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Are virtual particles really constantly popping in and out of existence? Or are they merely a mathematical bookkeeping device for quantum mechanics?

Virtual particles are indeed real particles. Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. These predictions are very well understood and tested.
Quantum mechanics allows, and indeed requires, temporary violations of conservation of energy, so one particle can become a pair of heavier particles (the so-called virtual particles), which quickly rejoin into the original particle as if they had never been there. If that were all that occurred we would still be confident that it was a real effect because it is an intrinsic part of quantum mechanics, which is extremely well tested, and is a complete and tightly woven theory--if any part of it were wrong the whole structure would collapse.

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Any vibrating object should be slowed by quantum "virtual" particles that permeate empty space, according to theory. This weak "friction" should spew photons like sparks from a dangling bumper. In the 26 May PRL researchers propose detecting this effect by vibrating one end of a small reflective cavity in which the photons would rebound and be amplified by ultracold atoms. The experiment, which lies at the limit of technical feasibility, would expose a direct effect of virtual particles on moving objects.

Quantum field theory demands that seemingly empty space be filled with virtual photons, particles of light that constantly flit into and out of existence. One measurable effect of these particles is the so-called Casimir force between two objects placed nanometers apart. The weaker "dynamical" Casimir effect occurs when a small object vibrates rapidly: An ideal conducting surface, for example, has no electric field parallel to itself and no perpendicular magnetic field. Around it is the quantum vacuum, replete with electric and magnetic fields associated with virtual photons. As the surface moves back and forth, it imposes a regularly changing pattern of electric and magnetic fields--in other words, a photon is born. The surface gives up some of its vibrational energy in the process, damping its vibration.
The effect generates relatively few photons, says Roberto Onofrio of Dartmouth College in New Hampshire and the University of Padova in Italy, so "the only hope to detect them is in a resonant cavity, as they can 'pile up' together." Onofrio and coworkers imagine creating and trapping photons inside a reflective cavity by vibrating a thin film on one end like a drum. To make the experiment practical, the researchers had to identify a mechanical device that would produce photons suitable for amplification.
The highest reported frequency they could find for a thin film was 3 gigahertz, achieved by an aluminium nitride device. Because vibrations should tend to produce pairs of photons of equal energy, the device would emit 1.5 gigahertz photons, in the microwave spectrum.
Although the photons would be too low in energy to be detected in such small numbers, they could be amplified by an ultracold state of a cloud of atoms called a Bose-Einstein condensate or BEC, the authors point out. The 1.5 gigahertz photon energy would just match the difference between two energy levels in sodium atoms. To amplify the Casimir photons a sodium BEC would first be pumped up to the higher energy level with laser light. Then, struck by Casimir photons, the entire condensate would be triggered to fall back into the lower energy level en masse, emitting a burst of photons. This so-called superradiance--an effect already observed by others--would amplify the Casimir signal one billion times, the researchers calculate.

"The experiment proposed is indeed a marvelous idea. it pushes the experimental envelope in nanofabrication and optical detection" - Umar Mohideen of the University of California, Riverside, and a challenging one:

Carrying out the experiment is plausible, says Charles Sukenik of Old Dominion University in Norfolk, Virginia. His main concern would be the difficulty in maintaining a high-quality resonance cavity for the photons.

"If the experiment were successful, it would be another wonderful demonstration that the quantum mechanical vacuum is not just a convenient theoretical construct." - Charles Sukenik.

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