Thanks for explanation. But the thing is that the speed of particles of light is slowed down forever which means even in vacuum(not any medium) the speed of light is slower than the original one. You may take a look at the link: http://www.bbc.com/news/uk-scotland-glasgow-west-30944584.
I've recently read a report about a group of Scottish scientists have managed to slow down the speed of light, not in a certain condition but through a special mask and the particles of light can be forever slowed down. Does that means the speed we need to achieve to make time travel is slower...