90 million kilometers per second. Other than that, everything is normal
Tamara Traubman
According to physics textbooks, nothing can move faster than the speed of light in a vacuum - 300 thousand kilometers per second. Einstein's theory of relativity says that as soon as an object, a person or any information moves faster than the speed of light, all the laws of physics will be broken and a paradoxical situation will be created: a person will be able to go back in time, kill his father and thus prevent his birth with his own hands.
However, in a study published in the latest issue of the scientific journal "Nature", American researchers reported that they were able to make a pulse of light break the light speed barrier. This stunt, while confusing, does not violate any physical principle.
The researchers, Dr. Lijun Wang and his colleagues from the NEC Research Institute in Princeton sent a pulse of light into a glass chamber filled with gas of the chemical element cesium. A series of interactions between the two caused the light pulse to move at a speed 300 times greater than its speed in a vacuum.
Physicist Prof. Jacob Bekenstein from the Hebrew University says that the study "contradicted the popular, but erroneous, assertion that nothing can move faster than the speed of light." Most physicists today believe that a pulse of light - which is a group of massless light waves at different frequencies - can exceed the speed of light in a vacuum.
Wang agrees that his experiment does not usher in a new era of time travel. The method by which he and his colleagues accelerated the speed of the light pulse would also not be able to make a spaceship pass the speed of light, and a person would not be able to send information to his friends about a future accident.
Because in the experiment no information passed at a speed higher than the speed of light in a vacuum, none of the physical principles that explain the world as we know it were violated. A light pulse is, as mentioned, a cluster of light waves at different frequencies. In most cases, a light wave travels faster in a vacuum than in matter. The ratio between the speed of the wave in the material and its speed in a vacuum is called the "refractive index". The refractive index depends on the frequency of the wave and the chemical and physical structure of the mediating material. The higher the refractive index, the slower the wave travels through it. Normal glass, for example, has a refractive index of about 1.5. But if you add a little lead to it, it increases to 1.6.
To speed up the light pulse, the researchers needed a material with a low refractive index, so that the light waves would pass through it faster. They built a six-inch-long glass chamber and filled it with cesium gas. Cesium has a special property: in a certain frequency range, it causes the waves of different frequencies to move at very different speeds from each other.
The pulse broke when it passed through the cell, as happens when any light pulse passes through any medium. But contrary to what usually happens in nature, at the end of the cell all the wavelengths reunited, and a new light pulse was created, exactly similar to the pulse that entered the cell.
The experiment is one of a series of experiments in the research of optics, which show that certain light pulses can travel faster than the speed of light in a vacuum, when they are passed through special materials. According to Dr. John Marangos, of King's College London, who wrote a commentary article accompanying the study, physicists have been waiting a long time for the results of the new experiment. The previous experiments, he said, attracted less attention because the accelerated light pulses were distorted when leaving the chamber, "and therefore it was difficult to interpret their results."
{Appeared in Haaretz newspaper, 27/7/2000}
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