The multiverse (or meta-universe) is the hypothetical set of infinite or finite possible universes (including the historical universe we consistently experience) that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The various universes within the multiverse are sometimes called parallel universes. A multiverse has been envisaged within the multi-dimensional extension of string theory or M-theory.
The structure of the multiverse, the nature of each universe within it and the relationships among the various constituent universes, depend on the specific multiverse hypothesis considered. Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology and fiction, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "parallel worlds", "alternative realities", "alternative timelines", and "dimensional planes," among others. The term 'multiverse' was coined in 1895 by the American philosopher and psychologist William James in a different context.
The multiverse hypothesis is a source of disagreement within the physics community. Physicists disagree about whether the multiverse exists, and whether the multiverse is a proper subject of scientific inquiry. Supporters of one of the multiverse hypotheses include Stephen Hawking, Steven Weinberg, Brian Greene, Max Tegmark, and Alex Vilenkin. Read more ...
In contrast, critics such as David Gross, Paul Steinhardt, and Paul Davies have argued that the multiverse question is philosophical rather than scientific, or even that the multiverse hypothesis is harmful or pseudoscientific.
In 1949, on the occasion of Einstein's 70th birthday, Kurt Godel
presented him with a mathematical proof of the nonexistence of time.
Nature, it appears, is governed by eternal laws that stand outside time.
Time is part of the illusion of physical reality.
As far as I know, there is no proof whatever of the existence of an
objective reality apart from our senses, and I do not see why we
should accept the outside world as such solely by virtue of our senses.
Reality is a consciousness hologram with endless possibilities.
The Physical Universe
Physics: Why there could be many identical copies of you BBC - August 7, 2017
We ask one of the hardest questions in the Universe, is there another you out there and if so, can we find our cosmic twin?
Evidence of a parallel universe? 'Cold Spot' in space suggests there are alternate worlds with their own versions of reality Daily Mail - May 17, 2017
In the desolate darkness of space it is difficult to imagine we are anything but alone in the cosmos. But imagine for a moment that we are not alone, but in fact one of an infinite number of parallel universes that contain infinite versions of ourselves. In one universe you might be president of the United States, while in another you could be made of gelatin. This is the theory of the 'multiverse', and a new study has found that a mysterious 'Cold Spot' in space could prove our universe is merely one of an everlasting string of realities.
Strange behavior of quantum particles may indicate the existence of other parallel universes PhysOrg - June 6, 2015
Quantum mechanics is a strange realm of reality. Particles at this atomic and subatomic level can appear to be in two places at once. Because the activity of these particles is so iffy, scientists can only describe what's happening mathematically by "drawing" the tiny landscape as a wave of probability.
Scientists propose existence and interaction of parallel worlds: Many Interacting Worlds theory challenges foundations of quantum science Science Daily - October 30, 2014
Academics are challenging the foundations of quantum science with a radical new theory on parallel universes. Scientists now propose that parallel universes really exist, and that they interact. They show that such an interaction could explain everything that is bizarre about quantum mechanics.
Is the universe a bubble? Let's check PhysOrg - July 18, 2014
Never mind the big bang; in the beginning was the vacuum. The vacuum simmered with energy (variously called dark energy, vacuum energy, the inflation field, or the Higgs field). Like water in a pot, this high energy began to evaporate - bubbles formed. Each bubble contained another vacuum, whose energy was lower, but still not nothing. This energy drove the bubbles to expand. Inevitably, some bubbles bumped into each other. It's possible some produced secondary bubbles. Maybe the bubbles were rare and far apart; maybe they were packed close as foam. But here's the thing: each of these bubbles was a universe. In this picture, our universe is one bubble in a frothy sea of bubble universes. That's the multiverse hypothesis in a bubbly nutshell.
How Would Humans Know If They Lived in a Multiverse? Live Science - June 4, 2014
Spotting a multiverse ...
Is free will dead?
Some theories in physics give rise to the idea of multiple universes, where nearly identical versions of the known universe exist. But if such a multiverse does exist, how would people know, and what would it mean for humanity? There may be ways to find out if the known universe is one of many, said Brian Greene, a theoretical physicist and author at Columbia University in New York.
Quantum twist could kill off the multiverse New Scientist - May 14, 2014
A radical new view of quantum mechanics does away with an eternal "bubble" multiverse, and suggests how the "many worlds" multiverse will draw to a close. A popular view of the multiverse says that our universe is just one of an ever-inflating multitude of discrete "bubble" universes. These bubbles are eternally budding off new universes even as individual universes age and die. But a new view of quantum effects Ð the brainchild of Sean Carroll at the California Institute of Technology and his colleagues Ð challenges this picture. It is also potentially very useful to quantum theorists, as it does away with some thorny issues that currently dog cosmology, including a particularly baffling paradox involving disembodied consciousnesses known as "Boltzmann brains".
That Signal From the Beginning of Time Could Redefine Our Universe Wired - March 18, 2014
The physics world was on fire yesterday after an announcement that astronomers had detected a signal from the beginning of time. This is exactly as cool as it sounds. Maybe even cooler. And it might lead to us learning further crazy things about our universe. Besides coming as a shock to most of the community, the discovery once again proved that we don't quite know many things about our universe. Ordinarily sober-minded scientists went to hyperbolic lengths to describe just how significant the results were. Depending on who you ask, they were as important as finding the Higgs boson, directly detecting dark matter, or discovering life on other planets. Nobel Prizes are already being discussed.
Cosmic Microwave Map Swirls Indicate Inflation NASA - March 17, 2014
Did the universe undergo an early epoch of extremely rapid expansion? Such an inflationary epoch has been postulated to explain several puzzling cosmic attributes such as why our universe looks similar in opposite directions. Yesterday, results were released showing an expected signal of unexpected strength, bolstering a prediction of inflation that specific patterns of polarization should exist in cosmic microwave background radiation -- light emitted 13.8 billion years ago as the universe first became transparent. Called B-mode polarizations, these early swirling patterns can be directly attributed to squeeze and stretch effects that gravitational radiation has on photon-emitting electrons. The surprising results were discovered in data from the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) microwave observatory near the South Pole. BICEP2 is the building-mounted dish pictured above on the left. Note how the black polarization vectors appear to swirl around the colored temperature peaks on the inset microwave sky map. Although statistically compelling, the conclusions will likely remain controversial while confirmation attempts are made with independent observations.
Cosmic inflation: Spectacular discovery hailed BBC - March 17, 2014
Scientists say they have extraordinary new evidence to support a Big Bang Theory for the origin of the Universe. Researchers believe they have found the signal left in the sky by the super-rapid expansion of space that must have occurred just fractions of a second after everything came into being. It takes the form of a distinctive twist in the oldest light detectable with telescopes. The breakthrough was announced by an American team working on a project known as BICEP2.
Our Universe May Exist in a Multiverse, Cosmic Inflation Discovery Suggests Live Science - March 18, 2014
Big Bang Discovery Opens Doors to the "Multiverse" National Geographic - March 19, 2014
This illustration depicts a main membrane out of which individual universes arise; they then expand in size through time.
Weird! Our Universe May Be a 'Multiverse,' Scientists Say Live Science - August 12, 2011
Is our universe just one of many? While the concept is bizarre, it's a real possibility, according to scientists who have devised the first test to investigate the idea. The potential that we live in a multiverse arises from a theory called eternal inflation, which posits that shortly after the Big Bang that formed the universe, space-time expanded at different rates in different places, giving rise to bubble universes that may function with their own separate laws of physics. The idea has seemed purely hypothetical, until now. In a new study, researchers suggest that if our universe has siblings, we may have bumped into them. Such collisions would have left lasting marks in the cosmic microwave background (CMB) radiation, the diffuse light left over from the Big Bang that pervades the universe, the researchers say.
'Multiverse' theory suggested by microwave background BBC - August 3, 2011
The idea that other universes - as well as our own - lie within "bubbles" of space and time has received a boost. Studies of the low-temperature glow left from the Big Bang suggest that several of these "bubble universes" may have left marks on our own. This "multiverse" idea is popular in modern physics, but experimental tests have been hard to come by.
A measure for the multiverse New Scientist - March 4, 2010
When Cosmologist George Ellis turned 70 last year, his friends held a party to celebrate. There were speeches and drinks and canapes aplenty to honor the theorist from the University of Cape Town, South Africa, who is regarded as one of the world's leading experts on general relativity. But there the similarity to most parties ends.
For a start, Ellis's celebration at the University of Oxford lasted for three days and the guest list was made up entirely of physicists, astronomers and philosophers of science. They had gathered to debate what Ellis considers the most dangerous idea in science: the suggestion that our universe is but a tiny part of an unimaginably large and diverse multiverse.
To the dismay of Ellis and many of his colleagues, the multiverse has developed rapidly from a being merely a speculative idea to a theory verging on respectability. There are good reasons why. Several strands of theoretical physics - quantum mechanics, string theory and cosmic inflation - seem to converge on the idea that our universe is only one among an infinite and ever-growing assemblage of disconnected bubble universes.
What's more, the multiverse offers a plausible answer to what has become an infuriatingly slippery question: why does the quantity of dark energy in the universe have the extraordinarily unlikely value that it does? No theory of our universe has been able to explain it. But if there are countless universes out there beyond our cosmic horizon, each with its own value for the quantity of dark energy it contains, the value we observe becomes not just probable but inevitable.
Despite the many virtues of the multiverse, Ellis is far from alone in finding it a dangerous idea. The main cause for alarm is the fact that it postulates the existence of a multitude of unobservable universes, making the whole idea untestable. If something as fundamental as this is untestable, says Ellis, the foundations of science itself are undermined.
One of the guests at Ellis's party doesn't see it that way. Raphael Bousso of the University of California, Berkeley, has also been grappling with the multiverse, and in the past few months he has found a way round the troubling problem of unobservable universes. At a stroke, he has transformed the multiverse from a theory so problematical that it threatens to subvert science, into one that promises predictions we can test. His insights are steering physicists along the path to their ultimate goal of uniting quantum mechanics and gravity into one neat theory of everything.
Bousso's achievement is all the more impressive because he has succeed where so many others have tried and failed. The problem they all encountered boils down to this: like quantum mechanics and thermodynamics, multiverse cosmology is an exercise in statistics. Given a universe within the multiverse, you cannot predict what its key characteristics will be - how much dark energy it contains, say. The best you can do is calculate the probability that it looks the way it does based on how likely it is that a universe with its particular set of characteristics will occur in the multiverse. Calculating probabilities, though, requires a "measure" - a mathematical tool that tells you how to define relative probabilities. And finding the right measure for the multiverse is far from easy.
The trouble is that in an infinite multiverse, everything that can happen will happen - an infinite number of times. In such a set-up, probability loses all meaning. "How do you compare infinities?" asks Andrei Linde of Stanford University in California.
Everything that can happen will happen in the multiverse - an infinite number of times.
Prior to Bousso's work, the favoured approach was to pick a snapshot of the multiverse at a particular time and calculate the characteristics of all the bubble universes inside, noting how many different values for the amount of dark energy crop up. From there, you extrapolate the relative probabilities to the multiverse as it develops over time with its infinite number of bubble universes.
Unfortunately, there's a nasty hole in this approach, in the shape of the phrase "at some particular time": according to Einstein's theory of relativity, it renders the whole exercise utterly meaningless. The problem arises from Einstein's insight that clocks run differently for different observers. Two events that are simultaneous for me are not simultaneous for you, so there are an infinite number of ways you can slice up the multiverse. None is more "true" than any other, so there's no reason to choose one time slicing over another - and different slices can yield dramatically different results.
Implicit in previous approaches was the idea that the multiverse can be described from an observerless, God's-eye-view, and Bousso realized that this was what lead to all those intractable infinities. So he decided to calculate probabilities based on what any one observer can see from within their own universe.
Quantum mechanics tells us that the vacuum of space is not empty; instead, it crackles with energy. It also tells us that, sooner or later, any given universe will decay spontaneously into another one with lower energy. Indeed, most cosmologists envisage our big bang as precisely such an event, during which the vacuum we live in emerged from a higher-energy vacuum that constituted a universe before ours. What matters here, though, is that there are a plethora of possible universes that can be produced in this way - each one with its own probability. By adding up these probabilities, Bousso was able to work out the various probabilities of the observer ending up in a universe with a particular set of characteristics.
Using this approach, Bousso was able to derive probabilities for things like the amount of dark energy in any particular universe, without ever have to resort to a God's-eye point of view, or speculation about what might be happening in disconnected bubble universes beyond our view. He calls this approach the causal patch measure, and the important thing is that it works. He has used it to predict the value of the dark energy we ought to see in our own universe, and it turns out to be remarkably close to the observed value (arxiv.org/abs/hep-th/0702115).
So, job done? Not quite. The problem with the causal patch measure is that the result depends on the vacuum energy of the universe the calculation starts with. And such arbitrariness is anathema to physicists.
While Bousso was working on his observer's-eye view of the multiverse, cosmologist Alexander Vilenkin of Tufts University in Boston was formulating another approach to the global picture. Vilenkin, too, had become dissatisfied with past approaches to measure making, and had decided there had to be a better way. Together with Jaume Garriga of the University of Barcelona in Spain, Vilenkin thought there might be some clues in an earlier breakthrough made by Argentinean physicist Juan Maldacena at the Institute for Advanced Study in Princeton.
Maldacena had been working with string theory to build model universes when he made a startling discovery. He found a model in a bizarrely shaped universe with five dimensions that is exactly equivalent to a simpler model on its four-dimensional boundary. This is a classic example of what is known as the "holographic principle", the idea that for a space in any number of dimensions, all the physics inside that space can be encoded on its outer boundary in much the same way that a two-dimensional hologram on a credit card can encode all the information about a 3D object.
Vilenkin and Garriga figured the entire multiverse must similarly have a holographic image living on its boundary (arxiv.org/abs/0905.1509). In the case of the multiverse, though, the boundary is not a frontier in space, but in time, infinitely far into the future. Could it hold a uniquely defined measure for the multiverse?
Bousso was intrigued. While he believed his causal patch measure was more promising, he decided to see what would happen if he tried to derive a measure for the multiverse by studying its boundary instead. "I wanted to figure out a straightforward way of transferring what we had learned from Maldacena to the multiverse," he says.
It turns out that zooming in on part of the boundary is equivalent to selecting different, finite slices of time in the interior of the multiverse (see diagram). To see how it works, imagine you are standing in a dark room with your back against one wall and facing another wall. You switch on a flashlight, which illuminates a large oval on the far wall. As you walk towards the wall ahead, the illuminated oval shrinks.
The further away you move from the back wall where you started, the smaller the area of illumination becomes. In other words, there is a clear relationship between areas on your future boundary and distance from your starting point. In a similar way, a particular area on the boundary of the multiverse is associated with a particular time inside it.
What is so powerful about this approach is that it sidesteps the problem Einstein raised about time being relative to different observers. Here the boundary tells you which bubble universes existed at a particular time. Knowing this, you can start comparing universes and calculating the probability of finding one with a particular value of dark energy, for instance.
As Bousso studied this measure, something astonishing came into focus. The global measure he had discovered using the holographic representation of the multiverse and its future boundary turns out to be exactly equivalent to the causal patch measure he had already derived by simply considering what a single observer can see. The two dramatically different approaches turned out to be two different ways of looking at the same underlying reality: one considers an ensemble of possible histories for a single observer; the other, the entire infinite history of an infinite number of disconnected bubble universes.
"That was really stunning," says Bousso. "It was amazing to me when I realized that the two measures reproduce the exact same probabilities."
Their equivalence turns out to be extremely useful, as weaknesses in one measure are strengths in the other, and vice versa. "They are like two people on crutches holding one another up," Bousso says.
So while in the causal patch measure your answers depend strongly on the universe in which your observers start out, the global measure does not suffer from this ambiguity. In the multiverse, bubbles beget bubbles beget bubbles, so that initial conditions are quickly lost in the crowd and no longer matter when it comes to calculating probabilities. In fact the global picture actually defines what the starting vacuum for the causal patch approach should be.
On the other hand, while the global picture suffers from the problem of "duplicate information" (see "What black holes can teach us"), Bousso's causal patch measure successfully circumvents this.
The implications might be immense. The two equivalent measures have not only provided a prediction for dark energy in our own universe that closely matches observations, they were both inspired in different ways by the holographic principle. This suggests that the holographic principle is profoundly significant, and could lead us to a theory of quantum gravity - the long-sought theory of everything that mirrors the dynamics of the multiverse. "By thinking about the measure problem, we seem to be learning, perhaps unexpectedly, about another, equally deep mystery, namely how to formulate the quantum gravity theory of the multiverse," says Bousso.
We are learning how to formulate a quantum gravity theory of the multiverse.
Even Ellis is impressed by Bousso's results, if not exactly sold on the multiverse. "It is a useful and intriguing kind of consistency test based in fascinating but speculative physics," he says. And there is another far-reaching consequence. If Bousso's equivalence holds, then not only can the resulting measure be used to make real, testable predictions, they can also make calculations in the multiverse without ever referring to unobservable universes lurking beyond our cosmic horizon. Everything we need to know about the multiverse might be right here in our own universe.
When Stephen Hawking calculated that black holes radiate away energy and eventually evaporate, he left a nagging question: what happens to the information about all the stuff that has fallen in? If it escaped back into the universe, it would have to be traveling faster than the speed of light, violating Einstein's theory of relativity. If it vanished from the universe, it would be violating a fundamental tenet of quantum mechanics. This conundrum became known as the black hole information loss paradox.
The answer comes from the idea known as the holographic principle, which says that the physics inside a region of space-time is equivalent to the physics on the region's boundary. You can think of a black hole as equivalent to a hot gas of ordinary particles on the boundary of the universe. And since a hot gas of ordinary particles never loses information, neither can a black hole.
The lesson from the holographic picture is that no observer should ever see information disappear from the universe. If Alice is watching from a distance as an elephant falls into a black hole, she will see it approach the black hole's event horizon, at which point it is incinerated by the Hawking radiation, which sends it streaming back towards her as a sad, scrambled heap of ashes. Meanwhile, Bob, who falls into the black hole along with the elephant, sees the elephant cross the horizon safely, and live happily for some time before hitting the singularity in the black hole's core.
According to the holographic principle, both stories must be true. But how can the elephant be in a heap of ashes outside the horizon and alive and well inside the black hole? It would seem the elephant has been cloned, but the laws of physics prohibit such duplication of information.
Cosmologist Raphael Bousso explains the paradox results from the mistaken idea that we can describe what's happening both inside and outside the horizon simultaneously, when in reality no single observer can ever see both at once. In other words, for physics to make sense, you must restrict your description of the universe to what a single observer can see. It's a profoundly different approach from the old idea that we can describe the entire universe from an observerless, God's-eye-view.
Talking about the multiverse as if it can all be directly observed at once, Bousso says, leads to an even greater nonsense than trying to simultaneously describe what's happening inside and outside a black hole horizon.
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