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The 30 January 2013 Physics World article Neutrons on a Lab Bench says:

Yin and Albright calculated that a very intense laser beam should be able to boost the speed of electrons in a plasma to such an extent that their increased relativistic mass significantly reduces the electrons' frequency to below that of their infrared laser.

Huh? They are increasing both the speed and the mass of particles, yet their frequency goes down?

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"electrons' frequency" does sound ambiguous. It should be the plasma frequency. For example explained in here:

Testing one of those promising mechanisms became possible by operating in the relativistically induced transparency (RIT) regime. RIT comes about in a laser-heated electron plasma when, because of the increased relativistic electron rest mass, the plasma frequency decreases below the laser frequency, so that there is no longer a critical surface to reflect the light.

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    $\begingroup$ If this is what is intended by the original source, then Kurt would be well advised to go back through the text asking "Did the author actually tell me that and I just missed it?". This kind of things will help you to improve your skills in reading trechnical manuscripts. Alternately, of course, they serve as an examples of ambiguous technical writing, and Kurt can aim to avoid doing the same to his future readers. $\endgroup$
    – dmckee --- ex-moderator kitten
    Oct 3, 2019 at 0:17
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This is not really increasing the mass but changing the Lorentz factor, which in turn changes the cyclotron frequency. The plasma frequency is a Lorentz invariant. Perhaps a better way to state things is that they are increasing the energy of the particles but it's not incredibly important for this.

I have always found it troubling to think of the mass changing as it causes problems. For instance, if I move extremely close to the speed of light this will not cause a nearby planet I happen to pass to suddenly collapse into a black hole. The inference here would be in your rest frame, the planet is moving incredibly fast. In your rest frame it is moving incredibly fast, but that does not mean the planet's mass suddenly exceeds the critical mass for core collapse. It just means its momentum and energy are increased by the Lorentz factor.

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