1 hour ago, TheVat said:

Like I said, we can only dimly imagine a K3 society and struggle to postulate what their technosignatures might be.

Life is a particularly efficient mechanism for converting high grade energy (eg. solar radiation) into waste heat (eg in extremis, CMBR). The more life (and associated low entropy structures like Dyson swarms) the more waste heat. The thing about waste heat is, by definition, it's useless. Even for K3 civilisations.

Like c, the 2nd Law appears to be another fundamental hard limit to the spread of advanced technology societies.

Red Shift explained!

To me the Kardashev Scale has always sounded like something for an SF character to say in an infodump. I'm not convinced any sane civilisation would be that insatiable and it sucks as a measure of a civilisation's technological advancement.

Edited May 31May 31 by Ken Fabian

16 hours ago, swansont said:

Assuming you have a bunch of antimatter, because getting and keeping a bunch of antimatter is simple.

Getting is fairly expensive ( at least at our technological level ), but 'keeping' seems fairly straightforward; apparently CERN ships it to other research facilities in trucks.

3 hours ago, Ken Fabian said:

Red Shift explained!

To me the Kardashev Scale has always sounded like something for an SF character to say in an infodump. I'm not convinced any sane civilisation would be that insatiable and it sucks as a measure of a civilisation's technological advancement.

I have wondered if K3 just missing some key inflection point where civs realize they need their biosphere and move towards technology that is more efficient (like molecular computers instead of entire-star-sucking stacks of silicon) and producing less waste heat. And/or their energy goals and demographics shift massively away from the large-scale. @sethoflagos makes the point about 2nd law limits - I only toss "funneling waste heat" in there hoping sharper engineering brains will clarify where that's "rubber science."

For all we know there's some arc that bends from K 0.8 to K2 to gentle eco-yogis sitting on rotting logs swapping droll anecdotes about their past brute technology phase. Unless of course Skynet goes live and takes over.

9 hours ago, sethoflagos said:

Life is a particularly efficient mechanism for converting high grade energy (eg. solar radiation) into waste heat (eg in extremis, CMBR). The more life (and associated low entropy structures like Dyson swarms) the more waste heat. The thing about waste heat is, by definition, it's useless. Even for K3 civilisations.

I need to dig out Charles Stross's mindbending novel "Accelerando," where almost every bit of mass in the solar system is converted to computronium. IIRC he is one of the very few speculative fiction writers who at least tries to address the waste heat issue. (But given his civilization is just our solar system, I guess he can just punt)

5 hours ago, MigL said:

Getting is fairly expensive ( at least at our technological level ), but 'keeping' seems fairly straightforward; apparently CERN ships it to other research facilities in trucks.

They “shipped” 92 antiprotons, and it wasn’t exactly simple

https://www.smithsonianmag.com/smart-news/in-a-first-the-worlds-most-expensive-and-volatile-substance-antimatter-traveled-by-truck-180988431/

It was a trip around the site, too. Nothing was shipped to another lab.

The current apparatus – which includes a superconducting magnet, liquid helium cryogenic cooling, power reserves and a vacuum chamber that traps the antiparticles using magnetic and electric fields – weighs 1000 kilograms: much more compact than BASE or any other existing system used to study antimatter.

“To reach our first destination – our dedicated precision laboratory at HHU in Germany – would take us at least 8 hours,” says Christian Smorra. “This means we’d have to keep the trap’s superconducting magnet at a temperature below 8.2 K for that long. So, in addition to the liquid helium , we’d need to have a generator to power a cryocooler on the truck. We are currently investigating this possibility.”

https://home.cern/base-experiment-cern-succeeds-transporting-antimatter/

1 hour ago, swansont said:

They “shipped” 92 antiprotons, and it wasn’t exactly simple

IOW, it has been done, and you admit the technology is available ?

Maybe, in a few centuries we ( or a much more advanced civilization ) will be able to 'get' antimatter more efficiently and in larger quantities ( perhaps even aboard a spaceship ), and I'm sure it will be easier to 'keep' by then.
We may not even need to 'keep' it, only producing it on-board as needed, once we figure out the mechanism that produces matter or antimatter in excess of the other, as in the early universe, that led to matter predominance.

All speculative, of course, but it has a basis in accepted science.

@TheVat Getting a bit off topic here, but an nteresting discussion.

Civilisations that cannot ever have enough or be enough don't sound healthy to me. Whilst I too like more (something built into our biology? Defense against hard times?) my idea of adequacy and sufficiency is relatively modest - extravagant compared to someone a century ago, yes, or even compared to the current global average but forever striving for more and more, into perpetuity? It doesn't appeal that much. An EV and some more solar to power it is something I would like, yet getting that wish fulfilled would reduce my energy consumption.

Good food - fresh baked sourdough included - good company, interesting and fulfilling work and hobbies, music, art, garden, nature... feeling economically secure and safe with accessible healthcare... Sure, lots of people do want muscle cars and monster trucks and homes with twice as many (designer) bathrooms as residents and no limits but I'm not convinced such consumerism is a natural and unavoidable condition, let alone healthy or even, ultimately, is able to provide genuine deep gratification. The epitome of civilisation? I don't think so.

Living sustainably within closed systems - which elevates energy efficiency above gross consumption - seems an unavoidable challenge for any intelligent species and succeeding doesn't look like inadequate ambition to me. On the contrary it looks like critical measure of technological advancement to my mind. And I'm inclined to be philosophical about mortality, both individually and for our species.

@MigL Like with fission and fusion I think using anti-matter for energy means more 'dry mass' and that will undercut some of the gains. Undercut all the gains, unless the ratio of 'tank' to antimatter is vastly improved. It sounds like another of those hypothetically possible but practically impossible things. Suitable for speculative thought experiment style fiction, not for a tangible goal to commit to.

Why CERN hasn't moved to the far easier dilithium crystal containment is truly perplexing.

10 minutes ago, Ken Fabian said:

Living sustainably within closed systems - which elevates energy efficiency above gross consumption - seems an unavoidable challenge for any intelligent species and succeeding doesn't look like inadequate ambition to me. On the contrary it looks like critical measure of technological advancement to my mind.

If there were a editing button for hearty applause, this summation would have me pushing it. Smaller and green/autonomous homes and rakes instead of leaf blowers would be baby steps away from thermodynamic crisis, but they are the sort we need to start taking. (I find it interesting recently reading that the nations with the largest houses (US, Oz) are among those with currently the most social isolation and least socializing in those houses - we seem to have vast districts of lonely and carbon-intensive castles now)

To bring this (sort of) round to topic, I have to wonder if there are alien civilizations which are felled by the culture of plenty (built into their biology, too) and then emerge from the ruins with a distinct technophobia. Which could be how space faring ambitions end for some sentient species. Or some species could turn back even sooner, say after a catastrophe at the WW1 stage of chemical weapons. Or WW2's urban obliterations. With zero data, I couldn't be sure but what humans are a rare extreme of stubbornness and self-destructiveness in the galaxy (and I acknowledge this is also a well-worn sci-fi trope). And there's also the Dark Forest types of theories, where civs suppress technology that would show visible technosignatures and attract potential "wolves."

1 hour ago, Ken Fabian said:

Civilisations that cannot ever have enough or be enough don't sound healthy to me. Whilst I too like more (something built into our biology?

When you look at it from that point of view, interstellar travel makes even less sense.
The cost-to-benefit ratio is astronomical. It doesn't benefit a civilization at all; it benefits the travelers ( or their descendants ) and spreads their genes to neighboring stars ( vanity ? ).

The only motivation that makes sense is curiosity and knowledge.

@MigL The sorts that consume planets and stars may better suit being the Great Enemy of SF and fantasy storytelling than shining examples to emulate. Whilst natural physical phenomena seem amenable to a sound understanding without needing to go and see everything (do we need to visit a black hole to understand it?) biology seems to be the phenomena most likely to surprise - and it will be a rare example that announces itself so freely as humans have been doing. Curiosity seems saner motivation than colonising and traveling between stars may be better to have transporting observation and communications technologies and knowledge as primary intention than live crews. As some SF character would say, aliens are alien. Biology throws up some really odd things, but I do think a species that sought unconstrained expansion would not be comfortable neighbors.

@TheVat A serious societal collapse would be hard to fully recover from even without gaining an aversion to advanced technologies. I think the dependence on the things taken for granted may be more critical than the things that are hard - without them the hard becomes impossible. Advanced technological self sufficiency in isolation, without global trade seems difficult. A wealthy economy without the easy, never before exploited mineral ores - high dependence instead on recycling from landfills and abandoned cites, when, with all our capabilities we still find recycling our wastes back to virgin quality materials mostly uneconomic - would be difficult to achieve. We rely a lot on natural soils and the foods from un-irrigated dryland agriculture, the least cost sorts; without them I think many kinds of technologies - products of economies with resources to spare - would be harder again.

13 hours ago, MigL said:

IOW, it has been done, and you admit the technology is available ?

Yes. My objection was to the “simply” as it glosses over the details, which is my main objection in these discussions.

8 hours ago, Ken Fabian said:

Advanced technological self sufficiency in isolation, without global trade seems difficult.

Effectively limitless solar energy and an efficient means of waste heat disposal is enough 'free income' to drive a stable economy of any degree of technological complexity. Ask the termites.

8 hours ago, Ken Fabian said:

A wealthy economy without the easy, never before exploited mineral ores - high dependence instead on recycling from landfills and abandoned cites, when, with all our capabilities we still find recycling our wastes back to virgin quality materials mostly uneconomic - would be difficult to achieve.

... within an effectively unchecked capitalist system that prioritises short-term growth over long-term sustainability regardless of the long-term consequences. Other forms of societal organisation are possible.

"Society exists to transcend itself" - Proudhon

11 hours ago, Ken Fabian said:

Living sustainably within closed systems - which elevates energy efficiency above gross consumption - seems an unavoidable challenge for any intelligent species and succeeding doesn't look like inadequate ambition to me. On the contrary it looks like critical measure of technological advancement to my mind

With @TheVat , that makes three of us. Only a few billion to go.

3 hours ago, sethoflagos said:

Effectively limitless solar energy and an efficient means of waste heat disposal is enough 'free income' to drive a stable economy of any degree of technological complexity. Ask the termites.

How much solar energy is avaliable in interstellar space?

6 hours ago, dimreepr said:

How much solar energy is avaliable in interstellar space?

An order of magnitude estimate would be what’s available at night during the new moon (but without the atmospheric losses if you use a measurement on earth)

200 microlux, according to https://en.wikipedia.org/wiki/Orders_of_magnitude_(illuminance)

That’s of order 10 microwatts/m^2, so it will be a bit higher in space.

14 hours ago, swansont said:

An order of magnitude estimate would be what’s available at night during the new moon (but without the atmospheric losses if you use a measurement on earth)

200 microlux, according to https://en.wikipedia.org/wiki/Orders_of_magnitude_(illuminance)

That’s of order 10 microwatts/m^2, so it will be a bit higher in space.

I'm guessing that number wouldn't be enough to realistically sustain, edit/ intelligent, life on an interstellar craft, even an unmanned one.

Edited June 2Jun 2 by dimreepr

56 minutes ago, dimreepr said:

I'm guessing that number wouldn't be enough to realistically sustain, edit/ intelligent, life on an interstellar craft, even an unmanned one.

You need a thermoelectric generation from a nuclear source. That's what's available atm.

Edited June 2Jun 2 by StringJunky

8 minutes ago, StringJunky said:

You need a thermoelectric generation from a nuclear source. That's what's available atm.

Indeed, how much do we need, to last forever?

Without a source?

Though interesting, discussions on "realistic" interstellar travel based on our current understanding of mainstream physics and technology is a bit pointless. Delving into the engineering requirements alone, aside from human biological limitations and vulnerabilities shows that its basically practically impossible to go any significant distance.

Its pretty obvious that "if" aliens are doing such then they have discovered either physics we are yet unaware of and technology which is beyond our understanding.

All our realistic assumptions on this subject are, and can only be based on human experience.

The reality being that if interstellar travel is practically possible in a meaningful and useful way for humans, then traditional reaction propulsion systems are not the way forward.

This then brings us back to what we currently consider as "sci-fi speculation", warp drives, anti gravity mechanisms and so forth. This is what Bob Lazar's claim has been since the 1980's to this day. Whether you believe his story or not is irrelevant. What is relevant is that he claims a type of "gravity warp field" is the mechanism which the craft operates with. Sci-fi, hoax or other, objectively this makes more sense for interstellar travel than slow, inefficient reaction propulsion systems (ironically what Bob was an accomplished engineer in).

The point being, as outlined originally, reaction propulsion systems are a pointless mode of interstellar travel. You can tackle the physics and engineering all day long but the conclusion will remain the same. Either aliens have never been here or ever will, or they have and we have yet to learn how to do it ourselves.

1 hour ago, Intoscience said:

Though interesting, discussions on "realistic" interstellar travel based on our current understanding of mainstream physics and technology is a bit pointless. Delving into the engineering requirements alone, aside from human biological limitations and vulnerabilities shows that its basically practically impossible to go any significant distance.

Its pretty obvious that "if" aliens are doing such then they have discovered either physics we are yet unaware of and technology which is beyond our understanding.

All our realistic assumptions on this subject are, and can only be based on human experience.

The reality being that if interstellar travel is practically possible in a meaningful and useful way for humans, then traditional reaction propulsion systems are not the way forward.

This then brings us back to what we currently consider as "sci-fi speculation", warp drives, anti gravity mechanisms and so forth. This is what Bob Lazar's claim has been since the 1980's to this day. Whether you believe his story or not is irrelevant. What is relevant is that he claims a type of "gravity warp field" is the mechanism which the craft operates with. Sci-fi, hoax or other, objectively this makes more sense for interstellar travel than slow, inefficient reaction propulsion systems (ironically what Bob was an accomplished engineer in).

The point being, as outlined originally, reaction propulsion systems are a pointless mode of interstellar travel. You can tackle the physics and engineering all day long but the conclusion will remain the same. Either aliens have never been here or ever will, or they have and we have yet to learn how to do it ourselves.

Quite. Putting it another way, if our models of physics are correct aliens have not visited us and, we can predict, will not do so in future either.

Edited Thursday at 08:44 AM4 days by exchemist

On 6/2/2026 at 9:19 AM, StringJunky said:

You need a thermoelectric generation from a nuclear source. That's what's available atm.

And this issue with those is that to be longer-lived they need to be bigger because longer half-lives mean lower activity for a given number of atoms.

1 hour ago, swansont said:

And this issue with those is that to be longer-lived they need to be bigger because longer half-lives mean lower activity for a given number of atoms.

Yes.

5 hours ago, exchemist said:

Quite. Putting it another way, if our models of physics are correct aliens have not visited us and, we can predict, will not do so in future either.

Yes, so fundamentally based on our current understanding of physics and technological capabilities any meaningful and practical interstellar travel is near on impossible. Unless you can consistently harness huge amounts of resource over a very long period of time.

So therefore we can conclude that IF E.T has visited Earth from outside our solar system then it must imply that either the warping of space and/or the dilation of time can be practically harnessed in some way that we have yet to discover.

53 minutes ago, Intoscience said:

So therefore we can conclude that IF E.T has visited Earth from outside our solar system then it must imply that either the warping of space and/or the dilation of time can be practically harnessed in some way that we have yet to discover.

Not an either/or, given other possible scenarios which don't involve rubber science. Present physics has options, and we also have yet to discover if those have engineering feasibility. E.g. we don't know if we could collimate a laser beam well enough to photonically push a lightsail ship through interstellar distances. Or suspend humans cryonically for centuries or millennia.

BTW, Bob Lazar was exposed and debunked as a hoaxer decades ago, wasn't he?

This dropped into my feed today. Are we being seduced by Fully Automated Luxury Interstellar Communism?

I find his arguments difficult to dismiss.

3 hours ago, TheVat said:

Not an either/or, given other possible scenarios which don't involve rubber science. Present physics has options, and we also have yet to discover if those have engineering feasibility. E.g. we don't know if we could collimate a laser beam well enough to photonically push a lightsail ship through interstellar distances. Or suspend humans cryonically for centuries or millennia.

BTW, Bob Lazar was exposed and debunked as a hoaxer decades ago, wasn't he?

Collimation is a problem but the real issue is how do you not melt the sails or ship with the intensity you need for a light sail to work with any realistic payload. You need a really big sail.

For light F=Power/c; to get 1N of force (1g of acceleration for ~0.1 kg) you need ~150 MW - you get a factor of 2 from perfect reflection, but it’s not going to be perfect, so whatever absorption you have will heat the target.

You probably don’t want your laser to have to slice through the atmosphere, but if it’s on a craft in space it’s recoiling with that same photon momentum effect you’re using for the solar sail.