4 ms·

A rocket launch from a planet, sure. But something using electric propulsion might not. The exhaust plasma is not super hot, and there's just not that much of

A rocket launch from a planet, sure. But something using electric propulsion might not. The exhaust plasma is not super hot, and there's just not that much of it coming out.

api 7y ago
Nope. If you are doing anything you are doing work and thus converting energy. The second law is inescapable, and it's one of the most solidly established of all physical laws. If we assume the same laws exist everywhere (quite reasonable) then ET must follow those same laws too. First, you have to be generating the energy. No reactor or conversion process can be 100% efficient (second law), so you must reject some heat. If it's solar, you have big-ass solar panels that get hot because they are not 100% efficient. The ISS looks like an orbiting Christmas tree in the infrared. Second, the "rocket" exhaust might be a directed particle beam but whatever is accelerating that exhaust cannot be 100% efficient (second law again). More rejected heat. Then you have all the other stuff going on. If the inhabitants are biological, life support for a long-duration large scale space flight is going to be pretty energetically expensive. Think of growing crops, bulk biosynthesis of nutrients, and a whole lot of recycling which usually involves a lot of endothermic reactions to break down waste products. If the inhabitants are post-biological (IMHO more likely for star travelers) then you have a whole lot of computronium of some kind. That's a big flying data center in space, and everyone knows how much heat those put off. ... I could keep going forever. You can reclaim a bit of heat like we do here on Earth with combined-cycle power plants, but there are absolute, hard, inescapable limits to how much. You also hit diminishing returns pretty fast, and keep in mind that a spacecraft has mass constraints it must work within due to delta-V budget and all that. At some point the graphs are gonna cross where it makes more sense just to reject heat, and any rational engineer will pick that point. You're gonna need a radiator, and there is no way to make that not visible. If you want to hide from the crazy-ass monkeys on the third planet you could try flying with a reflector in the way to block your heat from the Earth-Moon system and reflect it elsewhere, but the problem is that now your reflector will reflect a bunch of incident solar radiation! Last but not least: we are in theory talking about interstellar visitors here. Anything capable of interstellar travel is by definition capable of generating absolutely vast mind-boggling amounts of energy. The only physical power planets we know of that could theoretically propel you to the stars (at a reasonable enough speed) are some mix of fusion and matter-antimatter annihilation. Even turning that monster of a power plant on to "idle mode" is going to say "here I am!" in the infrared... if someone is actually looking. AFAIK we are not currently looking.
- lnanek2 7y ago
  I think you are right they would have ridiculous amounts of energy like you say, to be able to cross space between stars. I don't think that's as much a penalty to detection as you, however. If you have an infinite energy budget, you could easily remove all the heat from the front of your ship via a Peltier cooler and radiate it out the back, for example. Then point your ship at Earth and you are stealthed from heat detection. Not all ships are merchant ships designed to get where they are going as efficiently as possible, after all. Some are designed to get somewhere as quietly as possible.
- tzs 7y ago
  > Last but not least: we are in theory talking about interstellar visitors here. Anything capable of interstellar travel is by definition capable of generating absolutely vast mind-boggling amounts of energy. The only physical power planets we know of that could theoretically propel you to the stars (at a reasonable enough speed) are some mix of fusion and matter-antimatter annihilation. You are assuming that they bring the power plant with them. What if the power plant is back home, used just to accelerate the ship to relativistic speeds, and the only power plant for propulsion on the ship just needs to overcome friction from interstellar dust? For example, they could use a laser assisted light sail for departure. When approaching a destination solar system, they could turn off the on-board propulsion while figuring out if that solar system has any life capable of detecting that propulsion system. If it does, they could adjust their route so that they only use active propulsion when the system's sun is between them and the planet of those lifeforms.

A rocket launch from a planet, sure. But something using electric propulsion might not. The exhaust plasma is not super hot, and there's just not that much of it coming out.

api 7y ago

Nope. If you are doing anything you are doing work and thus converting energy. The second law is inescapable, and it's one of the most solidly established of all physical laws. If we assume the same laws exist everywhere (quite reasonable) then ET must follow those same laws too. First, you have to be generating the energy. No reactor or conversion process can be 100% efficient (second law), so you must reject some heat. If it's solar, you have big-ass solar panels that get hot because they are not 100% efficient. The ISS looks like an orbiting Christmas tree in the infrared. Second, the "rocket" exhaust might be a directed particle beam but whatever is accelerating that exhaust cannot be 100% efficient (second law again). More rejected heat. Then you have all the other stuff going on. If the inhabitants are biological, life support for a long-duration large scale space flight is going to be pretty energetically expensive. Think of growing crops, bulk biosynthesis of nutrients, and a whole lot of recycling which usually involves a lot of endothermic reactions to break down waste products. If the inhabitants are post-biological (IMHO more likely for star travelers) then you have a whole lot of computronium of some kind. That's a big flying data center in space, and everyone knows how much heat those put off. ... I could keep going forever. You can reclaim a bit of heat like we do here on Earth with combined-cycle power plants, but there are absolute, hard, inescapable limits to how much. You also hit diminishing returns pretty fast, and keep in mind that a spacecraft has mass constraints it must work within due to delta-V budget and all that. At some point the graphs are gonna cross where it makes more sense just to reject heat, and any rational engineer will pick that point. You're gonna need a radiator, and there is no way to make that not visible. If you want to hide from the crazy-ass monkeys on the third planet you could try flying with a reflector in the way to block your heat from the Earth-Moon system and reflect it elsewhere, but the problem is that now your reflector will reflect a bunch of incident solar radiation! Last but not least: we are in theory talking about interstellar visitors here. Anything capable of interstellar travel is by definition capable of generating absolutely vast mind-boggling amounts of energy. The only physical power planets we know of that could theoretically propel you to the stars (at a reasonable enough speed) are some mix of fusion and matter-antimatter annihilation. Even turning that monster of a power plant on to "idle mode" is going to say "here I am!" in the infrared... if someone is actually looking. AFAIK we are not currently looking.

lnanek2 7y ago
I think you are right they would have ridiculous amounts of energy like you say, to be able to cross space between stars. I don't think that's as much a penalty to detection as you, however. If you have an infinite energy budget, you could easily remove all the heat from the front of your ship via a Peltier cooler and radiate it out the back, for example. Then point your ship at Earth and you are stealthed from heat detection. Not all ships are merchant ships designed to get where they are going as efficiently as possible, after all. Some are designed to get somewhere as quietly as possible.
tzs 7y ago
> Last but not least: we are in theory talking about interstellar visitors here. Anything capable of interstellar travel is by definition capable of generating absolutely vast mind-boggling amounts of energy. The only physical power planets we know of that could theoretically propel you to the stars (at a reasonable enough speed) are some mix of fusion and matter-antimatter annihilation. You are assuming that they bring the power plant with them. What if the power plant is back home, used just to accelerate the ship to relativistic speeds, and the only power plant for propulsion on the ship just needs to overcome friction from interstellar dust? For example, they could use a laser assisted light sail for departure. When approaching a destination solar system, they could turn off the on-board propulsion while figuring out if that solar system has any life capable of detecting that propulsion system. If it does, they could adjust their route so that they only use active propulsion when the system's sun is between them and the planet of those lifeforms.