AI-generated illustration for Galileo Whispers. Symbolic microscopic photon-interaction concept; the visible traces, colors and geometry are illustrative, not observed particle paths or a cosmic explosion.
Distant gamma rays can interact with lower-energy background photons and produce an electron and a positron when the combined energy and collision geometry permit it. The gamma ray then no longer travels onward as that original photon. Over a sufficiently long path, this process can reduce the gamma-ray signal reaching a telescope.
The process is called photon-photon pair production. It is an interaction between light quanta, not a gamma ray bending away because it carries electric charge.
Empty-looking space contains other light
Light emitted by stars and processed by dust contributes to an extragalactic background extending from ultraviolet through optical and infrared wavelengths. The background is faint beside the objects we readily see, but a high-energy photon can encounter it during a long journey.
NASA’s account of a 2012 Fermi study describes this radiation as a cosmic fog for gamma rays. The analogy concerns attenuation: it does not imply a cloud of ordinary droplets or a uniform, opaque wall at one distance.
The background’s spectrum matters. A gamma ray of one energy does not have the same interaction probability with every possible background photon. Distance, the changing radiation field through cosmic history and the directions of encounters also enter the calculation.
Two photons supply the incoming energy and momentum
An electron and a positron each have rest mass. Creating the pair requires sufficient energy in the collision’s center-of-momentum frame, while conserving total momentum as well as energy. A photon’s high energy measured by one observer is not, alone, a complete statement of those conditions.
The second photon matters because it changes the incoming system’s energy-momentum balance. The angle between the photons matters for the same reason. Head-on and nearly parallel encounters are not equivalent, even when the individual photon energies are the same.
The Dwek and Krennrich review of gamma-ray opacity sets out the interaction and its dependence on photon energies and angle. It also shows why describing every collision as automatic conversion would be wrong. Pair production has an energy-dependent probability, not a guarantee attached to any two crossing rays.
Keep the energy fixed and change the angle
The threshold can be written as Eγ × ε × (1 − cos θ) ≥ 2(mₑc²)², where Eγ and ε are the two photon energies in the same local frame, θ is their encounter angle, mₑ is the electron mass and c is the speed of light. The expression sets a kinematic condition; it is not an interaction probability.
For a head-on encounter, θ is 180 degrees and the angle factor is 2. At 90 degrees the factor is 1. With the background-photon energy held fixed, the gamma ray therefore needs twice the energy to reach threshold at a right angle as it does head-on.
A hypothetical gamma ray just above the head-on threshold can consequently fail the right-angle test even though neither photon’s energy has changed. Parallel photons have a zero angle factor and cannot meet this pair-production threshold. Meeting the energy condition still does not guarantee a conversion, and a cosmological path requires accounting for the changing photon energies and radiation field.
A missing photon is not a missing energy budget
After a successful interaction, energy and momentum reside in the resulting particles, including their motion. The initial gamma ray is removed from its original beam, but its energy has not simply ceased to exist.
This distinction helps interpret the word “absorption.” In an astronomical spectrum, absorption means fewer photons reaching the detector in the relevant beam and energy range than a model without the intervening interaction would predict. It does not require a solid surface to catch them.
The resulting particles can participate in further processes. That does not mean a detector receives an unchanged replacement gamma ray along exactly the same path, at the same energy and time. Following those later products requires additional propagation models, beyond merely identifying the initial interaction.
How astronomers recognize attenuation
Astronomers do not observe each intergalactic collision individually. They measure surviving gamma rays and compare source spectra, distances and models. A source could also emit fewer high-energy photons intrinsically, so separating source behavior from propagation is essential.
The Fermi-LAT Collaboration’s original 2012 paper reported an absorption feature in combined blazar spectra. Blazars are active galaxies whose jets are directed near our line of sight. The study used an ensemble and a statistical comparison, rather than declaring that one faint source alone had measured all background light.
That method creates both an opportunity and a limit. Attenuation can constrain the background radiation field, but the inference depends on source modeling and observational uncertainties. Different energy ranges probe different parts of the radiation field; one inferred value is not a universal transparency rating for space.
Propagation is separate from detection
Fermi’s detector also uses pair production to measure incoming gamma rays, as NASA’s explanation notes. In the instrument, gamma rays interact with detector material and the resulting charged-particle tracks help reconstruct the arrival direction. That is a different setting from photon-photon interactions along an intergalactic path.
Keeping the settings separate prevents a common confusion: the instrument does not need to recover the electron and positron created far away. It measures the photons that survived to reach it.
