Doorway to the past? Some theories imagine wormholes as portals to different points in space and time.
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A Stop Sign for Travel to the Past?
Traveling back in time would be much, much more difficult.
Some theories around this idea require humans to travel faster than the speed of light, which is generally believed to be impossible.
And past time travel might need to rely on a passage or tunnel between two space-time locations—a “wormhole.” These theoretical tubes cutting through the fabric of space-time are often referred to as Einstein-Rosen bridges. One idea is that, under certain conditions, an astronaut could enter a wormhole and emerge at another point in space and time, which could be in the past.
But the general thought has been that wormholes are too unstable to allow anything to travel through them. However, some interesting recent work questioning this idea has been done by physicist Pascal Koiran. In a 2021 paper published in the International Journal of Modern Physics D, Koiran used a specific metric to follow the path of a particle through a hypothetical wormhole.
Looping Back
Another idea for visiting the past is to create a kind of curved loop path through space and time that returns to its original starting point—a shape called a closed time-like curve.
The thought is that future astronauts flying along this curve would eventually find themselves back at the same moment in time and same position in space where they first took off, no matter how long the journey took to get there.
The challenge with this concept is that closed time-like curves are just a theory, and haven’t been proven to exist anywhere in the universe. And the technology we would need to create one artificially is far beyond our capabilities, (so far).
Donuts and Spheres
Despite how far-fetched the reality of constructing a time machine may be, some scientists are proposing intriguing concepts.
One interesting idea was developed by Amos Ori, a theoretical physicist at the Technion-Israel Institute of Technology, who imagined creating a structure based on closed time-like curves that would be a donut-shaped vacuum surrounded by a sphere of matter.
The concept, Ori told Live Science, would be to “create an area with a warp like this in space that would enable timelines to close on themselves, it might enable future generations to return to visit our time.”
To go back in time using this structure, astronauts would speed around and around inside the donut, with each lap meaning they are going further back in time until they reach the moment the time machine was created, which would be their limit for reverse time travel.