According to our current understanding of physics, light travels at a constant speed in a vacuum, which is approximately 299,792,458 meters per second, often rounded to 300,000 kilometers per second. However, in certain mediums like water, glass, or other transparent materials, light can be slowed down due to interactions with the atoms or molecules in those materials.
Slowing down light does not cause it to stop completely. Instead, it reduces its speed to a lower value. This phenomenon is known as "slowing down of light" or "reduced group velocity." The speed at which light propagates in a medium is determined by its refractive index, which is a property of the material. When light passes through a medium, it interacts with the atoms or molecules, and this interaction can cause delays in the light's propagation.
If light were somehow brought to a complete stop, it would have significant implications for our understanding of physics. It would mean that all the energy and momentum carried by the photons, which are particles of light, would be concentrated at a single point. This would violate fundamental principles of conservation of energy and momentum.
Regarding matter, if light were to stop, it would not have an immediate impact on matter itself. Matter would continue to exist and behave according to its own properties. However, the absence of light would have profound effects on our perception and interactions with matter since our ability to see and interact with objects depends on the reflection, absorption, and emission of light by matter.
It's important to note that currently, there is no known method to bring light to a complete stop in a conventional sense. While scientists have managed to slow down light considerably in certain experimental setups, achieving a complete stoppage of light remains a subject of ongoing research and exploration in the field of physics.