<\/td><\/tr><\/table>In my previous post on Portal<\/a>, some people were talking about how the Portal Device (PD) could easily be made into a perpetual motion machine. This is something that struck me about Portals as well. I can accept various inexplicable magical technologies, but I can’t accept violations to the first law of thermodynamics. I have rigorously adhered to the laws of physics my entire life, and I’m not about to change that over some sexy new portal technology. Let’s see if we can’t get a portal that respects the laws we hold so dear.<\/p>\nLet’s assume that the PD is maintaining the portals. It’s got a power source that can supply some finite level of power, and the PD itself is required to keep the portals open so that they are not self-sustaining. The technology is pretty magical, but we want to keep it from obviously and flagrantly violating physics.<\/p>\n
<\/td><\/tr><\/table>The obvious and simplest way to get free energy from a portal is to place one at the bottom of a swimming pool and the other somewhere above the swimming pool. Add a waterwheel between them and enjoy the free energy. Some people suggested that you could avoid this violation by just having the portal consume lots of energy to remain open. But swimming pools and water wheels are just the simplest way to make free energy. You could put something of greater mass through the portal – say, depleted uranium ball bearings – and get more<\/i> energy. You can also move the exit portal higher to increase energy yield. So, you can’t just say that the portal gun uses “a lot” of energy in an attempt to balance out the free energy you’re getting, because once the portal is open there is no hard limit on how much mass you can cram through it, and thus no fixed limit on how much energy you can derive from it. There are practical limits, sure. But physics doesn’t care about what is practical. <\/p>\n
<\/td><\/tr><\/table>Let’s instead think about what we’re doing when moving stuff through the portal. We’re “creating” potential energy, which, if we want to obey the laws of physics, needs to be “paid for”. If your bike is at the top of the hill, you can ride down without exerting yourself because of the potential energy. But of course, you had to get the bike up there<\/em> in the first place. You paid<\/i> for that potential energy, and in fact the second law of thermodynamics means that you overpaid<\/i> for it. You spent more getting to the top than you saved getting down. (Riding your bike in a vacuum would help, but then nobody could hear your bell.)<\/p>\n
<\/td><\/tr><\/table>In order to solve this, let’s assume the Portal expends energy as stuff goes through<\/i>. So, holding a portal open doesn’t require “much”, just some arbitrary (but non-zero) overhead so that we can give lip service to the second law of thermodynamics. But any mass you put in will require as much energy as would normally be needed be move the object from A to B within the existing gravitational field. So, if you stuff a brick in and it comes out ten meters above you, then the portal will consume enough energy to lift a brick ten meters. (Don’t stand under it, BTW.) A better way to think of it (so that you don’t get confused worrying about how<\/i> the brick is transported up) is to assume we need to expend as much energy as you could hope to capture from the now-falling brick with a 100% efficient machine.<\/p>\n
<\/td><\/tr><\/table>In the game, objects halfway through a portal are never bisected if the portal disappears. The object is chucked out of one side or the other. Turning off the PD should cause the portals to slam shut. But now we have a paradox. Something<\/i> must happen to an object occupying a portal as it closes. The object needs to move to either side (which takes energy) or it needs to be sliced in half (which also takes energy) or it needs to hold the portal open. Expelling objects can’t work, because some objects could be