K^2

Horizon is modeled as a potential in this paper. It's like if you shine a beam of light into a mirror. Physically, electromagnetic wave induces a change in the electrostatic potential on the metallic coating of the mirror, which influences electric field near it, generating a reflected wave. Mathematically, we describe this as an incident wave entering a mirror and a reflected wave exiting the mirror. Just because that's how the math works out, doesn't mean the beam physically entered some sort of a mirror dimension that happens to have an identical beam shining back out of it into ours. Light simply got reflected without going past the boundary. It's a bit more complicated here, because the boundary is asymptotic due to the expansion, but the end result is the same. We model it as outgoing waves reaching towards the horizon and inbound waves from it, because infinities are hard, and sometimes you have to do strange things with math to get useful predictions out. The whole paper just talks about the universe being a resonance cavity that's expanding, producing interesting effects in the wave spectrum. And this only works because there is a boundary that nothing can reach us from beyond. If gravitational waves could pass and go freely, there would be no resonance, and no amplification.

That the spectrum of the background gravitational waves is going to have features corresponding to the change in dominant mode of expansion of the early universe. It's all in the paper. I have no idea why you think this has something to do with propagation of waves from beyond the horizon, seeing how the paper treats the horizon as a boundary.

The source of the primordial waves isn't beyond the horizon. It's the early inflation of the universe itself. Meaning, the source is effectively local. The article merely talks about the fact that the observable universe acts as a resonance chamber. It's not allowing any waves or information from beyond the horizon to reach us.

No, that doesn't work this way. Gravitational waves obey locality, just like everything else. You can create a space-time in which you can traverse the distance faster, but you have to do so in advance, because the initial change has to propagate through the space-time with the initial speed of light. That's why a gravitational wave, despite being able to modify space-time behind it, can't reach you faster than a light beam would. Anything that's beyond Cosmological Horizon for light, is also beyond the horizon for gravitational waves. We cannot interact in any way with matter that's beyond that point, which means that whether or not it even exists is entirely academic unless entirely new principles are discovered. Within classical General Relativity, the Cosmological Horizon is absolute.

Theoretically, sure. There isn't really an upper bound on how much compression and temperature you can get from accelerating matter towards a common focal point, as in a perfectly spherical case, it is indistinguishable from the action of gravity, and we know stars work. Practically, that's another matter. There's only so much pressure you can generate in the chemical explosion stages, so you have to rely on inertia and hydrodynamics of materials you normally consider solid for the rest. That seems to be the direction the linked article suggests, but any imperfection will lead to a high pressure zone that's going to turn your perfectly focused explosion into a spray. It's a bit like balancing a skyscraper on a needle-point. I'm pretty sure that not only do your shaped charges and plates have to be flawless, you can't even rely on cylindrical symmetry, as the device's own weight will produce enough density variation to cause it to fail. Unless you're going to detonate it in space, you'll have to account for how this device sits on the ground. The fact that no nation has built one, despite the fact that it perfectly circumvents many of the nuclear treaties, is a strong indication that even with modern simulation methods, this is a little too precise.

Yes, it's called a supernova. Short of that, a particle accelerator works, but if you need more than a handful of atoms, you're out of luck.

I'm pretty sure Obsidian kept the rights to The Outer Worlds IP and Hades II is listed as published by the Supergiant, so they definitely kept the rights. I'm sure PD had a lot of IP besides KSP, but most of it wasn't all that big. That said, the fact that the buyer didn't share the news immediately suggests that PD properties will be sold off piece-meal, rather than used to publish games in the near future. So on the KSP front, we'd be waiting to see who picks it up next. If anyone. On the bright side, Unlike T2, who felt like they need to get back what they spent for KSP IP, whoever bought PD is probably not going to have any particular attachment, and will just try go get the best bid. So it's much more likely that the price will be more reasonable, making it more likely that somebody will pick it up, rather than IP just collecting dust.

At a scale height of 11km, it would take 56km before the pressure hits atmospheric. Of course, the temperature will be higher down there, increasing scale height, so call it ~60km. In contrast, you can dig down a few dozen meters and put in an airlock to achieve the same effect.

If you're going to be using ethanol for fuel, you should just have a normal turbofan engines. Ethanol makes for a crap fuel for a number of reasons, but nearly all of them are going to impact your fuel cells the same way as a turbofan.

Legend has it, the topic of reversibility of time is what drove Boltzmann insane. So if all you got is a headache, you got off easy.

Fine grain entropy is conserved. Coarse entropy is growing. In context of a system governed by quantum mechanics, it allows for chaotic observables while being entirely on rails. The easiest (though, not the only) way to interpret this is through the lens of MWI, where it's pretty easy to show that the fine grain entropy of the universe is simply fixed at zero (there is exactly one state), while any particular timeline becomes more chaotic over the course of evolution. The increase in available state-space as the universe inflates leads to significantly more entropy/chaos in the "future" direction, setting up the arrow of time. Consequently, there is no practical difference if our universe is expanding from a point due to a Big Bang, or if it's collapsing in on itself. The future universe is in the future because it's bigger. And all of the chaos is a consequence of observation, while all underlying physics is fully deterministic, because the universe as a whole doesn't have such silly notions as order or chaos or time or entropy. It's just a multi-dimensional manifold with some symmetries, and we're just too small to perceive it that way. Physics is the most depressing of sciences.

Honestly? Why? The KSP2 source isn't really worth working with. Whether it's restarting the project or making a fan game out of it, there isn't anything in KSP2 source worth keeping. Option 1, you want to stick to Unity, in which case, KSP had a lot of fixes that KSP2 never got based on when they branched. Just start with KSP and add KSP2 features. Option 2, forget all of that, and start with Unreal. That way you get a modern physics engine (no problems with bendy rockets, if you set it up correctly) and you get planet tech for free. But then you do have to implement all of the space stuff from scratch, but IMO, worth it. Either way, KSP2 is not what you want. As for the assets, while raw assets are a bit better to work with than ripping baked assets, odds are, you're either going to make small changes to them, or just add new stuff. In which case, ripping KSP2 assets is just as good. In other words, you have everything you need. The big problem is that recycling KSP2 assets is going to set you up for being sued by T2, who'll probably do it just on principle even if the game is fully abandoned. So realistically, if you think KSP2 had good ideas that should be turned into a game, the correct path is to make a new game. New engine, new art, the lot. Yeah, we are all going to miss the Kerbals, but I doubt it's going to make a critical difference to any good stab at the genre.

Ah, yes, landing. Nonetheless, it seems that because the problem happened as the engines were getting re-lit, NASA opted to ground the flights, presumably out of abundance of caution. https://www.npr.org/2024/08/28/g-s1-19934/faa-spacex-falcon9-grounded-polaris-dawn

Pressure isn't as big of a factor for fire risk as it is for, say, breathing. Most of the reduction of reactivity of oxygen in air isn't due to the partial pressure of oxygen being lower, so much as the ballast of nitrogen etc. While it's true that fire hazard of pure oxygen at 0.2 bar is going to be lower than at 1 bar, it is still way higher than for air at 1 bar. A lot of things that won't catch fire in an air mix will catch fire in O2 at 0.2 bar. Unrelated, at least in any direct way, due to the failure of the most recent Falcon 9 launch, NASA has temporarily grounded all F9 flights until further notice. As far as I can tell, nobody really knows how long the grounding is likely to be in effect, but the general consensus seems to be that we should expect Polaris Dawn to be further delayed.

Somebody said this mission gives Titan vibes, and I kind of agree. I have no reason to doubt the capsule itself, but everything about the spacewalk plan is sounding very flimsy.

Which you might be able to achieve by strategically painting the comet.

Not a bad concept, but CO2 kind of sucks to store. You need a bit over 5 bar in a tank to keep it liquid even at cryogenic temperatures. It's not a huge pressure, but it's more than you typically want in a tank, so it is going to contribute to the weight. You can freeze it solid, obviously, but then getting it out of the tank in a controlled manner becomes a bit problematic. (Solid fuel NTR with radiative heat transfer from core to the fuel is a thought, but that would require a very uniform chunk of ice, so I don't think it's an ISRU kind of idea.) So I'm kind of looking back at carbon monoxide now. That'll sit liquid at 1 bar and 81K. Much lighter tank, enough pressure to feed it into the tank with just boil-off, and you'll get better ISP with NTR that way. Oxygen is now a byproduct, and not something you have to hoard for ascent, so you can potentially store it for manned missions (assuming this is like a cargo shuttle or w/e) and you want your nuclear reactor to be able to produce power to condense CO2 out of atmo anyways, so splitting it down to CO doesn't seem like that much overhead. A CO NTR might be the best ISRU cargo shuttle we can reasonably design with tested tech.

I really want an underground bunker, but I'm not allowed to dig here for "earthquake safety reasons." *kicks sand*

Well, FWIW, my Steam copy is marked up-to-date, and the executable is marked as last updated on July 14th, which matches the last patch notes posted on July 11th. In fact, I don't see any files that have been modified after that date. So if some change took place in the depot, it wasn't a change to any of the game's files.

You are forgetting about gyroscopic effect. Torque is additive to the angular momentum. If your rotor spins counterclockwise viewed from above, and advancing blade is generating extra lift, the angular momentum of the rotor points straight up and torque points back. Since torque is the rate of change of angular momentum, that causes precession of the rotor back away from direction of movement. On an articulated rotor of a real helicopter, the related behavior is the phase delay, which is generally significantly less than 90°, so you do get a rolling moment as well. On a rigid rotor of a drone, however, it's pretty much exactly 90°, and if the drone is moving "forward", the increased lift on the advancing blade is causing a "pitch up" moment. I understand why people say that. From a purely pilot perspective, I'd also call it unstable. But holding to physics terminology, a helicopter is a dynamically stable system. It kind of has to be, and precisely because of how unstable it feels. All of the controls on a helicopter are linked to all the other. If you input a cyclic correction, it changes the overall lift, requiring a compensating collective input, which changes rotor torque, requiring a pedal input, which produces a sideways force, requiring a collective correction, which alters lift requiring.... There is a feedback loop going through all of your controls. If the helicopter dynamics was unstable w.r.t. cyclic input, this whole feedback loop would have a growing oscillation. The only way to keep helicopter under control would be to enter the exact amount of correction for all 3 controls at the same time for every possible deviation. A sophisticated flight computer can do this - a human pilot simply cannot, no matter how well trained. And certainly, you wouldn't be able to go from one helicopter to another and fly it with any success at all without complete retraining. Given that human helicopter pilots exist, and they can fly a different helicopter model after training on their first one, we can exclude this. Human ability to fly a helicopter relies on this dynamic stability, however nominal. You are confusing controlled flight with a stable one. You absolutely cannot have controlled flight with two fixed rotors. You are going to drift in a direction perpendicular to the line connecting the rotors, and whatever power you end up dialing for the two remaining rotors (assuming they are counter-rotating), you are likely to have a small amount of net torque left over, which will cause the craft to turn. That means your decent profile will be a spiral that you don't really have any control over, except maybe how rapidly you descend (if there is sufficient power.) But you can keep the craft mostly upright and going down at a reasonable pace, limiting the velocity with which it impacts the ground. It's going to be a crash landing, but if we're talking about saving camera equipment, it's going to be something much more likely to keep it intact than if you just shut off the motors and let the drone drop.

Rotors are actually self-stabilizing - to a point. Consider a rotorcraft with a fixed rotor that is slightly out of balance. It begins to pitch over in the direction I'm arbitrarily going to call "forward". After all, that's where the rotorcraft will begin to accelerate the moment its thrust vector is off from vertical. As the craft picks up speed, the advancing blade is going to generate more lift, producing torque in the exact opposite direction. (Follow right hand rule from advancing blade to the axis of rotation.) As the angular momentum of the rotor is roughly aligned with the line of lift, this produces a moment trying to restore the orientation of angular momentum back to vertical. The faster the craft moves, the higher the torque countering the imbalance. This is actually useful when flying a helicopter, because the pilot doesn't have to constantly adjust the collective as if trying to balance a broom on its end. For a given steady position of the cyclic, there is a steady state of air speed that balances out the off-center thrust. The further the CoM is from CoL, the more air speed the aircraft will have to gain before the torques balance out. In an extreme case, this will lead to a complete loss of stability and a crash. But for a small imbalance, the aircraft will find an equilibrium merely drifting in that direction at a speed that will be entirely acceptable for an emergency landing.

I've never seen that actually implemented. One out is effectively a two out, as any amount of thrust on a diagonally opposite rotor will put you out of balance. And the remaining two rotors are usually spinning in the same direction. So you just lost your counter-torque, putting the drone into an uncontrolled spin, and that will lead to a loss of lift in addition to losing any chance for any sort of attitude adjustment. I don't see how a motor loss can lead to anything other than a crash here. If safety is a major factor, you can set up the orientations differently and have, say, both front rotors turning in the same direction. That would give you opposite rotation on the diagonals, letting you, in theory, perform an emergency landing without a total loss of control. But I've never actually seen a quad with that setup. In practice, every drone I've seen where redundancy was important has been either a hex or an oct.

The entirety of what you need to understand a non-orthogonal DoF control theory is understanding pseudoinverses in linear algebra. In practice, I've never seen this category of problem that wasn't made trivial with Moore-Penrose inverse. If you're getting bogged down in trig, you're overcomplicating things.

Definitely not early on. The surface of the white dwarf is already too hot to glow red, and certainly wouldn't get any cooler in the nova. Of course, most of the light will be produced by the cloud of the debris that is expanding, and that will cool over the several days, maintaining the brightness, for a bit, by the simple virtue of getting a larger and larger cross-section area to radiate from. What I don't know and haven't seen references to is how much it will cool before it stops being visible to the naked eye, so I don't know if we'll see a color starting to shift a few days in or if it will become too dim to see long before that.

Nah, I mean pantograph rides on the wire via maglev. Probably using induced eddies in the wires to create a "cushion" so that there is no direct mechanical contact. I'm sure it would introduce additional constraints on how the overhead wires are constructed, like the material, tension, etc. But maybe that's still viable for high speed trains? Maglev track for the train doesn't, unfortunately, solve the main issue. While moving induction coils to the track would make the train itself almost entirely passive, it would make the track construction way more expensive. Prohibitively so, almost certainly. By the way, in this setup, both the train and the tracks can still have permanent magnets taking up most of the weight, with the induction coils only used for stability adjustments and propulsion. And the track itself can simply be providing a constant AC to the coils, with train's logic adjusting phase differences between on-board coils as well as tapping into them to power lights, electronics, etc on board. Other than, "Each segment of the track now needs a sizable coil of low-resistance wire," this is very workable. Unfortunately, the coils are still pretty expensive and when you start looking at thousands of miles of track, it's a non-starter without something drastically changing in metal prices. Yeah, that's why I suggested hydrogen. You'd be able to cut it down to just a few tons of hydrogen, which should just about fit in a larger tanker car. Not that much, unfortunately. Average freighter is about 4,000 tons. It would take about 100kg of DT at 30% efficiency to get it to 60mph. Going with the above quote of 16T of DT on a locomotive, it'd take you 16 starts and stops just to get to 10% saving. And I'm sure the schedules for freighters are already optimized around having them have to stop as infrequently as possible specifically to save on the fuel. Inclines might be a bigger factor, but here you can't get away with just partial storage. If most of your fuel was used to go uphill, you do need to store all of it back into the batteries on the way down. That's an amazing fuel saving, of course, but it doesn't help you any with reducing the total weight of the necessary batteries. Honestly, a battery-free diesel hybrid that can tap into the grid is probably the best we can do right now. Using the grid as source or storage for both regenerative breaking and going up/down hills where available, and managing on diesel everywhere else. This lets you place wires more strategically, drastically reducing the fuel use, as well as allowing the train to travel slower in these areas - again such as going up or down an incline, while maintaining a higher speed in areas where overhead wires are not available.