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 A pair of unpublished papers by Yoshifumi Hyakutake and colleagues (available for free download here and here) describes a set of computer simulations that starts with a model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.
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