I'm writing a story about an icy alien culture which applies ultracold helium bombs that they can make appear at any place. The battle between good and evil translates in hot and cold. Though of course it remains to be seen which of the two is good or evil,

Helium bombs are just large balls of zero-kelvin helium. They can have a maximum radius of 1000 meter and can appear only in uniform spaces like liquids, gases, and empty space, which have to be extensive enough.

The main character tries to escape in a deep dark sea. The sea is 100 kilometers deep. If they mean, vicious aliens make a helium bomb appear, will the hero be able to escape? Will the ice that forms around the ball keep everything that's inside inside the crust? Or will no crust form so our hero gets haunted by the cold?

I asked this question in three different increasingly well-defined forms, on the physics site. It was said to be better suited here (it was even suggested that I tried to "beat the system" by asking it because it got closed three times and the reason given was that it was an arbitrary situation, and when moderators have their mind made up you indeed can't beat them...). The last form of the question can be seen here:


I didn't accept both answers though. The question about an ice layer forming wasn't touched upon properly.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 4, 2021 at 15:17

5 Answers 5


The ball would shoot upwards through the water.

Helium at 0 K will be solid. The projected density for solid helium is 0.187 g/cm3. . The density of water is 1 g/cm3. Maybe 100 km deep it is a little more than that but just a little.

The helium ball will shoot up through the water. The Leidenfrost effect produced by helium (vigorously!) boiling and bubbling away on all sides will prevent an ice rind from forming and also decrease water resistance that might slow the rise of the ball. The gas bubbles will still be cold and some water ice particles will form in the wake of the rising ball and be swept along, which would be kind of cool.

It is not stated where in the 1000 km sea your hero is. If they are 2 km below the ball they will not know it is there. Or 2 km off to the side. If they are directly above the ball they will see a giant mass of helium gas bubbles below them, rising ahead of the ball which is below the bubble. The ball is more dense than the gas bubbles and also has more surface area, so is rising slower.

If there are enough bubbles your hero might fall through them. They might land on top of the ball. They could get frostbite from this but again the Leidenfrost effect will protect them from freezing solid, as a cushion of helium gas will form between their warm extremities and the 0K ball.

If your hero is in a boat above this ball the boat could sink because the bubbles will disrupt buoyancy - an effect posited to happen in the Bermuda Triangle via bubbling methane clathrate deposits. Also all the helium will make their voice high, and their stogie might go out.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 5, 2021 at 3:43

100 000m down in an ocean on Earth(-like planet)

Pressure = 1GPa
Water phase = Ice XV from 0K to 130K, and Ice VI between 130K and about 330K(57C), above that liquid.

Unless your ocean is very warm, the water will already be a solid.

Assume solid:
The Helium sphere will just sit there, further cooling the ice around it. It will have massive buoyancy though, so if the ice cover over it is thinner than several km, it will break through and form an ascending sphere.
YES, sphere. Under that pressure, Helium is also a solid, even up to 50K or so. Maybe a bit of "molten" helium around the sphere from initial contact.
Slowly, very slowly, the heat from the surrounding water ice will permeate into the solid Helium, melting it all, and a plume of molten Helium will ascent up in a straw of frozen water, until it reached the surface.

Assume water hot enough (>57 Celcius) to be liquid:
The Solid Helium Sphere will very rapidly form a thin ice shell around it.
This ice layer will insulate the Helium sphere, keeping it mostly frozen, and the whole Helium+some ice ball will ascend rapidly. It might even remain frozen until it breaches the surface like an iceberg! But Helium has quite low specific heat capacity, and the molten helium will be an excellent convective heat mover, so I expect the whole sphere will melt in the 8 hours or so it takes to get to the surface. Once the Helium gets to within about 500m of the surface, it will flash to gas and make a huge surface explosion!

  • $\begingroup$ So even if the bombs contain small energy they can indeed be bombs! That is in fact the reason I uused these bombs. Like the aliens they draw energy. $\endgroup$ Aug 3, 2021 at 8:03
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    $\begingroup$ @DescheleSchilder By removing a ton (actually about 524 million tonnes!) of water and replacing it with a less dense material, the aliens have actually injected a HUGE amount of energy into the ocean at that spot. The temperature of the Helium is of secondary concern, it is the mass displacement that is interesting. You now have a column of water 100 km tall that really, really wants to fall down another 700m or so towards gravity, and all that is preventing it is some very squishy Helium. If they could replace that space with Void, it would be an enormously powerful bomb.(184kiloton nuke) $\endgroup$
    – PcMan
    Aug 3, 2021 at 8:09
  • $\begingroup$ Tha energy to displace the water I didn't yet consider! Interesting. Maybe they derived that energy by using other helium balls to extract energy. Or is this circular? $\endgroup$ Aug 3, 2021 at 8:15
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    $\begingroup$ @DescheleSchilder correction: 13 MEGAton nuke energy equivalent $\endgroup$
    – PcMan
    Aug 3, 2021 at 8:15
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    $\begingroup$ They are even more vicious than I imagined! 15 megaton is huge. Thanks! $\endgroup$ Aug 3, 2021 at 8:25

The helium would melt at a fantastic rate, and bubble off disappating the cold. Helium evaporates at a very low temperature: 4 degrees Kelvin. The hero's problem is the thermal equilibrium is achieved by dragging in surrounding heat. It is not a crust of ice but a ball. Not to mention that his sub can not be that insulated.

If their aim is perfect, he can not escape. Distance and speed are his only hope.

  • $\begingroup$ Wouldnt the water form a crust of ice around the ball of helium? $\endgroup$ Aug 3, 2021 at 9:03
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    $\begingroup$ The ball would not last long enough for that. It would explode. $\endgroup$
    – Mary
    Aug 3, 2021 at 12:06
  • $\begingroup$ Well thats the question. $\endgroup$ Aug 3, 2021 at 12:13
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    $\begingroup$ 4 degrees Kelvin is cold beyond human comprehension. It would explode anywhere where water is liquid. Like throwing a globe of water into the sun. $\endgroup$
    – Mary
    Aug 3, 2021 at 12:17
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    $\begingroup$ @DescheleSchilder anything below about 70km depth (100km was a bit ridiculous, Earth does not go that deep, max on Earth is more like 12-km-ish). for helium, look to researchgate.net/figure/… $\endgroup$
    – PcMan
    Aug 3, 2021 at 17:43

Note that a 1 km rock slamming into the earth from space could be a very bad day for everyone within a few hundred kilometers of it, but the physics aren't the same as suddenly displacing that much sea water and solid helium. Large extinction events on Earth are strongly correlated with 10km+ objects. They are so deadly, because they are large enough to impact the surface in tact. Smaller objects tend to break up and vaporize before impacting the surface. In this scenario, we have a large object that bypassed the Earth's atmospheric shield, and it's not anything like a natural object.

Unless this imaginary tech can first remove an equal volume of the water, before beaming in the 1km diameter sphere, it would have to displace all of that water, instantaneously. This would indeed be a very effective bomb. It would literally vaporize nearly a cubic kilometer of water, causing sufficient heat to rapidly melt the helium, which would add to the explosive effect and increase heating and pressure. A shockwave would travel back to the center of the sphere, heating more helium as it travels, and then bounce back outwards. Depending on the depth of the water, this shock wave will probably have sufficient force to vaporize or at least liquify all the remaining helium. How much water that would displace is a mathematical exercise I'll leave for someone else.

The total volume of displaced water would increase substantially as all that superheated fluid/gas expands. The shockwave from such an explosion would cause a rather large tsunami that would radiate in all directions at something like 1200 km/hr. and the shock wave passing through the lower levels of the ocean would probably kill everything for a considerable distance. The deeper your hero is, the more likely it is that the shockwave will destroy whatever vessel they are hiding in, or kill them directly if they are some kind of aquatic.

A rather large mushroom shaped cloud would rise up to at least the stratosphere, and a shockwave would travel outwards through the atmosphere. There would be a bright flash of light as a large volume of the atmosphere is suddenly ionized from the heat. All of this is nearly equivalent to a 1 km body of ice slamming into the earth, minus the momentum, but rather than being slowed, heated and exploding in the relatively thin atmosphere, it would encounter high pressure water immediately.

If any of the sphere survives in a solid or liquid state after the initial explosion, the sea will slam back into it and the subsequent shock wave would cause another explosion. The process would repeat until you had a very warm volume of sea water and helium bubbles, slowly dissipating heat to the surrounding water.

It's just a guess, but I'd say you probably don't want to be within a few thousand kilometers of such an event. There would probably be a large magma filled crater at the bottom of the ocean, that would also contribute to heating the area for quite a long time, and might spew sufficient toxic gases to cause a localized extinction event.

Come to think of it, the displacement of that much water will dissociate the water into mostly hydrogen and oxygen ions. The hydrogen would certainly fuse and possibly the oxygen as well. A 1/3 km hydrogen bomb core would be devastating.

So let's assume that the plot limits the rate of appearance of the object. We're already using more energy than a large cluster of starts give off in a lifetime, just teleporting that much mass over any distance. But it's not much of a bomb if it appears too gently, so you get to decide just how devastating this bomb is. In fact, it's the perfect weapon in that regard. Not only can you vary the size, you could vary the rate from say small fractions of the speed of light, all the way up to 99.999..%.

A more reasonable limit would a few tenths of the speed of light. Lowers the peak power requirements for the teleporter. Might even vary depending on the size. A large mass would have to have a lower rate than a small one.

So now you can dial-in the attack to a level that almost, but not quite extinguishes your hero.

Final addendum:

If the hydrogen fuses, that might cause a sufficiently forceful implosion to fuse the helium! Now we're talking shattered planet.

  • $\begingroup$ Do you think the sudden appearance of a huge massive ball is the same as that mass hitting the ocean with a high velocity? $\endgroup$ Aug 3, 2021 at 20:33
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    $\begingroup$ There's dynamical differences. "Slamming into" implies motion, which implies that there is a finite rate of displacement. An infalling object will have higher kinetic energy than one that simply appears somewhere, but that depends on whether these bad-guys can impart velocity on it as well right? I think the speed at which the water is displaced by the sudden appearance of the object, may in fact impart more kinetic energy to the surrounding medium than if it were falling into the gravity well, but I think there's an infinity in there somewhere that mucks up the calculations. $\endgroup$
    – jwdonahue
    Aug 3, 2021 at 20:40
  • $\begingroup$ A 1km helium ball suddenly appearing in empty space, just isn't much of a bomb. It's going to start slowly melting, but suddenly appearing inside another mass, seems like a very good bomb to me. In fact, if you targeted a uranium deposit, you might get a big enough blast to shatter a continent. $\endgroup$
    – jwdonahue
    Aug 3, 2021 at 20:44
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    $\begingroup$ Yup, speed of light is the limit, but that's more kinetic energy per gram than any 10km rock that ever hit the earth right? $\endgroup$
    – jwdonahue
    Aug 3, 2021 at 20:50
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    $\begingroup$ Your last edit is very enlightening. A thermonuclear bomb kilometers big. Mister Teller would be rubbing his hands! $\endgroup$ Aug 3, 2021 at 21:00

This is a fascinating question! There's a lot more to it then first meets the eye!

First, to answer the boring question: Will this kill the hero?


However, it won't be a very good story if the protagonist dies midway through, so as with any grand alien invasion, there's one aspect of Earth's environment that the aliens didn't count on, and it's going to be the Achilles Heel of their horrific weapon.

So, let's get started with this roller coaster of a science question:

There are multiple factors that come into play when dumping a solid, supercooled gas into a liquid substance, esspecially one as unique as liquit water!

  1. Water is one of the only substances in the known universe that reduces in density when it freezes. This is the reason why lakes freeze from the top down, rather than bottom up. So regardless of how you cool a body of water, it will always freeze from the top down!

  2. This counter-intuitive density behavior is made even weirder by the introduction of salt: Salt water has a lower freezing point then pure water and it also has a higher density. When you try to freeze sea water, this causes a natural desalination effect. As a result, the pure desalinated water rises as ice and the leftover highly-dense, sub-freezing brine shoots down in jets creating downward-growing ice spikes. Anything caught by these ice tentacles will freeze and die. Visual reference: YouTube: Brinicle, Underwater Icicle "Finger of Death"

  3. Now this sounds pretty bad, but if the hero can evade the slow-moving ice tentacles, he still has a decent chance of surviving due to one really cool physics effect:

  4. When you cool a gas to a liquid or solid state, its boundary layer boils very easily and turns to gas the moment it comes in contact with a warmer substance. This creates what's known as the Leidenfrost effect, where the boiling gas at the boundary layer forms a thermal barrier between the hot and cold substances. As a result the water won't freeze instantly! Here is a cool experiment where a guy tried to freeze seawater by pouring liquid nitrogen on it: YouTube: What Happens When You Freeze The Ocean? Pouring Liquid Nitrogen in the Ocean to Stop Global Warming

  5. But wait! There's more! The Leidenfrost effect is imperfect. When the object is big, the huge amount of cold matter will still have a sufficient thermal transfer rate to freeze the surrounding water (though not instantly). In this case, you will have ice forming around the sphere, and that’s where the hero’s situation suddenly becomes very dire. Ice and gas are good insulators, but not perfect, and the frozen helium inside will continue to slowly warm up. Eventually the increasing gas pressure will cause the ice shell to rupture...kinda like in this video: YouTube: Underwater Explosions - Slow Motion Dry Ice Bomb.

  6. Luckily, the explosion won’t be big since the whole sphere can't be uniformly encased in ice (due to the non-laminar effects of real-world turbulence in water), so it won't kill the hero. Instead, the hero is going to suddenly find themselves inundated by hundreds of back-to-back concussive blasts as the small helium pockets throughout the ice expand and fracture their ice blocks. These micro-explosions will probably feel like gut punches, damaging internal organs and rupturing ear drums. (It’s going to be a bad day for anyone in the vicinity when this starts happening.)

  7. If somehow the hero escapes the ice tentacles and manages to retain consciousness throughout the micro-explosions, their troubles aren’t over yet. Far from it. They will then need to swim up to the surface through a meat grinder. You see... all that ice we’ve been talking about, would have floated up to the surface and turned into the most dangerous form of ice you’ve ever seen: razor sharp ice sheets, crushing and grinding everything that gets caught in them (YouTube).

  8. Suppose the hero somehow climbs through the ice flow and claws their way up on to one of the ice blocks. They miraculously get through with only crushed ribs, broken arms and legs, and a badly shattered pelvis, but otherwise they're totally fine! However... now they will face their final two obstacles:

  9. First, all of the helium gas boiling off the sphere would have expelled most of the breathable air from the area, and the hero will probably suffocate.

  10. Second, if there is enough oxygen left for them to survive, the air will be really cold. This will cause edema, and (sorry for the graphic content) they will literally drown in their own bodily fluids as their longs fill up with water. Here (YouTube) is a good explanation from the 2000 movie Vertical Limit, and here (YouTube, graphic content) is another scene from the same movie where one of the characters is suffering from both broken ribs and edima, like your hero probably will be.

  11. Lastly, as the hero fades from life, staring up at the vast sky, they will be treated to an amazing sight. The intense outgassing from the sphere will create an unimaginably powerful storm over the ocean. Helium will rise rapidly as it warms, and normal air will drop as it cools. This will create waterspouts (tornados over the ocean—as opposed to hurricanes) the likes of which have never been seen.

But didn’t I say the hero has a chance to survive?


But, 'How???’ you ask.

  1. Well, here’s the kicker! Planets with a magnetic field like the Earth are extremely rare. So there is a very reasonable chance that an alien civilization won’t be familiar with the miraculous barrier that our magnetic field creates: the Ozone Layer.

  2. You see... as soon as the large volumes of helium gas begins rising up in the atmosphere, it will first punch a circular hole in the cloud layer and then shortly after through the ozone and upper atmosphere. When this happens, the local area will lose its shielding from the full power of the sun.

  3. The unexpectedly intensified solar heat will undermine the aliens’ attempt to kill the hero via freezing. It will help to maintain a survivable temperature and save the hero from getting edema! Moreover, the intense local heat (along with a nearly-endless supply of gas that's being heated by it) will cause strong winds to radiate from the center of this would-be-weapon-of-doom and blow away the blocks of ice like little sailboats carrying the hero with them.

  4. What’s even better is that amid the crazy raging storm this will cause, the aliens will lose any chance of tracking down the hero. The hero is then free to be picked up at sea and rescued!

So in the end, it is the weapon's own frightening power that offers our hero the chance for escape! :D

(Thank you for asking this question! I very much enjoyed thinking through this scenario in my head, and I'd be very excited to read your story about it!)

  • $\begingroup$ Note: Here I've assumed the sphere is floating on the surface. As one of the other answers pointed out, solid helium would be less dense than water. $\endgroup$
    – Vladimir
    Aug 22, 2021 at 18:33

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