Physicists from Hong Kong have determined at least one theory of time travel is impossible.

The researchers from Hong Kong University of Science and Technology led by Prof Shengwang Du surmised this theory when they showed how a single photon could not surpass the speed of light in a vacuum. This study reaffirms Einstein's theory that nothing is faster than the speed of light. Einstein's law stated that the speed of light was the traffic law of the universe - nothing could be faster than it.

"By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon. Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information," Du said.

This revelation, the scientists say, closes the book on time travel. One theory from 10 years ago said time travel may be possible through superluminal propagation of optical pulses in some specific medium. However, Du and his crew said this phenomenon is only a visual effect and could not be used for transmitting any real information.

There was also hope that a single photon may be able to travel faster than the speed of light. However, there was no experimental evidence. Du and his colleagues decided to test this and measured the ultimate speed of a single photon with controllable waveforms. The results confirmed Einstein's theory.

The team decided to use a demonstration which required not only producing single photons, but separated the optical precursor as well. The optical precursor is the wave-like propagation at the front of an optical pulse, from the rest of the photon wave packet. The team did this by generating a pair of photons and then passing one through a group of laser-cooled rubidium atoms with an effect called electromagnetically induced transparency (EIT).

This is where the team discovered that the main part of a single photon could not travel faster than the speed of light.

Time travel has long been a subject of science fiction and as a concept has been around since 700 B.C. There are other theories on time travel, such as wormholes, that have yet to be disproven. The wormhole is when space and time are intertwined aspects that can be bent to join two different times.

Du's work was published recently in Physical Review Letters.