Suppose a person, standing at sea level, could use telekinesis to exert a force around a sphere with a 10 cm diameter so that it becomes one-way permeable to air. As time goes on, the sphere becomes more and more filled with air and therefore hotter, more dense, and exerts pressure outwards.

How much force would the person have to use on that sphere:

  1. to keep the air in there?
  2. for that air to turn into a plasma or for it to cause a fusion explosion?

Bonus questions:

  1. How much time would it take?
  2. What would it look like?

To be clear, I need an answer in Newtons or a similar unit to compare against other forces in my story.

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    $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. Please note that using only pressure, you're more likely to succumb to fusion before the air turns to plasma. Yes, you get heat with pressure, but you also become a liquid and then a solid. Plasma is what you get when you superheat a gas. $\endgroup$ – JBH Sep 3 '18 at 4:52
  • $\begingroup$ @JBH Thanks and thanks for the fast response. I'm also interested in force needed for a fusion explosion but decided not to ask that in the same question. If you know the calculation I'd love to know it. $\endgroup$ – Logical Sparrow Sep 3 '18 at 4:57
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    $\begingroup$ first, please review this question of mine to see if it answers your question. Changing this question to fusion will likely get it closed as a duplciate. If you don't see an answer to your question, then edit this one to reflect the "new question" including a brief explanation as to why it's not a duplicate. $\endgroup$ – JBH Sep 3 '18 at 5:01
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    $\begingroup$ I reformatted and slightly edited the question to improve clarity. If I changed the meaning of the question, please revert. $\endgroup$ – RonJohn Sep 3 '18 at 6:13
  • $\begingroup$ @JBH Using the answer from the question you linked and checking some numbers with Google, I found that given the ratio of the density of the core of the sun to the density of air is about 122449, in order to get my sphere to that density, I will need to compress about 513 m^3 of atmosphere to get to core of the sun density, which isn't all that much. The force needed to do it is a mystery to me though. $\endgroup$ – Logical Sparrow Sep 3 '18 at 18:51

You Need the Core of a Star, Preferably a large one

Our atmosphere is comprised of primarily nitrogen, oxygen, carbon monoxide, and faint traces of noble gasses. The amount of energy to fuse these elements is only found within the hearts of stars. Most small star's fusion cycles do not incorporate these elements except fairly large ones, so I'm not really sure you can measure said pressure in newtons. Especially since the amount of energy required to fuse an element is not measured in newtons of pressure but electron volts.


How much force would the person have to use on that sphere:

  1. to keep the air in there?

That would keep increasing as more atoms diffused into your sphere. (Remember that standard atmospheric pressure is 1.03 kg/cm2, and your sphere has a surface area of 314.16 cm2.

Just doubling the air pressure would require this many Newtons:

2 * 1.03 * 314.16 * 9.8 = 6,324
  1. for that air to turn into a plasma or for it to cause a fusion explosion?

As mentioned by @TCAT117, the same pressure exerted by the core of a large star.

Bonus questions:

How much time would it take?

Depends on how permeable the sphere is. A long time, though.

What would it look like?

A few thousand of Tsar Bomba.enter image description here


There won't be any fusion and it won't get hot. Apart from building up a pressure of approximately 100 atmospheres nothing will happen.

In physics we have the concept of the mean free path (MFP) which means how far a molecule/particle can travel until it collides with another particle. We normally know that this path cannot be lowered anymore because the air liquifies at this point (the molecules begin to stick together).

For air the MFP is 68 nanometers and given that the molecules have a "size" (it is a bit more complex than that) of approximately 1 nanometer, you can estimate that molecules cannot enter anymore at pressure of approximately 68 atmospheres. Room temperature is far too high to liquify air, so what you have is a supercritical liquid which is indistinguishable from gas, but has the same density as liquid.

So what happens is once your sphere is full, incoming molecules will be prevented from enter because there is no space anymore. No fusion, nothing.

  • $\begingroup$ But the thing about the mean free path is that it's a mean, not a hard value. The molecules around the sphere also having a gaussian spread of velocities means there should always be a case where the incoming molecule is fast enough to squeeze in - it's just an incredibly slow process. $\endgroup$ – bukwyrm Sep 3 '18 at 15:23
  • $\begingroup$ @bukwyrm Correct, you are referring to the Maxwell-Boltzmann distribution. But as the molecules with the necessary speed are getting rarer and rarer with increasing pressure, so the filling of the sphere will grind to a halt for human lifetimes. $\endgroup$ – Thorsten S. Sep 3 '18 at 15:31
  • $\begingroup$ @ThorstenS. I did some lookups and calculations comparing densities of the core of the sun and normal air. The sun's core is 122449 times more dense. If I need my 10 cm diameter sphere to contain that much atmosphere, I only need about 513 cubic meters of atmosphere compressed in my sphere, which is about the size of a house. Such a small volume compared to the atmosphere would suggest that the overall rate that my sphere would be filling with air would not be asymptotic so long as nothing escapes. I am of course guessing that that density would be enough for fusion. $\endgroup$ – Logical Sparrow Sep 3 '18 at 19:20
  • $\begingroup$ Could you clarify your answer? Taking it at its word, stars shouldn't exist because it would require violating the MFP restriction. The OP is forcing the air into the sphere (or, perhaps more accurately, he's putting a sphere arounda boat load of air and then contracting the sphere....) $\endgroup$ – JBH Sep 3 '18 at 19:28
  • $\begingroup$ @LogicalSparrow Erm, what part was not clear? There won't be any further compression than 100 bar, it does not matter that it is a neglible amount of air in the atmosphere. If you as human have a room with an one-way door, you still cannot go through if the other side is completely filled with persons. $\endgroup$ – Thorsten S. Sep 3 '18 at 19:31

How much force would the person have to use on that sphere:

1.to keep the air in there?

I'm not sure force is the right way to conceptualize this one. If it is a 1 way permeable membrane somehow only made up of energy I think it is more reasonable to think of it as a different dimension than a physical container. If it were a physical container there would be relationship related to the pressure difference between its inside and outside and it would cease being permeable when the inside was greater than the outside. However, if you considered it more like a 4th dimensional rift then it sounds a bit more plausible simply because we know less about how that would actually work.

2.for that air to turn into a plasma or for it to cause a fusion explosion?

Fusion and plasma are different. Fusion is when an atom gains atomic mass in the form of a proton thereby releasing energy. Plasma is a highly energetic state of matter.

Hydrogen, being the smallest atom, is the easiest to fuse which drives the majority of stars specifically the smallest. The higher in mass you go down the periodic table, the more energy is required to fuse elements. Elements like Nitrogen (7) and Oxygen (8) form at end of life of small stars and are more prevalent in mid size stars. To fuse oxygen requires temperatures of 1 billion Kelvin or 3.299999e+8ºN. I provided the link so you can look up the requirements for fusing air. Off the top of my head air is roughly 70% Nitrogen, ~20% oxygen, and then various other gases.

Bonus questions: 1.How much time would it take?

Time is relative, however long it takes you to capture the amount of mass you want and raise it to the right temperature and pressure. This is very much based on the limitations of your magical system.

2.What would it look like?

Since a 4th dimensional rift has never been found, there is no knowing how it would react with physics as we know it. Though if light and energy are allowed to escape the sphere then I would expect it to appear much like a star, most likely igniting the atmosphere or in the very least burning every onlooker with ultraviolet radiation.


I think that the process could be improved by making the sphere permeable only to some compounds or elements.

Maybe the psychic person could heat and energize the air around the sphere and split up the molecules of elements and compounds in the air into separate atoms, and make the hydrogen atoms from water vapor the only atoms that can penetrate the sphere. So there will be a gradual build up of hydrogen inside the sphere, until hydrogen is by far the most common element inside the sphere, and oxygen, nitrogen, carbon dioxide, argon, etc. are only traces.

The free hydrogen that doesn't go into the sphere will react with oxygen and burn, producing heat and water vapor. There should be a very impressive zone of fire and glowing hot air around the sphere, and possibly an impressive zone of fire and devastation on the ground below and around the sphere. The psychic person might get caught in the fire and killed, ending the process.

I suggest that the upper limit of the density within the sphere would soon be reached because the atoms within the sphere would repel any atoms crossing through the permeable sphere. Thus the sphere wouldn't fill up with hydrogen enough to fuse.

Therefore, I suggest a more complicated course of action involving enlarging and shrinking the sphere and changing how permeable it is.

First, create a vast sphere in the air, and make it totally impermeable to molecules, atoms, and ions. Then shrink the sphere to a fraction of its former volume, thus multiplying the density and temperature of the air inside it.Then make the sphere permeable from the inside to the outside. Almost all the super hot air will rush out of the sphere, burning everything around it, and making the interior of the sphere almost a vacuum.

Then make the surface of the sphere impermeable and enlarge it again, making the vacuum inside even thinner. And when the sphere is greatly enlarged, make it permeable only from the outside in, and let only hydrogen in. Then heat up a vast volume of air around the sphere, so that molecules break down into atoms and ions. Only hydrogen atoms can pass through the surface of the sphere into the vacuum inside.

Some of the hydrogen atoms will remain outside the sphere and combine with oxygen. There should be a very impressive zone of fire and glowing hot air around the sphere, and possibly an impressive zone of fire and devastation on the ground below and around the sphere. The psychic person might get caught in the fire and killed, ending the process.

So the almost vacuum within the sphere will fill up with hydrogen atoms until they become so dense that they repeal all hydrogen atoms from entering the sphere. Then rapidly shrink the diameter of the sphere , thus increasing the temperature and pressure, until eventually the temperature and pressure is enough for an instant massive fusion reaction, a hydrogen bomb.

Or maybe stop the shrinking at the exact moment that a slower fusion reaction starts and use it as a power source.

I haven't done any calculations but it would take an immense amount of energy for all the heating up of outside air and all the compression of the size of the sphere.

And I assumed from your question that it would be done using psychic powers, so presumably the psychic person would be magically drawing energy from some unspecified outside source.

Or they could be using a highly advance science and some vast technological machines to do it.

  • $\begingroup$ Thanks for your answer. You've made some interesting assumptions that don't quite fit with my telekinesis mechanics, such as the ability to tell individual atoms apart. The sphere can be shrunk and grown so that can speed up the process and I have thought of using some similar techniques. I do like your idea of using explosions and vacuums in certain ways though. $\endgroup$ – Logical Sparrow Sep 3 '18 at 18:33

Your first problem is that you'd be mainly trying to fuse Nitrogen not Hydrogen. This only happens in latter stages of nucleosynthesis and requires immense pressure.. I believe it would be the pressure required to cause electron degeneracy. You should investigate the 'Chandrasekhar limit'. You might even be able to work that in somehow. Your next problem is that a ball of degenerate matter that large would probably require the whole atmosphere. One teaspoon of the stuff weighs the same as Everest.

  • $\begingroup$ The core of the sun has a density of 150 grams per cubic cm. A 10 cm diameter sphere of that would weigh 78.5 kg. I am of course assuming that the density of the core of the sun is enough for fusion of air. $\endgroup$ – Logical Sparrow Sep 3 '18 at 19:56
  • $\begingroup$ Our star.. in fact any main series star won't fuse anything other than hydrogen until it has first swollen into a red giant, exploded and collapsed into a white dwarf. A white dwarf's core density is many orders of magnitude denser than the core of our sun. And I'm not sure the small amount of hydrogen in this ball would fuse when polluted with that much nitrogen.. but there my knowledge runs out. Perhaps the hydrogen would raise to the surface as mentioned above and fuse there? $\endgroup$ – Richard Sep 3 '18 at 20:20
  • $\begingroup$ Actually.. anything heavier than helium.. helium burning is what causes stars to swell I think. $\endgroup$ – Richard Sep 3 '18 at 20:21
  • $\begingroup$ That's a fair point but the question being as ridiculous as it is supposes that the character has arbitrarily powerful telekinesis so is stronger than what is needed to get even carbon, nitrogen, and oxygen to fuse. Alternately, if hydrogen starts fusing first, the extra temperature and pressure from that can be useful too, either for its explosive value or to trigger fusion of denser elements. I need to know how much force needs to be applied to get the explosive effects. $\endgroup$ – Logical Sparrow Sep 3 '18 at 20:27
  • $\begingroup$ I redid the math with the density of a white dwarf. The weight of my sphere with that density would be 524,000 kg, rounding up, which is short of the weight of Everest by a factor of 300 million. $\endgroup$ – Logical Sparrow Sep 3 '18 at 20:40

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