How fast do lasers travel

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.

Isn't it possible that, in a sufficiently large room, the point of light could appear to move faster than the speed of light? Nemiroff said this is possible, and no laws of physics would be broken. Einstein's theory of special relativity says it is impossible for an object like a single photon of light to look as though it is traveling faster than the speed of light. Nemiroff's experiment doesn't break this physical law because the laser pointer produces a stream of photons, not a single photon.

No single photon in the stream ever moves faster than the speed of light, but collectively, they can create the illusion of faster-than-light travel. Another way to understand this illusion is to imagine an electronic ticker tape, which creates the illusion of a word moving horizontally across the screen.

In reality, the word is created by a series of flashes from many individual, stationary lights. In theory, it would be possible to have a word move across the screen faster than the speed of light — because nothing has actually moved.

Each flash of light is an isolated event, and they only create the illusion of movement collectively. Now, imagine shining a laser pointer straight out into infinite space. You don't see the laser dot projected anywhere because there is no end to this room. One of the most sacred laws of physics is that nothing can travel faster than the speed of light in vacuum.

But this speed limit has been smashed in a recent experiment in which a laser pulse travels at more than times the speed of light L J Wang et al. However, the laws of physics remain intact because Lijun Wang and colleagues at the NEC Research Institute in Princeton in the US are able to explain the results of their experiment in terms of the classical theory of wave propagation.

Special relativity prevents any object with mass travelling at the speed of light, and the principle of causality — the notion that the cause comes before the effect — is used to rule out the possibility of superluminal faster-than-light travel by light itself.

However, a pulse of light can have more than one speed because it is made up of light of different wavelengths. The individual waves travel at their own phase velocity, while the pulse itself travels with the group velocity. 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.

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. But when passed through the specially prepared chamber, light emerged 62 nanoseconds earlier than it would have had it passed through the chamber in a vacuum.

This unusual phenomenon is the result of "anomalous dispersion", an effect not seen in nature in transparent materials and is created by the non-natural thermal state of the cesium gas used in the chamber. A laser is electromagnetic radiation which never has mass and will only travel at lightspeed. So unless that medium or material is inbetween you and your target all the time you are out of luck. However, what if you change this around?

Rather than fire a beam of light, you fire a carrier that holds the light? Say you use a way to stop the light using a metamaterial, and at impact this light is released? It would have the added benefit of being potentially bullet-shaped and capable, but all the lightbeams and pulsed flashes you expect are gone. Plasma is a "soup" of free electrons and ionized atomic nuclei. Lasers are coherent beams of photons. End of the story. Light is slower when travelling through a gas medium such as an atmosphere.

However, the difference in speed between a vacuum and an atmosphere such as ours is practically negligible. As an alternative, consider laser powered rockets. Basically you shoot a rocket, but instead of the rocket burning fuel in order to accelerate, you push it with lasers. This allows them to go arbitrarily much further and faster, and depending on the power you spend they might hit the target as a red or white hot stream of molten metal that from a distance might look like a laser beam as seen in some videogames.

Not an actual laser, no. Laser is light , and by definition, the speed of any light is the speed of light. But you can cheat. Your "laser" might be some mixture of waves hands quickly light ions in an electromagnetically bound self-contained matrix , sort of a Kugelblitz.

The energy contents of the "package" is several orders of magnitude beyond that of a simple laser, which explains why it is preferred to the latter, even though its speed is lower and avoiding a hit is possible.

You could call this a Kugelblitz device, a Rydberg projector, or a "soliton gun". As the other answerers have already pointed out, lasers are by definition light, and light by definition always travels through space at the speed of light in whatever medium it's travelling through.

So you have two options: either it must travel through some medium with a lower speed of light, or it must travel through more space than just the Euclidean straight-line distance from the start to the end of its trajectory. Other answers have already proposed either changing the medium or changing the trajectory to not be a straight line.

I think there is only one more possibility: warp space around the laser beam, so that the straight line trajectory covers more spatial distance than it would in a flat spacetime. An Alcubierre warp drive is a hypothetical device which warps space in order to achieve "faster-than-light" travel for something which cannot travel through space faster than light; imagine something like this in reverse, which warps space to achieve "slower-than-light" travel for something which cannot travel through space slower than light.