One of the most important tools in the theoretical physicist’s toolkit is the thought experiment. If you study relativity, quantum mechanics, or any area of physics applying to environments or situations in which you cannot (or should not) place yourself, you’ll find that you spend a lot more time working through imaginary scenarios than setting up instruments or taking measurements.
Unlike physical experiments, thought experiments are not about collecting data, but rather about posing an imaginary question and working through an ‘if/then’ logical sequence to explore what the theory really means.
Asking “what has to happen if the theory is true?” is invaluable for developing intuition and anticipating new applications. In some cases, a thought experiment can reveal the deep philosophical implications of a theory, or even present what appears to be an unsolvable paradox.
Probably the most famous of all physics thought experiments is that of Schrödinger’s Cat – both because it involves (purely hypothetical!) carnage, and because its implications for the nature of reality in a quantum world continue to challenge students and theorists everywhere.
The basic – again, purely hypothetical – experimental setup is this. Imagine you have a radioactive material in which there is a 50 per cent chance of a nuclear decay in some specified amount of time (let’s say, one hour).
You put this material in a box along with a small glass vial of poison and a device that will break the vial if a radioactive decay is detected. Then, you put a live cat in the box, close the lid, wait an hour, and then open the box once again.
Based on this setup, it’s straightforward to deduce that since the chance the atom decays and triggers the poison is 50 per cent, half the time you do the experiment, you should find a living cat, and half the time, you should find a dead one, assuming you’re not re-using the same cat each time.
But when Erwin Schrödinger described the thought experiment to Albert Einstein in 1935, he did so to highlight an apparent consequence of quantum theory that seemed to both scientists to be complete nonsense: the idea that before you open the box, the cat is both alive and dead at the same time.
Ultimately, it comes down to the principle of uncertainty in quantum mechanics. Unlike classical mechanics (the kind of physics that applies to our everyday experiences), in quantum mechanics, there seems to be a fundamental uncertainty built into the nature of reality.
When you flip a coin (a classical event), it’s only “random” because you’re not keeping careful enough track of all the motions and forces involved. If you could measure absolutely everything, you could predict the outcome every time – it’s deterministic.
But in the quantum mechanical version of a coin flip, the radioactive decay, nothing you measure can possibly tell you the outcome before it occurs. As far as an outside observer is concerned, until the measurement of the quantum coin flip occurs, the system will act like it’s in both states at once: the atom is both decayed and not decayed, in what we call a superposition.
Superposition is a real phenomenon in quantum mechanics, and sometimes we can even use it to our advantage. Quantum computing is built on the idea that a quantum computer bit (or qubit), instead of being just one or zero, can be in a superposition of one and zero, massively increasing the computer’s ability to do many complex calculations at once.
In the case of Schrödinger’s Cat, the apparently absurd conclusion that the cat is both alive and dead comes from considering the whole apparatus – the atom, the trigger device, and the poison vial, and the cat – to be a single quantum system, each element of which exists in a superposition.
The atom is decayed and not, the device is triggered and dormant, the vial is broken and intact, and the cat is therefore simultaneously dead and alive, until the moment the box is opened.
Whether this conclusion is actually absurd is an open question. What both Schrödinger and Einstein concluded was that true, fundamental uncertainty simply cannot apply to the real, macroscopic, world. These days, most physicists accept that uncertainty is real, at least for subatomic particles, but how that uncertainty 'collapses' when a measurement is made remains up for debate.
In one interpretation, any measurement that’s performed fundamentally alters reality – though it is usually argued that the trigger device, or, at least, the cat itself, provides a measurement for that purpose. In another interpretation, called Many Worlds, the entire Universe duplicates itself every time a quantum coin is flipped, and the measurement simply tells you whether you’re in the dead-cat or alive-cat universe from now on.
While we can’t say how long it will take before we fully understand what’s really going on in the black box of quantum superposition, applications of quantum theory are already bringing us incredible technological advances, like quantum computers. And in the meantime, clever thought experiments allow us to follow our curiosity, without running the risk of killing any cats.
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