Limits on Lasers

Physicists are planning to create a laser so powerful that it will tear apart spacetime, well, it won't destroy spacetime, but it will tear particles out of the vacuum with dire consequences for the laser. I first made the statement that 'lasers will tear apart spacetime' when referring to future ambitious projects planned by the Extreme Light Infrastructure (ELI) when I was writing for Lindau Nature on 50 Years of Lasers. It is a bold claim, perhaps a colorful interpretation of the physics, but none the less recent experimental and theoretical work indicates that there is a fundamental limitation on the attainable intensity of lasers.

The vacuum that makes up spacetime is teeming with virtual particles that are inconsequential to low energy phenomena. Particles and their antiparticles, such as electrons and positrons (e^- and e⁺),  can be produced in pairs under certain conditions when energy is converted into matter. When enough energy is focused with laser pulses the peak electromagnetic field strength of the laser is enough to pair produce e^- e⁺ pairs which will cause an avalanche-like quantum electrodynamics (QED) cascade which will instantly disrupt the laser pulse.

A paper recently submitted to the arXiv (this paper hasn't been peer reviewed yet) by A Fedotov, N. Narozhny, G. Mourou, and G. Korn, Limitations on the attainable intensity of high power lasers, outlines how there is critical QED field strength that the authors state is unattainable and it is creeping up on experiments very fast. The idea that lasers could create particles or that there is limit in nature on the magnitude of the electromagnetic field is not new. Neils Bohr first suggested that a maximum field of E_s=2πm²c³/eh was physically unrealizable from theoretical considerations and the vacuum production e^- e⁺ pairs by a massive electromagnetic field was hypothesized in 1950 by J. Shwinger (who later received the Nobel prize for fundamental work in quantum electrodynamics). On the experimental front the limits to the laser was hinted at some time ago. In 1997 the Stanford Linear Accelerator (SLAC) collided what was then the worlds most powerful laser with electrons from the Stanford accelerator. The photons from the laser were boosted to produce backscattering gamma-ray photons which interacted with the oncoming laser beam. The energy of the laser and the gamma-ray photons was so high that real particles of matter and antimatter were created from the vacuum.

In this recent paper the authors argue that simultaneous pulses of lasers could reduce the maximum E_s field that may occur by two orders of magnitude to a mere ~10²⁵W/cm². The new analysis relies on the production of e^- e⁺  pairs at the Shwinger limit, but also takes into account the effect of secondary effects which the SLAC experiment did not have enough energy or speed of pulses to observe. Optimistically  the ELI project or the XFEL project could reach the maximum laser intensity within the decade. A super high power facility is planned by the ELI with intensities of ~10²⁹ W/cm² and the European XEFL, pictured above, will create extremely short and intense X-ray laser flashes they may also reach this limit by 2014.

The authors point out that the critical difference with future experiments and previous analysis of electromagnetic field strengths produced by lasers is that the most powerful lasers will play not only the role of the target, but will also be responsible for the acceleration of any new particles created. Thus at high laser intensities electron and positron pairs will be created and will immediately be accelerated to relativistic energies and emit hard photons, which will in turn produce new e^- e⁺ pairs. Thus a back-reaction, an avalanche of new particles, will develop from the vacuum by short focused laser pulses. The authors show that creation of even a single e^- e⁺ pair may result in complete destruction of the laser field.

This year is the 50th anniversary of the first successful laser built by Theodore Maiman and so it is rather fitting that we may have come full circle from the first laser to a theory of the ultimate laser. Yet, hurtles remain in the theory with respect to actually calculating the back-reaction of particles within the laser field (my hunch is that the particle avalanche may act to defocus some energy thus restoring the maximum E_s QED field to a an immense energy...) and in experiment with respect to actually building the ultimate laser.