The Time Window is a temporal device that appears in the 2006 film, Déjà Vu.
The time window was created by pure accident. A complete fluke. Scientists at Cambridge University were working for the National Reconnaissance Office on an R&D grant. They were attempting to use concentrated bursts of energy to enhance the sensitivity of optical telescopes. In the process they had a breakthrough. Given enough energy they could warp the very fabric of space. They found a way to fold space back unto itself.
We view space as flat wherein we see something from a distance light always has to travel the long way across the flat space in between. But the Time Window can fold the space, bring the target closer to the viewers, create what's known as an Einstein-Rosen bridge (otherwise known as a wormhole) suspended via gravitational field. Basically folding space in a higher dimension to create an instantaneous link between to distant points. Huge amounts of energy is used to create the bridge.
The bridge is not visible to the human eye, but it's just as real and solid as a cellphone signal or a radio wave. The viewers can look back can four and a half days within a limited radius. They can look back in the past. (In a sense we're always looking in the past. Even light reflected from yourself in a mirror takes some time to get back.)
Light can go back. But a person cannot go back because they would transit across what's known as the "wheeler boundary." An EM pulse annihilates all electrical activity, such as the heartbeat and brain waves. The field would even scramble a radio signal.