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 mere fiction. In reality, things do not change just because we are looking at them. If a tree falls down in a forest with nobody around, it will still make a noise. If you leave a pencil on your desk, it will not vanish when you look away. Right?

Well, not exactly, say physicists. They say that particles - the extremely tiny pieces of matter which make up everything - may be able to be in two or more places at once, but only until we look at them.

This is called quantum theory. It holds that wave functions allow particles to be in multiple places at once, which is called a quantum superposition. Physicists say that when the particles are observed, the wave function "collapses", forcing the particle to randomly "choose" a location.

So it could be said that there is no definitive reality until an event or object has been observed. The hypothetical particle split across several locations has not "chosen" where it will place itself before you have looked, so reality is still in fluxContinuous change..

A classic example is Schrodinger's cat. This thought experiment, devised by Austrian physicist Erwin Schrodinger in 1935, 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 in a superposition of being 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, the cat will be in either of the two states.

Of course, these are only the barest bones of quantum physicsThe study of matter and energy on the atomic and subatomic level. . But it gets weirder.

For example, if you watched the 2022 Oscar Best Picture winner Everything Everywhere All at Once and thought "what a load of nonsense", quantum mechanics specialists may want to have a word with you.

Physicists have long debated what happens to all of the other possible outcomes that are not observed, and some have suggested that they do occur - in infinite parallel universesThe idea of separate universes or worlds that exist alongside our world.. They say that this could prove the existence of the multiverse.

In another quirk that Einstein described as "spooky action at a distance", two particles can become "entangled" so that even if they are light-years apart in distance, they can communicate to change their properties in tandem. This is called quantum entanglement.

Quantum mechanics 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 an immeasurable gulf in what is real and what is not.

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.