3 ms·

I think what they must be referring to is the fact that particles are only rigorously defined in the free theory. When coupling is introduced, how the free theo

I think what they must be referring to is the fact that particles are only rigorously defined in the free theory. When coupling is introduced, how the free theory relates to the coupled theory depends on heuristic/formal assumptions. We're leaving my area of understanding, but I believe Haag's theorem shows that the naïve approach, where the interacting and free theories share a Hilbert space, completely fails -- even stronger than that, _no_ Hilbert space could even support an interacting QFT (in the ways required by scattering theory). This is a pretty strong argument against the existence of particles except as asymptotic approximations. Since we don't have consensus on a well-defined, non-perturbative gauge theory, mathematically speaking it's difficult to make any firm statements about what states "exist" in absolute. (I'm certain that people working on the various flavours of non-perturbative (but still heuristic) QFT -- like lattice QFT -- would have more insights about the internal structure of non-asymptotic interactions.)

codethief 2y ago
> This is a pretty strong argument against the existence of particles except as asymptotic approximations. I think it's also a pretty strong argument against the mathematical well-definedness of typical (interacting) QFTs in the first place.
westurner 2y ago
Though it doesn't resolve whether a "quanta" is a particle or a measurable convergence of waves, Electrons and Photons are observed with high speed imaging. "Quantum microscopy study makes electrons visible in slow motion" https://news.ycombinator.com/item?id=40981054 https://news.ycombinator.com/item?id=40981054 There exist single photon emitters and single photon detectors. Qualify that there are single photons if there are single photon emitters: Single-photon source: https://en.wikipedia.org/wiki/Single-photon_source https://en.wikipedia.org/wiki/Single-photon_source QFT is not yet reconciled with (n-body) [quantum] gravity, which it has 100% error in oredicting. random chance. TOD IIRC, QFT cannot explain why superfluid helium walks up the sides of a container against gravity, given the mass of each particle/wave of the superfluid and of the beaker and the earth, sun, and moon; though we say that gravity at any given point is the net sum of directional vectors acting upon said given point, or actually gravitational waves with phase and amplitude. You said "gauge theory", "Topological gauge theory of vortices in type-III superconductors" https://news.ycombinator.com/item?id=41803662 https://news.ycombinator.com/item?id=41803662 From https://news.ycombinator.com/context?id=43081303 https://news.ycombinator.com/context?id=43081303 .. https://news.ycombinator.com/item?id=43310933 https://news.ycombinator.com/item?id=43310933 : > Probably not gauge symmetry there, then.

codethief 2y ago

> This is a pretty strong argument against the existence of particles except as asymptotic approximations. I think it's also a pretty strong argument against the mathematical well-definedness of typical (interacting) QFTs in the first place.

westurner 2y ago

Though it doesn't resolve whether a "quanta" is a particle or a measurable convergence of waves, Electrons and Photons are observed with high speed imaging. "Quantum microscopy study makes electrons visible in slow motion" https://news.ycombinator.com/item?id=40981054 https://news.ycombinator.com/item?id=40981054 There exist single photon emitters and single photon detectors. Qualify that there are single photons if there are single photon emitters: Single-photon source: https://en.wikipedia.org/wiki/Single-photon_source https://en.wikipedia.org/wiki/Single-photon_source QFT is not yet reconciled with (n-body) [quantum] gravity, which it has 100% error in oredicting. random chance. TOD IIRC, QFT cannot explain why superfluid helium walks up the sides of a container against gravity, given the mass of each particle/wave of the superfluid and of the beaker and the earth, sun, and moon; though we say that gravity at any given point is the net sum of directional vectors acting upon said given point, or actually gravitational waves with phase and amplitude. You said "gauge theory", "Topological gauge theory of vortices in type-III superconductors" https://news.ycombinator.com/item?id=41803662 https://news.ycombinator.com/item?id=41803662 From https://news.ycombinator.com/context?id=43081303 https://news.ycombinator.com/context?id=43081303 .. https://news.ycombinator.com/item?id=43310933 https://news.ycombinator.com/item?id=43310933 : > Probably not gauge symmetry there, then.