If The Universe Is A Hologram, We May Soon Gaze Into A Black Hole

If you fell into a black hole, your journey might look something like this.

First, you'd stare into the rich, red event horizon of the abyss. Beyond this barrier, light cannot escape. As you get closer, your body would stretch out like chewing gum until it spaghettifies into the void. If you're still conscious at this point, you'd peer out the entrance and watch a warped universe grow smaller by the second. That wouldn't be your universe anymore. The black hole would be.

In all probability, though, you'd quickly be ripped to shreds.

Because of this absolutely horrifying disaster, we'll likely never receive firsthand evidence of what lies within these cosmic mysteries. But in a paper published this month in the journal PRX Quantum, scientists are working toward the next best thing. They developed computing algorithms to help solve a mind-bending theory in physics called "holographic duality." 

In a nutshell, holographic duality suggests that the three-dimensional universe, like space inside black holes, is mathematically strung to the two-dimensional universe, like particle planes and magnetic fields. It basically presents the fabric of spacetime as a 3D hologram "projected" by 2D webs. 

I know what you're thinking. No, this wouldn't be like the Star Trek holodeck. Unlike classic sci-fi holograms projected by light from a screen, holographic duality is bound by pure mathematics. 

"It has not been proven formally, under the point of view of rigorous mathematics, but we know many examples where this duality actually works," says lead author Enrico Rinaldi, a research scientist at the University of Michigan, based in Tokyo and hosted by the Riken Center for Quantum Computing and the Theoretical Quantum Physics Laboratory.

If holographic duality truly dictates the universe, scientists wouldn't have to go inside a black hole to take a picture of it. Instead, they could study easy-to-handle 2D space around the beast, then extrapolate the 3D architecture lurking inside. "It is often the case that things difficult to compute on one side are easy to compute on the other side," Rinaldi says. "That is why this duality is very important and useful." 

He compares the idea to having a dictionary where you can look up a word on one page and find its meaning on another. We just need some sort of index to bridge the 2D space-words with their 3D space-definitions -- aka, the mathematical connection. And that's precisely what Rinaldi's algorithms are poised to do.

However, before we can use them to unlock the inside of a black hole, there are several, pretty trippy, steps to take. "The duality, as it is right now, applies to a specific spacetime, which is different from the spacetime of our universe," Rinaldi says. 

In other words, holographic duality is confined to a sort of alternate, theoretical world that scientists use as a sandbox. 

A spacetime playground

1916 was a big year for physics. Albert Einstein had published the first of many papers that would forever alter the field: a holy grail chronicle of general relativity. Since then, the theory has earned a reputation for being unbreakable. I could go on forever about its spectacular consequences, but here's the important part for holographic duality. 

Suppose you have a trampoline and drop a soccer ball into it. The flat surface will morph inward, depending on where the ball settles. Now, add a tiny marble to the scene. It'll fall along the trampoline's curve and nestle next to the soccer ball. 

In this analogy, the marble is you, the soccer ball is Earth and the trampoline is the intangible fabric of space and time -- spacetime. According to general relativity, gravity is this "curve" we fall along until we're planted on the ground. 

In our universe -- which, per experts, is known as the "de Sitter" universe -- spacetime's curvature is positive. That's a problem. A positive model isn't great for math equations, Rinaldi explains, especially when it comes to ultra high-dimensional ones. But there's an easy fix. Scientists simply calculate stuff in a theoretical universe with negative curvature: the anti de Sitter universe. Then they translate their results back to our realm.

Fast-forward to the late 1960s. String theory is born. 

Allowing for simplification, string theory says if you break down atoms, the building blocks of our universe, into elementary particles, then pulverize those into even smaller specks, and so on, you'll eventually get to infinitesimal vibrating "strings." 

Presumably, these strings make up all we know: particles, fields, spacetime. If string theory is true, even you and I are made up of the wiggling bits. That's why this concept is such a big deal. It might well be the closest we've gotten to a theory of everything. On the flip side, however, some physicists consider string theory a dead end because we still haven't found concrete evidence for its premises.

But regardless, string theory requires unfathomable 11-dimension equations -- as you might've guessed, that means it's rooted in the anti de Sitter universe. And per Rinaldi, holographic duality relies on string theory. Thus, it's also rooted in the anti de Sitter universe. 

"Black holes we can investigate right now, with this duality, are not the same black holes that we imagine being out there," Rinaldi says. "These black holes are a sort of mathematical playground that we can use to formulate this duality and test it."

Simply put: In this mathematically ideal universe, Rinaldi is observing theoretical black holes to understand holographic duality. It's like playing a game in tutorial mode before the real level starts. Our universe. 

Getting to that level, though, is the crux of this whole procedure. "If we can do it for anti de Sitter," Rinaldi says, "then we should be doing it for de Sitter."

"The final goal is still to be able to describe gravity and black holes in our universe." 

The road into a black hole

OK, here's where it all comes together. 

First, a quick recap: Holographic duality can show us what's inside a black hole because it suggests the 2D universe is connected to the 3D universe via mathematics. We just have to construct an index to bridge the two dimensions. But holographic duality is based on string theory. So, first, we have to make the index's blueprints in our sandbox universe -- the theoretical, anti de Sitter universe.

How do we make the blueprints? Well, Rinaldi says, start with the easier side. That's the 2D half. But even though this side hurts less to think about, it isn't that simple; we still need strong numerical methods to analyze it. "That's what we're doing," Rinaldi says. "The numerical part."

Think of the universe as a blanket knitted by strings that have a bunch of points. Rinaldi's algorithms use quantum computing and deep learning to help calculate where these points are on the blanket and how they're attached to each other. The goal is to sort of draw out the "strings" of string theory, then put them all together, like cosmic connect-the-dots.

However, the researchers are still in the proof-of-principle stage. They  solved a few prototype points with their method, but these points don't really represent anything. In the future, though, Rinaldi says the method can scale up to study complex points really present on anti de Sitter strings, including those relevant to anti de Sitter black holes. 

Then, we'll be on our way to making the anti de Sitter 2D-to-3D index that'll reveal the insides of these theoretical black holes. 

Then, if the index is precise enough, it can be translated to our true-to-the-bone, observable universe. 

Then… we can use the final index to learn about the threatening insides of real, de Sitter black holes from the comfort of our homes and tucked away from terror. 

A new theory of everything?

When you think about the steps Rinaldi and tons of other researchers are taking to realize the insides of a black hole -- study prototype theoretical universe strings, scale up to learn about the full theoretical universe's geometry, zero in on theoretical black holes, take all of that and filter the real universe through it, and probably more we can't even comprehend -- a jarring question might be… why? 

Why does this all matter?

"We think we are very close to explaining the information paradox of black holes," Rinaldi says. "If information goes inside a black hole, general relativity says, OK, whatever goes in is gone forever." 

But quantum mechanics, the other founding principle of our universe, says you cannot lose information. It says information is always maintained. Perhaps it can change, transform or adapt, but it cannot go away. So what's happening to the information plunging into these massive space-borne voids?

"Stephen Hawking came up with this idea of the evaporation of a black hole and said 'Look, actually there is stuff coming out of a black hole, it's just slowly coming out'," Rinaldi says. 

But even those bits coming out don't look like what went in. Stuff still seems lost in the process. "This is a very, very big problem in physics," Rinaldi says. "And people are using the duality to understand that paradox." If we can understand what's inside, then maybe we can prove so-called lost information is actually, well, inside. 

"Maybe it's not lost, it's just in a different configuration. It's not particles anymore; it's not spacetime anymore; it's something else."