Physicists have applied classical mechanics rules to light – with success.
Light can behave both as a wave and a particle, a
head-scratcher that confused scientists for centuries before the fact became
obvious. This duality is a cornerstone of quantum mechanics, and the peculiar
behavior of the quantum world has mostly left classical mechanics theorems
behind in the realm of things our own size.
A research team has now used classical mechanics to explain
two particular properties of light: polarization and entanglement. The first is
the ability of light waves to have an orientation – a fact that is used in
sunglasses to filter out some light. The second is the ability of entangled
photons to form a quantum system whose parts remain connected even if separated
by vast distances. Changes to one would mean instantaneous changes to the
other.
These don’t sound like classical mechanics at all, but the
team considered whether there could be an analog to the behavior of
polarization in the Huygens–Steiner theorem. That 350-year-old theorem is about
how a solid body rotates with respect to an axis that doesn’t go through its
center of mass, and it is useful in both technical applications and studying
celestial objects.
"This is a well-established mechanical theorem that
explains the workings of physical systems like clocks or prosthetic
limbs," lead author Xiaofeng Qian, from the Stevens Institute of
Technology, said in a statement. "But we were able to show that it can
offer new insights into how light works, too."
The researchers used the intensity of light as an analog for
the mass of a physical object, and the rest of the properties were able to be
mapped out following the structure of the theorem, even though light is not a
classical body.
"Essentially, we found a way to translate an optical
system so we could visualize it as a mechanical system, then describe it using well-established
physical equations," explained Qian. "This was something that hadn't
been shown before, but that becomes very clear once you map light's properties
onto a mechanical system. What was once abstract becomes concrete: using mechanical
equations, you can literally measure the distance between 'center of mass' and
other mechanical points to show how different properties of light relate to one
another."
The reason why these relationships exist and why the mapping
works so well is currently not clear. Understanding this connection might have
important implications for our understanding of quantum properties, as well as
how we use them in applications.
The study is published in Physical Review Research.