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I have a giant sword, and a human sized person able to wield it. This person decides to cut through a planet. Mass extinction followed.

For this problem, I will have to clarify a few things:

  • The setting isn't realistic about the power-levels involved, but I do want the consequences of those powers being thrown around to happen.

  • The sword is made from unobtainium, unbreakable and unable to be changed (it won't heat up or conduct electricity).

  • The sword style is like Sephiroth's Masamune. But longer.

  • This means the sword has a standard sword thickness of about 1cm-1.5cm around the spine area, and very thin <1mm at the blade part.

  • The length is about 15.000km. Since earth is about 12.740km, this should be enough to slice through the earth with some sword left over on both sides.

  • The person swinging the blade is about 2000km away from the planet. Just out of satellite orbit.

  • The person swing the sword does so within human swing speed limits for a normal sides sword and won't slow down during the cut. (because they are strong enough not to.)

    • The sword moves at about 90° in 0,04 seconds.
    • I do not believe the mass of the sword is relevant, as it basically functions as an unstoppable object.

Now my question is: How wide of a scale destruction should I expect when this sword cuts through the planet?

Here is my train of thoughts about the matter. These aren't questions to be answered, but what caused me to ask the question. I added those to clarify my expectations of the consequences:

  • I expect a giant sword cutting a planet in two to do some mayor damage, but also that gravity will keep the halves of the world together.
  • When moving through space nothing much would seem to happen, but when it starts moving through the atmosphere, would the heat created by friction be enough to create a plasma like edge that will burn it's way through the materials in front before it hits the actual blade?
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  • $\begingroup$ How long does it take to cut through the planet? $\endgroup$
    – M S
    Dec 14, 2023 at 15:09
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    $\begingroup$ Just to be clear, are you asking whether it acts like a lightsaber or what scale of destruction it would cause? $\endgroup$
    – value1
    Dec 14, 2023 at 15:35
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    $\begingroup$ Please remember that there's a strict limit of one question per post. You're currently asking will a sword going fast enough act like a lightsaber, and the scale of destruction if this sword is used to cut through a planet, and whether the speed of the sword enough to create a plasma around it. Please edit this down so that you're asking a single specific answerable question. There's enough physics defying stuff going on that you'll need to be especially clear where you're handwaving for coolness, and where you're willing to sacrifice coolness for the sake of being possible in the real world $\endgroup$
    – sphennings
    Dec 14, 2023 at 16:25
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    $\begingroup$ Dimensions of the sword? Mass (or preferably, density of your material), length, width? Can you give us more precision on the speed (human speed does not mean much, we shouldn't have to do the research for you to figure out the number, especially since you didn;t describe the nature of the motion)? Geometrically, how is the human-sized creature able to make the motion to carve through the planet? I picture him standing relatively far away and spinning on himself, is that correct? If yes, how far? You're asking for quantitative results. We need quantitative inputs. $\endgroup$ Dec 14, 2023 at 17:10
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    $\begingroup$ Clearly this is a worldbuilding question. I mean: magic sword; long enough to cut a planet; superdude up in space wielding it. Straight out of CD Comix. The question asked for a "science based" answer, not and overly physics heavy answer. Please, feel free to try giving a creative answer to a creative question, rather than just demanding data points that can be inferred from the question itself! VTR $\endgroup$
    – elemtilas
    Dec 14, 2023 at 21:17

2 Answers 2

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You are breaking physics, not Earth.

15000 km turning by 90 degrees in 0.04 seconds means that the tip of the sword is moving at 590000 km/s, almost double of the speed of light.

Before the sword does any appreciable twitch, relativity effects will have kicked in and stopped its action.

Even if you handwave all of that, a 1.5 cm cut will close itself under gravity shortly after it has been done.

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    $\begingroup$ The gravity closing up the cut I expected, but would the air/rocks not heat up from the friction, and be pushed apart because of the phase change? And would it not cause cavitation bubbles in the air (or even in the molten rock?) $\endgroup$
    – vinzzz001
    Dec 19, 2023 at 14:37
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    $\begingroup$ @vinzzz001 Obligatory XKCD: what-if.xkcd.com/1 $\endgroup$
    – Mermaker
    Dec 19, 2023 at 14:51
  • $\begingroup$ @Mermaker That XKCD article is about half of what prompted my thought about the sword forming a plasma-like leading edge, and why I expected a sword swung fast enough to likely form one to do more damage than just cutting a line through the world. $\endgroup$
    – vinzzz001
    Dec 20, 2023 at 21:20
  • $\begingroup$ Relativity won't magically stop the blade from moving, it will just prevent the far end of it from moving FTL, which will either manifest as A: the swordsman swinging more slowly, or B: the sword curving enormously during the swing, and reassuming its shape once it's stopped moving. $\endgroup$
    – M S
    Dec 20, 2023 at 21:57
  • $\begingroup$ +1 for the relativistic baseball reference. The far side of the sword is moving at the kind of speeds that made physicists realize that neutron stars couldn't be more than 15 miles in radius. $\endgroup$ Dec 20, 2023 at 22:25
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While the blade isn't very thick, at the speeds it's moving it will cause a wave of heat and plasma to spread through the atmosphere, outward from the circumference of the Earth through which the blade passed. No hard math here, but I imagine the wave would cook any surface life and also emit tons of harmful radiation.

From the far end of the planet,I would expect an ejecta of magma and plasma to shoot out of the ground, following close behind the swinging blade, shooting far out into space and perhaps giving our planet a temporary new ring, similar to Saturn's.

The strike would not blow the Earth to pieces, but would likely cause massive earthquakes and tsunamis, a new ring of volcanoes spanning the entire circumference of the planet through which the blade passed, and would superheat the atmosphere, killing everything but underground bacteria and perhaps some ocean life.

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