Portals are a popular transportation device in many fantasy and sci-fi books and movies.
Some representative examples are Star Gate, Monsters Inc, Portal the video game and the Hyperion series.
Portals are often represented as doors or holes in spacetime in which people can simply take one step and travel light years to a new planet.
One thing that is often missed, or just plain ignored, is that when the travelers step through a portal they will be moving from one gravitational field another. The odds that the two gravitational fields will match perfectly will be close to nil.
Moving Between Different Gravity Fields
If a traveler is moving say from Earth to a super-Earth which would have a stronger gravitational field then that person will suddenly lose a lot of potential energy which should show up as kinetic energy. It will feel to them like falling as they travel through the portal and they should end up gaining significant velocity. Unprepared portal travelers will end up stumbling and falling if they are not prepared. Maybe they will even smash into a nearby wall and get splatted.
The air near the portal will also tend to flow from the weaker gravitational field of the Earth to the stronger gravitational field of the super-Earth. There will be a constant wind flowing through the portal which if not closed off with a door will eventually drain the Earth of most of its atmosphere.
On the other hand, if a traveler is moving from Earth to a space station (that is not spinning) then they will be moving from a strong gravitational field to a near-zero gravitational field. To that person it will feel like climbing a very tall mountain over a space of one step. They will need to somehow gain a heck of a lot of potential energy. Such a trip might simply not be possible with just human muscle power. The air will also flow from the space station through the portal to the Earth and since a space station does not have much volume it will be quickly drained of atmosphere.
I can only imagine that portals will therefore need to have the capability to automatically take care of these large differences in potential energy when they are linking up different gravitational fields.
Moving Between Locations That Have Different Velocities
Speed is a scalar whereas velocity is a vector.
Speed being a scalar is just a simple number and can be used to calculate the energy that a physical object may have via the kinetic energy equation E = mv2.
Velocity is speed plus direction and is therefore termed a vector. Both components of velocity are important as it takes effort to change both and object's speed as well as its direction.
So, if a portal is linking two planets each planet will most definitely be a different velocities. Not only will they have different speeds but the direction of travel of these planets will also be different (each planet will have different spins and the latitudes of the locations on each planet also come into play but let's ignore all that for now as a needless complication).
A traveler stepping through a portal will therefore need to worry about the different velocities of the two locations. If one planet is moving much faster than the other the traveler will either gain or lose a lot of kinetic energy within the space of one step.
It takes time to step through a portal so one part of the person's body will briefly be moving faster than the other part. The traveler will feel like they are being pulled apart and if the velocity differences are large enough they will be.
Essentially a portal could be a futuristic sci-fi way to draw and quarter a person.
Finally, if the direction of travel at the destination is different then an unaware traveler will no doubt stumble and fall as they get jerked in an unexpected direction at the new location.
A Portal Above Another Portal Which Links Back to The First Portal
Okay, that was an awkward mouthful.
Let's imagine we have two portals, a blue portal and a green portal, set up in the manner shown in the image in below. The blue portal has an inlet and an outlet which are connected through something like hyperspace. Ditto for the green portal.
The outlet of the blue portal is above the inlet of the green portal. The outlet of the green portal is set up above the inlet of the blue portal. The direction of gravity is shown by the black arrow.
Hold an object halfway between the blue portal and green portal on the left hand side of the image and let it go. It will fall through the inlet of the green portal and reappear below the outlet of the green portal.
Then it will gain kinetic energy (and speed) as it falls towards the inlet of the blue portal and it will reappear below the outlet of the blue portal.
Then it will gain even kinetic energy (and even more speed) as it falls towards the inlet of the green portal.
You see where this is going. The object will gain speed with every cycle, eventually reaching relativistic speeds and approach the speed of light.
This is clearly a free energy device which is scientifically impossible so the gravitational field must therefore need to be somehow active through the 'hyperspace' of a portal. The object will therefore have to decelerate and/or accelerate rapidly as it is transported from a portal's inlet to outlet.
If this acceleration of deceleration is too violent it could crush and object in one direction and rip it apart in the other direction. Neither scenario is a fun way to travel that's for sure.
Closing Words
Science fiction is a fun diversion and a great way to explore radical new intellectual concepts but the laws of physics must still be obeyed. The farther a story strays from the laws of science the less it is 'science' fiction and the more it is just straight up fantasy (which is also fine).
Portals are great story devices but to work they will need some serious potential energy, kinetic energy and gravitational field compensators built in to them to make things work so as to not rip unwary travelers apart.
Thank you for reading my post.
Post Sources
https://en.wikipedia.org/wiki/Teleportation
https://en.wikipedia.org/wiki/Portal_(video_game)
https://en.wikipedia.org/wiki/Vector_field
https://en.wikipedia.org/wiki/Scalar_field
https://en.wikipedia.org/wiki/Classical_mechanics
https://en.wikipedia.org/wiki/Kinetic_energy
https://en.wikipedia.org/wiki/Potential_energy
https://en.wikipedia.org/wiki/Wormhole