One of the most popular tropes in science fiction is the idea of time travel. Wouldn’t it be great if we could witness the heyday of the Roman Empire? Or even the dinosaurs? Or jump ahead to a future time to find out how our great-grandkids’ grandchildren turn out? One of the best-known early fictional treatments of the idea is H.G. Wells’ The Time Machine, and one of the most popular recent efforts is Audrey Niffenegger’s The Time Traveller’s Wife, but the concept has inspired countless novels, movies, and TV shows.

So will time travel ever be possible?

In a sense, jumping into the future just requires us to go somewhere at really high speed, because of the effects of relativity. In Orson Scott Card’s book Speaker For The Dead his main character, Ender Wiggin, exists in a world more than 3000 years after his birth, but has aged only 36 years because he’s spent so much of his life travelling between stars at near light speed. But is that really time travel? After all, you can never go back! What we really want is a way to go back and forth in time, isn’t it?

The idea doesn’t belong to fiction alone—lots of legitimate scientists have looked into it. The laws of physics don’t rule it out, and there are some phenomena that might do the trick.

One such is a wormhole in space—kind of like a black hole, but with an entrance and an exit. Star Trek fans will remember a wormhole as the setting for the series Deep Space Nine, but a wormhole might provide a shortcut through time as well as space.

Some scientists are even trying to make time machines. One of those is Dr. Ronald Mallett at the University of Connecticut. Mallett’s concept involves making a circular beam of high-energy light that would stir empty space like a spoon in a cup of coffee, making it theoretically possible for a particle in that space to travel faster than light and, hopefully, into the past. Mallett isn’t saying he’ll be able to send humans physically into the past, but perhaps information at least. There are advocates of time travel who believe that information is enough: that we might be able to experience other eras through a kind of virtual reality using information from those other times.

So far, the concepts that do appear theoretically possible have their drawbacks. A wormhole couldn’t take you back to a time before the wormhole existed. Similarly, Mallett’s time machine wouldn’t allow matter or information to travel to a time earlier than the moment the machine was switched on. Does that make his machine useless to the impatient types among us? Not really. The moment Dr. Mallett gets his machine working, he might be flooded with messages from people in the future (or even himself) trying to contact our time. That could be pretty useful.

One of the questions most deeply-ingrained into the human psyche is: what if I had done something differently? How would my life have turned out? From there it becomes: what if the world had done something differently? That question has generated a whole sub-genre of SF: the alternate history story.

That’s why even if time travel never becomes a reality, for science fiction it will always be necessary just the same.