Robotic swimming in curved space via geometric phase

https://www.pnas.org/doi/full/10.1073/pnas.2200924119

by obert • 4 years ago

50 16 4 years ago

pnas.org

gsatic• 4 years ago

Moonwalking on a curved surface eh?

jogundas• 4 years ago

This is about a robot that moves around in a curved space by merely changing its shape. Very cool.

upwardbound• 4 years ago

Could this principle be used to make a reactionless drive for spacecraft? I would assume it would only be effective in an absurdly deep gravity well, like right next to a star, so it's probably impractical; I'm just curious.

skykooler• 4 years ago

Sort of, but not really. This is like reorienting yourself in midair by waving your arms in that it doesn't change your velocity, just your position. So while something like an ion engine can get away with absurdly low accelerations because they build up to large values over time, this gives you an extremely low velocity with no way to increase it. And in space with a high enough degree of curvature for this to work, you need to be moving at a very high speed in the first place to not immediately fall into the source of the curvature.

sacrosancty2• 4 years ago

On Youtube https://www.youtube.com/watch?v=BZF1nelgPNs

sacrosancty2• 4 years ago

Could someone explain who their robot isn't violating conservation of angular momentum? That seems key to understanding it.

ben_w• 4 years ago

IANAP, but I think conservation laws are always equivalent to some symmetry (space translation, time translation, rotation, etc), so if one of those symmetries doesn't hold in a curved space, the corresponding conservation law just doesn't hold.

meltyness• 4 years ago

They simply argue that by maximizing the inertia of the sphere during the motion along the lateral arms, motion can be favored in that direction.

Here's the concept you're most likely missing, as taught and demonstrated by Walter Lewin https://www.youtube.com/watch?v=lvfzdibrUFA

MauranKilom• 4 years ago

A cat righting itself in free fall also manages to do so without violating conservation of angular momentum:

https://en.m.wikipedia.org/wiki/Falling_cat_problem

The more general term/field is https://en.m.wikipedia.org/wiki/Nonholonomic_system.

est31• 4 years ago

Does this also work for curved space time? Or is that type of curvature not going to help because it's relativistic? Or is the shape wrong? They mention that it doesn't work for cylinders.

omnicognate• 4 years ago

The abstract says "the noncommutativity of translations permits translation without momentum exchange in either gravitationally curved spacetime or the curved surfaces encountered by locomotors in real-world environments."

The idea isn't new. This is an experimental verification of it in the case of curved surfaces, not gravitationally curved space, but it should work for both.

bilsbie• 4 years ago

Maybe this how the EM drive could work.

kalimanzaro• 4 years ago

If cats are a good thinking aid for you, this seems very similar to the Falling Cat Problem:

From wikipedia: "The apparent contradiction with the law of conservation of angular momentum is resolved because the cat is not a rigid body, but instead is permitted to change its shape during the fall owing to the cat's flexible backbone and non-functional collar-bone."

https://en.m.wikipedia.org/wiki/Falling_cat_problem

quickthrower2• 4 years ago

This seems similar to one of the kids playground toys where you free spin (think record player with handlebars kind of thing) and I am pretty sure you can get somewhere you want (angle wise) by jigging side to side with technique, but I have never though about the physics of it.

anonymous344• 4 years ago

what if we use burst of energy (rocket motor) at the "hot point" when this is shape shifting and helping the most ?

danfickle• 4 years ago

My comment from duplicate post[3]:

So a scientific paper claims to have invalidated known physics. History tells us this will go one of two ways. Either, the authors will get a Nobel prize (eventually) or it will turn into a cold fusion level debacle.

I leave you with a quote from the lead researcher with commentary from Cosmos magazine[1].

“ We let our shape-changing object move on the simplest curved space, a sphere, to systematically study the motion in curved space,” says lead researcher Zeb Rocklin, assistant professor in the School of Physics at Georgia Tech. “We learned that the predicted effect, which was so counter-intuitive it was dismissed by some physicists, indeed occurred: as the robot changed its shape, it inched forward around the sphere in a way that could not be attributed to environmental interactions.”

To make sure that the effects induced by the curvature of the robot’s space dominated, the physicists had to isolate the system as much as possible from external forces. Only then could the team ensure minimal interaction or exchange of momentum with the environment.

The curved space was produced by placing a set of motors drive on curved tracks. The tracks were then attached to a rotating shaft to produce a spherical space.

Friction was curtailed using air bearings and bushings – low heat and low mess alternatives to ball bearings. Gravity was diminished by aligning the rotating shaft with Earth’s gravity.

——

Edit: There is some critical commentary in The Register’s comments section[2].

[1] https://cosmosmagazine.com/technology/curved-space-robot-def... [2] https://forums.theregister.com/forum/all/2022/08/11/curved_s... [3] https://news.ycombinator.com/item?id=32409959

Robotic swimming in curved space via geometric phase

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