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Consider a highly advanced civilization that is almost ON THE VERGE of developing futuristic (for us) technology like FTL travel. (I insist on the "on the verge" aspect, because I envision this civilization to be only a Millennium ahead of humanity, technologically. So, they might still NOT be at a point where they're confident about FTL and similar stuff).

Two questions:

  1. What is the minimum distance that such a civilization can exist from Humanity, and STILL evade discovery by humans?

  2. And for a civilization at such distances,what technology can it utilize to destroy (vaporize/blow it up into bits) the Earth?

PS: Additionally, I would also like it if this super-weapon were also super-sneaky. That is,no one detects this projectile/directed-energy-weapon being fired,until the large explosion that is the Earth going "phut".

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    $\begingroup$ Welcome to worldbuilding. Please note we follow a 1 question per post policy, while I see at least two unrelated questions here. Please take the tour and visit the help center to find more information on how to rework your post to fit our standards. $\endgroup$
    – L.Dutch ♦
    Commented Feb 26, 2019 at 10:44
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    $\begingroup$ For question #1, you may be interested in How far from the Sun could we detect an alien spacecraft similar to the Voyagers? (my question) and my answer to How far away would an alien civilization need to be for us to not notice them? For #2, we do have a number of questions already that involve the gravitational binding energy of planets; those may be instructive. $\endgroup$
    – user
    Commented Feb 26, 2019 at 10:49
  • $\begingroup$ I think we need to know more. (a) If they are far away enough for us not to detect them then how did they detect us? (b) They must be light years away. Aiming a physical weapon from light years away and expecting it to hit the target after centuries with no course correction seems highly improbable. Even if they send radiation it will take years to arrive and probably miss. Example: They are 10 light years away. They detect the position and orbit of the Earth - but this is 10 yrs out of date. They fire a death-ray at light speed. 20 years later they know if they were successful. Total 30 yrs. $\endgroup$
    – chasly - supports Monica
    Commented Feb 26, 2019 at 11:39
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    $\begingroup$ I think your point 1 is "minimum" rather than "maximum". Because "maximum" would be the size of the universe, if it's not infinite, and we surely can't detect anything beyond the observable universe. $\endgroup$
    – Rekesoft
    Commented Feb 26, 2019 at 11:50
  • $\begingroup$ A fundamental problem is that "a thousand years ahead" tells me nothing about what technologies they actually have. What devices do they have that can e.g. blow up a planet ? What kind of space propulsion do they have ? Energy generation ? All too vague. $\endgroup$
    – StephenG - Help Ukraine
    Commented Feb 26, 2019 at 14:08

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About the minimum distance, well, it can be also as near as alpha centauri. If they are a thousand years more advanced than us, starting from the idea that they used the same communication medium (radio wave) we use and that they used it at more or less our development level, their waves reached us about 1000 years ago, when we were not able to receive them (well, any time before the last 250 years). Now they can simply use some transmission method we are simply not able to intercept or even understand.

While saying "a millennium ahead of humanity, technologically" don't really say anything on their technological level, I suppose that they are more than able to mount a big engine (with some smaller ones for trajectory correction) on a small asteroid and launch it against us. As they are on the verge of FTL, a not so out of reach supposition is that they can aim well at the long distance or their FTL will end badly.

If they are able to apply to the asteroid a small acceleration (let's say just 0.5 g) for a year you end with about 5% of c (a little less of 15800 Km/s). Keep doing this for some more years and you end with a pretty decent fraction of c.

At this point, you can simply turn off the main engine and let the asteroid go, eventually allowing for some course correction every now and then.

It is not super sneaky (we can find out it, eventually) but its biggest advantage is that we can find it only if we know where and when to look. Even if the engine is on, we must look precisely at it to be able to detect it. The demonstration are all the asteroids we found just days before they fly-by the earth or even after this.

But in the end it don't really matter if we can see it, we have no way to stop it.

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  • $\begingroup$ No, I think your idea works for me. After all, an asteroid hitting a planet could be written off as a natural occurence. So,yeah, it's definitely a good idea! $\endgroup$
    – Gamma-ray-burst
    Commented Feb 27, 2019 at 5:56
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As for methods for destroying the Earth from multi-light year distances, there are two I know of: 1) Nicoll-Dyson Beam - laser weapon that utilizes power harvested from a star by Dyson Sphere. 2) Relativistic Kinetic Kill Vehicle - vehicle travelling at relativistic velocities with a purpose of hitting a planet, releasing its enormous kinetic energy on it. Nicoll-Dyson beam, although travelling faster than RKKV, has one potential flaw related to the first question, the Dyson Sphere might be detected by human race.

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    $\begingroup$ Yup. The Relativistic Kinetic Kill Vehicle seems a more effective candidate in this regard. $\endgroup$
    – Gamma-ray-burst
    Commented Feb 27, 2019 at 5:58

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