What happens in parallel universes?
The ‘many-worlds’ isn't the only type of multiple cosmos considered by physicists
Danish physicist Niels Bohr was instrumental in developing the Copenhagen interpretation of quantum theory
Aidan Chatwin-Davies, from Caltech in California, is another theoretical physicist not fond of firewalls. He has recently found an alternative way to abandon a blazing event horizon. He says all we have to do is think of black holes in terms of the many worlds interpretation of quantum physics - an idea first devised by physicist Hugh Everett in the 1950s as an alternative way of thinking about the weird sub-atomic world.
Quantum physics famously says that a particle can be in two places at once, or in two different states simultaneously. The original interpretation of this idea, favoured by Niels Bohr and devised in Denmark, is known as the Copenhagen interpretation. It argues that only once the particle is measured does it ‘decide’ which state to appear in. However, fellow physicist Erwin Schrödinger devised his famous Schrödinger's Cat thought experiment to show up holes in this argument.
The eponymous feline is trapped in a sealed box with a hammer and a vial of poison. Whether or not the hammer falls to crack the vial depends on the outcome of a measurement on a quantum particle. The Copenhagen interpretation says that the particle is simultaneously in both states at once until the measurement is made. That means the hammer falls and doesn't fall and the cat is alive and dead until the particle is measured. But why does the act of measuring force nature to choose? Everett's alternative ‘many-worlds‘ picture was to suggest that it doesn't – both outcomes occur.
The universe splits into two distinct versions (or branches) – one where the cat lives and another where it perishes.
“If you are trying to describe the formation and evaporation of a black hole truly quantum gravitationally then you would expect multiple versions of the black hole,” says Chatwin-Davies, just like there are two versions of the cat. The implications for the information paradox are profound. “If you're sitting around monitoring the Hawking radiation coming out of a black hole you should expect to see a loss of information,” he says. That's because you're limited to one of the many branches the black hole now exists in. The information about an infalling object isn't destroyed, it is simply shared out across the many branches of reality. Throw Hamlet into a black hole and Act I may emerge in this universe's Hawking radiation, but Act II in another.
Nomura agrees. “Focus on one world and clearly you cannot recover all the initial information,” he says. What effect does this have on the firewall? “The statement that you have to go smoothly into a black hole applies only to each branch of the many worlds,” says Nomura. “Whereas the rules about quantum information apply to the whole set of worlds.” According to Nomura, the Firewall Paradox results from confusing these differences. Chatwin-Davies is on the same page. Comparing the two “is like comparing apples and oranges,” he says.
So, as with many times in the history of physics, answering one question has thrown up several others. Information falling into a black hole may be imprinted as a hologram on the event horizon and carried back into space by Hawking radiation. It could be that severing the link between the quantum particles responsible for Hawking radiation incinerates you to a crisp as you enter, or that information could be hidden in the Hawking radiation after all. Finally, it could even be possible that information falling into a black hole is shared out among the many versions of reality that splinter off as a black hole evolves. Until we crack the elusive code of quantum gravity, it is hard to know who is right.
“If you're monitoring the Hawking radiation from a black hole you should expect to see a loss of information”
Level 3 Where your future self exists One approach says that the universe splinters into multiple copies every time a quantum event takes place. This could make you immortal. Imagine hooking a gun to a machine that fires upon a positive result of a 50:50 quantum measurement. Every time a measurement is made your universe would splinter. As you're only able to perceive a universe in which you didn't die, you'd believe you'd survived every measurement. Level 1 Where an identical Earth exists There is a limit to how far we can see into space. We can only see places from which light has had chance to reach us since the Big Bang. If you could venture beyond this cosmic horizon you might end up reaching another part of the universe where atoms are arranged in precisely the same fashion as they are here – another Earth and another you. Level 2 The expanding universe we can’t reachString theory – the idea that everything around us is made up of tiny vibrating strings – was theorised to in attempt to combine the general theory of relativity and quantum theory. String theorists need there to be seven additional dimensions to the three of space and one of time that we experience. Level 4 The universe next door Cosmologists introduced a modification to the Big Bang theory in the 1980s to address some of its failures. This patch is known as inflation, yet when they looked at what could have caused this to happen they found that they couldn't get it to happen just once. Instead, eternal inflation is constantly creating neighbouring universes.Level 1Level 2Level 3Level 4
The ESA's Planck space telescope observed the CMB (inset) for nearly 4.5 years