Kurzgesagt – In a Nutshell
Kurzgesagt – In a Nutshell
Embedded Files
Sources – Quantum Immortality
Sources – Quantum Immortality
We thank the following experts for their comments:
Dr. Matthew Caplan
Illinois State University
Dr. Sean Carroll
Johns Hopkins University, Santa Fe Institute
—But if you kick an electron ten times exactly the same way, it shows up in different places.
Modern interpretations of quantum mechanics, including the Copenhagen and the many-worlds interpretations, consider that there is an inherent randomness in quantum systems that means that predictions can only be probabilistic even with complete knowledge of the system.
#Nath Bera, Manabendra et al. (2017): “Randomness in Quantum Mechanics: Philosophy, Physics and Technology”, Reports on Progress in Physics, vol. 80, 12
https://arxiv.org/abs/1611.02176
Quote: “[I]n quantum physics there is a new form of randomness, which is rather intrinsic or inherent to the theory. Namely, even if the state of the system is pure and we know it exactly, the predictions of quantum mechanics could be intrinsically probabilistic and random! Accepting quantum mechanics, that is assuming that the previous sentence is true, we should consequently accept that quantum mechanics could be intrinsically random. [...]
Intrinsic randomness is the randomness that persists even if we have the full knowledge about the state of the system in consideration. Even exact knowledge of the initial state does not allow to predict future evolution exactly: we can only make probabilistic predictions. Probabilities and stochastic processes are used here as a necessary and inevitable tool to describe our knowledge about the system and its behavior”
—A particle like an electron is not like a tiny marble but like a shapeshifter – a diffuse thing we call a wave, that ebbs and flows. But to make it much weirder: It’s not a wave of matter or energy, but a wave of probability – an immaterial essence that has values between 0% and 100%.
In quantum physics, particles like electrons are represented by a wave function. The wave function is not itself a “wave of probability”, but the square of the wave function gives the probability of finding an electron at each point in space.
#Encyclopedia Britannica: “Wave function” (retrieved 2025)
https://www.britannica.com/science/wave-function
Quote: “[W]ave function[: I]n quantum mechanics, variable quantity that mathematically describes the wave characteristics of a particle. The value of the wave function of a particle at a given point of space and time is related to the likelihood of the particle’s being there at the time.”
—Say you shoot an electron gun in your apartment. You shoot it 100 times – 80 times the electron shows up in your living room, and 20 times in your kitchen. So the probability wave of our electron is 80% in the living room and 20% in the kitchen.
“Electron guns” are a narrative device of this video and do not correspond directly to any real-life instrument. When you fire one, you find that 80 out of 100 times the electron goes to the living room, and 20 out of 100 times to the kitchen.
We have illustrated the electrons as "shooting out" of the gun for clarity and image readability, but they have no trajectory and appear instantly either in the living room or in the kitchen.
—What is deeply annoying is that this story of probability waves works insanely well in reality. It's not just some brainchild that only works on paper. Scientists have tested it in countless experiments and it works every single time. It explains how a lot of real things work, from information passing through microchips to the atoms fusing in the center of stars.
Quantum mechanics is one of the most experimentally supported theories in history.
Perhaps the most famous experiment that supports it is the double slit experiment, of which there are many versions. Here is one example that includes a historical review of other versions of the experiment:
#Kaur, Manpreet; Singh, Mandip (2020): “Quantum double-double-slit experiment with momentum entangled photons”, Scientific Reports, vol.10 , 11427
https://www.nature.com/articles/s41598-020-68181-1
While support for the intrinsic randomness of quantum mechanics is best exemplified by experimental violations of Bell’s inequalities (though a minority of physicists question this interpretation of the experiments).
#Nath Bera, Manabendra et al. (2017): “Randomness in Quantum Mechanics: Philosophy, Physics and Technology”, Reports on Progress in Physics, vol. 80, 12
https://arxiv.org/pdf/1611.02176
Quote: “In 1964, Bell showed that all theories that satisfy locality and realism (in the sense of EPR) are incompatible with quantum mechanics (Bell, 1964, 1966). In a simple experiment, mimicking Bell’s scenario, two correlated quantum particles are sent to two spatially separated measuring devices (see Fig. 1), and each device can perform two different measurements with two possible outcomes. The measurement processes are space-like separated and no communication is possible when these are performed. With this configuration a local-realistic model gives bounds on the correlation between the outcomes observed in the two measurement devices, called Bell inequalities (Bell, 1964). In other words, impossibility of instantaneous communication (no-signaling) between spatially separated systems together with full local determinism imply that correlations between measurement results must obey the Bell inequalities.
Strikingly, these inequalities are violated with correlated (entangled) quantum particles, and therefore have no explanations in terms of deterministic local hidden variables. Infact, the correlations predicted by the no-signaling and determinism are exactly the same as predicted by EPR model, and they are equivalent. The experimental violations of the Bell inequalities in 1972 (Freedman and Clauser, 1972), in 1981 (Aspect et al., 1981) and in 1982 (Aspect et al., 1982), along with the recent loophole-free Bell-inequality violations (Giustina et al., 2015; Hensen et al., 2015; Shalm et al., 2015) confirm that any local-realistic theory is unable to predict the correlations observed in quantum mechanics. It immediately implies that either no-signaling or local determinism has to be abandoned. For the most physicists, it is favorable to dump local determinism and save no-signaling. Assuming that the nature respects no-signaling principle, any violation of Bell inequality implies thus that the outcomes could not be predetermined in advance.”
Quantum mechanics has tangible consequences on our everyday lives. For example, the band structure of semiconductors and their ability to carry electric charge, a critical factor in the function of almost all electronics today, is a quantum behaviour.
#Goldsman, Neil; Darmody, Christopher (2021): “Quantum and Semiconductor Device Physics: A Concise Introduction”
https://user.eng.umd.edu/~neil/enee307/Quantum_and_Device_Phyiscs_Goldsman_&_%20Darmody.pdf
Quote: “Electrons and holes, the basic charge carriers in electronics are quantum mechanical entities. While virtually all electronic devices are influenced by quantum mechanics, some devices operate virtually totally on quantum mechanical principles. These devices include Flash Memory Sticks, Solar Cells, solid state Lasers, LEDs and LED lighting.”
#Orzel, Chad (2025): “What Has Quantum Mechanics Ever Done For Us?”, Forbes
https://www.forbes.com/sites/chadorzel/2015/08/13/what-has-quantum-mechanics-ever-done-for-us/
Quote: “At bottom, the entire computer industry is built on quantum mechanics. Modern semiconductor-based electronics rely on the band structure of solid objects. This is fundamentally a quantum phenomenon, depending on the wave nature of electrons, and because we understand that wave nature, we can manipulate the electrical properties of silicon. Mixing in just a tiny fraction of the right other elements changes the band structure and thus the conductivity; we know exactly what to add and how much to use thanks to our detailed understanding of the quantum nature of matter.
Stacking up layers of silicon doped with different elements allows us to make transistors on the nanometer scale. Millions of these packed together in a single block of material make the computer chips that power all the technological gadgets that are so central to modern life.”
And the nuclear fusion reactions that power the Sun only happen due to quantum tunnelling.
#Siegel, Ethan (2025): ”The quantum reason that explains why the Sun shines”, BigThink
https://bigthink.com/starts-with-a-bang/quantum-reason-sun-shines/
Quote: “Because of the quantum nature of these protons, the wavefunctions of two protons can overlap. Even protons which don’t have enough energy to overcome the repulsive electric force between them can see their wavefunctions overlap, and that overlapping means they have a finite probability of experiencing quantum tunneling: where they can end up in a more stable bound state than their initial, free state.”
—The position of the linguists is “shut up and calculate”. They basically claim that quantum mechanics is not a story about reality, but just a kind of grammar for the theoretical language of the universe. With this language we can predict experiments. Nothing else.
The “linguists” in our video represent the proponents of the Copenhagen interpretation:
#Internet Encyclopedia of Philosophy: “Interpretations of Quantum Mechanics” (retrieved 2025)
Quote: “The problematic nature of quantum mechanics stems from the fact that the theory often represents the state of a system using a sum of several terms, where each term apparently represents a distinct physical state of the system. What’s more, these terms interact with each other, and this interaction is crucial to the theory’s predictions. If one takes this representation literally, it looks as if the system exists in several incompatible physical states at once. And yet when the physicist makes a measurement on the system, only one of these incompatible states is manifest in the result of the measurement. What makes this especially puzzling is that there is nothing in the physical nature of a measurement that could privilege one of the terms over the others.
According to the Copenhagen interpretation of quantum mechanics, the solution to this puzzle is that the quantum state should not be taken as a description of the physical system. Rather, the role of the quantum state is to summarize what we can expect if we make measurements on the system.”
#Stanford Encyclopedia of Philosophy (2024): “Copenhagen Interpretation of Quantum Mechanics”
https://plato.stanford.edu/entries/qm-copenhagen/
Quote: “The Copenhagen interpretation was the first general attempt to understand the world of atoms as this is represented by quantum mechanics. The founding father was mainly the Danish physicist Niels Bohr, but also Werner Heisenberg, Max Born and other physicists made important contributions to the overall understanding of the atomic world that is associated with the name of the capital of Denmark.
[...] A further issue is then how to interpret a physical theory. Does or doesn’t the quantum formalism, according to Bohr, represent the world over and above being a tool for prediction?
Here are four statements which seem to show that Bohr was an instrumentalist concerning scientific theories in general and the quantum formalism in particular.
“The purpose of scientific theories “is not to disclose the real essence of phenomena but only to track down, so far as it is possible, relations between the manifold aspects of experience” (APHK, p. 71).
“The ingenious formalism of quantum mechanics, which abandons pictorial representation and aims directly at a statistical account of quantum processes …” (CC, p. 152).
“The formalism thus defies pictorial representation and aims directly at prediction of observations appearing under well-defined conditions” (CC, p. 172).
“The entire formalism is to be considered as a tool for deriving predictions of definite and statistical character …” (CC, p. 144).
In these four statements Bohr mentions the absence of “pictorial representation” twice in relation to the quantum formalism. The term “pictorial representation” stands for a representation that helps us to visualize what it represents in contrast to “symbolic representation”. A pictorial representation is a formalism that has an isomorphic relation to the objects it represents such that the visualized structure of the representation corresponds to a similar structure in nature. Conversely, a symbolic representation does not stand for anything visualizable. It is an abstract tool whose function it is to calculate a result whenever this representation is applied to an experimental situation.”
It must be noted that some scholars argue that physicists like Bohr, traditionally associated with the Copenhagen interpretation, may have considered quantum physics to describe the world in a more subtle sense, instead of strictly holding the “shut up and calculate” views we associate with the Copenhagen interpretation today.
—They are “many-worldians” and they are convinced that quantum mechanics IS a story about reality. And they want to interpret the story. The many-worlds interpretation of quantum mechanics is not the same as the multiverse by the way, which is a whole different can of worms, but we’ll get to that another time.
#Internet Encyclopedia of Philosophy: “Interpretations of Quantum Mechanics” (retrieved 2025)
Quote: “The problematic nature of quantum mechanics stems from the fact that the theory often represents the state of a system using a sum of several terms, where each term apparently represents a distinct physical state of the system. What’s more, these terms interact with each other, and this interaction is crucial to the theory’s predictions. If one takes this representation literally, it looks as if the system exists in several incompatible physical states at once. And yet when the physicist makes a measurement on the system, only one of these incompatible states is manifest in the result of the measurement. What makes this especially puzzling is that there is nothing in the physical nature of a measurement that could privilege one of the terms over the others. [...]
According to the many-worlds interpretation, the quantum state is to be taken as a description of the system, and the solution to the puzzle is that each term in that description produces a corresponding measurement outcome. That is, for any quantum measurement there are generally multiple measurement results occurring on distinct “branches” of reality.”
We thank our expert Matt Caplan for the following comment on the difference between “many-worlds” and “multiverse”:
Quote: “As a very simple analogy, in the 'many worlds' interpretation there are many games of chess all happening on the same board, e.g. black vs white, red vs blue, yellow vs green, etc, but the pieces from each game only play among themselves. For simplicity, we would only call it a 'multiverse' if the pieces were also on different boards.
To clarify a bit more, physicists use the word 'universe' in a lot of different ways that often differ from how the public uses the word, and that can at times be confusing. But this ultimately ends up being semantics. For example, in 'black hole cosmologies' our universe is the interior of a black hole in another universe, and so this is a 'multiverse' of universes within black holes. That is clearly distinct from the "many worlds" quantum mechanical interpretation that is the subject of this video. You can certainly find physicists who prefer to call our state of the wavefunction in 'many words' the 'universe', with every other state forming the 'multiverse,' but this is likely to cause confusion with the public given the cosmological associations the word 'universe' has (and the comic book associations the word 'multiverse' has!), so we're avoiding that usage here. In short, we're reserving 'multiverse' to mean separate spacetime patches rather than separate states of the universal wavefunction.”
—Each of these different versions of you and the electron are equally real, equally true. And all of them exist at this moment, in your house. But they can’t communicate or interact in any way, so they are totally invisible to each other. Which means that, whoever “you” are right now, you are just one of your versions, experiencing just one thing – either seeing the electron in the living room, or in the kitchen.
#Internet Encyclopedia of Philosophy: “Everettian Interpretations of Quantum Mechanics” (retrieved 2025)
Quote: “What is arguably the most common interpretation of Everett’s formulation is what Bryce DeWitt called the many-worlds interpretation [MWI] (DeWitt 1968 and Wheeler 1998: 269–70). In his 1967 lecture on what he calls the “Everett-Wheeler interpretation” of quantum mechanics, DeWitt takes Everett’s claim that:
. . . with each succeeding observation (or interaction), the observer state “branches” into a number of different outcomes of the measurement . . . for the object-system state. All branches exist simultaneously in the superposition after any given sequence of observations (Everett 1957a: 25–6; Everett 1957b: 146).
to imply that we are forced:
. . . to believe in the ‘reality’ of all the simultaneous ‘worlds’ represented in the superposition [in which we find the universe after a measurement interaction] . . . in each of which the measurement has yielded a different outcome (DeWitt 1968: 326).[...]
[Everett argues that]
From the viewpoint of the theory, all elements of a superposition (all “branches”) are “actual,” none any more “real” than another. It is completely unnecessary to suppose that after an observation somehow one element of the final superposition is selected to be awarded with a mysterious quality called “reality” and the others condemned to oblivion – they won’t cause any trouble anyway because all the separate elements of the superposition (“branches”) individually obey the wave equation with complete indifference to the presence of absence (“actuality” or not) of any other elements.”
—Every possible quantum process means there are other possible worlds. A radioactive atom decays? Another world exists where it didn’t. A cosmic ray hits one of your cells? Another world exists where the ray just passed through. Each second, bazillions upon bazillions of new worlds exist on top of each other.
#Internet Encyclopedia of Philosophy: “Everettian Interpretations of Quantum Mechanics” (retrieved 2025)
Quote: “The branching that occurs in DeWitt’s MWI can be interpreted in several different ways. One possibility is DeWitt’s way, which suggests that:
[o]ur universe must be viewed as constantly splitting into a stupendous number of branches, all resulting from the measurement like interactions between its myriads of components . . . every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies of itself (DeWitt 1970: 178).
DeWitt takes a strong realist position in regards to the worlds that are the result of the branches splitting. He takes each branch to be “a possible universe-as-we-actually-see-it” (DeWitt 1970: 163) and believes that in spite of the fact that “all branches must be regarded as equally real” (DeWitt 1970: 178; see also Everett 1957b: note added in proof), we inhabit only one of the worlds that go to make up reality and we have no access to other worlds (DeWitt 1970: 182).”
—All we need are two electron detectors connected to a nuclear bomb in your living room. If the detector in your living room is activated the nuke explodes. If the one in the kitchen is activated you are safe.
This is a version of “quantum suicide”, an experiment that, if performed, would theoretically allow the experimenter to distinguish between the many-worlds and the Copenhagen interpretations of quantum mechanics. Another version is illustrated in this classic paper:
#Tegmark, Max (1999): “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?”, Fortschritte der Physik - Progress of Physics, vol. 46, 855-862
https://arxiv.org/abs/quant-ph/9709032
Quote: “Is there then any experiment that could distinguish between say the [many-worlds interpretation] and the Copenhagen interpretation using currently available technology? The author can only think of one: a form of quantum suicide in a spirit similar to so-called quantum roulette. It requires quite a dedicated experimentalist, since it amounts to an iterated and faster version of Schrödinger’s cat experiment [35] — with you as the cat. The apparatus is a “quantum gun” which, each time its trigger is pulled, measures the z-spin of a particle in the state (|↑⟩+ |↓⟩)/ √2. It is connected to a machine gun that fires a single bullet if the result is “down” and merely makes an audible click if the result is “up”.The details of the trigger mechanism are irrelevant [...] as long as the timescale between the quantum bit generation and the actual firing is much shorter than that characteristic of human perception, say 10−2 seconds. [...] She now instructs her assistant to pull the trigger ten more times and places her head in front of the gun barrel.”
— If there is only one universe, you’ll die fairly soon.
#Tegmark, Max (1999): “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?”, Fortschritte der Physik - Progress of Physics, vol. 46, 855-862
https://arxiv.org/abs/quant-ph/9709032
Quote: “In interpretations where there is an explicit non-unitary collapse, she will be either dead or alive after the first trigger event, so she should expect to perceive perhaps a click or two (if she is moderately lucky), then “game over”, nothing at all. “
—But if the many worlds interpretation is true, then every time your assistant shoots the gun, there are five versions of you and four instantly die. Only one of your versions is alive and there’s only one thing you can experience – your survival.
#Tegmark, Max (1999): “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?”, Fortschritte der Physik - Progress of Physics, vol. 46, 855-862
https://arxiv.org/abs/quant-ph/9709032
Quote: “She now instructs her assistant to pull the trigger ten more times and places her head in front of the gun barrel. This time the shut-up-and-calculate recipe is inapplicable, since probabilties have no meaning for an observer in the dead state, and the contenders will differ in their predictions. In interpretations where there is an explicit non-unitary collapse, she will be either dead or alive after the first trigger event, so she should expect to perceive perhaps a click or two (if she is moderately lucky), then “game over”, nothing at all. In the MWI, on the other hand, the state after the first trigger event is
Since there is exactly one observer having perceptions both before and after the trigger event, and since it occurred too fast to notice, the MWI prediction is that [the experimenter] will hear “click” with 100% certainty. When her assistant has completed his unenviable assignment, she will have heard ten clicks, and concluded that collapse interpretations of quantum mechanics are ruled out at a confidence level of 1 − 0.5n ≈ 99.9%.”
—In a universe with just one “reality”, your odds of surviving 100 times in a row are about 1 in ten duovigintillion – 1 followed by 70 zeroes.
As stated earlier in the script, the electron has a 80% chance of showing up in your living room and triggering the bomb, so you have a 20% chance of surviving each time the electron is measured. This means that the odds of surviving after 100 measurements are:
0.2100 ~ 1.27 × 10-70
Or, alternatively, one in 8 × 1069, or eight duovigintillion. For ease of representation, we approximate this number to 1070.
—Yes, you killed hundreds of versions of yourself. But you now know for sure that the many worlds are true. Because you are still here experiencing things.
It is important to note that you only “survive” in one of the world-branches.
#Tegmark, Max (1999): “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?”, Fortschritte der Physik - Progress of Physics, vol. 46, 855-862
https://arxiv.org/abs/quant-ph/9709032
Quote: “Note, however, that almost all terms in the final superposition will have her assistant perceiving that he has killed his boss.”
—A tumor starts, but a cosmic ray kills it before it spreads. A lightning strikes at you, but a quantum fluke makes it miss by a meter. A washing machine falls from a roof, but all its atoms quantum-tunnel through your body. No matter how extremely high the likelihood is that you will die, there may always be a branch in which you survive.
In the many-worlds Interpretation of quantum physics, each event that can happen does happen, but in different world-branches of the same world-tree.
If you always follow the best outcome of each branching, you’ll observe a reality where you keep surviving everything that comes at you. Even events like quantum tunneling of large objects (like washing machines) still have a very tiny possibility of happening,
#Strassler, Matt (2014): “Tunneling: A Quantum Process”
so they have their own multiverse branch.
What is more, for very sudden possible causes of death, like the one in our quantum suicide experiment, the only “you” that could perceive anything on any branches after the deed is the one that survives!
This is not so for most ordinary causes of death, which take place over time in multiple steps, explaining why we have a different ordinary experience of death and dying:
#Tegmark, Max (1998): “Quantum immortality”
https://space.mit.edu/home/tegmark/quantum.html
Quote: “Here's a brief comment on the issue of whether the MWI implies subjective immortality. [...] . After all, dying isn't a binary thing where you're either dead or alive - rather, there's a whole continuum of states of progressively decreasing self-awareness. What makes the quantum suicide work is that you force an abrupt transition. I suspect that when I get old, my brain cells will gradually give out (indeed, that's already started happening...) so that I keep feeling self-aware, but less and less so, the final "death" being quite anti-climactic, sort of like when an amoeba croaks.”
This, of course, excludes situations where there is a completely zero chance of survival even from a quantum-mechanical point of view. For example, if your assistant in the quantum suicide experiment is evil and has tricked the bomb to explode with 100% likelihood, no matter where the electron is detected. In those cases, you would die in all world-branches. Choose your assistant carefully.
—So should you start wingsuit-flying today? Well, not so fast. For every version of you that survives, bazillions don’t.
It is important to note that, even if the many-worlds interpretation is true, highly dangerous actions will result in correspondingly high numbers of world-branches in which you are dead.
#Tegmark, Max (1999): “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?”, Fortschritte der Physik - Progress of Physics, vol. 46, 855-862
https://arxiv.org/abs/quant-ph/9709032
Quote: “Note, however, that almost all terms in the final superposition will have her assistant perceiving that he has killed his boss.”
—And there is another good reason to move. While the many-worlds interpretation of quantum mechanics feels beautiful and elegant, that doesn’t make it true.
#Internet Encyclopedia of Philosophy: “Everettian Interpretations of Quantum Mechanics” (retrieved 2025)
Quote: “There is one other debate that ought to be considered before embarking on our project. And that is the debate over the appropriate way to explain the results of quantum mechanical experiments. Everett’s proposal for pure wave mechanics [from which the many-worlds interpretation derives] is but one way physicists explain what seem to be counterintuitive outcomes of quantum mechanical experiments. [...] Whether or not the unitary dynamics proposed by Everett are the correct laws for describing the world is a question that is far from decided.”
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