\nAdS/CFT suggests that our three-dimensional universe can be perfectly modeled by a two-dimensional surface. On one side is AdS, or Anti-de Sitter space, which is a kind of imaginary universe with some unusual properties - negative...\nAdS/CFT suggests that our three-dimensional universe can be perfectly modeled by a two-dimensional surface. On one side is AdS, or Anti-de Sitter space, which is a kind of imaginary universe with some unusual properties - negative curvature and a boundary, whereas our real universe is “flat” and expanding. One way to picture it is to imagine living inside a snow globe—where the interior of the snow globe corresponds the “bulk” or AdS, where people, planets, everything in the universe is; and the glass represents the CFT or boundary of that space.
Conformal Field Theory, or CFT is just a fancy word for a kind of quantum field theory - one without gravity – defined in a space that has one fewer dimension than the AdS bulk we just described. “Field Theory” means it’s a framework like those used to describe particles, forces, and fields, like electromagnetism or other quantum forces in physics. “Conformal” basically means the laws in this theory look the same even if you stretch or rescale things. Crucial to this is that a CFT has no gravity in it. CFT is analogous to the code of a video game or the film of a hologram. It’s a description of something, written in what looks like fewer dimensions.
Now here’s the mind-blowing part: AdS/CFT says that the bulk, the AdS space with gravity, and the boundary CFT without gravity are actually the same thing in two different forms. This idea is often called the holographic principle because it’s just like a hologram: a 3D image that’s stored on a 2D surface. But it’s an exact equivalence.
This was first discovered in 1997 by physicist Juan Maldacena, who showed that a gravity-filled universe described by string theory in AdS is mathematically equivalent to a gravity-free CFT on its boundary.
So if gravity can be described perfectly by a theory with no gravity, is gravity real? Yes it’s real because we can measure it and we have very accurate theories that model it. But it may not be fundamental. AdS/CFT provides a mechanism for gravity to emerge from a system that doesn’t contain gravity. On the boundary side (the CFT), we have a quantum system of fields and particles evolving with no gravitational force – it lives in flat spacetime and obeys ordinary quantum mechanics. Yet, this system is equivalent to a picture of one higher-dimensional world where gravity and curved spacetime do exist. In other words, gravity in the AdS bulk is an emergent phenomenon, a holographic image of the quantum field dynamics on the boundary. #holographicprinciple #quantummechanics But how exactly does gravity “come out” of the CFT? Modern research shows that quantum entanglement in the CFT is the key to building the spacetime geometry in AdS. Patterns of entanglement can be related to the connectivity and shape of space in the bulk. No single quantum of the CFT corresponds to gravity, but interactions of entangled particles create the effect of a smooth curved spacetime with a gravitational force. AdS/CFT provides a way for space, time, and gravity to emerge from a lower-dimensional quantum world.
In physics classes, we always learned that there are 4 fundamental forces – Strong force, weak force, electromagnetism, and gravity. But the AdS/CFT duality implies that gravity might belong in a different category: it could be a macroscopic phenomenon that isn’t fundamental in itself, but arises from deeper laws.
What would this mean for the future of physics research? First, it would mean there are no Gravitons at the Fundamental Level. Second, it would show that Einstein’s equations can come out of quantum information principles. Third, spacetime is not fundamental. And fourth, we might finally unite all of the known forces into a theory of everything.
Note that our real universe is not AdS. Our universe is closer to a de Sitter (dS) space with positive curvature due to expansion. Physicists are actively searching for a dS/CFT correspondence theory. \n
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