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How can we access the Multiverse if it is truly real?

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How can we access the Multiverse if it is truly real?

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What would be the boundaries between universes?

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What would be the boundaries between universes?

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How do scientists think we can prove this theory?

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How do scientists think we can prove this theory?

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A few months before physicist Stephen Hawking died, he published a paper that several media outlets touted as a way to finally prove (or disprove) the existence of parallel worlds.

But that claim may be a bit of cosmic inflation, said several physicists who were not involved in Hawking's research.

"The paper makes no statements about observational tests. It's not entirely uninteresting, but it's one of literally several thousand ideas for what might possibly have happened in the early universe" many of which include parallel worlds, said Sabine Hossenfelder, a physicist at the Frankfurt Institute for Advanced Studies in Germany, who blogs at backreaction.blogspot.com. [ The 18 Biggest Unsolved Mysteries in Physics]

The study, by Hawking and Thomas Hertog, a physicist at the Catholic University Leuven (KU Leuven) in Belgium, first appeared in the preprint journal arXiv in July 2017 and was modified just a week before Hawking passed away. The study is an attempt to isolate the types of universes predicted by one of Hawking's unproven theories, the "no boundary" model to explain the Big Bang.

However, it relies on several speculative and unproven theories and uses them in bold ways to make vague conclusions, said Frank Wilczek, a Nobel laureate and theoretical physicist at MIT.

No boundaries

The roots of the paper go back to on of Hawking's best-known, yet unproven theories: the "no boundary proposal" to explain the Big Bang. According to Einstein's theory of general relativity, before the Big Bang, there must have been a singularity, or an infinitely dense, extremely hot speck of matter for which the laws of physics break down.

In this conception, which was formulated with University of California, Santa Barbara physicist James Hartle, this singularity was one in both space and time - meaning that before the Big Bang, time as we know it didn't exist. Try to visualize the universe in time then, and it is as if it had no boundary - like a globe that can be traversed over and over without reaching an edge.

As scientists analyzed the idea, they realized that Hawking's no-boundary theory predicts that this baby universe, as it inflated super fast, would pop off other universes and ultimately an entire multiverse. In fact, Hawking's theory ultimately posited that our humble universe is just one of infinitely many parallel universes that exist in this infinite, fractal-like multiverse. This multiverse would then lead to a paradox: Because there would be an infinite number of universes, no one would be able to make any testable predictions about the particular universe we happen to live in, Hertog told Live Science in an email. (With an infinite number of possibilities, anything becomes possible, and no particulars about a universe could be determined.) [ Top 5 Reasons We May Live in a Multiverse]

"Hawking was not satisfied with this state of affairs," Hertog told Live Science in an email. "'Let's try to tame the multiverse,' he told me a year ago. So we set out to develop a method to transform the idea of a multiverse into a coherent testable scientific framework."

To cut down the infinite number of parallel worlds, the researchers had to find a bridge between the quantum physics that dominated the teensy-tiny singularity of the fetal universe and the classical laws that govern the big universe we all live in. In the new study, Hawking and Hertog used a method known as holography to unite the two sets of ideas. By doing so, they were able to cut this vast forest of multiverses into a countable number. Once they were working with a finite number of universes, they could make predictions about what these universes would look like, Hertog said.

For instance, "they find that universes which are smooth and like our own, basically, are probable," Hossenfelder told Live Science.

Hawking's no-boundary theory posits that after the Big Bang, the universe went through a burst of rapid expansion called cosmic inflation, amplifying the primordial gravitational waves that emanated from the Big Bang, Hertog said. This ancient echo of the universe's birth is recorded in the faint, cold microwave radiation that permeates every region of our universe, known as cosmic microwave background radiation (CMB). If future satellites show that the energy signal in the CMB data matches the inflation predicted by Hawking's model, it could conceivably provide strong evidence for the existence of a multiverse, Hertog said.

Weak test

But other scientists are skeptical. There are other models of inflation, and most of these models also include some primordial gravitational waves, said Katie Mack, a cosmologist at North Carolina State University who was not an author on the new paper.

"The ideas brought up in this paper are not giving any new kind of signature that's different from other models of inflation," Mack told Live Science.

In other words, there's no way for CMB data to reveal whether Hawking's theory, or one of thousands of others, is right.

On the other hand, if the CMB signals do not match Hawking and Hertog's predictions, "that would falsify the very specific set of assumptions and approximations used in the paper," Wilczek told Live Science in an email. "So it's testable in that very weak sense," he added.

Toy model

What's more, Hawking and Hertog relied on a mathematical framework to connect quantum theory and gravity, but that framework relies on a number of unproven conjectures and is, essentially, not complete, Mack told Live Science.

"What they've done in this paper is to use what they call a toy model - it's not fully rigorous and complete," Mack said. "They admit that there's a lot more work to be done."

Before anyone can say that this paper is valid, scientists need to have a better theoretical understanding of quantum gravity, Mack said, referring to a unified theory that unites both quantum mechanics and gravity as described by Einstein's theory of relativity.
Originally published on Live Science.

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Stephen Hawking's Last Paper (Probably) Doesn't Prove We Live in a Multiverse

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A few months before physicist Stephen Hawking died, he published a paper that several media outlets touted as a way to finally prove (or disprove) the existence of parallel worlds. But that claim may be a bit of cosmic inflation, said several physicists who were not involved in Hawking's research.

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What possibilities can come from accessing another universe?

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What possibilities can come from accessing another universe?

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"We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct." Niels Bohr spoke these words to Wolfgang Pauli about the latter's theory of elementary particles, but it could just as easily apply to many of today's most controversial modern physics ideas. One that's gotten a lot of attention recently is that of a Multiverse. In short, it's the idea that our Universe, and all that's contained within it, is just one small region of a larger existence that includes many similar, and possibly many different, Universes like our own. On the one hand, if our current theories of physics are true, the Multiverse absolutely must exist. But on the other hand, as Sabine Hossenfelder rightly points out, it's unlikely to teach us anything useful.

Why must the Multiverse exist? Quite simply: there must be more Universe than the part that is observable to us. If you look just at the portion of the Universe we can see, you can measure its spatial curvature, and find that it's incredibly close to flat. No regions repeat; no locations connect or loop back on one another; no large-curvature regions show themselves on a scale approaching that of the Universe we can observe. If the Universe were a hypersphere, the four-dimensional analogue of a sphere, it must have a radius of curvature hundreds of times the size of what we can observe. There must be more Universe out there than what we can access.

But this isn't just a conclusion from observations; it's the same conclusion that we'd draw from our leading theory of the Universe's origin: cosmological inflation. Prior to the hot Big Bang, the fabric of the Universe was expanding at an exponential rate, where every 10-35seconds or so, it would double in scale in all dimensions. Inflation went on for at least as long as 10-33 seconds or so, but could have lasted far longer: seconds, years, millennia, trillions of years or an arbitrarily long length of time. When inflation ends, the Universe we're left with is stretched flat, the same temperature everywhere, and far, far vaster than anything we can ever hope to observe. Considering the finite nature of all we can see, inflation is the natural way to create a Multiverse of possibilities.

Without a solid knowledge of how inflation began, or if it ever had a beginning, we cannot know how much "Multiverse" there is out there beyond our actual Universe. But based on the properties of inflation that imprint themselves on the Universe we inhabit, we can draw a few conclusions about it. In particular:

The lack of spatial curvature,

The adiabatic nature and spectrum of fluctuations imprinted on the cosmic microwave background,

The magnitude of imperfections that gave rise to the large-scale structure we see,

The constraints on the gravitational waves inflation could have created,

And the superhorizon fluctuations that we observe (on scales larger than the visible Universe),

all give us some important constraints on the type of inflation that occurred, and teach us two very important lessons, if the implications of these verified and validated theories are correct, about our Multiverse.

1.) Inflation did not occur at arbitrarily high energies. There's an energy scale at which the laws of physics no longer make sense: the Planck scale, or about 1019 GeV. This is about 100 trillion times larger than the maximum energies the LHC achieves, and a factor of about 100 million higher than the highest energy cosmic particles we've ever detected in the Universe. From the imprints of inflation, we can conclude that the temperature at the start of the hot Big Bang never got higher than about 1015 or 1016 GeV, safely below the Planck scale. This implies that inflation likely occurred below that scale as well. If true, this would mean that the inflationary epoch obeyed the current laws of physics, as well as every region of the Multiverse that inflation created.

2.) There are countless regions where inflation did not end, and still continues today. The idea that the Big Bang happened everywhere at once may apply to our Universe, but certainly ought not to apply to the vast majority of Universes existing in the Multiverse. Assuming that inflation is a quantum field, like all fields we know of, it must spread out over time, meaning that in any region of space, it has a probability of ending at a certain time, but also a probability of continuing on for a while longer.

In the region that became our Universe, which may encompass a large region that goes far beyond what we can observe, inflation ended all-at-once. But beyond that region, there are even more regions where it didn't end. Those regions grow and inflate as time goes on, and even though many of those new regions will see inflation end, the ones where it doesn't will continue to inflate. Inflation, therefore, should be eternal to the future, at least in some regions of space. This is irrespective of whether it was eternal to the past or not.

Accepting all of this leads to an inescapable conclusion: we live in a Multiverse, and our Universe is just one of countlessly many that exist within it. However, the standard predictions that come out of this are difficult to do science with. They include:

That different regions where inflation ends should never collide or interact.

That the fundamental constants and laws in different regions should be the same as they are here.

And that unless inflation was truly eternal to the past, there isn't enough "space" to contain all the parallel Universes that the many-worlds interpretation of quantum physics would require.

It's always possible to construct a contrived model that defies these generic predictions, and some scientists make a career of doing so. Writing in NPR, Sabine Hossenfelder is right to criticize that approach, stating, " Just because a theory is falsifiable doesn't mean it's scientific." But just because variants of the Multiverse are falsifiable, and just because the consequences of its existence are unobservable, doesn't mean that the Multiverse isn't real. If cosmic inflation, General Relativity, and quantum field theory are all correct, the Multiverse likely is real, and we're living in it.

Just don't expect it to solve your most burning questions about the Universe. For that, you need physics you can put to an experimental or observable test. Until that day arrives, the consequences of a Multiverse will likely remain in the realm of science fiction: where they presently belong. It's okay to speculate, but if you insist on attributing a physics problem's solution to an untestable feature of the Universe, you're essentially giving up on physics. We all know that the mysteries of the Universe are hard, but that's no reason to not even try to find a solution. The Multiverse is real, but provides the answer to absolutely nothing.

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Yes, The Multiverse Is Real, But It Won't Fix Physics

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"We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct." Niels Bohr spoke these words to Wolfgang Pauli about the latter's theory of elementary particles, but it could just as easily apply to many of today's most controversial modern physics ideas.

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Who created the theory for the multiverse, how do we know if it is possible for this theory to be true?

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Who created the theory for the multiverse, how do we know if it is possible for this theory to be true?

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Hugh Everett's many-worlds interpretation of quantum mechanics arose from what must have been the most world-changing drinking session of all time. One evening in 1954, in a student hall at Princeton University, grad student Everett was drinking sherry with his friends when he came up with the idea that quantum effects cause the universe to constantly split.

Read more&colon; " Multiverse me&colon; Should I care about my other selves? "

He developed the idea for his PhD thesis - and the theory held up. According to his work, we are living in a multiverse of countless universes, full of copies of each of us. It was sensational.

Max Tegmark of Massachusetts Institute of Technology has said that Everett's work is as important as Einstein's work on relativity. But the leading physicists of the Everett's day, in particular Niels Bohr, one of the fathers of quantum mechanics, couldn't stomach it. They couldn't cope with the idea that every decision we make creates new universes, one for all possible outcomes. Everett had to publish a watered-down version of his idea. Thoroughly disgruntled, he left physics.

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What he did next fascinates me, in the light of what he had discovered. Everett joined the Pentagon, and worked in a team calculating potential deaths in the event of nuclear war. His job was to calculate how to maximise the death toll for the Soviets while minimising it for Americans by looking at fallout.

If the many-worlds view is right, our actions shape our counterparts' lives in parallel worlds. What does this mean for how should we behave in this world? It's a question I explore - with the help of leading physicists, as well as Hugh Everett's rock star son - in this in-depth article&colon; " Multiverse me&colon; Should I care about my other selves? ".

Mutually assured destruction

But what about Everett himself? Did he consider what working for the Pentagon would mean in parallel worlds? Wasn't he facilitating nuclear war in the multiverse? We can't ask him, because he died in 1982, aged only 51. But we can ask those who know his work.

"He wrote arguably the first ever serious report on just how devastating a nuclear war would be for the US," says Tegmark. It helped devise the concept of mutually assured destruction (MAD). This is a concept appropriately summed up by its acronym - in that we'd be insane to start a nuclear war - but MAD might actually have prevented the cold war from overheating.

"MAD might have in a major way contributed to the extra caution that might explain why we're still here," says Tegmark. "My guess is that Everett's work helped drive home the full horror of war, and this reduced the fraction of the multiverse that saw global nuclear war."

The work Everett did for the Pentagon, then, arguably had a net positive effect on the multiverse. It's tempting to speculate that these actions were influenced by his physics work, but we'll never know.

Unstoppable vs immovable

Everett's life was fascinating and tragic. Peter Byrne's book, The Many Worlds of Hugh Everett III provides a detailed account. Many moments stand out, here are a few.

Everett wrote to Einstein when he was 12 - and Einstein replied. Young Hugh was already keen at challenging the very highest figures of physics authority. His letter was an attempt to solve the paradox of what happens when an unstoppable force meets an immovable object.

He was recommended for Princeton by his old professor of mathematics, who wrote, "This is a once-in-a-lifetime recommendation for I think it most unlikely that I shall ever again encounter a student I can give such complete and unreserved support."

When Everett died of a heart attack, his teenage son Mark discovered the body. Mark recalls that trying to revive his dead father was the first time he could remember ever touching him.

Everett was a keen atheist. Following his instructions his widow, Nancy, threw his ashes out with the garbage.

Everett's son Mark referenced his father in a 2005 song Things the Grandchildren Should Know&colon;
I never really understood what it must have been like for him,
Living inside his head,
I feel like he's here with me now,
Even though he's dead.
There is also a comprehensive Everett archive here, and an award-winning BBC documentary on his work called Parallel Worlds, Parallel Lives.

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Hugh Everett: The man who gave us the multiverse

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Hugh Everett's many-worlds interpretation of quantum mechanics arose from what must have been the most world-changing drinking session of all time. One evening in 1954, in a student hall at Princeton University, grad student Everett was drinking sherry with his friends when he came up with the idea that quantum effects cause the universe to constantly split.