Albert Einstein is known to be one of the best physicists in history, whatever theories he produced are undeniably precise and still practised. The most popular theory, he gave was the theory of power, E=mc², in this theory, E is defined as power, m defines mass of a matter and c defines the speed of light.
The speed of light is constant and nothing can go faster than the light. The speed of light always remains the same which was true in the past. But now, with the modernisation of science, it has been proved that the speed of light is not actually constant, it may vary in mediums. Light slows down while travelling through air, glass or water, and this effect is used in lenses and prisms. The constant speed of light was supposed to be same in a vacuum. But, that also is not true.
A group of scientists of Glasgow University has shown that we need to rethink about the constancy of the speed of light. They have proved that even in vacuums, the speed of 
light is not constant. They manipulated the wave structure of a photon and sent them to a path of the same length as unaltered packets of light. The surprising news is, the photons arrived later proving that they were travelling slowly.
The slowing occurs at a rate of about one part in a hundred thousand. So in the time it takes unmodified light to travel a meter, the adjusted light makes it 0.01 millimetres less. “Measuring the arrival time of single photons with femtosecond precision is challenging,” said the scientists. “The team achieved this by producing strongly correlated photon pairs and having them meet at the destination, so that tiny variations in their arrival times would be revealed as phase differences. The manipulation of the photon occurred by twisting a plane wave into a conical wave. If this research goes on, then we might find some new theories which would alter the most precise theories of Albert Einstein. However, this research will help a lot to work on microscopic optics.”
