However, Dr Gunter Nimtz and Dr Alfons Stahlhofen, of the University of Koblenz, say they may have breached a key tenet of that theory.
The pair say they have conducted an experiment in which microwave photons - energetic packets of light - travelled "instantaneously" between a pair of prisms that had been moved up to 3ft apart.
Being able to travel faster than the speed of light would lead to a wide variety of bizarre consequences.For instance, an astronaut moving faster than it would theoretically arrive at a destination before leaving.
The scientists were investigating a phenomenon called quantum tunnelling, which allows sub-atomic particles to break apparently unbreakable laws.
Dr Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of."
But scientists claim to have demonstrated there is the possibility of travel faster than the speed of light.
The feat contradicts one of the key tenets of Albert Einstein's special theory of relativity - that nothing, under any circumstances, can move faster than 300,000km a second, or the speed of light.
Travelling faster than light also, in theory, turns back time.
According to conventional physics, a person moving beyond light speed would arrive at his destination before leaving.
But two German physicists claim to have forced light to overcome its own speed limit using the phenomenon of quantum tunnelling.
Their experiments focused on the travel of microwave photons - energetic packets of light - through two prisms.
When the prisms were moved apart, most photons reflected off the first prism they encountered and were picked up by a detector.
But a few appeared to "tunnel" through a gap separating them as if the prisms were still held together.
Although these photons had travelled a longer distance, they arrived
at their detector at the same time as the reflected photons.
This suggests the transit between the two prisms was faster than the speed of light.
Dr Gunter Nimtz, of the University of Koblenz, told the magazine New Scientist: "For the time being, this is the only violation of special relativity that I know of."
The Daily Mail, in The Courier-Mail
Electric signals can be transmitted at least four times faster than the speed of light using only basic equipment that would be found in virtually any college science department.
Scientists have sent light signals at faster-than-light speeds over the distances of a few metres for the last two decades - but only with the aid of complicated, expensive equipment. Now physicists at Middle Tennessee State University have broken that speed limit over distances of nearly 120 metres, using off-the-shelf equipment costing just $500.
Jeremy Munday and Bill Robertson made a 120-metre-long cable by alternating six- to eight-metre-long lengths of two different kinds of coaxial cable, each with a different electrical impedance. They hooked this hybrid cable up to two signal generators, one of which broadcast a fast wave, the other a slow one. The waves interfere with each other to produce electric pulses, which can be watched using an oscilloscope.
Any pulse, whether electrical, light or sound, can be imagined as a group of tiny intermingled waves. The energy of this "group pulse" rises and falls over space, with a peak in the middle. The different electrical resistances in the hybrid cable cause the waves in the pulse's rear to reflect off each other, accelerating the pulse's peak forward.
By using the oscilloscope to trace the pulse's strength and speed, the researchers confirmed they sent the signal's peak tunnelling through the cable at more than four billion kilometres per hour.
"It really is basement science," Robertson said. The apparatus is so simple that Robertson once assembled the setup from scratch in 40 minutes.
While the peak moves faster than light speed, the total energy of the pulse does not. This means Einstein's relativity is preserved, so do not expect super-fast starships or time machines anytime soon.
Signals also get weaker and more distorted the faster they go, so in theory no useful information can get transmitted at faster-than-light speeds, though Robertson hopes his students and others can now rigorously and cheaply test those ideas.
Physicist Alain Hache at the University of Moncton in Canada adds that it may be possible to use this reflection technique to boost electrical signal speeds in computers and telecommunications grids by more than 50 per cent.
Electrical signals usually travel at about two-thirds of light speed in wires. Hache says it may be possible to send unsable electrical signals to near light speed.
"Briefly, tachyons are theoretically postulated particles that travel faster
than light and have 'imaginary' masses.
|[Editor's note: imaginary mass is a
bizarre theoretical concept that comes from taking the square root of a negative
number; in this case, it roughly means that a particle's mass is only physically
meaningful at speeds greater than light.]|
"The name 'tachyon' (from the Greek 'tachys,' meaning swift) was coined by the late Gerald Feinberg of Columbia University. Tachyons have never been found in experiments as real particles traveling through the vacuum, but we predict theoretically that tachyon-like objects exist as faster-than-light 'quasiparticles' moving through laser-like media. (That is, they exist as particle-like excitations, similar to other quasiparticles called phonons and polaritons that are found in solids. 'Laser-like media' is a technical term referring to those media that have inverted atomic populations, the conditions prevailing inside a laser.)
"We are beginning an experiment at Berkeley to detect tachyon-like quasiparticles. There are strong scientific reasons to believe that such quasiparticles really exist, because Maxwell's equations, when coupled to inverted atomic media, lead inexorably to tachyon-like solutions.
"Quantum optical effects can produce a different kind of 'faster than light' effect (see "Faster than light?" by R. Y. Chiao, P. G. Kwiat, and A. M. Steinberg in Scientific American, August 1993). There are actually two different kinds of 'faster-than-light' effects that we have found in quantum optics experiments. (The tachyon-like quasiparticle in inverted media described above is yet a third kind of faster-than-light effect.)
"First, we have discovered that photons which tunnel through a quantum barrier can apparently travel faster than light (see "Measurement of the Single-Photon Tunneling Time" by A. M. Steinberg, P. G. Kwiat, and R. Y. Chiao, Physical Review Letters, Vol. 71, page 708; 1993). Because of the uncertainty principle, the photon has a small but very real chance of appearing suddenly on the far side of the barrier, through a quantum effect (the 'tunnel effect') which would seem impossible according to classical physics. The tunnel effect is so fast that it seems to occur faster than light.
|"Second, we have found an effect related to the famous Einstein-Podolsky-Rosen
phenomenon, in which two distantly separated photons can apparently influence one
anothers' behaviors at two distantly separated detectors (see "High-Visibility
Interference in a Bell-Inequality Experiment for Energy and Time," by P. G.
Kwiat, A. M. Steinberg, and R. Y. Chiao, Physical Review A, Vol. 47, page
R2472; 1993). This effect was first predicted theoretically by Prof. J. D.
Franson of Johns Hopkins University. We have found experimentally that twin
photons emitted from a common source (a down-conversion crystal) behave in a
correlated fashion when they arrive at two distant interferometers. This
phenomenon can be described as a 'faster-than-light influence' of one photon upon
its twin. Because of the intrinsic randomness of quantum phenomena, however, one
cannot control whether a given photon tunnels or not, nor can one control whether
a given photon is transmitted or not at the final beam splitter. Hence it is
impossible to send true signals in faster-than-light communications.
"I refer interested readers to our paper 'Tachyonlike Excitations in Inverted Two-Level Media' by R. Y. Chiao, A. E. Kozhekin, and G. Kurizki, Physical Review Letters, Vol. 77, page 1254; 1996, and references therein.