Container of Vacuum

Suppose you have a ball made of a material with the same density as water, i.e. if submerged in water, the ball would neither sink nor rise. So if the ball is a sphere of the material, it stays in place in a column of water. If it’s full of a material with a lower density than water (e.g. air), it rises. If it’s full of a material with a greater density than water (e.g. iron), it sinks.

The question is, without changing the size of the ball, what happens if the middle of the sphere has no material at all, just vacuum? (Assume the material is strong enough to withstand the associated forces etc rigid and impermeable, so it doesn’t collapse the vacuum or fill it with anything, it’s a hard hollow ball with a vacuum inside – don’t buck the hypo)

My guess is entabbed below, but I am still uncertain.
[tab]It would rise, and faster/with more force than it did with air. My reasoning is that the sphere as a whole is less dense, as measured by mass/volume, i.e that vacuum is a very low density.

I confused myself (and hopefully some readers) by framing the setup as filling the ball with a ‘material’ that has higher density or lower density than water. That suggests a rule: whether it rises or falls is a function of the material’s density. In that frame, it’s uncertain what will happen to the ball when you remove all the ‘material’, because my rule was shitty and narrow – I think this is an example of overfitting, but I don’t know if I’m using that concept correctly.

But the rule should have been about the density of the ball as a whole. As far as the water is concerned, the ball is a black box; it doesn’t matter what the material is, it matters how much water the ball displaces and how much mass is contained in the displaced volume. In that framing, it seems that it would rise.

I’m still a little uncertain because it doesn’t seem like vacuum should float. It kind of makes sense that vacuum should repel matter that way, because that seems like another way to say “gravity”, but it’s still weird.[/tab]

[EDIT: clarified that the ball is rigid, per comment here]

It will sink to water level.

Very simple.

Vacuum or no.

After all, for all practical purposes, it IS water.

The ball is less dense, it goes up. I can’t see any problem with that.

A vacuum suddenly appearing ‘in the middle of water’ - if the water in a sphere shape was teleported away, would instantly fill with water, and upward faster than downward, I would guess, since below is under higher pressure than above, though gravity might make a race of it and lead to differing directional filling rates at different depths. But your ball prevents the water from filling the ball, so up it goes, even the uppermost portions of the water, being pulled down by gravity, wanting it more up than them. … -sea-level … this-be-an

I think the sphere would act the same, as it would when filled with air… though the vacuum might cause it to act differently in/on the water though, but I’ll have to think about how.

I haven’t read your tabbed answer btw… I’m saving it for later, till an end conclusion has been arrived at.

are you assuming for some reason that the ball wouldn’t immediately collapse into itself becoming once again density neutral?

Yes, sorry if that wasn’t clear. I’m picturing the material that the ball is made of as rigid and impermeable, and sufficiently so to keep the vacuum in place and not lose volume.

I’ll edit the OP to make that clearer.

Vacuums are by definition: weightless (neutral weight) although there’s probably an atom or 2 kicking around.

Being that it’s weightless does not mean lesser or greater weight.

Your exotic material has all the same properties as water in terms of mass and density.

That means it will act just like water.

It will sink to the top.

Water is obviously more dense as you go down, so it won’t sink to the bottom.

Let me expand on this: your exotic material has the property of not being saturated or super-saturated. That’s why it won’t sink to the bottom

why should it behave any different if it collapsed or not, anyway
in both cases, it is filled with emptiness
or you can say it is emptied

the only properties at play are the properties of the ball’s material
and if the material is density neutral
the whole set is in fact density neutral

What would happen to this ball is what would happen to a sphere shaped ball of water with a vacuum in its centre
The ball will behave exactly like the the water that it is in because they both have the same density
So the fact that it is a ball made of a different material does not mean it will behave any differently

The answer given by phoneutria directly above mine says the same thing albeit worded differently so I agree with what she has said
The density of the water and the ball are the same so it neither sinks or floats but simply blends in [ allowing for the fact that the ball is visually distinctive ]

I have not looked at your tabbed answer because I do not want it to influence mine in any way so that is it for what its worth [ excellent question by the way ]
But I would be very surprised indeed if there is no empirical evidence by way of experiment to actually determine what does happen in this particular scenario

A standard air-filled ball, would bob around and finally settle… a vacuum-filled ball, might remain in continuous motion and never settle.

I agree Surrep… great question. What conjured up such a question, Carleas?

If it collapses completely, no vacuum remaining then it does not float. If it does not collapse it will float. The ball, when not collapsed, is less dense than water. A ball filled with air rises because it is less dense. A ball with just a few atoms of oxygen nitrogen etc. inside it is even less dense and rises. A ball with a pure vacuum rises and is even less dense as a total object in the water.

It’s not less dense
The inside of it does not have the property of density
It’s filled with emptiness

A helium balloon flies
An empty balloon does not

I wish you had responded to the middle step of having very few atoms.
First off, yes, a ball with a vaccuum inside is less dense then one with metal in it and less dense than one with air in it. density = M/V. So, the ball Carleas mentioned has some mass, the outside, but no inner mass. Compared to that same ball exterior around air or water, it gets a lower density. Yes, it is strange to talk about the density of a vacuum. But we are talking about the overall density of an object with a vacuum in it. And it has a lower density than an object with that exterior with air in it or water. Just as if you lower the pressure of the air the density goes down. Keep lowering the pressure inside the ball and you lower the density.

And an empty balloon is NOT the equivalent, because it is flaccid. It has a much lower volume than one filled with air.

And any object has spaces inside it with nothing, given that matter is mostly empty space. Carleas’ ball would just have much less, because its inside would be devoid of matter. The objects density is lower, by the formula we use to determine density.

Carleas is assuming that he can maintain the volume. That is the entire point of his set up. Same volume but less inside to the point there is nothing inside. Fill a balloon with air and the volume goes way up. If we accept Carleas’ premise that it maintains its volume, then the density goes down Of the object.

You could argue that currently we cannot make a perfect vacuum in things, but we can come very close, and further it is an assumption in the OP.

Hm I see your point, i wasn’t considering the volume at all. I stand corrected.
So if you can make a large enough container of a lightweight material able to withstand the pressure, and “fill” it with vacuum, will it launch into space?

It was never a trick question.

Thanks Mags and Surreptitious (though Karpel Tunnel points out that I am sadly not the first to come up with it). I don’t remember the precise train of thought, but I can tell you that it was inspired by some… medicinal herbs… And I was watching Captain America at the time, but I don’t think that’s related.

I think this is the most compelling way to show that it must rise. Decreasing density of what’s inside the ball will always make it rise, and rise faster, and that continues to 0. It would be even weirder if there were a discontinuity at 0 where you take out the lasts molecule of nitrogen and suddenly it stops.

Very weird, right?! But that seems to be the conclusion. Also weird is that if you take away the ball, so it’s just vacuum, it sort of teleports to the surface of the atmosphere, as the water collapses into it, and the atmosphere collapses in to where the water was. By keeping the ball at a constant volume, it forces the vacuum to travel up to the surface of the atmosphere more slowly.

There are a few things going on with intuitions here. First, our intuitions around vacuum are understandably bad, because we don’t experience vacuum or vacuum-filled objects most of the time. We also think of the earth as repelling air, but really it’s just that water displaces air because it’s more strongly attracted to the earth – but air is still attracted to the earth. Then there’s some weirdness in treating vacuum, which is a non-thing, as though it were a thing, and giving it attributes like volume and density, and then mistakenly imagining that it’s moving through the atmosphere, or teleporting from the water up to space, when really “it” isn’t doing anything, “it” isn’t a thing, it’s an absence of things. It makes for a fun and unexpected mind-fuck.

A good method to formulate an argument, as any.


You folks have it all wrong. “A lightweight material able to withstand all the pressure”???

So let’s assume for a moment that you create a perfect vacuum in a material that never scrunches.

Like I said before… weightless is a different concept than lesser or greater weight. It doesn’t effect the buoyancy of the object relative to its volume, mass or density.


Atoms are shedding every second from every object…

So let’s say we have some exotic material that is a contained vacuum. The external forces will cause this exotic material to shed atoms on all sides equally when exposed to the environment.

Since it’s equal on all “sides“ of the sphere, it will have neutral buoyancy.

It won’t float up and it won’t sink.

Sounds like you are talking about your bubble of belief - a vacuous bubble with no weight that never grows up nor settles down.

Maybe the ionesphere. I mean, the outisde has some mass, and high up there, the density is very low. Perhaps some kind of nanosphere might make it to space.
It seems ridiculous to thank you for ‘standing corrected’ but it is so rare in forums like this that someone can manage to say something like this, it makes me grateful, despite the silliness of thanking.