To invite as many people as possible to discover the thrill of grappling with reality at its most fundamental, we are making seven of our most popular in-depth articles on quantum mysteries free to read until 11 June.

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## Your beginner’s guide to the quantum world

The “classical” world we see and experience emerges from that of subatomic particles, all of which are governed by quantum theory. This is the universe at its most fundamental, and with its famous strangeness, it can seem more than a little bewildering. Fear not! In this special package, we invite you on a whistle-stop tour of the known particles and forces; the laws and phenomena that earned quantum physics its reputation for bizarreness; the hypothetical particles that would solve some of physics’ biggest mysteries; and the maddest ideas about what might lurk at some even deeper layer of reality. I promise you, it is more thrilling – and less confusing – than you might think.

## Does reality exist without us?

According to quantum theory, we can’t know whether something is here or there, or this or that, until we look at it. Only upon “measurement” does a particle, say, assume definite properties. But what about bigger things? In this story, we look at a bunch of experiments probing the murky borders between the quantum and “classical” worlds by seeing if macroscopic objects exhibit quantum behaviours. The stakes are high, because the work might prove once and for all that we are mistaken if we think that things exist regardless of whether we are looking at them.

## Confronting quantum theory’s greatest mystery

This idea that things only become recognisably “real” when we observe or measure them seems utterly mad and yet most physicists have long since swallowed any philosophical qualms about that. The maths works so, as the saying goes, just shut up and calculate! But what happened before there were any conscious observers to distil the quantum fog? Intriguingly, as this feature explains, there is an alternative interpretation that not only gets rid of the observer problem, but also makes sense of cosmological mysteries from how black holes work to why time flows only in one direction.

## A new and even stranger twist on Schrödinger’s cat

Erwin Schrödinger’s famous thought experiment says that a cat trapped in box with some gubbins that could kill it is simultaneously dead and alive, until you open said box. It was devised to illustrate the absurdity of quantum theory’s insistence that things only become real when we observe them. The unease never went away, even if most people just moved on. But a few years ago, physicists came up with a new version of Schrödinger’s cat that coughs up a paradox even more bamboozling than the original – one that seemingly undermines the notion of absolute, objective truth. Meow!

## Why everything ever could be predetermined after all

What happens when the quantum fuzz of probabilities manifests as something definite? The long-held assumption is that the individual outcome of a measurement occurs at random and therefore that the universe at its most fundamental is indeterministic, or governed by chance. As this provocative story reveals, however, some physicists reckon the quantum world might be deterministic after all, and its apparent randomness merely an artefact of our limited perspective. This “superdeterminism” is super controversial, as you will discover, and well worth your time.

## How entanglement in time can take us to the centre of a black hole

Quantum entanglement is where two widely separated particles appear to influence one another despite having no way of communicating. But what if entanglement can also occur in time, such that a particle can be affected by events that happen in its future? Here, Vlatko Vedral, a physicist at the University of Oxford, explains how experiments suggesting just that inspired him and his colleagues to drastically rethink time in the quantum realm. By doing so, he says, we can finally reconcile Albert Einstein’s theory of gravity with quantum theory and figure out what goes on at the centre of a black hole, where the two theories must come together.

## Quantum theory, but even weirder

Einstein referred to quantum entanglement as “spooky action at a distance”, but it is far from the only hauntingly weird feature of the subatomic world. To get a grip on just why it is so strange, physicists have long prodded at the foundations of quantum theory – and in recent years, they have come up with a new version that seems to be even more bizarre than the original. As we explain in this story, the potential successor fits with everything we already know, but also predicts an extreme version of entanglement that is even more of an affront to our intuitions. It also happens to boast a mathematical structure that would support one candidate for the enigmatic theory of quantum gravity. And believe it or not, we are about to put it to the test.

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