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This is somewhat vaguely related to a old question of mine involving a alien species who spend three thousand years developing a post singularity spaceship armada in the mantle of a lava planet (Industrious). I try being realistic.

So realistic that I figured that might somehow not work as planned. My logic at the time was so sound too! "Can't hide in space because of heat signature? Well, let's find some extremely hot place and hide in it where looking for heat signatures is impossible and no one will look anyways". Turns out trying to make a massive underground base in a lava planet has lots of problems. Problems that you can only throw so much technology at.

So I wonder.

Where in the galaxy is a safe place to hide from some genocidal spaceship entities who wanna kill everyone? And I mean anywhere in the galaxy. Empty space is not a option, too easy to be seen. Cold planets I presume would be useless because of the lack of heat. However, where in the Milky Way galaxy is a safe place to hide from swarms of genocidal aliens that kill anything they detect on sight while building a giant space fleet?

-Will a lava planets work? -What about a hot jupiter? -What about near the galactic center? -Are globular or open clusters good or do they come with problems like being too much of landmarks? -Do red dwarfs or brown dwarfs hide things well enough or are they too cold to hide heat signatures?

Technological limits are very few here, but hyper-spatial options aren't available. FTL speeds are around 10-20 Ly per day. Wormholes would require moving to other star systems if used.

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  • $\begingroup$ Just an idea and I don't know if its even remotely technically possible but you said not limits to technology so why not. How about they orbit the supermassive black hole in the centre of the milky way, maybe travelling FTL and orbit low using the infinite power of technology to hold the structure together. Cause, damn, that would be hard to spot. $\endgroup$ – Jack.Ramsden Feb 5 '16 at 8:55
  • $\begingroup$ What sort of ships are you talking about? If they are smaller then Jupiter and don't radiate more heat then a brown dwarf, empty space is just what you need. You see noone - regardless of his technology and computational capacity - can categorize all tiny, cold objects in the galaxy, so if you put your fleet into the darkness and nobody will see it. $\endgroup$ – mg30rg Feb 5 '16 at 15:15
  • $\begingroup$ The Heechee, hiding from a sapiencidal unknown entity stick a load of stars close together which Frederick Pohl reckons creates enough mass in a small area for a black hole-like event horizon to protect them, even though inside that it's just a load of planets and stars you can live on normally. I've always wondered if that were actually possible. $\endgroup$ – Whelkaholism Feb 5 '16 at 15:38
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    $\begingroup$ What kinds of sensors do the bad guys have? If you travel ten light-years away, it'll take them ten years to see you, which is plenty of time to move even further. If you're saying these genocidal aliens can detect their prey at FTL speeds, you'll have to explain how that's possible, and define the limitations of such impossible powers. $\endgroup$ – DaaaahWhoosh Feb 5 '16 at 16:07
  • $\begingroup$ @ user16295 "assuming instant communication regardless of distance..." well there wd be no lag at all, it's just that u wd have to correct for positioning $\endgroup$ – user17892 Feb 5 '16 at 23:07
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Empty space is a great option! Actually multiple points in empty space. Move around every year or so to random points in the space between stars where nobody has any reason to be (space is really big and really empty) and you can't be found.

Why? At the speed of light it will take years for the light from your fleet to reach the nearest star (and heat is light). By that time you'll have moved to the next random spot.

The aliens can try to search for you based on where you were years ago, but the volume of space they have to search will just be too big. If your fleet moves 10 days every year in a random direction at 10ly/day that's 100ly per year. The bad guys have to search a volume of space 4x106 ly3 every year. That's 4 million cubic light years. That's a lot of space!

It gets better (for you). For every year out of date the bad guy's information is, the volume of space they have to search gets larger by eight times. With two year old information your fleet will have moved 200ly giving a possible search volume of 3.2x107 ly3. With three year old information it's 2.6x108. And so on.

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    $\begingroup$ This is cool because adds some suspense in the novel. There is a risk of accidental found. $\endgroup$ – Florin Ghita Feb 5 '16 at 10:58
  • $\begingroup$ You can also pick places to go to that are even farther from the nearest star systems, should you wish to do so. $\endgroup$ – a CVn Feb 5 '16 at 16:00
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TL;DR: Space is big, you wouldn't really have too try to hard - It could take the aliens at least 27 years just to find you, or as much as as 1.75E19 years, but if you must, try hiding on something fast.

It really depends on how bigger an object you want to hide. Remember, it's called 'space' for a reason.

Anyhow, you want to hide from genocidal alien berserkers, so let's assume it's a whole planet for the sake of the argument. That's perhaps a 30,000Km radius - The earth is about 6,000Km radius, so that's a conservative number for a terrestrial planet. As you suggest you need a decent energy supply, I'll allow for that to.

The ultimate factor in where you hide will the the sensitivity of your enemy's sensors - IF they use some form of radar or electromagnetic phenomena that by definition propagates at the speed of light, you don't even have to hide. - The Milkey Way is 100,000Ly diameter, so 'hiding' (in plain sight) on the opposite of the galaxy gives you 100,000 years to build super weapons before they even know you're there. However you've specified FTL speeds of 20 Ly / Day are available, so let's assume they dispatch thousands (trillions) of remote probes that must find you then fly back to report. That gives you 10,000 days (27 years) before the probe can find you and report back to the far side of the galaxy. Therefore: Scenario A: The other side of the galaxy, where ever is most convenient in terms of resources and energy.

Again, if you sensors have a (relatively) low resolution or sensitivity, anywhere with sufficient background will do. If we use a real science base, it just has to be the right background - no information can propagate faster than light (even gravity waves), as we currently understand physics, if your enemy uses electromagnetic sensors you hide near (or in) a bright star (Say a blue supergiant - which has a luminosity [brightness, basically] of 1,900,000 times that of our sun.) The heat would be a problem at 10-50 thousand kelvin, but at least you'd have your energy sorted.

On the surface of most planets, it would depend on how far away your enemy was - I can't give you an exact number because it would depend on the sensitivity of the sensors to electromagnetic flux, but consider that many chemicals produce an EM spectrum very similar to that of industrial activity and metabolic processes (IE the presence of elevated quantities Nitrogen, Oxygen, Carbon, Phosphorous, and Sulfur) exist on planets that could hide a smaller colony unless a ship got quite close relative to the resolution of their sensors - Consider that our best pictures of Europa - a moon of Saturn, have a resolution of about 1,000 Km^2 per pixel - easily enough for a small colony to go undetected. Carbon Planets (https://en.wikipedia.org/wiki/Carbon_planet) would be a good bet assuming you didn't vent phenomenal quantity of Oxygen into the atmosphere (That means in excess of 1E10 Kg, where it would start to be visible on a current technology spectroscope from about 20 Ly). You mentioned terrestrial planets with a large amount of volcanic activity.. I wouldn't bet on it, as they would (terrestrial) planets would be a priority in my xenophobic hunt if I was running it, because they tend to be ideal for life to evolve - remember their are 100,000,000,000 planets in the milky way, so searching each on a closest first basis could still take 36,986,301 years (assuming 10 planets per solar system, 27Ly average between each system, 20 Ly per day travel with instant acceleration and deceleration). So by extension of that logic, B: Any planet you wouldn't expect to find life, perhaps the atmosphere of a gas giant - It might be hard to get materials there.

Galactic center... Good option at first: High flux makes it hard to identify individual targets, lots of energy and mass for industry.... Except - when you hostiles start to get close, they can get more fidelity from given sensors - Imagine physically moving closer to an object that can't be photographed in any meaningful way, because it's 10,000 Km away and you're using your cellphone camera. Thus you start to get seen again.

[Incorrect relativity deleted]

My favorite place to hide would simply be on a large moving space dock, making no particular effort to hide. As I've already pointed out, a procedural search of the galaxy could take an unreasonably long time, but the could also get lucky in the first planet they go to. However, if you make yourself a moving target, you can lower the numbers even further. Using the same numbers as before, if they can check (on average) five planets a day, your change of being found on any given day is 5E-11. To make this easier, let's say they enemy can check any planet within 20Ly of their location instantly. This increases your chance of being found to about 1.25E-11. If you then cover 20Ly in a random direction per day, your chance of being randomly located is 1.25E-11 * 1.25E-11 (Your chance of being on a given 'sector' on the day, times theirs.) per day. 1.56E-22 per day change of being found give you a predicted stealth time of 6.4E21 days = 1.75E19 years. Of course, their is a low chance of you randomly encountering them a lot earlier, or alot later, but with that kind of margin, I'd be willing to bet my life on lasting at least the 10,000 years you want.

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  • $\begingroup$ Very nice answer, right up to the point where you get time dilation reversed. If your ship is the one moving at near-lightspeed, you'd barely get your first ship done before you run into an enemy fleet, that spent many years tracking you down by FTL jumps. The faster you move, the slower your time passes. $\endgroup$ – Cyrus Feb 5 '16 at 9:15
  • $\begingroup$ Ops... That'll teach me to write answers at three in the morning. $\endgroup$ – Vera F W C Feb 5 '16 at 9:18
  • $\begingroup$ You've missed a trick. Assuming instant communications regardless of distance. It takes 27 years for the probe to get to you, then another 27 years for the enemy fleet to get to you. If the probe has to fly back to report then you can add yet another 27 years into that cycle. Giving a total of 81 years to get your ship moved. $\endgroup$ – Separatrix Feb 5 '16 at 9:24
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The best place to hide in this universe would be orbiting just outside the event horizon of one of the supermassive black holes near the centre of the galaxy. You will economize on your logistics, since near the surface of a black hole the immense gravitational warping brings time close to a stop. You will also be orbiting very close to c so you have a similar effect in terms of time dilation. You hide there for a few subjective hours, and the aliens will have had millennia to evolve into something new, or become extinct, or find a new quest to occupy them.

Galactic Black Hole

The accretion disc will asset you, since it will be radiating immense amounts of energy at every wavelength, making it difficult to shield against. Any enemy fleet will have to find a way to "see" through that in order to find you. (How you actually get in and out of the disc and near the surface of the black hole is an exercise for the reader.

This brings us to two somewhat related ideas. If you create a massive cylinder of a material like neuronium and rotate it about the long axis near the speed of light, it creates a "frame dragging" effect, tipping light cones and allowing you to "escape" through a timeline loop. In effect, since the light cone is pointing somewhere else than a cone at rest would, you would be escaping to "elsewhen"

Tippler Machine

Finally, if you are truly motivated, you could warp space-time so immensely that you pinch off a "pocket universe". Going inside and closing off the wormhole puts you forever out of reach, and if you have the ability to bend space-time like that, you should also be able to tweak the physical constants and fine structure of your new universe to make it fit your needs even better than the current one.

Pocket Universe

Just remember, don't leave a forwarding address, that's how they always manage to find you.....

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As others stated, space is really big and passive sensors could only detect you from within your light-cone, so between normal circumstances there is no point hiding, but - just for the fun - let's assume there is.


Why hide?

You only have to hide your fleet if your enemies are actively searching for it by the means of gigantic superluminar fleets. In that case they might patrol all trade routes and solar systems in your galaxy. we are talking about a fleet able to patrol ~400 000 000 000 stellar systems regularly, so I don't think any fleet that can be hidden on the surface of a planet is sufficient enough to massacre them.

Where to hide?

If I had to hide a fleet of attack ships huge enough to attack an enemy which is logistically capable of controlling all (400bn) stellar systems in the Milky Way, I would hide my fleet outside the galaxy. I know you said in the galaxy, but I will assume, you said that because of the distances. Now think about the shape of our galaxy! It is a double spiral following an accretion disk of the gravity, so if you put your fleet "over" or "below" (not as if any of those actually means anything in space) the galactic lens, they will be closer to the stars within than i.e. in the peripheral systems, but you will literally be "in the middle of nothing". The huge intergalactic space what nobody can control or even see-through. If you park a few dozen light years away from the lens, you don't have to worry about detection for a few dozen years, so you are good.

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In some of Alistair Reynolds stories, the ships hide from such an enemy as you describe by chilling to match the background temperature of space. He uses an entropy-reversing algorithm on a quantum computer: running the program makes it colder. There is nothing like that in reality.

Now I recall more than one author describing missions into a star, and a sci-fi solution of using a cooling laser. It turns out there really is such a thing! It works by carrying away entropy, as the laser is highly organised. Cooling yourself when surrounded by a hot media like a star also plays trick with the population statistics: a normal hot gas will have a curve with a peak and taper off elsewhere; if you emit energy in a narrow band away from the peak, the surrounding media is colder in that frequency.

Now cooling something down to the temperature of empty space while making an adjacent beacon is fine for physics experiments but lousy for hiding. If you need to remain invisible from any direction, the beam is a give-away. But what if you hide next to a star, and beam the laser towards the star? Well, that changes the situation since you don't want to leave a cold spot against the star: you want to match the outside temperature but make a cold interior. That's more like your original question, I think.

Now back to empty space: what if the cooling beam is incisible? Emit nutrinos or dark matter or something. This is also my favorite idea for a seemingly reactionless drive, so you kill two birds with one stone. You can both move stealthily and keep your heat signature camouflaged by emitting a dark-matter beam.

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You could hide under the surface of a planet close to the sun that show always the same face to the sun. The temperature will not go far down under the surface, you will find a fresh area to live. You can produce geothermal energy for 3000 years. The difficulty is to keep a pressurized environment underground for that long but i think that it's easier than floating in lava.

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