Coherent light, such as laser light, can theoretically travel an extremely long distance in free space without significant degradation. Unlike non-coherent light sources, coherent light is composed of waves that have a fixed phase relationship, meaning the crests and troughs of the waves align.
In a perfect vacuum with no obstructions, coherent light can propagate indefinitely. This is because light waves in free space experience minimal interactions and do not suffer from scattering, absorption, or significant degradation over vast distances.
However, even in space, various factors can affect the propagation of light over extremely long distances. These factors include:
Diffraction: Light waves spread out or diffract as they travel, causing the beam to gradually widen. The rate of diffraction depends on the wavelength of the light and the size of the beam aperture.
Interstellar medium: In the interstellar medium, there are extremely dilute amounts of gas, dust, and other particles. While the density is low, these particles can scatter or absorb light to some extent, particularly at certain wavelengths.
Gravitational lensing: Intense gravitational fields, such as those near massive objects like galaxies or black holes, can bend light through gravitational lensing. This phenomenon can slightly alter the direction of light rays, potentially affecting their trajectory over large cosmic distances.
Cosmic expansion: The universe is expanding, and the space between galaxies is also expanding. This expansion can cause a redshift of light, stretching the wavelength and potentially affecting its coherence over extremely large cosmological scales.
Nevertheless, under ideal conditions and over relatively short astronomical distances, coherent light can travel vast distances without significant degradation.