## physics, portal, and reddit

Of course, there are a few caveats to any discussion about the physics of portals. Portals don’t actually exist and even in the game world they can’t move (except for one special example). So, not only could this scenario not happen in real life, it couldn’t even happen inside the game. That being said, it’s still a fun thought experiment.

I would tend to argue that B is the correct answer and it completely comes down to relative velocities. The cube has a velocity relative to the orange portal, which means it also has a velocity relative to the blue portal. So when the cube pops out of the blue portal, it should have that same relative velocity.

Some of the comments arguing for A use people jumping through hula hoops or door frames in their response. The space on either side of a hula hoop or door frame maintains the same relative velocity; the space around each portal has a different relative velocity in the scenario depicted above.

I like this argument but I don’t agree with it. Reddit user Falconhaxx uses the means of transmission between the two portals to argue for A. Basically, the cube can either instantaneously appear at the blue portal or move through in infinitesimal slices. Either it pops from one portal to the other or it gradually moves through. In the case that it instantaneously appears at the other portal, option A is correct. Rather than doorways, think of the portals as transporters. Whatever state an object is in when it hits the first transporter will be carried over as it instantly teleports to the other. So, if the cube has no velocity when it reaches the first teleporter, it would have no velocity at the second transporter.

On the other hand, if portals gradually move objects through (which is the stance I tend to take), then the cube would be ripped apart by changing forces. As pieces of the cube go through the portal, each feels an infinite force differential which would rip it apart. Think of dividing by 0. The forces on the cube change over no distance at all (gravity is pointing one way when the cube goes into the blue portal but gravity is pointing a different direction when it comes out), which is more than any piece of matter can handle. As a result, the cube would be sliced into infinitesimally thin slices as slice after slice of it moves from one side of the portal to the other (I bet you didn’t know you could use a variant of the word ‘slice’ four times in one sentence).

Of course, the latter argument could be used for any object going through portals. If the game played by those rules, Portal would hold the record for being the world’s first deli meat slicer simulator. Sure, the game breaks a few laws of physics (that sounds like a good topic for a post…), but it wouldn’t be fun if it didn’t.

In the end, relative velocities make much more sense. If the cube has a velocity relative to the entry portal, it should have a velocity relative to the exit portal.

Think I’m wrong? Comment and let me know!

Edit: This guy has a good point too. Does the relative momentum of the blue portal to the cube matter?

## Why teach with Portals?

Physics classrooms are behind the times. At school, kids use 1990s technology (if they use technology at all) but then go home and play ultra realistic games. [Sidenote: if you clicked on the first link in this sentence, you found PhET, run by CU Boulder, which I actually use all the time. I don’t mean to bash PhET at all because it’s actually awesome and shows some great examples of physics simulators. They’re perfect in many cases for their ease and utility in the classroom. But there’s a big difference between them and modern games.] There’s a huge disconnect between what kids are exposed to and used to in terms of technology and what we give them in school. It’s no wonder that students are falling behind in science.

As teachers, we’re receiving students with the built-in capability to be fully immersed within game worlds. Why not use those same skills and game worlds to teach them physics? Give them something they’re used to and simply reroute their attention from killing each other (which is, unfortunately, what normally happens within games) to building physics experiments. It isn’t much of a leap, especially considering that Portal 2 is, at its core, one big physics puzzle after another.

We’re wasting valuable resources when we don’t harness our students’ innate ability to use technology and lose themselves in game worlds. Using some careful planning, teachers can create a classroom environment where students manipulate digital worlds to create measurable scientific experiments that run on laws of physics. It’s a unique opportunity for educators. For the first time, we can let students play god and design their own worlds. We can put students in impossible situations (for safety concerns or otherwise) that run on actual laws of physics and instruct them to run tests and see what’s going on behind the scenes. They can build their own virtual experiments as valid as their physical counterparts in less time and with less effort. Teaching with Portal 2 is about students actively building and applying what they’ve learned.

Basically, Portal 2 is a tool that allows for easy creation, manipulation and sharing of virtual worlds that run on actual laws of physics. What can be done with it is only limited by the creativity of the educator.

## Why Portal 2?

Really, the question should read, “Why the Source engine?”

Over the years, game developers have recognized that the easiest way to make ultra realistic games is to build a game world that follows laws of physics. Rather than using a blank slate and adding objects with imbued laws of physics, game developers have created a physics backbone that determines how objects should behave. Let’s say a game developer wanted to build a game with two boxes of different masses and dimensions that both follow the laws of physics. Being that they have different densities and surface areas, they should fall through the air differently. Either code written for the boxes themselves can handle their respective behaviors, or the game itself can analyze the two boxes and use laws of physics to determine how they should behave. Instead of worrying about intrinsically applying laws of physics to each object individually, game developers simply create the objects and allow the physics engine to determine how the laws of physics affect each object.

Essentially, modern game developers use specialized physics simulators, called physics engines, to make games.

There are numerous physics engines in use by modern games. One of which is Valve’s Source engine.

While Source doesn’t stand apart in its technological capabilities, Valve has inadvertently added features that make it the perfect solution for classroom physics simulations.

1. Source is free and widely distributed. You’ll notice in the Wikipedia list above that it’s listed under freeware. Creating games or modifying games under the Source engine is free and is actually encouraged. If you acquire any game made by Valve (some of which are free), you get all the tools necessary to mod with Source.
2. Source is a little old. The first iteration of the Source engine was announced back in 2004, and while it has been updated many times since then, it’s reasonable to say that any computer bought within the last three years or so can run it. Though schools don’t have the most up-to-date technology, it’s much, much easier to run Source on school computers than it is to run other modern physics engines which require nothing less than dedicated gaming PCs.
3. Source is easy to use. A few months ago this statement wouldn’t have been true. Applying the tools game developers use to make video games is a tedious and time consuming process, for good reason. Games are meticulously designed, with all the nuances game developers need to include to create an immersive world. It takes about an hour for the uninformed to learn how to create a basic room with four walls, a ceiling and a floor using Hammer, Valve’s world creator for Source. A similar process for any other physics engine would take about as much time. You’re looking at hours of tedious work to make something simple because of the amount of freedom given to the user. However, in May, Valve released the Portal 2 Puzzle Maker, which shrinks the process of learning level creation basics down from an hour to about 30 seconds. Of course, it isn’t nearly as powerful or detailed as Hammer, but it’s unmatched in terms of ease and intuitiveness.

Though Valve didn’t have education in mind, it created a physics simulator that is powerful, accessible, free and easily modifiable. In other words, it’s perfect for the classroom.