Leonard Susskind
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Theoretical physicist, author of The Theoretical Minimum lectures
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Claims by Leonard Susskind (20 of 29)
Quantum Computers Need Error Correction Due to Fragility
Because quantum bits have so much capacity for complexity, they are extremely delicate and susceptible to random errors that would be insignificant for classical bits (which are robust like a coin you sneeze on), so quantum computers must build in redundancy and error correction that detect and correct errors automatically.
Black Hole Physics Yielded Quantum Error Correction Insights
Some of the most interesting recent theoretical constructions about quantum error correction have emerged from thinking about black holes, exemplifying a robust two-way dialogue between the gravitational 'It from Qubit' community and the technological quantum computing community in which lessons flow in both directions.
Quantum Information Cannot Be Cloned
Quantum mechanics forbids faithfully duplicating a quantum state, a principle Susskind independently dubbed the 'no quantum Xerox principle' and which is formally known as the no-cloning theorem; this rules out simply saving black hole information by Xeroxing one copy inside and one in the radiation.
Information Has No Unique Location in Spacetime
It is not a good question to ask where information is located absolutely; different protocols for gathering information may legitimately conclude it is in different places without contradiction, because no single observer can do both—undermining the usual assumption that information is located somewhere fixed in spacetime.
Be Nervous About Unobservable Theoretical Constructs
Physics is an empirical science, and one should be nervous when introducing into a theory mathematical things that in principle cannot be detected or empirically confirmed; if the very laws you trust always frustrate confirmation of something, it may be a signal you are thinking wrong, as with position and velocity in quantum mechanics.
Quantum Mechanics and Gravity Can Coexist Consistently
String theory and its spin-offs provide highly precise mathematical examples of theories containing gravity, electrodynamics, particles, fermions, and bosons, which establishes with certainty that quantum mechanics and gravity can fit together sensibly and consistently, overturning the prior belief that the two could not coexist.
Black Hole Complementarity Frustrates Detecting Duplication
Even if information were duplicated across the black hole horizon, the laws of quantum mechanics and gravity ensure any attempt to witness the duplication is always frustrated: to collect the information in Hawking radiation takes so long that by the time an observer jumps in, the infallen copy has already hit the singularity, so no one can ever measure both copies—analogous to the uncertainty principle for position and momentum.
Quantum Computers Could Simulate Gravity in the Lab
Quantum computers will be able to quantum-simulate Maldacena-type shells so that the quantum mechanics running inside the computer directly reflects the quantum mechanics of a system containing gravity; interacting with such computers would yield results that, taken literally, describe gravity and black holes even though opening the computer reveals only circuits—meaning quantum gravity may turn into a tool for quantum computational science.
Gravity Should Not Be Quantized Like Other Theories
Unlike quantum electrodynamics, gravity cannot be obtained by Dirac's recipe of starting with a classical theory and applying quantization rules—attempts always produce infinities or information loss—because quantum mechanics and gravity are so closely connected that separating them into 'classical theory' plus 'quantization' pulls them too far apart; gravity should instead be treated as quantum from the start.
AdS/CFT Made Holography a Precise Tool
Juan Maldacena's AdS/CFT construction, derived from string theory, turned the holographic principle from a wild speculation into a mathematically precise and convincing result—a theory in anti-de Sitter space with gravity is equivalent to a quantum field theory on its lower-dimensional boundary—and thereby into an actual tool of physics.
Holographic Bound: Information Scales With Area Not Volume
The maximum information that can be stored in a region of space is bounded by its surface area measured in Planckian pixels, not its volume, because trying to pack information at every point of a volume eventually concentrates enough energy to form a black hole larger than the region itself—implying a description in terms of degrees of freedom living on the boundary (the holographic principle).
Computational Complexity Defined as Minimal Steps
The computational complexity of a thing or process is the absolute minimal number of simple steps required to go from a starting point to it—not the number some person happened to use; a theorem's difficulty is its minimal number of logical operations from the postulates, so a simply stated theorem like the four-color theorem can be highly complex while Pythagoras is simple.
Black Hole Horizons Behave Like Materials
Black holes are a kind of material whose horizons have physical properties like viscosity and electrical conductivity and that wobble, and the mathematics of black hole surfaces has turned out to be very similar to the mathematics of fluids and superconductors, which is why condensed matter physics now heavily uses black hole mathematics.
Wormhole Traversal Equals Quantum Teleportation
Sending information into one entangled black hole and having it emerge from another through a wormhole is the exact same mathematical phenomenon as quantum teleportation—a lab-demonstrated technique using entangled systems for ultra-secure communication—and like teleportation it cannot send signals faster than light or build time machines.
Convergent Mathematics Signals a Right Track
A reliable sign that a line of theoretical work is on the right track is when its mathematics or principles turn up unexpectedly useful in other areas; really good ideas penetrate many directions beyond those they were intended for, as black hole mathematics now appears in condensed matter physics, fluids, and superconductors.
Black Hole Interior Volume Equals Computational Complexity
A black hole evolving in time is carrying out a quantum computation, and the computational complexity of that evolution has a direct meaning as the growing volume of the interior of the black hole—linking the geometric growth of a black hole's interior to the increasing complexity of its quantum state.
Questions Can Outlast Their Attempted Answers
A scientific question can be more important and leave a bigger legacy than the attempted answers to it, as with Hawking's question about black hole information, which has dominated areas of physics for decades regardless of whether his own answer was correct.
Universe Is Far Larger Than the Observable Patch
We know the universe is much larger than the part we can see—at least a thousand and more likely billions of times bigger in volume—because it appears very flat, just as a flat field implies the Earth extends well beyond the visible horizon; this leaves room for many diverse environments, only a small habitable fraction of which require special parameter values, and we necessarily find ourselves in a habitable one.
Quantum Foundations May Need Gravity First
We will not reach the end of the story on the foundations of quantum mechanics until we understand the relationship between gravity and quantum mechanics, because the two seem to influence and connect to each other in surprising ways, making it premature to give final answers about quantum mechanics in isolation.
Multiverse Is Best Available Idea for Fine-Tuning
The cosmological multiverse—where distant regions of space have different local laws of physics—is the best idea we currently have for explaining the strange fine-tunings of nature's parameters, and Susskind's recurring challenge to critics ('You got anything better?') has always been met with no alternative.
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