It is impossible to travel a the spped of light :
The infinite energy argument
When Einstein wrote down his postulates for special relativity, he did not include the statement that you cannot travel faster than light. There is a
misconception that it is possible to derive it as a consequence of the postulates he did give. Incidentally, it was Henri Poincare who said "Perhaps
we must construct a new mechanics, . . . in which the speed of light would become an impassable limit." That was in an address to the International
Congress of Arts and Science in 1904 before Einstein announced special relativity in 1905.
It is a consequence of relativity that the energy of a particle of rest mass m moving with speed v is given by
E = mc2/sqrt(1 - v2/c2)
As the speed approaches the speed of light the energy approaches infinity. Hence is should be impossible to accelerate an object with rest mass to
the speed of light and particles with zero rest mass must always go at exactly the speed of light otherwise they would have no energy. This is
sometimes called the "light speed barrier" but it is very different from the "sound speed barrier". As an aircraft approaches the speed of sound
it starts to feel pressure waves which indicate that it is getting close. With some more thrust it can pass through. As the light speed barrier is
approached (in a perfect vacuum) there is no such effect according to relativity. Moving at 0.99999c is just like standing still with everything
rushing past you at -0.99999c. Particles are routinely pushed to these speeds in accelerators so the theory is well established. Trying to get to
the speed of light in this way is like trying to get to the pot of gold at the end of a rainbow.
This explains why it is not possible to exceed the speed of light by ordinary mechanical means, however, it does not in itself rule out FTL travel.
It is really just one way in which things cannot be made to go faster than light rather than a proof that there is no way to do it. Particles are
known to decay instantly into other particles which fly off at high speed. It is not necessary to think in terms of the particles having been
accelerated so how could we say that they could not go faster than light? What about the possibility of particles which might have always been going
faster than light and which might be used to send information if they can be detected and deflected without ever slowing down to less than the speed
of light? Even if such particles don't exist there may be ways of moving matter from A to B, faster than light can get there by the usual root, but
without anything having to go at a FTL speed locally.
Or maybe it is possible?
Faster Than Light? Well …
PRINCETON, N.J. -- The dreams of warp drive and faster-than-light communications will have to remain just that. According to the scientists who have
demonstrated superluminal group velocity, the reports of the death of special relativity have been greatly exaggerated. The group has found that the
superluminal speeds it demonstrated in the lab can be explained with theories in accordance with relativity and are unlikely to apply to information
carried by light.
In its report in the July 20 issue of Nature, the team at NEC Research Institute Inc. describes how it used laser pumping to excite cesium gas in
a 6-cm-long, paraffin-coated glass cell to one of its 16 possible quantum mechanical states. A near-Gaussian, 3.7-µs probe pulse from a diode laser
was injected into the cell and detected at the far side with a high-sensitivity avalanche photodiode. The researchers found that the peak of the light
pulse emerged from the cell 62 ns earlier than it would have had it traveled the same distance through a vacuum. Effectively, it took negative time to
make the journey, showing a pulse advance 310 times the vacuum transit time.
They attribute the effect to interference of the frequency components of the pulse as it passes through the gas. The excited cesium gas is a
so-called anomalous dispersion medium that modifies light waves in such a way that, as shorter wavelengths enter the cell, they are elongated and
longer wavelengths are shortened. When a wave exits the medium, its original wavelength is restored. Considering a pulse as a number of waves in
phase, the effect of the gas is to bring the light's components back into phase ahead of where they would have been had they passed through a normal
dispersion region or a vacuum.
Lijun Wang, a research scientist at the institute and lead author of the report, said his team's rough calculations show that the phenomenon seen
in the smooth pulses does not apply to light conveying information. Citing Leon Brillouin's assertion that the speed of information should be
understood as the frontal velocity of a step function on a pulse, the researchers believe that it may accelerate information transfer speeds up to c,
but not beyond it. Wang said that his group, like that of Raymond Y. Chiao of the University of California in Berkeley in 1998, has demonstrated the
effect in an electronic system, where typical speeds are far less than c.
Because the superluminal effect can be understood in terms of the wave nature of light, Wang stressed that it is not at odds with special
relativity. "It is not our intention to claim that we have defeated Einstein's theory," he said. "Some members of the popular press have reported
that we are rebels tearing down relativity, but this is not the case. The results are consistent with relativity."
The team plans to perform a similar experiment, using a small number of photons to see what effects anomalous dispersion will have. "We suspect
that there is going to be additional noise," Wang said. "This will most likely reduce the signal-to-noise ratio, preventing a real signal being sent
faster than c
( source: Daniel S. Burgess / www.photonics.com...