- Hossenfelder ranks ten physics paradoxes by personal importance
- Her top pick is the transporter problem: identity, not technology
- A 2025 paper confirms her view that entropy arguments are circular
Sabine Hossenfelder has been thinking about the Star Trek transporter for thirty years. Not as science fiction, but as a genuine physics problem: if you scan every atom in a person's body, transmit the information, and rebuild them from fresh atoms elsewhere, does the original person survive?
She ranked it number one on her list of unsolved physics problems in the video above.
The Unsolved Physics Problems Physicists Cannot Escape
The list is idiosyncratic. It skips some famous candidates and includes problems that most popular accounts ignore entirely.
At number ten, she begins with Boltzmann Brains: the unsettling consequence of eternal cosmic expansion. If matter persists long enough, random fluctuations will, given infinite time, assemble a functioning brain from stray atoms.
Key figure
10100 years
The approximate timescale before random atomic assembly could produce a Boltzmann Brain in an eternal universe
Not once. Infinitely often.
"What does this mean?" Hossenfelder asks. "I have no idea."
Boltzmann Brains
In a universe that lasts forever, random atomic collisions will eventually assemble any structure by pure chance, including a conscious brain complete with false memories. If the universe is eternal, these "Boltzmann Brains" would outnumber real observers by an incomprehensible margin, raising the question of whether your memories reflect reality at all.
Where Mathematics Locks the Door
Her next entry is less familiar to popular audiences. Quantum theory relies on complex numbers, values built from the square root of minus one. Yet every measurement, every observable outcome in physics, yields only real numbers.
The imaginary component never appears in what we can actually see.
Hossenfelder finds this genuinely strange, as if quantum physics hides part of reality behind a mathematical curtain.
"Is there a deeper reason for why this is so?" she asks. "Or does it just mean that there's part of reality which we haven't yet figured out how to observe?"
The remaining entries cover more widely discussed terrain: the black hole information paradox, quantum gravity, the Fermi paradox, the growth of complexity, the grandfather paradox, the arrow of time, and Schrödinger's cat. Hossenfelder is characteristically blunt about what physicists actually know versus what they claim to explain.
The Arrow of Time Has No Clean Explanation
Her treatment of the arrow of time, ranked third, is particularly pointed. The standard account holds that entropy was low at the Big Bang and has increased ever since, giving time its direction.
Hossenfelder rejects this as circular. Entropy, she argues, depends on arbitrary choices in how we define a system's states.
"Fundamentally, the entropy of the universe was zero at the beginning and it's still zero today," she says. On this reading, rising entropy is not an explanation for time's direction. It is simply a different label for the same observation.
Is there a deeper reason for why this is so? Or does it just mean that there's part of reality which we haven't yet figured out how to observe.
Sabine Hossenfelder, theoretical physicist
A late-2025 paper from the Santa Fe Institute lends unexpected weight to her scepticism. David Wolpert, Carlo Rovelli, and Jordan Scharnhorst, writing in the journal Entropy, showed that standard arguments connecting memory, entropy, and the direction of time contain circular reasoning. Physicists, their analysis suggests, routinely assume facts about the past to justify conclusions about entropy, then use those conclusions to validate the original assumptions.
The pattern is reminiscent of older debates in the foundations of thermodynamics, where Ludwig Boltzmann himself struggled to explain why entropy should increase if the underlying laws are time-symmetric. More than a century later, the question remains open.
The Question That Outlasts the Physics
The list builds toward Hossenfelder's real preoccupation.
The transporter paradox, her number one, is not about teleportation technology.
Related reading
The 60-Year Addition Problem That Fell to a Graduate Student
Oxford student Benjamin Bedert solves a 60-year-old Erdős conjecture about sum-free sets, revealing hidden structure in the simplest arithmetic.
→It is about identity. Quantum mechanics tells us that copying a quantum state necessarily destroys the original. But that formal constraint, Hossenfelder notes, does not answer the question that actually matters: what is the internal experience of the person being transported?
She has been circling this problem for three decades without resolution. The video ends there, with a gentle instruction to sleep well.
The ten paradoxes she chose share a common feature. Each reveals a place where our best mathematical descriptions of reality stop making contact with experience.
The paradoxes do not live in what we can calculate.
They live in what the calculations leave out.
Sources
- Primary Source: The Top 10 Physics Paradoxes and Unsolved Problems (Sabine Hossenfelder, 2025)
- Additional Context:
- Disentangling Boltzmann Brains, the Time-Asymmetry of Memory, and the Second Law (Wolpert, Rovelli & Scharnhorst, Entropy, 2025)
- List of unsolved problems in physics (Wikipedia)
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