Academic Paper: On the Redshift of Spectral Lines through Interstellar Space

Overview
Fritz Zwicky proposed a radical alternative to the idea that cosmological redshifts are produced solely by recessional motion of galaxies. He argued that light traversing interstellar and intergalactic space could lose energy by interactions with matter or the gravitational field of that medium, producing a systematic shift toward longer wavelengths. This notion , later dubbed "tired light" , aimed to account for the observed correlation between redshift and distance without invoking a global expansion of space.
The proposal arose at a time when observations were revealing a clear redshift–distance relation, and Zwicky sought a mechanism grounded in processes occurring along a photon's path. He emphasized simple, testable consequences that differ from an expanding-universe interpretation, framing redshift as a cumulative propagation effect rather than a kinematic Doppler shift of entire galaxies.

Main proposal and physical idea
Zwicky envisioned that each photon would gradually surrender a small fraction of its energy while traveling through the diffuse matter filling interstellar and intergalactic space. That energy loss would lengthen the photon's wavelength and produce a redshift directly proportional to the distance traveled. The mechanism he sketched did not require a single catastrophic interaction but rather many minute interactions or a steady drag effect, so that the observed linear relation between redshift and distance could emerge naturally.
He discussed plausible physical agents for the energy drain, including collisions or near-collisions with particles and gravitational interactions with mass concentrations in space. An important element of Zwicky's argument was that the net scattering angles could be sufficiently small that images of distant objects would retain sharpness even while photons lost energy, thereby avoiding obvious observational contradictions such as severe image blurring.

Predicted observational consequences and possible tests
The tired-light picture yields a number of observational predictions that differ from an expanding-universe model. A primary expectation is a linear redshift–distance law, consistent with contemporary observational trends. However, surface brightness, time dilation of variable sources, and spectral-line behavior are predicted to diverge from expansion-based expectations. Zwicky noted that if energy were gradually removed from photons and deposited into the intergalactic medium, detectable heating or other secondary effects might follow.
He suggested direct empirical tests: checking whether distant sources show the time-stretching of light curves expected from cosmic expansion, examining whether surface brightness scales match tired-light or expansion predictions, and analyzing line profiles for telltale broadening or asymmetries that energy-loss mechanisms might produce. Zwicky argued that careful attention to such diagnostics could distinguish propagation effects from recessional motion.

Reception, challenges, and legacy
Zwicky's proposal stimulated debate and motivated observational scrutiny, but it confronted significant theoretical and empirical challenges. The mechanism for continuous, non-scattering energy loss without producing unacceptable image degradation or violating well-established conservation laws was never worked out in a physically compelling, quantitatively consistent way. Subsequent observational tests , in particular measurements of surface brightness versus redshift, detection of time dilation in distant supernova light curves, and the broader success of predictions from an expanding-universe framework , favored cosmic expansion and largely displaced tired-light ideas.
Despite being eclipsed by expansionary cosmology, Zwicky's contribution remains historically important. It exemplifies a willingness to question prevailing interpretations and to propose alternative, testable hypotheses. The discussion it provoked helped sharpen observational strategies and theoretical expectations for distinguishing propagation effects from cosmic dynamics, and it stands as a notable episode in the development of modern cosmology.