The speed of light may be manipulated depending on the medium that it is transmitted through.
- The rate of Light is 3 x 10*6 m/s. The light’s speed in air is 3×10*8 m/s and the light’s in common glass is 2*10^8 m/s.
- That is precisely 3,00000 km per second in a vacuum.
The speed of light is decreased when travels through transparent media such as air, water, glass etc. The ratio by which the speed of light is decreased is called the refractive index of the medium and is always greater than one.
The light travels at maximum velocity in a vacuum. The vacuum has less refractive index than Air. Air has a refractive index of 1.003.
The speed of light is actually higher when light passes through cesium.
These results are not brand new, but have been examined since 2000 or earlier.
In the view of General Relativity light travels on a geodesic; there should be no surprise if some geodesics are shorter than others. There can be more geodesics that connect two points. But the speed of light will still be c during the time of the travel, only that the road will be shorter. This is if truly we talk about shorter geodesics and not about the effect that
Also, "In 1967, acting on Einstein's proof that the speed of light is the most constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre. Since then, caesium has been widely used in highly accurate atomic clocks."
In Einstein's view the geodesic is not just the result of gravity but of the non-symmetric field, the unified field of gravity and of electromagnetism.
The speed of light in a gas is very close to, but slightly less than, the speed of light in a vacuum. Depending on how you define "speed" (group velocity, phase velocity,...), it is possible to get materials in which the speed of light is greater than in vacuum. However, there is no way that would happen in a gas made out of neutral atoms. None of this has any implications for relativity.
Constancy of c in relativity is the constancy of the speed of light in vacuum. Actually, it would have consequences for relativity if a signal could propagate at >c in a medium. There would then be frames of reference in which the signal was received before it was transmitted, leading to problems with causality. But cases like this one don't actually allow signals to propagate at >c. This is why it's important to make distinctions like group velocity versus phase velocity.
1 Let be the angular velocity and k the wavenumber , then the phase velocity Vf is
The group velocity is the velocity with which energy propagates and is defined by
Vg= δπ/δk (2)
2. The case of light
If a wave satisfies the dispersion relation then:
π(k)= k2c2 +const (3)
Expression (3) is exactly the case for light. From (1), (2) and (3) we obtain
In (4) Vf or Vg can be greater than c or even negative. Though Vf or Vg may exceed c no energy or information actually travels faster than c. Experiments showing group velocities greater than c include that of Wang et al. , who produced a laser pulse in atomic cesium gas with Vg between (-310+5)c and (-310-5)c.
Vf is in the range –c/305 to –c/315.
, in simple words, can be described as a mere disturbance along any plane surface. For instance, consider an ocean, where the Waves move along the ocean surface from one place to another. The crest of a Wave moves at intervals and therefore travels a specific distance over a specific time period. In this chapter, you are dealing with Phase and Group Velocity
, which are properties of Waves. When a Wave moves in Groups, the speed or Velocity at which it moves is known as the Group Velocity.
What is Group Velocity?
The Group Velocity of a Wave is defined as the Velocity at which an entire envelope of Waves moves through a medium. A most common Example, in this case, can be that of throwing stones in a water body which causes multiple Waves on the surface of water.
On throwing the stone, a ripple is created around the point where the stone drops. The ripple is formed of small Wavelets which propagate away from the dropping point in multiple directions. Here, a Wavelet having the shortest Wavelength propagates faster than others.
However, to understand what Group Velocity is, you should also have an idea of simple harmonic motion too. This will help you in understanding the concepts better and in a more natural way.
What is Phase Velocity?
Considering the fact that a Wave consists of two significant parts crest and trough, its Phase Velocity is also dependent on the same.
It is the Velocity at which a specific component of a Wave, say crest, propagates in space.
This feature or Velocity is directly dependent on the time period and Wavelength. Alongside, you should also be clear about the relation between Phase Velocity and Group Velocity.
- Feynman, Leighton and Sands (vol1) Addison-Wesley
- Wang, L. J.; Kuzmich, A.; and Dogariu, A. "Gain-Assisted Superluminal Light
Propagation." Nature 406, 277-279, 2000.
Not everyone is convinced the NEC scientists did what they claim.
Aephraim Steinberg, a physicist at the University of Toronto, said the light particles coming out of the cesium chamber may not have been the same ones that entered, so he questions whether the speed of light was broken. Still, the work is important, he said: ''The interesting thing is how did they manage to produce light that looks exactly like something that didn't get there yet?'' The achievement has no practical application right now but experiments like this have generated considerable excitement in the small international community of theoretical and optical physicists.
''This is a breakthrough in the sense that people have thought that was impossible,'' said Raymond Chiao, a physicist at the University of California at Berkeley who was not involved in the work. Chiao has performed similar experiments using electric fields. In the latest experiment, researchers at NEC developed a device that fired a laser pulse into a glass chamber filled with a vapour of cesium atoms. The researchers said the device is sort of a light amplifier that can push the pulse ahead.
Previously, experiments have been done in which light also appeared to achieve such so-called superluminal speeds but the light was distorted, raising doubts as to whether scientists had really accomplished such a feat. The laser pulse in the NEC experiment exits the chamber with almost exactly the same shape but with less intensity, Wang said. The pulse may look like a straight beam but actually behaves like waves of light particles. The light can leave the chamber before it has finished entering because the cesium atoms trade energy with the leading edge of the waves as they pass through. This produces an almost identical light pulse that exits the chamber and travels about 18 metres before the main part of the laser pulse finishes entering the chamber, Wang said.
Wang said the effect is possible only because light has no mass; the same thing cannot be done with physical objects. The Princeton experiment and others like it test the limits of the theory of relativity Albert Einstein developed nearly a century ago. The special theory of relativity states the speed of particles of light in a vacuum, such as outer space, is the only absolute measurement in the universe.
Ultimately, the work may contribute to the development of faster computers that carry information in light particles.
The speed of everything else - rockets or inchworms - is relative to the observer, Einstein and others explained. In everyday circumstances, an object cannot travel faster than light. The Princeton experiment and others change these circumstances by using devices such as the cesium chamber, rather than a vacuum.