http://www.nature.com/nsu/031013/031013-10.html
A team of US physicists has struck a blow for common sense: they have shown that an effect cannot precede its cause1.
The experiments confirm that recent work in which light appeared to travel faster than its speed in a vacuum does not conflict with the notion of causality, in which effects follow their causes.
Einstein’s theory of special relativity asserts that nothing can travel faster than the speed of light in a vacuum, conventionally denoted as c. If something were to outstrip c, this would lead to the possibility that an event could take place before the event that caused it. You could read these words before I’ve written them, for instance.
This troubling scenario seemed, at face value, to be implied by experiments carried out three years ago2,3, in which a light pulse was measured as travelling through a cold gas of metal atoms at a speed greater than c.
Information overload
At the time, most physicists did not greatly fear for relativity or causality. They argued that what really matters is how fast information can be sent. ‘Information’ here means any signal that can affect an object or system - a light pulse that can trigger a device, for example.
A light pulse is a bunch of light particles - photons - moving at a variety of speeds. The bunch can be assigned an overall group velocity.
Researchers argued that if some photons at the leading edge of a pulse travelled faster than c, such that the group velocity were greater than c, the pulse could not have any effect - it would not convey any information - until the bulk of the photons arrived some time later.
But what exactly is the speed of the information (denoted vi) in such a pulse? Physicists struggled to agree on an answer.
Some argued that information speed should be identified with the group velocity, so that it can exceed c, violating causality. Others said that vi must be less than or equal to c in all situations.
“Most people recognize the difference between the group velocity and the speed at which information can be propagated,” insists Arthur Dogariu, a physicist at the NEC Research Institute in Princeton, New Jersey.
Lene Hau of Harvard University in Cambridge, Massachusetts, who has also profoundly altered the speed of light in super-cold gases, agrees, adding: “The real question is: what defines information?”
The new study, by Daniel Gauthier of Duke University in Durham, North Carolina, and colleagues, provides an answer1. The researchers looked at how fast information can pass through a medium in which some photon speeds exceed c considerably.
Signal failure
Gauthier’s team points out that transmitting information isn’t just a matter of sending a signal. The signal has to be encoded at one end and read at the other. To convey information, a signal has to change - for example, light must get brighter.
The maximum speed of information transfer, the researchers say, corresponds to the earliest time at which this point of change can be detected
Detection takes a finite time, depending, for example, on the shape of the pulse and the amount of background noise. A detector must distinguish a change that denotes information from one that is due to random fluctuations in the signal, like radio static.
Gauthier’s team found that information encoded in a pulse travelling through a gas of potassium atoms takes longer to be detected than information in a pulse travelling through a vacuum at speed c. Even if the pulse’s group velocity far outstrips the speed of light, the information velocity can never exceed c.
In other words, the pulse arrives sooner but takes longer to announce its arrival.
now and probably most of the stuff went over my head… I must say it’s a very interesting read ..made me think 