- 4105 - LIGHT SPEED - is it constant? The phenomenon of light slowing down as it passes through a material like glass or air is one of the most fascinating areas of physics, involving a complex interaction between light and materials. There are three ways to look at the same situation, and each employs a different understanding of physics.
-------------- 4105 - LIGHT SPEED - is it constant?
- The first way is thsat
it is all waves. This first perspective comes
from James Clerk Maxwell, the 19th-century Scottish physicist who discovered a
unified theory of electricity and magnetism, and also found that light is made
of waves of electricity and magnetism.
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- When these waves
encounter a material like glass or water, they see a whole bunch of charged
particles. The molecules in the material are made of atoms, which have protons
and electrons, both charged particles. And charged particles respond to
electromagnetic waves passing by them by wiggling along with them.
-
- But moving charged
particles also create electromagnetic waves of their own. The result is a giant
mess, with the original electromagnetic waves interfering with all the waves
generated by all the charged particles in the material, and there are a lot of
particles.
-
- Most of those waves,
except the waves traveling in the original direction of the light, cancel each
other out. But because the waves generated by the particles are a little
delayed, the entire ensemble moves more slowly. The end result: The light moves more slowly.
-
- The second explanation
of light's behavior is that it is all particles. Light is made of tiny particles known as
“photons”. Maxwell's theory of light is a classical picture of radiation. A
much more sophisticated view based on quantum mechanics, where light is made of
countless tiny particles known as photons. Photons can act individually, but
when enough of them get together, they display all of the same properties as
electromagnetic waves.
-
- A fully quantum
treatment of photons interacting with materials can get pretty nasty, but
thankfully, we have an approach developed by the famed physicist Richard
Feynman to guide us through it. We can imagine all the photons of the incoming
light slamming into the material. Once inside, they begin interacting with all
the charged particles.
-
- Those charged particles
can absorb those photons and emit their own, because that's what charged
particles do. But these photons are a little different. In physics, they're
known as “virtual photons”.
-
- Photons can can roam
freely through the universe, existing as independent entities (this is light),
and they do the legwork of mediating the electromagnetic force (like the force
holding a magnet to a fridge). We call
them "virtual" photons, they exist only in our math to help us
account for the electromagnetic force.
-
- 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. But all of
those interactions come at a cost: It takes time for an electron to absorb and
reemit a photon, and those delays add up.
-
- The end result: The
light moves more slowly.
-
- The third alternative
explanation. is called“polaritons”. The properties of light, viewing it through a
particle-based lens and a wave-based lens. But the material is more than a
simple collection of charged particles that just do whatever they are
electromagnetically ordered to do.
-
- 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 the speed of light.
-
- That speed depends on
the properties of the material's phonons. In this view, it's not light that's
passing through a material, with the material responding to it, but a new
object, a polariton, passing through. This view is especially useful, because
in many situations, it's very easy to discard all the cumbersome math of
conflicting waves or bouncing photons and just deal with a straightforward,
simple entity that already encodes all the information you need.
-
- Light goes in, a
polariton travels through and light goes out.
-
- The end result: The
light moves more slowly. Any way you
look at it light is an amazing thing.
-
July 25, 2023 LIGHT
SPEED - is it constant? 4105
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