# Thinking with PortalsOctober 1, 2011 12:14 PM   Subscribe

A physics question about the climax of the game Portal 2. Spoiler alert!

In the climactic scene of Portal 2, Chell, the protagonist, opens a portal from the Earth to the Moon. There is explosive decompression between the two portals, sucking Chell and Wheatley through the portal and to the surface of the Moon. Chell is rescued by GLaDOS, the portal is closed and the game ends.

What would happen if that portal were left open indefinitely? Would the Earth's atmosphere eventually be sucked into space?

That sounds right ... but wait a minute ... the Earth's atmosphere is already open to the vacuum of space, with gravity holding it down to the Earth's surface. Moreover, gravity is shown to work just fine between portals, so why do we think there would be explosive decompression happening in this fictional scenario?

What's going on here, physics-wise?
posted by Cool Papa Bell to Science & Nature (36 answers total) 5 users marked this as a favorite

The analogy would be a water-filled tank with two valves: one near the top and one near the bottom. If you open the valve near the top, little or no water will escape, because it's held in place by gravity and will not flow "up" to leave the tank. Opening the valve near the bottom causes all of the water to escape. The fact that the portal leads to the moon matters less than the fact that it leads to an environment where the air pressure is effectively zero. Finally, the moon's gravity is not strong enough to retain an atmosphere. Although the rate of air loss will slow down as more air goes from the Earth to the moon, the moon will constantly lose it to interplanetary space.
posted by Nomyte at 12:21 PM on October 1, 2011 [1 favorite]

Earth's atmosphere IS open to space. Earth's oceans are also "open" to the air at the surface. That said, do you see how a portal to the bottom of the ocean from, say, your living room, might cause some... difficulties? It's a question of pressure.

Another consideration is that the moon is inside the gravitational field of Earth; would air sucked out of Aperture eventually come back down to Earth?
posted by Rinku at 12:22 PM on October 1, 2011 [2 favorites]

But yeah, the pressure differential would definitely drive air out at first, but I don't think it would degas the whole planet. I'm really not sure. It would be fascinating to work this out, though.
posted by empath at 12:46 PM on October 1, 2011

I think you can clarify the problem by positing a box full of an idea gas equal to the pressure of earth's atmosphere inside of a vacuum of infinite size and punching a tiny hole in the box.
posted by empath at 12:48 PM on October 1, 2011 [1 favorite]

(ideal)
posted by empath at 12:48 PM on October 1, 2011 [1 favorite]

Best answer: ...the moon's gravity is not strong enough to retain an atmosphere.

Per Wikipedia, the moon's gravity is greater than that of Titan and Titan has an atmosphere.

I think that what would happen is that the Earth's atmosphere would flow through the portal; the moon's gravity would be insufficient to hold all of it and most of it would overflow into space, but some of it would be retained; and it would stop flowing when the pressure was equalized on both sides of the portal. The Earth would end up with an air pressure equal to whatever the maximum possible atmospheric pressure the moon can hold.

My analogy would be if you had two different rubber balloons inflated to the same size but made of different thicknesses of rubber and you connected them with a straw. Air would flow from the balloon made of thicker rubber, which is squeezing the contents more tightly and thus contains more air at a higher density and pressure, into the other balloon until the pressure was equalized. The relative thicknesses of rubber are equivalent to the relative gravity between the Earth and moon.
posted by XMLicious at 1:12 PM on October 1, 2011 [3 favorites]

I wonder if the air is shooting out away from the lunar surface at some velocity would be a factor? Would it all just geyser out into space rather than forming a nice pool of air around the hole?
posted by Artw at 1:24 PM on October 1, 2011

If it is a portal that one can step easily through, then you have to assume that the traveller is not asserting the entire energy required to lift themselves from the earth to the moon. That would mean that something else was doing the work for the traveller, lest they encounter the portal as a hill as tall as the distance from the surface of the earth to the earth-moon laGrange point (the first step is the hardest).

Such a portal would strip the atmosphere from the earth, if left open. If we knew the diameter of the portal, and its altitude above mean sea level, , high-school physics would allow for calculation of an approximate timescale.
posted by the Real Dan at 1:35 PM on October 1, 2011

I wonder if the air is shooting out away from the lunar surface at some velocity would be a factor? Would it all just geyser out into space rather than forming a nice pool of air around the hole?

The average speed of nitrogen molecules in Earth's atmosphere (at the surface) is about 500 m/s. Escape velocity from the surface of the moon is about 2400 m/s. Some fraction of the molecules in our atmosphere are travelling faster than this, of course, but not very many; most of them would be pulled back to the surface, at least initially, by the Moon's gravity.

Longer-term retention of the Moon's new atmosphere after the portal closes, of course, is a different story. Some small fraction of molecules will, by random chance, end up with enough energy to escape the Moon's gravity entirely. Since the Moon's gravity is much weaker than Earth's, this fraction will be larger than it would be on the Earth, and so the atmosphere wouldn't last nearly as long as it has here.
posted by Johnny Assay at 1:37 PM on October 1, 2011 [1 favorite]

Per Wikipedia, the moon's gravity is greater than that of Titan and Titan has an atmosphere.

Yes, but the moon's gravity is not strong enough to retain an atmosphere given how close to the sun it is. Atmospheric pressure on the moon is effectively zero. See more here on some interesting interacting factors.
posted by Nomyte at 1:56 PM on October 1, 2011

Response by poster: Is this an accurate statement? The air pressure differential between the Earth and the Moon is sufficient to overcome the gravity that holds the air against the surface of the Earth in the first place.

If so, that makes NO sense to my (addled and uneducated) brain. It's like getting free energy. Gravity holds you down ... until it doesn't? Wha...?

That would also infer that at some point, after years and years of outgassing, you might reach a point where the air pressure isn't strong enough to overcome gravity, leaving a thin atmosphere on earth.
posted by Cool Papa Bell at 2:14 PM on October 1, 2011

Response by poster: I think that what would happen is that the Earth's atmosphere would flow through the portal; the moon's gravity would be insufficient to hold all of it and most of it would overflow into space, but some of it would be retained; and it would stop flowing when the pressure was equalized on both sides of the portal. The Earth would end up with an air pressure equal to whatever the maximum possible atmospheric pressure the moon can hold.

This starts to make sense. There's a portal between two worlds, creating one "connected system," with the Moon as weak point in the system. Earth's gravity holds it all down ... until it hits the weak point of 1/6 gravity, and then it escapes right there.
posted by Cool Papa Bell at 2:20 PM on October 1, 2011

I haven't actually played this game, but I think I've seen a little video clip from it of a character passing through a portal, and if I'm understanding it correctly it basically works like the "stargate" depicted in the television series of that name.

If it's like that, as the Real Dan says the atmosphere flowing through the portal doesn't have to work against gravity at all - anything that passes through the portal is instantaneously transported from the Earth to the moon, so it doesn't require any force or energy the way it would if you were to fill a steel tank with air on Earth and physically transport that tank to the surface of the moon.
posted by XMLicious at 2:42 PM on October 1, 2011

Actually, come to think of it, there ought to be lots more wind when they open the stargate than they actually depict in the television show, assuming that the air pressure is different on each planet they connect to.

When they connect to another stargate with a higher pressure than Earth's, there would be a headwind as they walk into the gate, with the air coming in to Earth; whereas when they connect to a location with a lower air pressure there would be a tailwind as Earth's atmosphere flows onto the other planet.

Another thing is, you could have a portal where both ends are on Earth, but one end is at sea level and the other is at the top of Mount Everest or something and wind would gush through the portal out the Mount Everest end where the pressure is much lower. In that case the wind would just blow continuously even if the portal was open permanently because all of the air would be dumped back into the Earth's atmosphere. (This would effectively be a perpetual motion machine, because the portals are fictional devices that violate conservation of energy among other things.)
posted by XMLicious at 2:54 PM on October 1, 2011

Well, if the portal acts as a tube between points, air wouldn't gush through it any more than it would gush through an enormous hose with one end at sea level and the other on Mt. Everest.

Except that a hose has multiple (infinite) pressure gradients working on the air inside the hose, from one opening to the other. A portal has just one pressure gradient - the immediate difference in pressure between the top of Everest and the one at sea level. Airflow would definitely happen in the Portal case.
posted by bfu at 3:13 PM on October 1, 2011 [1 favorite]

Portals travel is instantaneous. It's as if a circle in space suddenly joined onto another, distant circle in space with no intervening distance.

The energy blob that creates the portal, however, usually takes some time to get where it is going... Perhaps it accelerates, so we don't have to wait 3 days for it to get to the moon.
posted by Artw at 3:17 PM on October 1, 2011

(on preview, as bfu says) The reason why air wouldn't gush through a hose like that is because the hose itself is full of a column of air, in a gradient from sea level pressure up to the mountaintop pressure.

Think of one end of the portal being underwater and the other on dry land: the water would gush through the portal.

Or, one end at the bottom of a grain silo half-full of styrofoam packing peanuts and the other end at the bottom of a silo half-full of steel ball bearings. The ball bearings would flow into the packing peanut silo, at least to some degree (that example being more complex due to friction.)
posted by XMLicious at 3:19 PM on October 1, 2011

Best answer: The air pressure differential between the Earth and the Moon is sufficient to overcome the gravity that holds the air against the surface of the Earth in the first place.

Why doesn't all of the air on earth rest against the crust in a thin line? Obviously, pressure is enough to overcome gravity.
posted by empath at 3:50 PM on October 1, 2011 [1 favorite]

"If so, that makes NO sense to my (addled and uneducated) brain. It's like getting free energy. Gravity holds you down ... until it doesn't? Wha...?"

If portals like the ones used in the game were possible, they would indeed be a source of free energy and the first law of thermodynamics would be invalid. Consider the case where you put a portal in the floor and and connect it to a portal in the ceiling directly above. You could drop an object through the floor portal and let it continue falling through in an endless loop until it reaches terminal velocity, then catch it with a device that uses or stores its kinetic energy. Your object now has the same potential energy it started with, but you've generated a whole ton of free kinetic energy.
posted by tdismukes at 4:33 PM on October 1, 2011

Assuming that the portal didn't take energy to hold open, I guess.
posted by empath at 6:09 PM on October 1, 2011

Why doesn't all of the air on earth rest against the crust in a thin line? Obviously, pressure is enough to overcome gravity.

I'm not quite sure by what you mean by pressure overcoming gravity; the gravity is what causes the pressure by exerting a force on all of the molecules of the atmosphere.

Compared to the density of gas out in space, the atmosphere of Earth is crushed into a thin film against the Earth's surface. If it were all released off into space - if the Earth suddenly and magically disappeared leaving just the atmosphere behind - the atmosphere that currently fills just a hundred miles near the crust would fill a staggeringly immense volume, bajillions of times the size of the Earth, once gravity wasn't compressing it and it expanded back to the density of gas in the solar wind or interstellar space.

Or, if you took all the gas making up the Earth's entire atmosphere and dumped it into a black hole where the force of gravity is effectively infinite, it would be squished down into an infinitesimally small volume of infinitely high pressure, much smaller than what it takes up within the Earth's gravity.
posted by XMLicious at 2:25 AM on October 2, 2011

And your point is? Gravity exerts a force. Pressure exerts a force. If one is greater than the other it overcomes it.
posted by empath at 4:04 AM on October 2, 2011

Best answer: Part of my point was answering your question, "Why doesn't all of the air on earth rest against the crust in a thin line?"

For the rest - looking at the bit of CPB's comment you quoted which you seemed to be agreeing with, I'm guessing that you guys are saying that in the case of air flowing through the portal from the Earth to the moon is an example of pressure iovercoming Earth's gravity. This is what I don't understand, because the pressure and Earth's gravity aren't opposing forces in that case - gravity isn't trying to resist the air flowing through the portal, gravity is ultimately what's pushing it through the portal: it pulls down on all of the air molecules above the ones immediately next to the portal and that force is what pushes the latter molecules through the portal. (Like in my analogy with the ball bearings here, although that's an imperfect analogy in some respects.)
posted by XMLicious at 4:36 AM on October 2, 2011

Gravity exerts a force. If all of the air doesn't immediately collapse to the earth's surface, there must be an equal opposing force holding it up. That force is pressure. And it would be the same force that pushes air through the portal. Gravity pulls in the opposite direction, so it can't be that.
posted by empath at 4:47 AM on October 2, 2011

I guess I don't understand how gravity pulls in the opposite direction of the air flowing through the portal. That doesn't seem to be true to me.

I agree that there's a force which causes molecules in general to repel each other by one degree or another (which I guess would be electrostatic force or something like that? I don't know exactly, and it may be a combination of multiple forces) which might be what you're calling pressure, but if that didn't exist Earth wouldn't have a surface at all, and I don't think that this force overcoming or exceeding the force of gravity is the reason that air flows through the portal in the scenario we've been examining.
posted by XMLicious at 5:10 AM on October 2, 2011

Molecules don't repel each other, they're generally neutrally charged.

Everything that moves either moves because it was already moving or because something is exerting a force on it. Gravity is a force that pulls everything towards the center of mass. If something isn't moving towards the center of mass of the object, then it's being moved by a force that isn't gravity.
posted by empath at 5:51 AM on October 2, 2011

I'm a bit skeptical about your claim that movement away from the center of mass of an object can't be the result of gravity; for example, the slingshot effect results in movement away from the center of mass of a body but that movement is caused by gravity. I think you may be tangling up concepts of movement, acceleration, and inertia. But in any case, none of what you say above appears to explain how air flowing through the portal in this scenario would be the result of pressure overcoming gravity.

Whether or not it's related to charge, there is some reason why molecules bounce off of each other when they collide rather than passing through each other as happens when a weakly-interacting particle like a neutrino collides with most matter. That is what I was referring to when I described moleculesa as repelling each other.
posted by XMLicious at 6:40 AM on October 2, 2011

Before starting to inquire too deeply into Portal physics, it is important to remember that Portal physics is broken from the get-go in that it allows effects that break real-world physics (e.g. connecting a portal in the ceiling with one in the floor, mounting a water wheel in the room, and pouring a bunch of water down the floor portal will get you a perpetual motion engine). It's important to keep clear in your mind just which bits of physics you're willing to set aside in order for your Portal world to behave the way you want it to.

Since any analysis of gravitational effects near a portal is going to go badly wrong, it's best left out completely. Just concentrate on air pressures and be done with it.
posted by flabdablet at 9:20 AM on October 2, 2011 [1 favorite]

Gravitational affects don't appear to pass through the portal - so as a persons or objects in the vicinity of a portal are subject only to local gravity, and upon passing through are subject only to the gravity on the other side. If that were not the case it could allow for some interesting effects - would standing under a portal that enters onto a roof looking down have an antigravity effect?
posted by Artw at 9:29 AM on October 2, 2011

I'm a bit skeptical about your claim that movement away from the center of mass of an object can't be the result of gravity; for example, the slingshot effect results in movement away from the center of mass of a body but that movement is caused by gravity.

The sling shot effect is due to space time being curved and an object following a geodesic.
posted by empath at 1:09 PM on October 2, 2011

(which yes, is gravity, as is an object orbiting, but that wouldn't be what would be happening in the case of air flowing through a portal).
posted by empath at 1:10 PM on October 2, 2011

Look, if you open a balloon upward, what happens to the air inside? Why?
posted by empath at 1:14 PM on October 2, 2011

Oops. Not high-school physics. At least, not my high-school physics.
Line integral in a scalar field. Navierâ€“Stokes equations. Spherical portals.
posted by the Real Dan at 3:48 PM on October 2, 2011

Look, if you open a balloon upward, what happens to the air inside? Why?

The same thing that happens if you open a balloon facing downwards, or sideways, or on a space station in zero g where there's no up or down at all. The way it behaves - the phenomenon of gas in a region of high pressure flowing to a region of lower pressure - has nothing to do with pressure overcoming the Earth's gravity. Just as flabdablet points out, in the case of the air flowing through the portal pressure overcoming the Earth's gravity has nothing to do with the result.
posted by XMLicious at 9:05 PM on October 2, 2011

The airplane doesn't take off, because the air is being sucked out of the atmosphere.
posted by DoctorFedora at 12:39 AM on October 3, 2011

Response by poster: The way it behaves - the phenomenon of gas in a region of high pressure flowing to a region of lower pressure - has nothing to do with pressure overcoming the Earth's gravity.

Well, after reading the answers above, I think the best way to communicate it is the "two systems become one system with a weak point" metaphor.

Gravity on the Earth holds the atmosphere to ~14 psi. Suddenly, we "punch a hole" in that system to a spot where it's ~0 psi, because the Moon's gravity can't maintain 14 psi.

So, it's not so much as pressure overcoming gravity, so much as it is a hole in the system.
posted by Cool Papa Bell at 9:58 AM on October 4, 2011

« Older Having regrets about moving...it's eating me up...   |   Love of place Newer »