Gemini laser shows that Einstein's flying mirror thought-experiment is the real deal

May 10th, 2013
Experimental results obtained using the Gemini laser in STFC's Central Laser Facility and published in Nature Communications are the first to demonstrate that laser light reflected from a ultrathin foil mirror moving close to the speed of light can be upshifted in energy, a demonstration of one of Einstein's special relativity concepts first published in his 'On the electrodynamics of moving bodies' 1905 paper.

The experiment explored the idea that the interaction of intense laser light with a surface of densely packed electrons moving close to the speed of light will lead to a momentum exchange between the 'mirror' and the incoming light wave. This momentum exchange can result in the peak power of the reflected light being substantially increased, via a combination of the pulse being compressed in time and the reflected wave being of shorter wavelength (and therefore higher energy).

The experimental conditions needed in order to observe this effect are extremely hard to achieve, requiring one super intense laser to ionise the target and accelerate a dense packet of electrons (the flying mirror) and then a second to arrive head-on to the electron packet within the few femtoseconds timeframe that the mirror exists, to reflect from the surface. Add to this the need to use a solid target that is only a few nanometres thick and a laser of sufficient intensity contrast quality and you've got the makings of a somewhat challenging experimental proposal.

However a collaboration between the Max-Planck-Institute of Quantum Optics in Garching, the Ludwig-Maximilians-Universität München, the Queens University Belfast and the Central Laser Facility (CLF) used the dual-beam capability of the Gemini laser along with ultrathin, 50 nm-thick foil targets to achieve these conditions. They observed a downshift of the laser beam wavelength from 800 nanometres down to ~60 nanometres, with the reflected pulse being compressed from 50 femtoseconds duration down to the order of a few hundred attoseconds (1 attosecond = 10-18 s).

This observation not only supports Einstein's theory of special relativity, but also paves the way for a new technique of generating intense, attosecond flashes of light, which are required in order to study the ultra-fast dynamics of electron motion and gain further understanding of fundamental physics on the atomic scale.

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