Can We Travel through Time to the Past?

black holes merging
Could black holes create a portal to the past?. LIGO Consortium

Going back in time to visit an earlier era is a fantastic dream. It's a staple of SF and fantasy novels, movies, and TV shows. However, could someone travel to a previous era to right a wrong, make a different decision, or even completely alter the course of history? Has it happened? Is it even possible? The best answer science can give us right now is: it's theoretically possible. But, no one has yet done it. 

Traveling Into the Past

It turns out that people time travel all the time, but only in one direction: from the past to the present. And, as we experience our lives here on Earth, we are constantly moving into the future. Unfortunately, no one has any control over how quickly that time passes and nobody can stop time and continue to live. 

This is all right and proper, and fits with Einstein's theory of relativity: time only flows in one direction—forward. If time flowed the other way, people would remember the future instead of the past. So, on the face of it, traveling into the past seems to be a violation of the laws of physics. But not so fast! There are theoretical considerations to take into account if somebody wants to build a time machine that goes back to the past. They involve exotic gateways called wormholes (or the creation of such gateways using a technology not yet available to science). 

Black Holes and Wormholes

The idea of building a time machine, like those often depicted in science fiction films, is likely the stuff of dreams. Unlike the traveler in H.G. Wells's Time Machine, no one has figured out how to build a special carriage that goes from now to yesterday. However, one could possibly harness the power of a black hole to venture through time and space.

According to general relativity, a rotating black hole could create a wormhole—a theoretical link between two points of space-time, or perhaps even two points in different universes. However, there's a problem with black holes. They've long been thought to be unstable and therefore un-traversable. However, recent advances in physics theory have shown that these constructs could, in fact, provide a means of traveling through time. Unfortunately, we have almost no idea what to expect by doing so.

Theoretical physics is still trying to predict what would happen inside the wormhole, assuming one could even approach such a place. More to the point, there's no current engineering solution that would allow us to build a craft that would let make that trip safely. Right now, as it stands, once you enter the black hole, you're crushed by incredible gravity and are made one with the singularity at its heart.

But, if it were possible to pass through a wormhole, it would probably be a lot like Alice falling through the rabbit hole. Who knows what we would find on the other side? Or in what time frame?

Causality and Alternate Realities

The idea of traveling into the past raises all sorts of paradoxical issues. For instance, what happens if a person goes back in time and kills their parents before they can conceive their child?

The common solution to this problem is that the time traveler effectively creates an alternate reality or parallel universe. So, if a time explorer did travel back and prevent her birth, a younger version of her would never come to be in that reality. But, the reality that she left would carry on as if nothing had changed.

By going back in time, the traveler creates a new reality and would, therefore, never be able to return to the reality they once knew. (If they then tried to travel into the future from there, they would see the future of the new reality, not the one they knew before.)

Warning: This Next Section May Make Your Head Spin

This brings us to another issue that is rarely discussed. The nature of wormholes is to take a traveler to a different point in time and space. So if someone left Earth and traveled through a wormhole, they could be transported to the other side of the universe (assuming they are even still in the same universe we currently occupy). If they wanted to travel back to Earth they would either have to travel back through the wormhole they just left (bringing them back, presumably, to the same time and place), or journey by more conventional means. 

Assuming the travelers would even be close enough to make it back to Earth in their lifetimes from wherever the wormhole spat them out, would it still be the "past" when they returned? Since traveling at speeds approaching that of light makes time slow down for the voyager, time would proceed very, very quickly back on Earth. So, the past would fall behind, and the future would become the past... that's the way time works flowing forward

So, while they exited the wormhole in the past (relative to time on Earth), by being so far away it's possible that they wouldn't make it back to Earth at any reasonable time relating to when they left. This would negate the whole purpose of time travel altogether. 

So, Is Time Travel to the Past Really Possible?

Possible? Yes, theoretically. Probable? No, at least not with our current technology and understanding of physics. But perhaps someday, thousands of years into the future, people could harness enough energy to make time travel a reality. Until that time, the idea will just have to stay relegated to the pages of science-fiction or for viewers to make repeated showings of Back to the Future