Most of us grow up familiar with the prevailing law that limits how quickly information can travel through empty space: the speed of light, which tops out at 300,000 kilometers (186,000 miles) per second.
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While photons themselves are unlikely to ever break this
speed limit, there are features of light which don't play by the same rules.
Manipulating them won't hasten our ability to travel to the
stars, but they could help us clear the way to a whole new class of laser
technology.
Physicists in the US have shown that, under certain
conditions, waves made up of groups of photons can move faster than light.
Researchers have been playing hard and fast with the speed
limit of light pulses for a while, speeding them up and even slowing them to a
virtual stand-still using various materials like cold atomic gases, refractive
crystals, and optical fibers.
But impressively, last year, researchers from Lawrence
Livermore National Laboratory in California and the University of Rochester in
New York managed it inside hot swarms of charged particles, fine-tuning the
speed of light waves within plasma to anywhere from around one-tenth of light's
usual vacuum speed to more than 30 percent faster.
This is both more – and less – impressive than it sounds.
To break the hearts of those hoping it'll fly us to Proxima
Centauri and back in time for tea, this superluminal travel is well within the
laws of physics. Sorry.
A photon's speed is locked in place by the weave of
electrical and magnetic fields referred to as electromagnetism. There's no
getting around that, but pulses of photons within narrow frequencies also
jostle in ways that create regular waves.
The rhythmic rise and fall of whole groups of light waves
moves through stuff at a rate described as group velocity, and it's this 'wave
of waves' that can be tweaked to slow down or speed up, depending on the
electromagnetic conditions of its surrounds.
By stripping electrons away from a stream of hydrogen and
helium ions with a laser, the researchers were able to change the group
velocity of light pulses sent through them by a second light source, putting
the brakes on or streamlining them by adjusting the gas's ratio and forcing the
pulse's features to change shape.
The overall effect was due to refraction from the plasma's
fields and the polarized light from the primary laser used to strip them down.
The individual light waves still zoomed along at their usual pace, even as
their collective dance appeared to accelerate.
From a theoretical standing, the experiment helps flesh out
the physics of plasmas and put new constraints on the accuracy of current
models.
Practically speaking, 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.
Lasers would be the big winners here, especially the
insanely powerful variety. Old-school lasers rely on solid-state optical
materials, which tend to get damaged as the energy cranks up. Using streams of
plasma to amplify or change light characteristics would get around this issue,
but to make the most of it we really need to model their electromagnetic
characteristics.
It's no coincidence that Lawrence Livermore National
Laboratory is keen to understand the optical nature of plasmas, being home to
some of the world's most impressive laser technology.
Ever more powerful lasers are just what we need for a whole
bunch of applications, from ramping up particle accelerators to improving clean
fusion technology.
It might not help us move through space any faster, but it's
these very discoveries that will hasten us towards the kind of future we all
dream of.
Reference: Physical Reviews Letter
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