Is it possible to travel backward in time?

Is it possible to travel backward in time?
To answer this question, we first have to address why one direction of time is forward and the other backward. But must we always move in the direction of arrow of time? Could our descendants someday built a time machine that would allow them to visit the past? It doesn’t seem possible to reverse the arrow of time, but perhaps we could build a “tunnel” that permits time travel. A wormhole, for example, is a “tunnel” that connects two different places in space. We could use wormholes, an idea that dates to Einstein, who imagined a bridge in space-time that connects two arbitrary points in the universe. Although he found solutions to the equations of general relativity for wormholes, do they actually exist? Wormholes might have formed in the early, turbulent universe, stabilized by exotic physics. The best way to reach the stars is to find nearby wormholes. We could spot them by the flickering as a wormhole passes in front of a distant star. But space and time are part of the 4-dimensoinal fabric, so a wormhole could also connect two different places in time. If you entered one and day, you could come out yesterday. But could we ever construct a wormhole or some other exotic arrangement that would permit time travel? Einstein’s general theory of relativity tells us that matter and energy curve space and time, and curved space-time is exactly what we need. In fact, if you have the right kind of matter, you can curve space-time any way you want. This concept looks good for time travel, but there’s a hitch. The matter you need to build a time machine is very strange: it has to have negative mass. We don’t know whether there’s any system in which quantum-mechanical effects produce the right amount of negative mass to permit travel.  But we find it unlikely that travel into past is possible, but we can’t say yet for certain.
~ Astronomy’s Greatest Mysteries~