Stephen Hawking thought for Time Travel

"If one made a research grant application to work on time travel it would be dismissed immediately," writes the physicist Stephen Hawking in his posthumous book Brief Answers to the Big Questions. He was right. But he was also right that asking whether time travel is possible is a "very serious question" that can still be approached scientifically.

Arguing that our current understanding cannot rule it out, Hawking, it seems, was cautiously optimistic. So where does this leave us? We cannot build a time machine today, but could we in the future?

Let's start with our everyday experience. We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now. But this is something we can never actually know. The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us.

Our inability to access the "now" of someone far away is at the heart of Albert Einstein's