Can one electron go through two slits at the same time?
Send electrons through an interference experiment one at a time, and their landing points build a wave pattern. Each electron still arrives as one dot. Explaining that combination leads to a question: why do we see one result when the quantum description includes several?
The many worlds interpretation answers by keeping all the outcomes allowed by the quantum state. Each occurs in a branch whose observers see one result. The experiment itself doesn't tell us whether this interpretation is right.
The double slit experiment
Comparison: send a water wave toward a barrier with two openings. Beyond it, each opening sends out a wave. Where two crests meet, the waves add; where a crest meets a trough, they cancel. This produces alternating regions of strong and weak waves. That pattern is interference. Light passing through two slits produces a similar pattern of bright and dark stripes on a screen.
Thomas Young showed this kind of interference with light in 1803, by splitting a thin beam of sunlight with a narrow card, in a lecture published as Experiments and Calculations relative to physical Optics. The drawing with two slits comes from his lectures published in 1807. Light makes stripes, so light behaves like a wave.
Electrons can go through the same experiment, one at a time. An electron is a particle. When it hits a screen, it makes one small dot in one place. If electrons were tiny balls, each would go through one slit or the other, and you'd get two bright bands on the screen, one behind each slit.
In 1989, Akira Tonomura and his team at Hitachi sent electrons through an interference experiment one at a time. They used a thin wire between two plates to split the electron wave into two paths. Each electron landed as a single dot. As thousands of dots accumulated, they formed an interference pattern.
The pattern requires contributions from both paths, even with only one electron in the apparatus at a time.
When a detector leaves a distinguishable record of which path was taken, the interference disappears. In 1998, Stephan Durr, Tilman Nonn, and Gerhard Rempe demonstrated this with atoms. Their apparatus recorded the path without giving the atoms a large mechanical push. Nobody had to read the record for interference to be lost.
Superposition and the wave function
Quantum physics describes the electron with a wave function. It assigns an amplitude to each possible position. An amplitude has a magnitude and a phase, which describes where it is in its wave cycle. The phases determine how contributions from different paths add or cancel.
Before it hits the screen, the electron's wave function goes through both slits. It's in a superposition: a combination of "went through the left slit" and "went through the right slit" at the same time. Those two parts interfere with each other, which produces the stripes.
Two rules tell you what to do with the wave function.
The first is the Schrodinger equation, published by Erwin Schrodinger in 1926 in Quantisierung als Eigenwertproblem. It tells you how the wave function changes over time. It's smooth and exact: if you know the wave function now, the equation tells you what it will be later, with no randomness at all.
The second is the Born rule, proposed by Max Born the same year. It tells you the probability of each result when you measure: take the size of the number attached to that result, and square it. Born shared the Nobel Prize in Physics in 1954, "especially for his statistical interpretation of the wavefunction".
These rules predict the observed probabilities. They leave open how a measurement produces the single result that an observer records.
The measurement problem
Apply the Schrodinger equation to both an electron in superposition and its detector. The interaction correlates each possible electron result with a corresponding detector state. The equation doesn't select one of those records and discard the rest.
Yet the detector records one dot at one position.
Introductory textbooks commonly add collapse, often associated with the Copenhagen interpretation. On measurement, the wave function changes to a state corresponding to one result, with probabilities given by the Born rule.
That answer works in practice. It also raises a question nobody has answered to everyone's satisfaction: what counts as a measurement? A detector is made of atoms, and atoms follow quantum physics. So why should a detector collapse anything when a single atom doesn't, and where's the line between them? Physicists call this the measurement problem.
In 1935, Schrodinger illustrated the problem with a thought experiment in Die gegenwaertige Situation in der Quantenmechanik. A cat shares a closed box with a radioactive atom, a detector, and poison released if the atom decays. Apply quantum evolution to the whole box and the description contains both a decayed atom with a dead cat and an undecayed atom with a living cat. The problem already exists before anyone opens the lid.
Schrodinger used the cat to question how the quantum description connects to an ordinary object with a definite observed state.
Everett's many worlds
In 1957, a Princeton graduate student named Hugh Everett III proposed a radical answer. His advisor was John Wheeler. His paper was titled "Relative State" Formulation of Quantum Mechanics, published in July 1957.
His idea was to drop the collapse rule. Keep only the Schrodinger equation, and apply it to everything: the electron, the detector, the cat, and the person who opens the box.
The atom is in a superposition of decayed and not decayed. The detector interacts with it, so the detector ends up in a superposition of "clicked" and "didn't click". The cat ends up in a superposition of dead and alive. You open the box, and you end up in a superposition too: one version of you who sees a dead cat, and one who sees a living cat.
Each version records one result and remembers it consistently. In this interpretation, the full wave function contains both versions. A branch is one of the resulting histories, containing the cat, detector, and observer with matching records.
Everett called this the "relative state" formulation. In 1970, the physicist Bryce DeWitt wrote Quantum mechanics and reality in Physics Today. He described the theory as worlds that physically split, and that description, along with the name, became the most popular way to understand it: the many worlds interpretation.
Everett left academic physics after his PhD and worked for the Pentagon. Peter Byrne's account of his life describes how little recognition the proposal received at first.
Why we don't notice the branches
Decoherence explains why the different records stop interfering in practice. H. Dieter Zeh described it in 1970 in On the interpretation of measurement in quantum theory. Wojciech Zurek later reviewed how it connects quantum behavior to familiar objects in Decoherence, einselection, and the quantum origins of the classical.
An isolated electron can retain interference between its two paths. A cat interacts with air molecules and light constantly. Information about its state spreads into those surroundings, tying the different parts of the superposition to different states of the environment.
Recovering interference would require reversing the changes in the surroundings as well as in the cat. For an ordinary object, that information spreads too quickly and widely for us to control it. The different records then behave as separate histories for practical purposes.
In 1996, Serge Haroche's group in Paris watched it happen step by step, in Observing the progressive decoherence of the meter in a quantum measurement. Haroche shared the 2012 Nobel Prize in Physics with David Wineland "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems".
Physicists use decoherence whatever interpretation they prefer. It explains why big things look classical. What it doesn't explain, on its own, is why you see one result. In the Copenhagen picture, you still need collapse for that. In the many worlds picture, you don't: each branch sees its own result, and decoherence is the reason the branches stop affecting each other.
Branches in Git
Comparison: Git is a tool for keeping versions of software. Two branches share a history, then accumulate different changes. That resembles the separate histories in many worlds. Git also lets you merge branches; recombining the quantum histories of a cat would require control over the environment that recorded them.
Quantum evolution is reversible, and experiments can recover interference in controlled systems. A cat's surroundings record so much information that reversing the process is beyond any practical control.
I work on payment systems, where a transfer must happen exactly once. Many worlds wouldn't change that requirement for the people using the system. Each branch would still have to balance its own books.
The probability problem
Keeping every allowed outcome raises a problem about probability.
In the Copenhagen picture, the Born rule gives probabilities. If a measurement has a 70 percent chance of one result and a 30 percent chance of the other, you'll see the first result about 70 percent of the time.
In many worlds, both results always happen. So what does "70 percent" even mean, if both branches exist?
Defenders of many worlds have several answers. David Deutsch, in Quantum theory of probability and decisions in 1999, and David Wallace, in his 2012 book The Emergent Multiverse, argued that a rational person who knows they'll branch should make decisions as if the Born rule gives probabilities. Charles Sebens and Sean Carroll argued in Self-locating uncertainty and the origin of probability in Everettian quantum mechanics that after branching, but before you look, you're uncertain about which branch you're in, and that uncertainty follows the Born rule.
The disagreement concerns whether those arguments derive the Born rule or rely on assumptions that already contain it. Agreement with quantum experiments alone doesn't settle that question.
Does the universe split when I make a choice?
A common picture of many worlds goes like this: you decide between two jobs, and the universe splits into one branch where you took each job.
In many worlds, branching follows physical interactions that spread quantum information into the environment. A conscious decision has no special role in starting it. In a 2000 calculation of brain processes, Max Tegmark estimated decoherence times of 10−13 to 10−20 seconds, compared with neural activity at 0.001 to 0.1 seconds. He argued that the processes he studied could be treated classically. That calculation doesn't assign probabilities to the jobs a person might choose.
A radioactive decay or another small quantum event can have larger consequences over time. In many worlds, different resulting histories could include different lives. The available histories depend on the quantum state and its evolution; the theory doesn't make every imagined life possible.
What about Days of Future Past?
In Days of Future Past, the future gets rewritten. One timeline replaces another, and the old one stops existing.
Many worlds keeps the different recorded outcomes. It doesn't provide a way to send Wolverine into the past, erase a history, or visit another version of himself. The film's time travel needs assumptions beyond the interpretation.
How many physicists believe it?
Two surveys found substantial disagreement about how to interpret quantum physics.
In July 2011, Maximilian Schlosshauer, Johannes Kofler, and Anton Zeilinger asked the 33 participants of a conference on quantum foundations in Austria which interpretation they preferred. In A snapshot of foundational attitudes toward quantum mechanics, they report 42 percent for Copenhagen and 18 percent for Everett's many worlds.
In 2025, for the hundredth anniversary of quantum mechanics, Nature emailed more than 15,000 researchers and got more than 1,100 answers. According to the report, reprinted in Scientific American, 36 percent chose Copenhagen and 15 percent chose many worlds. Only about a quarter of the respondents were confident that their favorite interpretation is correct.
These were surveys of particular groups of respondents. The percentages describe their preferences; they don't measure whether an interpretation is true.
What I think
The appeal of many worlds is that it applies quantum evolution to the observer as well as the thing observed. It avoids having to specify a separate collapse process.
The probability problem is the part I can't resolve. I still think parallel universes exist, and applying the same quantum rules to large and small things is one reason I take many worlds seriously.
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