There is no limit on the value of a proper speed as a proper speed does not represent a speed measured in a single inertial frame. A light signal that left the Earth at the same time as the traveller would always get to the destination before the traveller. Possible distance away from Earth[ edit ] Main article: Space travel using constant acceleration Since one might not travel faster than light, one might conclude that a human can never travel further from the Earth than 40 light-years if the traveler is active between the age of 20 and A traveler would then never be able to reach more than the very few star systems which exist within the limit of 20—40 light-years from the Earth. This is a mistaken conclusion: because of time dilation , the traveler can travel thousands of light-years during their 40 active years.
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For generations, physicists believed there is nothing faster than light moving through a vacuum - a speed of , miles per second. But in an experiment in Princeton, N.
The pulse traveled times the distance it would have covered if the chamber had contained a vacuum. But the findings--the long-awaited first clear evidence of faster-than-light motion--are "not at odds with Einstein," said Lijun Wang, who with colleagues at the NEC Research Institute in Princeton, N.
But physicists now believe that a pulse of light--which is a group of massless individual waves--can. As a general rule, light travels more slowly in any medium more dense than a vacuum which, by definition, has no density at all. The ratio between the speed of light in a vacuum and its speed in a material is called the refractive index.
The index can be changed slightly by altering the chemical or physical structure of the medium. Ordinary glass has a refractive index around 1. But by adding a bit of lead, it rises to 1. The slower speed, and greater bending, of light waves accounts for the more sprightly sparkle of lead crystal glass. The NEC researchers achieved the opposite effect, creating a gaseous medium that, when manipulated with lasers, exhibits a sudden and precipitous drop in refractive index, Wang said, speeding up the passage of a pulse of light.
The team used a 2. They then trained several laser beams on the atoms, putting them in a stable but highly unnatural state. In that condition, a pulse of light or "wave packet" a cluster made up of many separate interconnected waves of different frequencies is drastically reconfigured as it passes through the vapor. Some of the component waves are stretched out, others compressed. Yet at the end of the chamber, they recombine and reinforce one another to form exactly the same shape as the original pulse, Wang said.
That is, the peak of the pulse is, in effect, extended forward in time. As a result, detectors attached to the beginning and end of the vapor chamber show that the peak of the exiting pulse leaves the chamber about 62 billionths of a second before the peak of the initial pulse finishes going in.
That is not the way things usually work. This happens because each frequency moves at a different speed in glass, smearing out the original light beam. Blue is slowed the most, and thus deflected the farthest; red travels fastest and is bent the least. That phenomenon produces the familiar rainbow spectrum. It bends red more than blue in a process called "anomalous dispersion," causing an unusual reshuffling of the relationships among the various component light waves.
Gain-assisted superluminal light propagation