Light Sprints


light pulse

Small light pulses can now be modified so they slow down

Remember any physics lessons during your high school years? How it was always said the speed of a light was the most unchangeable constant of all? Well… keep reading. In a perfect example of how science changes to perfect itself, scientists at the University of Glasgow have carried out a very interesting set of experiments which ultimately showed that the speed of light can in fact change.

Now, we all know that when light enters a medium, such as water or glass, it slows down. Whereas the speed in a vacuum is said to be 299 792 458 m/s, it can go down to 225 056 264 m/s in water and even 124 018 189 m/s in diamond. This is due to the increased density through which the light has to pass through, so the light particles suffer more collisions which slow it down. But the news come from the idea that speed can also change in a vacuum, even if there is no change in medium and therefore in density.

However, there is a trick. This change in speed won’t happen spontaneously, it has to be slightly triggered. Although it is usually simplified as ‘straight’ or plane waves, where every point travels parallel to each other, light is a bit more complex than that. Two points in a ray of light can actually converge and join, bending their shape. When this effect happens, light speed is affected.

The experiment consisted of a source emitting only two photons. One of them was directed to flow through an optical fibre, so its journey was not interrupted and was as smooth as possible. The other one was passed through a series of apparatus which changed its structure for a short period of time and then restored it back to normal. The time it took each of the photons to arrive to the finish line was measured very precisely. No matter how many repeats the team conducted, the modified photons were always slightly slower and arrived after the untouched one.

The change is speed is not immense, and will have no effect on day-to-day calculations, but it could be have some importance on experiments which use short pulses of light. The fact this effect exists is already worth noting, as it is theoretically obvious but no one had proved it before.

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One comment on “Light Sprints

  1. Wow, this was very interesting to read and it’s incredible that they have been able to prove that speed of light can actually change! It definitely will be very interesting to follow up this story and see how this new discovery affects experiments which use short pulses of light. Great job! 🙂

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