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Simple Rules ‘Bootstrap’ the Laws of Physics

quantamagazine.org

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Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#11

Post by ISL »

hn_throwaway_99 wrote: post #3

Wondering if anyone could recommend a layman's introduction to "spin". I've read the Wikipedia article, I broadly understand it, but just curious as to how it was discovered, why it is considered an "intrinsic" form of angular momentum, and why it is so fundamental a concept in our universe.

Objects with spin can be point-like but carry angular momentum. The electron is one such particle.

It is "intrinsic" because it has no known moving parts. If it had moving parts, and that motion had net angular momentum, we would refer to it in many contexts as "orbital angular momentum".

Spin should seem counterintuitive. There are no macroscopic objects that I am aware whose spin you can feel with your hands. Our research group has an object with 10^23 polarized spins (and negligible net magnetic moment) -- observing the angular momentum effects directly requires a sensitive modern torsion balance. https://arxiv.org/abs/0808.2673

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#12

Post by gus_massa »

If it is inevitable, why does the weak force only affect the left-handed fermions and not the right handed? Most of the article is just a discussion of the applications of the symmetry, not inevitability.

U(1) [aka electromagnetism] is probably inevitable, the rest is more controversial.

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#13

Post by zamalek »

LoSboccacc wrote: post #6

this is the opposite of how science works. until grand unification is proven, the four forces simmetry is a nice and convenient hypothesis

> this is the opposite of how science works

Yep, this is dogma. People seem to be hard-wired to believe in something, for some it's a god, for others it's a theory or hypothesis. As for the grand-unification surrounding the the four forces, aren't we currently questioning the existence of a fifth? That's a pretty short-lived god.

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#14

Post by knzhou »

This is a good article, but I think it's a little confusing -- it doesn't specify the scope of the bootstrap programme.

It's true that general considerations can give you very strong constraints on how particles of various spins can interact. For example, it's been known for decades that at low energies, massless spin 1 particles inevitably give you Yang-Mills and massless spin 2 particles inevitably give you general relativity. There is also a separate idea called the conformal bootstrap which seeks to use general principles to pin down everything about a theory, but my impression is that it only works well for simple conformal field theories in lower dimensions.

In particular, these ideas don't tell you anything about the detailed structure of the Standard Model: why it has the gauge groups it does, why there are 6 quarks, the values of the quark masses, how the spontaneous symmetry breaking works, the gauge couplings, and so on. That's the way with almost all theoretical tools: the flashier and more general in scope they are, the less details they actually pin down. So while formal theorists may be excited about this, experimentalists can't really use it for anything.

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#15

Post by castratikron »

hn_throwaway_99 wrote: post #3

Wondering if anyone could recommend a layman's introduction to "spin". I've read the Wikipedia article, I broadly understand it, but just curious as to how it was discovered, why it is considered an "intrinsic" form of angular momentum, and why it is so fundamental a concept in our universe.

It was called spin because for a time the model of an electron was a solid sphere of charge that was spinning on its axis. It was known that the electron made a magnetic field, and since charge in motion makes a magnetic field that could be explained by a spinning electron. But if you actually do the math you'll be off by a factor of about two. Quantum mechanics had to be invented to explain this discrepancy and the name stuck.

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#16

Post by LoSboccacc »

zamalek wrote: post #13
LoSboccacc wrote: post #6

this is the opposite of how science works. until grand unification is proven, the four forces simmetry is a nice and convenient hypothesis

> this is the opposite of how science works Yep, this is dogma. People seem to be hard-wired to believe in something, for some it's a god, for others it's a theory or hypothesis. As for the grand-unification surrounding the the four forces, aren't we currently questioning the existence of a fifth? That's a pretty short-lived god.

yeah there's the x17 thing too, but it's still quite to early for that.

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#17

Post by [deleted] »

jetrink wrote: post #8

The content of the article is very interesting and worth reading, but the framing feels circular. Some laws of nature are fundamental and other laws are inevitable consequences of those laws. If the inevitable laws were not inevitable, then they would be fundamental. As our knowledge and explanatory power grows, some laws that first appeared fundamental have been explained in terms of deeper laws, but we must still a…

[deleted]

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#18

Post by chongli »

ISL wrote: post #11
hn_throwaway_99 wrote: post #3

Wondering if anyone could recommend a layman's introduction to "spin". I've read the Wikipedia article, I broadly understand it, but just curious as to how it was discovered, why it is considered an "intrinsic" form of angular momentum, and why it is so fundamental a concept in our universe.

Objects with spin can be point-like but carry angular momentum. The electron is one such particle. It is "intrinsic" because it has no known moving parts. If it had moving parts, and that motion had net angular momentum, we would refer to it in many contexts as "orbital angular momentum". Spin should seem counterintuitive. There are no macroscopic objects that I am aware whose spin you can feel with your hands. Our r…

It's totally counterintuitive. Classical angular momentum requires a choice of origin. All angular momentum is then relative to that origin. Any point particle lying at the origin should not be able to have any angular momentum.

Imagining the classic "figure skater experiment", the figure skater speeds up as she draws her arms inward, due to conservation of angular momentum. If she could shrink down to a point, her angular speed would tend to infinity. This is what I imagine happening to a black hole.

Re: Simple Rules ‘Bootstrap’ the Laws of Physics

#19

Post by The_rationalist »

jetrink wrote: post #8

The content of the article is very interesting and worth reading, but the framing feels circular. Some laws of nature are fundamental and other laws are inevitable consequences of those laws. If the inevitable laws were not inevitable, then they would be fundamental. As our knowledge and explanatory power grows, some laws that first appeared fundamental have been explained in terms of deeper laws, but we must still a…

Are there some sound theories that explain this why?
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