The Strangest Idea in Science: Quantum Immortality

Source: The Strangest Idea in Science: Quantum Immortality, Cool Worlds, 36:26, uploaded 2025-03-03, Watch Later position 564.

Quantum mechanics seems to describe particles that have no fixed position until a measurement makes them choose one. Cool Worlds uses that familiar strangeness to reach a much larger question. If every possible quantum outcome continues in a separate branch of reality, then there should always be a branch in which a person survives. The thought experiment is called quantum immortality. It sounds like a claim about living forever, yet the video spends most of its time asking what counts as survival and which version of a person could experience it.

From superposition to the measurement problem

The double-slit experiment supplies the opening. Fire electrons one at a time at a wall with two narrow slits and an interference pattern appears on the other side. Each electron lands as a single mark, while the pattern builds as though each electron travelled through both slits and interfered with itself. A detector that records which slit the electron uses removes the interference pattern and leaves two bands. The measurement changes the result from wave-like behaviour to something that resembles a particle taking one path.

Richard Feynman once said that nobody understands quantum mechanics. The point is less that the mathematics fails than that the mathematics works while the picture of reality remains unsettled. Before measurement, a system evolves as a wave function. The Copenhagen interpretation says that the wave function collapses when an observation selects one outcome. The Born rule then assigns each possible result a probability given by the square of its wave function. This account agrees with experiments to extraordinary precision, which makes its missing story about what collapse is feel more serious. The interpretation predicts the results without giving an ontological mechanism for the transition.

The video separates this problem from the old idea that human awareness causes collapse. Most physicists reject that view. An interaction with the environment, such as a stray photon passing near one of the slits, can count as a measurement. That move creates a new problem because the photon should then become entangled with the electron. The superposition would spread through the measuring device, the room, the cat in the room, and eventually the observer. Everything is quantum at the fundamental level, yet ordinary life contains no visible superpositions.

Schrödinger’s cat makes the tension hard to ignore. A quantum measurement determines whether a poison bottle opens, so a cat in a sealed box should become entangled with the measurement and occupy a state that combines life and death. Opening the box appears to resolve the situation, although the Copenhagen interpretation leaves the boundary between the quantum system and the classical observer unexplained. The video treats the cat as a sign that the theory needs an account of why one part of the universe receives a definite result whilst the rest remains in the calculation.

Everett and the universal wave function

In 1957, the Princeton graduate student Hugh Everett proposed taking the Schrödinger equation at face value. The wave function never collapses. Every possible result continues in a separate, non-interacting branch of reality. Everett’s many-worlds interpretation spent decades at the edge of the physics community before gaining prominent advocates such as Sean Carroll, David Deutsch, and Max Tegmark.

The electron, the apparatus, the cat, the observer, and the universe then belong to one universal wave function. The classical appearance of the world comes from decoherence. A simple quantum system can keep the phases of its possible states in step, which allows interference. In a complex system, interactions with countless particles scramble those phases. Interference between the resulting branches becomes exponentially small, and the branches behave as separate worlds. Branching is a spreading process rather than a single moment of division. The worlds cannot later exchange information or form a council of alternate versions of the same person.

Bryce DeWitt is said to have admired the mathematics of Everett’s proposal whilst feeling that he did not experience himself splitting into countless copies. Everett’s reply was that DeWitt also did not feel the Earth orbiting the Sun at roughly 30 kilometres per second. The exchange shifts the standard of evidence. A physical process can take place without entering conscious experience, which becomes important once the process in question is a person’s death.

Quantum Russian roulette

Max Tegmark popularised the next step in his book Our Mathematical Universe. Imagine a machine that makes a quantum measurement once per second. In one outcome it fires a projectile fast enough to kill the person operating it before the person can perceive the event. In the other outcome it makes an audible click. After one measurement, many worlds contains a dead version and a living version. The living version hears the click and continues into the next measurement.

From the point of view of the surviving stream of consciousness, every result is a click. After forty clicks, the Copenhagen interpretation assigns survival a probability of one in a trillion. Many worlds contains a branch in which the person survives with certainty, and the person in that branch could treat the sequence as private evidence for Everett’s interpretation. The other branches contain observers who see the person die and learn nothing from the supposed proof. The experiment therefore cannot persuade the rest of the world. Its evidence would belong only to the branch that continues to contain the experimenter.

The video attributes a strong version of this belief to Everett through Eugene Shikhovtsev’s biography. Everett is said to have believed that his consciousness would follow each branch that did not lead to death. His life on this branch ended at 51 after academic rejection, heavy drinking and smoking, and a fatal heart attack. The source treats that fact as a warning about the difference between a claim about other branches and a claim about the life experienced here.

Tegmark gives three conditions for the thought experiment. The random event must be genuinely quantum. It must kill or remove consciousness faster than the person can perceive. It must cause death rather than severe injury. The video explicitly warns against trying anything like it. The conditions matter because they later decide whether the same reasoning applies to illness, ageing, sleep, and ordinary accidents.

From one instant of survival to a life of deterioration

Philosopher David Lewis extended quantum Russian roulette to ordinary mortality in a 2001 lecture. Every physical cause of death ultimately depends on quantum events, he argues. A car about to hit a person therefore has some tiny branch in which the vehicle quantum-tunnels through without harm. Many worlds should contain a version of the person who survives the event. After enough such events, that person would seem to accumulate personal evidence for the interpretation by escaping crashes, illness, and other threats whilst outliving friends and family.

Tegmark rejects this extension because most deaths take longer than the time needed to perceive them. A person usually experiences illness and the gradual loss of mental faculties. Lewis accepts the objection and gives the idea a darker form. Survival requires passing through each moment, not remaining healthy. A person could continue through repeated injury and deterioration, like the immortal Struldbrugg people in Gulliver’s Travels or the protagonist of Death Becomes Her. Lewis describes the result as eternal life accompanied by the loss of loved ones, eyesight, limbs, mental powers, and health.

The video tests this prediction against sleep and anaesthesia. Anthony Aguirre points out that Tegmark treats death and unconsciousness as equivalent because neither state supplies a perception. If the rule simply selected branches that contain experience, it should favour the branch in which a person stays awake all night rather than the branch in which they sleep. Dreaming gives sleep a reduced form of consciousness, although the comparison still exposes the problem. Eight hours later, both branches contain an awake person. The absence of experience during one interval does not amount to permanent death.

The mediocrity principle creates another difficulty. If quantum immortality produced an ever-growing sequence of decrepit years, the number of those years would vastly exceed the years of ordinary youth. A person should therefore expect to find themselves in an old and damaged state rather than in the relatively healthy present. The argument resembles the Doomsday argument and Boltzmann-brain reasoning, which both depend on how a reference class is chosen. A future state might belong to a different class from the present one, so the argument weakens without delivering a final refutation.

The corrected intensity rule

Lewis’s central move concerns the measure, or intensity, assigned to each branch. Suppose a quantum event has four equally likely outcomes, three of which kill a person. Copenhagen gives each outcome a 25 per cent probability and only one outcome occurs. Many worlds contains all four outcomes, each with a 25 per cent measure. Every result occurs in the sense that it has a branch, while the Born rule still gives the branches different weights.

Lewis discards the branches in which the person dies because “death is oblivion”. A dead person cannot experience being dead, so those branches should receive no weight from the first-person perspective. Renormalising the surviving branches produces the corrected intensity rule. The person then expects survival with certainty even when most of the original branch measure contains death.

David Papineau objects that Everettians have no clear reason to modify the intensity rule in life-and-death cases. David Deutsch makes the objection more sharply. He says that ignoring histories in which the decision maker is absent adds an assumption that does not follow from quantum theory, and he guesses that the assumption is false. The video accepts the force of both objections whilst keeping the logical point open. Lewis has added a premise, yet the premise could still be true. The disagreement concerns the relation between physical branch measure and the experience of a person within one branch.

Transporters, death, and the branches we count

A Star Trek transporter gives the problem a classical form. One person enters the machine and one person rematerialises elsewhere. If a malfunction creates two copies, each copy has the memories and consciousness of the original and each insists that it is real. The split resembles many worlds, where one person becomes multiple branch-relative successors. The question then becomes which copy, if any, owns the original identity.

The case becomes harder when one rematerialisation fails and leaves a body on the transporter pad. A person might reasonably fear a 50 per cent chance of dying, even though the failed copy never becomes conscious. The crew could also detect the failure and delete the second file before attempting to rematerialise it. The situation then resembles the safe one-to-one transporter, which shows how much the intuition depends on which possible histories count as part of the person’s future.

Sean Carroll rejects Lewis’s attempt to set aside death branches. A person who is killed by a gun still has reason to object even though the dead person cannot feel the killing. The future fact of being dead matters to the living person before the event. Lewis’s quantum case differs only if the living stream of consciousness continues without interruption in every branch that it can occupy. The video leaves that distinction unresolved. It also reports a further objection from philosopher Charles Sebens: the universe branches as soon as the quantum trigger fires, before any bullet reaches the person. Both successors then count as versions of the person, so one of them dies even when the process is faster than perception.

Branch measure creates a separate objection. After many rounds of quantum roulette, the surviving branches make up an astronomically small portion of the total. Lev Vaidman treats that large measure of dead branches as a good reason to avoid the experiment. Lewis’s reply would make the concern look less decisive because the branch in which a person is born and reaches the present is already a tiny fraction of all possible histories since the Big Bang. The response exposes a problem with counting worlds and with treating the number of branches as the same thing as their physical weight.

A question about the self

Quantum immortality finally turns into a dispute about personal identity. Brian Greene says that each copy is the person, provided the word you expands to include every branch-relative consciousness. The totality of copies forms the larger self. Lev Vaidman takes a more practical position. Many different people called Lev exist in different worlds, yet it is meaningless to say that another “I” exists. They share an origin at the moment of splitting and then become separate beings.

David Wallace pushes the question further. The first-person sense of I may only make sense when identity remains confined to one branch. If many worlds cannot explain how that confinement occurs, the theory may be dismantling the ordinary idea of a self rather than multiplying it. David Aguirre describes the result as a reductio ad absurdum against current accounts of many worlds and the mind. The video compares this collision between quantum mechanics and selfhood with the black hole information paradox, where quantum mechanics and general relativity also seem to imply incompatible conclusions.

The Cuban Missile Crisis supplies a historical example of how the interpretation changes the meaning of survival. On 27 October 1962, a Soviet submarine near Cuba was surrounded by American destroyers. The Americans dropped practice depth charges to signal the submarine, while its captain and political officer believed that a nuclear attack had begun. Vasily Arkhipov, the second officer, refused to approve the launch of a 10-kiloton weapon and persuaded the captain to stand down. In a many-worlds account, branches in which the launch went ahead contain no living observers like us. Our existence selects a history in which enough unlikely decisions avoided that outcome. The source turns the example into a loose form of anthropic reasoning: the chain of events leading to a person is visible only from branches in which that person remains.

What the idea can and cannot say

Cool Worlds treats it as easy and lazy to dismiss quantum immortality because it feels wrong. A rigorous dismissal has to keep the many-worlds interpretation and explain why its branches do not generate the first-person result. The thought experiment therefore works as a diagnostic for gaps in the interpretation, especially around probability, consciousness, and identity.

David Kipping remains unconvinced by Lewis’s version. Many worlds permits every outcome allowed by the physical rules, so it does not make arbitrary survival possible. The idea also predicts that a person should much more often find themselves in a far older state than the present one, which conflicts with the ordinary experience from which the argument begins. A weaker claim survives the criticism. Many worlds would contain a living successor of a person at any future time, even when that successor is no longer the same person in any useful first-person sense.

The final comfort comes from that weaker claim. Other branches could contain versions of people who made different choices, lived longer with their loved ones, or found a happier history. Those branches remain inaccessible, and the video does not treat them as evidence that personal immortality exists here. They serve as a way to imagine that the lives people lose through choice and chance still occur somewhere within the interpretation.

Limits

The note follows David Kipping’s presentation and the complete English caption track for the 36-minute video. The source presents many-worlds as a serious interpretation of quantum mechanics, while the video does not establish that interpretation as settled physics. Quantum immortality depends on extra claims about branch measure, the status of death and unconsciousness, and the identity of successors. The source presents the corrected intensity rule as a disputed assumption and leaves the dispute open.

The video attributes historical claims about Hugh Everett, the 1962 submarine incident, and the arguments of Papineau, Deutsch, Lewis, Vaidman, Wallace, Greene, Aguirre, and Sebens to its narration or named works. The reported one-in-a-trillion figure, the three criteria, and the descriptions of branch measure belong to the video’s explanatory framework. The captions contain occasional transcription errors, so the note uses conventional spellings where the intended person or work is clear. The historical sources and the underlying physics would need separate checking before these details could support a scholarly account.

Further reading / references

• Max Tegmark, “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?”, Fortschritte der Physik 46 (1998), 855–862. The video lists this paper in its description as a reference for the many-worlds interpretation.