Whenever I read a description of the multiverse, it always talks about human decisions causing branches. What about animals and the decisions they make? Does my hamster deciding to eat the apple first rather than the pea cause a new branch?
鈥� The idea that human decisions cause branches is a literary rather than a scientific one (perhaps due to the idea that time travellers can create a new timeline through their actions in the past).
The scientific view, put forward by Hugh Everett III in 1956, is that quantum events cause branching. This was an attempt to explain why the Schr枚dinger wave equation describes the probabilities of various outcomes rather than one fixed outcome, as occurs in classical physics. For example, when a photon hits a semi-silvered mirror, the equation describes a 50 per cent chance of reflection and a 50 per cent chance of transmission. Various ideas had been proposed, but Everett suggested taking the equation at face value.
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If it describes two outcomes, that鈥檚 because there are two outcomes and both occur 鈥� they just occur in different branches of the multiverse. This has the advantage of not needing any new physics, but the disadvantage of having to explain exactly how the probabilities give rise to branches (this is still being argued over).
In addition, it posits a vast number of almost undetectable parallel universes. But the existence of unseen infinities has never put scientists off 鈥� at least it hasn鈥檛 since they recovered from being convicted of heresy for suggesting something very similar 400 years ago.
Charles Goodwin, Auckland, New Zealand
鈥� Hugh Everett III鈥檚 relative state formulation of quantum mechanics is the basis of the 鈥渕any worlds鈥� interpretation (you can see his original PhD thesis at ).
This makes no reference to conscious decision-making and doesn鈥檛 need to invoke the concept of mind or free will at all. Instead, Everett imagined purely physical objects interacting, such as machines and recording devices 鈥� indeed any physical interaction that leaves a trace.
Each of these interactions then 鈥渃auses鈥� a branching of the universe. So you interacting with your environment, or your hamster with its surroundings, or even a single photon scattering off a macroscopic mote of dust, will lead to branching in the many worlds picture.
Although this appears to give rise to a dizzying infinity of branches (worlds), all observers can only ever be aware of one world: the one associated with all the recorded outcomes of all the interactions they themselves could, in principle, go and check. All the other possible worlds with different outcomes don鈥檛 interact with, or have any influence on or observable consequences for, the observed world.
Nick Canning, Coleraine, Co. Londonderry, UK
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