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Right now, I am developing a weapon of mass destruction dubbed 'The World Laser'. Basically, the antagonist is trying to destroy the world completely by cutting it in half in a singular motion using a laser which is focused out of a very small point, I'm talking a nanometer wide concentrated laser.

Let's say this is far in the future, with universal exploration and all the technology you could possibly ask for. How feasible would a laser of this magnitude be? I've heard of gamma rays which seem to be a possible candidate for this, although I really am not sure of the physics behind them. While its powers are based on really excited nuclei, I am unsure if this is feasible in a laser.

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    $\begingroup$ The far distant future can only be hypothesised about, and no matter how hard you try, you will see it, to at least som extent, through the eyes of a present day person. Any hypotheses about the future are just guesswork based on hypothetical extrapolation from today. Combine this with your statement that you have "all the technology you could possibly ask for", and I'd say this is either impossible by the laws of physics, or perfectly doable. Exactly how it is done, is probably best handwaved to fit the rest of your world. $\endgroup$
    – user95279
    Dec 1, 2023 at 9:54
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    $\begingroup$ Even if the laser and the cutting work, I don't think anything would happen. Planets aren't held together by glue or mechanical strength, but rather gravity. Simply slicing it in half wouldn't do anything and besides those on the surface who get sliced, nobody would even notice since the small crack would immediately be filled by the surrounding material. Maybe a small earth tremor, cnsider that a 1 nanometer slice is VERY thin $\endgroup$
    – Dragongeek
    Dec 1, 2023 at 13:07
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    $\begingroup$ Note to commenters : recall the question is "Can I make a laser that can cut very precisely a planet". So if you don't think this is possible, it is perfectly viable to tell "no" and explain why in an answer instead of using comments 🐶. Comments should not answer the question 😉. $\endgroup$ Dec 1, 2023 at 13:29
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    $\begingroup$ What's the point? Even if you slice clean through with magic, the two halves are still gravitationally bound. $\endgroup$
    – BMF
    Dec 1, 2023 at 14:51
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    $\begingroup$ Slicing it in half would require many order of magnitude more energy than just using the laser to boil the oceans and make the planet uninhabitable. $\endgroup$ Dec 1, 2023 at 15:57

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destroy the world completely by cutting it in half in a singular motion (...)

Cool.

(...) using a laser which is focussed out of a very small point.

Nope.

As Separatrix explained, you won't really separate the halves of the world with a laser. Gravity will keep binding everything together.

In fact, with a laser, you would more likely vaporize the planet rather than chopping it in half.

If you want two planet slices, you need to use something with mass that will push both halves away as it passes through. Luckily for you, such a thing may already exist. It's called a relativistic blade, and it happens sometimes when a magnetar forms. Checkout this study uploaded to Arxiv, a preprint database. It is awaiting for peer review before actually being published in a journal but there is serious consideration among dome astrophysicists that it may be right.

Stars Bisected by Relativistic Blades

We consider the dynamics of an equatorial explosion powered by a millisecond magnetar formed from the core collapse of a massive star. We study whether these outflows -- generated by a priori magneto-centrifugally-driven, relativistic magnetar winds -- might be powerful enough to produce an ultra-relativistic blade ("lamina") that successfully carves its way through the dense stellar interior. We present high-resolution numerical special-relativistic hydrodynamic simulations of axisymmetric centrifugally-driven explosions inside a star and follow the blast wave propagation just after breakout. We estimate the engine requirements to produce ultra-relativistic lamina jets and comment on the physicality of the parameters considered.

Some gamma ray bursts we've detected are hard to explain. According to the authors, an explanation would be that sometimes a magnetar forms a relativistic blade inside itself, which then bisects it - that's nerd for cutting in two. Such blades have more energy than a supernova and cause particularly long lasting GRB.

If this is true, and these things can slice a star that is second in density only to black holes, then they sure can slice a planet. All you need to do is to move an humongous star close enough to the planet (doesn't need to be within our solar system to slice the Earth, for example), and with the right positioning - the planet has to be aligned with the star's equator - you would slice the planet in half.

A couple caveats, though:

First, a magnetar is formed after a star goes supernova. This means your planet will probably be sterilized in a very infernal way slightly before or while being cut in half. The fact that the blade itself is also a moving gamma ray burst stronger than a supernova also does not bode well for any form of life in the vicinity of the stellar system where this happens.

Second, due to the way that planets do their planeting, those halves won't be perfect domes floating in space. For an Earth sized planet, as soon as the halves are disconnected they will immediately collapse into two new spheroids. I imagine the energy involved in the rearrangement alone will also melt them.

For comparison, Ceres is like 960 km wide (compare with Earth's 12,800 km diameter) and is still massive enough to have collapsed into a planetoid shape. Only small things like asteroids and comets would mostly maintain their shape after being torn in two.

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    $\begingroup$ A thing that is capable of bisecting a star is probably going to vaporize a planet (should much of the planet survive the supernova) $\endgroup$
    – fraxinus
    Dec 3, 2023 at 0:18
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    $\begingroup$ absolutely upvoting for the term "relativistic blade" - that's terrifying, and kind of cool. $\endgroup$
    – lupe
    Dec 4, 2023 at 10:15
  • $\begingroup$ Could it maybe be combined with a wormhole, perhaps? So the magnetar remains on one side of the wormhole, while the blade passes through it? $\endgroup$ Dec 4, 2023 at 12:48
  • $\begingroup$ @MatthewDaly that could be a way to focus the blade, yes. $\endgroup$ Dec 4, 2023 at 16:58
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Taking the assumption that it is possible, what effect do you think it would have?

Planets aren't like buildings, they're not held together by structural integrity that could be damaged. They're held together by their own mass, held in shape by their own mass. The effect of this weapon would be damage to some buildings, a few trees, not a lot on the whole. It would be the most futile attempt to damage a planet ever devised. If you have weapons with the kind of power needed, just boil an ocean.

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    $\begingroup$ Yes, even if you were to make a meter-wide cut, the two halves would just come back together. Although a meter-wide cut might make quite the earthquake. Also, somehow, relevant xkcd $\endgroup$ Dec 1, 2023 at 17:40
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    $\begingroup$ For a more intuitive perspective, imagine what would happen if you tried to cut a pile of sand in half with your nanometer wide concentrated laser -- for a brief microsecond there may be a microscopically thin plane separating the two halves ... but afterwards? Any separation fills in due to gravitational slumping and it's practically indistinguishable from the original pile. $\endgroup$
    – R.M.
    Dec 1, 2023 at 17:44
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    $\begingroup$ @BillThePlatypus By the time I got to "the bottom half of Darth Maul" I was laughing so hard I had trouble reading the rest. That was a relevant XKCD. $\endgroup$
    – JBH
    Dec 1, 2023 at 18:02
  • $\begingroup$ re: meter wide cut, possibly see "Seveneves" as something similar happens to the moon, and the results are a bit more spectacular (as more and more bits break off when gravity brings the whole thing back together) $\endgroup$
    – njzk2
    Dec 2, 2023 at 16:46
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    $\begingroup$ @njzk2 that wasn't a nanometer scale laser, it was something (I think it was supposed to be a singularity, right?) that resulted in the Moon breaking up into multiple pieces. Not really relevant here. $\endgroup$
    – terdon
    Dec 3, 2023 at 12:48
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It cannot work: a laser work at cutting through matter by turning the superficial layer it hits into vapor that goes away.

While that's a no brainer on a flat surface, as soon as you start digging deeper into the material you get more and more atoms on the path of the laser, which will affect the beam propagation and effectiveness, defocusing, scattering and absorbing it.

In particular with a 1 nm broad hole, the fluid dynamic of extracting the vaporized matter out of the hole will become terribly complicated and won't allow cutting through the thousands of kilometer of the planet.

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Laser physicist here. There's no such thing as a laser beam with width 1 nm over 1000 km. The smaller your beam diameter at the aperture, the more divergent (cone-shaped) your beam becomes. Beams that need to be almost parallel over a large distance must be expanded. E.g. to hit the retro-reflectors on the moon, a beam of a few mm is expanded to the telescope aperture of about 2 m. Even then the beam diameter at the moon is on the order of kilometers.

This is a consequence of light being a wave and undergoing diffraction at the aperture. If your world is placed in this universe, cutting planets will need a 10 meter wide beam and will not make the hemispheres fly apart (assuming you solve the problem of getting the plasma out of the way). They will attract each other gravitationally. The 10 m gap will fill quickly, about as fast as falling 10 m would take. Imagine dropping half a planet from a height of 10 m.

You'll get quite some quakes, though.

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  • $\begingroup$ What if the beam waist is located at the target, so it's focused from a much larger beam, and the beam converges on the target rather than diverges? Could you say focus a 1000 km aperture laser into a 1 nm beam waist a long distance away? $\endgroup$
    – causative
    Dec 2, 2023 at 22:24
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    $\begingroup$ Is the diffraction wavelength-dependent? $\endgroup$ Dec 3, 2023 at 7:36
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    $\begingroup$ @fyrepenguin Yes. gentec-eo.com/blog/spot-size-of-laser-beam $\endgroup$
    – causative
    Dec 3, 2023 at 17:13
  • $\begingroup$ @fyrepenguin Yes, the angle is proportional to wavelength over aperture. $\endgroup$
    – Jens
    Dec 3, 2023 at 21:02
  • $\begingroup$ @causative The problem with focusing high energy photons is that there are no materials that would survive. There are no lenses for gamma rays. You can somewhat deflect roentgen photons with concentric metal plates, which is what roentgen telescopes do. A gamma laser would likely be a one-shot device, where you pump a laser medium (think large elongated cloud of unobtainium) and stay away from both business ends. $\endgroup$
    – Jens
    Dec 3, 2023 at 21:11
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Whether or not it's feasible is irrelevant

This is a Frame Challenge.

Feasibility is a question of Real Life and here on Worldbuilding we focus on imaginary worlds. Your imaginary world is sometime in the future where pretty much anything is possible, so it is obviously feasible. Therefore, let's modify the question...

I have an orbital laser strong enough to pierce through the center of the planet and it can be sustained long enough to draw the beam entirely through the planet pole-to-pole. Would this destroy the world?

No

The majority of what the beam is cutting through is heated liquid to begin with. Passing the laser through it will vaporize a (compared to the planet) minuscule amount of mass, certainly not enough to cause the planet to separate.

Those portions of the solid crust the laser passes through would liquefy and vaporize, causing some pollution, but rock cools and with the surrounding mass being much cooler than the comparatively small amount heated by the laser it would cool too quickly to allow separation.

OK, but what if we do something like split the laser into a fan that wholly divides the world?

This would take a monstrous amount of energy... kinda Death Star level energy, but let's roll with it.

This still wouldn't destroy the world. Yes, you'd have a complete separation... but the nature of gravity is to hold everything together and when the laser is shut off the world would go on as it did before. Remember that everything is spinning and orbiting at the same velocities and with the same inertia. What variations may exist due to things like imperfect density and shape would be so minor that you'd need to hold the laser in place for a very long time (measured in centuries at least, I'd guess... possibly millennia) before the two halves began to slide apart and then you'd need to hold it a whole lot longer (approaching the million year mark here) before they could be truly deemed separated.

OK, how do I destroy the earth with my big old laser?

Vaporize a big chunk of the upper core. You don't need to vaporize the center and you really don't want to. It would be very difficult to rationalize. But you could believably superheat a boat load (thousands or tens of thousands of cubic kilometers) of mass about 2,000 km beneath the surface. The rapid expansion would believably crack apart the planet.

Yeah, but could such a laser exist?

Not our problem. Our problem is suspension of disbelief and if I can enjoy watching the original Star Wars (which I did... I kinda miss 1977) then I can just as easily enjoy reading about a mad scientist feral government radical terrorist group crazy loon threatening to blow the world apart. It has a James Bondesque feel to it. Smashing!

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A laser isn't going to cut it! (ba-dump psh!)

As others have pointed out, cutting the Earth in half does almost nothing at all. The two halves are still held together gravitationally, and will quickly melt back together. We need to beat out gravity. And to do that, we need to exceed the gravitational binding energy of the Earth. This is the amount of energy released as the dust that formed the Earth came together. It's also the amount of energy needed to permanently tear it apart.

Assuming that the Earth is a sphere of uniform density (which it is not, but is close enough to get an order-of-magnitude estimate) with $M = 5.97\cdot10^{24} \text{kg}$ and $r = 6.37\cdot10^6 \text m$, then U = $2.24\cdot10^{32} \text J$. This is roughly equal to one week of the Sun's total energy output. It is $37.5 MJ/kg$, 60% of the absolute value of the potential energy per kilogram at the surface.

That's a lot of energy. And unfortunately, we will need even more.

To make this work, the two halves of the earth need to be blown apart. Your laser can't do this directly. It needs to use the hot gases to push the halves apart. The faster the cut is made, the more efficient this process can be. Any gases spewed backwards along your cut line form wasted energy.

Earlier I quoted Wikipedia's number of $37.5 \text{MJ/kg}$. All of that energy needs to be kinetic energy. Using $E=\frac 1 2 m v^2$ with a mass of the earth at $M = 5.97\cdot10^{24} \text{kg}$, we find we need to accelerate both halves to $8660 \text{m/s}$. Now we run into a problem. What you have effectively created is an incredibly inefficient light-gas gun. These are tools NASA uses to explore high-velocity effects. Herein we find some problems:

  • Gas guns are limited by the speed of sound in the medium. They can exceed it, thanks to the kinetic theory of gas, but their efficiency falls off as they go faster than the speed of sound.
  • A paper by Lawrence Livermore Laboratories indicates a velocity limit of roughly 10 km/s for a 1 stage light-gas gun. Higher speeds require multi-stage guns, which are carefully constructed laboratory apparati which are unlikely to be constructed by waving around a planetary-scale light saber.
  • Light gas guns depend on the lightness of the gas. Lighter gasses like hydrogen and helium have a faster speed of sound (by 3-fold or more). Contrasting with this, you're about to make a gas-gun out of vaporized rocks, with very heavy atoms.

It is unlikely you could accomplish this. The best laboratory environments in the world can barely reach the velocities you need, and the cut you make in the planet just isn't going to have the carefully machined shapes we use in the labs. And even if you found a clever way to do so, light-gas guns are not very efficient. Laboratory gas guns have an efficiency of around 0.6%. Multiplying this through, we are now talking about the total energy released by the sun in three years. And you will only see worse efficiency than that.

So how much energy are we talking? Let's set up a scaling value and cross reference with one of my favorite Wikipedia pages, Orders of Magnitude (energy). I'll reference a few entries, and a sense of how efficient we would need to be. We'll set an efficiency of 100 to be "laboratory efficiency," of 0.6%.

  • $1.2\cdot 10^{34} \text J$ -- Total energy output of the Sun each year (required efficiency = 320)
  • $1.5\cdot 10^{39} \text J $ -- Energy of the giant flare (starquake) released by SGR 1806-20 (required efficiency = 0.0025)
  • $6.6\cdot10^{39} \text J$ -- Theoretical total mass–energy of the Moon (required efficiency = 0.00058)

These are probably more realistic efficiency levels. You wont get laboratory results, or even close to them. So you're talking about an amount of energy on par with the starquake of a magnetar, or turning all of the mass of the moon into energy.

And, in the end, it's an absurd goal in the first place. The energy required to cut the planet in half is at least the amount of energy it takes to vaporize it in the first place. Rather than trying to do something exotic, just put a beam spreader on your laser, and start vaporizing the planet from one side to the other like a bad tattoo you want removed. It requires nothing but brute force.

...which is something your villain most certainly has!

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This isn't much of an answer (QUOTE: Note to commenters : Comments should not answer the question 😉. – Tortliena 2 days ago). Kurzgesagt has an answer to your question, though.

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There are a number of problems with the proposed laser-based doomsday weapon, most fundamental of which is that "cutting a planet in half" isn't really an intelligible phrase.

I'm sure you're receiving lots of other suggestions, so I'll just link to the the textbook, where many ideas, similar and dissimilar to your own, are discussed and compared.

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    $\begingroup$ In case the link you provide becomes dead, can you give a quick summary of one interesting idea that my help the querent as closely as possible to their original goal? Don't need much, but it would really help your answer. $\endgroup$ Dec 2, 2023 at 11:04

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