Quantum Mechanics is a strange and counterintuitive theory. Dealing with the behaviour of subatomic particles, it is extremely difficult to reconcile with our experience of everyday life. Niels Bohr, one of the theory’s father figures, famously said, “Anyone who is not shocked by quantum theory has not understood it.”

The inherent counterintuitiveness of Quantum Mechanics has led some scientists to adopt an approach termed “shut up and calculate;” intentionally resisting the urge to attribute meaning and interpretation to the predictions of  the mathematical equations behind the theory. Others have attempted to ascribe meaning to Quantum Theory, attempts which have resulted in the multitude of interpretations prevalent today.

Quantum Suicide is a fun, quirky thought experiment that I particularly like because it makes an introspective argument, rather than an observational one, casting us, the readers, as both experimenter and experimentee.

Published independently by Hans Moravec in 1987 and Bruno Marchal in 1988, and developed further by Max Tegmark in 1998, it is the only practical experiment capable of distinguishing between the two leading interpretations of quantum mechanics: the Copenhagen (and a host of similar interpretations) and the Many-Worlds interpretations. The distinction is accomplished by means of a variation on Schrödinger’s cat thought experiment, this time from the cat’s point of view.

In classical physics, the solutions to mathematical equations that describe physical systems are specific, discrete values. For example, a planet orbiting a star has a specific measurable direction and angular momentum. On the other hand, the equations that govern quantum physics assign probabilities to the set of possible outcomes. It is only when we attempt to measure the actual state of a system that it reverts to a classical behaviour, and we obtain a discrete value. For example, a fundamental particle can spin clockwise and counterclockwise at the same time – until you look at it, at which point it definitely becomes one or the other. More precisely a quark, a fundamental particle with a property called spin, which can have values of both up and down at the same time until a measurement is made, after which it can only be one or the other, up or down. When a measurement is made and the system shifts from indeterminism to determinism it is said that the function used to describe the set of possible outcomes collapses.

This odd property of Quantum Mechanics is called Quantum Indeterminacy, which both the Copenhagen and the Many-Worlds interpretations attempt to explain.

The Copenhagen Interpretation of Quantum Mechanics (developed by Niels Bohr, Werner Heisenberg and collaborators in the 1920s) claims that observed indeterminacy (in our example, particles spinning both ways at the same time) is a statement not about the limits of measurement, but about the nature of reality — that is to say, that determined positions and velocities simply do not exist for fundamental particles before a measurement is made. Measurements select from the many possibilities and narrow them down to one. The theory claims that observing reality fundamentally changes it. Several additional interpretations are similar to the Copenhagen interpretation in that they also create a link between observervations and reality.

The Many-Worlds Interpretation (Hugh Everett, 1957), ignored for years after its appearance, states that the equations used to predict quantum phenomena continue to hold after observation is made. Every time a measurement is made, all of the possible outcomes actually occur in different branches of reality, creating a multitude of parallel worlds (one world where the particle is spinning clockwise and another where it is spinning counterclockwise).

The Many-Worlds interpretation presents a more elegant alternative to the Copenhagen Interpretation in terms of its underlying assumptions because it does not assert a relation between observer’s consciousness and reality.

A more intuitive explanation of the Many-Worlds Interpretation is provided by Andrew Zimmerman Jones and Daniel Robbins:

“The universe is continually splitting apart as every quantum question is resolved in every possible way across an immense multiverse of parallel universes.

This is one of the most unusual concepts to come out of quantum physics, but it has its own merit. Like the work of Einstein, Everett arrived at this theory in part by taking the mathematics of quantum theory and assuming it could be taken literally. If the equation shows that there are two possibilities, then why not assume that there are two possibilities?

In the case of the Schrödinger’s cat experiment, when you look inside the box, instead of something odd happening to the quantum system, you actually become part of the quantum system. You now exist in two states — one state that has found a dead cat and one state that has found a living cat.”

But how are we to determine which of the two interpretations accurately describes the world we live in? This is where the Quantum Suicide thought experiment steps in. It attempts to offer an empirical way distinguishing between the two types of interpretations.

Tegmark describes the ”Quantum Suicide Experiment” as follows:

note that this is a simplified version of the text without the mathematical proofs

“The apparatus is a “quantum gun” which each time its trigger is pulled measures the spin of a particle (particles can be spin up or spin down, seemingly at random). 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 experimenter first places a sandbag in front of the gun and tells her assistant to pull the trigger ten times. All Quantum Mechanics interpretations predict that she will hear a seemingly random sequence of shots and duds such as “bang-click-bang-bang-bang-click-click-bang-click-click”.

She now instructs her assistant to pull the trigger ten more times and places her head in front of the barrel. This time the non Multiple World interpretations have no meaning for an observer in the dead state… and they will differ in their predictions. In interpretations where there is an explicit 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 Multiple World Interpretation, on the other hand, the prediction is that the experimenter will hear “click” with 100% certainty. When her assistant has completed this unenviable assignment, she will have heard ten clicks, and concluded that the collapse interpretations of quantum mechanics (all but the Multiple World Interpretation) are ruled out to a confidence level of 1-0.5n ˜ 99.9% .

Note, however, that almost all instances will have her assistant perceiving that he has killed his boss.”

The experiment really only works from the point of view of the experimenter. In most worlds, there is one less experimenter, but the experimenter herself does not experience death.

In other words, the two interpretations differ in how they view the interaction between quantum phenomena and human consciousness. In the experiment above, the machine gun is triggered by a quantum phenomena, whilst the bullet hitting the experimenter’s head is equivalent to an observation made by a continuous being.

According the the Copenhagen Interpretation; when the trigger is pulled and a measurement is made, the machine gun enters a state where it is has both fired and not fired, reflecting the two possible spin value measurements, each with 50% probability (the technical term for this state is “superposition”). But because the experimenter’s head is in the flightpath of the bullet, it is in a way conducting a measurement, admittedly not in the most elegant fashion. This results in a deterministic outcome where the machine gun either has or has not fired, and the experimenter is either dead or alive. The quantum behaviour of the system is hidden from everyone, including the experimenter. All observers see a classical system in which each round results in either a live or a dead experimenter.

The Many-World interpretation differs from the point of view of the experimenter. When the trigger is pulled and a measurement is made the universe splits into two, reflecting the two possible spin value measurements, each with 50% probability. In one universe the machine gun has fired and the experimenter has died, whilst in the other she is still alive. Of course, the experimenter only lives to experience the second universe.

By repeating the experiment 10 times, the experimenter can conclude with 99.9% confidence that, since she is still alive, the Many-Worlds interpretation is true: she can repeat the experiment a greater number of times to improve her confidence. The experimenter is privileged over all other observers since only she has direct access to her own consciousness.

An interesting implication of the experiment is the notion of Quantum Immortality:

“Quantum immortality, which posits that no one ever dies, they only appear to. Whenever I might die, there will be another universe in which I still live, some quantum event (however remotely unlikely) which saves me from death. Hence, it is argued, I will never actually experience my own death, but from my own perspective will live forever, even as countless others will witness me die countless times. Life however will get very lonely, since everyone I know will eventually die (from my perspective), and it will seem I am the only one who is living forever — in fact, everyone else is living forever also, but in different universes from me.”

Another related concept is that of the Doomsday Device, described in Hans Moravec’s book Mind Children – The Future of Robot and Human Intelligence:

“Two builders of a future super (immensely expensive) particle accelerator have a problem. The machine has been completed for months, but so far has failed on each attempt to use it. The problem is not in the design but seemingly just in the designer’s bad luck. Lightning caused a power outage just at turn on, or a fuse blew, or a janitor tripped over a cable, or a little earthquake triggered an emergency cutoff; each incident was different, and apparently unrelated to the others.

But perhaps the failures are an enormous stroke of luck. New calculations suggest that the machine is powerful enough to trigger a collapse of the vacuum to a lower energy state. A cosmic explosion might radiate out at the speed of light from the accelerator’s collision point, eventually destroying the entire universe. What a close call!

Or was it? If the universe had been destroyed, there would be no one left to lament the fact. What if the many-worlds idea were correct? In some universes the machine would have worked. For all practical purposes those worlds would have ceased to exist. Only in the remainder would a pair of puzzled physicists be scratching their heads, wondering what had gone wrong this time.

Given so many nearly identical universes, the destruction of a few seams of small consequence. An idea strikes them. Why not reinforce the weak points in the machine so that a random failure within it is extremely unlikely, then wire it to a detector of a nuclear attack, like the doomsday machine in Stanley Kubrick’s film Dr. Strangelove? An attack would be met by the destruction of the offending universe. Only those universes in which the attack had not happened, for some reason (the commanding general had a heart attack, the missile launch system failed, the premier had a fit of compassion…), would live to wonder about yet another close call.

The machine in Strangelove was ineffective as a deterrent unless the other side was aware of it. Not so the many-worlds version. No attack (that anyone will notice) can occur so long as it operates, no matter how secret its existence.“

In the movie Dr. Strangelove, the Russians create a doomsday device which will render the world uninhabitable for many years if a nuclear attack is launched against them. It is an absurd yet effective deterrent, mocking the stockpiles of nuclear weapons held by the US and Russia with the aim of achieving roughly the same goals. Much like these aging stockpiles, the doomsday device only works as an effective deterrent if the other side knows it exists. The advantage of the device described in the above experiment, which destroys the known universe upon an attacked, is that the enemy does not need to know about it’s existence for it to work. Although destroying the known universe might sound like a bad idea if our objective is to survive, if the universe indeed splits into many universes at every quantum decision point, and we stay alive in at least one of them, then who cares? That’s the beauty of Quantum Indeterminacy.

A note on consciousness

Several readers pointed out on reddit and in the comments below that that the Copenhagen interpretation does not make an explicit connection between consciousness and reality. Reddit user Quoggle commented:

“People really need to stop saying that the Copenhagen interpretation implies that consciousness causes wave function collapse.”

ForgetfulPotato elaborated:

“What “observe” means is more or less “interacts with.” Does the electron hit a piece of paper? That’s an “observation” because the electron is interacting with the paper. It’s not clearly defined what requires the wavefunction to collapse (there are plenty of things that don’t cause it to collapse), but it certainly doesn’t require “consciousness.” If a photon hits film, the wavefunction collapses. It doesn’t matter if there’s any consciousness there to “observe” it, the “observation” in the QM sense is the interaction with the film.

tl/dr: observe in this case means interacts with another object in a certain way, not be watched by a person.”

Both have a point, and I should have been far more careful before stepping into the consciousness trap. But there’s a good argument that the role of consciousness in Quantum Indeterminism is far from decided. What constitutes an “observer” or an “observation” is not directly specified by the Copenhagen interpretation. Reddit user person594 replies to the above comment:

“It’s not nearly as simple as that though, and while the Copenhagen doesn’t necessarily say anything about consciousness, it certainly leaves the notion of observation much vaguer than you make it seem. You mentioned that observation is just interaction, but most interactions don’t cause quantum states to collapse. […] It seems only certain kinds of interactions cause quantum states to collapse — generally ones that would involve the state becoming entangled with a macroscopic system. The Copenhagen interpretation calls these sort of interactions measurements, but there is definitely a gap to be filled in explaining why a photon hitting a piece of film is a measurement, while a qubit entangling with its neighbor isn’t.”

PM_ME_UR_Definitions added:

“And the reason people think that consciousness causes collapse has nothing to do with a misinterpretation. There’s two good reasons to think that it’s possible that conscious observation causes collapse:

  • There’s no clear end point for what counts as an observation. One electron interacts with a proton, that’s the most basic form of an observation, but that doesn’t cause a collapse. What about a bunch of protons and neutrons and other electrons? At what number does it count as an “observation”? If there is a number/amount it should be fairly easy to determine experimentally. If an electron hits a photosensor, what’s to stop the entire photo sensor from being in superposition? Afterall, it’s just made up of particles interacting with each other.
  • Complex QM experiments. Take a delayed choice quantum eraser. Pairs of photons are interacting with a dozen or more pieces of apparatus (mirrors, beam splitters, detectors, etc.) and yet an interference pattern still shows up, which means that the photons must be in superposition will being “observed” by several non-conscious things that interact with them.”

If you find this debate interesting, or feel strongly about either position then I strongly suggest reading through the discussion on Reddit. I was amazed by the wealth of information and points of view contributed. Thanks for everyone who pointed out the oversight on my part.

There is however one perspective I’d like to add to the debate, namely the historical one. It is true that consciousness might be far to strong of an assumption regarding the nature of observers as prescribed by the Copenhagen Interpretation in light of the current understanding of Quantum Mechanics. But that may not have been the case for the scientists who first conceived that interpretation.

Heisenberg explicitly claimed that the collapse of the wavefunction takes place when the result of a measurement is registered in the mind of an observer (Physics and Philosophy, 1958):

“The transition from the ‘possible’ to the ‘actual’ takes place as soon as the interaction of the object with the measuring device, and thereby the rest of the world, has come into play; it is not connected with the act of registration of the result by the mind of the observer. The discontinuous change in the probability function, however, takes place with the act of registration, because it is the discontinuous change of our knowledge in the instant of registration that has its image in the discontinuous change of the probability function.”

Bohr on the other hand believed that it was not a question of physics, but one of philosophy or convenience (The Quantum Postulate and the Recent Development of Atomic Theory, 1928):

“Ultimately, every observation can, of course, be reduced to our sense perceptions. The circumstance, however, that in interpreting observations use has always to be made of theoretical notions entails that for every particular case it is a question of convenience at which point the concept of observation involving the quantum postulate with its inherent “irrationality” is brought in.”

The Von Neumann–Wigner interpretation explicitly claims that the consciousness of an observer is the demarcation line which precipitates collapse of the wave function. Interestingly, Hugh Everett took mathematical physics classes with the same Eugene Wigner, and was himself greatly concerned by the role consciousness played in the theory. He proposed that the problem of “conscious observers” can be simplified by noting that the most important element in an observation is the recorded information about the measurement outcome in the memory of the observer. He even proposed that human observers could be replaced by automatic measurement equipment that would achieve the same result. A measurement would occur when information is recorded by the measuring instrument (“Relative State” Formulation of Quantum Mechanics, 1957):

“As models for observers we can, if we wish, consider automatically functioning machines, possessing sensory apparatus and coupled to recording devices capable of registering past sensory data and machine configurations.”

It’s worth noting that both of the terms “Observer” and “Consciousness” are far from having a precise definition. It is therefore unlikely that the question will be decided one way or another anytime soon.

In a follow up post I discuss the Many Worlds interpretation of Quantum Mechanics, which examines the consequences of the Everett Many Worlds from the perspective of the mind. It is based on a thought experiment suggested by Everett, which again opens up the question of consciousness.

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