Slowing down light is not as easy as it sounds.
. But these photons don't roam freely; they have a job to do. So we call them"virtual" photons — they exist only in our math to help us account for the electromagnetic force.
So all of these charged particles start emitting copious amounts of virtual particles, and once again, there's a giant, confusing mess. Feynman came to the rescue. He developed a technique of averaging out all of the possible paths that those photons can take. That averaging process eliminated all the wayward photons, leaving behind only the ones traveling in the original direction of the light.
Materials can be interesting, too. Specifically, all materials can support vibrations — little ones, big ones, ones that last a long time, ones that fade away quickly. All material is constantly in motion, and that motion affects how that material interacts with everything else. To help physicists grapple with the complexities of all the kinds of vibrations that are constantly racing through materials, they proposed an entity known as a phonon.
A phonon is another kind of fake particle, but like virtual photons, it's very useful. It allows physicists to use the language of quantum mechanics to describe the vibrations in a material. This new language comes in handy when light, which is made of photons, enters that material. When photons and phonons get together, they create something new: a polariton. In this view, when light enters a material, it disappears. And so do the phonons in the material itself. Instead, they get replaced by polaritons. These polaritons share a lot of properties with their parents, but they have one crucial property: They travel more slowly than theThat speed depends on the properties of the material .
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