Scientists can hold and move living bacteria using pure beams of light
Light carries physical momentum. When a laser beam is tightly focused through a microscope lens, the steep gradient of light intensity pulls microscopic particles toward the beam's center. Known as optical tweezers, this technology allows scientists to grab, hold, and manipulate individual living bacteria, viruses, and strands of DNA without physically touching or puncturing them, earning Arthur Ashkin the 2018 Nobel Prize in Physics.
The Tangible Pressure of Illumination
We rarely think of light as something that can push against physical matter. Sunlight feels warm against human skin, but it does not feel heavy or forceful. Yet, electromagnetic radiation possesses momentum. Whenever light bounces off a reflective mirror, passes through a clear lens, or is absorbed by a dark surface, that momentum must be conserved. The change in the momentum of the incoming light imparts an equal and opposite kick to the physical object it strikes. Johannes Kepler famously noticed this phenomenon centuries ago when observing that the tails of comets consistently point away from the Sun, correctly inferring that sunlight exerts an outward push.
Under ordinary conditions, radiation pressure is far too weak to move ordinary objects because everyday light sources are diffuse and incoherent. A standard household bulb casts its photons across an entire room, spreading its momentum so thinly that the resulting force is completely imperceptible. The development of the laser fundamentally changed this dynamic. By concentrating a vast number of coherent photons into an extremely narrow, collimated beam, physicists suddenly possessed light powerful enough to exert measurable mechanical forces on tiny, microscopic targets suspended in liquids or gases.
Balancing the Push and the Pull
Manipulating microscopic matter with light requires more than merely shoving it forward. An unshaped laser beam acts like a miniature fire hose: the stream of photons strikes an object and drives it downstream in the direction of the beam's propagation. In optical physics, this forward thrust is known as the scattering force. If the scattering force were the only mechanism at play, a laser beam would simply blow tiny particles away indefinitely, making precise physical capture impossible.