Using lasers to freeze matter
While we usually think of lasers as heating things up, scientists use them to cool atoms to temperatures just a fraction above absolute zero. By aiming laser beams from multiple directions at moving atoms, the light particles collide with the atoms and slow them down. Since temperature is a measure of atomic motion, slowing them freezes them.
The Direct Link Between Motion and Heat
In everyday experience, laser beams are associated with concentrated energy and intense heat. They can cut through industrial metals, cauterize biological tissue, and ignite combustible materials. Yet in atomic physics, precisely tuned lasers serve the opposite purpose: bringing particles almost entirely to a standstill. Understanding how light can freeze matter requires looking at what temperature actually represents at the microscopic scale.
Temperature is simply a measure of the average kinetic energy of atoms or molecules. In a warm gas, particles zip around in every direction at hundreds of meters per second, colliding chaotically with one another. When an object cools down, its constituent particles lose kinetic energy and slow their pace. To achieve temperatures near absolute zero—the theoretical threshold where all thermal motion ceases—physicists do not use conventional refrigeration, but instead use the momentum carried by light to halt fast-moving particles.
How Light Exerts a Mechanical Push
Although photons have no mass, they carry momentum. When an atom absorbs a photon of light, it absorbs that photon's momentum as well, receiving a tiny mechanical kick in the direction the light was traveling. Shortly after absorption, the atom returns to its lower energy state by spontaneously emitting a photon in a completely random direction. While each emitted photon also gives the atom a recoil kick, these emissions occur uniformly across all directions over many cycles. The random recoil kicks average out to zero net change, whereas the absorption kicks from a directed laser beam consistently push the atom along the beam's axis of travel.
If a laser simply blasted an atom with continuous light, it would accelerate the atom away rather than slow it down. To achieve cooling, the atom must selectively absorb photons only when it is traveling directly toward the laser source. This selective absorption relies on a fundamental quantum property: an atom can only absorb light whose frequency precisely matches the energy difference between its internal electronic states, known as its resonant transition frequency.