Me, I'm not a physicist and have no idea what I'm talking about, but I do know that if you're looking for a campfire in the desert and the desert is the size of Utah and Nevada, you're going to have a hard time.

Conclusion: you can have stealth ships, and will only be able to counter them by blanketing the volume you patrol with sensors or by deploying your own stealth platforms.

  > You have a stealth ship, only detectable by occlusion

From every other angle, the gas/plasma/etc flying out of the cone would still contain the same amount of heat energy, just dispersed over a larger area. Anything more than a few degrees above absolute zero will stick out like a sore thumb, and you'd have to disperse the heat from a single thruster over thousands of square meters to get close to that.

That's where cheap remote sensor networks probably win. For the cost of your single "thermally stealth" warship, your enemy can likely afford thousands of cheap dumb probes that can spot you from many angles.

"Stealth" will probably staying cold as long as possible. An attack ship/missile could be launched on an initial trajectory from somewhere in space by kinetic (rail gun-like?) means. It then remains dark/cold for most of its journey, until it is extremely close to the target. At that point it could launch chaff/flares and close in for the kill.

Alternately (or complimentarilty? is that a word?) another effective strategy might be to park a bunch of dormant ships/weapons around the area you wish to attack/defend. Dark and cold, they'd be nearly undetectable... until they're needed.

You've jumped to conclusions about the kind of drive it would have. Once launched it would only use mass drivers for course correction. Remember: think ahead. Remember also, that these are only supposed to have limited maneuverability.

That's where cheap remote sensor networks probably win. For the cost of your single "thermally stealth" warship, your enemy can likely afford thousands of cheap dumb probes that can spot you from many angles.

If they are so cheap that they are easily detectable, then enemy HQ can calculate a geometry that would preclude your detecting the ship. Also, I'm not necessarily envisioning a full-on warship. Being unmanned, small, and low energy as possible is a win for craft such as these, so they would also tend towards being inexpensive. On the other hand, the "more, dispersed" counter-strategy is disadvantaged by a factor of N^3. An optimal strategy would consist of multiple stealthed units to achieve dispersal/coverage. This way, your enemy is always guessing whether all of their detection platforms have been located or compromised in some way, opening an attack vector to their vital in system transfer orbits. (Lines of commerce.)

"Stealth" will probably staying cold as long as possible. An attack ship/missile could be launched on an initial trajectory from somewhere in space by kinetic (rail gun-like?) means. It then remains dark/cold for most of its journey, until it is extremely close to the target. At that point it could launch chaff/flares and close in for the kill.

Indeed. But without special provisions, such an attack mothership would be highly visible from vast distances, so again there is a possible application of stealth.

  > If [the sensors] are so cheap that they are easily detectable

I'm thinking there's no reason that the cheap sensors would be anything but tiny, dark, and cold.

Relatively speaking, this is true. But would basic physics render them tiny, dark, and cold enough in the face of a technological arms race? Remember: think ahead. The opposite side would counter by building IR telescopes with enough sensitivity to detect even the minimal differential in IR emissions and correlate that with other emissions and starlight occlusion. Thus the arms race begins.

Remember that we're dealing with societies with several decades of technological advancement over our own. They would have access to vastly greater computational resources and technological manufacturing. The most esoteric and sensitive sensors available to NASA now would be somehow outmoded by units that could be mass produced. Moore's Law and the like still obey basic physics, but there is nothing in basic physics that ultimately limits detection or stealth. (Maybe getting the blackbody emissions of a unit down to the same level as ambient gas in the solar system?) There would eventually be some balance reached between numbers and individual unit potency, subject to one horrendous cloud of war.

  > But would basic physics render them tiny, dark, and cold 
  > enough in the face of a technological arms race? Remember: 
  > think ahead. The opposite side would counter by building 
  > IR telescopes with enough sensitivity to detect even the 
  > minimal differential in IR emissions and correlate that 
  > with other emissions and starlight occlusion. Thus the 
  > arms race begins.

As IR detection improves, it gets easier to detect tiny dark sensors - but the tiny sensors also get the improved IR detection ability, and it really, really gets easier to detect attack ships or missiles that are going to produce heat as a byproduct of acceleration.

Attack ships/missiles that need to carry a payload and/or accelerate will always be orders of magnitude more expensive and easier to detect than a sensor that essentially remains motionless.

And even if Side A knows where all 10,000 of Side B's sensors are located around the solar system, then what? It's a pretty expensive proposition to destroy them. One side could even bleed the other dry that way - for every $1 that Side B spends deploying sensor probes, Side A has to spend $10 or $100 or $1,000 to destroy it.

Ultimately, what the ease-of-detection means in space is that battles will much tougher to win for the underdogs, as it will be harder for them to overcome a deficit in resources with the use of stealth or surprise.

In general. I'm basically arguing that this can be subverted in limited geometric/temporal contexts. Given a sufficiently motivated arms race, it will be exploited.

And even if Side A knows where all 10,000 of Side B's sensors are located around the solar system, then what?

Then if Side B has completely discounted stealth, Side A has the opportunity to circumvent all of Side B's sensors and mount attacks with units that "stay quiet until nearby," as you outlined. Also, if Side B's sensors are so minimalist and cheap, what's to keep Side A from destroying them all, or enough of them to make them moot? If you've completely discounted stealth from a simplistic thermodynamic argument, you've not thought through the nuances.

Ultimately, what the ease-of-detection means in space is that battles will much tougher to win for the underdogs, as it will be harder for them to overcome a deficit in resources with the use of stealth or surprise.

Stealth as I've outlined it isn't going to benefit the underdog. It will benefit the side with the greater industrial and technological resources.

EDIT: Alternately (or complimentarilty? is that a word?) another effective strategy might be to park a bunch of dormant ships/weapons around the area you wish to attack/defend. Dark and cold, they'd be nearly undetectable... until they're needed.

It seems like we're thinking along the same lines. Only in space, there is no reason to limit yourself to distance proximity. There will also be proximity by delta-v. If you extrapolate improving detection technology, there will be a point at which a little bit of active effort will be required to maintain stealth in the face of enemy reconnaissance.

Er, and what does it do with the absorbed radiation? If you assume it can magically not re-radiate the absorbed EM radiation, then its heating up without limit, and so you also need to make it magically able to absorb limitless energy without failing.

Er, previous comment was written for people who think one or two steps ahead. A parabola is great for directing EM energy. (Think searchlight.) The rest was left as an exercise for the reader.

and so you also need to make it magically able to absorb limitless energy without failing.

There is indeed a fail, but not the one you envisioned. (EDIT: Don't feel too bad. That discussion on spacebattles was presaged by similar discussions on USENET in rec.arts.sf.science from over 25 years ago. I have a bit of a head start.)

So its a magic surface where the inside is a perfect reflector and emitter, the outside is a perfectly absorbent and transmitter (to the inside)? That (though it requires multiple levels of magic) solves one problem, but of course then its also a drive system you can't control or turn off that turns all of your internal energy use and all incoming EM energy into thrust in the direction opposite the outlet of the cone, in addition to creating a detectable (by reflection from anything that passes through it) plume of EM emissions from the back end of your ship that will expand in such a way as to be visible even in the area from which your ship itself is shielded from direct observation.

Not a surface. It's a paraboloid shaped machine. (With a "stamen" in the center.) It seems magical to us now, but within the realm of physical possibility, so it's fair game for science fiction speculation.

its also a drive system you can't control or turn off that turns all of your internal energy use and all incoming EM energy into thrust in the direction opposite the outlet of the cone

Yes. It will be tricky to use. Swarms of these units will be implementing strategies devised by AIs in cooperation with interfaced human geniuses, heuristically solving the horrendous geometric and continuous thrust trajectory puzzles formed by the partially and probabilistically known configuration of enemy units. (EDIT: There will also be some mass driver propulsion available, but this will also be limited.)

plume of EM emissions from the back end of your ship that will expand in such a way as to be visible even in the area from which your ship itself is shielded from direct observation.

I must admit I haven't worked out the numbers for how hot these things can get or how concentrated their waste EM plumes can be before it's counterproductive because the near-vacuum gas/plasma starts emitting too much. As mentioned elsewhere in these threads, I'm not envisioning a Star Trek or Star Wars style ship. These are clouds of small autonomous drones. Small size, unmanned operation, and larger numbers are going to be a big win for such technology. In other words, I'm not trying to stealth something big and hot in the first place. (Unless it's the "mother ship" and that will only operate far outside of enemy space.)

Start with clouds of small, dim, but potent drones, then take the arms race in a particular direction.