Basically, in my novel, there is a magic power that involves creating portals that lengthen or shorten the distance between two places.

To make a portal, the user must first have a large amount of two things: "Sunlight", a form of energy that diffuses through all things, particularly Sunstones, and "Moonlight", something similar, but not a form of energy, can be used to turn one form of energy to another.

Two portals of equal size must first be in existence before either of them can be opened. While not opened, they are grey discs on both sides that are either fixed to an object, fixed in a particular position relative to the Earth, or in freefall.

When something goes through the portal, and loses gravitational potential energy in the process, the portals gain Sunlight. When the reverse happens, the portals lose Sunlight. When the portals are running out of Sunlight, they draw heat from the surroundings and spend Moonlight. When the portals run out of Moonlight, or cannot siphon heat away from the surroundings, they close.

The trouble is, what happens if the portals close with something still inside them? If they cut whatever is inside them, would that violate the conservation of enegy? How do I do this in a way that doesn't violate the conservation of energy?

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    $\begingroup$ Here are some interesting ideas about the game "Portal" which you could use as some reference. Generally I think you use a lot of magic by allowing such portals to exist, so in my opinion there is no reason to stop there. Just say "My portals cut things in half" or "My portals just expel everything" or "My portals check before if they have enough energy and wouldn't leave anything else through". $\endgroup$
    – Secespitus
    Feb 19, 2017 at 17:35
  • $\begingroup$ You say they can also lengthen the distance between two places? How does that work? $\endgroup$
    – Zxyrra
    Feb 19, 2017 at 18:56
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    $\begingroup$ the molecules affected by the severed bonds would just suck in energy from their surroundings cooling the matter around them, it is just an unusual endothermic reaction. Unless your portals can cool things to absolute zero before closing there is no real issue. $\endgroup$
    – John
    Feb 19, 2017 at 23:54
  • $\begingroup$ @Zxyrra A portal between two places can be made, possibly even what is basically a hoop. However, it is possible to make it so that the sides of the portal are the grey substance the circles are made of, and to essentially 'borrow' space from the Aether, (a parallel universe) which is how most magic works. The result is the distance between two places being longer. $\endgroup$
    – Piomicron
    Feb 20, 2017 at 17:57
  • $\begingroup$ I want to cover all bases. What would happen if the molecules were cooled to absolute zero? $\endgroup$
    – Piomicron
    Feb 20, 2017 at 17:57

4 Answers 4


You won't violate the conservation of energy.

It states that "the total energy of an isolated system remains constant"; "Energy can neither be created nor destroyed". Counting the universe as a truly isolated system, cutting something in half doesn't create or destroy energy. It merely separates the "net" energy of the object into two halves.

For example, if a blacksmith cuts a hot piece of metal in half, each piece conserves its own heat. They do not create any heat, and although the blade they use may get warmer, they do not destroy any energy whatsoever. It just separates.

Similarly, two halves of a separated object will share what used to be the energy preserved by one.

Energy is a prerequisite

Don't allow objects to pass through a portal as it powers down! In order to complete any task, you need to expend energy. If the portal is losing energy, it can lose the ability to teleport matter before the matter enters.

If an object is still somehow caught, the portal can draw energy from it - making it colder (thermal), or slower (kinetic / mechanical), or less electrically charged, or less magnetic, or less radioactive - etc.

  • $\begingroup$ This is not correct. Splitting an object in half cuts the chemical bonds that hold it together, and that costs energy. That's why objects don't fall apart spontaneously. $\endgroup$
    – kjgoebel
    Feb 21, 2017 at 22:50
  • $\begingroup$ @kjgoebel Keyword is split, not destroy, and that's why energy is conserved. For instance, bonds may be broken when you burn wood, but you don't destroy the energy contained - you release it as heat and light. Read up on the law of conservation of energy for more info. $\endgroup$
    – Zxyrra
    Feb 21, 2017 at 22:57
  • $\begingroup$ When something burns, bonds are broken but new bonds are formed, with a net release of energy. That's why wood can burn. Just splitting a solid object into two parts requires energy to be added to the system. The question was about when the portals run out of energy, so there's nowhere for the energy to come from. See en.wikipedia.org/wiki/Surface_energy $\endgroup$
    – kjgoebel
    Feb 22, 2017 at 0:09


If you don't have sufficient energy, you have to supply it by pushing. So the first 200 of your troop go through, and the last guy gets halfway through and can't go forward. This would be unhealthy for more than a few seconds, because blood can't move past the the interface either. If your head was in the interface, I think you would lose consciousness almost instantly. You could have crude jokes about 'portal disease' from having the wrong bits of you in the portal.

If you have a portal between a high and a low place can you recharge it by running stuff from high to low?

Can you connect two port sets with a 'sunlink' so that surplus energy from one can be used by the other. This would mean that you could put a portal in a river on a mountain, with the other end in a desert, and irrigate the desert at the same time you are charging the portal system.

Perhaps it is difficult to connect more than 2 sets together. Then for an invasion you are constantly disconnecting/reconnecting

Can a spell be made that shows the colour of a portal that is low on sunlight. So one that can take a man in full armour or a larger one that can pass a horse and coach shows up with faint gold colour, and as the charge gets lower goes to copper and then black or grey. (Black no sunlight, grey, not connected)

If you are worried about conservation of energy, you should worry about conservation of momentum. This is trickier since it's a vector quantity. Move 10 km west and the surface of the earth is moving at something like a meter per second up relative to where you were. The curve is approximately quadratic. Similar motions affect travel in other directions. At the extreme, a portal to the opposite side of the world would drop you out at between mach 2 and mach 3. This has weapon potential. Imagine the effect of a 4" water jet moving at 2000 miles an hour. Pressure washer indeed.

One way around this: Sunlight is your energy, moonlight is your momentum bank. You are using the angular momentum of the moon as a damper. This would imply short portals wouldn't need moonlight.

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    $\begingroup$ Firstly, yes, you can actually charge a portal by dropping something high into something low. In fact, that's such a good idea, I think I'll make that the main way of getting Sunlight, from a device that rotates a weight through a portal using solar power (you get an upvote just for that). You can connect two portals, but not directly (another magic power is needed),same with monitoring amount of Sunlight/Moonlight. As for momentum, direction is changed in relation to the portal, the difference in rotation is negated, giving or taking Sunlight from the portal in the process. $\endgroup$
    – Piomicron
    Feb 20, 2017 at 18:14
  • $\begingroup$ So the velocity relative to the exit portal equals the velocity relative to the entrance portal regardless of the relative velocities of the two portals. Hmm. *** I have a garden hose stream of water moving down at 2m/s. have 6 portal rings mounted on a wheel, spinning at 50m/s so the rings are intercepting the flow like anemometer cups, but open. Water exits the other end of the portal at 52m/s? $\endgroup$ Feb 20, 2017 at 21:58
  • $\begingroup$ Water would exit the portals at 2m/s, I believe. It's difficult to understand exactly what you were saying. You can only link two portals together, so I'm not quite sure what you were getting at. Unless the water is at all deflected, it would keep its velocity. The only rotational velocity that gets negated is the difference in rotation of the Earth as a result of being at a different latitude. $\endgroup$
    – Piomicron
    Feb 21, 2017 at 17:15

Spit it back out again.

If something needs energy expenditure going one way, it charges the portal going he other way. So if the portal reaches zero, any object in transit gets expelled from the origin portal again like an appleseed between your fingers, recharging the portal with a bit of energy.

For added fun, the spitting could be rather vehemently, smearing a person over the nearest wall, or smoothly, as required by the story.

Also, depending on whether you know how much charge a portal possesses and needs for the next transport or not the ride could a thrilling gamble.

  • $\begingroup$ Creative, but doesn't that also take energy? $\endgroup$
    – Zxyrra
    Feb 19, 2017 at 23:34
  • $\begingroup$ @Zxyrra not necessarily. It could be the energy of the portal itself being expelled. Think like a building collapse. When the base gets chewed through, it falls, releasing a lot of kinetic energy. $\endgroup$
    – SRM
    Feb 20, 2017 at 5:41
  • $\begingroup$ According to the OP, one direction gains the portal energy, the other expends it. $\endgroup$
    – ths
    Feb 20, 2017 at 7:33
  • $\begingroup$ That's if there's a height difference. Also, portals can change size, and there are ways of depleting Sunlight besides losing gravitational potential energy. $\endgroup$
    – Piomicron
    Feb 20, 2017 at 18:47
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    $\begingroup$ It spits 'downhill' If it's an uphill climb, it backfires, but if downhill it would suck it through. Consider a portal on the front of a push wagon, with the other end facing down a cliff nearer to sea level. If your battle is well above the exit portal then you drive back and forth. Enemy trips on the entrance, falls into the portal and falls to their death. $\endgroup$ Feb 20, 2017 at 22:02

Can a portal be selective? E.g. Can you set it up that only living things will go through it. Send in armoured cavalry, and get naked men riding bareback on the other side, while the back side of the entrance portal accumulates inventory?

Can a portal be made so that it only functions one way? If the entrance is very high, and the portal isn't charged, then in effect it's one way.

Can a portal be charged from either end, or is one end a master to the other? This can prevent a one way portal from being charged at the downhill end to allow infiltration 'uphill'.

Do both ends of a portal have to be the same size? What happens to a person who walks through a 6' portal if the other end is 4". Extruded like toothpaste? Are there other properties that come up from mismatched end sizes? E.g. is a big to small portal pair in effect a nozzle, with the exit speed multiplied by the relative area?

Tune it specific metals, place one end in the ocean in a current, and harvest gold. Or salt. Could you use this to get pure water from otherwise undrinkable sources?

One issue would be that for sparse materials (gold in seawater) the material coming through would be a gas. You may want the exit ring to be fastened to the neck of a large jar. Or you start off with a sample of the material, and it is flush to the portal, and the material plates out on the sample. It would act very much like vacuum depositing without the vacuum. Later on they could make integrated circuits this way.

Make a tiny portal. Surgically implant it in a person you want to control, or give a pill that disolves in the stomach releasing some mechanism that prevents it passing through. You can pass strychnine through the portal any time. Tiny doses will show him how painful the death can be.

Variations on a spell:

Can you make a portal that only passes light? This would be a magic viewing glass. Consider a gift of a partially silvered mirror with a lightportal on the back side. From the front it looks like a mirror. From the back you see what's in front. Make these up into spectacles. Wearers would be, "The Eyes of the King" Similar ones, bi-directional worn near the ear would be communicators.

If it's 2 way, and built into a pair of crystal balls, you have in effect reinvented Tolkien's palantir.

Can a light portal 'remember' and fast forward? In effect be a video recorder than can only playback once, as it lets the light it received back out in the same order.

Can a simpler light portal just delay light. Hold a 2" light portal up to the sun all day, and have an all night torch?

Can a light portal discharge all of it's light in a single flash? Take a 1 foot light portal, fill it with sunlight all day, keeping it oriented so that the light is always at right angles to the portal. Discharge all at once. This is in effect badly collimated 1 shot war laser. (it would spread at an angle of 1/2 degree) If nothing else this should be sufficient to set a ship's sails on fire from a distance, or temporarily blind an enemy.

Can a portal entrance be passed through a portal. If portals can be made elliptical then one can be passed through another. If you can quickly change the pairing between entrance and exit portal you have a method than an army can travel as fast as a team can run carrying the portal. You run forward until you are exhausted. Set the portal up. Next carry team comes through carrying a portal and goes another mile, sets it up. Meanwhile the end portal of the chain is tuned to the first set up portal, and everyone at the end comes through to the beginning. The end portal is then carried through to the beginning of the line. I think it takes a minimum of 4 to make it work. There are constraints on the relationship between the time you can carry fast, and the time it takes to pass an army through a portal. Probably better to have just the running team., and one more portal from the entrance to the exit at the end.

E.g. a 4 team relay. Portal D is matched to Portal A. A team sets out and goes a mile at good pace. Stops sets up portal. B team comes from start through portal D, carrying portal B and carries on. A team is reasting. Portal D is retuned to B. C team passes through D and keeps running. B and A are both resting. D team immediately picks up their portal and runs it to A, which now is tuned to ... The logistics of doing this get messy. You end up with 2 sets of of sprints leap frogging. You would need a set of interlinked communication portals to make it work.

Because of the gravitational/atmosphere equilibrium you don't end up with a hurricane blowing from a sea level portal to an alpine one. But normal storms coming through produce a typical variance of a few percent. If your end has a low in place while the other end has a high, you will have a strong headwind.

This might be commercialized with small portals. You have a farm of them in a part of the world that is chronically low pressure. The other end then is a good vacuum cleaner. If the size differential can be turned into a speed or pressure differential, then you can get better suction.

  • $\begingroup$ No, it can't be selective, although silver going through it will drain it; perhaps, but someone could charge it, and it would just absorb heat energy; the charging effect is from gravitational potential energy disappearing, therefore the entrance portal must be higher than the exit portal to work; they have to be the same size; you can't do any of the suggestions that involve specific transportation; yes you could surgically implant a portal; you would need to use another magic power; portals can be passed through other portals (you could just shoot a portal out of a cannon to travel like that $\endgroup$
    – Piomicron
    Feb 25, 2017 at 15:05
  • $\begingroup$ ); and yes you could get a commercial use out of it such as that. $\endgroup$
    – Piomicron
    Feb 25, 2017 at 15:06

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