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As a completely sideways note, mass-gap in physics also refers to the property of some particles that they need a larger-than-zero minimum energy to start exist

As a completely sideways note, mass-gap in physics also refers to the property of some particles that they need a larger-than-zero minimum energy to start existing (their "rest mass"), as opposed to for example photons that can (in principle) get going from any available energy.

raattgift 7y ago
I don't mind your sketch, except I guess that "in physics" was meant to be "in particle physics" or perhaps more specifically "in QED". In QFTs (and more broadly in gauge theory) it's the infimum at which gauge freedom cannot remove an excitation that produces a state orthogonal to the vacuum. You can have a mass-gap in a massless scalar or vector field, for instance. Explaining mass gaps gets hairier in specific gauge theories (Yang-Mills theory notably) or when considering interacting fields or when stepping outside the safety of purely Lorentz transformations. It's almost impossible to work on compact objects without engaging with gauge theory, so I think it's safe to bet that whoever originated this astrophysical "mass gap" (MG) terminology would have been aware of the uses of "mass gap" in particle physics. However, I don't see a straightforward path from there to an analogy. I like the (year-old) chart that appears after the animations at https://media.ligo.northwestern.edu/gallery/mass-plot https://media.ligo.northwestern.edu/gallery/mass-plot since it shows the MG pretty starkly. One is free to wonder whether some strange (pardon the pun) nuclear process shrinks compact neutron stars into ultracompact objects (the latter are defined as having a photon sphere but more generally low-ellipticity orbits of photons and neutrinos within the object and a cloudy atmosphere of photons and neutrinos just above the surface; there are theoretical descriptions of ultracompact neutron stars, ultracompact quark stars, and even ultracompact string stars), whether progenitor star physics prevents the formation of MG compact objects as in silicon-oxygen rebounds, whether there are semiclassical effects in the gravitational backreaction around relativistic stars generally that become strong in the MG (and then does that lead to loss via gravitational radiation rather than an MG-mass equilibrium state?), whether MG BHs are always formed by mass transfer from Roche lobe overflow with the relativistic accretion spectrum (high X-Ray luminosity tending towards burstiness) frustrating binary mass-ratio estimates, or whether it's just blind luck that nothing in an apparent-but-not-real MG has been observed yet. Or some combination. Ultimately further "multi-messenger" observations will help resolve these issues. There are a lot of compact objects in the sky to be found.

raattgift 7y ago

I don't mind your sketch, except I guess that "in physics" was meant to be "in particle physics" or perhaps more specifically "in QED". In QFTs (and more broadly in gauge theory) it's the infimum at which gauge freedom cannot remove an excitation that produces a state orthogonal to the vacuum. You can have a mass-gap in a massless scalar or vector field, for instance. Explaining mass gaps gets hairier in specific gauge theories (Yang-Mills theory notably) or when considering interacting fields or when stepping outside the safety of purely Lorentz transformations. It's almost impossible to work on compact objects without engaging with gauge theory, so I think it's safe to bet that whoever originated this astrophysical "mass gap" (MG) terminology would have been aware of the uses of "mass gap" in particle physics. However, I don't see a straightforward path from there to an analogy. I like the (year-old) chart that appears after the animations at https://media.ligo.northwestern.edu/gallery/mass-plot https://media.ligo.northwestern.edu/gallery/mass-plot since it shows the MG pretty starkly. One is free to wonder whether some strange (pardon the pun) nuclear process shrinks compact neutron stars into ultracompact objects (the latter are defined as having a photon sphere but more generally low-ellipticity orbits of photons and neutrinos within the object and a cloudy atmosphere of photons and neutrinos just above the surface; there are theoretical descriptions of ultracompact neutron stars, ultracompact quark stars, and even ultracompact string stars), whether progenitor star physics prevents the formation of MG compact objects as in silicon-oxygen rebounds, whether there are semiclassical effects in the gravitational backreaction around relativistic stars generally that become strong in the MG (and then does that lead to loss via gravitational radiation rather than an MG-mass equilibrium state?), whether MG BHs are always formed by mass transfer from Roche lobe overflow with the relativistic accretion spectrum (high X-Ray luminosity tending towards burstiness) frustrating binary mass-ratio estimates, or whether it's just blind luck that nothing in an apparent-but-not-real MG has been observed yet. Or some combination. Ultimately further "multi-messenger" observations will help resolve these issues. There are a lot of compact objects in the sky to be found.