The Casimir effect, as it's commonly called, is all about attraction. Broadly speaking, it says that two mirrors, when placed in a vacuum such that they face each other, will be attracted to each other. It's a quantum physics notion, and in quantum physics vacuums are not "empty," as they are popularly understood to be in normal daily life. In quantum physics a vacuum is always filled with electromagnetic waves, in all wavelengths. The waves suggest there is ever-present energy in the vacuum. Some of the waves will fit between the two mirrors and will bounce from one to the other. The energy between the mirrors is higher when longer wavelengths can fit in the space, but if the mirrors are brought closer to each other the longer waves won't fit the space; the energy between the mirrors will therefore lessen, causing the mirrors to be drawn to each other. In 1948, the Casimir Effect was predicted by Hendrick Casimir, but it would take until 1996 for the small force of the effect to be measured in a lab. Steve K. Lamoreaux was able to perform that measurement [source: Scientific American].
It should be noted that these waves between the mirrors are considered virtual waves, but if the mirrors move really fast then some of them can transform into real waves, which is called the "dynamical" Casimir effect. As it happens, scientists late in 2011 observed the dynamical Casimir Effect when they were able to get photons to go from being virtual to real photons, or real and measurable light. (The mirror they used wasn't your average bathroom mirror though -- instead it was something called a superconducting quantum interference device.) The key significance of the experiment was to deepen our knowledge about the virtual-to-real particle fluctuations that can happen in a vacuum. It is theorized that similar vacuum fluctuations may have something to do with dark energy, the unseen force powering the expansion of the universe [source: Science Daily].
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