yes this is a physics post
so, I have an annoyance with Arthur C Clarke's "third law," as it's used by a lot of authors. "any sufficiently advanced technology is indistinguishable from magic", I read as meaning "technology more advanced than you understand will appear to be magic to you, but will follow sensible rules if you can learn them." a ton of authors seem to read it, instead, by kinda turning it backwards, getting: "any magic can be handwaved away by saying it's sufficiently advanced technology." this has the effect of answering any weirdness in a setting with "a wizard alien tech did it!" which, in a "softer" setting, is perfectly fine! but in a claimed hard sci-fi setting, it's kinda disappointing.
I want to say "this isn't how Clarke would do it", but then I read up on Clarke's Space Odyssey series.
so! in 2010: Odyssey Two, Clarke's now very well-known autonomous alien artifacts, the "monoliths", do something wild: they ignite Jupiter into a new second sun, which humans dub Lucifer. the reason they do this is to nurture life on Europa, like they did humanity 3 million years prior. the stated way they do this is by increasing the density of Jupiter until it ignites, producing fusion energy, just on a smaller scale than the Sun.
so that leaves the question of... how exactly did they do that?
the known monolith masses are pretty light, with the first one discovered being claimed to have a density just slightly higher than air. no other masses were stated, but we'd assume if they could be incredibly dense, that would have come up.
I had thought, "what if we drop some degenerate matter into Jupiter to make it denser?" and it turns out that doesn't really work. for one thing, Jupiter is already there; its core is made of electron degenerate matter. without the pressure of heat from fusion to make a ball of stuff expand, ordinary matter that gets past Jupiter's mass doesn't really get much bigger; all brown dwarves are around Jupiter's size in radius. as you add more mass, the ball starts compressing, to the point a Sun-mass that can't fuse would be around the size of Earth.
When this hits a point beyond 1.44 solar masses, the object can't hold itself together through electron degeneracy pressure alone (this is the Chandrasekhar Limit) and it will compress further, down to a neutron star, dominated by neutron degeneracy pressure instead. unless some more exotic matter we've yet to see shows up, beyond that second limit is where black holes form.
we know that Lucifer in the stories couldn't be anywhere near this massive. it's supposed to have no extra mass to work with, it's just denser; but there's no way that ordinary gravitational pressure alone could cause Jupiter to get small enough to ignite like this. manipulating Jupiter's mass to effectively make it act heavier than it is also would make its gravity stronger away from its surface, so that won't work either (even if the science of Clarke's day couldn't say as much as we can now, about how messing with the Higgs field would be a pretty bad idea).
so what could work? well... either some kind of force being applied that we don't know about yet, that is somehow stronger than the electromagnetic force and can be applied from outside the planet... or inserting a tiny black hole into the core of Jupiter through some means. both of these have problems. the first is just straight up space wizard shit, let's face it. meanwhile, the second, well... making a black hole artificially is very very difficult. trying to do it using the "kugelblitz" method you sometimes hear about, where you just fire enough lasers at one spot that they form a black hole from the light energy alone, is also very hard to make work even for microscopic black holes that would evaporate almost instantly.
the example I saw proposed, claiming it would generate a pretty-cold-for-a-star outside temperature of around 1000K on the surface of Jupiter (after the compression near the event horizon causes fusion to occur in the accretion plasma), required a black hole with a mass of 1,000,000,000,000,000,000 tons. a million million million. something around 170 millionths of the mass of the entire Earth. which as far as I can tell, still would require putting pressure on that much of Jupiter's mass, using that same unknown force that's way stronger than electromagnetism, hard enough that it makes a black hole that can last longer than the universe has existed by an enormous margin. so we're back to space wizard shit again.
and even if this did work, on the time scales that the monoliths work on, this might also be unacceptable. over time, the black hole will keep accreting mass, getting bigger and more powerful, and causing the pressure and heat it applied to increase along with that. in around 100 to 200 million years, Lucifer would become way too hot for any of the worlds orbiting it to survive. this might seem like a long time, but given the monoliths are willing to bury themselves for 3 million years just to send out an alarm signal, they would definitely be thinking on this scale.
so, from what I can tell, the monoliths compressed Lucifer to ignite it, creating a new star from around two orders of magnitude too little mass, by replicating themselves enough to exert pressure on the planet by some unknown means, and in doing so, make Jupiter permanently into a new star without needing to keep pressing inwards on it. the monoliths are supposed to work according to laws of physics, and are limited by the speed of light as a result, so maybe the issue here is that Clarke bought the idea that Jupiter is "a failed star" and just needed a little more mass or pressure added to it, instead of needing 80x its current mass to ignite.
or, maybe I need to accept that Clarke has space wizards in his most famous series*, and I just need to accept that he didn't mean his tongue-in-cheek "laws" quite the way I was thinking.
I still think it's OK to be annoyed at hard sci-fi going "a space wizard did it" tho.
*yes, the Firstborn that made the monoliths are even more of this, being unrestricted by matter or by spacetime anymore and seemingly able to do whatever they want. but the monoliths are supposed to be physical objects bound by our universe's laws... beh