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Comment by colordrops | Hacker News Reader

Spinning motion is an illusion and doesn't exist as a fundamental form of movement, similar to how lack of gravity in orbit is an illusion, in that you are continuously falling downward but missing the earth.

Spinning objects are made of particles that are all actually going in straight lines but forces against each other are causing them to change direction, cummulatively appearing to be spinning, like an extremely coherent eddie current or vortex. Their movement is in relation to each other, and there is no need for an objective space in which to spin.

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> Spinning motion is an illusion and doesn't exist as a fundamental form of movement

If there is no objective space, then "movement" in general is not a fundamental concept; but you are assuming that at least some kinds of movement are fundamental (tarikjn points out something similar to this).

> similar to how lack of gravity in orbit is an illusion, in that you are continuously falling downward but missing the earth.

Actually, you have this backwards; it's the "falling downward" because of "gravity" that is the "illusion", according to our best current theory, general relativity. In a free-fall orbit, you are moving in a straight line through spacetime; you can tell this by the fact that you are weightless.

Regarding gravity, that was probably a bad analogy. Regarding movement, movement relative to other objects is fundamental. You are begging the question by assuming objective space is required for movement.

Mind you, I'm not arguing that space doesn't exist, only that Newton's argument is flawed.

> movement relative to other objects is fundamental

I agree that "objective space" in the sense of "a space that is the same for everyone", is not required to give an account of movement of objects relative to other objects. However, the only reason we know that is that we have an alternative: a theory of objective spacetime that gives an account of movement of objects relative to other objects.

In other words, according to our best current classical theory, general relativity, Newton's argument was flawed not because nothing objective is needed to account for the bucket experiment, but because he got wrong what thing was objective: he thought it was space, but it's actually spacetime. More precisely, it's the geometry of spacetime.

On the GR viewpoint, we can indeed view the rotation of Earth, for example, as relative in this sense: we can adopt a "cosmic" frame in which the Earth is spinning and the rest of the universe is at rest; or we can adopt a "geocentric" frame in which the Earth is at rest and the rest of the universe is spinning. Both frames are valid; neither one is "preferred" by the laws of physics. But both frames are describing the same underlying objective thing: the same geometry of spacetime. They are just describing it from different viewpoints.

Physicists are rather aware of these things, seeing as you they are the ones who taught them to you in the first place. This is an old, venerable question that, as the article says, dates back to before Einsteinian relativity and remain unresolved to this day. Physics 101 is not capable of resolving the matter.

Appeal to authority is worse than actually trying to figure out a problem from first principles, especially in the field of physics.

"Appeal to authority" is more than "mentioning the existence of authorities". This is not an appeal to authority.

The point is, if the answer was as simple as that, it wouldn't be a problem. This is not news. This is a century+ old conundrum, and you can't solve it by simply reciting Physics 101 as if that's it, book closed. It's the equivalent of listening to a operating systems researcher describe a thorny problem in the creation of a multitasking operating system on a hundred-core NUMA system, and then pedantically, slowly, patronizingly explaining back to them how functions work by pushing things on to a stack and then popping them afterwards.

When they favor you afterwards with a deeply shocked expression, it's not in amazement at your penetrating insights.

Still sounds like "Appeal to authority" to me. To speak to your analogy, I am involved in the field of computers, and I've seen first hand "experts" in this field build big complicated systems, that maybe had some merit at some point in time, but as the context changed with new developments and technologies, a young upstart built something much more simple that made the expert's system look like a mess. This is relevant because the context of what we understand is very different from Newton's time. School children have a more accurate depiction of the laws that govern our reality than the tops minds at the time, who believed in such things as phlogiston and the earth-centric universe.

I am actually not sure that this is true. I used to think exactly like you, that rotations are just piles of small translations with time varying directions, but there is a problem with this, linear momentum and angular momentum are two different conserved quantities. If it were the case that rotations are just translations then you should be able to get rid of angular momentum and its conservation and explain it in terms of linear momentum and I don't think that you can do that. Linear momentum is tied to the homogeneity of space while angular momentum is tight to the isotropy of space. You could actually do the exact opposite, say that translations are just rotations about a point infinitely far away, but I think that won't work either. I really don't know for sure, I mean I am not really, really, really confident about it, but as far as I can tell rotations and translations are fundamentally different.

Unaccelerated translations make only sense as relations between two objects but you don't need any reference to detect acceleration, you can just release an object from your hand and see if it drops to the floor because of gravity, hits the wall behind you because your space ship is accelerating or flies away tangentially because you are on a carousel. Rotation requires continuous acceleration (I am avoiding the term constant acceleration because only the magnitude is constant but not the direction) of all the particles towards the center of rotation or otherwise they would fly away tangentially. But if rotation requires continuous acceleration and you can always detect acceleration without reference to some other outside objects then you can always detect rotations and that is not true for translations, you can not detect unaccelerated translations.

It is the difference between velocities and accelerations and the fact that rotations always require accelerations that makes rotations more than just a pile of translations. You won't notice any difference if you translate everything in space by the same amount, you won't see any change if you rotate everything in space by the same amount, there is also no difference if you change the velocity of everything by the same amount but you will immediately notice if you change the acceleration of everything.

Or maybe that is at least partially nonsense, unfortunately I haven't yet mastered that topic myself.

Angular momentum is an abstract concept, and an emergent property. It doesn't exist inherently in an object.

Centrifugal force is an abstraction, but angular momentum has physical reality at the subatomic level. See Feynman, chap. 17.[1] Photons have angular momentum, even though they don't have a meaningful size. This is strange, but it's reality.

[1] http://www.feynmanlectures.caltech.edu/III_17.html

Angular momentum could be encoded in the wave function.

http://arxiv.org/abs/0807.3930

That is a trivial fact, the wave function of a system completely describes the state of the system and that of course includes the angular momentum. But the wave function has nothing to do with a physical wave with physical properties, it is a pretty abstract construction and the physical properties are essentially encoded in the base vectors of the Hilbert space. I only skimmed the paper but I didn't come across something that plausibly argued that the wave function is the fundamental physical entity.

Not meaning to argue that the wave function is a physical entity but rather that particles aren't required to "exist" as a physical entity to explain experimental data.

I would say nothing is required to exists. Just take the idea that our universe may be a simulation, then nothing in the universe is real. It could then be that the real universe out there is similar to ours or we could be the Game of Life of the real universe which could be totally incomprehensible to us with concepts that aren't even imaginable in our little world. Or take Last Thursdayism, the idea that the universe somehow came into existence last Thursday with fake memories of the past in our all heads and so on. Or Solipsism. There is no way you could ever find out that something like that was true or not.

We just have to make some assumptions about the nature of the universe and accept them as axioms, otherwise you can not even start to draw conclusions. Some assumptions may look more reasonable then others but I guess that is more or less an illusion, last Thursdayism looks only unreasonable because you have already rejected the idea and accepted some other assumptions.

So what is real? Particles? Fields? Wave functions? We pretty surly don't know. Some will say the fields are the real things because they give us virtual particles and that matches experiments, others will say that fields are not real because they have gauge symmetries - gauge redundancies - and how can something that is underdefined be real? It seems certainly possible that we will be able to rule one or another view out in the future because they can not accommodate some new discovery but we are probably not yet there and there is definitely no consensus.

Not to forget that there are quite a couple of new directions in the last years and decades, like space and time emerging from entanglement, the holographic principle, gravity as an entropic force and what not. There is probably a good chance that some or all our fundamental concepts of physics as of today will not survive the next century, millennium or million years. I mean they will still exist as useful approximations but no longer been seen as fundamental.

I'm not in disagreement. I certainly could have been more careful with my original wording.

I had a gut reaction to the original commenter's statement that angular momentum was a "physical reality at the subatomic level" and should have been more direct in my response.

I mean angular momentum of macroscopic objects, which the spin of particles has little to do with.

No, they're the same thing. Feynman: "Now we would like to discuss the idea of angular momentum in quantum mechanics—or rather, the characteristics of what, in quantum mechanics, is called angular momentum. You see, when you go to new kinds of laws, you can’t just assume that each word is going to mean exactly the same thing. You may think, say, “Oh, I know what angular momentum is. It’s that thing that is changed by a torque.” But what’s a torque? In quantum mechanics we have to have new definitions of old quantities. It would, therefore, be legally best to call it by some other name such as “quantangular momentum,” or something like that, because it is the angular momentum as defined in quantum mechanics. But if we can find a quantity in quantum mechanics which is identical to our old idea of angular momentum when the system becomes large enough, there is no use in inventing an extra word. We might as well just call it angular momentum. With that understanding, this odd thing that we are about to describe is angular momentum. It is the thing which in a large system we recognize as angular momentum in classical mechanics."[1]

Yes, it's weird. Yes, it's not intuitive. But it's very real. Many experiments, such as the classic two-slit experiment, have confirmed the stranger predictions of quantum mechanics.

[1] http://www.feynmanlectures.caltech.edu/II_34.html

I think something is being lost in translation here. An object made of multiple particles could potentially have one at dead center that is spinning perfectly, and this would be of the same sort of angular momentum as that at the quantum level. But a particle sitting out on the edge of an object is subject to the forces of those surrounding it, keeping it moving in a circle instead of a straight line. Instead of microscopic particles, imagine a bunch of marbles connected by loose springs in a 3D mesh. If you spin this around, are you suggesting that something is happening besides the simple fact that springs are tugging and pushing at marbles to constantly affect their linear motion to appear circular? It's like suggesting somehow that a line on a screen is not made of pixels.

Spin angular momentum seems to be a pretty intrinsic property of (fundamental) particles. What do you understand as inherent properties of objects? Position? Charge? Mass? Velocity? Energy? Linear momentum? Spin? And why? What if the object is composite? What about quantum physics, e.g. superposition states?

The angular momentum of a spinning bucket of water has approximately nothing to do with the intrinsic angular momentum of the particles, though. Statistically, we'd expect all of the intrinsic spins of the electrons and quarks to cancel out entirely.

Rather, the angular momentum is due to the fact that the straight-line motion of all the atoms on the left side of the bucket is opposite in direction to the straight-line motions of all the atoms on the right side of the bucket. (With respect to the reference frame of the center of gravity of the bucket, etc, etc.)

They are nonetheless related, if you would align the spins in the water and the (metal) bucket you would make the bucket and/or water spin to conserve the angular momentum. [1] They are surly different but it is not some accident of history that both are called angular momentum, they are really both angular momentum.

[1] https://en.wikipedia.org/wiki/Einstein–de_Haas_effect

What is the point you're arguing against? I know perfectly well that conservation of angular momentum is a thing. I know perfectly well that the intrinsic property that we call 'spin' of an electron is really angular momentum. I know that a spinning bucket also has real angular momentum.

So, I'm not sure what you're trying to tell me.

The central point made by colordrops is that angular momentum in a macroscopic object is 100% (accurate to at least ten decimal places) due to synchronicity of linear momenta.

What colordrops wrote at least suggest that he thinks that circular motion and angular momentum are not fundamental but can be expressed or understood in terms of linear motion and linear momentum. As far as I can tell today this is not true, they are independent concepts and one can not be fully understood in terms of the other, neither by looking at circular motion as piecewise linear motion nor by looking at linear motion as circular motion about a point infinitely far away. I only brought up spin because it makes the point pretty clear - or maybe not - that angular momentum is a fundamental concept that can not be recast in other terms, especially it is not just the sum of many linear momenta.

Look at the 2-body gravitational system -- let's say Earth-Moon. Let's put our non-rotating reference frame at the barycenter. At any given moment, Earth has a definite position and velocity, and the Moon has a definite position and linear velocity. If we know these positions and velocities (and masses), we can derive anything else we'd like to know about the system. Including its angular momentum.

This would also be true of any system such that the size is large enough to render the quantum spins statistically close to zero.

Sure but you make an arbitrary choice here, you choose to use Cartesian coordinates. Lagrangian and Hamiltonian mechanics make it more clear that this is more or less an arbitrary choice and that there are other sets of generalized coordinates to describe the problem, something that is in some sense less obvious in Newtonian mechanics because things usually become pretty hard to deal with.

Yes, every choice of coordinates is capable of describing the physics of a system. Yes, you could certainly argue that Cartesian coordinates are a natural choice or in some sense special, derivatives are especially simple and whatnot. But I still think that something is lost when one disregards rotations and angular momentum, they seem to capture an important aspect of the structure of space, its isotropy.

Then again every equivalent description should capture the same things, just maybe not in an obvious way. And then again the existence of spin angular momentum hints at the fact that there is really more to it. As I said, I am really not sure. I used to think of rotations as emergent from translations and I kind of changed my mind but I am probably still on the edge.

Suppose your object is the only thing in space, how do you tell that it is spinning? How do the particles in the object can tell that the object as a whole is spinning?

What you describe are particules going in straight lines, but over what space/grid are they going on a straight lines? If they are going over straight lines on something then your description is one of an objective space.

If pulling forces are still exerted on its edges in proportion to said "spinning", then there exist a static field in space for which the object is in static alignment when the pulling force are at minima.

Interestingly the same observation could be made of two objects in orbit or the electrons orbiting individual atoms in the object, with the static alignment being observed when the acceleration between the objects is at a maxima i.e. they are falling into one another.

They are going in straight lines in relation to the other particles in the object. Then the force of one particle exerts on another particle as they get closer or further from each other, causing them to change direction. This happens for innumerable numbers of particles, appearing like a singular object that is spinning, since the forces are too great to pull the object apart. If you spin it fast enough, the particles will overcome the atomic forces holding it together, and the object will break apart, with the individual pieces continuing to move in a straight line.

So you're saying there's a final fractional digit to PI? That's good news.

That's a non sequitur.

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