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May the force be with us? Precise measurements test quantum electrodynamics, constrain possible fifth fundamental force

Posted on June 6, 2013
A schematic layout of the experimental setup. The oscillator cavity is seeded by a cw Ti:Sa laser, the pulsed output of which makes multiple passes in an amplifier stage. The amplified output is frequency up-converted in two frequency doubling stages leading to fourth harmonic generation of ~211 nm. The deep UV radiation is sent to the experiment, where molecules in the X1 Σg+ ν´ = 1 state, populated by electrical discharge, are optically excited in a two-photon Doppler-free configuration . The cw-seed light is compared to a frequency comb while the frequency offset between pulsed and cw-seed light is measured via on-line chirp analysis to obtain an absolute frequency calibration. See text for further details. SHG: second harmonic generation; PMT: photomultiplier tube; and YAG: yttrium-aluminum garnet. Reproduced with permission from G. D. Dickenson et al, Phys. Rev. Lett. 110, 193601 (2013)

A schematic layout of the experimental setup. The oscillator cavity is seeded by a cw Ti:Sa laser, the pulsed output of which makes multiple passes in an amplifier stage. The amplified output is frequency up-converted in two frequency doubling stages leading to fourth harmonic generation of ~211 nm. The deep UV radiation is sent to the experiment, where molecules in the X1 Σg+ ν´ = 1 state, populated by electrical discharge, are optically excited in a two-photon Doppler-free configuration . The cw-seed light is compared to a frequency comb while the frequency offset between pulsed and cw-seed light is measured via on-line chirp analysis to obtain an absolute frequency calibration. See text for further details. SHG: second harmonic generation; PMT: photomultiplier tube; and YAG: yttrium-aluminum garnet. Reproduced with permission from G. D. Dickenson et al, Phys. Rev. Lett. 110, 193601 (2013)

Quantum electrodynamics (QED) – the relativistic quantum field theory of electrodynamics – describes how light and matter interact – achieves full agreement between quantum mechanics and special relativity. (QED can also be described as a perturbation theory of the electromagnetic quantum vacuum.) QED solves the problem of infinities associated with charged pointlike particles and, perhaps more importantly, includes the effects of spontaneous particle-antiparticle generation from the vacuum. Recently, scientists at VU University, The Netherlands, published two papers in quick succession that, respectively, tested QED to extreme precision by comparing values for the electromagnetic coupling constant1, and applied these measurements to obtain accurate results from frequency measurements on neutral hydrogen molecules that can be interpreted in terms of constraints on possible fifth-force interactions beyond the Standard Model of physics2. In addition, the researchers point out that while the Standard Model explains physical phenomena observed at the microscopic scale, so-called dark matter and dark energy at the cosmological scale are considered as unsolved problems that hints at physics beyond the Standard Model.

Prof. Wim Ubachs discussed the research he and his colleagues (at University of San Carlos, Philippines; Mickiewicz University, Poland; and University of Warsaw, Poland) undertook, citing some of the challenges they faced, in a conversation with Phys.org. “The challenges in testing QED to extreme precision by comparing values for the electromagnetic coupling constant are twofold,” Ubachs says. “Using lasers, we measured transition frequencies as accurately as possible. These measurements, in turn, had to be compared with calculations, which also had to be performed at the highest accuracy levels, involving many steps: First, solving the Schrodinger equation for the H2 molecule, and secondly calculating the relativistic corrections and the terms associated with quantum electrodynamics.” The latter, he notes, involves calculating the interaction of the particles with the quantum vacuum – that is, with the spontaneously generated particles from the void.

Read more at: Phys.org

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