Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

"The Portal problem" (Classical Mechanics solution)

  1. Jan 20, 2016 #1

    Lately I've seen many different explanations to the following problem based on "Portal" and "Portal 2" videogames (which I personally played and enjoyed).
    Since the explanations didn't convince me, I drafted my own.
    Hope you find it intersting and we can discuss it :)


    http://41.media.tumblr.com/tumblr_mbbrvkhFEz1ro2gsmo1_400.jpg [Broken]

    Proposed solution:

    Proposed explanation:
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Jan 20, 2016 #2


    User Avatar
    Gold Member

    Wouldn't portals make free energy possible?
  4. Jan 20, 2016 #3

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    ... where? What were they?
    ... why didn't you find them convincing and how does your explanation address the issue?

    You'll get a bigger audience by providing this sort of information.

    I'd go for B on the grounds that you go through a portal exactly as you go through a hole in a thin partition ... only the exit result is displaced and rotated to the position and orientation of the exit portal.
  5. Jan 20, 2016 #4
    I don't think so, as I said, they're active dispositives; so, in fact, they would require energy to opperate, as far as I'm concerned.

    In 9GAG; sources:





    And there are many more.
    All those explanations don't seem to work with the physics rather than try to distort them to prove their points.

    I'm trying to give a sensible explanation to the phenomenon using a transformate approach to the conservation of linear momentum.

    And for your solution, think that the block itself has no linear momentum associated; hence, as I pointed out in the PDF, I think it should be A.
    Last edited by a moderator: Jan 20, 2016
  6. Jan 20, 2016 #5


    User Avatar
    Gold Member

    So if I put a portal at the bottom of a pool and one on the ceiling and put a waterwheel inbetween what happens?
  7. Jan 20, 2016 #6
    As you would expect, you'll generate power from the waterwheel. However, the portal itself would have given that potential energy to the water on the first place.
  8. Jan 20, 2016 #7


    User Avatar
    Gold Member

    But the water goes through the ceiling and back into the pool and back through the ceiling.... how could it have the potential energy for infinite trips?
  9. Jan 20, 2016 #8
    It's not that the water has energy for many trips; but just that every time it passes through the portal, the portal itself does the required amount of work to put the water to a non-equilibrium state.
  10. Jan 20, 2016 #9

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    The game rule is "Portals do not move along the portal direction." (I thought it was that they do not move at all, a rule broken in the neutrotoxin room in Portal 2, but I was corrected by Gabe Newell himself) You can test this yourself in Chamber 10 in Portal 1.

    So this goes back to the question "What do the laws of physics say if we break the laws of physics?" only here it's not real physics but game physics.

    And the Cake is a lie.
  11. Jan 20, 2016 #10
    Yet I think we can derive an insteresting discussion from my assertion, if you're willing to give it a look :)
    Last edited by a moderator: Jan 20, 2016
  12. Jan 21, 2016 #11

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    ... where from?

    Presumably there is something catching the difference in momentum too ... basically, the part of the portal operation that makes it obey conservation of energy and momentum is hidden from the player.
    You can test the pmm in-game by setting entry portal in the ground and firing an exit to the ceiling directly above, then dropping a cube through.
    The cube will wizz by for a bit but is quickly ejected ... but the portals are not diminished in any way. They would need to be drawing energy from someplace.
    Possibly, since only some surfaces support portals, the surface itself contains conduits for the portals to get their energy, with a large power source hidden someplace else.

    Try attaching the coordinate system to the orange portal, see if the result is consistent.

    What would make the discussion "interesting" in the scientific sense is if there is some way to test the ideas experimentally, within the game.
    As it is, the long-fall boots are tricky enough... gravity does not appear to work the way we are used to.

    Is it not possible to summarize your "assertion" so we don't have to follow links?
    There are several flaws in reasoning - the most important I think being that you assert that the object emerges through the blue portal at the same rate that it enters the orange one but has "zero valued linear momentum" once it has completely emerged.
    How does something emerge at speed but afterward have zero velocity?

    Apparently the situation given in the problem statement is one that does not appear in the game (check?) ... if a portal were projected on a surface that moved in the direction of the portal, then the portal would, presumably, fail (This is what seems to happen in P1.10). The question posed in the problem statement does not arise. However, moving portals have been observed in portal 2, is that correct? Therefore an experiment to see if the portal motion (that would be relative motion between the endpoints) affects the velocity of the object passing through.
    Last edited: Jan 21, 2016
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook