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> I read somewhere (don't remember where) that if a ship constantly accelerates in space, it can reach another star system in a few decades and another galaxy in less than a century.

The problem is, it also has to slow down and stop at the destination. To do that, it would have to take all the deceleration fuel with it, and the amount of fuel needed goes up exponentially with the amount of deceleration required.[1] There’s no known or even theoretical form of propulsion with the specific impulse needed to get us to the next star system in under 600 years.

[1] https://en.m.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

I skipped over deceleration because 1) our probes can transmit something significant about a galaxy even at that speed and 2) over time, we can probably figure out a way to decelerate using a galaxy's natural resources (similar to how we use a planet's gravity for slingshot)

Surely, given a century, our scientists will figure out something.

Sorry this may seem a little snarky but - "They will think of something" - the universal band-aid for I want it to be true but I rationally don't know how.

When ever I hear that, the optimist in me wants it to be true. The pessimist thinks otherwise and nowadays the pessimist usually wins.

We will have hotels in orbit by the 1970's - they will think of something! That was a legitimate thing that the Hiltons proposed back in the 1960's.

I used "they will think of something" as a all-encapsulating solution for what humanity will do in the next century. Predicting the next 10 yrs is almost impossible, let alone the next century.

Your example of hotels in orbit is an incorrect example. If humanity wanted to build a hotel in orbit, we would have a hotel in space (we already have a crappy hotel - ISS). I don't consider Hilton's incompetence to represent humanity's incompetence.

I don't want to defend the Hiltons but that is just one example of something that was very common during that time period. In the 50's/60's - the idea that we would not solve the fundamental issues of the era over the coming decades would have been laughable.

When it comes to material shortages, fusion power, limits of computer chip scaling, global warming, over fishing, plastic waste, hyper effective battery storage, nuclear deescalation etc... have all been answered a lot with - they will think of something. Yes, there have been improvements but with most of these things, we have also missed the mark by a long shot. And decades later, we haven't. Sometimes it is right, look at what we did with CFC's. But it is by no means something I would hitch my wagon to.

John Michael Greer calls phrases like these 'Thought-stoppers' Little saying that we have that mean we don't have to face the ugly truth of something or are just trying to be a little too optimistic about the future in the face of hard times. "Housing prices only go up", "They will think of something", "Stock prices only go up", "It is different this time", "The fundamentals don't matter any more" etc.

If it is just a probe that blasts past, that does not count as interstellar travel.

There would be very little left of a vehicle after being blasted by relativistic hydrogen nuclei for 3M years on its way to Andromeda. And, who would be listening for its report 6M years later, if it could get to Andromeda intact? Or even 500,000 years later, from one of the Magellanic Clouds?

It would make sense to gather up your whole civilization and set out at a statelier pace, perpendicular to the galactic plane, to get clear of the numerous hazards that all threaten to sterilize your whole solar system when you are in the thick of a galaxy. Then it wouldn't matter if it took a million years to get clear. Probably you induce a polar jet from the sun for propulsion, and bring the whole solar system with you. (You would leave the Oort cloud behind, sadly.)

There aren’t any resources in interstellar space. It’s a near-perfect vacuum and the temperature is a uniform 3 Kelvins.

There are huge swaths of interstellar dust (mostly hydrogen) which can be used to slow the probe down.

If not, then we can use a galaxy's resources to slow us down. Basically a complex version of planetary-slingshot.

My argument is, given a century, we are likely to be able to find some way to decelerate. Consider our accomplishments in the last century (airflight, spaceflight, computers, nuclear, internet)

The amount of energy required to beam a signal through galaxies far exceeds the capabilities of any probe.

>A century ago, almost everyone thought that stepping on a divine body like the moon is absolutely impossible for humanity.

A century ago we DEFINITELY conceived of visiting a divine body. 121 years ago a science fiction movie about landing on the moon was released, based on a scifi book released _158_ years ago.

>if a ship constantly accelerates in space, it can reach another star system in a few decades

Try a few dozen thousand years, my dude... https://www.universetoday.com/141407/how-big-would-a-generat...

Even Project Orion, which is literally strapping many successive nuclear bombs onto the bottom of your craft, would take 130 years to get to our closest star.... and that's not taking into account stopping time, so it'd take 130 years to do a flyby of Alpha Centauri at 3% of the speed of light.

Not to mention the fact that ALL spaceships bleed air even under optimum conditions. A generational ship would need to be HUGE and have even bigger reserve supplies of air and water than you'd ever think possible. 50,000 years worth of air and water for 100-500 people? That's a LOT of mass.

Beyond ion propulsion and Project Orion we start to get into wormholes, black-hole-gravity-assists, remote-quantum-entanglement, time travel. Stuff that's theoretically possible kinda sorta if you completely ignore some fairly fundamental laws... the tricky thing is EVEN IF you choose to ignore those fundamental laws, the amount of sheer energy you're talking about is like the many multiples of the energy output of our sun over its _entire lifetime_.

> A century ago we DEFINITELY conceived of visiting a divine body. 121 years ago a science fiction movie about landing on the moon was released, based on a scifi book released _158_ years ago.

I said "almost everyone". If you go to 1920s and ask a million people if humanity will be able to walk on the moon within their lifetimes, not a single one would say yes. And yet, we were able to do it. Similarly, we have been conceiving of interstellar travel for decades.

> Try a few dozen thousand years, my dude

With constant acceleration, ship time is very different. With constant acceleration using controlled nuclear fusion, we can do it in less than a century. Project Orion is not constant acceleration. Now before you point out issues in constant acceleration, remember that if you were in 1920s, you (and I) would also point out issues in moon landing. How can humanity ever leave Earth's gravity, go accurately to the moon, decelerate, LAND without crashing, walk, LAUNCH from moon again, go accurately to the earth, handle extreme atmospheric temps, and LAND without crashing.

Well, we did all of that. So surely, given a century, we can figure out interstellar travel.

I suspect you're getting ion propulsion confused with nuclear fusion. In order for deuterium and tritium to be useful for fusion, you're going to need kilograms of it, not individual atoms. And tritium is required to be radioactive for it to be useful for fusion, but it has a halflife of about 12 years, meaning any fuel we take with us will be useless pretty quickly. And that's even before you take into account the idea of "constant acceleration"... some sort of magical sustained explosive reaction that would go on for 100 years? And that's after sourcing the unobtanium required to shield/support the "constant" detonation.

I suspect you're thinking of an ion drive, which uses fancy magnets to accelerate mass (pea-sized pellets all the way down to individual molecules), which is a slow start but is fine for constant acceleration over tens or thousands of years.

The problem there though is that the acceleration is SLOW, much less than 1G, hence the 20-80-thousand-year estimate to get to our closest star.

>If you go to 1920s and ask a million people if humanity will be able to walk on the moon within their lifetimes, not a single one would say yes.

I'm not sure I agree with that. Spaceflight and lunar (and even Martian!) colonisation was a pretty common scifi idea by 1920. Cars and planes were already 20 years old by that point. If you asked a million people in 1920 I'm pretty confident you'd get a decent chunk of people saying "yes".

Any serious fusion proposal will breed tritium. Power to weight is going to be abysmal though.

Also slow acceleration is not the millenia long problem you're making it out to be, 0.01g will get you to 0.1c in under a century. Ion drives are largely limited by the weight of the power source even with current tech.

Any fusion reactor capable of working without prohibitively large radiatiors will have enough control over the plasma to just eject it for thrust directly. ISP will be your limit, not thrust.

Q-drive is a fairly compelling proposal compare to other options. Use some kind of magnetic solar sail to soar along the heliopause until you can make the trip in decades.

>0.01g will get you to 0.1c in under a century

Okay, cool, but that's TWO centuries including the flip-&-burn halfway through.

Also, an NSTAR thruster gives 0.000092 m/s2 for a single-ton payload. That'll take roughly 10,000 years to get to 0.1c, not 100 or 200 years.

Try this : https://en.wikipedia.org/wiki/Project_Daedalus

Also, I agree that the idea for interstellar travel has some holes. Let me rephrase my argument as follows : for every accomplishment of mankind (airplanes, spaceflight, moon landing, nuclear, computers, internet), there have been just as many critics claiming that it isn't possible (they had their own set of reasons). And all of these technologies were much more infeasible compared to interstellar travel (and yet we did that).

5 years ago, would you have imagined ChatGPT or Stable Diffusion? Tech moves fast. There is no doubt in my mind that in a century, we will see interstellar travel like we do airplanes today. But this may well be wishful thinking because I really want this to be true.

>Project Daedalus

That's not a manned craft, which is what we've been talking about so far. It also doesn't have the deceleration stage - Daedalus would flyby Barnard's Star at 12% the speed of light. If you hit a rock along the way at that speed, you're not gonna get any pretty pictures :-)

>5 years ago, would you have imagined ChatGPT or Stable Diffusion?

Absolutely! I think most people in my industry would have imagined ChatGPT or Stable Diffusion as they appear now 20-30 years ago, let alone 5.

>And all of these technologies were much more infeasible compared to interstellar travel (and yet we did that).

See... again, most of the problems faced by the "smaller" feats like airflight are engineering problems, not "this breaks Einsteinian physics" problems. When you're talking about interstellar flight you're running up hard against many fundamental constants of the universe, not just "we can't do that yet"

Side note: Sorry to spoil the apparently impeccable record, but for completeness:

A half-century ago, civilians could experience supersonic flight. Well, it has since become impossible.

A half-century ago, people walked on the Moon. And not once since.

Just a reminder then that unfortunately the passage of time alone is insufficient to guarantee technological progress. Now, let's get back to work, I guess...

Absolutely. I wonder if say, cheap energy, will be as readily available for non gov / army purposes in 50 years.

The comment your responding too sound like someone born in the 60´s or 50’s. ( meaning : tech is always progressing, no limit, forward and upward . It’s nice. But I don’t think it match 2023 )

Energy is available more cheaply now than at any time in history, and the cost is still falling at an exponential rate.

You can just set out a solar panel and collect hundreds of watts. The only way that could be lost is if civilization collapses so solar panels can't be made anymore. Even if that happens, most of humanity would quickly starve, and whoever is left would find plenty of panels to salvage.

Yes, it is absolutely the cheapest in history. Or maybe that was in 2008 actually, when the oil producing countries organisation let us know that we reached peak oil production. Most likely that was the cheapest in human history.

What I’m saying is that it’s not going to be cheaper.

We’re gonna have to work more and more for it. That’s fine.

Back to the solar panel. Can we produce, operate, service them at scale without the support of cheap petrol?

Can we sustain the industrial process without the gas to carry stuff from one place to another?

Can we build those solar panel in a lighter industrial society?

I think so yes. But we need to be drastically more frugal.

Specially in place like North America and Western Europe. I also welcome that as a nice change.

To put some context : let think about the effect of COVID-19 on the global logistic. I think that was not even a warming round.

More specifically to your point :

- yes, you can collect hundred or watt during the day. Again, it’s great.

- yes, you can salvage them and operate them. For 10 years. Then you need to find a fabric of photovoltaic cells to replace them :)

For example of low tech long term energy I would rather pick wind ( basic wind turbine ) or water ( good old dam and turbine )

My point in one sentence : solar is great, but you need the whole modern industrial complex to use them and that complex is about to be shocked by the scarcity of gas.

Solar and wind are still getting cheaper, with no end in sight. Solar, in particular, will soon drop below $10/MWh. Printed thin-film perovskite cells delivered in rolls will sharply undercut prices for rigid panels.

Silicon cells last well beyond 30 years. Even after 40 years, most will still produce 80% of nameplate power; longer if they are kept cool by floating on reservoirs.

Manufacturing relies on energy available. That will increasingly come from renewables as that fraction of generation increases, so dependence on fossil fuel for that will fall in direct proportion. Transportation still relies on petroleum, but it will soon be cheaper to synthesize fuel using renewable energy; ocean shipping will move to ammonia.

There is no scarcity of gas. There will instead be a surfeit as demand falls off, to the point where it will not be worth extracting at a price competitive with renewables. It will still be used where its carbon is needed, although it will see competition from captured carbon.

We do not need to be frugal. We just need to build out renewables faster.

> there is no scarcity of gas.

Is gas not by default, scarce? In the sense that we know that their is limited reserved ? ( as opposed to infinite reserve of solar energy for instance )

I thought OPEC was more clear on that, I found peak oil date in the 2030/2050 range. My understanding was that it was behind us already. Good.

What about storage of solar? We know demands is higher when production is lower ( during winter, in the evening after work and so on )

Don’t we need a second source to complement with something like gas/coal/nuclear ?

> shipping will move to ammonia.

Never heard of it. Great.large ship do consume a lot and our logistic need them.

Solar panels require some pretty advanced supply chains, but anyone with an engineering textbook, some wire, and some bamboo can gather a few kW of wind for years with a few weeks work.

you are right about this, but my argument is that we can do this if we want to. Whenever humanity set its sights on a big project such as Apollo or Manhattan, we have accomplished "impossible" things.

Human history is one of both pessimism and optimism. Definitely the optimism of the US in the 1950s and 60s was helped along by the destitution of the Great Depression (1930s+) and WWII. And there was still a lot of suffering and reason to be pessimistic in that time. (And the 30s were a direct result of the 20s.)

I’ve also seen the studies on human cycles suggesting a link between how long ago a period of massive suffering occurred and the propensity for people to entertain ideas likely to bring about great suffering. Simply, people unable to forget the horribleness of war will fend off the little and big things that can lead to war much better than people who have never seen war.

I hope humanity can regain some optimism and direction without having to go through a world war. Or similar.

I find it worrying how much modern fiction is dystopian. I would call Game of Thrones dystopian, for instance, and it was immensely popular. Utopias like Star Trek are often ridiculed, for counterpoint.

Granted, internet comments are generally worthless. The signal to noise ratio in HN comments has gotten extra horrible of late, for that matter. It could be just the low lying fruit for people to get upvotes. But it degrades the site.

I think the difference is that your historical examples of skepticism (which are a bit exaggerated, btw. Da Vinci was a pretty bright guy who thought human flight was possible; Churchill summed up the current understanding of nuclear physics 99 years ago as "Might not a bomb no bigger than an orange be found to possess a secret power... to concentrate the force of a thousand tons of cordite and blast a township at a stroke?"; Konstantin Tsiolkovsky described the science of space travel by rocket in The Exploration of Cosmic Space by Means of Reaction Devices back in 1898)... your examples of skepticism are based on ignorance, not science. As far as I'm aware, nobody ever had a scientific rationale to think humans couldn't achieve flight. If so, it was obviously extremely flawed. We have well-supported science to support the idea that interstellar travel at human timescales isn't possible.

But any scientist will also tell you that there are potential methods to achieve interstellar travel that are consistent with our laws of physics. It's just that, unlike your examples, it's not a matter of human ingenuity to find them. It's a matter of the very nature of our universe whether things like wormholes or negative energy exist or not.

I think the big thing is that all those original achievement don't defy physics to achieve them. It was a case of ignorance and the belief that we just were not that smart. And while there was a big period of exponential growth - there is an upper limit.

We aren't going to Galaxies in a few hundred years - the time to Andromeda is 2.5 Million Light Years. Unless we fundamentally have physics wrong, we aren't doing that any quicker no matter how hard we try.

2.5 million years earth time but ship time is totally different.

Imagine being blasted with 2.5M years' worth of relativistic IGM protons in only a few years of ship time.

A couple centuries ago, the brightest minds thought that humans flying in the air would take a million years.

Two centuries ago humans actually flew.

A century ago, almost everyone thought that stepping on a divine body like the moon

Nobody seriously thought the moon was a 'divine body' a century ago. The rocket equation was known and understood to form a possible theoretical basis for interplanetary travel technology.

A century ago, everyone thought it was impossible to fling a tiny device to blow up an entire city.

HG Wells thought it was possible as did many others.

https://www.bbc.com/news/magazine-33365776

"Churchill grasped the danger of technology running ahead of human maturity, penning a 1924 article in the Pall Mall Gazette called "Shall we all commit suicide?". In the article, Churchill wrote: "Might a bomb no bigger than an orange be found to possess a secret power to destroy a whole block of buildings - nay to concentrate the force of a thousand tons of cordite and blast a township at a stroke?""

Now, we think that it would be impossible to travel very long distances in space. Well, this challenge seems a piece of cake compared to flying in the air, walking on the moon, or generating unimaginable energy from atoms.

We think it's very difficult compared to the challenge of flying through the air or walking on the moon or generating energy from atoms in part because we understand those past challenges and can reasonably compare it to something like interstellar travel. It is decidedly nontrivial.

I agree with your conclusion, but I always find that these discussions miss the role of medical science.

The key parameters governing feasibility of human interstellar travel are the average human lifespan in years and typical interstellar distance in lightyears. These are roughly 72 years and 5 lightyears, respectively. So it takes about 70% of a human lifespan to travel a typical interstellar distance at 10% of the speed of light. This is... not great.

The key realization is that the relationship of these two numbers is in a sense an accident of biology and astronomy.

It is usually assumed that future technological progress might enable travel at a large fraction of the speed of light, but it is often missed that future medical progress might extend human lifespan to say 400 or 1000 years.

A scarier realization is that intelligent aliens might have natural lifespan that make interstellar travel seem far more practical.

Human hibernation should be able to solve that problem. I know I am asking for too many things but the challenges for the Apollo mission were even more extravagant.

They had to figure out how to launch from earth's gravity, go exactly to the moon, decelerate, land without accident, launch back again, go exactly to the earth, decelerate and land without accident. Atmospheric reentry is almost 25000 degree Celsius, hotter than almost anything we had ever experienced.

So, given a century, surely medicine will figure out hibernation. And then humans can continue their lifespan when they reach their destination.

https://cantrip.org/slow.pdf

I think the situation with interstellar travel is well summed up in this conversation between TARS and Cooper as they attempt to dock at Endurance (intentional pun hiding in movie title):

TARS: It's not possible.

Cooper: No, it's necessary.

It seems impossible, but if we don't pull it off we die. And I don't mean we the few folks on HN. I mean everyone.

Earth is a graveyard of species. Nearly all (99% IIRC) species that ever existed have gone extinct (most before we showed up). As a species, we can't stay here and live. A 9-mile thin layer of gas over a single small planet is just not a reliable long-term basis of survival. In fact, the fragility of our tiny little habitant is on display right in front of us this century.

That’s a very anthropocentric, exceptionalist take.

Earth is as much a graveyard as it is a womb and a home and a paradise.

It’s frail compared to what’s outside of it. But it has quite a record (the only one known so far) of making life possible, for millions of years.

There’s nothing we know of this scale and capacity and resilience and welcoming to us (not only humans).

tbh I never really understood that conversation. TARS is a machine and if it's impossible from an advanced machine's standpoint, it's impossible from a physics standpoint. Cooper's maneuver was pretty simple that TARS should have been able to simulate and figure out.

Regardless, I agree that being multi-planetary is absolutely a requirement for humanity. We are too risky here. And the risk is not from outside. Humanity has survived for 100s of millions of years without as asteroid annihilating us, and I don't see it changing now).

The biggest risk to humanity is humanity. We will probably figure out a way to destroy ourselves, either with something much more powerful than nuclear bombs or some new unimaginable tech.

Single planetary system isn't much more of a protection than a single planet against some of the possible extinction events, e.g. ones associated with the activity of the Sun or intelligent aliens.

> Humanity has survived for 100s of millions of years without as asteroid annihilating us, and I don't see it changing now).

Homo sapiens is thought to have existed for mere 300,000 years. Our last common ancestor with the chimpanzee [1] existed no more than 13 million years ago. K-T extinction event [2] that wiped out dinosaurs happened 66 million years ago. There have been numerous extinction events before and after that [3].

On the timescales of species this planet isn't safe. At these timescales rare events become regular occurrences.

[1]: https://en.wikipedia.org/wiki/Chimpanzee%E2%80%93human_last_...

[2]: https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_e...

[3]: https://en.wikipedia.org/wiki/List_of_extinction_events

Every time this topic comes up somebody suggests constant acceleration, and every time somebody has to point out that even if you don’t want to decelerate the relativistic rocket equation is still a very harsh mistress. Remember you don’t just have to carry the fuel to accelerate your payload, you have to accelerate the fuel as well.

You may say, “Oh, but I could use a laser to accelerate my craft, that way I don’t need to carry the fuel on board,” and the amount of energy required is still going to be a hell of a lot more than you think.

Interstellar travel is downright easy on time scales of hundred to thousands of years. We just need to make humans last long enough.

[1]> According to wikipedia, interstellar travel at 1G would take approximately 1 year + the distance in lightyears. Proxima Centauri (4.2 light years) for example would take 5.2 years.

This^ is from the traveller's standpoint. I am not completely sure about how long it would take from the perspective of someone on Earth. But this definitely seems plausible if we can solve the other problems with constant acceleration such as sufficient fuel and protection from interstellar dust (at close to light speed, dust would hit with unimaginable force).

[1] https://space.stackexchange.com/questions/840/how-fast-will-...

Maintaining a 1G acceleration for years is either impossible or so hard it may as well be impossible. Even if you had a magic drive that could do it you would soon reach velocities where colliding with the smallest spec of dust would be like a small nuclear bomb. 1G to Proxima Centauri would reach a peak velocity of .95 c and hitting a 1g particle would release 48451 tons of TNT of energy. So you would need BOTH a magic drive and a magic shield.

There’s one thing about interstellar travel you should bear in mind. If you plan on visiting an alien world you should take into consideration the time needed to decelerate. And once you start decelerating, time dilation will gradually stop and you’ll spend an eternity getting to your destination.

Using fusion for interstellar travel could work, but it would be extremely slow due to the low availability of fuel. Maybe it could be done for synthetic lifeforms or cryonic preservation. Generation ships and such are out of the question though.

> low availability of fuel

Isn't hydrogen one of the most common elements in the universe? And nuclear fusion needs only hydrogen (technically deuterium and tritium, but where there's hydrogen, deuterium and tritium will also be there)

Sorry, no tritium.

If interstellar travel is as "trivial" as you argue, then the great filter starts to look a lot more scary to me.

With the assumption that interstellar travel is trivial for a sufficiently advanced civilization, Fermi Paradox becomes a true problem. The best solution for Fermi Parafox, in my opinion, is interstellar travel being impossible.

I don't think Dark Forest is possible because consider how different our thought process is from any other life form on Earth. And due to butterfly effect, every civilization's thinking will be completely different. The chances that every civilization will converge their thinking into dark forest seems impossible. Some stupid civilization is bound to broadcast their existence and location. Some other stupid civ might try to control all galaxies using self-replication (exponential growth). So we should have seen some such sign (considering how many alien civs are likely to be there)

Great Filter is another possibility. But it feels unlikely : there are trillions (10^12) of galaxies and trillions (10^12) of planets in each galaxy. So a total of 10^24 (1,000,000,000,000,000,000,000,000) planets out there. Consider the number of civilizations. Is it possible for every single advanced civilization to be destroyed by the Great Filter? If it is, then it must be one hell of a filter. I am very excited to find out what it is, if it happens in my lifetime.

Magical thinking unfortunately.

what is magical thinking? Interstellar travel being possible or impossible?