On portals, momentum and inertia.

[Butters]

Another silly science question I’ve puzzled for a while.

A common device in science fiction is a magic portal, put to good effect in recent movies. A portal is opened to intercept projectiles and redirect them toward the enemy, or to bamboozle the enemy in combat with multiple portals to launch attacks.

Question is, magical elements aside, if you entered a portal in one direction that led to another portal facing the opposite direction. Would you retain your current momentum that you shared with the planet beneath your feet, putting you at odds with the ground facing the other way? Or if you left the pole to arrive immediately on the equator, where the surface is moving much faster. I imagine the effect being something like stepping off of a moving bus, but with much higher speeds involved.

 

[Christopher]

Yes -- if both portals are within the same overall frame of reference so that their orientation relative to each other is measurable, like teleporting from one part of Earth to another, then you would retain your original momentum. Larry Niven's teleportation stories cover this a lot. If you came out at a point where the Earth's surface was moving at a very different speed, you'd go flying. If you teleported to a much lower altitude, the gravitational potential energy would be converted to heat and you'd burn up. I think Niven's teleport booths had some sort of momentum correction system that dealt with the issue.

 

[Butters]

Thanks, and thanks for the link.

I don’t think they ever touch on this in Trek, except for that touch and go down warping thing they did once.

 

[Christopher]

I think the TNG Technical Manual has some handwaves about momentum cancellation in the transporters.

 

[rahullak]

Yeah, momentum cancellation exists in Trek. Kirk and Sulu wouldn't have survived their dramatic last minute transport onto the Enterprise while dropping into the heart of Vulcan.

 

[Kemaiku]

Transporters can obviously try to remove inertia to some degree. Otherwise Kirk and Sulu would have splattered all over the transporter room in the 2009 movie.

 

[Christopher]

^You both mentioned the fall in the 2009 movie, but keep in mind that a starship in orbit is literally falling around the planet, while also moving sideways fast enough to avoid hitting it. So the same physics you're both talking about there would apply in spades to every routine transport from orbit to the planet surface. If all that kinetic and gravitational potential energy weren't siphoned away somehow, anyone who got beamed down would instantly turn into a superheated ball of plasma blasting through the atmosphere at orbital velocity.

 

[Kemaiku]

The synchronising element of the transporter seems to account for orbital movement from the earliest model, even the NX-01.

Linear movement away from the ship seems to be a lot harder to compensate for, in most all of Star Trek, even with a planet rotating wildly fast, is don't move *relative* to the two fixed transport points around the same center of gravity once the ACB has locked. The terminus doesn't like sudden movement from it's lock on point, and can't arrest additional momentum after that.

Carol probably took a tumble off the Vengeance transporter once on that ship given running up an incline that suddenly became a downward ramp.

 

[XCV330]

one of those things like "why don't people transported back to the ship suffer the bends or at least severe sinus pain since there's no way every planet they visit has the same air pressure" kind of thing.

 

[JirinPanthosa]

Yes -- if both portals are within the same overall frame of reference so that their orientation relative to each other is measurable, like teleporting from one part of Earth to another, then you would retain your original momentum. Larry Niven's teleportation stories cover this a lot. If you came out at a point where the Earth's surface was moving at a very different speed, you'd go flying. If you teleported to a much lower altitude, the gravitational potential energy would be converted to heat and you'd burn up. I think Niven's teleport booths had some sort of momentum correction system that dealt with the issue.

That about gravitational differences is debatable, we can probably assume that gravity transfers through the portal.

Otherwise, can’t imagine the effect of transferring immediately between space with different amounts of curvature.

 

[Christopher]

That about gravitational differences is debatable, we can probably assume that gravity transfers through the portal.

It's not gravity, it's gravitational potential energy. If you do the work to lift something to a high altitude against Earth's gravity, you instill it with potential energy proportional to its altitude. If it then moves to a lower altitude, then that potential energy has to go somewhere, because energy cannot be created or destroyed. If you drop it, it's converted into kinetic energy as it falls faster, or thermal energy from its friction with the atmosphere. This is the kind of energy that's harvested by a water wheel or a dam turbine, the energy released when something at high altitude descends to lower altitude. But if you suddenly teleport that mass to a lower altitude, then that excess potential energy doesn't magically go away. It still has to be released, probably in the form of heat, since that's pretty much the final, most randomized form that all energy takes.

 

[JirinPanthosa]

Yeah, but those equations consider that you have to move through space to get to that lower energy level. If you have a shortcut the potential energy is less. The height part of the equation is smaller because you have a shortcut. Distance from center of mass takes the portal into account.

 

[XCV330]

It's not gravity, it's gravitational potential energy. If you do the work to lift something to a high altitude against Earth's gravity, you instill it with potential energy proportional to its altitude. If it then moves to a lower altitude, then that potential energy has to go somewhere, because energy cannot be created or destroyed. If you drop it, it's converted into kinetic energy as it falls faster, or thermal energy from its friction with the atmosphere. This is the kind of energy that's harvested by a water wheel or a dam turbine, the energy released when something at high altitude descends to lower altitude. But if you suddenly teleport that mass to a lower altitude, then that excess potential energy doesn't magically go away. It still has to be released, probably in the form of heat, since that's pretty much the final, most randomized form that all energy takes.

the problem is that the transporter has been depicted in one of two basic ways: as a straight up teleport system, which, if some kind of stable wormhole might be thought to behave that way

but especially since TNG aired, maybe influenced by TRON, the transporter become more of the ultimate 3d scanner and replicator, which is even more magical and problematic for reasons I wont enumerate here, except to say I don't think the show runners realized how storybreaking that potentially could be).

Abrams movies, does show inertia transferring back with being beamed in a spectacular way in the 09 movie, but that's just as problematic, as you have mentioned due to GR and the fact that while the beamed subjects were falling DOWN, relative to world they were on, they were also travelling in several other directions at various velocities which were simply not the same as the landing pad they were beamed to. But Trek has always had magic devices to handwave inertia and gravity, so I guess not that great to get hung up about.

And as usual, we'll find Trek was probably right in a few decades, and you can beam someone in and control all their properties of matter. , oh wait someone's working on it https://arxiv.org/pdf/1712.08680.pdf

 

[Christopher]

Yeah, but those equations consider that you have to move through space to get to that lower energy level. If you have a shortcut the potential energy is less. The height part of the equation is smaller because you have a shortcut. Distance from center of mass takes the portal into account.

That's not how it works. It may appear to be a shortcut relative to the frame of reference of the person or object being teleported, but you cannot ignore how it would appear in the frame of reference of the external system, e.g. the gravitational field of the planet. The discrepancy between the two frames of reference is exactly the problem, because you're taking the object between different gravitational energy levels relative to the planet without doing the work of moving it through the intervening distance and thereby releasing its energy. So that energy has to be released all at once instead, and that's a mess.

Not to mention that it only even counts as a "shortcut" if you're assuming some sort of dimensional portal or wormhole. If it's a Star Trek- or The Fly-style teleporter, then there is no spatial shortcut, merely an energy signal or a particle stream being transmitted from one point to another. It may seem that taking an object apart and putting it back together would negate the energy/momentum discrepancy issue, but it wouldn't do so automatically. Presumably the goal of a teleporter is to put every particle back in exactly the same state it started in, including its momentum and energy levels. Put it together at a destination where the potential or kinetic energy is lower and it ends up containing too much energy -- unless you somehow program the reassembly system to compensate for the shift and bleed off the excess.

Here's Larry Niven's essay "Exercise in Speculation: The Theory and Practice of Teleportation." It's a useful overview of the subject.

 

[Butters]

So. Two of my favourite fantasy device/inventions I came up with while I should have been doing something important, wouldn’t work. And the lack of portal technology is the lesser problem.

A shaft, much like a lift might use, but with a portal at the bottom that exits at the top, creating an infinite shaft. Take that shaft and drop some thing, like water, you’ve got infinite fuel for a water wheel. Free energy.

Take the same shaft and drop a lift carriage. It would fall indefinitely, but inside you’d have zero g because of the free fall. Add an extra portal so that you can enter that carriage, but instead of enjoying a zero g chamber, you’d become mush against the roof of the fall lift as you went zero mph to terminal velocity.

But would planetary gravity work through the portal? Would there be any downward gravity if the bottom of the shaft was also the top?