?

The observable universe is ~93B LY - unless you're assuming FTL (and MUCH faster than light), I don't see how that's possible?

[omnicognate]

Time dilation means that you can get anywhere while experiencing an arbitrarily small amount of time. You can cross the galaxy in a second as far as special relativity is concerned. (With the expenditure of insanely vast amounts of energy, ofc.)

To an observer back home you'd look like you're travelling at merely extremely close to the speed of light, but to you the journey would take a second.

[mr_toad]

> You can cross the galaxy in a second as far as special relativity is concerned.

Sure, but the rest of the universe will keep on changing. In 90 billion years it’s going to be a very old universe. Galaxies will become consolidated and isolated, fewer young stars will be born. Only the dim light of red dwarf stars will shine among a graveyard of dead stars.

[omnicognate]

Hey, I didn't say it would be fun :-D

Tau-Zero by Poul Anderson explores this, BTW. It's a great little sci fi novel about a spaceship doomed to accelerate at 1G indefinitely.

It is significant from a colonisation PoV. With sufficient acceleration capability and the ability to survive travelling through the interstellar medium at extreme velocity (rather than getting vaporised by a mote of dust), a single generation of humans could in theory colonise the whole galaxy within their own lifespans. Indeed, some of them could even come back together and meet again after visiting those distant worlds, on an Earth many millions of years older, if their worldlines end up with similar proper times.

[Sharlin]

Yes, my brain totally froze. Added a correction.

[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.

[xoa]

>According to some calculations, it should in principle be possible to colonize the entire observable universe in less than a hundred million years

...what? That doesn't seem right, just from a really quick gut check it looks like the observable universe has a radius of 45.7 billion light years [0]. Even if the universe wasn't expanding nobody could get to everything any faster than that number of years right? Maybe you saw something that was talking about the local (Virgo) supercluster, which I think has a radius of around 55 million light years, so that sounds more like something that could be done on that timescale "in theory". But there are millions and millions of superclusters in the observable universe overall.

----

0: https://en.wikipedia.org/wiki/Observable_universe

[Sharlin]

Oops, yes, I don't know what I was thinking. A total brain fart. The paper I referred to is Sandberg and Armstrong's 2012 "Eternity in Six Hours", and of course they don't claim such a thing. Only that it's possible to start a colonization wave that has plenty of time to spread to everything visible now before they slip outside of our future light cone. The ~100M years refers to the colonization of the Milky Way. Sorry!

[1] https://www.sciencedirect.com/science/article/abs/pii/S00945...

[palmotea]

>> According to some calculations, it should in principle be possible to colonize the entire observable universe in less than a hundred million years

> ...what? That doesn't seem right, just from a really quick gut check it looks like the observable universe has a radius of 45.7 billion light years [0].

I guess it depends on whose hundred million years you're talking about: the colonists' or those who stay home's. I don't know how to do the calculations, but it seems plausible that you could traverse the entire observable universe at near light-speed in 100 million years ship time.

[grvbck]

You need ridiculous speeds for time dilation to really kick in though. Mathematically, it starts as soon as an object moves. But if a spaceship travels at 90 % of light speed (0.9 c), their local time moves just approximately at half speed compared to local time on earth. A year for the astronauts is just over 2 years on earth.

At 0.995 c, the ship clock runs 10 x slower.

At 0.999 c, 22 x slower. Then if you push the turbo button to 0.9999 c, 71 x slower.

The fastest man-made object to date is the Parker Solar Probe, at 0.059 c.

[oofbey]

That limitation only counts for visiting other galaxies. Travel within the galaxy is always possible, regardless of the universe’s expansion. And Alpha Centauri is super close, even within our galaxy.

[dtech]

Specifically the local group, so Milky way + Andromeda and some dwarf galaxies

[Sharlin]

Dozens of dwarf galaxies, even! Also, Triangulum is sort of borderline at around 70% of the Milky Way's diameter, although admittedly only 10% of its mass. But Mars is also around 10% of Earth's mass, for a comparison.

[indoordin0saur]

The universe was always only expanding between galaxies, not within them.

[Razengan]

So wait, individual stars aren't getting further apart? Galaxies aren't getting "bigger"/more diffuse?

[kmeisthax]

Galaxies have enough gravity to counteract the expansion of the universe.

[Razengan]

So do we see the expansion cancelled out by the gravity, or do we only see the gravity?

I mean, is it

    change = gravity

or

    change = expansion - gravity

Because this just made me wonder.. is "dark energy" simply the absence of gravity? i.e. just in regions where there is next to no matter/activity?

[JumpCrisscross]

> do we see the expansion cancelled out by the gravity, or do we only see the gravity?

We see gravity overpowering expansion. Same way you can’t launch yourself into orbit by throwing lots of pennies at one a second.

[recursivecaveat]

Imagine the universe as a giant balloon. Inside are little miniature balloon stars floating around, tied with string into balloon galaxies. If we heat the air: the big balloon expands, the clusters of mini-balloons spread out from the other clusters, but the clusters don't get any more diffuse. The string is way way too strong to be overpowered by the separating force from the expansion of the gas over short distances.

[Razengan]

I mean, this is tricky to even ask: is there still expansion INSIDE galaxies, BUT it's countered by gravity?

Or is there no expansion within galaxies at all?

i.e. is dark energy or whatever that causes expansion only present in the absence of matter, or is it present everywhere regardless of matter, but because matter also has its own gravity the expansion is not visible/relevant?

[indoordin0saur]

No, stars stay just as close to each other. Also even the nearest galaxies (Andromeda) are either getting closer or staying at roughly the same distance.

[jfengel]

The limit to space travel is the Rocket Equation, which says that you require exponential fuel to reach higher speeds. Alpha Centauri isn't going anywhere, but it will take millennia of travel even with wildly optimistic assumptions.

Also note that there isn't any "container" to fill up. It could well be infinite. It's just that we will be forever limited to a finite subset, even in theory.

[mr_toad]

There are theoretical designs using antimatter (pion rockets, or beam core engines) that could reach 0.4c - 0.7c, which puts Alpha Centauri at decades away, not millennia.

https://arxiv.org/pdf/1205.2281 https://ntrs.nasa.gov/api/citations/20200001904/downloads/20...

[floxy]

Roundtrip Interstellar Travel Using Laser-Pushed Lightsails

https://ia800108.us.archive.org/view_archive.php?archive=/24...

"The third mission uses a three-stage sail for a roundtrip manned exploration of Eridani at 10.8 light years distance."