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Generally, when someone uses telekinesis to fly, it's either by riding an object that can be levitated, or by applying the power to their body. Would there be any advantages to alternative techniques, such as the titular vacuum balloons?

Since "telekinesis" is vague and overpowered if unconstrained, I'd focus on something vaguely resembling Green Lantern Constructs: "hard light" force-fields that must be physically connected to the caster in some way, which can be shaped by the caster. The more force exerted, the more energy is used, so it is more costly to move more massive objects, to resist greater forces, or to affect a larger area.

So, were a telekine to create a balloon-shaped force-field, and expand it so that it is empty of air, would this have any advantages as a means of levitation, over just standing on a force-field and lifting that directly? If it depends on the precise numbers, what would be some general estimates of those?

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  • $\begingroup$ Since its a force field it doesnt actually have any weight and moves with the user's mind. You'd be better off creating a forcefield inside a hollow object to force the air out and create a vacuum in there. The advantage is ofcourse less energy and focus wasted that can be focused elsewhere. $\endgroup$ – Demigan Oct 10 at 13:03
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If you can move the force field, then the energy to move oneself by standing / riding on the force field would be much less than to create a vacuum capable of lifting yourself.

A benefit, assuming you don't need to be conscious to keep the shape of the force field, is you could float while asleep to travel all night.

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Physical advantage? Uncertain. Storywriting potential? Certainly.

"Lifting yourself by your own bootstraps", while a typical comic trope to give hypothetically non-flying characters flying abilities, ignores momentum and the laws of motion. But more importantly, it's boring. If you can stand on an object and lift it, you might as well just wrap yourself in the field like a suit and fly wherever you want. Similarly you can wrap up another object in the field and move it wherever you want, as long as you have the energy. This pretty much negates the whole point of a construct-maker, since almost anything you can do with a construct can be done more easily by just moving the thing directly.

A regular telekinetic who can ignore momentum has basically 3 responses to every situation - fly out of the way, lift the thing and move it, or throw something else at it. BORING.

A shapeshifting field with a stable mass, but otherwise obeying the laws of physics and momentum, makes for a much more interesting story and allows for more creative uses of powers. For example, let's say someone throws a large rock at the character. A "regular" telekinetic is simply limited by the weight of the rock - if it's light enough they can lift it, otherwise they can fly out of the way. But a construct-maker has to get creative. You can create a spring to absorb the shock - but this will still throw you backwards, so is only viable if there's empty space behind you, or you'll get crushed. If there's a wall behind you you can make a stick between the rock and the wall, but this may damage the wall. Maybe you want to damage the wall? Or you can dodge. Maybe create a rapidly-lengthening stick underneath you - or a grappling rope to pull you out of the way. Or maybe you'll turn it against your enemies by creating a pipe that redirects it or a lasso to swing it around - but you have to brace the construct against something that can handle the strain, or the force of the boulder will fling you around as the laws of physics demand. All interesting solutions that will be ruined if you just let the characters move things freely.

You can fly by creating a vacuum-balloon, but this is slow. Perhaps in some situations it may be more practical to construct a pole underneath you and lengthen it, lifting you in a single direction more quickly. The ability to use your powers creatively, while avoiding the simpler "I lift the thing and move it where I want" can make the story more interesting.

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Let's assume that your telekinetic bubble is massless, so in order to lift a human of mass $m$ you need a bubble which displaces that mass of air. At standard temperature and pressure air has a density $\rho = 1.225 \mathrm{kg/m^3}$, so we need to displace a volume of $m/\rho \approx 57 \mathrm{m^3}$.

Inflating your telekinetic bubble would not be quite the same as inflating a balloon, because you're not pressurising a fluid inside the boundary that causes the boundary to move, but moving the boundary despite the pressure differential across it. But we can use a thermodynamic argument to say that the energy you put into the bubble must at least equal the energy (kinetic and potential) of the air that you're forcing out of it. Otherwise you would have the heart of a perpetual motion machine *.

The internal energy of the gas is given by:

$$U=c_vmT$$

Where $c_v$ is the specific heat capacity (about $0.7\mathrm{kJ/kg \cdot K}$ for dry air at room temperature). For our bubble this works out to be around 15 megajoules.

Conversely, the energy required to levitate a mass $m$ directly is just the gravitational potential energy you have to give to them, which is $E=mgh$. To levitate the same human, say, 100m, you need to expend 68 kilojoules. You must also do work against the air resistance, which depends on the shape and size of the human and the speed you want to move them at. Let's say you want to move at $5\mathrm{m/s}$, ie a fast run, good for escaping bad guys and sticky situations. The air resistance force is:

$$F = \frac{1}{2}\rho AC |{v}|^2 \approx 3 \mathrm{N}$$

Where $AC$ is the area and 'drag coefficient' of the body. To move 100m against this force requires an energy input of 300J. A trivial component at this speed, but would rise quadratically with the speed; moving at superman speeds of 50m/s the drag energy would quickly overtake the potential energy.

This clearly makes it substantially less efficient to lift a mass by telekinetic bubble than it would by just levitating directly.

However, the bubble approach may be efficient for very high lifts. The energy required for direct levitation scales linearly with height, whereas the bubble approach is probably roughly constant: having created a buoyant bubble, it will continue to rise with no additional energy input as long as it remains buoyant. Even with this crude calculation, raising an object to a height of 20km would have the same overall energy input and would require the telekinetic to apply force continuously for over an hour, compared to a one-time hit to create the bubble at the beginning and then able to just maintain it as it rose.

In short, I agree with IndigoFenix that this is probably not the most efficient application of telekinesis, but it would make for a great world.

(* I think you could make such a perpetual-motion machine by having a bubble-generator inside an engine cylinder. Expanding the bubble pushes the air out into the rest of the cylinder, increasing the pressure and raising the piston, then you equalise the pressure and then seal the cylinder again before collapsing the bubble, which then drives the piston down with the same force. But I'm not completely clear what the 'over energy' is.)

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