  Only three extragalactic supernovae have been detected at late times at
millimeter wavelengths: SN 1987A, SN 1978K, and SN 1996cr. SN 1978K is a
remarkably luminous Type IIn supernova that remains bright at all wavelengths
40 years after its explosion. Here we present Atacama Large
Millimeter/submillimeter Array (ALMA) observations taken in 2016 using Bands 3,
4, 6, and 7 that show a steepening in the spectrum. An absorbed single power
law model broadly fits all the radio and millimeter observations, but would
require significant chromatic variability. Alternatively, a broken power law
fits the radio-millimeter spectrum: this can be explained using an
ultra-relativistic spherical blast wave in a wind scaling with a cooling break,
as in a gamma-ray burst afterglow. Using updated Australia Telescope Compact
Array (ATCA) light curves, we show the non-thermal radio continuum continues to
decay as $t^{-1.53}$; in the fireball model, this independently defines the
power law indices found in the radio-millimeter spectrum. Supernovae such as SN
1978K might be important contributors to the Universal dust budget: only SN
1978K was detected in a search for warm dust in supernovae in the transitional
phase (age 10-100 years). Using Spitzer Space Telescope observations, we show
that at least some of this dust emission has been decaying rapidly as
$t^{-2.45}$ over the past decade, suggesting it is being destroyed. Depending
on the modeling of the synchrotron emission, the ALMA observations suggest
there may be emission from a cold dust component.
