Darkness is the condition resulting from a lack of illumination. It is not a substance, but the perception of when photons are absent or entirely absorbed. This concept extends beyond everyday experience, encompassing extreme conditions where light is nearly or completely absent. To understand the “darkest” things, it is helpful to explore how light interacts with matter and the vastness of the universe.
Understanding Darkness
The perception of darkness hinges on how light interacts with objects. When light encounters a surface, it can be reflected, transmitted, or absorbed. Reflection occurs when light bounces off a surface, while transmission means light passes through a material. Conversely, absorption happens when a material takes in light energy, converting it into another form, heat.
Objects that appear dark absorb a high percentage of the light that strikes them, reflecting very little back to our eyes. Scientists quantify this light absorption using concepts like reflectivity or albedo. Albedo measures the fraction of incoming light that a surface reflects, ranging from 0 for a perfect absorber to 1 for a perfect reflector. Therefore, the “darkest” objects are those with an albedo close to zero.
Earth’s Darkest Creations
Human ingenuity has engineered materials designed to achieve extreme darkness by maximizing light absorption. Among the most prominent is Vantablack, an acronym for Vertically Aligned NanoTube Arrays. This material is not a paint but a coating made from a “forest” of microscopic carbon nanotubes. Each nanotube is about 20 nanometers in diameter and grown perpendicular to a surface.
When light strikes Vantablack, instead of reflecting, photons become trapped and continually deflect within the dense network of these nanotubes. The light bounces around inside this structure until it is almost entirely absorbed as heat. This unique design allows Vantablack to absorb up to 99.965% of visible light, making objects coated with it appear as two-dimensional voids, devoid of any discernible features or shadows. Such materials find applications in aerospace to prevent stray light in telescopes, in optical instruments to enhance camera performance, and even in art.
The Universe’s Ultimate Dark Objects
Beyond terrestrial creations, the universe hosts phenomena that represent ultimate darkness: black holes. A black hole is a region in spacetime where gravity is so intense that nothing, not even light, can escape once it crosses a specific boundary. These cosmic entities typically form from the gravitational collapse of massive stars at the end of their lives, compressing a vast amount of matter into a small space.
The boundary surrounding a black hole, from which escape is impossible, is called the event horizon. This is often referred to as the “point of no return” because the escape velocity required to leave this region exceeds the speed of light. Since black holes do not emit or reflect light, they are invisible to direct observation. Scientists detect them indirectly by observing their powerful gravitational effects on nearby stars and gas, the X-rays emitted by superheated matter spiraling into them, and through gravitational lensing, where their immense gravity bends light from more distant objects.
The Vastness of Cosmic Darkness
Despite the countless stars and galaxies, the vast expanse of space appears largely dark. This observation, known as Olbers’ Paradox, questions why the night sky is not uniformly bright if the universe is infinite and filled with stars. The resolution to this paradox lies in several aspects of our universe.
Firstly, the universe has a finite age, meaning light from very distant stars and galaxies has not had enough time to reach Earth. Secondly, the universe is expanding, and this expansion causes the light from distant objects to stretch to longer, less energetic wavelengths, a phenomenon called redshift. This can shift visible light into infrared, microwave, or radio wavelengths, thus contributing to the perceived darkness. Finally, the immense distances between celestial bodies mean there is simply not enough matter to scatter light effectively throughout the vacuum of space, preventing light from filling every line of sight.