There is no actual description of this video on YouTube, so I'll just try writing up a bit of an introduction myself.
Sean Carroll is a physicist at the California Institute of Technology, and the author of several scientific books. This is a lecture he gave at the Linda Hall Library -- a lecture in which he explains how what we know about the concept of time has changed over the ages since Isaac Newton, and how Albert Einstein first developed what is now known as the Special Theory of Relativity, and then later integrated this theory with the concept of gravity, thereby creating the General Theory of Relativity.
After explaining the theory of it all and the apparent limitations imposed upon us by the laws of physics, Sean then goes on to hypothesize how time travel could in theory become possible, even though practice forbids us from exploring the possibilities.
It's a very interesting lecture, and I sincerely recommend it to anyone seeking to understand why time behaves the way it does.
1 hour 7 minutes