Time need not end in the multiverse

No longer a worry

GAMBLERS already had enough to think about without factoring

the end of time into their calculations. But a year after a group of cosmologists argued that they should, another team says time need not end after all.

It all started with this thought experiment. In a back room in a Las Vegas casino, you are handed a fair coin to flip. You will not be allowed to see the outcome, and the moment the coin lands you will fall into a deep sleep. If the coin lands heads up, the dealer will wake you 1 minute later; tails, in 1 hour. Upon waking, you will have no idea how long you have just slept.

The dealer smiles: would you like to bet on heads or tails? Knowing it’s a fair coin, you assume your odds are 50/50, so you choose tails. But the house has an advantage. The dealer knows you will almost certainly lose, because she is factoring in something you haven’t: that we live in a multiverse.

The idea that our universe is just one of many crops up in a number of physicists’ best theories, including inflation. It posits that different parts of space are always ballooning into separate universes, so that our observable universe is just a tiny island in an exponentially growing multiverse.

In any infinite multiverse, everything that can happen, will happen – an infinite number of times. That has created a major headache for cosmologists, who want to use probabilities to make predictions, such as the strength of the mysterious dark energy that is accelerating the expansion of our own universe. How can we say that anything is more or less probable than anything else?

One procedure physicists are fond of is to draw a cut-off at some finite time, count up the number of events – say, heads and tails – that occur in the multiverse before the cut-off time, and use that as a representative sample.

It seems reasonable, but when tackling the casino experiment, something strange happens. Wherever the cut-off is drawn, it slices through some of the gamblers’ naps, making it appear as if those gamblers simply never woke up. The longer the nap, the more likely it is to be cut off, so if you do awaken, it’s more likely that you have taken a shorter nap – that is, that you flipped heads. So even though the odds seemed to be 50/50 when the coins were first flipped, heads becomes more probable than tails once you and the other gamblers wake up.

“This thought experiment was unbelievably perplexing at first, because it seemed like probabilities were changing from one instant to the next without any explanation,” says Alan Guth of the Massachusetts Institute of Technology, who along with Vitaly Vanchurin of Stanford University in California, came up with the conundrum two years ago.

Last year, Raphael Bousso at the University of California, Berkeley, and colleagues devised an explanation that was effective, if unsettling. The changing probabilities were behaving as if time ends at the cut-off, they said, because time really does end at the cut-off. That’s why the initial 50/50 odds change when you wake up from your nap.

Upon waking, you have new information: you know that time didn’t end. That now means it is more likely that you only slept for a minute than for an hour. After all, time could end at any minute, and an hour has an extra 59 of those to spare. Heads wins.

The idea that time must end for the probabilities to make sense has been bugging Guth and Vanchurin for the last year. Now they say they have developed a mathematical explanation for the multiverse that saves the fourth dimension (arxiv.org/abs/1108.0665).

The essence of the argument is that you don’t need any new information, in this case the fact that you woke up, to understand why the odds are no longer 50/50. In a multiverse that grows exponentially, where each new generation of universes is far larger than the last, younger universes always outnumber older ones. Waking up, you will either be in a universe in which 1 minute has passed (heads), or in a universe in which 1 hour has passed (tails). “The experiment sets up a 59-minute ambiguity in the age of the universe,” Guth says. “You should always bet on the younger one.”

But Bousso doesn’t feel safe just yet: “Nature has often seemed crazy as we discovered how far removed its workings are from our everyday intuition. The end of time may sound crazy, but it is by far the simplest interpretation.” Whether or not time is going to end, there’s a lesson to take from the debate: should you wake up in Las Vegas, bet heads.  http://www.newscientist.com

This Week’s Hype

The idea that other universes – as well as our own – lie within “bubbles” of space and time has received a boost.

After taking a look at the PRL and PRD papers that are behind this, it’s clear that a more accurate title for the story would have been “‘Multiverse’ theory suggested by microwave background – NOT”. As usual, the source of the problem here is a misleading university press release, one from University College London entitled First observational test of the ‘multiverse’. Somehow the press release neglected to mention something one might think was an important detail, the fact that this “First observational test” had a null result.

It’s well-known that one can find Stephen Hawking’s initials, and just about any other pattern one can think of somewhere in the CMB data. The authors of the PRL and PRD papers first put out preprints last December (see here and here). In these preprints they essentially claimed to have found four specific features in the CMB where the hypothesis that they were due to bubble collisions was statistically preferred. A guest post by Matthew Johnson at Cosmic Variance explained more about the preprints. I didn’t understand their statistical measure, so asked about it in the comment section, where Matthew explained that, by more conventional measure, the statistical significance was “near 3 sigma“.

It turns out that the PRL and PRD papers differ significantly from the preprint versions. In the acknowledgements section of the PRD paper we read that:

A preprint version of this paper presented only evidence ratios confined to patches. We thank an anonymous referee who encouraged us to develop this algorithm into a full-sky formalism.

and the result of the new analysis asked for by the referee is summarized in the conclusion of the paper:

The posterior evaluated using the WMAP 7-year data is maximized at Ns = 0 [Ns is the average number of observable bubble collisions over the full sky], and constrains Ns < 1.6 at 68% confidence. We therefore conclude that this data set does not favor the bubble collision hypothesis for any value of Ns. In light of this null detection, comparing with the simulated bubble collisions… [various bounds ensue]

So, the bottom line is that they see nothing, but a press release has been issued about how wonderful it is that they have looked for evidence of a Multiverse, without mentioning that they found nothing. As one would expect, this kind of behavior leads to BBC stories about how the Multiverse has “received a boost”, exactly the opposite of what the scientific evidence shows.

First observational test of the ‘multiverse’

The theory that our universe is contained inside a bubble, and that multiple alternative universes exist inside their own bubbles – making up the ‘multiverse’ – is, for the first time, being tested by physicists.

Two research papers published in Physical Review Letters and Physical Review D are the first to detail how to search for signatures of other universes. Physicists are now searching for disk-like patterns in the cosmic microwave background (CMB) radiation – relic heat radiation left over from the Big Bang – which could provide tell-tale evidence of collisions between other universes and our own.

Many modern theories of fundamental physics predict that our universe is contained inside a bubble. In addition to our bubble, this `multiverse’ will contain others, each of which can be thought of as containing a universe. In the other ‘pocket universes’ the fundamental constants, and even the basic laws of nature, might be different.

Until now, nobody had been able to find a way to efficiently search for signs of bubble  collisions – and therefore proof of the  – in the CMB radiation, as the disc-like patterns in the radiation could be located anywhere in the sky. Additionally, physicists needed to be able to test whether any patterns they detected were the result of collisions or just random patterns in the noisy data.

A team of cosmologists based at University College London (UCL), Imperial College London and the Perimeter Institute for Theoretical Physics has now tackled this problem.

“It’s a very hard statistical and computational problem to search for all possible radii of the collision imprints at any possible place in the sky,” says Dr Hiranya Peiris, co-author of the research from the UCL Department of Physics and Astronomy. “But that’s what pricked my curiosity.”

The team ran simulations of what the sky would look like with and without cosmic collisions and developed a ground-breaking algorithm to determine which fit better with the wealth of CMB data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). They put the first observational upper limit on how many bubble collision signatures there could be in the CMB sky.

Stephen Feeney, a PhD student at UCL who created the powerful computer algorithm to search for the tell-tale signatures of collisions between “bubble universes”, and co-author of the research papers, said: “The work represents an opportunity to test a theory that is truly mind-blowing: that we exist within a vast multiverse, where other universes are constantly popping into existence.”

One of many dilemmas facing physicists is that humans are very good at cherry-picking patterns in the data that may just be coincidence. However, the team’s algorithm is much harder to fool, imposing very strict rules on whether the data fits a pattern or whether the pattern is down to chance.

Dr Daniel Mortlock, a co-author from the Department of Physics at Imperial College London, said: “It’s all too easy to over-interpret interesting patterns in random data (like the ‘face on Mars’ that, when viewed more closely, turned out to just a normal mountain), so we took great care to assess how likely it was that the possible bubble collision signatures we found could have arisen by chance.”

The authors stress that these first results are not conclusive enough either to rule out the multiverse or to definitively detect the imprint of a bubble collision. However, WMAP is not the last word: new data currently coming in from the European Space Agency’s Planck satellite should help solve the puzzle.

More information: ‘First Observational Tests of Eternal Inflation’ and ‘First Observational Tests of Eternal Inflation: Analysis Methods and WMAP 7-Year Results’ published online in Physical Review Letters and Physical Review D
http://arxiv.org/abs/1012.3667
http://www.physorg.com/news/2011-08-multiverse.html

The multiverse and quantum physics: Other worlds may not be so far away

A new paper simplifies – slightly – our view of the cosmos by reconciling two theories relating to multiple universes , writes Roger Highfield.

Two of cosmology's most mind-blowing ideas can be combined to solve some of physics' hardest problems, scientists suggest.

Let’s take two of the biggest, most bizarre and mind-blowing ideas in cosmology. First, the multiverse, the hypothetical set of possible universes that comprise all that exists. Second, the suggestion that the cosmos constantly divides into parallel universes in which every conceivable outcome of every event happens somewhere, a concept that is adored by science-fiction writers.
What happens if we blend the two? Well, it happens to solve one of the most persistent problems in physics. At the moment, our most successful theory describes a universe that is larger than any observer inside it can see: to view the whole thing, you’d need what cosmologist Raphael Bousso calls a “godlike” view of the cosmos….. Continue reading The multiverse and quantum physics: Other worlds may not be so far away

When the multiverse and many-worlds collide

ΤWO 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 this,” he says…. Continue reading When the multiverse and many-worlds collide

Multiverse = Many Worlds

Two of the most bizarre ideas in modern physics are different sides of the same coin, say string theorists
The many worlds interpretation of quantum mechanics is the idea that all possible alternate histories of the universe actually exist. At every point in time, the universe splits into a multitude of existences in which every possible outcome of each quantum process actually happens.
So in this universe you are sitting in front of your computer reading this story, in another you are reading a different story, in yet another you are about to be run over by a truck. In many, you don’t exist at all.
This implies that there are an infinite number of universes, or at least a very large number of them.
That’s weird but it is a small price to pay, say quantum physicists, for the sanity the many worlds interpretation brings to the otherwise crazy notion of quantum mechanics. The reason many physicists love the many worlds idea is that it explains away all the strange paradoxes of quantum mechanics.
For example, the paradox of Schrodinger’s cat–trapped in a box in which a quantum process may or may not have killed it– is that an observer can only tell whether the cat is alive or dead by opening the box.
But before this, the quantum process that may or may not kill it is in a superposition of states, so the cat must be in a superposition too: both alive and dead at the same time.
That’s clearly bizarre but in the many worlds interpretation, the paradox disappears: the cat dies in one universe and lives in another.
Let’s put the many world interpretation aside for a moment and look at another strange idea in modern physics. This is the idea that our universe was born along with a large, possibly infinite, number of other universes. So our cosmos is just one tiny corner of a much larger multiverse.
Today, Leonard Susskind at Stanford University in Palo Alto and Raphael Bousso at the University of California, Berkeley, put forward the idea that the multiverse and the many worlds interpretation of quantum mechanics are formally equivalent.
But there is a caveat. The equivalence only holds if both quantum mechanics and the multiverse take special forms.
Let’s take quantum mechanics first. Susskind and Bousso propose that it is possible to verify the predictions of quantum mechanics exactly.
At one time, such an idea would have been heresy. But in theory, it could be done if an observer could perform an infinite number of experiments and observe the outcome of them all.
But that’s impossible, right? Nobody can do an infinite number of experiments. Relativity places an important practical limit on this because some experiments would fall outside the causal horizon of others. And that would mean that they couldn’t all be observed.
But Susskind and Bousso say there is a special formulation of the universe in which this is possible. This is known as the supersymmetric multiverse with vanishing cosmological constant.
If the universe takes this form, then it is possible to carry out an infinite number of experiments within the causal horizon of each other.
Now here’s the key point: this is exactly what happens in the many worlds interpretation. At each instant in time, an infinite (or very large) number of experiments take place within the causal horizon of each other. As observers, we are capable of seeing the outcome of any of these experiments but we actually follow only one.
Bousso and Susskind argue that since the many worlds interpretation is possible only in their supersymmetric multiverse, they must be equivalent. “We argue that the global multiverse is a representation of the many-worlds in a single geometry,” they say.
They call this new idea the multiverse interpretation of quantum mechanics.
That’s something worth pondering for a moment. Bousso and Susskind are two of the world’s leading string theorists (Susskind is credited as the father of the field), so their ideas have an impeccable pedigree.
But what this idea lacks is a testable prediction that would help physicists distinguish it experimentally from other theories of the universe. And without this crucial element, the multiverse interpretation of quantum mechanics is little more than philosophy.
That may not worry too many physicists, since few of the other interpretations of quantum mechanics have testable predictions either (that’s why they’re called interpretations).
Still, what this new approach does have is a satisfying simplicity– it’s neat and elegant that the many worlds and the multiverse are equivalent. William of Ockham would certainly be pleased and no doubt, many modern physicists will be too.
Ref: arxiv.org/abs/1105.3796: The Multiverse Interpretation of Quantum Mechanics
http://www.technologyreview.com/blog/arxiv/26787/