Scientists have fired lasers at a plasma that appears to travel faster than the speed of light – without violating the laws of physics.
The speed of light is not always the same. Light traveling through plasma can appear to be moving at slower and faster speeds than what we call “the speed of light” – 299,792.458 meters per second – without violating any laws of nature. So did physicist Clement Guyon and colleagues from the Lawrence Livermore Laboratory in California get it done By pointing two lasers at each other in a beam of hydrogen and helium plasma.
One laser beam, the pump beam, was 100,000 times more powerful than the other, the probe beam. Each laser emits a short light pulse. At the place where the two pulses collided, it was confusion of light as a wave through the plasma.
Change this wave refractive index of plasma, a property that determines how light travels through a medium. By changing the wavelength of the light beams, the researchers were able to manipulate the refractive index. Then they measured how long it took a light pulse from the probe’s beam to travel to the camera.
When the probe beam had a wavelength longer than the pump beam, the light traveled 12 percent slower than the speed of light in a vacuum. When the situation was reversed, the light from the probe’s beam seemed to move slightly faster than the standard speed of light.
Because of the way the researchers measured light, the result does not violate the laws of physics. They tracked the brightest region of the pulse rather than the movement of the entire pulse or individual photon.
“We’re not violating Einstein’s principles because we’re not saying that information travels faster than the speed of light,” Guyon says. The pulse peak may move faster than the speed of light, but this is due to energy fluctuations along the beam, not the actual movement of the entire pulse. If you were to measure a single photon or particle of light, it would always be moving more slowly.
delicate or sensitive
Understanding how light is manipulated by plasma could open the door to the kinds of experiments that are now impossible because they require an extremely powerful laser. “When a large force hits an optical device such as a lens, it melts,” Guyon says.
Guyon argues that the plasma in this experiment actually behaves like conventional optics. This means that in the future it may be able to replace the micro-lenses and other devices that many physics experiments now use.