How the man whose contribution to theoretical physics rivals Einstein’s admitted that the main highway of modern physics may be a dead end – and why, in his ninth decade, he draws new paths that the young avoid
📖 Introduction
We will speak of Leonard Susskind. The man who created string theory. The man whose contribution to theoretical physics measures up to Einstein’s. And the man who said loudly:
“I can tell you with absolute certainty – string theory is not a theory of the real world we live in.”
This is not an attack on string theory. This is an acknowledgment by its creator. And at the same time – a cry for help.
🧵 What Did String Theory Actually Accomplish?
Before we say where it got stuck, we must pay tribute to where it succeeded.
String theory is the first mathematical construction that placed into a single framework:
- General relativity (gravity, curved spacetime)
- Quantum mechanics (probabilities, discreteness, fields)
That is no small feat. Before it, attempts at unification shattered against infinities that emerged from equations. Strings – one-dimensional objects whose vibrations give rise to all particles and forces – stitched those wounds on paper.
When string theory incorporated supersymmetry (SUSY), it became superstring theory. Elegant. Mathematically perfect. And so powerful that, through the work of Maldacena and others, it gave birth to the holographic principle – the idea that everything happening inside a certain space can be described by a theory on its boundary.
Susskind was one of the fathers of that principle. And for a long time, it seemed the goal was near.
But then nature began to show its teeth.
⚠️ First problem: Supersymmetry – it’s not there
For string theory to describe fermions (matter, from electrons to quarks), supersymmetry must exist. It is a connection between matter particles and force particles. Every known fermion would have its supersymmetric partner – a boson. And vice versa.
Elegant. Predictable. And – nothing.
For decades we raise accelerator energies. The LHC searched for the lightest SUSY particles. Zero. We push the limits higher – maybe they are heavy, maybe they are yet to be found. But every new upper limit without a discovery whispers quietly: maybe supersymmetry does not exist.
And without it, string theory loses fermions. Loses matter. Loses us.
Susskind knows this. And that is why he says: this is not a theory of our world – at least not in its current form.
🌌 Second problem: We live in de Sitter space, not anti-de Sitter
This is more technical, but crucial.
String theory found its paradise in anti-de Sitter space (AdS). This is a negatively curved space and – most importantly – it has a boundary. A distant edge on which you can place a theory. The holographic principle says: physics inside AdS is fully described by a theory on that boundary. A mathematical gem.
The problem? Our universe is not anti-de Sitter.
Our universe is de Sitter (dS). Positively curved, expanding at an accelerating rate, and – it has no boundary. It has a horizon, similar to a black hole. But a horizon is not a boundary on which you can place a holographic theory in the same way.
Susskind and his colleagues have tried to move the entire construction from AdS to dS. The results are lukewarm. What works in AdS falls apart in dS.
Thus: a theory that claims to describe our cosmos – does not work in the geometry of our cosmos.
The irony is almost painful.
🎲 Third problem: A landscape of 10⁵⁰⁰ worlds – and none of them ours
Perhaps the most famous problem. Susskind himself contributed to its discovery.
String theory does not give a single solution. It gives a landscape – a vast number of possible vacua, each with its own laws of physics. How many? About 10⁵⁰⁰. That is a number larger than the number of atoms in the visible universe.
In that landscape, our world is just one random solution. Nothing special. No Leibnizian best of all possible worlds. No necessity. And – crucially – the theory cannot predict why our constants are the way they are. It can only say: well, somewhere in the landscape, this too exists.
For physicists who for centuries sought a unique, elegant theory – this is an intellectual defeat.
Susskind admits it. And he admits something else.
👴 What now? Ninth decade, new blackboard
Most people in their nineties enjoy retirement. Leonard Susskind does not.
He continues to work. And not on the old string theory – but on what might come after it.
His path today:
- Generalized string theory without supersymmetry – if nature lacks it, we must build a theory that does not depend on it.
- String theory in de Sitter space – a direct attack on its greatest weakness.
- SU(5) and other grand unified theories – old ideas getting a new chance.
Susskind does not claim to have the answer. But he claims the question is correctly posed: we must step off the beaten paths.
And here we come to the saddest part of the story.
🤫 The silence of the young: Why does no one follow him?
Susskind often repeats one thing in interviews that hurts more than mathematical walls:
“Younger colleagues do not work on these questions.”
Why? Because they are risky. Because they do not lead to secure publications, projects, jobs, the prestige of the scientific community.
Beaten paths are beaten for a reason – they lead to academic success. And unexplored terrain leads to Tesla’s end: brilliant, but forgotten, dies in poverty while the world uses his patents.
Young physicists today choose security. Not because they are lazy – because the system rewards what is already recognized and punishes what is uncertain.
Susskind knows this. He has already received everything. He can afford the luxury of curiosity. The young cannot.
And that may be the greatest trial of this age of science: how to preserve curiosity when your career is at stake?
⚡ Return to the roots: Tesla’s spirit on Susskind’s blackboard
This story is not new.
Galileo renounced recognition to save his life – but he did not renounce the truth.
Tesla died alone, in a hotel room, while the world celebrated Marconi. His sin? He looked where no one wanted to look.
Susskind today stands at the same crossroads. Only he is not exiled – he is recognized, celebrated, awarded. And precisely because of that, his voice carries weight.
When he says “string theory is not a theory of our world”, that is not abandoning ship. It is a call to build a new one.
When he draws new diagrams for de Sitter space without supersymmetry in his ninth decade, he sends a message: science is not a race for publications. Science is a dialogue with what we do not yet understand.
✨ Conclusion: The theory of everything has not yet been written – and that is good news
Perhaps Susskind’s new theory will succeed. Perhaps it won’t. Perhaps someone else, a young physicist reading these lines, will find a path that unites quantum mechanics and gravity without strings, without SUSY, in de Sitter space.
But one thing is certain: that path will not be beaten.
It will be difficult, uncertain, and perhaps in the end – unrecognized in its creator’s lifetime.
Just like Tesla’s. Just like Galileo’s.
But if curiosity is stronger than fear – then it is worth it.
Leonard Susskind understood that. The question is whether we will.
“The most beautiful thing we can experience is the mysterious. It is the source of all true art and science.” – Einstein
And the mysterious still awaits us. On the horizon of de Sitter space. In the silence of accelerators that find no supersymmetry. On the blackboard of an old man who has not stopped drawing.
We will continue the search.
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