Some quantum physics experiments obviously depend on what scientists do in the lab. But, they shouldn’t depend on what happens outside of the lab, right? I mean, why should laser light bouncing around through crystals and mirrors care what the current value of the S&P 500 is, let alone what I had for breakfast?
The conditions under which an experiment is performed are called its context. In practice, the contexts we consider are very limited to a few settings on the devices in the lab. But, maybe the temperature of the room is important. Were the lights on? Was the door open? Especially when things go wrong — which is more often than not — the context is where you look for answers. But some parts of the context are so far removed from the experiment that there is absolutely no way they could affect the results, such as that delicious muesli. (Did I mention it was toasted with a hint of maple and paired with a pot set Greek yoghurt?)
A theory is a set of mathematical rules that make predictions about the outcomes of experiments. Most theories automatically rule out most contexts simply by ignoring them. Dependence on other contexts are ruled out by experimentation. If there is no possible experimental arrangement in the lab that can distinguish what I had for breakfast, then the theory shouldn’t make reference to that context. Think of it as an application of Occam’s razor. Indeed, quantum physics makes no mention of breakfast choices.
As successful as quantum physics is, it is merely an operational theory. It’s like a lab manual with instructions about the preparations and expectations of experiments. It’s remarkably accurate, allowing us to engineer materials and devices which form the basis of all modern technology. But, it doesn’t tell us anything about reality — and that bothers a lot of physicists.
What is reality? Nope. There’s no way we are going through that philosophical minefield. Let’s focus instead on scientific realism, the idea that a world of things exists independent of the minds that might perceive it and it is the world slowly revealed by progress in science. Scientific realism is the belief that the true nature of reality is the subject of scientific investigation and while we may not completely understand it at any given moment, each experiment gets us a little bit closer. This is a popular philosophical position among scientists and science enthusiasts.
A typical scientific realist might believe, for example, that fundamental particles exist even though we cannot perceive them directly with our senses. Particles are real and their properties — whatever they may be — form part of the state of the world. A slightly more extreme view is that this state of the world can be specified with mathematical quantities and these, in turn, obey equations we call physical laws. In this view, the ultimate goal of science is to discover these laws. So what are the consequences of quantum physics on these views?
As I mentioned above, quantum physics is not a realistic model of the world — that is, it does not specify quantities for states of the world. An obvious question is then can we supplement or otherwise replace quantum physics with a deeper set of laws about real states of the world? This is the question Einstein first asked with colleagues Podolski and Rosen, making headlines in 1935. The hypothetical real states of the world came to be called hidden variables since an experiment does not reveal them — at least not yet.
John Bell is the most famous of the trio Bell, Kochen, and Specker, and is credited with proving that quantum physics contained so-called nonlocal correlations, a consequence of quantum entanglement. Feel free to read about those over here: