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Understanding science: what we cannot know
Understanding science: what we cannot know

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3.3 Schrödinger’s cat

The most famous example of the puzzling implications of quantum theory is known as ‘Schrödinger’s cat’. This is a thought experiment in which an imaginary cat is sealed in a box, along with a device that will release a deadly poison once a radioactive atom has decayed. As quantum mechanics only tells us about the probability that an atom has decayed after a certain amount of time, it’s not possible to know for sure whether the cat is still alive without checking inside the box. All we know is that the longer we wait, the worse the cat’s chances become.

In the framework of quantum theory, this is described as a ‘superposition’ of two states: one in which the cat is alive, and one in which it is dead. So, in a sense, until the outcome is confirmed, Schrödinger’s cat is both alive and dead at the same time.

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Video 3 60-Second Adventures in Thought: Schrödinger’s Cat
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This popular paradox relies on a link between the quantum world of the radioactive atom and the device that releases the poison. They are linked in a way that transports the quantum probability related to the atomic scale to the macroscopic scale, by triggering the device.

The wider issue this thought experiment raises concerns the measurement process, by which we infer what happens at the atomic scale. Necessarily, this has to be done by linking the quantum system to a ‘macroscopic’ device, something that can be counted (e.g. the clicks of a Geiger counter) or otherwise recorded. Once this is done, what was a quantum system with a probabilistic description has been assigned a definite outcome – the atom has either decayed (i.e. we heard the click of the Geiger counter) or it hasn’t.

Understanding the measurement process and what happens is one of the key issues in quantum theory. One way out of Schrödinger’s cat paradox is to take the macroscopic device that detects the decay to be the observer that measures the quantum system. In this case, there is no more uncertainty about the outcome, whether or not you’ve looked in the box.