You heard it like me, someone just asked about the physics of Aperture Science's neat Portal Gun. I'll try to give an account of what we know from the material that Aperture recently made available and from what can be deduced from demonstrations of the product.
Unfortunately I've been unable to get any additional information or confirmation of my findings from Aperture themselves as the only person I was able to contact, a Mrs. Glados, told me on the phone that "all the persons who are still alive are currently unavailable, but as you seem to have an interest in doing science, I'd be happy to show you around our test chambers. We are open 24/7. I hope you like orange. Can you bring some cake? I don't want to lie to you: we seem to have run out."
So let's get going, shall we? Here's how the gun works:
The central part of the Portal Gun is a miniature black hole with a Schwarzschild diameter of a few centimeters. Now black holes this size usually radiate strong amounts of Hawking radiation, resulting in their rapid evaporation. This is counterbalanced by an extraordinarily simple device, in the form of a cooling fan that blows evaporated virtual particles back onto the black hole's horizon.
Above and below the black hole, there are two ring singularity rings that can be made to rotate one way or another in order to communicate angular momentum to the black hole (this is why operating the gun while moving creates the impression that it is being gently pulled out of your hands, by gyroscopic effect). Making the hole rotate results in the event horizon opening up and revealing the ring singularity inside.
The fan is then sped up suddenly behind the singularity to blow part of it forward into the portal intake manifold, focused by the three quantum shaping prongs that you can see moving around the front end of the gun when firing a portal. The focalization is necessary but not sufficient, as the singularity needs to be stopped by a surface charged with Z bosons. This is why Aperture is building special panels for their test chambers, the only surfaces that can stop portals in place. Whenever you attempt to fire a portal on a different surface, the portal continues its course through space unaffected while the dye is stopped by the wall and produces a spray of color at the impact point. This of course makes portal guns unusable in practice outside of a specially equipped test facility.
It's worth mentioning at this point that it is a trivial cosmetic detail that orange or blue dye is injected around the singularity at the precise moment when it passes through the intake manifold. The dye then enters into the gravitational orbit of the ring singularity and remains there as long as the portal remains active, creating the rotating colored shimmer that we all know and love.
Now here is the tricky part: when you fire the second half of the portal, it is emitted as a second ring singularity that rotates counter to the orientation of the first one and most importantly while being entangled with it. It is this quantum gravitational entanglement that does the job of bridging space-time on both sides of the portal. Contrary to common belief, Aperture's portals work by creating a quantum tunneling channel between two locations and not at all by creating a wormhole. Creating wormholes requires considerably more energy, making the process unsuitable to the construction of portable devices. Furthermore, as Black Mesa recently showed, we do not know how to control which universe the other end of the wormhole comes from.
It is often asked whether portals are limited to connecting points in space or if they could permit time travel like wormholes do. Well, if you can find a way to aim in the direction of the past or future, which are unfortunately orthogonal to the three dimensions of space that we are familiar with, then yes, it is possible. Good luck with that.
Now that we understand how the gun works, we can look at the energetic consequences of punching tunnels into space. One thing that all people who have operated a portal gun report is that if they create a portal between two places that present a potential gravitational energy gradient (in layman terms, different altitudes), there is wind coming out of the top portal and entering the bottom portal. This wind gets stronger with the altitude difference.
This is very easy to understand as the difference in potential energy between the two ends of the portal manifests itself like any potential energy gradient: by a force from highest to lowest potential.
That leads us to an interesting point which is that air is affected by portals in exactly the same way that we or any massive object such as a weighted cube is. If for example you punch vertically aligned portals on the floor and ceiling of a test chamber, the air in the room will start falling through, creating a strong wind that will blow downwards, attracting passers by in virtue of the Venturi effect.
Another amusing consequence is that falling through such a portal pair, you can reach speeds much higher than the normal terminal velocity in the same conditions of pressure and temperature.
This effect is actually used by Aperture Science to produce the considerable energy that their testing facility consumes: weighed cubes are left to fall between two portals in a tall well, and a turbine harvests the gravitational energy transformed into kinetic energy along the way.
Now of course the next question is where does this energy come from? Well, the portal itself carries little energy and it doesn't seem to decay as energy is harvested. Many speculations have been made about this but the most plausible explanation seems to be as follows.
Conservation of energy, as everyone knows, is not a principle in itself but rather is a consequence of the laws of physics being the same at all points in time. Ergo, all you have to do is to break the laws of physics in order for energy to stop being conserved. And you'll have to admit that a persisting quantum tunneling device is as weird as it gets.
The effect of this is that constants such as the fine structure constant or the gravitational constant are probably varying slightly as more stuff gets dumped into portals, possibly resulting eventually in atoms failing to retain electrons, galaxies to dissolve or the Sun to transform into a black hole. But don't worry too much, that should take at least a few centuries to happen if my calculations are right.
Well, this is it in a nutshell. I'm going to take questions now.