Imagen-The Universe as a Quantum Computer: Leonard Susskind's Revolutionary Insights on Information Processing
In episode #41 of the Lex Fridman Podcast, theoretical physicist Leonard Susskind shares his profound insights on quantum mechanics, string theory, black holes, and the nature of reality. As a professor at Stanford University and founding director of Stanford Institute of Theoretical Physics, Susskind is widely regarded as one of the fathers of string theory and one of the greatest physicists of our time. This conversation offers a rare glimpse into how one of physics' most brilliant minds approaches the deepest questions about our universe.
The discussion explores everything from the intuitive versus mathematical approaches to physics, the mysteries of quantum mechanics, the purpose of string theory, and even ventures into philosophical territory regarding consciousness and the nature of reality. For anyone interested in understanding how physicists think about the fundamental structure of our universe, this conversation provides invaluable insights.
The Intuitive Physicist: Learning from Richard Feynman
Susskind begins by reflecting on his relationship with the legendary physicist Richard Feynman and how Feynman's approach to physics influenced his own. While many physicists rely heavily on mathematical formalism, Feynman was known for his deeply intuitive approach.
"What I saw was somebody who could do physics in this deeply intuitive way," Susskind recalls. "His style was almost to close his eyes and visualize the phenomena that he was thinking about and through visualization outflank the mathematical, highly mathematical and very sophisticated technical arguments that people would use."
This validation of intuitive thinking had a profound impact on Susskind. Rather than feeling constrained to work primarily through equations, Feynman showed him that visualization could be a powerful approach to physics.
"I tend not to think about the equations, I tend not to think about the symbols, I tend to try to visualize the phenomena themselves," Susskind explains. "And then when I get an insight that I think is valid, I might try to convert it to mathematics."
This approach might seem paradoxical given the counterintuitive nature of quantum physics. How can one visualize something that defies classical intuition? Susskind addresses this by explaining that physicists develop new intuitions over time:
"After time and getting familiar with these things, you develop new intuitions... It's to the point where me and many of my friends find it, many of my friends can think more easily quantum-mechanically than we can classically. We've gotten so used to it."
Rewiring the Brain for Quantum Thinking
The conversation moves into how physicists "rewire" their brains to think in quantum mechanical terms—something that doesn't come naturally to humans evolved to understand the classical world of everyday objects.
Susskind suggests that while we can develop new intuitions for quantum phenomena, there may be fundamental limits to our ability to truly visualize certain concepts. For example, visualizing higher dimensions remains challenging:
"I think we're fundamentally wired to visualize three dimensions. I can't even visualize two dimensions or one dimension without thinking about it as embedded in three-dimensional space," he notes. "Whether we can ever completely rewire ourselves to be completely comfortable with these concepts, I doubt."
This limitation doesn't prevent progress, however. Using mathematical tools and developing specialized intuitions, physicists have been able to make remarkable advances in understanding reality at scales and dimensions that defy direct visualization.
The Delicate Balance: Arrogance and Humility in Science
When discussing Feynman's infamous ego, Susskind offers an insightful perspective on the personality traits required for scientific breakthrough:
"I think you have to have both arrogance and humility," he explains. "You have to have the arrogance to say 'I can do this, nature is difficult, nature is very hard, I'm smart enough, I can do it, I can win the battle with nature.' On the other hand, I think you also have to have the humility to know that you're very likely to be wrong on any given occasion."
This delicate balance allows scientists to pursue ambitious questions while remaining open to evidence that might contradict their theories. Susskind reflects on his own journey in academia, noting that he felt like an outsider until around age 50, when he suddenly found himself "at the very center" of theoretical physics.
"I came from a very working-class background, and I was uncomfortable in academia for a long time," he shares. "They weren't doubts about my ability... they were just the discomfort in being in an environment that my family hadn't participated in. I knew nothing about it. As a young person, I didn't learn that there was such a thing called physics until I was almost 20 years old."
Quantum Computers: Simulating Reality
The discussion turns to quantum computers, with Susskind offering a clear explanation of what makes them fundamentally different from classical computers:
"The quantum computer is truly a quantum system which is actually doing the things that you're programming it to do," he explains. "If you want to program a quantum field theory, if you do it in classical physics, that program is not actually functioning in the computer as a quantum field theory. It's just solving some equations."
Susskind illustrates the extraordinary power of quantum computation with a striking example: "In order to store the amount of information that's in a quantum state of 400 spins—that's not very many, 400, I can put 400 pennies in my pocket—to do that would take more information than can possibly be stored in the entire universe if it were packed so tightly that you couldn't pack anymore in."
While quantum computers might offer exponential advantages for certain specialized problems like factoring large numbers, Susskind believes their greatest power will be in simulating quantum systems:
"If you're interested in a certain quantum system and it's too hard to simulate classically, you simply build a version of the same system... The advantages you can run it much slower, you can poke into them, you can modify them in arbitrary kinds of ways."
This capability could revolutionize our understanding of chemistry, solid-state physics, material science, quantum gravity, and quantum field theory.
Black Holes and Information
Susskind has made groundbreaking contributions to our understanding of black holes, particularly regarding what happens to information that falls into them. When asked about viewing the universe as an information processing system, he notes that all systems process information in some sense:
"All systems are information processing systems. You poke them, they change a little bit, they evolve. All systems are information processors. There's no extra magic to us humans."
This perspective raises profound questions about consciousness and intelligence. Susskind suggests that introspection might not be the best way to understand how consciousness works:
"When we do introspection, when we imagine doing introspection and try to figure out what it is when we're thinking, I think we get it wrong," he says. "Everything I've heard about the way the brain functions is so counterintuitive."
He cites examples of specialized neurons that detect specific orientations of lines—a compartmentalization that feels nothing like our subjective experience of seeing. This leads him to suggest that perhaps computer scientists and machine learning researchers might eventually provide better insights into consciousness than introspection ever could.
String Theory: A Tool for Understanding Reality
When discussing string theory, Susskind offers a perspective that differs from how it's often portrayed:
"I don't like thinking of string theory as a subject unto itself with people called string theorists who are the practitioners of this thing called string theory," he explains. "I much prefer to think of them as theoretical physicists trying to answer deep fundamental questions about nature, in particular gravity, in particular gravity and its connection with quantum mechanics, and who at the present time find string theory a useful tool."
String theory has had several lives, Susskind explains. It was originally developed to understand hadrons (protons, neutrons, and mesons), and it remains a valid theory at that level. Its second life came as a theory of gravity, operating at scales 19 orders of magnitude smaller than a proton.
The greatest contribution of string theory, according to Susskind, has been demonstrating that quantum mechanics and gravity can coexist in a mathematically consistent framework:
"Forty years ago, thirty-five years ago, people very much questioned the consistency between gravity and quantum mechanics. Stephen Hawking was very famous for it, rightly so. Now nobody questions that consistency anymore," he notes. "They don't, because we have mathematically precise string theories which contain both gravity and quantum mechanics in a consistent way."
The Arrow of Time and Reversibility
The conversation explores the puzzling nature of time's directionality. In most fundamental physics equations, time is completely symmetric—there's no inherent arrow pointing from past to future. Yet we experience time as having a definite direction.
"It's only when the phenomena involves systems which are big enough for thermodynamics to become important, for entropy to become important," Susskind explains. "For a small system, entropy is not a good concept. Entropy is something which emerges out of large numbers. It's a probabilistic idea, it's a statistical idea."
Fascinatingly, Susskind suggests that in controlled laboratory settings, we can actually reverse the apparent arrow of time for small-to-medium sized systems:
"You can take a system which is somewhere intermediate between being small and being large and make it go backward, a thing which looks like it only wants to go forward because of statistical mechanical reasons, because of the second law... you can very carefully manipulate it to make it run backward."
He illustrates this with the example of billiard balls. If you start with balls arranged in a triangle, hit them to scatter them, and could then precisely reverse the motion of each ball, they would reassemble back into the triangle. This becomes exponentially more difficult as the system grows larger, which explains why we don't observe time reversal in everyday life.
Consciousness and the Mysteries that Remain
As the conversation concludes, Susskind reflects on the questions that science might never be able to answer:
"Is there an intelligence out there that underlies the whole thing? You can call it the G-word if you want... Are we a computer simulation with a purpose? Is there an agent, an intelligent agent that underlies or is responsible for the whole thing? Does that intelligent agent satisfy the laws of physics? Does it satisfy the laws of quantum mechanics? Is it made of atoms and molecules?"
These questions, while perhaps unanswerable with current scientific methods, still feel real to Susskind. "Some philosophers would say that a question is not a question unless it's answerable. This question doesn't seem to me answerable by any known method, but it seems to me real."
This sentiment perfectly captures the humble curiosity that drives great scientists—a recognition of both what we know and the vastness of what remains unknown.
Key Points:
- Intuition versus mathematics in physics: While mathematical formalism is essential, visualization and intuition play crucial roles in developing physical theories. Physicists like Susskind and Feynman rely heavily on intuitive understanding before translating insights to mathematical language.
- Quantum computers represent a fundamentally different paradigm from classical computation—they don't merely simulate quantum systems but actually function as quantum systems, making them exponentially more powerful for certain tasks.
- String theory's greatest contribution has been demonstrating that quantum mechanics and gravity can coexist consistently in a mathematical framework, resolving a fundamental tension that concerned physicists like Stephen Hawking.
- The arrow of time emerges from statistical mechanics rather than being fundamental to physics equations. Small systems can theoretically run backward in time if precisely manipulated.
- Consciousness and intelligence remain profound mysteries that may require insights from artificial intelligence and machine learning rather than pure introspection to understand.
- Scientific progress requires both arrogance and humility—the confidence to tackle seemingly impossible problems combined with the openness to recognize when you're wrong.
- Some fundamental questions about the nature of reality and existence may lie permanently beyond the reach of scientific methods while still being meaningful to contemplate.
For the full conversation, watch the video here