In a video game series called Portal, we are given a gun, which shoots two portals: an orange one and a blue one.

These portals have the following properties:

  • Anything that goes into the orange portal comes out of the blue portal (instantaneously), and vice versa.

  • Only two portals may be fired at any one time; if another blue (orange) portal is fired, then the already-existing blue (orange) portal will disappear and be replaced by this new portal.

  • Portals can only be fired onto flat, stationary $^1 \ $ surfaces. If a portal is fired onto a stationary $^1 \ $ surface, and the surface then moves $^1 \ $, then that portal will disappear.

  • Portals may not coincide (i.e. two portals can't be in the same place at the same time)

  • When an object travels from one portal to the other, momentum is conserved, relative to the portal normal. i.e. $p_{\text{initial}}=p_\text{final}$, or, in other words, if a body enters one portal with a given velocity, it will exit the other portal with the same velocity.

This picture succinctly demonstrates what's going on: enter image description here

Now, assuming that such we can create such a gun, does the existence of these portals contradict any of the laws of physics? Is their existence possible, in theory?

$^1$ Relative to the planet that both the gun and the portal are on. For some reason, if the gun and the portal are on different planets (say, if you shot a portal from the Earth to the moon) this rule doesn't apply.

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    $\begingroup$ You might want to note that momentum/velocity are conserved relative to the portal normal. Otherwise, you would be launched down the wall after jumping through the blue portal. $\endgroup$
    – wchargin
    Dec 20, 2014 at 21:12
  • $\begingroup$ The portal surface can't move relative to the gun? So you can't move the gun at all and can't take it with you. What about the Earth's rotation? $\endgroup$
    – JDługosz
    Dec 20, 2014 at 22:14
  • $\begingroup$ @jdlugosz Good point. The portal cannot move, but if I had said that, some physicist would have corrected me and said that there's no such thing as absolute speed. What I mean is that portals can't move relative to the Earth (or whatever planet they're on). $\endgroup$
    – alexqwx
    Dec 20, 2014 at 22:31
  • $\begingroup$ Related topic. $\endgroup$
    – Irigi
    Dec 21, 2014 at 9:43
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    $\begingroup$ Thinking about this, and the "free energy" problem in the scenario, why doesn't the air in the room form a vicious, ever accelerating wind until it hits lightspeed? Now that would be exciting. $\endgroup$
    – Zan Lynx
    Jan 15, 2015 at 0:41

7 Answers 7


There was a paper on this when portal came out, though I cannot seem to find it right now.

The end answer was Yes, portals are physically possible, but the picture is probably not. The worst hand waving has to be done in the process of making the portal. Defining why two sections of space-time would get cemented together in this way is a job for worldbuilders like us, not for scientists.

Instantaneous communication is no problem, because we could curve space-time to make the link. Effectively this would decrease the "straight line" distance between an emitter and a receiver to keep the speed of light rules in place. There would be some annoying General Relativity effects during the process of placing the portals to deal with, but I don't think it would violate anything. It'd just be nuisance.

Conservation of energy requires a bit more excitement. After all, Portal's ability to break the laws of physics in this way were legendary. Most energy is conserved without effort. Momentum in equals momentum out, and that accounts for a great deal of the potential conservation issues. However, there's always gravity.

These portals would need to interact with gravitational fields to avoid violating the conservation of energy. Whatever the underlying mechanism is for gravity, it would have to bend the direction of gravity around the portal. It would, in fact, have to bend the direction of gravity everywhere to ensure gravitational potential energy can still be modeled as a divergence free gradient field. Hopefully the mechanic would be localized, so it would have less and less of an effect as you walk away from the portals, otherwise people on the other side of the world would notice when you plopped them down. That being said, the more "evil" you are in your portal placement, the more the laws of physics will have to be "creative" to keep the laws intact.

The picture of how portals work would probably not apply. Because you simply jumped off a ledge and were carried by gravitational energy to the portal, through it, and off to the far ledge, the far ledge would need to be at a lower gravitational potential even though instinct directs us to assume that, because its "higher," its at a higher potential energy. We are used to working in fields where gravity always points down. I would expect most "reasonable" portal field implementations would not have this behavior (for a reason I'll mention at the end), so it is unlikely but not impossible that the picture would work.

Now for the hard parts

The edges of the portals will be tricky. As Peter Masiar put it, we like space time to be continuous. These sharp edges will end up being mighty torrents of quantum rage... not infinitely thin, but you'd have to spend a lot of work stabilizing them because... well... you cut a hole in the universe pulled itself through itself, and then glued it all together. What do you think would happen? Fortunately, the portal graphics clearly show this chaotic dangerous region around the edges. For your safety and the safety of others, please do not lick the edges of the portal.

The final issue would be the nastiest: the energy of creating a portal. When you drag spacetime around, the conservation of energy is STILL in effect. If you change the direction of the gravity field, you will need to effectively recalculate the potential energy of every object in the universe. If you "raise" the universe, you have to put energy into it as you make the portal to offset this potential energy. If you "lower" the universe, it will emit energy which will have to be reclaimed, or lost as gamma radiation. Don't have any tumors? Well, unless you're wearing lead lined underwear, we've taken care of that too!

So in the picture example, if you do the following:

  • Rest state A
    • Place the portals
    • JUMP!
    • Disassemble the portals
  • Rest state B

Then you MUST have put in mgh worth of energy into the portal to lift you from Rest state A to Rest state B. It looks like that's about 2m higher than before. The portal gun has to do the work... you don't get the energy for free.

Also worth noting is that the gun doesn't get to "know" how you plan to use the portals, so it probably wont get to be so efficient. It will probably have to warp a lot of gravity and "move a lot of objects." Likely it will require a nuclear power station's worth of energy to generate a consistent gravity field, and then unleash a nuclear bomb's worth of energy when you collapse it back to normal (with the difference between work in and "loud bang" out being exactly 1 Chel mass * 9.8m/s^2 * 2m. I tried to weigh her, but the scale couldn't display enough digits).

So sure, you can bend the laws of physics to make the portal work. You just can't break them.

  • $\begingroup$ I think the instanious traverse would cause problems, with time-travel issues. Make the traverse a good fraction of lightspeed to avoid the issue. In fact, transporter "stepping disks" from Niven's universe is probably a better implementation than wormholes. $\endgroup$
    – JDługosz
    Dec 20, 2014 at 22:20
  • $\begingroup$ @jdlugosz Could you expand on that first sentence? How would instantaneous travel cause problems? $\endgroup$
    – alexqwx
    Dec 20, 2014 at 22:38
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    $\begingroup$ I tried to weigh her, but the scale couldn't display enough digits. - GLaDOS'ed that for you $\endgroup$ Dec 21, 2014 at 10:32
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    $\begingroup$ What is simultaneous? Different obsevers have different time axes. I think this issue is a separate question: why do wormholes violate causalty? In general, if you can outrace light thennyou can find some way to send messages back in time. $\endgroup$
    – JDługosz
    Dec 21, 2014 at 10:46
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    $\begingroup$ OMG: "For your safety and the safety of others, please do not lick the edges of the portal." I have to remember that when I'm GMing SF RPGs. $\endgroup$
    – ShadoCat
    Mar 8, 2018 at 19:35

The main problem with the portals is that while they maintain kinetic energy (your speed going in) they do not maintain potential energy. That's why you can put one portal in the ceiling, one in the floor, and fall through going faster and faster.

That fundamentally breaks the law of conservation of energy. The only way it is possible is if the portal generator is adding energy into you to make up the difference.

  • 1
    $\begingroup$ correct. If no energy is going into it, you have essentially created an infinite loop and creating infinite energy. $\endgroup$
    – Keltari
    Mar 13, 2018 at 4:05

It's been assumed that the portals work as wormholes, changing the topology of spacetime. But, what if that's not how it works? It could be teleportation ports instead. I recall a book series where "fax" technology had nanomachines that disassemble and analyse what is pushed into it (the plane of nanotech seems to be a portal) and another one can reproduce it. That could implement a portal in principle without twisting physics at all, reducing it to normal stuff: communication bandwidth and power input. Going in and coming out at a substantial velocity would be a challenge to the engineering, but in principle does not require new physics. The receiving end applies velocity to the atoms it's arranging at the surface.

  • $\begingroup$ Interesting idea. However, then the portal would not be see-through since the nanites couldn't possibly reproduce photons, right? $\endgroup$
    – SK19
    Mar 12, 2018 at 19:58
  • $\begingroup$ @SK19 Why not? If you're not using some kind of accessibility device, you are staring at stuff reproducing photons (find online webcam for even better results). Make nanobots webcam + display and there you go! $\endgroup$ Jun 11, 2019 at 13:37

They...oh wow they violate so many rules, but really it's such a violation I'm having trouble even knowing where to begin to articulate all the rules they violate lol.

so they violate thermodynamics a tim said, effectively creating free energy. if you have a way to do that you can pretty much break every aspect of the universe, thermodynamics is the driving force of everything.

in addition the violate the laws of inert. They also allow instantaneous communication across indefinite distances which violates relativity and the speed of light limitations.

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    $\begingroup$ Instantaneous communication? Has it been established anywhere that there is no delay between two portals? (Not just no noticeable delay within the realm of a few hundred meters, but absolutely no delay?) $\endgroup$ Dec 20, 2014 at 22:08
  • $\begingroup$ without giving too much way I'll say the ending of the second portal game does seem to suggest instantaneous travel of the portals themselves. I would go into more detail about the numbers, but doing so would give something away so I'll leave it an exercise to the reader. $\endgroup$
    – dsollen
    Dec 21, 2014 at 17:23
  • $\begingroup$ Oh, I see - I have never played the Portal games due to the bundling with an online activation software, so I did not know enough specifics to completely know what you were referring to. If there is such a scene, the remark about instantaneous communication makes sense. $\endgroup$ Dec 21, 2014 at 20:17
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    $\begingroup$ I've actually never played the second either, too cheap to buy games now of days. However, I spend so much time online at places like tvtropes I can't help accidentally learning about it anyways :) $\endgroup$
    – dsollen
    Dec 21, 2014 at 22:37

The portal gun has a miniature black hole inside it. If Aperture Science has found a way to contain the gravity well, it makes sense to assume that the portal gun can "harvest" energy from the gravitational force itself. So the energy required to hold open the portals comes from the black hole contained in the projector.

  • $\begingroup$ Try elaborating on your answer. If you can provide some links to back up your statements. $\endgroup$
    – Jim2B
    Jan 7, 2016 at 4:59

Portals following those rules violate the laws of physics in several ways.

First, energy and momentum aren't just conserved globally, they're conserved locally. They obey continuity equations, which tell you that in order to get them from one place to another, they have to travel through the space between. There's a lot of quantum mechanics that sounds like it breaks that rule, but it doesn't.

Second, the portals don't conserve momentum at all. (Yes, GLaDOS was wrong about that.) Momentum is a vector. It's not just an amount of stuff; its x-, y- and z-components are separately conserved. Changing its direction disconserves it.

Third, any requirement that something be stationary violates Galilean relativity. It gives one frame of reference special status. The portals you describe conserve (specifically) kinetic energy in the frame of reference where they're stationary, but not in others. Consider the case where the two portals are side by side on a wall. In the frame of reference where the portals are stationary, an object moves with constant speed v directly towards one portal. It emerges from the other portal with speed v in the opposite direction. In the frame of reference where the object is initially stationary, it emerges from the second portal with speed 2v.

Fourth, the portals violate Einsteinian relativity. They permit instantaneous transmission of information between locations, and therefore they require time travel. Since there's some confusion about why those are related, let me explain. Suppose you travel instantly from point A to point B. According to an observer moving (rapidly) in the direction from A to B, you arrive at B before you leave A. Since all frames of reference are equivalent, you're free to travel from B back to A instantaneously in this other frame, thus arriving at A before you left, killing your own grandfather, etc.

Now, with general relativity you may (or may not) be able to create wormholes which change the topology of spacetime and bring previously distant locations close together. But I think it's sort of guaranteed never to be useful. I mean, we're talking planet-destroying bending of spacetime. Picture pinching a balloon to bring two distant points on its surface together. And you're still constrained to obey causality.


I think that the question we ask is all incorrect because if you think about the quantum physics, you wouldn't be able to sustain the load of particles inside of it. The only way to sustain a force of that kind of strength is to have negative amounts of energy inside, and is possible, but the force is powerful enough to destroy a moon.


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