Multiverse

Sabine Hossenfelder reflects on British science writer Jim Baggott’s 2013 book Farewell to Reality at her blog, Back(re)action: I largely agree with Baggott’s assessment, though I am less critical of research on the foundations of quantum mechanics and I could quibble with his take on black hole evaporation, but it seems somewhat besides the point. I share Baggott’s worry that presenting unfounded speculations, like that we live in a multiverse, as newsworthy research undermines public trust in science. … Baggott is a gifted science writer whose explanations seem as effortless as I’m sure they’re not. He knows his stuff and isn’t afraid of clear words. And having noted this, it is not irrelevant to mention that Baggott is no longer Read More ›

From Natalie Wolchover at Quanta: The controversial idea that our universe is just a random bubble in an endless, frothing multiverse arises logically from nature’s most innocuous-seeming feature: empty space. Specifically, the seed of the multiverse hypothesis is the inexplicably tiny amount of energy infused in empty space — energy known as the vacuum energy, dark energy or the cosmological constant. Each cubic meter of empty space contains only enough of this energy to light a lightbulb for 11-trillionths of a second. “The bone in our throat,” as the Nobel laureate Steven Weinberg once put it, is that the vacuum ought to be at least a trillion trillion trillion trillion trillion times more energetic, because of all the matter and Read More ›

From Philip Perry at BigThink: What’s inside a black hole? In most, there’s something called the singularity—an area of such density and intense gravitational force that not even light can escape. Don’t venture too close. Once you enter the event horizon—the outer rim—it’s all over for you. You’d be shredded to ribbons of atoms that’ll be sucked down into its depths. But there may be one exception. Mathematicians have recently unveiled a scenario even more mind-blowing. If you stepped into a Reissner-Nordström-de Sitter black hole, the deterministic nature of the universe would break down, and your past and your future would no longer be connected. The past would simply slide away, while an infinite number of futures would open up Read More ›

From some thoughts on by Peter Woit at : Part of the problem with this good vs. evil story is that, as the book itself explains, it’s not at all clear what the “Copenhagen interpretation” actually is, other than a generic name for the point of view the generation of theorists such as Bohr, Heisenberg, Pauli, Wigner and von Neumann developed as they struggled to reconcile quantum and classical mechanics. They weren’t solipsists with poor reasoning skills, but trying to come to terms with the extremely non-trivial and difficult problem of how the classical physics formalism we use to describe observations emerges out of the more fundamental quantum mechanical formalism. They found a workable set of rules to describe what Read More ›

From Adam Becker at Scientific American: Quantum physics, Everett pointed out, didn’t really reduce to classical physics for large numbers of particles. According to quantum physics, even normal-sized objects like chairs could be located in two totally separate places at once—a Schrödinger’s-cat–like situation known as a “quantum superposition.” And, Everett continued, it wasn’t right to appeal to classical physics to save the day, because quantum physics was supposed to be a more fundamental theory, one that underpinned classical physics. … Everett’s work fell into deep obscurity. It wasn’t revived until the 1970s, and even then, it was slow to catch on. Everett did make one last foray into the academic debate over his work; Wheeler and his colleague Bryce DeWitt Read More ›

From Sabine Hossenfelder at Back(Re)Action: Yesterday, the media buzzed with the revelation that Stephen Hawking had completed a paper two weeks before his death. This paper supposedly contains some breathtaking insight. About the multiverse (parallel universes). The paper is based on an old idea by Stephen Hawking and Jim Hartle called the “no-boundary” proposal. In the paper, the authors employ a new method to do calculations that were not previously possible. Specifically, they calculate which type of universes a multiverse would contain if this theory was correct. The main conclusion seems to be that our universe is compatible with the idea, and also that this particular multiverse which they deal with is not as large as the usual multiverse one Read More ›

From Henry Bodkin at the Telegraph: A final theory explaining how mankind might detect parallel universes was completed by Stephen Hawking shortly before he died, it has emerged. Colleagues have revealed the renowned theoretical physicist’s final academic work was to set out the groundbreaking mathematics needed for a spacecraft to find traces of multiple big bangs. Currently being reviewed by a leading scientific journal, the paper, named A Smooth Exit from Eternal Inflation, may turn out to be Hawking’s most important scientific legacy. More. This sounds a lot like grief talking but we’ll see. See also: The Universe Hawking Created from Nothing Eric Anderson: Lennox’ analysis of Hawking’s absurd pronouncement isn’t Earth-shattering or particularly difficult to grasp in its own Read More ›

A reader writes to ask, quoting Sabine Hossenfelder at her blog Back(Re)Action: What the particle physicists got wrong was an argument based on a mathematical criterion called “naturalness”. If the laws of nature were “natural” according to this definition, then the LHC should have seen something besides the Higgs. The data analysis isn’t yet completed, but at this point it seems unlikely something more than statistical anomalies will show up. I must have sat through hundreds of seminars in which naturalness arguments were repeated. Let me just flash you a representative slide from a 2007 talk by Michelangelo L. Mangano (full pdf here), so you get the idea. The punchline is at the very top: “new particles must appear” in Read More ›

From Ethan Siegel at Forbes: why, and when, can we use mathematics to learn something about our physical Universe? We don’t know the answer to why, but we do know the answer to when: when it agrees with our experiments and observations. So long as the laws of physics remain the laws of physics, and do not whimsically turn on-and-off or vary in some ill-defined way, we know we can describe them mathematically, at least in principle. Mathematics, then, is the toolkit we use to describe the functioning of the Universe. It’s the raw materials: the nails, the boards, the hammers and saws. Physics is how you apply that mathematics. Physics is how you put it all together to make Read More ›

From astrophysicist Brian Koberlein at Nautilus: The idea, from Caltech physicist Kimberly Boddy, and colleagues, is somewhat speculative, and it has an interesting catch. The argument that the true vacuum of the universe is stationary relies on a version of quantum theory known as the many-worlds formulation. In this view, the wave function of a quantum system doesn’t “collapse” when observed. Rather, different outcomes of the quantum system “decohere” and simply evolve along different paths. Where once the universe was a superposition of different possible outcomes, quantum decoherence creates two definite outcomes. Of course, if our minds are simply physical states within the cosmos, our minds are also split into two outcomes, each observing a particular result. In solving the Read More ›

From Tom Siegfried at ScienceNews:: Maybe, string/brane/M theory would explain the amount of that mysterious “dark” energy in space and all would be well. But no. Working with physicist Raphael Bousso, Polchinski found that string theory did not specify how much energy the vacuum of space contained. Instead the theory predicted a virtually countless number of vacuum states, with nearly any amount of repulsive energy you could imagine. In other words, string theory described a giga-gaggle of different universes — a multiverse. Polchinski’s modesty manifested itself in his reaction to this situation. He hated the idea of a multiverse, because it implied that some questions had no answers that physicists could calculate. No equation could specify the amount of dark Read More ›

From Columbia mathematician Peter Woit at his blog Not Even Wrong: Back in 2003-4 I never would have believed that the subject would end up in the state it finds itself in now. With the LHC results removing the last remaining hope for observational evidence relevant to string theory unification, what we’ve been seeing the last few years has been a concerted campaign to avoid admitting failure by the destructive tactic of trying to change the usual conception of testable science. Two examples of this from last week were discussed here, and today there’s a third effort along the same lines, Quantum Multiverses, by Hartle. Unlike the others, this one includes material on the interpretation of quantum mechanics one may Read More ›

From Lisa Grossman at ScienceNews: Not all fundamental forces are created equal. An alternate universe that lacks the weak nuclear force — one of the four fundamental forces that govern all matter in our universe — could still form galaxies, stars, planets and perhaps life, according to calculations published online January 18 at arXiv.org. Researchers have done calculations to that effect. Why? “People talk about universes like they’re very fine-tuned; if you changed things just a little bit, life would die,” Adams says. But “the universe and stars have a lot more pathways to success.” It soon becomes clear that this is a pitch for a multiverse: The paper does not help figure out if the multiverse is real, though. Read More ›

It’s good when they come right out and say that. From Sean Carroll at Arxiv: Cosmological models that invoke a multiverse – a collection of unobservable regions of space where conditions are very different from the region around us – are controversial, on the grounds that unobservable phenomena shouldn’t play a crucial role in legitimate scientific theories. I argue that the way we evaluate multiverse models is precisely the same as the way we evaluate any other models, on the basis of abduction, Bayesian inference, and empirical success. There is no scientifically respectable way to do cosmology without taking into account different possibilities for what the universe might be like outside our horizon. Multiverse theories are utterly conventionally scientific, even Read More ›

In response to growing disquiet with the concept of a multiverse, voiced by theoretical physicist Sabine Hossenfelder, astrophysicist Ethan Siegel assures us at Forbes: 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. … 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 Read More ›