The universe can seem bewildering at times. In the past century, we've learned an incredible amount about the cosmos: its 13.8 billion-year history, its structure (including the number and distribution of galaxies), and its possible future (increasingly rapid expansion forever). Yet two big mysteries still elude physicists: What happened to the universe in its first instants? And what is the connection between gravity and the other forces of nature?
Researchers entertain some fairly exotic ideas in an effort to understand the bits we haven't figured out yet. One of these ideas is the notion that our four-dimensional spacetime—three dimensions of space plus one of time, with gravity and everything else that is familiar to us—could correspond to a simpler cosmos with fewer dimensions. According to this line of reasoning, our universe could be like a multidimensional hologram, just as a hologram in our reality represents a three-dimensional shape on a flat surface.
That approach could be very promising, but nobody has figured out how to make the calculations work for the real universe yet. Instead, physicists have focused on making imaginary universes that might help guide our thinking. One such model has gotten a lot of attention after a write-up in Nature. Even though this imaginary universe does not resemble ours, subsequent coverage feels like a game of telephone, turning an interesting idea into headlines like “Physicists discover 'clearest evidence yet' that the Universe is a hologram” and “Mindblowing! Our Universe Might Just Be One Giant Hologram.”
Let's not get ahead of ourselves.
What does this research actually mean? A pair of unpublished papers by Yoshifumi Hyakutake and colleagues (available for free download here and here) describes a set of computer simulations that starts with a model 10-dimensional universe with a black hole. The researchers then demonstrate that this simulated universe corresponds numerically to a much simpler one-dimensional cosmos with no gravity. It's an interesting model that could be useful for future research, but it's a far cry from describing our real universe.
That's not the same thing as saying this 10-dimensional hologram model is nonsense—it's not. To appreciate what this far-out idea really means, we need to talk about a few other crazy, real things: black holes and quantum gravity.
Black holes are indisputably some of the freakiest objects in the cosmos. Their reputation for “sucking everything in” is a bit exaggerated—if you replaced the sun with a black hole of the same mass, Earth's orbit wouldn't change noticeably. But black holes stretch the limits of our understanding of the universe. When anything crosses into a black hole's interior—passing the event horizon—it can never return to the outside.
But that's where things get tricky. Black holes are defined by just their mass and rate of spin. They don't have lumps or various colors or differences in chemistry. The black hole apparently doesn't “remember” what falls in: Electrons, iron atoms, dark matter, and even photons can all contribute to its mass.
However, if the information about a particle is destroyed when it falls into a black hole, that means there’s a fundamental incompatibility between general relativity (our standard theory of gravity) and quantum physics. According to the basic rules of quantum mechanics, certain pieces of information about the identity and properties of particles need to survive. The solution to the conflict might lie in a complete quantum theory of gravity, but we don't have such a thing yet.
Over the decades, a variety of physicists, including Stephen Hawking, has worked to figure out what quantum gravity should be like. One thing they discovered is that the information encoded in a black hole is proportional to the area of its event horizon, not its volume. Think of it this way: Earth is roughly spherical, so it exists in three dimensions, but the surface we live on can be described with two numbers: latitude and longitude. Quantum gravity's description of black holes is akin to saying everything important about Earth's interior can be inferred from its surface features alone.
That’s one reason physicists employ the analogy of a hologram: a laser-constructed image of a three-dimensional object on a flat surface. The holographic principle was worked out by Gerardus 't Hooft (who won a Nobel Prize for unrelated work), Leonard Susskind, and Raphael Bousso. In its short version, it says that quantum gravity behaves differently than other forces of nature: Information inside a region is encoded on its surface.
The holographic principle by itself isn't a solution to the problem of black holes destroying information. However, physicist Juan Maldacena worked out a version of the holographic principle that could be a promising approach. This formulation replaces our reality with a five-dimensional cosmos filled with a kind of negative dark energy. Just as dark energy makes the expansion of the universe accelerate, this negative influence makes the model universe contract.
One weird feature is that this model universe has a kind of four-dimensional boundary if you look far enough into the future. That boundary is similar to a black hole's event horizon, and as with black holes, there's a holographic correspondence. All the physics in the imaginary five-dimensional universe corresponds directly to the physics on this surface, where there's no gravity.
If neither the gravity-free, four-dimensional, surface-only world nor the five-dimensional supergravity cosmos corresponds to ours, why make a big deal out of it? The answer lies in what the work informs us. By showing a connection between a complicated world with gravity and a simple one without, Maldacena's model reveals a possible way to solve a number of problems in quantum gravity and black hole information. In particular, a black hole in the five-dimensional universe would not destroy information: Anything falling into it would correspond to a completely different, gravity-free process in the lower-dimensional “hologram,” where information is preserved thanks to the laws of quantum physics.
That brings us back to the new model of Hyakutake and colleagues. In this case, they began with a 10-dimensional toy universe containing a black hole. The universe has a holographic correspondence to a one-dimensional, gravity-free cosmos (Additionally, eight of the 10 dimensions are “compactified,” curled back on themselves like a sphere.) They used computer simulations to show how behavior in these two very different worlds corresponded.
The simulation doesn't constitute a mathematical proof that these two systems are equivalent, and it doesn't demonstrate that similar methods can work for our real universe, but it's a step further along in showing that the holographic principle could be fruitful in understanding the cosmos. I realize this is a lot of weasel words, but that's the most we can say right now.
Physicists think up imaginary universes all the time, and not just because it can be fun. Sometimes these models correspond to an aspect of our cosmos or point to a new way of thinking. The holographic principle is in the latter category: It provides hope that our messy four-dimensional cosmos with gravity, dark matter, dark energy, and all that complicated junk could correspond to a much simpler gravity-free model governed by quantum physics. And it may even provide testable predictions; that's the principle behind the Holometer project at Fermilab.
To put it another way, even if the holographic principle turns out to be a useful way to think about the universe, it doesn't say we live in a hologram or that gravity isn't real. It says that there might be a simpler description of physical phenomena that currently require two incompatible theories. Just as a world map represents the same information that a globe represents, the “hologram” view is an alternative—and perhaps more fruitful—description of the cosmos we inhabit. These new simulations could bring us a little closer to understanding the quantum nature of gravity. And if you're feeling a little flat today, there's no need to fret: It's not because you're a hologram.
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