7 ms·

Robotic motion in curved space defies standard laws of physics

41 points by thedday 4y ago

8bitsrule 4y ago
Hmmmm ... time to look at the 'anomalies' again? (And then there's Oumuamua ...) [https://en.wikipedia.org/wiki/Pioneer_anomaly https://en.wikipedia.org/wiki/Pioneer_anomaly] [https://en.wikipedia.org/wiki/Flyby_anomaly https://en.wikipedia.org/wiki/Flyby_anomaly]
keyle 4y ago
Do article like these get peered review before publishing or is it a publish first, deny later type of approach?
- hither_shores 4y ago
  Press releases don't get peer reviewed: they're not by or for researchers. Hence the sensationalist title. The actual paper (https://www.pnas.org/doi/10.1073/pnas.2200924119 https://www.pnas.org/doi/10.1073/pnas.2200924119) was peer reviewed, and is perfectly reasonable.
  - dataflow 4y ago
    > Hence the sensationalist title. It's not just the title. The body itself seems to claim researchers have "proven" momentum isn't conserved: "Until recently, physicists believed this to be a constant, following the law of conservation momentum. Now, researchers from the Georgia Institute of Technology have proven the opposite—when bodies exist in curved spaces, it turns out that they can in fact move without pushing against something."
    - hither_shores 4y ago
      Sure, sensationalist copy as well. The point is that press releases are written by people who neither know nor care about the subject, and can't be trusted to accurately reflect the state of the field or the content of the papers they refer to.
      - defrost 4y ago
        Absolutely, read the paper and not the press release. By way of example, this linked press release contains the paragraph: > While the effects are small, [ ... ], just as the slight frequency shift induced by gravity became crucial to allow GPS systems to accurately convey their positions to orbital satellites. which is a /headdesk given the reality that ground based GPS receivers compute their positions using signals from several 'visible' transmitting GPS satellites; they do not "convey their positions to orbital satellites". This doesn't detract from the source paper but it does highlight the quality of the lay press release. (Note: It's essential to correct for gravity induced shifts in timing given these exist because of relativity .. but the shifts themselves are not "crucial" to the triangulation; in a hypothetical universe where relativity didn't exist | had a minor effect in this usage case, the triangulation of GPS against moving stations would still work with either no correction for relativity or a different correction)
        dataflow 4y ago
        > crucial to allow GPS systems to accurately convey their positions to orbital satellites. I actually think they may have been referring to the ground stations here, not the GPS devices you carry as a user, since they said "GPS systems" and not "GPS receivers". Presumably the satellites need to determine their positions based on the positions of the ground stations.
        defrost 4y ago
        > Presumably the satellites need to determine their positions based on the positions of the ground stations. The satellites broadcast a rough guide to their collective positions (an almanac) to recievers and a precise guide to their specific exact location (the ephemeris) to ground clients (phones, GPS receivers, etc). Master Ground control stations update the satellites with that information every four hours or so .. strictly speaking the satellites don't do any computation of their own position - they broadcast their weekly lapsed epoch signal and echo data frames from their last Master Ground Station update. Orbital changes can result from solar wind pressure, magnetic forces, passing objects with mass, etc. It's the GPS receivers that do the 'heavy' positional computations for individual fixes .. and the Master Stations that do the somewhat harder work of constellation updates. ( There's a wealth of accuracy processing that can be run in parallel or post - differential base station GPS, forward error correction, post aquisition corrections, et al ) [1] https://gssc.esa.int/navipedia/index.php/GPS_Ground_Segment https://gssc.esa.int/navipedia/index.php/GPS_Ground_Segment [2] https://en.wikipedia.org/wiki/GPS_signals https://en.wikipedia.org/wiki/GPS_signals
    - kergonath 4y ago
      This completely discredits the work being presented. I wish universities and institutions (and popular science media) stopped doing that. It is really annoying to have headlines about $whatever “breaking the laws of Physics” every other day when reading the abstract is enough to see that no, this is not breaking anything. It is also harmful in the long term because it reinforces pseudo-scientific tendencies in the general public. As an example, I once had a discussion with someone arguing that energy use was not a problem because we’ll soon have free energy because he read a badly written vulgarisation piece discussing arguments for local non-conservation of energy in some quantum interactions. This gives people in general a very distorted view of what is and is not feasible. Which would not be too much of a problem if people did not then vote and behave based on this inaccurate perception.
chaps 4y ago
Between the gulf of how the article's written (PR fluff?) and the paper's/video's kind of bland approach, I'm having a hard time understanding what's going on here or what the significance of this is.. or even if there is one. Are they suggesting a novel way to move forward in space without exerting anything in the opposite direction?
- hither_shores 4y ago
  That "swimming" in curved spacetime is possible is well-known. The paper describes the implementation of a system for doing so.
  - pmayrgundter 4y ago
    Can you elaborate or cite something about this? What does swimming mean?
    - codethief 4y ago
      "Swimming in spacetime" is a reference to the following paper by Jack Wisdom: http://groups.csail.mit.edu/mac/users/wisdom/swimming.pdf http://groups.csail.mit.edu/mac/users/wisdom/swimming.pdf The paper is quite accessible but here is a short summary: https://www.science.org/content/article/swimming-through-spacetime https://www.science.org/content/article/swimming-through-spa...
aabajian 4y ago
Edit: Here's the pre-print: https://arxiv.org/pdf/2112.09740.pdf https://arxiv.org/pdf/2112.09740.pdf I'm not sure others have read deeply into the article. While there's always some subtle ways friction can sneak its way, the authors account for the obvious. The article has a lot of definitions, which makes the crux difficult to pinpoint. They use the word "shape" many times to describe the configuration of the four motors. I think this adds confusion. Let me try to summarize as I understand it without using the word "shape." The authors are measuring the net position of the arm holding the two motor tracks. The arm can only rotate clockwise or counter-clockwise in the horizontal plane, so it suffices to measure the arm position using an angle - phi, with initial position being phi = 0. The two motor tracks are oriented perpendicular to one another. A key point is that the motors do not travel up, down, left, or right. That's Cartesian thinking. Rather, they travel clockwise or counter-clockwise along their respective tracks. One track is vertical, the other is horizontal. With this picture, the authors claim that the net horizontal position angle (phi) of the arm changes depending on how the motors move along their tracks. None of this is particularly impressive (e.g. the motion of the motors may cause the arm to "wobble" one way due to issues with synchronization, weight differences, and numerous other causes). The interesting part is this: If the vertically-oriented motors are made to move up-down on linear straight paths instead of curved paths, the net movement of the arm is not observed. Think about that for a moment...why should the path that the vertically-oriented motors take affect the horizontal rotation of the arm? In Newtonian physics, acceleration/force in one direction cannot affect acceleration/force in a perpendicular direction. At any given moment, the vertically-oriented motors are perpendicular to the arm's rotation direction, so their motion should not affect its horizontal rotation in space. In particular, both the curved-vertical and linear-vertical paths are perpendicular to the horizontal at all times. Newton would predict that changing the path of the vertical motors from linear to curved would not affect the horizontal rotation...but it does.
- waterproof 4y ago
  Thanks for the interpretation, very compelling if that's what they're showing. Sci-hub doesn't seem to have this paper so I haven't been able to read the whole thing. But from what I can see the data doesn't seem to agree with what you're saying - > The interesting part is this: If the vertically-oriented motors are made to move up-down on linear straight paths instead of curved paths, the net movement of the arm is not observed In the video, it looks like the same kind of net movement is observed with the cylindrical configuration as with the spherical configuration. With the cylindrical configuration the average position migrates by about 0.2rad and stays there, compared with 0.3rad for the spherical configuration. So the spherical configuration seems "better"(?) but it's hardly a convincing difference given how much slop there seems to be - there's even significant net rotation in the "plateau regime" where it's apparently supposed to stop migrating. And why is there a "plateau regime" anyway? If they can pull off a delta-V with no momentum transfer, I'd expect the robot to keep migrating around in a circle. But it stops. The abstract says: > While this simple geometric effect predominates over short time, eventually the dissipative (frictional) and conservative forces, ubiquitous in real systems, couple to it to generate an emergent dynamics in which the swimming motion produces a force that is counter-balanced against residual gravitational forces. I think this translates to "it just stops working after a bit". What's your take?
deleted 4y ago
[deleted]
rcthompson 4y ago
The title seems like a weird way to describe the phenomenon, given that the robot exists within our real space, which is essentially flat at the scales and velocities involved (specifically, non-relativistic ones). Is constraining motion to take place within a spherical slice of Euclidean 3-D space really equivalent to motion in an actually spherical space? Unless I'm missing something, it's fairly easy to understand why it works. The horizontal weights use Newton's 3rd law to rotate the bar in the opposite direction from their own motion. The vertical weights are used to alter the robot's moment of inertia relative to the constrained axis of rotation. (You could replace these weights with a single weight that slides along the radial bar toward and away from the center axis.) The moment of inertia determines how much the robot as a whole moves then the horizontal weights move, so if you move the weights one way, then change the moment of inertia and move the weights back the other way, you get a net change in position. This is why it doesn't work with a cylinder: because when the vertical weights are constrained to a cylindrical space, moving them doesn't change the robot's moment of inertia. I guess the point is that in a true spherical (or otherwise non-flat) space, any robot with the appropriate moving parts can do this "swimming" just by moving its parts in the right way in space without pushing on anything, essentially "pushing" against the curvature of the space itself. Now that I think about it, there's a conceptually simpler way to demonstrate the same thing. Imagine a "robot" made of two parts: a "gun" and a "bullet". The gun starts on the "equator" of a spherical space and fires the bullet due west. By Newton's 3rd law, the gun begins moving east. After some time, the bullet flies all the way around the equator and impacts the gun from the east. The gun catches it and its velocity once again becomes zero, but in the time between firing the bullet and catching it, the gun has progressed some distance to the east, and now the combined gun and bullet system that we started with has changed its position. The gun can reload and fire the bullet west again, repeating the process as many times as necessary to continue progressing around the equator.
- oreally 4y ago
  It's clickbait I guess. IIRC einstein had to dip into math for imaginary numbers and non-euclidean spaces for his theory on general relativity, but imaginary numbers aren't the most intuitive in the physical world and one at first conception one wouldn't think it should be in there. Yet imaginary numbers do help in many computations - for example, 3d rotations use quaternions that use imaginary numbers.
tbrownaw 4y ago
You can do something similar yourself with an office chair and a pair of weights. 1. Hold both weights out in front of you 2. Swing one to the side. 3. Swing the other one straight up over your head (or just pull it in next to your body) 4. Swing the first one (off to the side) back in front of you. 5. Swing the second one back down (or push it away from yourself) 6. You are now back at (1), but the chair you're in has turned slightly.
- aabajian 4y ago
  I get what you're saying, but this is why people are brushing off the article because they think it's either obvious, or due to friction. It isn't either. The phenomenon you're describing is a gyroscopic effect. The entire system (you, the chair and the weights) rotates. That is not what the authors have setup. The author's system includes the weights and their tracks, but not the arm connecting them. The arm is the "space" (or surface) in which the weights and tracts reside. In your example, planet Earth would be the "space" (or surface). In order for you to demonstrate their phenomenon, your chair would have to change position (translate) from where it started, not just rotate in-place. The source of the confusion is that they are measuring translation with an angle. They can do this because their universe is very small. You could also measure your translation relative to Earth with an angle, but it would be a tiny angle as the Earth has a very large radius.
  - colanderman 4y ago
    > The author's system includes the weights and their tracks, but not the arm connecting them. But the arm is a part of the system. Are they not accounting for the centripetal force through the arm? (EDIT: Answering my own question, the preprint does include the text: > Notably, the robot’s ability to move does not violate the usual rule against movement without forces in three-dimensional space because the full three- dimensional dynamics in fact include normal forces on the robot supplied by the boom arm that confines it to the sphere. So the authors are (of course!) aware of this in 3D space. Their experimental setup is functionally no different from the swivel chair example.) > In order for you to demonstrate their phenomenon, your chair would have to change position (translate) from where it started, not just rotate in-place. No, by the same logic above, why can I not simply keep my upper arms stiff and declare them not part of the system? My lower arms and the weights are then "translating" through "space".
colanderman 4y ago
No physics are being violated here -- the authors themselves point out: > Notably, the robot’s ability to move does not violate the usual rule against movement without forces in three-dimensional space because the full three- dimensional dynamics in fact include normal forces on the robot supplied by the boom arm that confines it to the sphere. (And I'm sure they would agree that you can even eliminate forces from the table itself from the equation, if you attach an identical robot on an opposite arm to supply that normal force from within the system.) What I don't understand is -- given that the Euclidean physics are known, why was an experimental setup involved in this paper, as opposed to simply mathematical proof? Or is the experimental demonstration the purpose in and of itself?
pontifier 4y ago
This seems to be using a kind of "reactionless drive" I.E. Movement without apparent exterior force. This kind of drive can't produce a net velocity, but can create repeated small translations. To imagine how one would work in a space ship, imagine a very long ship that acts like a particle accelerator. Take an atom from the front end, accelerate backward to near the speed of light so that it's mass increases say 1000 times. While that atom is flying backward, your ship that pushed on it moves in the opposite direction. Catch it again in the rear, and the ship must return to zero speed because of conservation of momentum, however the ship is now in a slightly different spot. Move that atom back to the front of the ship, but slowly so it's mass stays low. Your ship moves backward, but not quite as far as it moved before. Do this again and again to translate the ship. The net velocity of the ship will always be zero though.
- ianai 4y ago
  I wonder if there’s a way to do that on an integrated circuit? I’m guessing a little channel carefully designed to create a magnetic field pointing a consistent direction. Electron gun on one end and positive terminal the other. This also starts to resemble how a laser works internally (at least HeNe). Granted, that’s light and massless.
  - prox 4y ago
    Wasn’t there a magnetic/levitating robot that did this?
- b112 4y ago
  Catching the atom, and moving the atom, should result in the ship being returned to its original place. "Being sneaky" by moving things slowly, or catching things, just doesn't work. It will all add up. All movement will be undone.
  - Tuna-Fish 4y ago
    You ignored the part where the atom is fired at such high speeds that it sees relativistic increase in mass. I dunno if that works, but that's the part that matters in the explanation above, and that's the part you'll have to address.
jstanley 4y ago
The video is very unconvincing, unless I misunderstand what they're trying to achieve. Even with all the friction and air resistance in the environment, they didn't manage to rotate even a quarter of a turn. Even when it looked like it was working, the direction would sometimes invert for no apparent reason. You can do better than that just by shuffling around on an office chair.
fanick 4y ago
I suspect friction in the bearings is the culprit and allows for this "defiance of laws of physics".
- thethirdone 4y ago
  It does seem that friction in the bearing and tension of the wires is not sufficiently controlled for the physical demonstration to be convincing. However, the mathematics of it seems to have already been confirmed by a previous paper and the actual idea of changing orientation without exchanging momentum with anything is not novel. You can turn a satellite by spinning up a flywheel and then reabsorbing the angular momentum when you reach the desired orientation.
  - 6510 4y ago
    > You can turn a satellite by spinning up a flywheel and then reabsorbing the angular momentum when you reach the desired orientation. Wouldn't 2 of those allow for 2 different centers of rotation. If so, we can just walk though space. :-)
    - colanderman 4y ago
      No, the center of rotation is always that of the full system. It's not relative to which flywheel happens to be spinning.
- hither_shores 4y ago
  No, it really is possible to violate momentum conservation in suitably curved spacetimes. But this is a straightforward consequence of the laws of physics, not a "defiance" of them: it falls right out of general relativity. We still have a conservation law for the stress energy tensor, given by ∇_{μ}T^{μν}=0, it just doesn't give rise to separate global conservation laws on each component. The same applies to the system they actually built, except that the nonconserved "momentum" is not the same as the usual notion. It's the conjugate momentum(https://en.wikipedia.org/wiki/Canonical_coordinates https://en.wikipedia.org/wiki/Canonical_coordinates) for the coordinates on the surface.

41 points by thedday 4y ago

Hmmmm ... time to look at the 'anomalies' again? (And then there's Oumuamua ...) [https://en.wikipedia.org/wiki/Pioneer_anomaly https://en.wikipedia.org/wiki/Pioneer_anomaly] [https://en.wikipedia.org/wiki/Flyby_anomaly https://en.wikipedia.org/wiki/Flyby_anomaly]

keyle 4y ago

Do article like these get peered review before publishing or is it a publish first, deny later type of approach?

hither_shores 4y ago
Press releases don't get peer reviewed: they're not by or for researchers. Hence the sensationalist title. The actual paper (https://www.pnas.org/doi/10.1073/pnas.2200924119 https://www.pnas.org/doi/10.1073/pnas.2200924119) was peer reviewed, and is perfectly reasonable.
- dataflow 4y ago
  > Hence the sensationalist title. It's not just the title. The body itself seems to claim researchers have "proven" momentum isn't conserved: "Until recently, physicists believed this to be a constant, following the law of conservation momentum. Now, researchers from the Georgia Institute of Technology have proven the opposite—when bodies exist in curved spaces, it turns out that they can in fact move without pushing against something."
  - hither_shores 4y ago
    Sure, sensationalist copy as well. The point is that press releases are written by people who neither know nor care about the subject, and can't be trusted to accurately reflect the state of the field or the content of the papers they refer to.
    - defrost 4y ago
      Absolutely, read the paper and not the press release. By way of example, this linked press release contains the paragraph: > While the effects are small, [ ... ], just as the slight frequency shift induced by gravity became crucial to allow GPS systems to accurately convey their positions to orbital satellites. which is a /headdesk given the reality that ground based GPS receivers compute their positions using signals from several 'visible' transmitting GPS satellites; they do not "convey their positions to orbital satellites". This doesn't detract from the source paper but it does highlight the quality of the lay press release. (Note: It's essential to correct for gravity induced shifts in timing given these exist because of relativity .. but the shifts themselves are not "crucial" to the triangulation; in a hypothetical universe where relativity didn't exist | had a minor effect in this usage case, the triangulation of GPS against moving stations would still work with either no correction for relativity or a different correction)
      - dataflow 4y ago
        > crucial to allow GPS systems to accurately convey their positions to orbital satellites. I actually think they may have been referring to the ground stations here, not the GPS devices you carry as a user, since they said "GPS systems" and not "GPS receivers". Presumably the satellites need to determine their positions based on the positions of the ground stations.
        defrost 4y ago
        > Presumably the satellites need to determine their positions based on the positions of the ground stations. The satellites broadcast a rough guide to their collective positions (an almanac) to recievers and a precise guide to their specific exact location (the ephemeris) to ground clients (phones, GPS receivers, etc). Master Ground control stations update the satellites with that information every four hours or so .. strictly speaking the satellites don't do any computation of their own position - they broadcast their weekly lapsed epoch signal and echo data frames from their last Master Ground Station update. Orbital changes can result from solar wind pressure, magnetic forces, passing objects with mass, etc. It's the GPS receivers that do the 'heavy' positional computations for individual fixes .. and the Master Stations that do the somewhat harder work of constellation updates. ( There's a wealth of accuracy processing that can be run in parallel or post - differential base station GPS, forward error correction, post aquisition corrections, et al ) [1] https://gssc.esa.int/navipedia/index.php/GPS_Ground_Segment https://gssc.esa.int/navipedia/index.php/GPS_Ground_Segment [2] https://en.wikipedia.org/wiki/GPS_signals https://en.wikipedia.org/wiki/GPS_signals
  - kergonath 4y ago
    This completely discredits the work being presented. I wish universities and institutions (and popular science media) stopped doing that. It is really annoying to have headlines about $whatever “breaking the laws of Physics” every other day when reading the abstract is enough to see that no, this is not breaking anything. It is also harmful in the long term because it reinforces pseudo-scientific tendencies in the general public. As an example, I once had a discussion with someone arguing that energy use was not a problem because we’ll soon have free energy because he read a badly written vulgarisation piece discussing arguments for local non-conservation of energy in some quantum interactions. This gives people in general a very distorted view of what is and is not feasible. Which would not be too much of a problem if people did not then vote and behave based on this inaccurate perception.

chaps 4y ago

Between the gulf of how the article's written (PR fluff?) and the paper's/video's kind of bland approach, I'm having a hard time understanding what's going on here or what the significance of this is.. or even if there is one. Are they suggesting a novel way to move forward in space without exerting anything in the opposite direction?

hither_shores 4y ago
That "swimming" in curved spacetime is possible is well-known. The paper describes the implementation of a system for doing so.
- pmayrgundter 4y ago
  Can you elaborate or cite something about this? What does swimming mean?
  - codethief 4y ago
    "Swimming in spacetime" is a reference to the following paper by Jack Wisdom: http://groups.csail.mit.edu/mac/users/wisdom/swimming.pdf http://groups.csail.mit.edu/mac/users/wisdom/swimming.pdf The paper is quite accessible but here is a short summary: https://www.science.org/content/article/swimming-through-spacetime https://www.science.org/content/article/swimming-through-spa...

aabajian 4y ago

Edit: Here's the pre-print: https://arxiv.org/pdf/2112.09740.pdf https://arxiv.org/pdf/2112.09740.pdf I'm not sure others have read deeply into the article. While there's always some subtle ways friction can sneak its way, the authors account for the obvious. The article has a lot of definitions, which makes the crux difficult to pinpoint. They use the word "shape" many times to describe the configuration of the four motors. I think this adds confusion. Let me try to summarize as I understand it without using the word "shape." The authors are measuring the net position of the arm holding the two motor tracks. The arm can only rotate clockwise or counter-clockwise in the horizontal plane, so it suffices to measure the arm position using an angle - phi, with initial position being phi = 0. The two motor tracks are oriented perpendicular to one another. A key point is that the motors do not travel up, down, left, or right. That's Cartesian thinking. Rather, they travel clockwise or counter-clockwise along their respective tracks. One track is vertical, the other is horizontal. With this picture, the authors claim that the net horizontal position angle (phi) of the arm changes depending on how the motors move along their tracks. None of this is particularly impressive (e.g. the motion of the motors may cause the arm to "wobble" one way due to issues with synchronization, weight differences, and numerous other causes). The interesting part is this: If the vertically-oriented motors are made to move up-down on linear straight paths instead of curved paths, the net movement of the arm is not observed. Think about that for a moment...why should the path that the vertically-oriented motors take affect the horizontal rotation of the arm? In Newtonian physics, acceleration/force in one direction cannot affect acceleration/force in a perpendicular direction. At any given moment, the vertically-oriented motors are perpendicular to the arm's rotation direction, so their motion should not affect its horizontal rotation in space. In particular, both the curved-vertical and linear-vertical paths are perpendicular to the horizontal at all times. Newton would predict that changing the path of the vertical motors from linear to curved would not affect the horizontal rotation...but it does.

waterproof 4y ago
Thanks for the interpretation, very compelling if that's what they're showing. Sci-hub doesn't seem to have this paper so I haven't been able to read the whole thing. But from what I can see the data doesn't seem to agree with what you're saying - > The interesting part is this: If the vertically-oriented motors are made to move up-down on linear straight paths instead of curved paths, the net movement of the arm is not observed In the video, it looks like the same kind of net movement is observed with the cylindrical configuration as with the spherical configuration. With the cylindrical configuration the average position migrates by about 0.2rad and stays there, compared with 0.3rad for the spherical configuration. So the spherical configuration seems "better"(?) but it's hardly a convincing difference given how much slop there seems to be - there's even significant net rotation in the "plateau regime" where it's apparently supposed to stop migrating. And why is there a "plateau regime" anyway? If they can pull off a delta-V with no momentum transfer, I'd expect the robot to keep migrating around in a circle. But it stops. The abstract says: > While this simple geometric effect predominates over short time, eventually the dissipative (frictional) and conservative forces, ubiquitous in real systems, couple to it to generate an emergent dynamics in which the swimming motion produces a force that is counter-balanced against residual gravitational forces. I think this translates to "it just stops working after a bit". What's your take?

deleted 4y ago

[deleted]

rcthompson 4y ago

The title seems like a weird way to describe the phenomenon, given that the robot exists within our real space, which is essentially flat at the scales and velocities involved (specifically, non-relativistic ones). Is constraining motion to take place within a spherical slice of Euclidean 3-D space really equivalent to motion in an actually spherical space? Unless I'm missing something, it's fairly easy to understand why it works. The horizontal weights use Newton's 3rd law to rotate the bar in the opposite direction from their own motion. The vertical weights are used to alter the robot's moment of inertia relative to the constrained axis of rotation. (You could replace these weights with a single weight that slides along the radial bar toward and away from the center axis.) The moment of inertia determines how much the robot as a whole moves then the horizontal weights move, so if you move the weights one way, then change the moment of inertia and move the weights back the other way, you get a net change in position. This is why it doesn't work with a cylinder: because when the vertical weights are constrained to a cylindrical space, moving them doesn't change the robot's moment of inertia. I guess the point is that in a true spherical (or otherwise non-flat) space, any robot with the appropriate moving parts can do this "swimming" just by moving its parts in the right way in space without pushing on anything, essentially "pushing" against the curvature of the space itself. Now that I think about it, there's a conceptually simpler way to demonstrate the same thing. Imagine a "robot" made of two parts: a "gun" and a "bullet". The gun starts on the "equator" of a spherical space and fires the bullet due west. By Newton's 3rd law, the gun begins moving east. After some time, the bullet flies all the way around the equator and impacts the gun from the east. The gun catches it and its velocity once again becomes zero, but in the time between firing the bullet and catching it, the gun has progressed some distance to the east, and now the combined gun and bullet system that we started with has changed its position. The gun can reload and fire the bullet west again, repeating the process as many times as necessary to continue progressing around the equator.

oreally 4y ago
It's clickbait I guess. IIRC einstein had to dip into math for imaginary numbers and non-euclidean spaces for his theory on general relativity, but imaginary numbers aren't the most intuitive in the physical world and one at first conception one wouldn't think it should be in there. Yet imaginary numbers do help in many computations - for example, 3d rotations use quaternions that use imaginary numbers.

tbrownaw 4y ago

You can do something similar yourself with an office chair and a pair of weights. 1. Hold both weights out in front of you 2. Swing one to the side. 3. Swing the other one straight up over your head (or just pull it in next to your body) 4. Swing the first one (off to the side) back in front of you. 5. Swing the second one back down (or push it away from yourself) 6. You are now back at (1), but the chair you're in has turned slightly.

aabajian 4y ago
I get what you're saying, but this is why people are brushing off the article because they think it's either obvious, or due to friction. It isn't either. The phenomenon you're describing is a gyroscopic effect. The entire system (you, the chair and the weights) rotates. That is not what the authors have setup. The author's system includes the weights and their tracks, but not the arm connecting them. The arm is the "space" (or surface) in which the weights and tracts reside. In your example, planet Earth would be the "space" (or surface). In order for you to demonstrate their phenomenon, your chair would have to change position (translate) from where it started, not just rotate in-place. The source of the confusion is that they are measuring translation with an angle. They can do this because their universe is very small. You could also measure your translation relative to Earth with an angle, but it would be a tiny angle as the Earth has a very large radius.
- colanderman 4y ago
  > The author's system includes the weights and their tracks, but not the arm connecting them. But the arm is a part of the system. Are they not accounting for the centripetal force through the arm? (EDIT: Answering my own question, the preprint does include the text: > Notably, the robot’s ability to move does not violate the usual rule against movement without forces in three-dimensional space because the full three- dimensional dynamics in fact include normal forces on the robot supplied by the boom arm that confines it to the sphere. So the authors are (of course!) aware of this in 3D space. Their experimental setup is functionally no different from the swivel chair example.) > In order for you to demonstrate their phenomenon, your chair would have to change position (translate) from where it started, not just rotate in-place. No, by the same logic above, why can I not simply keep my upper arms stiff and declare them not part of the system? My lower arms and the weights are then "translating" through "space".

colanderman 4y ago

No physics are being violated here -- the authors themselves point out: > Notably, the robot’s ability to move does not violate the usual rule against movement without forces in three-dimensional space because the full three- dimensional dynamics in fact include normal forces on the robot supplied by the boom arm that confines it to the sphere. (And I'm sure they would agree that you can even eliminate forces from the table itself from the equation, if you attach an identical robot on an opposite arm to supply that normal force from within the system.) What I don't understand is -- given that the Euclidean physics are known, why was an experimental setup involved in this paper, as opposed to simply mathematical proof? Or is the experimental demonstration the purpose in and of itself?

pontifier 4y ago

This seems to be using a kind of "reactionless drive" I.E. Movement without apparent exterior force. This kind of drive can't produce a net velocity, but can create repeated small translations. To imagine how one would work in a space ship, imagine a very long ship that acts like a particle accelerator. Take an atom from the front end, accelerate backward to near the speed of light so that it's mass increases say 1000 times. While that atom is flying backward, your ship that pushed on it moves in the opposite direction. Catch it again in the rear, and the ship must return to zero speed because of conservation of momentum, however the ship is now in a slightly different spot. Move that atom back to the front of the ship, but slowly so it's mass stays low. Your ship moves backward, but not quite as far as it moved before. Do this again and again to translate the ship. The net velocity of the ship will always be zero though.

ianai 4y ago
I wonder if there’s a way to do that on an integrated circuit? I’m guessing a little channel carefully designed to create a magnetic field pointing a consistent direction. Electron gun on one end and positive terminal the other. This also starts to resemble how a laser works internally (at least HeNe). Granted, that’s light and massless.
- prox 4y ago
  Wasn’t there a magnetic/levitating robot that did this?
b112 4y ago
Catching the atom, and moving the atom, should result in the ship being returned to its original place. "Being sneaky" by moving things slowly, or catching things, just doesn't work. It will all add up. All movement will be undone.
- Tuna-Fish 4y ago
  You ignored the part where the atom is fired at such high speeds that it sees relativistic increase in mass. I dunno if that works, but that's the part that matters in the explanation above, and that's the part you'll have to address.

jstanley 4y ago

The video is very unconvincing, unless I misunderstand what they're trying to achieve. Even with all the friction and air resistance in the environment, they didn't manage to rotate even a quarter of a turn. Even when it looked like it was working, the direction would sometimes invert for no apparent reason. You can do better than that just by shuffling around on an office chair.

fanick 4y ago

I suspect friction in the bearings is the culprit and allows for this "defiance of laws of physics".

thethirdone 4y ago
It does seem that friction in the bearing and tension of the wires is not sufficiently controlled for the physical demonstration to be convincing. However, the mathematics of it seems to have already been confirmed by a previous paper and the actual idea of changing orientation without exchanging momentum with anything is not novel. You can turn a satellite by spinning up a flywheel and then reabsorbing the angular momentum when you reach the desired orientation.
- 6510 4y ago
  > You can turn a satellite by spinning up a flywheel and then reabsorbing the angular momentum when you reach the desired orientation. Wouldn't 2 of those allow for 2 different centers of rotation. If so, we can just walk though space. :-)
  - colanderman 4y ago
    No, the center of rotation is always that of the full system. It's not relative to which flywheel happens to be spinning.
hither_shores 4y ago
No, it really is possible to violate momentum conservation in suitably curved spacetimes. But this is a straightforward consequence of the laws of physics, not a "defiance" of them: it falls right out of general relativity. We still have a conservation law for the stress energy tensor, given by ∇_{μ}T^{μν}=0, it just doesn't give rise to separate global conservation laws on each component. The same applies to the system they actually built, except that the nonconserved "momentum" is not the same as the usual notion. It's the conjugate momentum(https://en.wikipedia.org/wiki/Canonical_coordinates https://en.wikipedia.org/wiki/Canonical_coordinates) for the coordinates on the surface.