Magical Realist
Valued Senior Member
Yes..but only a slight one.
"Physicists today claim that they reached 300 times the speed of light. But don't write off Einstein, and don't hold your breath for a time-travelling Star Trek universe, warns Paul Davies
On the face of it, today's announcement in Nature that a team of Princeton physicists have broken the light barrier demolishes what is arguably science's most cherished principle.
Ever since Albert Einstein formulated his theory of relativity nearly a century ago, it has been a central tenet of physics that nothing can travel faster than light. Now it is claimed that in certain circumstances, light itself can be accelerated up to 300 times its usual speed. But it's too soon to consign the textbooks to the dustbin. As always, the devil is in the detail.
Moving through a vacuum, light travels at 300,000 km per second. According to the theory of relativity, it is the ultimate speed limit for the propagation of any physical influence. That includes spacecraft, subatomic particles, radio signals, or anything that might convey information or cause an effect.
When light passes through a medium such as air, it is slowed. The effect is best explained by analogy with water waves. Try throwing a stone in a pond to make ripples. Focus on a particular wave crest, and it will appear to move fairly fast, but then take a wider perspective to view the group of waves as a whole, and it travels outwards from the point of disturbance noticeably more slowly. It is almost as if the waves are rushing to get nowhere fast. You can watch as new ripples rise up at the back of the wave group, whiz forwards, and fade away at the front.
The same thing happens to light in a medium. It comes about because atoms in the medium create outgoing ripples of light as the primary light wave sweeps by them. When these ripples overlap and combine with the primary wave, they obliterate the parts racing on ahead, suppressing the fast-moving wave front and serving to slow down the group. So light passing through a medium has two associated velocities: that of the group as a whole, and that of the individual wave crests, known as the phase velocity.
A normal medium always reduces the group velocity of light to below its phase velocity, leading to the familiar phenomenon of refraction - the effect that causes a stick to look bent when it is stuck in water. The special feature of the Princeton experiment was the creation of a peculiar state of matter in which this situation is reversed: the secondary ripples of light actually make the wave group travel faster than the phase velocity.
To achieve this odd state of affairs, the scientists used a gas of cold caesium, and then excited the caesium atoms with a laser. So energised, the atoms do more than cause secondary ripples of light, they amplify the light too. It is this amplification that is the key to boosting the speed of the wave group, reportedly to 300 times the speed of light in a vacuum. Bizarrely, the wave distortion achieved is so large, it causes the group velocity to become negative, which means the peak of the wave pulse appears to exit the gas before it enters. In other words, the light waves seem to run backwards.
What makes this result so sensational is the relationship between light speed and causality. The theory of relativity predicts that speed slows time. For example, time passes a bit slower in an aircraft than on the ground, an effect that has been verified using atomic clocks. The time warp is small for everyday motion, but grows without limit as the speed of light is approached. Cosmic rays, for example, travel exceedingly close to the speed of light, and their internal clocks are slowed millions of times.
Relativity theory predicts that if a particle could exceed the speed of light, the time warp would become negative, and the particle could then travel backwards in time.
As Dr Who fans are aware, travel into the past opens up a nest of paradoxes. For example, suppose a faster-than-light particle is used as a signal to explode a bomb in the very lab that the particle itself is created. If the bomb explodes yesterday, the particle cannot be made today. So the bomb won't explode, and the particle will be made.
Either way, you get contradictory nonsense. At stake, then, is the very rationality and causal order of the universe. Allow faster-than-light travel, and the physical world turns into a madhouse .
Timing the speed of a pulse of light is fraught with complications, not least because the shape of the pulse changes when it passes through a medium. To make a pulse of a short duration, it is necessary to mix together waves of many different frequencies, and in a medium each wave will propagate differently.
As for transmitting information, opinions differ about how to associate it with a pulse that has a complicated, changing shape. The inherent fuzziness in a light pulse made up of many different waves superimposed precludes a clean definition of how fast actual information travels.
The problem is closely related to the quantum nature of light, where each frequency component can be thought of as made up of pho tons that behave in some ways like particles. But photons are subject to Heisenberg's principle, according to which there is an inherent uncertainty in their whereabouts. In the pulses of light used in the experiment, it isn't possible to pick out a given component photon and observe it travelling at superluminal velocity.
The Princeton physicists believe this fundamental fuzziness associated with a finite pulse of waves prevents information from exceeding the speed of light, so in an operational sense the light barrier remains unbroken and the causal order of the cosmos is still safe. It is intriguing to see how the wave nature of light rescues the theory of relativity from paradox."===
Light goes backwards in time | Science | The Guardian
http://www.guardian.co.uk/science/2000/jul/20/technology2
