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Let's say that hundreds of years from now our posthuman descendants finally do good on NASA's promises and build an Alcubierre drive or a true reactionless drive (either one works for this example). Great! Now we can traverse the stars! And if any uppity aliens show up and try to get fresh with us, we can just use our ship's drive as an infinitely powerful weapon with utterly arbitrary destructive potential to blow them, their planet, and even their solar system to bits if we wanted.

But would we want to? Fortunately or not so fortunately for our descendants, it's not too difficult to spot the major flaw in having an infinitely powerful weapon. You could never fire it, even in the vacuum of space, because all that energy has to go somewhere, and it may as well be the person, ship, or planet the weapon is being fired by (as well as everything else within a few light hours of you). This is absolutely unavoidable, unless the weapon is truly of a bizarre and exotic nature and doesn't inflict damage via thermal or kinetic energy, much like the hypometric weapons from Alastair Reynold's Revelation Space series.

So having said that, my next question is of course going to be "how do you avoid that?" Assuming you could weaponize a Q-thruster or the kind of Alcubierre drive that accumulates massive amounts of energy inside the warp bubble, would there be any way to protect the person(s) firing the weapon that isn't purely wild speculation (and by that I mean even more wildly speculative than Q-thrusters or warp drives)? I assume not, but I'm no physics expert.

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    $\begingroup$ Aim the pointy end towards the aliens? $\endgroup$ – Innovine Oct 18 '16 at 16:48
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    $\begingroup$ This question reminds me of the Larry Niven [short story][3] (The Warriors) about first contact with a warrior alien species, where humanity had "no weapons"... but we're a brutally nasty species when it comes to defending ourselves. [3]: baen.com/Chapters/0671878794/0671878794___2.htm $\endgroup$ – Ghotir Oct 18 '16 at 17:18
  • $\begingroup$ Why not jut fly the weapon out to the middle no nowhere before you fire it, so that the side-effects destroy only a cubic lightyear of empty space? $\endgroup$ – Ninety-Three Oct 18 '16 at 17:21
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    $\begingroup$ what-if.xkcd.com/109 "Most of our equations don't really work when you put "infinity" in them." Infinity isn't a Real number. Our equatios work on Real and sometimes Complex numbers; you can't really plug infinity into Real-valued equations and get meaningful results, you can only take the limit as some variable(s) approach infinity. $\endgroup$ – zstewart Oct 18 '16 at 17:39
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    $\begingroup$ "All that energy has to go somewhere" - but it has to come from somewhere, right? So where it came from and why can't you dump it back? $\endgroup$ – Mołot Oct 18 '16 at 20:10
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Infinities are tricky to work with. Generally speaking it's better to work with "unbounded" or another term which captures the bigness really well without all the pathologies that come up with infinities.

An infinity in any term which shows up in one of our physics equations is the end of the world as we know it unless it's corrected. No, I mean that literally. Our equations are not designed to perform in these situations. Most of them are done over the set of real numbers, which does not include an infinity. You would have to expand those equations to include "all real numbers plus one or more numbers of infinite size." These would be different equations than what our physicists are claiming to be true today. They'd look similar, but changing the domain would make them look very different.

Probably your best solution would be to fight an infinity with an infinity. Before the energy is put into the system, we create an infinitely deep energy potential well, and we choose an infinity of a higher cardinality than that of our energy weapon (did you know infinities came in different sizes? If not, I recommend VSauce's video on the topic). This way, when we finally do put infinite energy into the universe, it's safely culled away. One might argue that a black hole would be enough, since nothing can escape from it, but "nothing can escape from it" was a statement made within the real numbers. When you add infinities, all sorts of things might happen, so I wouldn't trust a black hole to be enough until I saw the equations proving the weapon is "safe."

The effect would be remarkably like a lysome in biology. If an infectious agent tries to get into a cell, the cell membrane often pinches off around the agent, creating a sphere encapsulating it. The cell then pumps it full of all sorts of nasty acids and enzymes designed to eradicate whatever is inside the lysome for good.

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An infinite amount of power sustained for a non zero amount of time is an infinite amount of energy, which would collapse into a black hole of infinite mass. This would cause space-time to collapse in on itself. Lets assume that you have a finitely powerful weapon. To protect yourself either:

1) use more shielding

2) turn the power down to a manageable level

3) be a long long way away (missile)

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Wikipedia's entry on the Alcubierre drive will suffice for this, I think.

The Alcubierre drive [...] is a speculative idea [...] by which a spacecraft could achieve apparent faster-than light travel [...].

Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel.

@Innovine said in a comment "Aim the pointy end towards the aliens?" and that's exactly what I propose: if you have something that can contract or expand space, why not weaponize that? "Simply" configure your field to create gravitational sheering inside your target (be it a ship or weapon) and let physics do its thing. (The scientific name is spaghettification.)

I'll leave it as an exercise for the serious student to determine exactly how to work the equations... but note that this shouldn't destroy the drive, let alone the pilot.

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  • $\begingroup$ Doesn't the Alcubierre drive do no damage to the manipulated environment? (As is, someone along the path wouldn't notice it go by) $\endgroup$ – Tezra Oct 18 '16 at 17:40
  • $\begingroup$ @Tezra From the same Wikipedia link: "Enormous tidal forces, however, would be present near the edges of the flat-space volume because of the large space curvature there [...]." I believe precisely setting that on an enemy ship/planet would suffice. $\endgroup$ – Ghotir Oct 18 '16 at 17:43
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This question goes in so many directions. Here are some quick thoughts:

  • You have a space drive. It doesn't kill the people it's transporting. So there are safe places in close proximity to it, as a starting point.
  • You weaponise it. Why does this become "infinitely powerful" or even "arbitrarily powerful". What do these terms even mean, for a weapon?
  • Also think hard about it , a weapon that would destroy the user (who we assume isn't suicidally motivated and would like to avoid that) isn't infinitely powerful. Its no weapon at all, unless it can plausibly be used. Axiomatic.
  • There are likely to be extremely powerful weapons not needing to be made of future star drives. In the right circumstances a person, some words, a concept, can all be "infinitely powerful weapons_"...?
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