How fast are lasers

Lijun Wang, Dr. Alexander Kuzmich and Dr. Arthur Dogariu. In the experiment, NEC scientists measured the time taken by a pulse of light to pass through a 6cm-long specially prepared chamber containing cesium gas. The 3-microsecond long pulse of light would normally take only 0. In a vacuum all the phase velocities and the group velocity are the same.

In a dispersive medium, however, they are different because the refractive index is a function of wavelength, which means that the different wavelengths travel at different speeds.

Their experimental set-up is remarkably similar to that used to slow light to a speed of just 17 metres per second last year. It relies on using two lasers and a magnetic field to prepare a gas of caesium atoms in an excited state.

This state exhibits strong amplification or gain at two wavelengths, and highly anomalous dispersion — that is, the refractive index changes rapidly with wavelength — in the region between these two peaks. Wang and colleagues begin by using a third continuous-wave laser to confirm that there are two peaks in the gain spectrum and that the refractive index does indeed change rapidly with wavelength in between.

Next they send a 3. Moreover, unlike previous superluminal experiments, the input and output pulse shapes are essentially the same. There is no widespread agreement among physicists about the speed at which information is carried by pulses in such experiments.

Nothing can travel faster than the speed of light. Robert Nemiroff, a professor of physics at Michigan Technological University, doesn't dispute that fact. But he does have an idea for a scenario in which something would appear to travel faster than the speed of light to an observer. Appearances can be deceiving, but in this case, they may also have practical applications.

The basis of this faster-than-light scenario is fairly complicated, but Nemiroff explained it in a few short thought experiments during a Jan. For example, imagine a room with a ceiling 50 feet high and walls 50 feet wide 15 by 15 meters.

Suppose you lie on your back in the middle of the room with a laser pointer, which you shine upward so you can see the tiny dot on the ceiling. Now you move the laser pointer from left to right across the ceiling. To do this, you only need to move your hand a few inches — but in the short time it takes you to do this, the point of light on the ceiling travels 50 feet.

Now, expand this scenario to a much bigger scale. Imagine that the room were many miles tall and wide. Imagine the laser pointer were a much more powerful beam of light. The point of light projected onto the ceiling could effectively move at hundreds of miles per hour. Out of the thousands of photons bombarding the moon, few will return to Earth and be captured by the telescope.

A round trip journey of roughly , km that takes 2. While traveling through the vacuum of space, laser beams are invisible unless shot directly into your eye. The vacuum of space does not have anything to reflect the light back into your eye. Only by adding air, dust, or debris does a light beam become visible from the side.