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In Effects of being hit by an object going at FTL speeds, the effects of a spear going at FTL speeds are debated in detail.

For my setting, humanity's last hope of stalling an alien invasion comes from the creation of an FTL-capable missile launched to destroy the main alien fleet. It is based on the Alcubierre Drive (I use that because there is plenty of resources on that topic), the same type of warp drive used in the aforementioned post.

What would it look like? (since the setting I am currently developing will be mostly used in visual material)

Effects mentioned in the post include "a burst of Gravitational Cherenkov Radiation", creation of extremely hot plasma if used in an atmosphere etc.

Note: I use the science-based because I want something plausible to build the style of my universe.

EDIT: after reading the comment, it might be better to say that the fleet orbits a planet and that the missile is going to it the planet. I forgot to add the mass of the missile, probably several tons.

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    $\begingroup$ I wonder what will happen when that alien fleet would enter warped space segment of the missile's Alcubierre drive, and should they use the same, what would happen to the missile. Warp aside, this is no FTL, and normal physics laws apply. I expect disintegration of target if objects actually collide and about nothing if the missile would miss, probably some warp metrics fluctuation could cause some minor damage on the alien ship(s) when missile would pass by, but nothing too heavy. $\endgroup$
    – Vesper
    Aug 18, 2022 at 14:56
  • $\begingroup$ Commenting as I'm only guessing... but the actual 'explosion' would probably only behave like an extremely penetrative kinetic impact and would probably only affect a single object(kinetic impact forces imparted does not translate well over the vacuum of space between ships). As for the launch, a 'gravitational' 'burst' makes me think there's going to be inward optical(light) distortion similar to what a black hole does. $\endgroup$
    – Lemming
    Aug 18, 2022 at 14:58
  • $\begingroup$ @vesper No FTL collision but I imagine the heat, radiation, and junk from around the bubble would do something $\endgroup$
    – DKNguyen
    Aug 18, 2022 at 15:04
  • $\begingroup$ Unlikely the debris would do damage t9 the rest of the fleet, at least whatever plasma would appear wouldn't be any more damaging than neutral gas in space to a ship going at subluminal speed. Mayhe some radiation spike would occur, but the plasma cloud from the collision will very soon be left behind the advancing aliens, and anyway it takes time for the cloud to spread, which will not be enough to damage even adjacent ships (space-wise). $\endgroup$
    – Vesper
    Aug 18, 2022 at 16:07
  • $\begingroup$ So, if the fleet is orbiting a planet, and the missile hit the planet, what would happen then? The debris of the planet would damage the fleet for sure. Indeed, if the missile hits nothing, there won't be much happening. $\endgroup$
    – Elyo
    Aug 18, 2022 at 16:10

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Energy In = Energy Out

At relativistic speeds, any explosive warhead in a missile is pretty irrelevant, so we can treat it as a dumb impactor.

Gravitational Cherenkov Radiation

You mention this as being a symptom listed in the other question, but you need to read more closely:

What is dangerous though is the burst of Gravitational Cherenkov Radiation. This circular blast wave (perpendicular to the velocity of the spear) centred on the point at which the spear crossed the lightspeed barrier will carry almost all of the energy that went into accelerating the spear. Depending on how much of the crossover the handwavium applies to, this could be an astronomical amount

The two important points - the effect would be centred around the point at which the missile passes lightspeed, and it is wholly dependent on the energy used in accelerating it. (And it does not use an Alcubierre drive, but rather handwaves passing the infinite energy barrier of reaching lightspeed.)

Extremely Hot Plasma

This is mentioned in the other question in reference to the What If Relativistic Baseball. Notably, in the original article:

We'll suppose it's a normal pitch, except in the instant the pitcher releases the ball, it magically accelerates to 0.9c.

All the subsequent effects are a function of the energy required to accelerate the baseball, which is applied magically. Your postulated missile is being launched by energy provided by the human fleet.

So an FTL missile, as an impactor, will do damage proportionate to the energy used to accelerate it. Exotic matter of the sort required for an Alcubierre drive would react poorly with normal matter, so it would probably approximate an antimatter/matter explosion. With a 1 tonne payload, that would be somewhere in the vicinity of a two gigatonne explosion. Not small, but not special.

Any fleet that could launch such a missile can do better damage by other means.

(For reference, the Chicxulub Impact was about a hundred thousand gigatonnes.)

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