Introduction
You often confront mortality, the mystery of free will, and the search for cosmic meaning. When traditional beliefs fade, you might turn to spiritual claims or mystical interpretations of quantum mechanics for comfort. Sabine Hossenfelder argues that you should look to established physics instead.
Physics provides definitive answers to existential questions by stripping away pseudoscience. The book guides you through the fundamental nature of time, determinism, reductionism, and human consciousness. You will discover how the universe operates without relying on wishful thinking or supernatural ideas.
The following questions guide this exploration:
- Does your past permanently exist somewhere in the universe?
- Can your mind survive the biological death of your brain?
- Do you possess true free will, or are you just a machine?
- Was the cosmos deliberately designed for human existence?
1. Physics of Time and Determinism
The passage of time is not a universal constant that ticks forward for everyone at the same rate. You experience time depending on how fast you move through space and the strength of the gravitational field around you.
Einstein's theory of special and general relativity shows that proper time is the length of a curve in four-dimensional space-time. The more an object accelerates or the stronger the gravitational field it is near, the slower its proper time passes compared to an observer elsewhere. When you look at the Sun today, you see light that was emitted 8 minutes ago. When you observe the North Star, you look back 434 years into the history of its light emission.
Global positioning system satellites orbiting Earth must account for time passing differently on the satellites than on Earth due to motion and weaker gravity in order to correctly calculate locations.
The Block Universe
Different observers moving at different velocities cannot agree on whether two events happened at the same time. Because the speed of light is finite and constant for all observers in a vacuum, constructing a notion of simultaneity depends on an observer's state of motion. Observers moving relative to each other will obtain conflicting sequences for when events occur.
Using imaginary spaceships with friends named Sue and Paul, Einstein demonstrated that causally disconnected events like a birth and a distant supernova explosion can be registered as simultaneous by different observers.
According to special relativity, the past, present, and future all exist in the same way within a four-dimensional space-time.
Because all observations and notions of now from different moving reference frames are equally valid, and causally disconnected events can be simultaneous for someone, any two points in the universe exist together. This implies that past events and past versions of individuals are permanent features of space-time. John Lloyd compared time to a landscape where New York remains in existence even when you are not there.
Determinism and Information
Fundamental laws of nature are time-reversible and deterministic, meaning the future is fixed by the present and past states can be calculated from later states. Successful physical theories require an initial condition and an evolution law. Because the equations can be run both forward and backward like a movie, information about a system is conserved and no detail is ever truly lost.
In 1814, Pierre-Simon Laplace described his omniscient demon, an imaginary intelligence that could calculate the entire past and future of the universe given complete state data at one instant.
2. Thermodynamics and Human Aging
The Arrow of Time
Fundamental microscopic laws of nature are time-reversible and allow movies to run backward. Macroscopic processes are irreversible because systems start with low entropy and evolve toward the most statistically likely high-entropy equilibrium states. The second law of thermodynamics dictates that entropy cannot decrease. Useful free energy inevitably runs out and makes change impossible over time. This foundational rule relies on the past-hypothesis.
Isaac Asimov's 1956 short story "The Last Question" features a drunk man asking the Multivac computer how entropy can be decreased, receiving the persistent response of insufficient data.
Biological aging and death are fundamentally driven by entropy increase as our bodies accumulate inevitable micro-errors that cannot be perfectly repaired.
Cell-repair mechanisms fail to correct bodily errors indefinitely. Organs begin to function less efficiently as you age. Skin loses elasticity and vital systems eventually break down beyond repair.
Entropy and Ignorance
Entropy is formally a mathematical count of the microstates that give rise to a specific macrostate. It functions as a measure of our ignorance regarding exact particle configurations. A macrostate groups together many different microscopic configurations that you treat as equivalent because you cannot distinguish between them. Entropy relies on counterfactual states and human definitions of similarity.
A simple mixing model of 36 squares demonstrates how alternating adjacent flour and sugar molecules randomly transitions a separated initial state into a mixed equilibrium state. Macroscopic mixtures typically involve about 10 to the 25th power molecules.
The subjective experience of a special present moment is entirely compatible with the block universe where past, present, and future are equally real. The feeling of a unique Now is created by human memory, which stores records of the past but lacks records of the future. Objective physics does not require a special mathematical present moment to measure proper time.
Mica minerals act as natural particle detectors that retain permanent physical tracks of past radiation particles. This demonstrates that memory and the tracking of time are not exclusive to conscious brains.
3. Reductionism and Human Identity
The human body is roughly 60 percent water, 18 percent carbon, 3 percent nitrogen, 1.5 percent calcium, and 1 percent phosphorus, alongside trace amounts of potassium, sulfur, sodium, and magnesium. You might cope with speech anxiety by imagining your audience disassembled into these exact chemical elements.
Reductionism is a proven scientific fact stating that macroscopic reality emerges from microscopic physical parts through coarse-graining, making you an informational pattern rather than just particles.
Sabine Hossenfelder notes that many people reject this physical reality in favor of dualism. A 2019 survey showed that 75.8 percent of Americans and 88.3 percent of people in Singapore believed the human mind is more than a complicated biological machine.
Bags of Atoms
The atoms making up your body were forged inside stars through nuclear fusion. They were later released via supernova explosions or neutron star mergers and recycled by gravity.
An average human brain contains about a thousand trillion trillion atoms. Countless experiments confirm that these objects are made of smaller things, and understanding the small parts explains the large ones.
Although computing limitations prevent simulating a trillion trillion atoms in a brain, there is no evidence that complex systems violate reductionism.
Coarse-Graining and Substrate Independence
Physicists use coarse-graining as a rigorous mathematical averaging procedure where information below a certain cutoff is discarded. This creates effective models that describe high-level phenomena like temperature, pressure, or metallic conductivity, which do not exist at the fundamental subatomic level. Because information is permanently thrown away, deriving high-level laws from deep levels is a one-way street.
Your identity and consciousness depend on the information pattern rather than specific physical particles, which constantly change. The average age of a cell in the adult human body is seven years, based on a 2005 carbon-dating study. Skin cells are replaced every two weeks, and red blood cells are replaced every few months, making you akin to the Ship of Theseus.
In Zenon Pylyshyn's 1980 thought experiment, a person's brain neurons are replaced one by one with silicon chips while maintaining identical input-output functions. Because scales decouple, replacing biological neural substrates with functional silicon chips should preserve emergent consciousness and identity, provided the structural information and interactions remain identical.
4. Free Will and Brain Mechanics
Established physical laws are strictly deterministic with occasional quantum randomness, leaving no room for traditional uncaused free will. Future events are completely fixed by past states and the laws of nature. Because fundamental quantum events are random and humans cannot influence them, the brain merely acts as a sophisticated biological computer evaluating inputs.
A decision is the mechanical output of neural computations, not an uncaused choice.
Sabine Hossenfelder notes that 59 percent of professional philosophers in a 2009 survey identified as compatibilists. Other philosophical positions include libertarians, multiple-past compatibilists, and local-miracle compatibilists.
Determinism Rules Out Free Will
Societal moral responsibility and legal systems remain fully functional without free will because they serve as strategies to optimize human well-being and deter threats. Blaming individuals or locking away violent criminals does not require proving they possessed free will. Societies assign responsibility because dangerous behavior threatens public safety. Managing threats optimizes future outcomes regardless of whether human decisions are causally determined.
Lisa Marie Montgomery was executed in the United States on January 13, 2021. This occurred sixty-seven years after a woman was previously executed in the US. Montgomery befriended twenty-three-year-old victim Bobbie Jo Stinnett and committed the murder in 2004. She suffered extreme childhood abuse, severe brain injury, and mental illnesses, demonstrating how severe trauma shapes human actions while society still intervenes to neutralize threats after marrying her first husband at age eighteen.
Practical Rationality and Cognitive Biases
Accepting that the brain is a biological machine helps humans recognize cognitive biases and make more rational choices despite determinism. Abandoning the illusion of free will does not lead to fatalism. Instead, it allows individuals to study human cognitive shortcomings and guard against logical fallacies, loss aversion, and errors in judgment.
A 2019 survey of over 5,000 participants from 21 countries examined cognitive reflection and free will. A 2014 Scientific American article by Azim Shariff and Kathleen Vohs discussed free will doubt and moral behavior. A 2017 study on free will concluded disbelief can have a positive impact on morality.
The author shares how she delayed signing up for a frequent flyer card during years of heavy commuting due to loss aversion over past missed benefits. She overcame this bias only by recognizing her own irrationality.
You can apply these insights through concrete steps:
- pay closer attention to known shortcomings of human cognition, logical fallacies, and cognitive biases;
- evaluate input carefully and remain selective about what you read and listen to, acknowledging your brain works on collected inputs;
- recognize cognitive traps like loss aversion to avoid acting against your own best interests.
5. Cosmic Limits and Meaning
The known laws of nature rely on twenty-six constants whose values cannot be calculated from first principles. These include the fine-structure constant, Planck's constant, Newton's constant, and the cosmological constant. Human existence does not imply the universe was made for us.
The universe was not made for us, and fundamental constants cannot be calculated from first principles, yet basic research fulfills the universe's need to understand itself.
Fine-Tuning and Constants
Claiming these constants are fine-tuned for life is ascientific because you only have a single sample of data. You cannot compute a probability distribution for unobserved universes.
Sabine Hossenfelder compares pulling the constants of nature to blindly reaching into a bag. You pull out a single sheet of paper with the number 77974806905273. This tells you nothing about the probability of getting that number without knowing what else is in the bag.
There are seven billion people on Earth, sharing space with 100 billion planets in one of about 200 billion galaxies in the visible universe. About 85 percent of matter is dark matter.
The Universe and Religion
The universe cannot think or act as a giant brain. Its enormous size and the speed-of-light limit make rapid internal communication impossible.
Our current universe is 90 billion light-years in diameter. It contains 200 billion galaxies in our universe according to Hubble observations. About 3/4 of the brain is water, comparable to the 3/4 of the universe's matter-energy budget that is dark energy. It would take 11 million years to send a signal to the nearest galaxy cluster, the M81 Group, even at the speed of light.
Panpsychism, the idea that elementary particles possess rudimentary consciousness, conflicts with empirical data from particle physics. Elementary particles are classified by quantum numbers that uniquely determine their types and collision outcomes. An electron has a -1 electric charge and a +1/2 or -1/2 spin value. If quarks or electrons had internal experiences, particle collision experiments would produce vastly different variants than what is observed.
Science has fundamental limits that it cannot justify, leaving room for coexistence with religion and shared human community. Organized religion offers social integration and community that science currently lacks. The human drive to perform basic scientific research stems from a deep need to make sense of existence. Through building workable models, you contribute to the universe's ongoing self-awareness.
10 Key Ideas
- Einstein's theory of relativity shows that the passage of time depends on proper time, which is the length of a curve in four-dimensional space-time affected by an object's speed and gravitational field.
- According to special relativity, the past, present, and future all exist in the same way within a four-dimensional space-time, meaning past events and past versions of individuals are permanent features.
- Fundamental laws of nature are time-reversible and deterministic, meaning the future is fixed by the present and past states can be calculated from later states.
- The second law of thermodynamics dictates that entropy cannot decrease, and biological aging and death are fundamentally driven by this entropy increase as bodies accumulate inevitable micro-errors.
- Reductionism is a proven scientific fact stating that macroscopic reality emerges from microscopic physical parts through coarse-graining, making you an informational pattern rather than just particles.
- Your identity and consciousness depend on the information pattern rather than specific physical particles, which constantly change over time.
- Established physical laws are strictly deterministic with occasional quantum randomness, leaving no room for traditional uncaused free will.
- Societal moral responsibility and legal systems remain fully functional without free will because they serve as strategies to optimize human well-being and deter threats.
- The universe was not made for us, and fundamental constants cannot be calculated from first principles, meaning claiming these constants are fine-tuned for life is ascientific.
- The human drive to perform basic scientific research stems from a deep need to make sense of existence, fulfilling the universe's need to understand itself through building workable models.