Antigravity? We don’t use that word. Science fiction is two shelves over to the left.
Surprisingly, there is no upward-pushing force. It’s just that there is less downward-pulling force on the helium than there is on the air that surrounds it because helium gas is lighter than an equal volume of air.
Gravity pulls less strongly on the lighter helium atoms than it does on the heavier air molecules. The air will therefore tend to move down past the helium, or, same thing, the helium will be observed to move upward past the air. If you were inside the helium balloon, you might be wondering, Why is all that air rushing downward past me?
When you release a piece of wood under water, you’re not surprised to see it zoom upward through the water, are you? That’s because wood and water are such familiar materials that you expect wood to float on water.
Helium and air, however, being gases instead of solids or liquids, are not as familiar to us; we can’t see them, pour them, grab them, or throw them. But they are matter (substance) all the same, made up of tiny particles that are tugged upon by the Earth’s gravitational field, and they respond in the same way as solids and liquids do. The force of gravity is proportional to the mass of the particles, be they in the solid, liquid, or gaseous form.
When you let go of a helium balloon outdoors, several things happen. As it ascends, it encounters changing conditions of both air pressure and air temperature. As far as the pressure is concerned, it decreases pretty regularly as the altitude increases. That’s because the atmosphere is a layer of air enveloping the earth, held down tightly against the globe by gravity.
The higher you ascend into this layer, the less of it remains above you pressing downward, so you feel less air pressure. And so does the balloon.
At any given time, a rubber balloon is a certain size because the outward-pushing pressure of the gas inside is counteracted by the inward-pushing pressure of the atmosphere outside (plus, of course, the inward-contracting tendency of the rubber). When the atmospheric pressure decreases, the outward-expanding tendency of the helium gas can prevail, and the balloon will expand. So as the altitude increases, the balloon tends to get bigger. Hold that thought.
Now, what are the effects of decreasing temperature? We know that all gases will try to expand when heated and contract when cooled. That’s because the molecules of a hot gas are bouncing around faster and pushing harder against any walls that are attempting to contain them. Our particular container of helium is ascending into colder and colder air; the average temperature of the earth’s atmosphere decreases from about 65 degrees Fahrenheit (18 degrees Celsius) at sea level to about 60 degrees below zero Fahrenheit (-51 Celsius), at an altitude of 6 miles (10 kilometers). So as the balloon rises and gets colder, it will tend to shrink.
We now have two counteracting tendencies: an expansion due to the atmospheric pressure decrease and a contraction due to the atmospheric temperature decrease. Which tendency will win out?
The rules that govern the expansion and contraction of gases are well-known; scientists lump them into a mathematical equation called the gas law. Using this equation, they can actually calculate the effects of varying pressures and temperatures on a gas. If you do the calculations for our rising helium balloon (and I did), you find that the expansion due to the pressure decrease is a much bigger factor than the contraction due to the cooling.
So the net effect on the balloon is that it gets bigger and bigger as it rises until, popl, the rubber will stretch no farther and it bursts, eventually fluttering down into somebody’s picnic mustard. The helium gas, now unfettered, just keeps rising through the atmosphere until it reaches a level where the air is so thin that a balloonful of it would be just as light as a balloonful of helium, and that’s where it will remain until doomsday.
Well, not exactly until doomsday. Because of winds, weather, and other mixing phenomena, we can always find a little helium in the air at any altitude, on the average, about five helium atoms for every million air molecules. And at the top of the atmosphere, some of them even escape from the Earth entirely.
Moreover, we must admit that other happenings can interfere with our rather neat picture. Our balloon may not even get high enough to explode, because the amount of helium in it isn’t enough to carry its payload of rubber high enough, and it’ll settle out at a maximum altitude. Then, winds could blow it about for days, until enough helium has seeped out (helium atoms are extremely tiny as particles go, and can diffuse right through the rubber) that the weight of the rubber brings it down. You’ve probably seen that happen to a balloon left on your ceiling for a couple of days.
And by the way, many helium balloons these days are made out of aluminized Mylar, a tough plastic film coated with a very thin layer of aluminum, rather than rubber They will last a lot longer and go a lot higher before meeting their fate. Commercial jet airplanes have been known tc spot them miles high, speeding along in the jet stream.