The speed of light may not necessarily be constant. Light travelling through a plasma can appear to move at speeds both slower and faster than what we refer to as 鈥渢he speed of light鈥� 鈥� 299,792,458 metres per second 鈥�聽without breaking any laws of physics.
at Lawrence Livermore National Laboratory in California and his colleagues accomplished this using a pair of lasers fired into a jet of hydrogen and helium plasma. One laser, called the pump beam, was 100,000 times more powerful than the other, called the probe beam. Each laser released a short pulse of light, and at the spot where the two pulses collided, the interference in the light created a wave throughout the plasma.
This wave altered the plasma鈥檚 refractive index, a property that governs how light travels through a medium. By changing the wavelengths of the beams, the researchers modulated the refractive index. The researchers then measured how long a light pulse from the probe beam took to travel to a camera.
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When the probe beam had a longer wavelength than the pump beam, its light moved slower than the speed of light in a vacuum by about 12 per cent. When the opposite was true, the probe beam鈥檚 light appeared to move slightly faster than the canonical speed of light.
Because of the way the researchers measured the light, this doesn鈥檛 violate any of the laws of physics. They tracked the brightest area of a pulse rather than the movement of the entire pulse or any individual photon.
鈥淲e are not violating Einstein鈥檚 principles because we are not saying information travels faster than the speed of light,鈥� says Goyon. The peak of the pulse may move faster than the speed of light, but that is due to energy fluctuations along the length of the beam rather than the entire pulse鈥檚 actual movement 鈥� if you were to measure any individual photon, or particle of light, it would always move slower.
Understanding how to manipulate light with a plasma could open the door to types of experiments that are impossible now because they require extremely powerful lasers.
鈥淲hen you have a lot of power reaching an optic device like a lens, you鈥檙e just going to melt it,鈥� says Goyon. He says the plasma in this experiment actually behaves a lot like traditional optics, which means that, in the future, it may be able to replace the delicate lenses and other devices on which many physics experiments rely.
Physical Review Letters
