- String theory works in the wrong spacetime for our universe.
- String theory permits 10^500 vacuum solutions, making our universe unidentifiable.
- The LHC found no supersymmetric particles, deepening string theory's problems.
Leonard Susskind helped build string theory, and he co-founded the holographic principle. Now, late in his career, the Stanford theoretical physicist is watching it run out of road.
In a candid conversation on Curt Jaimungal's Theories of Everything podcast, Susskind lays out where physics has stalled. The interview spans de Sitter space, the landscape problem, wormholes, and why young physicists are too scared to work on the field's hardest questions.
What is de Sitter space?
De Sitter space is an expanding universe filled with a positive cosmological constant: the energy of empty space. Our universe appears to be evolving toward de Sitter geometry as dark energy drives its acceleration. Anti-de Sitter space is the mathematical opposite: it contracts, has a spatial boundary, and turns out to be far more tractable for string theory calculations. String theory is comfortable with one. We live in the other.
String Theory Built Its House on the Wrong Spacetime
Susskind's core complaint is architectural. String theory's mathematics works beautifully in a specific geometric setting: anti-de Sitter space. That geometry has a boundary, and the holographic principle (the idea that everything inside a region can be encoded on its surface) functions cleanly there.
Susskind and Gerard 't Hooft at Utrecht University developed the holographic principle together in the 1990s. It has since become one of the field's most durable results.
The problem is that we don't live in anti-de Sitter space.
Our universe is expanding, driven by dark energy. That makes it a de Sitter space. And de Sitter space has no boundary in the same sense. The holographic description breaks down. String theory, in its precise mathematical form, has no known version that works in de Sitter space.
We live in the wrong kind of world to be described by string theory. So we need to start over.
Leonard Susskind, Theories of Everything podcast
Ten to the Five Hundred Problems
The landscape issue compounds the trouble. String theory does not predict one unique universe. Its equations permit an enormous number of different solutions, or 'vacuum states', each with different particle masses, coupling constants, and physical laws. Susskind coined the term "landscape" himself to describe this proliferation of possibilities.
Key figure
10500
Commonly cited lower bound on the number of distinct string theory vacuum solutions, each with different physical constants.
Finding which solution matches our universe is, as Susskind puts it, like searching for a needle in a haystack. Except the haystack contains more solutions than there are particles in the observable universe.
The problem deepens when you drop supersymmetry, the mathematical symmetry that currently makes string theory calculable. Our universe does not appear to have supersymmetry. The Large Hadron Collider found no evidence of supersymmetric particles. And string theory without supersymmetry is, at present, not a theory anyone knows how to write down.
Susskind is blunt about this. String theory with a capital S, he says, cannot describe the world we live in. A generalization that drops supersymmetry might exist. Nobody has built it yet.
What String Theory Did Get Right
Susskind is not dismissing the theory he helped build. He argues that string theory achieved something genuinely important: it demonstrated that quantum mechanics and gravity can coexist.
For decades, physicists suspected the two frameworks were incompatible, and that quantum rules would dissolve at the scales where general relativity dominates. String theory proved, mathematically, that reconciliation is possible. That counts, even if the specific theory isn't our universe.
He also points to the ER=EPR conjecture, developed in 2013 with Juan Maldacena at the Institute for Advanced Study in Princeton. The conjecture proposes that quantum entanglement and wormholes are, in a deep sense, the same phenomenon. Two Einstein papers from 1935 - one about wormholes, one about entanglement - that nobody connected for nearly 80 years turned out to be about the same physics.
Neither ER=EPR nor the complexity-volume conjecture (which links the growing interior of black holes to quantum computational complexity) has been proven. Both have generated active research. Susskind regards them as genuine conceptual progress, not consolation prizes.
The Younger Generation Won't Touch the Hard Problems
The sharpest moment in the interview is about culture, not equations. Susskind says he is troubled that young physicists won't work on de Sitter quantum gravity, arguably the most pressing open problem in theoretical physics, because it is too risky for their careers.
Related reading
The Silence Between Black Holes
Black holes are colliding in silence across the cosmos. That silence could help explain cosmic expansion.
→"They're scared," he says in the video. "They might not get a job."
This, Susskind argues, is precisely backwards. De Sitter space is open territory. Anything you manage to say about it will be new. Thinking about problems that everyone else has already worked over is harder, not easier, he suggests. Most of the low-hanging ideas are already gone.
He acknowledges, with the directness of a physicist who no longer needs to worry about grant committees, that he probably won't solve it himself. He is 84. He doesn't know any young physicists actively working on a non-supersymmetric generalization of string theory or on de Sitter quantum gravity. That, he says plainly, is intolerable.
Active research programs at Cambridge, Princeton, and Stanford are circling the problem. The Hilbert space of de Sitter quantum gravity has received renewed attention in recent years, with proposals from several groups about how to formulate holography without a spatial boundary. Whether any of these approaches will generalize string theory into the real universe remains genuinely open.
Susskind's candor here is partly the freedom of late career. But it is also, perhaps, a fair accounting of where theoretical physics stands: with a theory that is mathematically beautiful, internally consistent, and built for a universe that turns out not to be ours.
Go Deeper
- Wormholes Untangle a Black Hole Paradox (Quanta Magazine) - How the ER=EPR conjecture emerged from the firewall paradox
- The String Theory Landscape (Wikipedia) - The 10^500 vacua problem explained in detail
- The Hilbert Space of de Sitter Quantum Gravity (JHEP, 2024) - Recent technical work on the problem Susskind describes
Fact Check: Claim-by-Claim Verification Verified
The article accurately summarizes Leonard Susskind's statements from the Theories of Everything podcast and correctly represents established physics concepts like the de Sitter crisis, string theory landscape, and holographic principle.
Commentary
- The article uses "String Theory with a capital S" for SUSY version, aligning with Susskind's precise terminology in the podcast.
- dS holography remains an open research area with ongoing proposals, not fully resolved.
- ER=EPR is a conjecture (2013, Susskind-Maldacena), not proven, but accurately presented as such.
Sources used for verification
Academic/Peer-reviewed:
- The Crisis in String Theory is Worse Than You Think | Leonard Susskind - youtube.com
- The Large N Limit of Superconformal Field Theories and Supergravity - arxiv.org
- The Hilbert space of de Sitter quantum gravity - springer.com
- Leonard Susskind Faculty Page - stanford.edu
Other reliable sources:
- Leonard Susskind - Wikipedia - wikipedia.org
- Wormholes, Quantum Entanglement Link - scientificamerican.com
Fact-checked by Perplexity Sonar Pro on 2026-03-03
