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Okay, so the ultimate villain of my story is a million-year-plus-old malevolent extraterrestrial believed (and for good reason) to be the most powerful being in the universe. He has incredible strength, being able to exert a force roughly equivalent to 112 MN with just one of his fingers, and is quite morphologically similar to a human.

One of his favorite weapons to wield is a sword made of a space age metal that is about as long as he is tall (and he stands 2.25 m tall), has a mass of about 3 kg (swords tend to be surprisingly light), and has a cutting edge that is an atom thick at its thinnest, and gets macroscopic thickness that allows it to cause both physical and chemical damage to whatever it cuts. And it's got enough sturdiness to go through centuries of combat without having to be maintained. And, another ability of his wielder is firing lasers from his eyes that can heat a human being up to 3000 K.

I calculated the power of those rays by figuring out how much energy the victim would be losing to emission at that temperature, and those rays have a power that's equal to the power of emission. And that blade can cut diamond, as well as practically anything. So, how will it be possible for the wielder of the sword to cause a nuclear reaction by either splitting an atomic nucleus in two or by pushing one nucleus into another?

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    $\begingroup$ Minor point - even if you split an atom or push one into another, you won't create a nuclear chain reaction. The resultant emissions will be tiny, and will not be sustained, because the adjacent nuclei will not be inclined to fission/fuse. $\endgroup$ – jdunlop Dec 12 '20 at 18:33
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This reminds me of a scene from Terry Pratchett's Mort where Death was honing his scythe with sunlight to get it sharp enough to sever lives

However... the ability to cause nuclear reactions?

Nope.

Nuclear forces — the ties that bind molecules together — are incredibly strong. Were we able to proverbially tie a rope to the three atoms in a molecule of water, the three strongest human beings in anything short of pure fiction would not be capable of pulling them apart (fission), much less push them together (fusion).

The nature of your weapon is irrelevant. It would simply push push things out of the way. The only way this would change is if your character could swing the sword at relativistic speeds.

With one possible exception...

If two characters, each having one of these super-sharp swords came together, the impact of those two swords might, just might, cause a nuclear reaction. That would be similar to the shaped charges used to create the implosion necessary to set of a nuclear bomb.

But I'd hate to be either guy. At best, they die of cancer in a couple of years. At worst, their shadows are burned onto nearby walls as a reminder that making more than one sword out of the space-age metal is a really bad idea.

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To achieve nuclear reactions your bad boy has to swing the sword at around the speed of light.

This can be better explained by quoting the cornerstone of any worldbuilding: XKCD's What if, in particular the relativistic baseball

What would happen if you tried to hit a baseball pitched at 90% the speed of light?

The ball is going so fast that everything else is practically stationary. Even the molecules in the air are stationary. Air molecules vibrate back and forth at a few hundred miles per hour, but the ball is moving through them at 600 million miles per hour. This means that as far as the ball is concerned, they’re just hanging there, frozen.

The ideas of aerodynamics don’t apply here. Normally, air would flow around anything moving through it. But the air molecules in front of this ball don’t have time to be jostled out of the way. The ball smacks into them so hard that the atoms in the air molecules actually fuse with the atoms in the ball’s surface. Each collision releases a burst of gamma rays and scattered particles.

And, to keep quoting the same page

Let’s set aside the question of how we got the baseball moving that fast.

Just swap baseball with sword.

I have the well founded feeling that not even 112 MN times 10 fingers will be enough to push the sword at those speed regimes. But that's up to you, someone was able to fly around the world so fast that he turned back time, so...

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  • $\begingroup$ Adding to that: in order to accelerate a 3kg object to light speed, you'd need to apply about 900MN of force, assuming no energy loss to friction and no other forces acting on the object. If you take friction into account, the numbers become much more absurd. $\endgroup$ – JANXOL Dec 12 '20 at 18:08
  • $\begingroup$ It's a 6.88 kg sword once it's up to 0.9c. It's still on the order of seconds to get it there with ~1GN in a vacuum, though. $\endgroup$ – sideromancer Dec 12 '20 at 18:13
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    $\begingroup$ Also worth noting that if you could exert that kind of force, you wouldn't need the sword. $\endgroup$ – jdunlop Dec 12 '20 at 18:34
  • $\begingroup$ @sideromancer, the sword moving at 0.9 c has a relativistic kinetic energy of 10^11 J, to impart it with 1 GN it would need to be swung over about 100 meters, not exactly at arm reach for a human like creature $\endgroup$ – L.Dutch - Reinstate Monica Dec 12 '20 at 19:19

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