Did you hear the one about the universe that moves backwards in time?

Scientists had been trying to study neutrinosA particle smaller than an atom that has no electrical charge and a tiny mass. They are therefore extremely difficult to detect. in the atmosphere above Antarctica. The problem was that the particles appeared to be going the wrong way, from the ice into space.

Because this violated the current picture of how the universe works, what physicists call the "standard modelThis is a theory that attempts to unify explanations for the behaviour of all particles. Theoretical physicists have therefore described particles that ought to exist for the model to work, and then have attempted to discover them. The discovery of the Higgs boson particle in 2012 was seen as a triumphant confirmation of the standard model.", scientists have scrambled to propose explanations.

One of the 40 offered so far is that the particles emanated from a parallel universe that mirrors our own while moving backwards in time. This explanation, understandably, made headlines this year.

The evidence for a mirror universe is thin on the ground, but it is not the first time physicists have reached beyond reality in order to make this world make sense.

Take the "many-worlds" interpretation of quantum mechanics. This theory interprets a famously insoluble problem called the "wave-function collapse".

Above a certain scale, the movement of objects seems to obey fixed laws, but subatomic particles' movements can only be described by the probability that a certain outcome will occur when observed. Once a particle is observed, however, this probability, which is calculated using a mathematical object called the wave-function, "collapses" into a single outcome.

Physicists have offered competing explanations. The most popular is still the CopenhagenThe Copenhagen interpretation is named after the city where Niels Bohr and Werner Heisenberg first described their interpretation of quantum mechanics. There are a range of other interpretations all of which attempt to explain the gap between the so called classical laws for the behaviour of objects and the quantum mechanical model. interpretation, which accepts that some things can only be described in terms of probability. Albert Einstein famously rejected it, saying "God does not play dice with the universe."

Einstein was unable to find a better rule. However, in 1957, a physicist named Hugh Everett suggested a new, deceptively simple solution.

Rather than there being two clashing sets of physical laws, he argued that the universe branches into multiple universes every time a subatomic "observation" occurs.

Thus universes endlessly proliferate and each one of them is a number on a dieThe singular form of dice.. Many of these will contain a different version of you.

For some, the implications are disturbing. They wonder how we can claim to be conscious if at every moment we split into new selves.

The very idea of a self would be an illusion. It is as if a new you were constantly being reborn. The physicists' equations would be more real than we are.

For the more relaxed believers, there is no need to think that these many worlds, which we will never be able to see, should change our self-understanding. We do wake up every time in the right place.

There is even an iPhone app that will "split" the universe for you by measuring a particle in a lab in Switzerland.

For the terminally indecisive, knowing they can split themselves in two might help. Others might caution that, if the theory is true, every decision really is the end of the world.

Everything would change, say some. If every moment that a subatomic particle is measured creates a new universe, then our whole sense of self must disappear. Objectively speaking, there would be no way to talk meaningfully about the consequences of our actions. Our existence would be more like a wave spreading out across the multiverse than a person living a single life.

Nothing would change, say others. What matters is the scale at which you experience things, not the scientific language in which we describe it. Many philosophers have long argued that continuity of experience is an illusion, and yet people have not acted as if that were true. It will always be hard to live with the scientific picture of the universe because that is not how we see the world.