https://www.ua-magazine.com/the-other-side-of-the-universe/April 29, 2019
Ovidiu Racorean
The Other Side of the Universe
Grab a book. Notice its length, width and thickness. They correspond, respectively, to the three dimensions of space and, we reasonably assume, to all objects in the three-dimensional space.
As you flip through the book, you note that its pages have two sides – in fact, all objects around you are two-sided. It may slip your mind that this trivial aspect is a feature of spacetime. And, yet, the spacetime itself is a two-sided surface.
The fact that the world is two-sided is sustained by an important property of two-sided spacetimes: a definite orientation for the arrow of time. Since we experience time as flowing irreversibly from the past to the future, we assume we are living in a two-sided world.
Still, there may be one-sided worlds out there, with one-sided objects in them. For the sake of imagination, picture yourself reading your book in a one-sided spacetime. Hard to believe it, but you would be able to read both sides of the pages at the same time. To see this, I encourage you to cut a sheet from the book (just kidding!) and glue it as in the picture below (fig 1), forming a trivial Mobius strip, the most known one-sided surface.
Fig. 1: Mobius Strip
There is more to the story. In a one-sided world, time has no preferred orientation; it may flow from the past to the future or vice versa.
Now, we may ask where the other side of the universe is and how can we get there. We may find the answers in the recent paper “Quantum entanglement, two-sided spacetimes and the thermodynamic arrow of timeâ€, posted in the physics online archive arXiv, and elected one of the best of the week by MIT Technology Review.
If we are going to delve into technical details, first we must describe the overall picture, by looking at prior scientific work, which resembles the pieces of a puzzle. Quantum entanglement happens to be the key to solve that puzzle.