Any fan of Doctor Who will have heard of the weeping angels. These uncanny beings look just like regular statues - until the watcher closes their eyes or looks away. Then, they become lightning-fast predators who can reach you in the duration of a blink.

But this is all mereSmall and unimportant. fiction. In reality, things do not change just because we are looking at them. Right?

Well, not exactly, say physicists. They say that particles - the extremely tiny pieces of matter which make up everything - can actually be in more than one place at once, but only until we look at them, at which point they "choose" a single location.

This is called quantum theory. This means we could say there is no reality until an object is observed, because it has not "chosen" where it will place itself yet.

A classic example is Schrodinger's cat. This thought experiment imagines a cat trapped in a box with a radioactive atom and a vial of poison. The atom could decay at any moment, smashing the vial to release the poison and instantly kill the cat.

This means that it could be said that before the box has been opened, the cat is both dead and alive. But we will never see a cat mystically suspended between life and death, because as soon as we open the box to look inside, we will see its "real" state.

It gets even weirder. Physicists have long debated what happens to all of the other possible outcomes we do not observe, and some have suggested that they do occur - in infinite parallel universesThe idea of separate universes or worlds that exist alongside our world..

Quantum mechanicsA fundamental theory in physics describing the properties of nature on an atomic scale. Unlike general relativity, which works when applied to large objects, quantum mechanics helps describe the world on a tiny level - at a scale where things can be in several places at once, and measurement can affect reality. is a controversial field full of conflicting views. But many claim that objective reality and quantum laws cannot co-exist.

Imagine your best friend observes a particle's location in a lab. You wait behind the door. Until they tell you where it is, their reality is "decided", whereas yours is not. Briefly, you are separated by reality itself.

Yes: Many quantum physicists believe in objective reality. The idea that reality does not exist until it is observed is only one interpretation of the theory.

No: By definition, reality has to be something that exists outside of the mind and something that can be established by consensus. Quantum physics disproves this, because it states that physical reality does not exist independent of the eye of its observer.

Or... There are explanations which could allow for objective reality to exist in quantum mechanics. For example, if everything in the universe were predetermined, objects would not be choosing their location randomly and subjectively after being observed, but in an objective and predestined pattern.