Cosmic ‘constant’ of light speed not so constant after all

Researchers show that light can be slowed down with modifications to their spatial structure.

The massless particle that defines the speed limit of the universe can be slowed down after all. Researchers at the University of Glasgow have devised an experimental procedure which allows them to experimentally show that the speed of light can be slowed down in free space. This slowing down of light in the condition of empty, vacuous space, was once thought impossible.

The research was led by Daniel Giovannini, a research assistant at the University of Glasgow, and conducted by a team of research students and physicists. They published their results in Science Express under the title “Spatially structured photons that travel in free space slower than the speed of light.”

In order to design an experiment to slow light down, the researchers placed a mask over an optical device in order to give the individual light particles a spatial structure as they were ejected from the laser. This modified the light particles to give them a particular shape before they are shot into space. The researchers then raced their modified particles against regular photons to see if they would finish at different times. They found that the modified light particles were slowed down and finished later than their regular counterparts. When the normal light appeared across the finish line, the slower light was about a metre behind.

Even when the machine stopped acting on the light, the particles remained slowed down and did not rebound to their original velocity. The perpetual slowed state of the modified light particles is quite significant. Normal photons usually bounce back when they face resistance, but even after the machine stopped working on them, the light particles remain slower than their unaltered counterparts.

The idea of slowing down light particles is not new. Every time light passes through a medium such as water or glass, it slows down. As soon as it comes out of the medium, it immediately returns to its original speed.

“This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum,” postdoctoral student Jacquiline Romero said in a press release.

The accepted calculation of c, the symbol for the speed of light, is 299,792,458 metres per second. It is often regarded as the cosmic speed limit and it is used as a universal constant in physics. Einstein’s equations utilize the constant in order to derive many significant results such as his special theory of relativity and E=mc2.

Slowing down the speed of light in a vacuum may force many physicists to rethink how they view the speed limit of the universe. Although it is doubtful it will have any immediate theoretical implications, Giovannini suggest it may have significant implications on how we use light in technology.

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