Helium Balloons · 4:14pm Sep 13th, 2015
Pinkie Pie clearly knows a thing or two about balloons, as her cutie mark would suggest. We can only speculate about the history of party balloons in Equestria. Did ponies play with blown up pigs' bladders in times past? Or did have more respect for their fellow hooved creatures and only start the party with the invention of rubber membranes?
Modern party balloons are a nice example of the parallel between child's play and experimental science. Invented by the British physicist and Royal Institution's top party planner Michael Faraday (who started the Christmas Lectures). In addition to being fun toys, balloons are the ideal tool for investigating the physics of buoyancy and the relative weights of gases.
The math behind this is pretty straightforward. The upwards force of a buoyant object (such as a helium filled balloon) is equal to the weight of the air it displaces, just as the force supporting a floating object is equal to the weight of the displaced water, as discovered by Archimedes in his bathtub in 3rd century BC Syracuse, presumably while playing with a rubber duck, or whatever bathtime toy was the fashion in the ancient world.
A helium filled balloon floats because the weight of the air which it pushes out of the way, exceeds the weight of the balloon, and the helium inside it. Helium is much lighter than the nitrogen-oxygen mixture we breath. The pressure pushing on the bottom is greater than that on the top, so up it goes.
Using this principle, the above picture shows us that Pinkie weighs the same as six balloons worth of air, or about 80g. Which is remarkably light considering the amount of cake she eats.
At this point the sensible comment is that Pinkie does not respect the laws of physics. Which is, after all, backed up by a substantial amount of other observational evidence. But before we get there let's have a bit more fun with this.
First, can we consider the possibility that the Equestrian atmosphere is unusually dense? Well if we take Pinkie's weight as closer to a real little pony, say 100kg, then we can show the density of the air to explain the above image is about 300kg per cubic metre. This is denser than cork, and would make moving through Ponyville more like swimming. When opening a bottle, the stopper would float off into space. While this could explain some features of pegasus flight, it doesn't quite fit the lightweight party atmosphere we see around Sugarcube Corner.
It does appear from the show that it is the balloons, rather than anything else, that has some sort of magic super-buoyancy. What are they filled with if not helium? Hydrogen is lighter, but that makes little difference as the gas inside is much lighter than the air outside (it's the difference in densities that counts). A zero-weight gas would have only a marginally higher lifting power.
Does Pinkie have access to some sort of special negative-density gas? Does she have some source of antigravity aerosol among her partillery? Or does it come out of her lungs? There is clearly a lot of magic going on in that pink pony to explain her metabolism. It's not unreasonable that she could breath out a lot more than hot air.
In our world, however, we have only helium to fill our balloons (you can use hydrogen, but that was a tendency to explode, and while that can be very cool in controlled circumstances, you have to be very careful to keep it away from the cake candles). This is a problem as helium gas is a limited resource. It's so light that it drifts away from the Earth into space. While it is very abundant in the Universe as a whole, spaceship technology is not yet at the point where it is economical to order new supplies from Jupiter. Instead we must make do with what is produced by radioactive decays underground and accumulates in gas fields.
But reserves are slowly running out, and prices are rising. And helium is required for more than party supplies. It's also invaluable in science in liquid form as it has the lowest boiling point of any element (-273C), making it the easiest way to cool superconducting magnets and other stuff for science or medical applications (such as MRI scanners). New reserves will be found, but in the long term we will need to make an effort to conserve this valuable resource. This has lead some to suggest that party balloons are a frivolous use, and helium should be reserved for serious science.
I'm not convinced by this argument. I've worked with liquid helium a lot. I once had a job running a helium liquifier in an underground laboratory. One liquid of liquid helium is equal to 1500 litres of gas. I've seen several hundred litres of liquid wasted in a day as inexperienced researchers make a mess of transferring it between containers. That's enough to fill some 50,000 balloons. And, even though I pride myself on being able to explain why science is cool, I'm not convinced that I could persuade a child with a balloon that my need is greater. To conserve helium reserves, it makes more sense to make more efficient use of it in laboratories than to raid childrens parties.
Here's a question for Pinkie:
If an object is immersed in a liquid in which it is otherwise buoyant, and both its bottom and the bottom of the container are atomically flat, will the object float if it is pressed flat to the surface of the container? It would seem that it shouldn't: the horizontal forces will cancel, and liquid being pushed from above can't flow below the object to induce a vertical force.
What say you, Pinkie?
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I'm inclined to think it would end up floating, though I don't know if it would be near-instantly or would take a while. Things to consider:
Buoyancy affects the sides of the submerged object too, so any irregularities or overhangs might be enough to let the liquid lift the submerged object.
The atoms of floor/object/liquid are all in balance on average, but not necessarily at any given instant. Even if an imbalance couldn't break the seal all on its own, it could weaken it so later imbalances would have more effect - kind of like van der Waals forces, where temporary fluctuations in electron distribution build up and create attractive forces in neutral particles.
The section of floor the object is resting on will be under less weight, since the object is lighter than the water it displaces. That difference might warp the floor enough to let the liquid get under the edges of the object and pry it up.
I'll admit I'm not an expert on fluid mechanics though, so I wouldn't be surprised if I'm missing something important. Hopefully someone with more knowledge will stop by and chip in.
He is simply a gas that has seen massive supply/demand spikes, and therefore has seen massive price swings. For a while, the US Government was paying above market prices to set up a massive reserve. Then in 1996, Congress (in its infinite wisdom) decided that the massive reserve needed to be sold off at a particular rate, which killed any new production. Now the reserve is running out of He to sell, and the 'World Wide He Shortage' doomsayers are once again in full voice. In truth, Kansas alone has a number of natural gas wells that crank out 2% He that haven't been commercially viable as He sources since '96, and now that the prices are headed back up to normality, so is supply.
It takes *time* to build any kind of manufacturing volume for *anything* where a swing in production can result in a double-digit swing in the world's need for a particular element or compound, oil included. I am constantly baffled by idiots who say thinks like "If the Keystone pipeline were approved today, it would take ten years for the first drop of oil to flow on it." Well, duh! It was first proposed and the financing started ten years ago, so if you nitwits would not have gone all sparse over the concept, you could be pumping gas from that pipeline into your SUV right now.
The same concept applies to the superconducting supercollider people who whine about balloons. You can bet that if Boeing were to build a factory that took 5,000 tons of liquid He to run, they would have purchased contracts for a steady supply before the first drop of concrete were poured. Somehow the genusi at these various research institutions seem to have forgotten the concept of 'Supply' and 'Sufficient Reserves' when they were putting together their fancy toys. Methinks a few Physics PhDs need to take a few Business 101 classes.
Bill: Ok, we've got the Superconducting Supercollider Smasher 6000 built and we're ready to throw the switch, right?
Phred: Yes, just as soon as we fill up the Helium tanks.
Bill: (pause) I thought you were going to do that last week?
Phred: Well, I had this research paper I was working on, and I've been binge-watching Game of Thrones--
Bill: Well, call up the company and have them deliver a few tanker trucks so we can get started.
Phred: Five hundred and seventeen of them.
Bill: Right. (pause) Five hundred and seventeen trucks? Won't that take some time?
Phred: Let me get out my slide rule. Carry the four. Just a little under a year.
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What would Pinkie say? Maybe: you mean if you stick a marshmallow to the bottom of your cup before you fill it with cocoa, will it stay stuck to the bottom or will it float to the top?
It stays at the bottom... until it comes unstuck
You're right in principle, if there is no pressure from below it won't move up. But in practice you would need a water-tight seal with the bottom, which is pretty difficult to achieve, unless you use some sort of sealant, in which case you could say you were sticking it to the base.
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An experiment one could try would be to place a fresh glass microscope slide at the bottom of a glass container (would need a very flat bottom), then pour mercury in very slowly so as not to disturb the slide. My hypothesis would be that the slide will not rise due to the strength of the van der Waals forces between the glass surfaces, even though its density is less than that of mercury (It's really hard to lift a slide from a microscope stage!). Imperfections in the slide might eventually allow mercury to seep underneath of course.
So yeah, essentially what Pinkie said
[Safety disclaimer: This experiment would be NSFL (not safe for lab)]