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The Physics of Portal

By Shamus
on Monday Oct 29, 2007
Filed under:
Video Games


In my previous post on Portal, 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.

Let’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.

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 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.

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 in the first place. You paid for that potential energy, and in fact the second law of thermodynamics means that you overpaid 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.)

Tossing a concrete block into the portal so that exits ten meters in the air – further from the Earth – is just like moving your bike to the top of the hill for free. You can’t do that. Once that heavy object is a little distance away, you can capture (some of) that potential energy when you let the object fall again. We need to make sure our PD pays for the potential energy we get for transporting objects up. (Lateral moves can, I believe, continue to be free1.)

In order to solve this, let’s assume the Portal expends energy as stuff goes through. 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 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.

If we need to define a rationale for the energy usage, it could be something along these lines: Imagine that mass passing through the portal makes it unstable. There is a difference between the gravity on the object as it enters the “bottom” versus when it exits the “top”. The object has unaccountably changed its relationship with other gravitational actors. This causes minute ripples in the surface of the portal. If both portals were made back-to-back on the same 2d plane (so that the portal didn't go anywhere) then the ripples on both sides would match and the portal would not be destablized at all. But once you move one above the other, the shape of the ripples change compared to one another. It order to maintain the portal within these gravitational “ripples”, you need to add power. It will work out that the power required is always more than you gained from moving the object up.

At any rate, if the portal device can’t come up with the power to allow the mass through, the portal needs to fail. We haven’t worked out how yet – we’ll get to that in a second. The important point here is that it needs to stop “giving” energy the moment we stop “paying” for it2.

Ok, we now have a portal which doesn’t let us create energy when we pass through, which is a step in the right direction. One problem left. What happens when the portal closes?

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 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 impossible to expel. We have to assume the object holding the portal open isn’t possible, or we get into the situation where you can prop the portal open and then start tossing mass through, and we’re right back where we started with free energy.

This leaves us with slicing. However, that means the edge of the portal needs to be capable of slicing through any object occupying the opening, and it must be able to do so without any further energy from the PD. We can say that opening a portal requires some energy, just as lifting the blade of a guillotine does, and that shutting down a portal is simply releasing that energy by allowing the blade to fall. I think this should allow a two-dimensional surface to cut through any given three-dimensional object regardless of mass, but I have no idea how to prove it. (Assuming doing so is even possible.)

That’s my take on how the PD needs to work. Now someone get out there and build me one. For the sake of science!3

1. Lateral moves can’t be completely free, of course. All objects exert gravitational force on each other, so moving a brick fifty feet away means moving it away from my gravitational field which, despite my recent weight loss, is still non-zero. However, the energy needed to overcome this miniscule. It needs to be paid for, but it’s nothing compared to Earth gravity.

2. I’m not suggesting that the portal should be designed with these limits, on purpose. I’m suggesting that if some clever researcher figured out how to make one, she should quickly discover that these limits are unavoidable. As in, “we can build a portal device, but we will have to provide this energy in such-and-such a manner in order for it to work.”

3. By “science” I mean this.

While not part of any test protocol, we are pleased to present you with this complementary Aperture Science bonus fact: Using a portal device, it would be possible to make a genuine Klein Bottle without resorting to the rather unsightly nexus required by the immersion of the Klein Bottle into our sadly limited 3d space. Unfortunately, one of the portals would need to go inside of the bottle (which is one-sided and therefore the concept of “inside” is a bit daft) and would therefore make the thing a royal pain to use. Not that I have any idea what you’d use it for.

Comments (105)

1 2

  1. Sharpe says:

    But what if you are not actually “moving” the object? What if the 2D plane of one portal is actually the plane of the other portal? Like a wormhole, where you just fold the paper so the 2 ends touch. therefor you are not using energy to move the object, you are using energy to bend space-time.

    This seems to be the case, because there is ALSO the energy of “redirecting” the momentum of whatever enters the portal, you are not just moving the object. This also solves the problem of “cutting” the object in half. It doesn’t, it just leaves the half of it behind when the portal closes.

    But where does that energy go when the portal close? Does the universe just snap back like an elastic? This must cause some sort of detectible ripple in the larger picture.

  2. Shamus says:

    Sharpe: That is how I imagine it as well.

    I’m not sure redirecting momentum is required in this case. From the object’s perspective, it is simply moving in a straight line. Much like an object orbiting the sun can be said to be moving in a straight line when viewed in 4 dimensions. There certainly does seem to be some 4-dimensional changes when making portals.

    If this is not the case, then yes, we would need to spend more energy re-directing the object and dealing with its momentum. I really hope we don’t have to go this route, as it sounds like a real headache.

    • sea says:

      i agree but from what i see light might be able to pass through i say lots of quantum energy will be used to make it pop into the plane of existence and entering might not be a good thing since at any giving time it could put to much stress on the energy its using and just close so if you’re passing through and it close’s your body will be imeditaly vaporized an teleported to third party dimension where nothing can exist and even if you manage to pass through to get to your destination once you close it out it will have servere consiquences and probaly erase any living matter that went through the portal. also it will make a huge ripple through the plane of existence probaly disorienting time and space ripping a hole in our universe allowing some anomalys to get through and probaly mess up history as we know as it may disrupt our time line and space making our existince not happen and if space is disrupted the planets may have never formed. but if sucsessful one day to much energy to make the portal appear into and out the field of existince it may erupt and warp time and space as we know it! it may also alter our universe or some idiot or retard will activate two portals one above the other and corrupt the time zone becuase it activates an endless time loop.

  3. DGM says:

    “We can say that opening a portal requires some energy, just as lifting the blade of a guillotine does, and that shutting down a portal is simply releasing that energy by allowing the blade to fall. I think this should allow a two-dimensional surface to cut through any given three-dimensional object regardless of mass, but I have no idea how to prove it.”

    But just as there’s no arbitrary limit to the amount of mass that can be shoved through a portal (for power generation purposes), there’s no arbitrary limit to how difficult an object can be to cut through (and thus how much extra energy will be required to do so). What if you didn’t spend enough energy “raising the guillotine” (opening the portal) to cover it?

    Let opening the portal be cheap, but let it draw and hold energy energy equal to the strength of the bonds passing through it and then release it as the object comes out the other side. You’ll get a spike in power draw as an object goes through but you’ll get it back afterwards, unless the portal closes on the object (in which case it’s guarenteed to have enough energy to cut). If you don’t have sufficient power when an object starts to pass through, it gets block or the portal instantly closes.


  4. Spider says:

    OK, I’m being drawn in to the geekery of it all.

    It seems to me that part of you equations start to treat the portal as a transportation device (one object moved to a different location) as opposed to a wormhole (bend space so that any object, if smaller than the apeture, can traverse it).

    If we put aside all other disbelief, I would suspect the energy output to create such a wormhole would likely be related to maximum mass that could pass through the portal, rather that the mass that actually was being passed through this. Afterall the beams of light are contually flowing through it (thus you can see on the other side), and if quantum packets can constantly flow back and forth through it, likely so could more substantial objects without adding additional stress.

    I’m sure others, who are more learned than I, will hop in and tell me differently, but that’s my thoughs.

  5. MintSkittle says:

    Being one of the non-learned peoples here, I just wanna say those portal tiles are awesome. Did you make them, Shamus?

  6. Shandrunn says:

    This is great Shamus, I love posts like this. Where did you get the Enrichment Center icons? They’re fantastic.

  7. Shamus says:

    Spider: Here is how I was thinking of that, to avoid the problem of a portal becoming a “transport” device:

    Imagine that mass passing through the portal makes it unstable. There is a difference between the gravity on the object as it enters the “bottom” versus when it exits the “top”. The object has unaccountably changed its relationship with other gravitational actors. This causes minute ripples in the surface of the portal. If both portals were made back-to-back on the same 2d plane (so that the portal didn’t go anywhere) then the ripples on both sides would match and the portal would not be destablized at all. But once you move one above the other, the shape of the ripples change compared to one another.

    It order to maintain the portal within these gravitational “ripples”, you need to add power. It will work out that the power required is always more than you you gained from moving the object up.

  8. BChoinski says:

    Larry Niven had a whole article on this (dealing with teleport disks used in some of his “Known Space” stories.

    One case was a planet busting bomb that was little more than an airtight case with a vacuum inside and a portal at top and bottom, filled with lead balls.

    Simply place the thing at the north or south pole and turn it on. Lead balls continue to be accelerated by gravity (falling from top portal to the bottom, and sent back up) After a while you have things moving at relativistic velocities. If you turn off the portal system (or if the speed/mass it to great for the system) the lead ball slam into the planet like a fist of god.

  9. Shamus says:

    Shandrunn: Thanks. The icons are my own crude imitations, made from drawing and cut & paste from the originals.

  10. Katy (the other one) says:

    This essay reminded me, very much, of Niven’s “The Theory and Practice of Teleportation”.

    And the illustration tiles are wonderful. Good job.

  11. Shadow2336 says:

    I think you’re thinking too much about this.

  12. rflrob says:

    I remember reading some SF short story at one point (although any details are now escaping me) that used probabilistic mass to energy conversion as a power source. In essence, you put something through, then randomly pick some tiny small fraction of the particles in that object to convert to energy.

    Thanks to the fantastically large conversion from the c^2, you wouldn’t need to pick very many to get enough energy to do something useful. For example, to lift an 80kg man 10 m high, you’d only need about .1 picoliters of water, so it only _looks like_ he’s coming through unchanged. This is the beauty of only taking 1 out of every 10^15 particles or so.

  13. Joe says:

    bchoinski: The title was “Exercise in Speculation: The Theory and Practice of Teleportation”, I read it in the compilation “All the Myriad Ways”, and I think that’s the only place it’s been published (if you like thinking about strange subjects, that’s the book to read… there’s also a theory and practice of time travel, and the ever-popular “Man of Steel, Woman of Kleenex”)

    Shamus, one of the things Niven points out in relation to what you’re saying: Define “lateral”. If I stay at the same “level” WRT gravitational potential, but move 1000 miles East, is my momentum conserved? When I step in one side, I’m moving at 1 earth circumference per 24 hours in a direction tangential to the earth’s surface. The vector of the tangent to the earth’s surface is decidedly not the same at the destination.

    And then there’s the question of whether you teleport instantly, or at the speed of light (leading to the question of where your mass is in the interim). If the former, there are… issues. If the latter, there are, well, different issues.

  14. Spider says:

    Fair question. Here is what I was trying to get at, to my mind a portal is an opening as opposed to a plane. Metaphorically, a door frame, but not a door. What I was trying to say was, given my understanding, the rules you were giving (based on what object was passing through) seemed to be a kind of transporter technology (because it is this object going here it will cost this much energy). The way I understood it in your response, though, is closer to a “true” portal, but still has less feal of altering time and space and more feel of shifting an object. Really, at that point, it could be semantics.

    I guess I like my portals free and obstruction, even if it’s just an infinately thin barrier to seperate gravational fields on a given object.

    This thought, actually led me to an interesting secondary thought. If you did infact have the clean portal I envisioned, you wouldn’t have the issue you mentioned, but they wouldn’t work like they do in the game either. The reason is the gravitational/energy forces would also leak though each side of the portal. Those portals with a greater gravitational force on them would amost always drawn bojects from those with lesser gravitational force and you would have ro “work” make objects traverse them backwords.

    Maybe I like your portal better.

  15. Shamus says:

    Joe: “lateral” and “up” are obviously used to simplify the explanation. The REAL thing that determines how much energy we need (in my hacked-together explanation) is the size of the gravitational force on the object. The more you change the shape of gravity around the object, the bigger the “ripples” I talked about above.

  16. Shamus says:

    Spider: Very interesting. Yes. Having gravity “reach through” would make the use of portals very complicated. Put one in the ceiling and one in the wall right in front of you. Get sucked through and bellyflop on the floor. Ouch!

  17. krellen says:

    Those icons have a Fallout-like feel to them. This says good things.

  18. Stephen says:

    Even with a perfect portal (a fold in space that doesn’t consume further energy once created), aren’t you still technically consuming potential energy from the planet? That is:

    You create two portals, one direct above the other.
    You drop a brick into the bottom portal.
    The brick appears at the higher portal.
    The mass of the planet attracts the brick, the mass of the brick attracts the planet.
    Normally, a brick lifted manually would have its gravity impact balanced by the energies involved in the lifting. This brick is being continually reset; it never hits the planet and, thus, the planet never stops pulling on it (and towards it).
    Over a very long length of time, the minuscule amount that the brick is pulling on the planet would add up, and the planet would be subtly pulled off its orbit.
    Even if you put a second portal at the opposite side of the planet to balance it, you’d still gradually distend the planet into an elliptical shape, due to the competing masses at either side.

    Of course, there’s probably a key conjecture about gravity that I’m forgetting in all of that.

  19. Jeremy says:

    The mass/energy question is interesting, but I think fairly straightforward to solve: for each unit of mass going through the portal, you need to spend X joules depending on the potential energy gradient between the ends.

    Similarly, I think you can solve the cutting problem by asserting that the ends of the portal always exist; you can never actually collapse the end of a portal, just move it somewhere else. ie, somewhere Aperture Science has a great big portal storage room, just waiting for you to place them somewhere useful. That means things half way through the portal will still be half way through the portal, just somewhere else… (Sure, this contradicts the experience of the game, but – Hey! Look at that!)

    But related to that:

    More interesting is whether the portals violate causality by allowing FTL travel. Given that you can collapse and reestablish one end of the portal tube at any time, does that happen instantaneously?

    For example: you make a portal here, and another a couple of metres away, and you shine a laser through it. Then you take your portal gun with you and travel to some distant place (say, 10 light mins away), and re-establish the out portal. Does the laser come through immediately? After a delay? Red-shifted?

  20. Shamus says:

    Added the “ripples” explanation to the main article, just because I liked it.

  21. scragar says:

    interesting point Shamus. I’m against the portal costing a set amount of energy to remain open, after all if nothing(say the portals are in zero gravity with no light and a vacuum) enters either end, but the device requires a set amount of energy isn’t that a breach of the whole energy cannot be created or DESTROYED?

  22. Zaghadka says:

    Oh crap. I failed my save versus dork.

    First off, the energy required to maintain a portal should probably be directly proportional to the square of the distance of the portal gates from each other (or similarly exponential scale). This would help conserve kinetic energy gained by simply “raising the exit point,” by making the costs of doing so exponentially approach infinity in short order.

    Secondly, we’d be foolish to ignore the significance of the red shift and blue shift. One possible interpretation is that it is gravitational forces and distortions caused by bending space-time, which you’d have to be doing to merge two discrete points in space-time.

    In this view, the color shift is an illusion. You’re not seeing the actual rush of photons away from or toward your point of view, but instead the literal stretching of space as it warps to the entrance. The photons appear to be accelerating to the speed of light, but the space itself is what is stretching/compressing. Relative to a stationary observer, the eye interprets it as a Doppler shift, but the apparent acceleration is merely a relative function of the vast space-time distortion.

    For a real mind blower, if you have your back to what you think is the blue gate, it is red. If you have your back to the red gate, it isn’t, it is blue. I’m basing that on a loose understanding of Heisenberg. If you can’t see both portals, they are both the same color. ;^)

    By equating this “Doppler shift” with a gravitational effect, we could exact heavy work/energy penalties based on the mass of something going through the portal. Anything of significant enough mass would literally be repelled by the entry point as it tried to enter, instantaneously approach the speed of light (relatively) at point of entry, and be expelled from the other side with the same converse difficulty. It takes force to get a significant mass to go through a portal, or sufficient portal energy to overcome the gravitational field of the mass. Both exits repel in direct proportion to the mass which enters. There is no slingshot effect to gain momentum as the transfer of position is instantaneous. You either provide enough energy to overcome the gravitational forces of the mass and provide enough force to get past the space-time warp (probably producing heat, which is where you satisfy the second law), or your object is stopped dead by it’s own gravitational forces. Complete conservation of energy is achieved, plus work to heat loss of efficiency at the point of transference.

    So everything comes through the portal warm. Bigger things might come through thoroughly cooked. Gabe would eventually catch fire doing that slide trick.

    The energy requirement for entry/exit (it’s relative)and/or the portal energy expended to overcome that gravitational resistance, and the transfer of some of that work to heat, which must be dissipated, and the distance between the two portals will exact a heavy penalty on any non-trivial portal operation. Thus we can see why they’re only really useful to solve trivial puzzles involving low masses at, by astronomical standards, minuscule distances.

    The worst case scenario is the mass being stuck between the two portals and the exact wrong moment, in which case your mass implodes, becomes a quantum singularity, and your portal ends all normal space time in a several light-year radius.

    It’s the equivalent of crossing the Proton accelerator pack beams. It’s bad, and you should only attempt it in the face of certain Armageddon.

    Otherwise, you might be able to bake a nice layer cake with the thing?

  23. quadir says:

    I still don’t see anyone looking at the actual gun. Your theory works really well for a stargate type mechanism that is obviously “powered”.

    However in Portal it “shoots” energy at a wall and forms a portal (like sludge splattering). If the wall has some properties that portals can’t form on (some kind of molecular thing?) then the energy dissipates. I see the gun investing the initial energy but not any kind of upkeep. Opening a portal of the same type could be seen as a collapse of the corridor (wormhole talk) and so the other dissipates.

    One point that hasn't been covered in HL2:EP2 and Portal is if the portals are confined to the facility. We see energy fields that portals cannot “Shoot” through, and which collapse any open portals if you bring the device through (this seems to indicate the gun is still doing something with the portals to maintain them). The facility could be powering the portals, in more a radio frequency way then a contained field (which explains why you can use portals outside the test chambers but still within the complex).

    Of course if HL2:EP3 has portals outside the facility that theory goes out the window. Or rather doesn't.

  24. Shamus says:

    Another problem: If we put in object in the top and it comes out the bottom, we should have to account for the extra energy, or as scagar points out, we will have destroyed energy.

    I think we can cut this corner with the excuse that the extra energy is converted into heat. My ripples explanation above gives us a handy way to add heat to the equation whenever we need. Either the ripple effect creates heat, or the PD generates heat during operation. Even better: Both.

    So if we chuck a boulder into a hole in orbit and have it come out a hole at sea level (in an airtight room, to avoid ejecting a bunch of atmosphere into space) then we just threw away a horrendous supply of potential energy. The portal or the device will need to radiate a lot of heat in the process.

  25. Shamus says:

    And obviously I’m simplifying the above. If the boulder is in orbit, then the sucker is moving fast. Sending it through a portal is going to obliterate your room if you don’t slow it down first. :)

  26. Dev Null says:

    But where does the energy come from, that the portal requires to raise your brick 10 feet? Obvious, of course; the same portal machine opens two portals somewheres else – one _under_ another brick of equal mass, and the other 11 feet lower…

    If I put one portal on one planet, at an altitude where a given level of gravitational acceleration is experienced, and the other portal on another planet entirely, but at an altitude with the same effective gravity, does it cost me any energy to chuck my brick through?

  27. Dev Null says:

    Hey Shamus, I would’ve sworn your old page layout had a per-post comment RSS feed, but I can’t find it anymore. (Yes, I see the one at the bottom of the page, but thats for all comments on the blog, isn’t it?) Any chance of getting the per-post version back and/or pointing out where it is now, always assuming I didn’t make it up in the first place?

    I like reading the comments here, but you’ve got too many readers now; I can’t follow the complete thread on all the posts.

  28. scragar says:

    I should think with the different planets thing you would calculate the total GPE for both objects and simply take the difference, after all you lose a lot of energy leaving Earth, but gain it back as you enter the new planet. what would be terrible is if the portal required more energy to allow the object to escape Earth, but then transferred the energy gained entering the new planet into heat like Shamus suggested…

  29. Phlux says:

    I know this very thought will enrage every physicist reading this article, but…. What’s to say that the first and second laws of thermodynamics cannot be “broken”.

    The equations which prove them to be true may simply not hold in a universe where portal technology is possible.

  30. Poet says:

    Clearly it works as an extension of PIXIE MAGIC!
    Either that, or…
    I recall Dr. Mossman mentioning that they were working with new teleportation technology–the poor man’s portal, in a way–that allowed them to slingshot other universes. Perhaps the portals work via a similar function, using the space of another universe to cut out transport time in our own. They could, then, be drawing their energy from anther universe to remain open, and by extension provide objects with potential energy appropriate to distance circumvented as they pass between the rings.
    It was also mentioned that teleportation functioned via string theory. Perhaps portals allow for shortcuts via string connections, thus drawing the energy from our own universe. This would, though, mean that every trip through a portal increases the level of entropy, speeding our rush to an eventual heat death.

  31. Zem says:

    I daydream about the pysics of hypotetical portals pretty often (I am weird that way).
    I also like to ramble on and on incongruently.
    You have been warned :)

    I find it more “natural” if forces act throught the portals, like spider mentioned before.

    Think about it:
    if I place a magnet next to one portal opening, I would expect iron next to the other opening to be attracted to the magnet.
    if I have an iron ball stuck to a magnet, I would expect it to remain stuck even when I push the magnet throught the portal.

    The same should apply to gravity, but this has some very interesting and counterintuitive implications:

    If you open a portal to the ground of the ocean, the ocean does not suddenly gush out of it to flood the area.
    Instead you would find yourself pulled strongly along the shortest path to the ground of the ocean, towards the portal.

    If you on the other hand open a portal to deep space, all the atmosphere does not suddenly ge sucked into it.
    If you try to pass throught this portal into space, you would need the same amount of energy that a rocket needs for liftof.
    You would perceive something like a strong repelling force from the portal, but in reality this would only be the pull of the planet, preventing you from gaining potential energy for free.

    Some more incoherent rambling:

    If you have one portal opening at a higher altitide than the other, then you would sense a strong attracting force towards the higher portal as you approach it.
    After all, the “ground” would attract you throught the portal.
    The shortest path to the bottom is throught the portal, and an attracting force pulls you along the shortest path towards it.

    If you drop something next to the higher portal, it would accelerate faster than 10m/s² towards the portal, so that after it has passed throught the portal it has the same kinetik energy as something falling the long way from high to low.

    if you drop something just below the higher portal opening, it would “fall upwards”, taking the shortest path to the ground.

    I think as a consequence the lower portal would also seem to exhibit a virtual repelling force, act as a perceived source of antigravity just as the upper portal acts as a perceived source of gravity.
    The only actual source of gravity is still the planet, the portals just seem to atracct or repell due to the planet attracting throught them.

    In this way, the energy to maintain a portal is totally independent from mass passing throught it. Potential energy is converted into kinetik energy and vice versa, with or without the portal.

    Sometime in the future, when I have lots and lots of time, and also somehow gained some talent in writing, I want to write a science fiction story using these phsysics.

    Sorry for the long post, if you had the patience to read this all, please tell me what you think

  32. Bogan the Mighty says:

    Yeah although I can freely say I don’t know very much about physics so most of my explanations are blaming magic, I have to say I like what Zem here is saying.

  33. John Marley says:

    Think of the good/evil uses for such a device.

    Good: open a portal with one end at the bottom of a major river/lakes = irrigate any desert, anywhere.

    Evil: Open a portal with one end at sea level, the other in space = drain off Earth’s atmosphere

    No extra energy would be needed, because the pressure difference would push the water/air through the portal.

  34. John Marley says:

    Damn, should have read Zem’s comment first.

    Speaking of which,

    Zem, why do you say this wouldn’t work? The atmospheric pressure difference between sea level and space is much greater than the gravitational difference.

  35. John Marley says:

    Dammit Shamus! Now I can’t stop thinking about this! I have work to do.

    Anyway, here’s how I see it working. It would still require the energy to move the mass to the new height.

    The pressure difference would push the air through the portal, the same as you pushing a solid object through.

    The portal generator would have to provide the energy to move the mass of the air.

    So if you opened a portal with one end at sea level and one in space, the air would gush through until the portal generator could’nt supply enough energy, at which point the portal would collapse.

    Extra evil! Deplete the atmosphere and cause rolling brownouts at the same time!

    Still, the irrigation idea would be pretty efficient.

  36. Vendrin says:

    …You thought way to much about this. Your trying to fit something that does not exist in the current model of physics, to the current model of physics. If and when such portals are ever discovered, much of physics will probably have to be changed.

    The cake is a lie.

  37. Zem says:

    John Marley:
    The atmospheric difference cannot be greater than the gravitational difference.
    Otherwise the atmosphere from sea level would already be pushed into space.

    But my comment only applies to my version of portals, I think the valve-portals would work just like you described.

    However, having not played the game myself yet, that version does not sound very consistent:
    The strong, weak and elektromagnetic forces clearly do work througth the portal.
    Otherwise all matter would fall appart when pushed throught a portal, since one atom can be on one side, while the rest of the molecule is on the other side. Same applies to atomic nuclei.

    Of course gravity could be an exception, but that seems arbitrary.

  38. Grant says:

    Not having played the game, I can’t comment on how the portals in the game work. Magic, I assume.

    However, in reality, I can think of only 3 possiblities: teleporter, wormhole, or quantum teleporting maybe.

    A teleporter would be something that moves the particles of the object in normal space time. Really quickly perhaps, so you don’t see it happen. This implies they would need to expend enough energy to push the thing wherever you want it to go, while it was going.

    A wormhole creates interesting problems. The gravity thing you mention would sort itself out because since space-time is bent, the “distance” between you and the earth would still be the shortest path between you and the earth, which might go through the wormhole. Which means if you put the portal on the roof and the other one the floor, if you were higher than halfway between them (the farthest point from the center of mass of the earth), you would fall into the one on the roof. So, as somebody implied, having space-time messed up like this would seriously disturb your perception of “down”. This is, of course, ignoring the fact that as far as we know, the only way to bend space-time significantly is to have a lot more mass than the earth somewhere nearby, which would probably make these effects negligible.

    I could imagine it done it with quantum teleportation, too, but I have no idea how that would work.


  39. Roxysteve says:

    [Shamus] You really do need to read the Niven article on this. There are a number of issues that haven’t been catered for in your model (nor in most “teleport” models from TV). The most obvious being the Earth isn’t standing anywhere near still, so even if you instantly teleport only a few inches there is an energy debt to be paid derived from “stolen position” caused by the earth rotating, translating along its orbit in the solar system which is itself moving in a complex orbit within the larger galactic gravitic system.

    You can forget Heisenburg. He says you can’t do the trick at all, at least by Star Trek methods (position and energy state of subatomic particles cannot be determined acurately: Do you want to be put together slightly scrambled or exacly as you were with your brain firing randomly?)

    Spacewarps don’t work either. Current physics suggests you need a significant portion of the total energy in the universe to make one and keep it stable. At least, it did when I last looked. These things are subject to almost daily change. :o)

    The only answer is…the D20 Ed 3.5 dimension door, that must have been manufactured, or at least marketed, by Apple because you aren’t supposed to ask what’s inside and it just works*.


    * Apple fans note: After screwing around with my Brother in Law’s Mac for a week and having had it break again on Sunday when I looked at it wrong (no lie, I was several feet from the thing and it was unplugged at the time, and the monitor support sheared), and having to sit here and listen to co-workers moan about the Leopard fiasco in the making, my irony filter is turned up full when I say that “it just works”.

  40. Roxysteve says:

    October 29th, 2007 at 12:13 pm
    Even with a perfect portal (a fold in space that doesn't consume further energy once created), aren't you still technically consuming potential energy from the planet? That is:

    You create two portals, one direct above the other.
    You drop a brick into the bottom portal.
    The brick appears at the higher portal.

    I can’t see a problem with your analysis, but it does mean the Earth would also get draged around by its bootstraps with such an arrangement. Consider if we use not a brick, but the moon, and position the it (by means of these same conservation of energy flaunting portals to where we need it to be) in “front” of the Earth. At the cost of a few Earthquakes, tidal waves and so on we can go a-voyaging any where we want without the need for tedious starships or other Heath-Robinson jury-rigs. If we use the Sun instead of the moon, we can be avoid the pesky “Freezing Solid” problem too.

    I like this idea. We go from Charles Stross soup-can starwhisps, probably the most thoroughly depressing idea I’ve read in a long time, to E.E. ‘Doc’ Smith-scale travelling solar systems. We could also do stuff like move th more interesting planets like Jupiter and Saturn in a bit so we could see them properly and remove the clutter like Mercury and that hell-hole Venus out to the boonies where they belong.


  41. ngthagg says:

    I think there are some serious problems with gravity propagating through the portal.

    But first, I want to emphasize “the portal”, not “portals”. There is only one portal per gun, and the user is choosing where each side is. Also, because the sides have to be placed on a surface, they are distinct sides, one-way only. This may seem to be confusing the issue, but it means when considering propagation of forces through the portal, we can think of the portal as being a simple hole, and then consider how forces would travel from one side of the hole to the other.

    Now, consider this situation: you place both sides on your bedroom ceiling. On one side of the portal, there is a larger massive object creating a gravitational field. On the other of the portal there is . . . a large massive object creating a gravitational field. Assuming the sides stay in position (since they are connected to your ceiling, which is connected to the walls of your house, which is connected to the earth), the earth will immediately begin accelerating towards the speed of light, in the direction of your bedroom ceiling. If you do this at noon, we’ll probably all be dead in a few minutes as we approach the sun. On the other hand, if you do it at night, we’ll have a bit more time to enjoy the ride.

    I have a poor understanding of relativity, but I’m pretty sure that mass is supposed to increase along with velocity. Hence the limit of the speed of light: a set force provides less acceleration as the velocity (and therefore mass) increase. But since the force of gravity is proportional to mass, the force will continue to increase as well . . . I think? Are there any physicists who post here?

    A more complicated situation is when you put the sides on the ceiling and the floor. This would create a space with no acceleration due to gravity inbetween the sides. Unfortunately, this would magnify the problem above. Looking down into the portal, you would see the earth, and another portal. Inside that portal you would see the earth, and another portal. Etc, etc. There would now be infinite multiple copies of the earth, spaced 10 feet apart, all exerting a force on infinite multiple copies of the earth spaced 10 feet apart. The force would then be an infinite sum of infinite sums.

    Anyways, this is why gravity probably shouldn’t propagate through the portal.

  42. Ozy says:

    I’m a physicist!

    An object’s proper mass (also called invariant mass or rest mass) is what determines and objects gravitational field, does not change as it accelerates, and is the same in all reference frames. “Relativistic mass” is dependent on reference frame and was used by physicists for a short time before they realized that proper mass is the important part and that relativistic mass is just a confusing way of saying “total energy.”

    You have to remember that the basic tenant of relativity is that all inertial reference frames are equally valid, and so you can see the absurdity of proper mass increasing with speed, since we are all already moving at very close to the speed of light in some reference frames, particularly those of cosmic rays or the particles in particle accelerators.

  43. Poet says:

    My fiancée watched me thinking about this issue. She’s not a big fan of FPS games (a forgivable offense, as she’s a huge fan of roleplay). I explained the situation to her, and she said, and I quote, “Have you considered that the problem could be caused by the number of people questioning game logic?”
    It occurred to me then that in a science fiction universe, we look for logical explanations for technology. Let’s apply this to a fantasy setting. If a wizard in D&D opens a portal between two locations, does it ever occur to anyone to question the method, and whether or not energy is being created or destroyed of he happens to open one further above the surface than his starting location.
    We are, therefore, the biggest nerds on the planet.

  44. Mike says:

    I agree with Shadow2336… my head is now in excruciating pain.

  45. beno says:


    I think I can solve the “where does the extra energy come from” problem. We have to supply it ourselves. Assuming the “wormhole” thesis of what the portal actually is, that means that spacetime is bent arond on itself so that the two places are “glued” together. (There was that Sam Neill movie called Event Horizon where he folds a piece of paper in half and punches a hole through it to show two separate places being “joined” – think of that.) The thing with spacetime is that such a distortion comes with some really strong gravity gradients, just as though you were right near a black hole or something.

    Whah? Okay let’s talk specifics

    Suppose there’s a portal on the floor you’re stnading near, and the other half is on the ceiling. If you try to drop a brick through the portal on the floor, the brick needs extra potential energy (from somewhere) to end up appearing on the ceiling. The brick has to go “uphill” in terms of potential energy. Therefore you will actually have to push the brick into the hole in the floor, with as much force as if you were lifting it directly up to the ceiling. The portal doesn’t do the work for you.

    But what happened to downward gravity? Well that’s the effect of the distortion of spacetime. Gravity is still “down” if you’re sufficiently far away from a portal. Near the portal, all bets are off.

    If you want the portal to provide the energy, you’ll have to think up a premise other than wormholes and spacetime distiortions to explain it. Wormholes are probably not your best bet anyway because you’d get spaghettified when you went anywhere near them. Head closer to portal than feet equals head subject to stronger gravity than feet equals spaghetti-person-you.

  46. ArchU says:

    The one major flaw I can see with a closing portal bisecting matter is in atomic theory. Surely it could happen but atoms can go boom when split (could that possibly empower the portal?).

  47. tussock says:

    You’re nowhere close to the real problems. You can’t translate or rotate things (everyday things, normal distances) without taking the time and using the force to do it properly. Any attempt to skip the intermediate steps violates causality.

    Basically, if you jump, there’s always a reference frame which showed you gaining infinite energy and another showing you traveling back in time (and able to jump back and prevent yourself from entering in the first place, and all that jazz).

    If you don’t jump, if there’s an intermediate tunnel structure that you must climb and rotate through, using up the required time, your energy is temporarily removed from reality, which can’t happen without, again, having reference frames in which infinite energies are dissipated into and then lost from the world.

    Unless the tunnel and your energy persists in reality too. However, you can’t overlap it with reality or you get black hole problems quite easily.

    So you’re left with the only possible portal being a real structure, two hoops with a plastic tunnel between.

  48. ArchU says:

    To clarify that last point, the plane is 2-dimensional which makes an atom infinitely wider than the portal. The bisection would be indescriminate of particle size.

  49. Sam says:

    Awesome, very interesting article

  50. Kacky Snorgle says:

    The one major flaw I can see with a closing portal bisecting matter is in atomic theory. Surely it could happen but atoms can go boom when split (could that possibly empower the portal?).

    I don’t think this is much of an issue. Boom happens when *lots* of *heavy* atomic *nuclei* split at once. But atomic nuclei are tiny compared to atoms, so probabilistically, the overwhelming majority of atoms would lose a few electrons at worst (and the resulting + and – ions would be equally numerous on both sides of the portal, and would sort themselves out quickly on each side with negligible consequences). The few odd bisected nuclei wouldn’t be enough to start a chain reaction even in fissionable material. And in the great majority of materials (i.e., those composed of light elements rather than heavy ones), the fission would absorb rather than release energy anyway.

    Now, if only I could make all the *other* issues go away so readily…. :)

  51. icekatze says:

    hi hi

    What I want to know is how the portal deals with the Second Problem of FTL travel. (http://www.physicsguy.com/ftl/html/FTL_part4.html#chap:unsolvableparadoxes) I mean, it appears that matter is instantaniously transmitted from point A to point B, but one was to place two portals very far apart, you could end up portaling yourself back in time.

  52. Duffy says:

    Just to add fuel to the fire (or help narrow it down) since we’re considering the game’s portal physics as true, let’s just stick to the basic assumption that there is conservation of energy between two portals, yet the portals do not break the laws of physics. “Speedy thing go in, speedy thing come out.” to simplify.

    Problem #1: A PD is not actually a perpetual motion machine. I think we can agree that the portals are not self-sustaining energy wise, they are powered by something. Therefore on an infinite time line a perpetual motion machine made with portals will both use and generate infinite energy, thus not technically violating physics.
    (I may be wrong on this point but it was stuck in my head from the moment I started reading, so I’m throwing it out there.)

    Problem #2: The method of teleportation. Any method based on moving quickly or shortcutting through space and time, such as wormholes, would need to be ruled out. Doing so would require interaction with outside forces, not to mention energy for traveling, that would probably effect the object’s energy by the time it emerges from the 2nd portal thus either losing or gaining energy.

    The method used would probably have to be quantum entanglement, all other options I know of being exhausted. As far as I can understand, and put in layman’s terms, it works by slightly modifying a particle’s (or several to make a portal in this case) information, thus you make one location the same as another location. No energy is lost or gained from the teleportation process.

    Sounds more like magic to me in all honesty, however problem #1 remains even with this model. Therefore I’m relatively confident a PD cannot be used to create free energy.

    Now I may be wrong, and if so oh well I gave it a shot. And if so I add this last example to attempt to help: two perfectly balanced objects that orbit each other due to gravitational forces in an isolated vacuum are not a perpetual motion machine since interacting with them to harness the energy would destabilize the system, eventually causing them to spiral into one another.

    Ok, I’m done now.

  53. xbolt says:

    First off, I’d like to say that this was the most brain-hurting thing I’ve read in a long time. Excellent topic, Shamus!

    Second, while reading through this, I thought: ‘What would happen if one portal passed through the other?’

    Suppose I opened portal #1 on the wall, and then opened #2 on the side of a box. Then I pushed the box, so that the side that is holding portal #2 passes through #1 first. Where would the box go?

  54. HeroForge says:

    Seen on a bumper sticker: Heisenberg may have slept here.

    I’m not going to start digging out my Michio Kaku books, not yet. I just have one comment, from way back in #22, when Zaghadka was talking about how things would get warm when they passed through the portal, and you could possibly bake a cake with it.

    The cake is a lie.

  55. ngthagg says:

    Here’s a joke that is somewhat relevant:

    Feynman and Heisenburg are taking a road trip. Feynman is driving and Heisenburg is navigating, when Heisenburg spots a speed trap up ahead.

    Heisenburg says, “Uh oh, you’d better slow down: there’s a speed trap up ahead.”

    Feynman says, “Don’t worry about it. Look, the speedometer says I’m going 55.”

    Heisenburg replies, “You idiot! Now we’re lost!”

  56. Primogenitor says:

    Well, since Valve try to write self-consistency into the physics of the Half-life universe, perhaps it has something to do with the “dark matter equations” (and/or dark energy) and “using Xen as a sling-shot” that Judith rambles about when you get to Black Mesa East. Sure that’s from Black Mesa not Aperture Science, but the Combine also have similar (if slightly inferior) teleporter technology; plus there was the portal-storm that brought the 7-hour war in the first Half-Life.

  57. conniemac says:

    From an astrophysics perspective, if we accept General Relativity as the “truth”, ignoring the need for a quantum theory of gravity, and treat the portal gun as a device which constructs “wormholes”, then a perpetual motion machine will not be possible.

    As an example, suppose you place one end of the wormhole in the celling of a room, and the other end on the floor directly beneath it. Now, our normal physical intuition would dictate that you could continue falling forever in this situation with increasing speed until you reached terminal velocity, if you just dropped through the hole in the floor. However, if you work out the equations, what happens is the (really large, that is as a lower bound, many millions of times larger than the sun puts out in a year) amount of energy required to make the wormhole, which is presumably supplied by the gun, actually acts to warp spacetime such that the two ends of the wormhole are at equal potential energy, thus prohibiting the net gain of energy no matter what causal (that is, with velocity less than the speed of light) path through spacetime you construct. As a result, if you were standing anywhere near the portals, gravity would act in a completely non-intuitive manner. Specifically, between the two ends of the wormhole, there would be no net gravitational force in the example I’ve described (provided you assume that the portals are in the center of the room, and there are no masses aside from the room’s walls). In more complex situations, there may be a gravitational force, but not in the sort of up/down sense that we humans living on the surface of the Earth are used to.

    Of course, this analysis ignores the fact that the game does not behave this way (go figure that games would have different physics from the real universe…), and that to create such wormholes that would allow passage of a human being without tidal disruption of our bodies would require openings to be insanely (as a lower bound, on the order of 100 million miles in radius) large.

  58. Spider says:

    Let me say, Zem, excellent work taking over where I left off. It might have been rambling, but it added a lot of clarity to what I was get at.

    Next, I think you’re going to have to put Quantum Entanglement as the method the portal uses aside. This method, as far as I know only works on light, and it requires manipulation of the “host” object in order to “transport” it. This is clearly in the tranportation camp, as opposed to the portal camp.

    Lastly, for our physicist friends. If we assume gravitational force is carried through the portal and normal gravitational current is down toward the earth). If we constuct a normal room on earth, then place a portal connecting the top and bottom of that room, what does the gravitational current of that room look like.

  59. Roxysteve says:

    I'm a physicist!

    An object's proper mass (also called invariant mass or rest mass) is what determines and objects gravitational field, does not change as it accelerates, and is the same in all reference frames. “Relativistic mass” is dependent on reference frame and was used by physicists for a short time before they realized that proper mass is the important part and that relativistic mass is just a confusing way of saying “total energy.”

    You have to remember that the basic tenant of relativity is that all inertial reference frames are equally valid, and so you can see the absurdity of proper mass increasing with speed, since we are all already moving at very close to the speed of light in some reference frames, particularly those of cosmic rays or the particles in particle accelerators.

    Let me offer up the following carefully reasoned observations in possible rebuttal of your fiendish and learnéd discussion:

    Physics shmisics!


    PS A very big :o) there.

  60. Segev Stormlord says:

    I’m a physicist by hobby and degree, and got into it for exactly these kinds of problems, so pardon me as I proceed to be at least as nerdy as the rest of you.

    The portals as presented in the game (from what I’ve seen of it – I have not been able to play it) are magic. That said, they’re neat magic.

    Actual wormholes, being the four-dimensional constructs that they are, would have three-dimensional apertures from the perspective of us 3D space beings. That is, from any side “outside” of it, we could look in and see out the “other” side. There would be no clear boundary, and it would probably appear as a sort of lensing effect to our human brains as they interpret the light-pattern scattered off its edges.

    Ignoring the tremendous amounts of tidal force due to the singularities forming the bridge – which we can do if we just assume sufficient amounts of mass arranged in precise formation in the internal 4D structure such that all the tidal forces “cancel” in the travel-able portion of the “tunnel” – we would feel “gravity” and other forces more or less as conniemac has suggested. That is, the potential energies would be the same at both ends, insofar as both ends are close together.

    There can be actual “distance” within an Einstein-Rosen Bridge such as we are discussing, and in fact must be, because there is no 2D aperture, but what that distance is is a function of the structure of the wormhole, not of “real” distance between the entrances according to “external” observation. This is the key to answering Shamus’s guillotine question: If a wormhole “shuts off”, the spatial effect is one of distance suddenly increasing between the two exits. If you’re unlucky enough to be close to the middle, you won’t have a physical object cut you in half, but you will be ripped apart by tidal forces as space warps about you. If you’re lucky enough to be near an exit, you’ll just get flung out of it by gravitational force. Because, by at least one theory, all gravity is is space bending around you, and your relative motion through it.

    From the perspective of one entrance near the top of a silo and one near the bottom, you’d find that, indeed, the gravitational force near the top one would be towards the entrance. You’d less find that there’s a “repulsive” force from the lower one, and more find that trying to enter it feels like climbing. The interesting bit is what happens between the two, or directly “above” the lower one. Gravity does indeed affect you according to the planet’s mass, but the portal entrance itself cuts off the direct force, so you’re only getting the pull from the planet below and to the side, rather than directly below you. Meanwhile, if the other end is sufficeintly near, you might be getting a pull from it as the planet is technically closer to you through it than it is directly below you. Either way, you’ll feel weird.

    Due to vagueries of internal 4D structure, you might even slide “around” the bottom entrance rather than through its center, and hit the ground below it.

    As for the magical 2D doorway portals of Portal… the physics engine of the game seems to treat them rather well, though it oddly cuts out gravitational effects directly at the interface rather than propagating them through. Directional orientation is another interesting issue, handled well by their engine because they ignore field effects passing through the interface. The guillotine problem is handled by ejecting it out one or the other, which makes perfect sense if you consider a “collapsing” portal to be losing its connection to one end, and the pull of the collapse towards the other drags the half-way-through object along for the ride. This neatly avoids infinite energy conditions for the “slicing” effect.

    In the end, it is the non-two-way nature of the portal entrances (it can be assumed, since their backsides are on surfaces, that there’s no way to go through them) that makes the infinite-falling-energy possible in-game. The gravitational field passes through the interface without any problem and without being shunted to the other side. Ultimately, that’s where the laws of thermodynamics are being broken. And ultimately, that’s why these are magic, and not science fiction. If gravity is a bending of space, then the bending of space to link the two portal entrances must have its own gravitational effect. If gravity is “gravitons” and the fourth fundamental force, the gravitons should pass through the portal just like any other paricle. That’s where the magic comes in: the portals just translate objects and light. They make a doorway that is selective in what it allows to pass through it. Namely, only mass and energy do; gravity (and maybe magnetism?) will not.

    All that said…it’s a tremendously fun-looking game, and I hope to get it for Christmas.

  61. alakar says:

    I’d like to toss the (instant?) transfer of “information” into the mix. Since light travels through the portals, (disclaimer: I’ve not played the game) it seems to me that this implies that there is a constant transfer of (quantum?) information in a portal. E.g. a portal could make for an “instant” remote “camera” view of a distant location.

    A tiny portal in a fiber optic cable splice could finally bring good ping times to all of those *.mars domains, once the “remote” side was sent(?) there.

    This would be mitigated by the possibility raised above of travel within the portal (entry to exit) occurring at the speed of light.

  62. Dev Null says:

    All of this dodges one of the issues that always amuses me the most; if you project the portal onto a large sheet of glass, say, what would you see from the other side? And if you projected them both onto sheets of glass, then wheeled them up next to each other and made a portal sandwich, what would happen?

    Probably a good thing noone has given me a real one, actually; bit too strongly of the “what does _this_ do” school of goblin engineering…

  63. J Greely says:

    Here’s my take, thought up this morning while driving into work: the portal gun shoots a pellet of nanobots. When it hits something, it attempts to adhere to the surface and construct a portal endpoint. If it succeeds, it opens a micro-scale portal to a control system, through which data and energy can be passed in both directions.

    When the control system detects two active, matched endpoints, it calculates their relative positions and orientations to determine the potential difference, and then feeds them enough power to open a macro-scale portal. The endpoints are kept in sync by exchanging energy with the control system through the micro portal.

    When an endpoint changes, the control system arranges a controlled shutdown of the old one, timed to match the startup of the new one. When both ends of a portal close, the control system supplies the extra energy necessary to push objects out one side or the other.

    The only snag is that the gun will eventually run out of “ammo”.


  64. Bruce says:

    A few random thoughts.

    If you can see the other side then light travels through it so why not gravity, water, air or anything else. In the case of gravity, does it cancel out. Gravity flows into the hole from both directions, or is the hole a kind of one way system?

    If you stick your head into the hole, you could see yourself looking through it which means the gravitational force on your head will be different from that exerted on the rest of your body. If the other force was strong enough, would you get pulled towards the stronger side or pulled apart?

    With regard to slicing in half, since it is theorised that, at its smallest level, everything in existence is constructed of little ribbons of energy, then slicing something in half may just be a matter of separating the ribbons into two piles – or perhaps using up their energy at the join as the portal closes which would achieve the same effect.

    Projecting the wall onto a surface, unless you are in a vacumn, causes problems too as as soon as you pull the trigger, whatever streams out of the end collides with the millions of air molecules long before it strikes the wall molecules.

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