If you ask a particle physicist or a cosmologist what they think about quantum physics, they will tell you that this theory has passed all experimental tests. Therefore, the universe is undoubtedly quantum.
They are wrong.
Due to the high cost of describing many-body quantum states, extracting predictions from quantum physics is typically impossible. If we used all the computers in the world to carry out numerical simulations, we would be stuck with quantum systems of around 60 particles or less.
So when people say: “quantum physics has passed all the tests”, what they actually mean is that quantum physics has passed all the tests in all situations where we are able to make a prediction. Those are not that many situations: essentially, the spectra of small molecules, the behavior of light and the outcomes of experiments where a few elementary particles interact.
This is the actual experimental status of quantum physics. Everything else is wishful thinking.
To say that “because we haven’t found any contradiction so far, quantum physics must be the ultimate theory” reveals a poor knowledge of the history of science. 150 years ago, all experiments performed by humankind were compatible with Newtonian mechanics. Until 19th century chemists started exploring the spectrum of hydrogen and found an anomaly that they couldn’t explain.
With quantum physics, it will be the same. One day, someone somewhere will conduct an experiment that cannot be described by any quantum theory. It is just a matter of time.
To this, there are two possible attitudes: we can sit and wait for this experiment to come… or we can actively search for clues of non-quantumness.
To challenge quantum physics, we must understand the meaning of the word “quantum” like no one has understood it before. Given an experimental scenario (be it a Bell test, the detection of neutron beams scattered by a condensed matter system or a contextuality game), we must identify how the Hilbert space structure, the very essence of quantum physics, imposes limits on what we can expect to observe in the lab.
This is what we do here. We identify the limits of quantum physics… so that one day we can break them.