9 ms·
Simple Rules ‘Bootstrap’ the Laws of Physics
- tus88 7y ago> When bootstrapping, physicists determine how elementary particles with different amounts of “spin,” or intrinsic angular momentum, can consistently behave. In doing this, they rediscover the four fundamental forces that shape the universe. They have derived gravity through quantum mechanics? What rock have I been living under?
- namirez 7y ago> They have derived gravity through quantum mechanics? Sort of! The problem is that the solutions of the existing quantum field theories are not renormalizable so infinities arise in the solutions. The whole point of quantum gravity is to find realizable solutions.
- qubex 7y agoAsymptotically safe quantum gravity may not be a dead end, incidentally.
- mcaruso 7y agoNot a physicist by any means, but from the article it talks about gravity in the context of the graviton (as one plausible solution to the problem mentioned). Which is still hypothetical, so it's not like they've proven gravity from first principle or anything.
- nsomaru 7y agoAs a layman reading this, it’s funny. Because I do not need gravity proven to me from first principles ;)
- maxnoe 7y agoProven is the wrong word. You cannot prove a theory. Deriving a description of gravity from first principles would be more appropriate
- hprotagonist 7y ago*an empirical theory. I can certainly prove things about, say, addition.
- solarist 7y agoif you mean you can prove 2+2==4, it's actually tautology: | | + | | == | | | |
- hprotagonist 7y agono like, "addition is closed over the integers" is something you can show to be True (for all members of an infinite set, no less -- so it's not empirical.)
- deleted 7y ago[deleted]
- bsder 7y agoAnd yet, funnily enough, your phone needs a more thorough understanding of gravity than you do. The GPS system must correct for relativistic effects that cause the atomic clocks to shift relative to ground.
- orbifold 7y agoThere is a nice derivation of Newton’s law from quantum field theory in Zee‘s book „Quantum Field theory in a nutshell“. You can also arrive at Einstein’s field equations by deriving the beta function of bosonic string theory and (2-2 scattering to be precise) and requiring that it is constant.
- Iwan-Zotow 7y agoFeynman lectures on Gravitation, 1962
- tus88 7y agoThanks.
- Iwan-Zotow 7y agoBasically, Feynman starts with microscopic approach. We want to describe interaction where each piece of mass/energy is attracted to each other. So lets introduce a boson (whole number spin - 0, 1, 2, ...) to carry interaction. Then he talks about spin properties, rejects spin-1 boson (aka photon), somehow rejects spin-0 boson (I don't remember arguments) and start building microscopic theory of interacting via spin-2 massless bosons. Because they carry energy there is a self-interaction. He builds interaction rules, Feynman diagrams, and then goes to classical limit, where he derives macroscopic classic GR from microscopic quantum interactions. That's what I remember from reading this book. Apparently he did it on the fly, lots of errors in derivations which were noted and corrected in footnotes. Nevertheless, very entertaining approach...
- hn_throwaway_99 7y agoWondering 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.
- deleted 7y ago[deleted]
- ISL 7y agoObjects 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 https://arxiv.org/abs/0808.2673
- chongli 7y agoIt'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.
- ISL 7y agoYou need not worry about infinity in the black-hole case. Known physics ends at the black hole's event horizon, for an external observer. Black holes of a given mass can only have so much angular momentum. As a rule of thumb, that angular momentum corresponds to that at which the equator of the black hole is moving at the speed of light. This has important consequences for the mergers of two high-spin black holes that have aligned spins. The merger is actually delayed for a little while, just before merger, as the binary dumps angular momentum through gravitational radiation.
- LoSboccacc 7y agothis is the opposite of how science works. until grand unification is proven, the four forces simmetry is a nice and convenient hypothesis
- zamalek 7y ago> 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.
- LoSboccacc 7y agoyeah there's the x17 thing too, but it's still quite to early for that.
- jetrink 7y agoThe 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 ask why the deeper laws exist.
- deleted 7y ago[deleted]
- The_rationalist 7y agoAre there some sound theories that explain this why?
- vanderZwan 7y agoNot quite what you're arguing, but related: you might be interested in the strong anthropic principle. PBS Spacetime did a fairly accessible video about the reasoning behind it recently: https://www.youtube.com/watch?v=q-6oU3jXAho https://www.youtube.com/watch?v=q-6oU3jXAho
- ilitirit 7y agoAs mentioned elsewhere in this thread it sounds as if what you're asking is related to the anthropic principle. Maybe it would be more appropriate to argue that "the set of physical laws that arise from the known fundamental constants are inevitable". But this itself is not strictly logical because changing the fundamental constants doesn't imply a change of the fundamental laws. The fundamental laws may very well work fine with different constants but we may just not be there to observe them in action because the outcomes may not result in a universe conducive to life (hence: anthropic principle). As a bad analogy, physics affects an egg falling to the surface of a planet in low gravity in the same way as an egg falling in high gravity. The outcomes may be different, but the laws are the same. And of course, in the latter case it's less likely for something to survive the fall and live to even ask these questions in the first place. There are some interesting possibilities that arise at this point. e.g. What if this Universe is only "somewhat" tuned for intelligent life? What if, on some more "appropriate" level of "tuning", more stable, fundamental particles are observable for longer periods of time yielding even more insight into the fundamental laws of the universe? Lots of interesting stuff to talk about around a camp fire...
- gus_massa 7y agoIf 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.
- knzhou 7y agoThis 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.
- Vysero 7y ago"That solution is the graviton: a spin-2 particle that couples to itself and all other particles with equal strength." So, has this been discovered recently? I was under the impression that the graviton was theoretical?
- qubex 7y agoWhat that means is that gravity, at the quantum level, must be conveyed by a spin-2 particle, known as a ‘graviton’. It has not been detected, and probably never will be (due to technical limitations). We can broadly pin down its properties, but we do not yet have a full theory of how it interacts.
- huu 7y agoThe existence of a solution to abstract equations that involve gravitons does not imply the existence of gravitons. Designing an experiment to measure and confirm the existence of the particle is a different endeavor.
- Iwan-Zotow 7y agoWhat was discovered is the existence of gravitation waves, long wavelength limit to the weak quantum gravity (aka propagating and interacting gravitons)
- jejones3141 7y ago"...bootstrappers find that many different spin-0 particles are possible." Is that a typo, or is it really possible to have spin -0?
- danbruc 7y agoNo typo, spin 0 is a thing.
- ketralnis 7y agoIt's a hyphen, not a minus sign
- Accujack 7y agoMaybe for small values of 0? ;)
- yummypaint 7y agoAn example is a helium nucleus in the ground state
- yters 7y agoI think with some equations there is a discontinuity at zero, like tan, but it still makes sense to talk about what happens asymptotically approaching zero and then it matters from which side you approach zero, hence positive and negative zero.
- mjcohen 7y agotan does not have a discontinuity at zero. |x|/x does, as does 1/x. tan's is at pi/2 in particular and (n+1/2)pi in general.
- duelingjello 7y agoAlthough tan has a set of discontinuities when it jumps between +∞ and -∞ through complex infinity (∞~). tan(π(x + 0.5)) = ∞~, x ∃ ℤ (π on my screen doesn't look much like greek pi.)
- Iwan-Zotow 7y ago"As the Nobel Prize winner Steven Weinberg showed in 1964, the existence of a spin-2 particle leads inevitably to general relativity" Huh?!? spin-2 particle leading to GR was the prime essence of Feynman lectures on Gravitation, 1962
- bonzini 7y agoThere is a link pointing to https://journals.aps.org/pr/abstract/10.1103/PhysRev.135.B1049 https://journals.aps.org/pr/abstract/10.1103/PhysRev.135.B10.... Some quick Googling shows quantum gravity being referred to as Feynman-Weinberg gravity, so both must have contributed parts of the theory.
- anigbrowl 7y agoN-gate theory offers the best hope of cutting through conflicting theories to get at the fundamentals.
- IshKebab 7y ago> And the hour hand on a clock must spin around twice before it tells the same time again. Err, not on any clock I've ever seen! But points for effort.
- ahuth 7y agoThis example definitely assumes familiarity with a 12-hour clock, which indeed spins around twice before getting back to the same time.
- minitech 7y agoBut only once before getting back to the same state, so it’s a bit confusing.
- BlueTemplar 7y agoI guess that the usual "model" was a bit too weird for a layman? https://en.m.wikipedia.org/wiki/Spin-%C2%BD https://en.m.wikipedia.org/wiki/Spin-%C2%BD
- scotty79 7y agoThis brings to mind how Noether showed that conservation laws of physics must exist if universe has some specific symmetries. Like translation symmetry, the simple fact that the laws of universe don't change as you move a bit to the side mathematically implies conservation of momentum.
- beaconstudios 7y agoSounds to me like the higher order laws of physics are emergent properties of the lower level properties. Is this surprising? Every system in existence is the result of the emergent organisation of lower systems - why would the laws of physics be different?
- mannykannot 7y agoIt is one thing to say that something is plausible, and something quite a bit more to be able to show that not only is it plausible, but also that it is the way things actually are, and in a very specific way.
- beaconstudios 7y agoyes, but the reporting is as if the fact that emergence exists is itself surprising. I'm sure the actual science is a lot more sober and focused on the genuine revelations, around how such emergence plays out.
- tabtab 7y agoI thought this was going to be about Twitter Bootstrap's UI library, and was thinking, "Oh good, I can read about simple consistent rules so that it's not so organic and unpredictable anymore!" It was not to be. Back to the UI trial-and-error slog...
- deleted 7y ago[deleted]
- eprparadox 7y agoMax Tegmark was on the Sean Carroll podcast recently, talking about the different types of multiverse we could postulate may exist[0]. I found the quanta article fascinating, and I don't believe it's strictly speaking 'circular'. It can also be the case that multiple solutions exist mapping the relationships between a given set of fundamental laws. Prof. Tegmark discusses this in the '4th level' of multiverse which examines whether it's right to talk about a multiverse of all possible mathematically consistent universes. [0]: https://www.preposterousuniverse.com/podcast/2019/12/02/75-max-tegmark-on-reality-simulation-and-the-multiverse/ https://www.preposterousuniverse.com/podcast/2019/12/02/75-m...