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Multiverse unites many worlds in single reality
A multiverse of parallel worlds
meyer/tendance floue
THIS WEEK
A new way to explain the universe’s observability is exciting and bewildering in equal measure Justin Mullins
together with Leonard Susskind at Stanford University in California, turned to the work of physicists who have puzzled over the same problem but on a much smaller scale: why tiny objects such as electrons and photons exist in a superposition of states but larger objects like footballs and planets apparently do not. This problem is captured in the famous thought experiment of Schrödinger’s cat. This unhappy feline is inside a sealed box containing a vial of poison that will break open when a radioactive atom decays. Being a quantum object, the atom exists in a superposition of states – so it has both decayed and not decayed at the same time. This implies that the vial must be in a superposition of states too – both broken and unbroken. And if that’s the case, then the cat must be both dead and alive as well. To explain why we never seem to see cats that are both dead and alive, and yet can detect atoms in a superposition of states, physicists have in recent years replaced the idea of superpositions collapsing with the idea that quantum objects inevitably interact with their environment, allowing information about possible superpositions to leak away and become inaccessible to the observer. All that is left is the information about a single state. Physicists call this process
TWO of the strangest ideas in modern physics – that the cosmos constantly splits into parallel universes in which every conceivable outcome of every event happens, and the notion that our universe is part of a larger multiverse – have been unified into a single theory. This solves a bizarre but fundamental problem in cosmology and has set physics circles buzzing with excitement, as well as some bewilderment. The problem is the observability of our universe. While most of us simply take it for granted that we should be able to observe our universe, it is a different story for cosmologists. When they apply quantum mechanics – which successfully describes the behaviour of very small objects like atoms – to the entire cosmos, the equations imply that it must exist in many different states simultaneously, a phenomenon called a superposition. Yet that is clearly not what we observe. Cosmologists reconcile this seeming contradiction by assuming that the superposition eventually “collapses” to a single state. But they tend to ignore the problem of how or why such a collapse might occur, says cosmologist Raphael Bousso at the University of California, Berkeley. “We’ve no right to assume that it collapses. We’ve been lying to ourselves about “It’s an explanation for how this,” he says. the entire universe may In an attempt to find a more collapse into the single satisfying way to explain the state that we observe” universe’s observability, Bousso, 8 | NewScientist | 4 June 2011
“decoherence”. If you can prevent it – by tracking all the information about all possible states – you can preserve the superposition. In the case of something as large as a cat, that may be possible in Schrödinger’s theoretical sealed box. But in the real world, it is very difficult to achieve. So everyday cats decohere rapidly, leaving behind the single state that we observe. By contrast, small things like photons and electrons are more easily isolated from their environment, so they can be preserved in a superposition for longer: that’s how we detect these strange states. The puzzle is how decoherence
might work on the scale of the entire universe: it too must exist in a superposition of states until some of the information it contains leaks out, leaving the single state that we see, but in conventional formulations of the universe, there is nothing else for it to leak into. What Bousso and Susskind have done is to come up with an explanation for how the universe as a whole might decohere. Their trick is to think of the volume of space that encompasses all the information in our universe and everything it might possibly interact with in the future. In previous work, Susskind has
In this section n Mind-reading technology understands your thoughts, page 11 n An “invisibility cloak” for coastlines, page 14 n The future of money, page 23
they say. They call this idea the multiverse interpretation of quantum mechanics and in a paper now available online they have proposed the mathematical framework behind it (arxiv.org/ abs/1105.3796). One feature of their framework is that it might explain puzzling aspects of our universe, such as the value of the cosmological constant and the apparent amount of dark energy.
“The global multiverse is a representation of the many-worlds idea in a single geometry”
dubbed this region a causal patch. The new idea is that our universe is just one causal patch among many others in a much bigger multiverse. Many physicists have toyed with the idea that the cosmos is made up of regions which differ so profoundly that they can be thought of as different universes inside a bigger multiverse. Bousso and Susskind suggest that information can leak from our causal patch into others, allowing our part of the universe to decohere into one state or another, resulting in the universe that we observe. But while decoherence explains
why we don’t see cats that are dead and alive at the same time, or our own universe in a huge superposition of states, it does not tell us which state the cat, or the universe, should eventually end up in. So Bousso and Susskind have also linked the idea of a multiverse of causal patches to something known as the “many worlds” interpretation of quantum mechanics, which was developed in the 1950s and 60s but has only become popular in the last 10 years or so. According to this strange idea, when a superposition of states occurs, the cosmos splits into multiple parallel but otherwise
The paper has caused flurry of excitement on physics blogs and in the broader physics community. “It’s a very interesting paper that puts forward a lot of new ideas,” says Don Page, a theoretical physicist at the University of Alberta in Edmonton, Canada. Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena and author of the Cosmic Variance blog, thinks the idea has some merit. “I’ve gone from a confused skeptic to a tentative believer,” he wrote on his blog. “I realized that these ideas fit very well with other ideas I’ve been thinking about myself!” However, most agree that there –Welcome to Parallel Alley– are still questions to iron out. “It’s an important step in trying identical universes. In one to understand the cosmological universe we might see the cat implications of quantum survive and in another we see mechanics but I’m sceptical that it die. This results in an infinite it’s a final answer,” says Page. number of parallel universes in For example, one remaining which every conceivable outcome question is how information of every event actually happens. can leak from a causal patch, Bousso and Susskind’s a supposedly self-contained contention is that the alternative volume of the multiverse. realities of the many worlds Susskind says it will take time interpretation are the additional for people to properly consider causal patches that make up the their new approach. And even multiverse. Most of these patches then, the ideas may have to be would have split from other refined. “This is not the kind of universes, perhaps even ancestors paper where somebody does a of our own. “We argue that calculation and confirms that the global multiverse is a we’re correct,” says Bousso. representation of the many“It’s the sort of thing that worlds in a single geometry,” will take a while to digest.” n 4 June 2011 | NewScientist | 9
meyer/tendance floue
THIS WEEK
A new way to explain the universe’s observability is exciting and bewildering in equal measure Justin Mullins
together with Leonard Susskind at Stanford University in California, turned to the work of physicists who have puzzled over the same problem but on a much smaller scale: why tiny objects such as electrons and photons exist in a superposition of states but larger objects like footballs and planets apparently do not. This problem is captured in the famous thought experiment of Schrödinger’s cat. This unhappy feline is inside a sealed box containing a vial of poison that will break open when a radioactive atom decays. Being a quantum object, the atom exists in a superposition of states – so it has both decayed and not decayed at the same time. This implies that the vial must be in a superposition of states too – both broken and unbroken. And if that’s the case, then the cat must be both dead and alive as well. To explain why we never seem to see cats that are both dead and alive, and yet can detect atoms in a superposition of states, physicists have in recent years replaced the idea of superpositions collapsing with the idea that quantum objects inevitably interact with their environment, allowing information about possible superpositions to leak away and become inaccessible to the observer. All that is left is the information about a single state. Physicists call this process
TWO of the strangest ideas in modern physics – that the cosmos constantly splits into parallel universes in which every conceivable outcome of every event happens, and the notion that our universe is part of a larger multiverse – have been unified into a single theory. This solves a bizarre but fundamental problem in cosmology and has set physics circles buzzing with excitement, as well as some bewilderment. The problem is the observability of our universe. While most of us simply take it for granted that we should be able to observe our universe, it is a different story for cosmologists. When they apply quantum mechanics – which successfully describes the behaviour of very small objects like atoms – to the entire cosmos, the equations imply that it must exist in many different states simultaneously, a phenomenon called a superposition. Yet that is clearly not what we observe. Cosmologists reconcile this seeming contradiction by assuming that the superposition eventually “collapses” to a single state. But they tend to ignore the problem of how or why such a collapse might occur, says cosmologist Raphael Bousso at the University of California, Berkeley. “We’ve no right to assume that it collapses. We’ve been lying to ourselves about “It’s an explanation for how this,” he says. the entire universe may In an attempt to find a more collapse into the single satisfying way to explain the state that we observe” universe’s observability, Bousso, 8 | NewScientist | 4 June 2011
“decoherence”. If you can prevent it – by tracking all the information about all possible states – you can preserve the superposition. In the case of something as large as a cat, that may be possible in Schrödinger’s theoretical sealed box. But in the real world, it is very difficult to achieve. So everyday cats decohere rapidly, leaving behind the single state that we observe. By contrast, small things like photons and electrons are more easily isolated from their environment, so they can be preserved in a superposition for longer: that’s how we detect these strange states. The puzzle is how decoherence
might work on the scale of the entire universe: it too must exist in a superposition of states until some of the information it contains leaks out, leaving the single state that we see, but in conventional formulations of the universe, there is nothing else for it to leak into. What Bousso and Susskind have done is to come up with an explanation for how the universe as a whole might decohere. Their trick is to think of the volume of space that encompasses all the information in our universe and everything it might possibly interact with in the future. In previous work, Susskind has
In this section n Mind-reading technology understands your thoughts, page 11 n An “invisibility cloak” for coastlines, page 14 n The future of money, page 23
they say. They call this idea the multiverse interpretation of quantum mechanics and in a paper now available online they have proposed the mathematical framework behind it (arxiv.org/ abs/1105.3796). One feature of their framework is that it might explain puzzling aspects of our universe, such as the value of the cosmological constant and the apparent amount of dark energy.
“The global multiverse is a representation of the many-worlds idea in a single geometry”
dubbed this region a causal patch. The new idea is that our universe is just one causal patch among many others in a much bigger multiverse. Many physicists have toyed with the idea that the cosmos is made up of regions which differ so profoundly that they can be thought of as different universes inside a bigger multiverse. Bousso and Susskind suggest that information can leak from our causal patch into others, allowing our part of the universe to decohere into one state or another, resulting in the universe that we observe. But while decoherence explains
why we don’t see cats that are dead and alive at the same time, or our own universe in a huge superposition of states, it does not tell us which state the cat, or the universe, should eventually end up in. So Bousso and Susskind have also linked the idea of a multiverse of causal patches to something known as the “many worlds” interpretation of quantum mechanics, which was developed in the 1950s and 60s but has only become popular in the last 10 years or so. According to this strange idea, when a superposition of states occurs, the cosmos splits into multiple parallel but otherwise
The paper has caused flurry of excitement on physics blogs and in the broader physics community. “It’s a very interesting paper that puts forward a lot of new ideas,” says Don Page, a theoretical physicist at the University of Alberta in Edmonton, Canada. Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena and author of the Cosmic Variance blog, thinks the idea has some merit. “I’ve gone from a confused skeptic to a tentative believer,” he wrote on his blog. “I realized that these ideas fit very well with other ideas I’ve been thinking about myself!” However, most agree that there –Welcome to Parallel Alley– are still questions to iron out. “It’s an important step in trying identical universes. In one to understand the cosmological universe we might see the cat implications of quantum survive and in another we see mechanics but I’m sceptical that it die. This results in an infinite it’s a final answer,” says Page. number of parallel universes in For example, one remaining which every conceivable outcome question is how information of every event actually happens. can leak from a causal patch, Bousso and Susskind’s a supposedly self-contained contention is that the alternative volume of the multiverse. realities of the many worlds Susskind says it will take time interpretation are the additional for people to properly consider causal patches that make up the their new approach. And even multiverse. Most of these patches then, the ideas may have to be would have split from other refined. “This is not the kind of universes, perhaps even ancestors paper where somebody does a of our own. “We argue that calculation and confirms that the global multiverse is a we’re correct,” says Bousso. representation of the many“It’s the sort of thing that worlds in a single geometry,” will take a while to digest.” n 4 June 2011 | NewScientist | 9