In what must be the most touching and useful tribute to departed author Arthur C. Clarke ever attempted, a group of scientists authors are trying to dare time travel into existence. Sir Clarke famously stated that "when a [distinguished scientist] states that something is impossible, he is very probably wrong". And a lot of distinguished scientists just told LiveScience "Time travel is absolutely impossible".

The authors, including Charles Liu (author of "One Universe: At Home In The Cosmos"), Brian Greene (of "The Elegant Universe") and Michio Kaku ("Hyperspace") float a raft of objections to trans-temporal travel. True to Clarke's statement, sometimes affectionately known as "Clarke's Law", each objection seems more like reason to expect time travel than rule it out. Professor Greene states that all time-travel theories operate at the very boundaries of known physics, and are therefore unlikely to work. As opposed to, say, the boundaries of our understanding being where new discoveries are made. As Sir Clarke said years ago: "The only way of discovering the limits of the possible is to venture a little way past them into the impossible".
The other chief objection is the incomprehensible amounts of energy required to punch a hole in spacetime, or stabilise a wormhole, or engineer a double-cosmic-string-ring (yes, that's a real astrophysical concept) capable of bending space hard enough to let us pop back to the past. One point eighty-one jigawatts just isn't going to cut it here, whatever "jigawatts" turn out to be, and most calculations show that powering a time machine with a lightning strike would be like powering a sixteen-wheeler with a bag of jelly babies. (So it seems Marty won't be getting back to the future after all). Of course, the idea of lighting up New York would have had you committed to a mental home in the early eighteenth century. Pre-electricity, schemes were being suggested to transport the increasing numbers of people to the scant available heat and light in times of need.
Understand: the amount of energy we now take for granted was so vast, so utterly unimaginable to people in the past that they were preparing to restructure their whole society rather than even attempt to generate it. Of course, this doesn't guarantee that we'll be able to pop back and tell them. The false argument of past scientific ignorance, the "didn't scientists used to think the world was flat" gambit fails because we know so much more now. The key to progress is our cumulative knowledge, developed and refined by generations of researchers into a vast, accurate body of knowledge. We are far more likely to be able to find what's possible than at any point in history. What we know so far is probably right, and allows us to make predictions about what might be possible.
But until we can explain absolutely everything, we should still steer clear of saying something is impossible.