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In Ender’s Game, the Battleroom is comprised of large, static objects called “stars” that are organized into patterns to create an environment for zero-gravity Laser Tag. Presumably the stars don’t move, as the players can land on them, jump off of them, and use them as rally points without worrying about their momentum moving the star around. They also don’t seem to be physically moored to anything.

I was thinking that if this tech would work, I could use it as a zero-gravity construction mechanic in my story.

Would there be a way for a specially-designed object, like a star, to be held static in a given location in a large, specially-designed space without physical contact? My first idea was magnetic induction from the “walls” onto the object, but I don’t think that would work for multiple objects in different locations in the same space.

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    $\begingroup$ Related question with in-universe (Ender's) answer: scifi.stackexchange.com/questions/46103/… $\endgroup$
    – CorwinCZ
    May 17 at 10:57
  • $\begingroup$ My understanding of the Ender's Game "stars" is that they were just very massive (in terms of mass, not necessarily size) objects, so the force of a couple of teenagers kicking off of them is not enough to move them all that much. Presumably a concerted effort by a whole bunch of people could move them more, but they've got more pressing concerns during a match. $\endgroup$ May 17 at 14:36
  • $\begingroup$ While it's very cool, I don't see what immovable stars buys you in terms of training. Just moor them with wires, and if necessary make it illegal (in terms of battle rules) for anyone to use the wires to move. $\endgroup$ May 17 at 16:23
  • $\begingroup$ Worth noting that in Ender's Game, they explicitly have gravity control, and that is what allows the Army leaders the "hook", a handheld device that can drag them through the Battle Room. Presumably they use that to make the "stars" static as well. $\endgroup$
    – jdunlop
    May 17 at 18:19
  • $\begingroup$ Sorry, @CorwinCZ - should've followed your link first, as it makes all those points and then some. $\endgroup$
    – jdunlop
    May 17 at 18:24

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Yes, you absolutely can use magnetic fields to do this. By manipulating the magnetic fields on the boundary of your construction zone using an array of electromagnets, you can create more or less whatever pattern of fields you want inside the volume. If your ‘stars’ are made out of diamagnetic materials, or you mount the right type of magnets to them, they will be repelled by the regions with stronger magnetic fields. You then create a pattern of magnetic fields in the zone that has low-points where you want the stars, and they should find their way there. A simple version of this was demonstrated here: https://www.pnas.org/doi/full/10.1073/pnas.1408705111

Alternatively, you could use sound waves to achieve a similar effect. Place speakers around the construction zone, use them to create a pattern of sound waves in the zone, and let the sound waves push things into position. Best to use frequencies outside the normal hearing range or things will be very loud. This has been done experimentally here: https://www.nature.com/articles/ncomms9661

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    $\begingroup$ The stationary electromagnets would have to be dynamically controlled, right? As far as I'm aware permanent magnets can't stably levitate objects. $\endgroup$ May 16 at 22:55
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    $\begingroup$ If you're thinking of Earnshaw's Theorem, it doesn't apply to diamagnetic materials - they can be stably levitated using static fields. If you want to levitate other things, then yes I believe you're correct. $\endgroup$
    – Mothas
    May 18 at 5:12
  • $\begingroup$ You're right. I had forgotten about superconductors, and didn't know about the other materials. $\endgroup$ May 18 at 16:25
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Dynamic position keeping: you define a position for each object, give it a mean to check it and means to keep it against external influences, probably something similar to rockets.

In this way when a player lands on one of them the, the system detects a movement from the desired location and activate the reaction to compensate for the movement. In this way the player's momentum is countered by a reaction in the opposed direction. Same trick when they jump off.

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What if you just made them REALLY heavy to minimize movement from students? And reset their position at the end of each round?

It would be less resistant against rotation but that is easily solved with reaction wheels or control moment gyros.

How big are those cubes? I can't remember. 3 foot cube seems a reasonable size for one person to be able to hide behind. If tungsten then it is 14,717 KILOGRAMS. Difficult to get moving by human sized momentum even with no friction. If multiple people can hide behind a cube, it gets a lot heavier.

If tungsten is too valuable then lead which halves the mass.

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  • $\begingroup$ Thought about this as well. If you cover it up well enough, depleted uranium would be an option too, cheaper than Tungsten. Though if you have the budget to build something like that just for training some kids (or whatever your reason is), the cost of tungsten is likely not make that big of a dent in that, since asteroid mining would be very well established by then $\endgroup$
    – Hobbamok
    May 17 at 8:14
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Dynamic redistribution of internal mass for a more-than-rigid shell

The "star" has to be heavy so as not to move around. But the weight is not distributed uniformly in the interior, nor in the shell, like we assume for many everyday objects.

Instead, there is a small, very dense mass (blue in the illustration) inside the larger "star". This mass is tethered in many directions to the outer faces. When a contestant lands on the star, pushing it, the tethering strands are stretched. Ordinarily, this would make the "star" act as a rigid body, moving slightly in response to the impact. However, in this case, the tether detects the tension from the outside contact, and compensates by increasing that tension with its own motor in near real time (yellow arrows on the gray lines) All linear and any angular momentum are transferred into the central mass.

enter image description here

The result is that the outer shell of the "star" moves scarcely at all, even though it absorbs the contestant's momentum. The momentum is now hidden in the internal motion of the dense inner mass.

The "star" is limited in the amount of momentum it can absorb this way, multiplied by the time. But of course it is also limited by the force of impact it absorbs before it crumples to avoid lethally injuring the contestant. I hope. Given the rules and gameplay, it may be designed so that the internal masses can be reset between matches, or it may have an unobtrusive propulsion system (such as a cable it can launch at the nearest "star" in the right direction, or to a nearby wall) that abruptly transfers the momentum when the action of the game has moved elsewhere. In extremis, the "star" can abruptly move its outer shell so that the mass inside is now heading toward the center; this would have to be included in the rules, since it affects game play.

Minutiae: because the negative feedback on the "star's" motion is accomplished by a positive feedback on the cable linking shell and core, you might think that this would be unstable. However, the system knows how much tension it has added to the system, and can subtract that to understand the true force on the outer shell. Also, the system only needs to add a small percentage to the tension normally needed to accelerate the inner core in order to ensure all the added momentum goes to that core. (The outer hair cells of the ear do this to a much more remarkable extent, greatly amplifying the sounds we hear) Also, because the core needs to absorb angular momentum, it may contain reaction wheels that adjust the core so that it doesn't literally need to rotate while remaining attached to its network of cables.

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  • $\begingroup$ This seems like it would work well, albeit with some high-level calculation. The only problem I can think of is that the star’s center of mass is moving, so angular velocity might be more difficult to control. Otherwise, it seems like a very effective method. $\endgroup$
    – Mark Price
    May 16 at 15:43
  • $\begingroup$ @MarkPrice - Ender's Game involved some very sophisticated computing power. However, I think this could work with a cable-autonomous method - each cable simply pulling with a little extra tension whenever tension is put on it. Dumb cables would do a purely physical calculation to put 99% of any sort of momentum on a heavy central mass; we just need to nudge that up to 100%. But without reaction wheels, the mass probably can't stay attached to cables long - with them, it should hold the same angular momentum no matter where it is in the shell. $\endgroup$ May 16 at 15:54
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A high density core.

These objects are way heavier than it seems and the player jumping off them would not give them enough momentum to move them.

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Water filled cubes acting as momentum stabilisers

... and also gyroscopic stabiliser.

If I size up the cubes in the scene right, they seem to be about 3m wide (I'm being a bit generous with the size for this to work). If you partially fill that volume with water you get a nice round 10 tons of water. Now the cube is separated into many flexible chambers with pumps between them. For a simple example imagine it's separated into halves and all the water is placed on one side. When the kid, 50kg, impacts the side with water at roughly jogging speed of 2m/s, it will need to absorb all the momentum and start pushing water to the other side. But since it's 10000kg of water it ends up moving at 0.01m/s. Since the cubes are 3m wide the water will have about 1.5m distance to move around, that gives it 2.5minutes of staying perfectly still from the outside. Even when it starts moving, the sheer mass of it makes it move at super slow rate 0.01m/s, that's 100 times slower than a slow walk. If anything hits it from the other side you can basically average out momentum over time. Finally it can also do gyroscopic stablisization by pumping water in a circle.

I used water in the example because it's guaranteed that any spaceship that size will want to have that much water available; for one they also have a pool. But this contraption would obviously work much better with mercury. With a density 13.5 times higher (but let's say you need much bigger motors) you can get at least 10x the time, ie 25minutes. And after all that time, the resulting move is 1000 times slower than walking. That's more than 10 times slower than a snail at 0.013m/s.

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