Simulation physics and theology for Qubitzers
A recent paper by physicist Leonard Susskind, titled “Dear Qubitzers, GR=QM,” outlines (among other things) intriguing physical models for the simulation hypothesis.
Susskind’s set up is simple: A laboratory where engineers Alice and Bob experiment with condensed matter shells. Their experiments are totally beyond reach of today’s technology, but Susskind seems to think that, if something is feasible in-principle, some day it will be doable in practice. I totally agree.
The paper doesn’t contain equations, but can be hard to understand. For example: “The shell has been engineered to be at a quantum critical point, where the excitations are described by a conformal field theory having a holographic bulk dual.” Don’t worry, a footnote explains it all: “By bulk I mean the AdS-like geometry dual to the CFT.”
Right, the paper is a letter to experts in Susskind’s league, not meant to be easy understandable to the rest of us. To understand the paper you need to know about string theory, general relativity, black holes, wormholes, quantum gravity research, the holographic principle, AdS/CFT duality, ER=EPR, condensed matter physics and whatnot, and you’ll have to go through all the references. I only have amateur-level understanding, but I’ll try to explain something.
Let’s start at the end. Susskind says (p. 10):
“[Suppose] we construct a large block of matter engineered to have the standard model (without gravity) as its excitations… Is the world in [the block] real? Sure it is; the block and its excitations are certainly real, and if the standard model was well simulated it may support observers who could communicate with laboratory observers.”
Here “the standard model” is the physics of our world, and Susskind is saying exactly the same thing that I say in “From Elon Musk to Joseph Smith: a material simulation hypothesis.” We could (in principle) engineer a material system to support excitations (think of waves in a quantum sea) and quasiparticles that correspond exactly to the quantum fields of fundamental physics in our world. It follows that the same physics could unfold in the quasiworld of quasiparticles in the material, and eventually result in sentient observers.
I have argued that this is a physical formulation of simulation theology: God organizes the universe like we organize a material system to give it desired properties (like superfluidity and superconductivity) and behaviors, operating in a deeper level of physical reality, which we could conceivably gain access to.
In passing: Susskind is a top physicist and a great teacher (see his highly recommended “Theoretical Minimum” books and video courses), and I have an enormous respect for him. He is often described as an atheist. So how do I dare interpreting his work in a theist key? Well, Susskind is actually an agnostic. I think he is a “cultural atheist” and I am fairly sure he wouldn’t like what I’m writing, not at all, but his work does have theological implications for those who want to see.
Let’s go back to the paper. String theory is somewhat messy, and very much work-in-progress with little experimental support. A recent book titled “Why String Theory?” includes the shortest chapter that I have even seen: The full text of Chapter 7 is “There is no direct experimental evidence for string theory.”
However, string theory is often considered as the most promising approach to a theory of quantum gravity that could encompass both the quantum field theory of the standard model and Einstein’s general relativity. Also, recent applications of string theory to condensed matter and nuclear plasma physics indicate that the mathematics of string theory does relate to things that can be measured in the lab.
One of the cornerstones of string theory is something called “AdS/CFT duality.” (Very) roughly, AdS/CFT means that a special type of quantum field theory without gravity (a Conformal Field Theory, or CFT) is equivalent to a string theory with gravity in a special type of space (an Anti-de Sitter space, or AdS) with one more dimension. The “special type of” qualifiers could be relaxed in future development of the theory.
The “with one more dimension” part is important: For a two/three dimensional analogy, imagine a surface (“boundary”) enclosing a volume (“bulk”). AdS/CFT means that a quantum field theory without gravity on the boundary is equivalent to a string theory with gravity in the bulk, where “equivalent” means that the two theories are different mathematical descriptions of the same physics.
In “The Black Hole War,” Susskind suggests to visualize AdS/CFT as “a can of soup.” Outside, a two-dimensional quantum field theory of particles that live on the boundary. Inside, a three-dimensional space-time bulk, with a soup of matter, energy, gravitational waves and black holes. The bulk physics inside is determined by the boundary physics (holographic principle).
So we have quantum field excitations on the boundary, black holes and gravitational waves in the bulk, and there is always a correspondence between what happens on the boundary and what happens in the bulk. For example it turns out that, if you heat the boundary, you create a black hole in the bulk.
It also turns out that when one of the two dual theories is strongly coupled (difficult to calculate), the other is weakly coupled (easier to calculate). So one can calculate using the easier theory and translate the results into the language of the more complex theory at the end. “[In] a matter of a few years condensed matter theory was rewritten in a different mathematical language. This language is the one that one would perhaps least expect: general relativity,” notes a recent book on “Holographic Duality in Condensed Matter Physics.”
Let’s go back to Alice and Bob experimenting with condensed matter shells in Susskind’s lab (p. 2):
“The shell has been engineered to be at a quantum critical point, where the excitations are described by a conformal field theory having a holographic bulk dual…”
I argue that the bulk with its gravitons, black holes, and bulk observers is just as real as the laboratory itself. It can be probed, entered, measured, and the results communicated to observers in the lab...
From the holographic AdS/CFT correspondence we may assume that observers, perhaps with human-like cognitive abilities, are possible in the bulk.”
Don’t forget that the bulk is NOT the interior of the shell, but a four-dimensional space bounded by the three-dimensional shell. The observers in the lab (Alice and Bob) and the observers in the bulk can communicate with each other by generating, respectively, excitations in the shell and gravitational waves in the bulk.
The shell can be engineered to correspond to a bulk where the speed of light (the maximum signal propagation speed) is much less than the speed of light in the lab. If so, Alice and Bob can engineer signals that would appear as faster-than-light (FTL) to the bulk observers. For example, Alice could extract a signal from one side of the shell and re-insert the signal immediately into the other side. (Note: this also applies to the “standard model” case above, where everything takes place in one universe. In both cases, engineers in a “base reality” can generate FTL correlations).
Then Susskind considers two quantum-entangled matter shells in the lab, which correspond to two entangled black holes in the bulk. There’s one bulk for the two shells because “this spacetime can be holographically described by considering two identical, non-interacting copies of the conformal field theory and picking a particular entangled state” (ref. 5).
Enter ER=EPR: Entangled physical systems are connected by wormholes. I like ER=EPR because it provides a physical picture for whatever “happens” in quantum entanglement.
According to Susskind the two wormholes, one between the two shells in the lab, and one between the two black holes in the bulk, are really one and the same.
Susskind says that a lab observer could mind-upload to the shell and the bulk:
“[What] if the bulk of such a shell has no observer? In principle the lab observer can create one by applying appropriate perturbations to the shell. In fact there is nothing to prevent her from merging her own quantum state with the shell and entering into the bulk.”
“If there is anything new here it is the idea that information may pass from a laboratory environment to the degrees of freedom of a physical realization of a CFT, thereby bridging the gap between the lab and the bulk. One can enter the bulk, observe it, and go back to the lab… The first step would be to act on the observer with a unitary operation that transfers his quantum state to a set of qubits. The qubits can then be combined with the circuit. An appropriate protocol, similar to quantum teleportation, can move the observer back to the lab at a later time.”
Quantum information can be teleported between entangled quantum systems, and ER=EPR provides a physical picture for quantum teleportation: “Quantum teleportation is teleportation through the wormhole.”
Enter Tom, a software being encoded in quantum bits (qubits) and teleported from Alice to Bob. Quantum teleportation requires transferring classical information as well, so it’s limited by the speed of light, but don’t forget that the speed of light in the shells and the bulk is much slower than the speed of light in the lab.
Susskind considers a quantum teleportation protocol that can be visualized as Tom falling into Alice’s black hole, traveling through the wormhole, and coming out from Bob’s black hole as scrambled qubits that can be unscrambled (ref. 10). So, Tom can tell Bob what happened in the wormhole beyond the black hole’s horizon. If Bob sends Tina, Tom and Tina can meet in the wormhole and have a chat.
Susskind thinks future advanced quantum computers could permit carrying out these experiments: “Instead of shells supporting conformal field theories, a more practical alternative might be quantum computers simulating the CFTs.” Here I stop following him (it seems to me that the simulations wouldn’t be the real physics to which Susskind’s arguments apply), but I don’t play in Susskind’s league. I guess he thinks a quantum (as opposed to classical) simulation would be the real physics.
These, and the possibility that general relativity and quantum mechanics could be two sides of the same coin, are Susskind’s main points.
Now let me try and interpret all that with a simulation theology slant. Alice and Bob, the engineers in the lab, create and control new universes from their base reality. Alice and Bob are like God (sorry Prof. Susskind). They send sentient agents Tom and Tina on a mission in a created universe, and retrieve them after the mission. Tom and Tina are like Jesus (sorry again).
God can use similar techniques to retrieve the native inhabitants of the universe and upload them to the base reality. This is resurrection. God can escape the physical laws of the universe by operating in the base reality where different laws apply. In particular, God can engineer FTL signals in the universe, which implies that God is not limited by time.
We can also imagine that future engineers could find ways to gain access to the base reality and operate like God. They could then teleport the dead (including you and I) from the past to the present (our future). This is uploading to the future, or technological resurrection. Eventually, future humanity could teleport itself out of the universe into the base reality.