The sci-fi landscape is littered with wormholes. From Douglas Adam's Hitchhiker's Guide to the Galaxy and Rick and Morty to the Marvel Cinematic Universe, these theoretical constructs allow characters to zip between distant points in the universe as easy as stepping through a doorway.
An Einstein-Rosen bridge is the simplest kind of wormhole. And while it can, in theory, allow you to meet a new friend from a distant part of the universe, there are some important reasons why it won’t let you travel back in time.
Black Holes, White Holes and Wormholes
Let’s start with everybody’s favorite astronomical mystery: a black hole. Despite their fearsome reputation, they’re actually rather simple creature. They have a point of infinite density, known as the singularity, in their centers. They are surrounded by a boundary called the event horizon.
The event horizon doesn’t exist in the same way that the surface of a planet exists. Instead it’s just a mathematical line in the sand that tells you one thing: if you cross within that special distance, you’re trapped forever, because you’ll have to travel faster than the speed of light to escape.
And that’s it. That’s a black hole. A singularity and an event horizon. All things that cross the event horizon will never escape back into the universe – things go in and never come out.
Mathematically we can also define the polar opposite of a black hole, which is conveniently called a white hole. White holes also have a singularity, but their event horizons act differently. Anything already on the outside of a white hole (like, the entire universe) can never, ever cross within it, no matter how hard it tries. And anything already inside the white hole will find itself ejected from it faster than the speed of light.
Now when we take a black hole and a white hole and connect their singularities together, we get an entirely new kind of object: an Einstein-Rosen bridge, better known as a wormhole.
What Is a Wormhole?
Wormholes are essentially hollow tubes through space and time that can connect very distant regions of the universe. A star may be thousands of light-years away, but a wormhole can connect that star to us with a tunnel only a few steps long.
Wormholes also have the somewhat mystical ability to allow backwards time travel. If you take one end of the wormhole and accelerate it to a speed close to that of light, it will experience time dilation — its internal “clock” will run slower than the rest of the universe.
That will cause the two ends of the wormhole to no longer be synchronized in time. Then you could walk in one end and end up in your own past. Voilà: time travel.
Can Humans Travel Through Wormholes?
There's just one, tiny, teensy problem with this setup: Einstein-Rosen bridges are indeed wormholes, but the entrance to the wormhole sits behind the black hole event horizon. And the number one rule of black hole event horizons is that once you cross them, you’re never allowed to escape. Ever.
Once you pass through a black hole event horizon, you are forced towards the singularity, where you are guaranteed to meet your gruesome end. In other words, once you enter an Einstein-Rosen bridge, you will never escape.
So, the unfortunate truth with Einstein-Rosen bridges is that while they appear to be magical doorways to distant reaches of the universe, they are just as deadly as black holes. When you enter you can meet other travelers who have fallen in from the other side, and you could even carry on a conversation…briefly, before you both struck the singularity.
There have been attempts to stabilize Einstein-Rosen bridges and make them traversable by somehow getting their entrances to sit outside the event horizon. So far the only way we know how to do this is with exotic matter. If you threaded the wormhole tunnel with matter that had negative mass, then in principle you could have a not-deadly-at-all wormhole.
Alas, negative matter does not appear to exist in the universe, and so our wormhole — and time travel — dreams will have to remain as mere mathematical fantasies.