The Sun is a giant fusion reactor at the center of our solar system that powers all life on Earth. Inside its core, immense heat and pressure cause hydrogen atoms to collide and fuse together, creating helium atoms and releasing energy in the form of photons. Photons are the quanta – smallest units – of electromagnetic energy and exhibit a quantum property known as wave-particle duality, existing simultaneously as both waves and particles. They are packets of light information and the fundamental particles that carry all frequencies of the electromagnetic spectrum. This spectrum includes, from longest to shortest wavelength: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Radio waves, with the longest wavelengths, oscillate at the lowest frequencies and carry the least energy. Gamma rays at the opposite end have the shortest wavelengths, highest frequencies, and contain the most energy.

Inside the Sun's core, the energy released from nuclear fusion initially emerges as high-energy gamma rays. As these gamma rays travel outward from the core and through the Sun’s layers, they continuously interact with matter, engaging with electrons – negatively charged subatomic particles that orbit the nuclei of atoms. When a gamma ray photon collides with an electron that is orbiting the nucleus of an atom, the electron absorbs the photon’s energy and jumps to a higher orbital state. Eventually, the electron releases the photonic energy and returns to its lower, more stable orbital state. However, the photon that is re-emitted is typically of a lower energy than the original gamma ray due to energy loss, primarily in the form of heat. This process of absorption and re-emission repeats, causing photons to progressively lose energy and be scattered across a wide spectrum of electromagnetic radiation that includes visible light. Due to the Sun’s dense and chaotic internal environment, it can take thousands to millions of years, on average, for energy to work its way out into space. Once they have escaped the Sun, the photons travel unimpeded through the vacuum of space and will reach the Earth in just 8 minutes.

Before reaching the Earth’s surface, photons must first pass through the magnetosphere—a magnetic field that surrounds the planet and protects it from streams of charged particles, such as cosmic rays and solar wind. Photons, being neutral and lacking a charge, are unaffected by the magnetosphere and continue their journey into the Earth’s atmosphere. In the atmosphere, shorter wavelength photons (like X-rays and ultraviolet light) are absorbed primarily by oxygen and nitrogen molecules. The high energy of these photons can cause the electrons surrounding these molecules to not only jump to a higher state but also to be ejected entirely, resulting in ionization. This process creates ions—charged particles—that form the ionosphere. The ionosphere protects us from harmful radiation and plays a vital role in enabling global communications, as it reflects radio waves back to Earth.

Lower-energy photons, with longer and more penetrative wavelengths, continue their journey through the atmosphere toward Earth's surface. As they reach the lower levels of the atmosphere, the shorter wavelengths of visible light (like blue light) are scattered by air molecules, causing the sky to appear blue. By the time photons reach the Earth's surface, they are comprised of approximately 49% invisible infrared, 43% visible light, and 8% invisible ultraviolet. However, this composition varies based on altitude, latitude, time of year, and weather conditions. The 43% that is visible light is all that we as humans can see with our eyes, though it is less than half of the total photonic energy that reaches our planet from the Sun.

On Earth, photons that escaped the Sun, traversed through space, and penetrated the planet’s atmosphere, will then be captured and consumed by photosynthetic life forms. When light hits a plant’s leaf, the electromagnetic energy is absorbed by chlorophyll molecules inside cell organelles called chloroplasts. This photonic energy excites the chlorophyll molecules’ electrons to higher energy states, leaving a positively charged hole where the electron previously was. The excited, negatively charged electron and the positively charged hole it left behind together create an effective battery due to their charge separation, known as an exciton.

Excitons are capable of storing energy, but they are inherently unstable. They need to travel swiftly from the chlorophyll molecule to a photosynthetic reaction center in the chloroplast where their light energy can be converted into the chemical energy that powers the plant’s growth. A single, random pathway might take too long, risking energy loss. To overcome this, excitons exhibit a quantum property known as quantum superposition. This allows them to explore every possible route simultaneously, as a wave rather than a particle, ensuring the most efficient path to the reaction center is taken. Inside the reaction center, the exciton is used to power the chemical reactions that synthesize glucose and oxygen from carbon dioxide and water. The glucose that is produced serves as both an energy source and a structural building block for the plant's growth. Through the process of photosynthesis, plants effectively harness electromagnetic energy and transform it into chemical energy, alchemizing light into matter on Earth.

Some of this plant matter will be consumed by herbivores. The herbivores, unable to photosynthesize themselves, depend on the chemical energy that was derived from photonic energy for fuel. When the herbivore consumes the plant, organelles called mitochondria inside their cells metabolize the plant carbohydrates to release the energy. Some of the herbivores will then be consumed by carnivores. Carnivores cannot photosynthesize or survive on vegetation, but they still depend on the alchemized photonic energy that is locked within the bodies of their prey. When carnivores consume meat, their own mitochondria break down fats and proteins to release the energy of the Sun into their cells. In this way, all food is a form of stored electromagnetic energy and all food webs boil down to the capture and storage of light. At the most fundamental level, we are consumers of light.

Our lives, and the lives of all living organisms, are sustained by the light of the Sun. Living beings absorb photonic energy, transform it through biochemical processes, and subsequently release it at reduced quantum energy levels, where it continues to drive reactions. Photons, through their continuous absorption and re-emission, drive the biochemical engines of life. Life on Earth gives its thanks to the photon’s incredible journey, spanning millions of years and millions of miles, across time and space, from star to cell, and from light to life.