Quantum physics inaccuracies in Seishun Buta Yaro (Rascal Does Not Dream of Bunny Girl Senpai)
I finished watching Seishun Buta Yaro (Rascal Does Not Dream of Bunny Girl Senpai) recently, and really enjoyed the anime. It explored themes of finding one’s identity, and dealing with social anxiety during adolescent years. In the show, students are inflicted by a fictional disease called Adolescence Syndrome when they are severely affected by the things happening around them, whose symptoms depends on the specific reason why a person is mentally distraught. Rio Futaba, the president of the science club in the school, tries to explain the reasons for Adolescence Syndrome using quantum mechanics principles throughout the show. Unfortunately, most of it was poorly applied, and could be harmful in reinforcing misconceptions of quantum mehanics. In this post, I examine each of her claims and describe why they don’t really make sense. This post is aimed at a general audience and requires no knowledge of quantum mechanics.
Mai’s Arc and Quantum States
In the first arc, Futaba explains that the invisibility of Mai to everyone in their school is analogous to the thought experiment known as Schrödinger’s cat. She says that because nobody in the school was actively observing Mai, she begins to become invisible to them. Sakuta, the protagonist, then refuses to sleep, believing that falling asleep will also cause him to be unable to perceive Mai afterwards.
The Schrödinger’s cat thought experiment is used to highlight the point that if you are handed a quantum system with an unknown state, it remains in a superposition of its possible states until you measure it, after which its state collapses into the measured state with 100% probability.
Therefore, just observing Mai-san once is enough to trigger a collapse in her quantum state (if that even makes any sense at all). One can argue that her quantum states can always be changing due to interactions with the environment, but then in practice stable quantum superpositions are extremely hard to maintain anyway (otherwise we will have quantum computers by now).
In Tomoe’s arc, Sakuta pretends to be Tomoe’s boyfriend in order to help with her reputation at school. However, when it is time for their fake relationship to end, Tomoe has regrets and hopes instead that their relationship could blossom into the real thing. Her disappointment causes time to loop back to the start of their fake relationship in hopes of a different outcome. Futaba explains this in terms of Laplace’s demon, which states that if an omniscient demon knows the positions and velocities of all particles in the universe, then the demon can predict everything that happens in the future. The analogy drawn is not really strong, and many arguments have been made against the possibility of Laplace’s demon from a quantum mechanics lens. This is because the inherent non-determinism in the outcomes of quantum measurements contradicts the deterministic model of the universe which the Laplace demon thought experiment requires.
Sakuta finds out that there is a Futaba doppelganger, and after talking to the both of them he learns that it is due to her split personality because of her self-loathing. Futaba says that quantum teleportation is the reason why there can be two Futabas’ in the world. There is a very serious misconception in popular culture that quantum teleportation is capable of teleporting atoms and objects, when in fact it is only quantum states which can be transferred from one party to another. Furthermore, quantum teleportation requires an entangled EPR (Einstein, Podolsky, and Rosen) pair to be set-up in advance between the two teleportation locations, which means entangling two qubits using a Hadamard and CNOT (controlled-NOT) gate and then separating them and bringing them to their respective locations, so there is a lot of setup work before quantum teleportation can take place. Finally, quantum teleportation also requires transmission of two classical bits, where the receiver in the protocol may have to apply certain gates to the results of their qubit after quantum teleportation based on the bit that was received. So it is certainly not the case that quantum teleportation can transmit large amounts of matter seemingly at random.
Nodoka and Mai swaps bodies in this arc because of Nodoka’s jealousy of Mai’s successes in life, which Futaba also attributes to quantum teleportation. This explanation is pretty silly for the same reasons.
Apart from the botched quantum justifications, I felt that everything else in the anime was really well done, and would definitely recommend it if you haven’t watched it (although you have already been spoiled quite a bit). I believe that studios should pay greater care to the scientific accuracy of the claims made in their shows, to avoid perpetuating wrong information that is difficult to reverse once captured in the public imagination. In any case, I heard that the movie is really good too, and I am looking forward to watching it!