
Mind the Mass Gap - raattgift
https://blogs.scientificamerican.com/observations/mind-the-mass-gap/
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l33tman
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
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.

