That’s what I thought; seems straightforward enough. But Atomic Rockets insists it’s a closed debate, as impossible as perpetual motion. Is there a deal breaker I’m unaware of, or can they really not have thought of something this simple?
True… David Brin wrote up the idea, and I was just citing him (it was fiction, but he really does have some decent science cred.) But, really, it doesn’t have to “lase.” Others have already suggested a “hot spot” with a reflector around it.
You can only conceal yourself in one direction using a refrigerated shield. If you put radiators behind such a shield the radiators will be visible from anywhere behind it. And you can’t wrap the refrigerated shield around the radiators, as the radiators will then heat the shield up, making the refrigerator work harder, which then creates even more waste heat.
The best you can do is a cold flattish plate with radiators concealed directly behind it, making you invisible in one direction only. This assumes that your target doesn’t have any remote, passive sensors dotted around the system that can peer behind your limited concealment - which seems foolhardy.
I’m not quite clear why you couldn’t have a cold hemisphere, making you invisible (to heat sensors, anyway) in half of all directions. Why does the plate have to be flat?
(And…if flat…how big could it be? It might cut off all detection in an arbitrarily large cone of directions.)
Even if I agreed with your major premise, I think you’re unnecessarily limiting the engineering.
Ok, I did the research to follow and go through a few links:
The Atomic Rockets main article on Detection: Detection - Atomic Rockets
A link from that page to a discussion about stealth vs. IR detection: Redirecting to Google Groups
Pretty much, they agree with the points I made in the OP: If you’re using reaction drives, you’re announcing to anyone in the system who wants to know where you are, where you’re going and how heavy you are.* And limiting the direction of your IR emissions is possible, but not useful in many cases- such as if you enter a region where you’re surrounded by remote sensor drones.
*With a possible exception: if your reaction mass is helium, which is a very poor IR emitter because it’s monatomic and has tightly bound electrons.
Also, it was pointed out that weak IR signals aren’t necessarily easy to spot, because the background space has red dwarfs, brown dwarfs, interstellar dust, etc. A detection system is going to have to compare scans against a perfectly enormous library of known IR sources, and if the enemy ship is line of sight with one it’s going to be very difficult to detect. Increasing resolution means increasing scan time.
So there’s no practical stealth for torchships; if you have something like an impulse drive, that’s another thing.
If you can power your spaceships with stars, and the hotter kinds of them at that, I shouldn’t think you really need to hide them any more 
I power my starships with baby universes, and draw upon expansionary phase Higgs field energy.
Well, imagine a radiator as a point source, at the centre of a refrigerated hemisphere. Half of the heat radiated by that source will be intercepted by the hemisphere, making the refrigerating system work harder, which will increase the heat output of the radiator, and creating a feedback loop.
Even if you reflect the heat back from the inner surface this will tend to heart the radiator further, and no reflection material is 100% effective anyway, so the hemisphere will absorb at least a fraction of the heat, creating the feedback loop I mentioned earlier.
Best to keep the invisibility shield as far away from the radiators as possible, which does allows a variety of shapes, but none of them can conceal as effectively as a hemisphere
Of course, if the hemisphere is big enough, you could use it as the radiator directly. A large enough sphere could radiate waste heat at the CMBR temperature, but would be detectable as it transits in front of distant stars.
How does an impulse drive work?
Of course, the ejected objects would have their own heat-signatures, which might be detectable as the spaceship’s trail or wake.
Frankly, though, I’m having a hard time envisioning future conditions that could lead to war in space. We have not already colonized the Solar System because there is no profit in space, nothing there that justifies the expense of getting to it and back, no industrial processes that can be done in space that cannot be done cheaper on Earth. It’s just a good place to put communications satellites, that’s all. All other space activity is pure scientific research, which is something you spend money on, and is never expected to be profitable immediately. Colonization for its own sake is theoretically possible, but it’s not going to happen, people aren’t going to go live out their lives in a sealed can just to expand the human race’s territory, nor others fund them; there has to be an economic incentive. Does anyone see any chance that will change? And without economic interests in space, how can there be war in space?
People might one day do so because they can. Who knows?
I do think most of the people here are making some odd assumptions, however, because stealth is a function of distance. And in space, everything is a distant speck.I don’t mean this to be snarky: even a ship travelling through space is really, really tiny, and we don’t have anything like Star trek sensors. If it doesn’t show up visually, on thermal, or on radar there ain’t much to see. And even something very hot can be quite hard to pick out. Worse yet, they likely won’t be duping a lot of waste heat once they get up to speed; it’s just not necessary to run your engines ona constant burn.
The best written article I ever saw about what a space military might look like involved as few technical advances as possible, and described a simple set of outer-space submarines, operating a packet of drones with guided missiles. Everything would be designed to look like either random debris or rocks, or being wrapped in radar-absorbent materials to hide. Either way, they mostly just sit somewhere.
You would only have a handful of people in your ships, they would spend most of their time operating on very little power (and hence almost no heat to radiate) and being exceptionally bored. You could have picket satellites which would actively sweep the area and fed you information, but mostly you just waited to see if anything showed up. If so, you sent a signal for one of your drones to boost towards the area, then it would unload nuclear missiles at the target.
An actual war, should it come to pass, would involve groups of these silently approaching each other’s bases and then trying to destroy or disable them. If you removed enough of the warning network you might be able to eventually approach to a planet (Earth, or maybe a terraformed Mars) or space colony.
What you’d do once you had was an open question, since you likely couldn’t create enough of an attack force to do anything unless you were willing to nuke a population into extinction or submission, which would invite open reprisal from everyone else and would likely result in your own population being nuked in return.
Right now we’re trapped in the vicious circle of “few people go there because it’s too expensive, and it’s too expensive because so few people go there”. Arguably, we’ve been locked for the past 50 years into a “Space Program” paradigm taken from the military/aerospace model, which also gave us the infamous $5000 toilet. With multiple vendors now offering launch services and even fully private endeavors, we may only now be truly taking the first baby steps of an incremental approach that is slower and more modest in goals but establishes a much firmer foundation.
For example, if limited “space tourism” like Virgin Galactic’s short up&down flights becomes a modest success (comparable to rich people paying to fly in the early 1920s)- then possibly suborbital hypersonic flight would be the next step; people have been proposing “boost glide” manned flight since Eugen Sänger. This would be easier than orbital flight while laying the groundwork for more ambitious plans.
If travel into Low Earth Orbit on a commercial basis becomes economically viable, then you’re already halfway there. It’s easier to get to Mars from LEO than it is to get from the earth’s surface to LEO.
With very little prompting or planning.
I’ll throw this out: Any “cold” shield would have to be very cold, like cryogenic. And it would still be visible in IR, just less so.
For example, cutting the ship’s surface temperature by half, you reduce IR emissions by factor of 16, and the detection range by a factor of 4. That’s not much of stealth.
It’s a mechanical process to convert narrative energy into kinetic 
You can’t base the industrialization/colonization of space on tourism. There has to be (1) a way to make money in space through mining/manufacturing/something that can’t be done on Earth as economically and/or (2) a reason why significant numbers of people, with money, would want to live in space and die there.
Oh, you mean a Dean drive!