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Why Understanding Space Is So Hard

nautil.us

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Re: Why Understanding Space Is So Hard

#81

Post by yaks_hairbrush »

danbruc wrote: post #79
yaks_hairbrush wrote: post #78

Earlier quoted context omitted.

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 obje…

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 linea…

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.

Re: Why Understanding Space Is So Hard

#82

Post by abecedarius »

ashark wrote: post #19
abecedarius wrote: post #8

Julian Barbour's The Discovery of Dynamics goes into great detail on absolute vs. relative pre-Einstein. His The End of Time reviews the ideas at a more popular level and continues the story into the 20th C.

Do you have the paperback? If so, is it well-constructed? I'm interested, but with so many pages I'm wary of the format. I'd usually buy something like this in hardcover but the paperback's not cheap and the hardcover's far more expensive, and seemingly out of print.

I'm afraid I don't know -- I read a pdf, and a library hardback for the latter book. That one shouldn't be hard to find from a U.S. library, and would tell you whether you want to delve into the former. (Which I found really interesting, though it's maybe too ready to valorize Ptolemy just because we know so little about his predecessors.)

Re: Why Understanding Space Is So Hard

#83

Post by danbruc »

yaks_hairbrush wrote: post #81
danbruc wrote: post #79

Earlier quoted context omitted.

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 linea…

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…

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.

Re: Why Understanding Space Is So Hard

#84

Post by ice303 »

danbruc wrote: post #71
CPLX wrote: post #43

Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.

52! is the number of different ways you can arrange a single deck of cards. Let's try to wrap our puny human brains around the magnitude of this number with a fun little theoretical exercise. Start a timer that will count down the number of seconds from 52! to 0. We're going to see how much fun we can have before the timer counts down all the way. Start by picking your favorite spot on the equator. You're going to wa…

Awesome explanation. Thank you for sharing

Re: Why Understanding Space Is So Hard

#85

Post by jerf »

colordrops wrote: post #47
jerf wrote: post #32

Earlier quoted context omitted.

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.

Re: Why Understanding Space Is So Hard

#86

Post by klank »

danbruc wrote: post #70
klank wrote: post #67

Earlier quoted context omitted.

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 univer…

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.

Re: Why Understanding Space Is So Hard

#87

Post by colordrops »

jerf wrote: post #85
colordrops wrote: post #47

Earlier quoted context omitted.

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…

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.

Re: Why Understanding Space Is So Hard

#88

Post by colordrops »

Animats wrote: post #69
colordrops wrote: post #66

Earlier quoted context omitted.

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 b…

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.
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