Light released by a distant star does not simply wink out after crossing some threshold of space. In a vacuum, photons keep moving indefinitely, and astronomers routinely detect light that has traveled billions of light-years to reach Earth. Yet the universe itself is expanding, and that expansion stretches every photon’s wavelength, saps its energy, and slows the rate at which successive photons arrive. The result is a kind of cosmic dimming that does not destroy light but makes the most distant sources progressively harder to see.
Photons in a Vacuum Never Stop
The simplest answer to whether light travels forever is yes, at least in principle. Unlike sound, which requires a medium and loses energy to friction within it, light waves propagate through empty space without any material resistance. The explanation from the Illinois Physics Van notes that light waves in a perfect vacuum encounter nothing to absorb or scatter them, so individual photons can keep going indefinitely. That is why photons emitted by stars billions of light-years away still register on modern detectors. Nothing in empty space absorbs or halts them.
But “traveling forever” and “arriving with the same brightness” are two different things. A photon that left a galaxy ten billion years ago still exists, still moves at the speed of light, and still carries some energy. The catch is that the universe has been expanding throughout that photon’s journey, and expansion changes what observers measure when the photon finally lands on a telescope mirror.
How Expansion Stretches and Weakens Light
Cosmological redshift is the most direct way expansion alters starlight. As space itself stretches, the wavelength of every photon traveling through it stretches too, shifting visible light toward the red end of the spectrum and, for the most distant sources, into the infrared or microwave bands. A NASA overview describes how this expansion increases wavelength and lowers photon energy. The effect is analogous to the falling pitch of a receding ambulance siren, except the “motion” here is the growth of space rather than the movement of an object through it.
Each individual photon therefore carries less energy by the time it reaches an observer than it did when it was emitted. That energy is not absorbed or converted into heat along the way. It is, in a real physical sense, diluted by the expansion of the metric through which the photon travels. A discussion on NASA’s Cosmicopia uses the geometric image of a shell of light expanding outward from a source: as the shell grows, its energy spreads over a larger and larger area, so any given patch of sky intercepts a smaller share. Light does not fade out locally, but it becomes steadily harder to detect as it redshifts and spreads.
On top of this geometric dilution, the drop in photon energy with redshift means that detectors tuned to visible light may simply fail to register the most distant sources, whose photons have been shifted into infrared or longer wavelengths. Instruments like the Hubble Space Telescope and its successors are designed with this in mind, using infrared-sensitive cameras to catch light that began its journey as visible or ultraviolet radiation billions of years ago.
Time Dilation Compounds the Dimming
Redshift is only half the story. Expansion also stretches the intervals between photon arrivals. If a distant supernova emits two photons one second apart, those photons will arrive at Earth separated by more than one second because space expanded during transit. This time dilation means that the rate at which photons reach a detector drops for more distant sources, compounding the per-photon energy loss from redshift.
The clearest observational proof comes from Type Ia supernovae. A study from the Supernova Cosmology Project found that supernova light curves broaden in proportion to a factor of (1+z), where z is the redshift. A supernova at redshift 0.5 appears to brighten and fade about 1.5 times more slowly than an identical explosion nearby. That broadening matches the prediction of general relativity for an expanding universe and serves as primary observational evidence for cosmological time dilation. Because the photon arrival rate is dilated alongside the per-photon energy drop, the total received flux from a distant source falls faster than redshift alone would predict.
This time stretching is not a quirk of supernova physics; it is a feature of spacetime itself. Any periodic signal, from pulsating stars to flickering quasars, will appear slowed by the same (1+z) factor when observed from far away. The effect is subtle at low redshift but becomes dramatic for objects seen when the universe was only a fraction of its current age.
Tolman Dimming and the (1+z) to the Fourth Power
Richard C. Tolman formalized these effects in a 1930 paper in the Proceedings of the National Academy of Sciences, titled “On the Estimation of Distances in a Curved Universe with a Non-Static Line Element.” Tolman derived how an expanding, curved universe would alter the observed flux and surface brightness of distant objects. His work established what is now called Tolman surface-brightness dimming, which predicts that surface brightness drops as the fourth power of (1+z). Two of those powers come from redshift (one for energy loss per photon, one for the reduced arrival rate), and two come from the angular-size effects of expansion on the apparent area of the source.
The original paper is archived in resources linked through the U.S. National Library of Medicine, which preserves many early twentieth-century scientific proceedings. In modern cosmology, Tolman’s reasoning is built into standard distance definitions. David W. Hogg’s widely cited work on cosmological distances lays out the mathematical relationships among luminosity distance, angular-diameter distance, redshift, and expansion. In that framework, distant sources appear dimmer because (1+z) enters the flux calculation through both redshift and time dilation.
The practical consequence is stark: a galaxy at redshift 1 has its surface brightness reduced by a factor of 16 compared to an identical galaxy at redshift zero. At redshift 3, the dimming factor becomes (1+3)4 = 256. Even if the galaxy is intrinsically bright, its light is spread and weakened so severely that only the largest telescopes can see it at all.
Hubble Telescope Data Confirms the Prediction
Tolman’s theoretical prediction remained difficult to test for decades because galaxy evolution complicates the comparison. Galaxies at high redshift are younger and may have been intrinsically brighter than their nearby counterparts, partially masking the dimming signal. Lubin and Sandage tackled this problem using Hubble Space Telescope imaging of early-type galaxies in distant clusters and carefully matched them to local samples. Their analysis, together with related work, showed that after accounting for evolutionary effects, the observed surface brightness decline closely followed the (1+z)4 law expected in an expanding universe.
These observations fit into a broader cosmological picture constrained by many independent measurements. A summary from NASA’s Wilkinson Microwave Anisotropy Probe mission on cosmological parameters describes how data on the cosmic microwave background, galaxy clustering, and supernova distances collectively support an expanding universe with dark energy driving an accelerated expansion. The same framework that explains Tolman dimming also explains the faintness of distant supernovae and the detailed pattern of temperature fluctuations in the microwave background.
Questions about where the “lost” photon energy goes often arise in this context. An earlier answer from NASA’s Imagine the Universe! service on redshift and energy emphasizes that energy conservation in general relativity is more subtle than in everyday mechanics. In an expanding universe, there is no single global energy conservation law that applies in the simple way it does in a static box. Instead, energy-momentum is conserved locally, while the global redshifting of photons reflects the changing geometry of spacetime rather than a transfer of energy into some other reservoir.
Does Light Ever Truly Disappear?
For practical astronomy, there are limits beyond which light might as well have vanished. Photons can be absorbed or scattered by dust, gas, or intervening galaxies. Over extremely long timescales, they can also be swallowed by black holes. But in the idealized case of a photon in empty space, no known process makes it simply stop. It will keep traveling, redshifting, and contributing ever more weakly to the radiation background of the universe.
In the far future, as expansion continues, the wavelengths of today’s visible and infrared photons will be stretched to such extremes that they become effectively undetectable to any conceivable instrument. Yet even then, the photons themselves still exist within the fabric of spacetime. Their fading is not a matter of running out of distance, but of the universe growing so vast, and their energy so dilute, that no observer can collect enough of them to notice.
From the first light of the early universe to the last embers of distant galaxies, the story of a photon is therefore not about a sudden end, but about an unbroken journey through an ever-expanding cosmos. In principle, light travels forever; in practice, the universe conspires through redshift, time dilation, and geometry to hide that light from view long before it truly ceases to be. For researchers tracking citations and archival records of the foundational work behind these ideas, tools such as My NCBI help organize the scientific literature that has illuminated how and why the cosmos dims its own light over time.
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*This article was researched with the help of AI, with human editors creating the final content.
