• Yahya Ashraf

Researchers use pulses inside hot plasma to break the speed barrier!

A plasma ball
A plasma ball

We all are aware of the fact that photons are the fastest substance in the world. They can travel at a whopping speed of 300 thousand kilometers per second, setting a speed limit to our universe.

Even though this fundamental rule of light can never be broken down, we can tweak with this rule under some conditions. By influencing some features of light, we can actually break down the speed limit of light. But this does not mean interstellar travel is possible although it indeed can pave the path to a new class of laser technology.

Physicists have been experimenting with the speed limit of light pulses, speeding them up and even slowing them to a virtual stand-still using various materials like cold atomic gases, refractive crystals, and optical fibers.

Now scientists have broken the speed limit of light through pulses sent past hot plasma. Researchers from Lawrence Livermore National Laboratory in California and the University of Rochester in New York used plasma to put the brakes on light waves and even speed them up, up to thirty percent!

Now before you doubt Physical laws, note that this superluminal travel is well within the laws of Physics.

The velocity at which pulses of light pass through a material can strongly differ from the speed c that light travels in a vacuum. This velocity, called the group velocity, can be both higher or lower than c, and it factors in how the shape of a light pulse spreads and distorts as it moves through a material.

For their demonstration, Goyon and colleagues first created a hydrogen-helium plasma by ionizing a jet of the gas with a polarized laser beam. They then aimed a second laser beam at the plasma. Where the paths of the two beams crossed, the horizontal component of the second laser pulse slowed in response to a change in the plasma’s refractive index. This slowing came from interactions between the two lasers and the plasma.

Measuring the time delay between the horizontal and vertical components of the second laser pulse, the team observed they had different speeds. By tuning the frequency difference between the two beams, they found they could tune that speed from 0.995c to between 0.12c and − 0.34c, showing that the peak of the pulse traveled faster than c.

Practically, this is good news for advanced technologies waiting in the wings for clues on how to get around obstacles preventing them from being turned into reality.

Even though this discovery won’t let us embark on interstellar travel but it indeed can pave the way to a new band of lasers. Using streams of plasma to alter the features of light, we could prevent the solid-state optical material of some lasers from getting damaged at higher levels.

Once again, do note, group and phase speeds arise from the quantum wave model. Not just for photons, all particles. The group speed of the wave represents particle speed; phase speed is considered physically irrelevant. Because there's no observed aspect to coincide with it. In the simplest case, they are related by u*v=c^2 (u group speed, v phase speed). Guess what happens when a particle is slower than light? Yes v > c, even for massive particles. The rest is about the changed speed of light in a material medium (lower than c). All of these are known. The novelty is about plasma used as such medium and measurement of these speeds. No physical law is broken.

It might not help us move through space any faster, but it's these very discoveries that will hasten us towards the future we all dream of.

The findings were published in Physical Review Letters.

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