The Multiverse from the Many Worlds Perspective
(Originally posted June 07, 2017 on Blogger)
In the epic poem, the Odyssey, Odysseus sailed away from Kalypsos' island of Ogygia, guided by the light from the Pleiades. The Pleiades would have been one of many Messier objects visible on clear nights throughout ancient times. Right up until the discovery of electricity, people would have been able to see everything from the phenomenon of air glow, to the zodiacal light with the naked eye. Today, all but perhaps the Pleiades are washed away behind the dome of artificial light glowing from every city in the world. To see what people could see in the night sky prior to the 19th century, we would have to venture off to some remote place unaffected by the light pollution of cities and towns. For most of us, this would be no small feat, as urbanization is nearly ubiquitous these days.
The maps below are based off the Bortle Scale; a scale devised just 16 years ago to map light pollution across Earth. Different colors represent different levels of light pollution. As can be seen in the images below, nearly all of Europe, the eastern half the United States and its west coast are now blind to heavens.
Millions may never know the true beauty of a pristine night sky, and this is tragic. Some wonder how the ancients knew so much about the night sky, but if we could see what they could see on any given night, we might have an inkling as to how they were so knowledgeable. Shows like the "History" Channel's Ancient Aliens might actually give credit to the ancients who imagined and created impressive structures and calculations, without having this absurd need to attribute these accomplishments to some superior extraterrestrial life. I think the success of such shows is a reflection of a general loss of perspective of who we and where we are in the cosmos.
And so it seems; as the laity continues to lose perspective of our night sky and all it has to offer, so too does the inspiration they might have otherwise had to desire knowledge of the cosmos. Inspiration from the heavens has driven great minds since the days of Hipparchus of Nicaea. If adults lose interest in a night sky they can't see (and who can blame them), then I fear their children will follow suit.
Imagine a child or young adult going outside at night and seeing the zodiacal light, the gegenschein, portions of the Milky Way so bright they could cast obvious shadows where you stand. Their hearts would race upon seeing such things, and any time a child is excited s/he is inspired. In the days before city lights, anyone with the gift of sight could look up with the unaided eye and see things that are all but hidden now; the Triangulum galaxy, a myriad of globular clusters, and of course the twinkling of thousands upon thousands of distant stars.
Proper perspective is foundational. When it's not skewed nor taken away, perspective can mold the kind of mindset that I believe can only ever benefit a person... and by extension, benefit all of humanity. Sadly, the only light most are interested in staring at are the photons emitted from laptops, televisions, cell phones, and obnoxious LED-illuminated billboards. I've been just as guilty as the next person. So let's utilize the photons from the laptop I'm on now, and the smart phone, iPad, or computer screen you're on, and change our collective perspective here and now!
A New & Growing Cosmic Perspective
Though the ancients were very familiar with the stars, they still had no idea what they were seeing. Even if we were able to go back in time and tell them, they'd scratch their heads in confusion... "Thermonuclear what now?" Cultures from around the world believed the stars and planets were divine lights hung from the heavens like ornaments on a mobile spinning over a baby's crib. Our ancestors believed Earth was at the center of it all; the nucleus of the universe. Or perhaps I should say, the cradle of the universe.
Of course, Copernicus realized this wasn't true. His observations and subsequent calculations were later elaborated on by the likes of Kepler and Galileo. Earth was not the center of all things; geocentrism was rubbish. I'll bypass the whole Catholic church fiasco and say that, even though people slowly began to accept Earth was not the center of the universe, they continued to believe the universe wasn't all that big. Too big seemed unnecessary and uncomfortable. In fact, up until less than a hundred years ago, the consensus across academia was that the entirety of the cosmos was encapsulated within our Milky Way galaxy. LESS than 100 years ago my friends! We believed the Milky Way was the universe, beyond which nothing existed.
But one chilly night in the year 1919 at an observatory atop California's Mount Wilson, a man by the name of Edwin Hubble realized something no one could have ever imagined. He determined that the Milky Way was not the entire universe. He realized after careful observations that it was merely one of billions of other galaxies, most of which were speeding away from us at an accelerating rate (he discovered the expansion in 1929). This is to say that the velocity at which distant galaxies are receding from us are continuously increasing with time. Faster and faster they recede such that there is a horizon at which their rate of recession is greater than the speed of light! Imagine realizing for the first time that our Milky Way is just a speck in the cosmos, and that the cosmos is growing exponentially with time. Talk about uncomfortable. That horizon I mention essentially means that we can never know the most distant celestial objects, because the light from those objects cannot outpace the velocity at which they're speeding away from us.
This horizon, or boundary at which the expansion rate is greater than the speed of light is called the cosmological horizon, or sometimes called the particle horizon. We can only ever know that which lies within that horizon. Anything beyond it is forever lost to us. We're in a massive bubble so to speak, and that bubble is the known universe. That part of the cosmos whose light can reach us.
An observant reader may ask how the universe can expand faster than light speed, and this is a valid question. The limit of light speed is not violated by the universe's increasing rate of expansion and here's why; First, I purposely stated that the rate of expansion is greater than the speed of light to make a quick-and-dirty point as to why we'll never know what lies beyond the cosmological horizon. If light cannot outpace the rate at which its source is receding, then we'll never see it and it's source will forever be unknown to us. However, for me to say the rate of expansion is greater than the speed of light is really tantamount to me saying "apple is longer than smile". It's so absurd that things that don't make sense make more sense than that. It's so wrong it isn't even wrong.
Galaxies are not moving away at any particular speed. In fact, they're not even "moving away". The galaxies are where they are. It's the expansion of the universe that is creating the increasing distances between galaxies. Imagine if Earth were to expand. I'm sitting in my vehicle on the 405 freeway (meaning I'm not moving), and my brother is sitting at his desk in New York City (also not moving). We're about 2,450 miles apart from each other, but Earth expands at some rate such that its volume increases with every passing second. Even though my brother and I aren't moving, we're getting further apart from each other because the surface of Earth's spher(oid) is getting bigger. However, our distance from each other has not changed in comoving coordinates. Comoving distances between objects is unchanged by the expansion of the universe. Though my brother and I are getting further apart, fact remains that we aren't moving at any speed ourselves. The ball of Earth is getting bigger and my brother, myself, and everyone and thing on it needs not move an inch to become further and further apart. The increasing distance growing between my brother and I has a rate we could measure, and this is what Hubble and others have done with the universe.
The current rate of cosmic expansion is about ~70 km per second per megaparsec. (A megaparsec is about 19 quintillion, 170 quadrillion miles... give or take a few inches.) In actuality, no one seems to know exactly what that rate is as there's an ongoing discrepancy between measured values, but that's perhaps best left for a future blog post. Anyway, what this rate means is that galaxies 1 megaparsec from us are currently receding at a rate of 70 km per second, on average. Galaxies 2 megaparsecs from us are currently receding at a rate of 140 km per second, on average. Galaxies 3 megaparsecs from us are currently receding at a rate of 280 km per second, on average... and so on... we notice the trend here is exponential. The rate of expansion increases with every additional megaparsec from the observer (meaning Earth).
Of course this is an average, and there are exceptions like the Andromeda galaxy, which is actually approaching us. And on this point, I should mention that generally the rate of expansion doesn't apply until about 10 megaparsecs out due to effects of the Local Group. But point here is to realize there is a point at which this doubling surpasses light speed (without violating it), and the light from galaxies beyond this horizon will never reach us. Remember, the galaxies themselves don't necessarily have a speed. They're just in an expanding spacetime.
Now that I've said all that, let me take another step further to say that my expanding-Earth analogy I just gave is probably not all that great. The reason is that as far as cosmologists can tell (so far), the universe appears to be flat. (Though I have a hunch that it isn't... but we'll get to that later.) For now, the better analogy might be to imagine an elastic chess board. All the pieces, including the rooks (!!) are in their starting positions. We'll move none of them, but grab the four corners of the board and stretch it outward such that the distances between each piece becomes greater as the surface area of the board expands. Though the distances between pieces increases, the pieces themselves remain in their starting positions unmoved. This is analogous to my Earth analogy, but considering flat space.
Though I digress, as even this analogy may not even be all that good! :( I have a hunch our universe is shaped like a saddle... Soooo, imagine a rubber saddle... no, no, I'll stop at chess! I think we get the point.
Before we go on, I just want to quickly (and likely inadequately) tackle the terminology, "the speed of light" as it's a bit of a misnomer, much like calling the Minoans "Minoans" is a misnomer, or the Hittites the "Hittites". I need to start my ancient history blog next! Anyway, it isn't simply the maximum speed at which light can travel, but the maximum speed at which information can travel too. Here I mean information in the sense of any physical interaction; causality. Hence the reason I've often put the term "speed of light" in scare quotes in previous blogs. It might better be termed the speed of causality.
At any rate (pun intended), these discoveries; light speed, expansion rate, the cosmic microwave background radiation... they've led us (us as in them) to more and more fascinating revelations about our universe. One being the universe's age; an age derived by taking the known expansion rate of the universe—called the Hubble constant—and extrapolating back in time to determine its beginning. Though I should probably mention the Hubble constant has yet to be precisely confirmed, and agreed to be fixed.
Now, whether or not time is an actual thing, or whether or not there was a beginning at all, we'll get to in a bit. For now, we know that having extrapolated the universe's expansion rate back in time, along with consideration of other factors such as density of matter within the known universe, as well as information we've obtained from some of its oldest (most distant) objects, that the age of the universe is about 13.82 billion years. We owe a debt of gratitude to the folks behind the WMAP and Planck missions for that number.
At 13.82 billion years old, the furthest any light or information could have traveled is 13.82 billion light years, ipso facto. A light year being the distance light travels in 1 Earth year (leap years excluded!). Light speed multiplied by the age of the universe gives us a distance, BUT as with all things in astrophysics, it isn't so simple to say we can calculate the age of the universe (time) by the speed of light to determine the maximum distance light or information could have traveled since the Big Bang.
What we're actually multiplying is the speed of light by conformal time. As I understand it, conformal time considers the fact that the universe is not static, but expanding as we discussed above. It "stretches" in all directions like that chess board (or saddle), thereby affecting the distances between celestial objects within it.
This may seem a bit mind boggling at first, but let's remember that the universe is expanding not because the galaxies are necessarily in motion, speeding away from each other, but because spacetime itself is expanding. Imagine spacetime being the chessboard, and all the galaxies in it being the chess pieces in their starting positions. The conformal distances between those pieces (the galaxies) are not changing, just as the conformal distances between the chess pieces, or my brother and I weren't changing. The increasing distances are a result of spacetime expanding. Again, this is because conformal time takes into consideration the expansion of the universe.
For any super nerds out there that want to read Robert Penrose's explanation of this conformal stuff, check out his concept as it relates to entropy in this paper (link downloads as a PDF).
We've come a long way over the past century; having gone from believing Earth was the cradle beneath a mobile of stars, to realizing Earth isn't the center of our solar system, and that our solar system is a tiny part of a much larger galaxy of which it is not the center. That galaxy is part of a much larger cluster, of which it is not center of that either (the Virgo Cluster), and that cluster is part of an even larger supercluster (the Virgo Supercluster); not the center there either. And that supercluster is part of an even larger universe, of which there is no center!
Does it end there? Well, I hope so, because that was one heck of a run-on sentence! But as for it being the end at the universe being the largest structure there is? I don't think so, and neither do most of the world's top cosmologists. In fact, there is no consensus that our universe is alone. Many cosmologists are in the camp that our universe may very well be only one of many, and perhaps infinitely many bubble universes in a behemoth multiverse structure.
In an eternal inflation scenario, a normal universe begins when a small patch of the inflating universe stabilizes. Specifically, its vacuum energy takes on a stable value at that point it stops inflating and starts expanding normally. This can happen spontaneously anywhere in the greater inflating spacetime such that there could be many, if not an infinite number of universes. They can occur at any rate depending on completely unknown details of the string theory parameter space. We'll get to all this in some future blog.
The idea of a multiverse isn't as far-fetched as some might think. It's something that can be tested for, and not merely some speculative, non-testable, overly-imaginative, hyperbolic discourse in philosophy and fuzzy mathematics.
A multiverse could be evidenced by certain features in the cosmic microwave background radiation; that faint afterglow from the Big Bang. Imagine two bubbles expanding, one into the other. If we were on the inside of one of those bubbles, then we'd see the other bubble's collision with our own as a disc in the sky. The sky in the universe's case would be the cosmic microwave background radiation (CMB), and bubble collisions would be imprinted on the CMB as disc-like anisotropies.
Another way to test for a multiverse, or at least provide evidence for it, is to measure for any curvature of our universe. I mentioned above that so far as we know, the universe appears to be flat. But just as the first version of LIGO was unable to detect gravitational waves, yet upgrades to it have since detected three gravitational waves (at the time of this writing), we can expect technological upgrades, along with all we might learn in the coming decades from observations with current technology, will invariably increase our ability to measure for curvature to a much higher degree of accuracy. An accuracy level that may reveal the flatness we see now is merely due to the fact we aren't looking from a big enough perspective. Indeed, a nanobe might believe the surface of a saddle is flat!
Negative spatial curvature would be supporting evidence of a multiverse. Negative curvature can be imagined as that saddle I keep bringing up throughout this blog. A flat universe, or a positively-curved universe would undermine the possibility of there being a multiverse.
Another way to test for curvature is to divide the average density of matter in the universe by its critical energy density; about 5 monatomic hydrogen atoms per cubic meter (based off WMAP data). The quotient derived from this is called the density parameter (denoted by the capital Greek letter Ω). If the density parameter is equal to 1, then the universe is flat and a multiverse isn't likely. If it is greater than 1, then the universe has positive curvature, and a multiverse is unlikely. However, if it is less than one, it's negatively curved and this would provide evidence for a multiverse.
In the artist renderings above that depicts bubble universes within a multiverse megastructure, we can see each universe (each bubble) as being finite. And from the perspective of the multiverse itself (outside those bubbles), this would be the case. However, we are presumed to live inside one of those bubbles. Our universe may appear finite from the perspective of the multiverse "outside" so to speak, but from the inside, our universe appears to be infinitely large. An infinitely large universe, to us, should appear to be curved.. negatively so from inside the bubble. And this is a reason why negative curvature would support the theory of a multiverse.
And though the artists' depictions above show bubble universes apart (or colliding) with each other, cosmologists theorize that there may also be bubble universes within larger bubbles, and those bubbles may be expanding within yet even larger bubbles. The combination of possibilities is endless (as one might expect in an eternally-inflating multiverse).
But I digress, because I've not really discussed these bubbles other than to say they're each a universe unto themselves. I'm certain everyone reading this has heard of the Big Bang Theory (not the sitcom!). Though it wasn't an explosion, and there was no bang, the unfortunately-named Big Bang gave rise to all the matter we observe across the cosmos. But on its own, the Big Bang theory gives rise to three major problems; the horizon problem, the monopole problem, and the flatness problem. I'll do the explanation of these issues no justice in this blog as they each deserve a blog of their own, but suffice it to say there are things we'd expect to see across the cosmos as a result of the Big Bang, that we don't and they're revealed as a major conundrums in those three aforementioned problems.
But cosmologist Alan Guth remedied these three problems with his theory of inflation. Anyone interested in learning more about this (and the multiverse) are invited to see my friend SkyDivePhil's documentary on these topics here: https://www.youtube.com/watch?v=QqjsZEZMR7I My friends might recognize me in this particular documentary as Phil asked me to interview cosmologists, Yasunori Nomura, and Anthony Aguirre for him. :) A great honor and experience I'll not forget.
Guth's theory of inflation not only resolved the three major issues with the Big Bang theory, but it has also given rise the very high probability that inflation is eternal. This means that time is both past and future eternal... no beginning, no end. This doesn't bode well for Genesis. Imagine, the multiverse has always been, and will always be. Again, I highly recommend my readers see Phil's video for more, because for me to follow this tangent here would increase the length of this blog by a factor of 10, and I fear it's already getting a bit lengthy.
Back to those bubbles... During the brief period of inflation (see the video linked above), regions of space would stop expanding at different times (their respective vacuum energies would stabilize), while others would have continued expanding. Each time a region stopped expanding, a bubble universe would form and, as mentioned earlier, go on to expand normally. And as I noted above, there could be an infinite 'number' of bubble universes. But just as the Big Bang gave rise to problems, the concept of infinite bubble universes has also gives rise to a problem all its own.
The problem is this, and I am paraphrasing Alan Guth here; anything and everything can happen in an eternally-inflating universe, and they can happen an infinite amount of times! Well dang. It sounds kind of cool at first, but when we think about it, this means we can't make any predictions. And as any scientists in any field will tell us, predictions are the very heart and soul of any theory. The inability to make predictions would be the metaphorical knife to the heart and soul of Guth's eternally-inflating multiverse theory. But all is not lost my friends!
If you watch Phil's video linked above (here it is again: https://www.youtube.com/watch?v=QqjsZEZMR7I), you'll see that Yasunori Nomura has come up with what I think is a brilliant idea as to how predictions can be made in an eternally-inflating multiverse. To understand his idea, we'll need to adjust our perspective of the cosmos once more...
A New & Growing Quantum Perspective
So far we've been discussing things on a macro scale; Earth, stars, galaxies, and bubble universes within a multiverse megastructure. But now let's delve into the world of quantum mechanics; a world that exists within the extremely-small scale of subatomic particles. It's a strange, strange world where intuition is best if thrown out the window! In this world we deal with wave-particle duality, the uncertainty principle, the quantization of energy, and the correspondence principle. But one thing in particular that I'd like to (hope to) illustrate here is superposition.
If I were to drop a #2 Ticonderoga pencil on the table, we could predict that it'd fall, hit the table at a certain point, and roll a bit before coming to a stop; in other words, we could predict to a degree of accuracy what would happen. Air friction, the acceleration due to gravity, the height of the drop, the shape of the pencil... all these factors could be calculated. And if we could exact these factors in subsequent drops (repeat the experiment), then we could expect and accurately predict the pencil would fall the exact same way and stop in the exact same position each and every time. But this is the macro world. Remember, we need to throw all that intuition and prediction stuff out the window in the quantum world.
If that pencil were a subatomic particle, it wouldn't matter how precisely we dropped it each time. Nor would it matter how exact the conditions were each time we dropped it. The outcome of its fall could be different every time. The outcome of each drop could only ever be probabilistic. In the quantum world, good-old cause and effect is never intuitive.
It gets stranger. In the macro world, if I drop a pencil, we see it fall through the air. But in the quantum world, the quantum pencil falls into superposition where it is neither here nor there. It exists as a wave function of probabilities; a superposition state of possible outcomes of falling at every point on the table from edge to edge, corner to corner. That is, if we don't observe it.
The instant we make an observation to see where it falls, it will appear on the table at some point; its wave function collapses and its outcome is instantly realized. We'll get to wave functions more shortly. Prior to us observing this outcome, the pencil existed in a quantum cloud of all possible landing spots to varying degrees of probability, none of which are certain; a probability wave. Our quantum pencil's wave function—in a superposition of several different probable outcomes—appears to reduce to a single outcome the instant we observe it. Observation, my friends, appears to collapse the wave function such that the pencil goes from a superposition state to a single outcome realized on the table.
In quantum mechanics, the act of observing is our attempt to measure that which we're experimenting with. But the weird thing about quantum mechanics, is that the very act of measuring affects the system. We can measure the length of a piece of paper without affecting it, but we can't do the same with subatomic particles. This is really weird. No matter how exact we make the conditions in subsequent quantum-pencil-dropping experiments, we'll never be able to accurately predict nor see the pencil appear on the table in the exact same spot every time. Instead, it'll appear on the table in entirely different locations non-intuitively.
This measurement-affecting-the-system weirdness comes out of the Coppenhagen interpretation of quantum mechanics; wherein physical systems lack definite properties. Quantum mechanics is only able to predict the probabilities that measurements will produce certain outcomes, and the very act of measurement affects the physical system, causing the set of probabilities to reduce to only one of the possible values. This reduction to one is known as wave function collapse.
It's the Coppenhagen interpretation of quantum mechanics that gave rise to the famous thought problem known as Schrödinger's cat. In this rather twisted thought experiment, a cat is in a box with a flask of poison, a radioactive source, and a hammer set to a trigger than is activated by said radioactive source. The box is closed shut such that no outside observer can see in, and only a Geiger counter can detect the radioactivity inside. If radioactivity is detected, then it is presumed the trigger has been activated causing the hammer to fall, and shatter the glass flask thereby releasing the poison and killing the cat... or is the cat dead?
In a world defined by the Coppenhagen interpretation, the cat is neither alive nor dead. Until an observer opens the box to see the outcome of this experiment, the cat would exist in a superposition state. It isn't until the box is opened and the observer looks inside that the wave function collapses such that the cat is either alive or dead.
The Coppenhagen interpretation is the most taught, most common, most accepted version of quantum mechanics, but there are others. Thankfully so in my opinion, because the Coppenhagen interpretation gives rise to some conceptual issues. For instance, why does the wave function collapse upon being measured (observed)? It doesn't answer this at all. Also, does observation have to be by humans, or can a wave function collapse upon being observed by a flea, gnat, elephant, or octopus? If not, then what is so special about human observation to cause a physical system to reduce to a single outcome? The answers to all these questions are completely lacking in the Coppenhagen interpretation.
However, the Coppenhagen interpretation isn't the only interpretation of quantum mechanics. There are many more, with one of the other interpretations being the many worlds interpretation of quantum mechanics. This interpretation has some incredible implications for all that macro-world stuff we discussed above regarding galaxies, universes, and the multiverse at large, and the person who made the connection between many worlds and the multiverse is Yasunori Nomura. I interviewed him over a year ago at Berkeley for that documentary my friend Phil and his wife made (here is that link again: https://www.youtube.com/watch?v=QqjsZEZMR7I
Before we get to Nomura's incredible theory, let's discuss briefly what the many worlds interpretation is in quantum mechanics. We can do this with the same thought experiment outlined above. Again, the (pissed) cat is in the closed box with the flask, hammer, trigger, and radioactive element. Again, the hammer falls, the flask is broken, the poison is released. We have learned that in the Coppenhagen version the cat exists in superposition until the box is opened, but the many worlds interpretation denies that the wave function collapse ever happens!
In the many worlds version, there is nothing special about observation. As Nomura states in his awesome article in this month's issue of Scientific American, "[a]ccording to this picture, humans making measurements have no special significance". Both alive and dead states of the cat exist after the hammer falls. But each are decoherent of each other, meaning there exists no communication, no information sharing, nor any interaction between the alive and dead outcomes whatsoever.
The cat, and the observer become entangled upon opening the box. As such, they split into two different worlds such that there is an observer looking at a box with a dead cat in it (boo!), and an observer looking at a box with a living cat in it (hoo ra!).
In summary, the many worlds interpretation allows that all possible outcomes (and histories) are real, and the branches made at each point represent an actual world or universe. As such there may be an infinite number of universes within the collective of which everything that could possibly have happened has happened in some other universe. Wish you made the decision to get two scoops of ice cream instead of one that day awhile back? Don't worry, in many worlds you did, and that version of you might be regretting it now in another universe!
Let's return to the quantum pencil for a moment; the Dixon Ticonderoga #2, which an old friend of mine has rightfully said is the greatest pencil of all time! In the many worlds interpretation, we must have a complete description not just of the air friction acting upon the pencil when it's dropped, the shape of the pencil, the height at which the pencil is dropped etc... but we must also include the observer, AND the ENTIRE universe and everything in it at the time of the experiment.
Because in many worlds, it isn't simply that the pencil exists in superposition of possible landing points on the table, collapsing to a single point upon measurement. In many worlds, each possible outcome exists in an entire universe. In other words, and in Nomura's words, "...the quantum state after the measurement is still a superposition—not just a superposition of [all] landing spots, but of ... entire worlds" in which every outcome is realized; each in a world all its own! Already this meshes incredibly smoothly with the idea of a multiverse. But it gets better...
In my interview with Nomura, and in his article in this month's issue of Scientific American, Nomura explains how and why the eternally-inflating multiverse and quantum mechanical many words are the same concept. I'm paraphrasing the following from both my interview with Nomura, and his write up in SA; the bubble universes we discussed above do not actually exist in a single real space. Rather, each is a representation of a different branch on the "probabilistic tree" (in Nomura's words) of many worlds. In other words, the rules of quantum mechanics don't just dictate what happens in the subatomic world, but they also play a fundamental role on the grand scale of the universe... indeed, the multiverse!
And Nomura isn't alone on this. Theoretical physicists Leonard Susskind at Stanford, and Raphael Bousso at Berkeley have independently arrived at this idea as well. It's in the math! In this month's issue of SA, as well as in our interview together, Nomura gives the example of information as it pertains to black holes to further illustrate his amazing idea.
I'll use his example here, so can take no credit for this analogy whatsoever: If we are outside of a black hole and observe a book falling into it, contrary to laymen belief, the information of that book is not lost. There was a time we thought it was, but Stephen Hawking realized this wasn't the case when he discovered black holes "leak" radiation (and information about what's inside along with it). This "leaking" radiation is now known as Hawking radiation.
As outside observers, we're able to retrieve information about the book from Hawking radiation as it slowly leaks out beyond the event horizon; the limiting rim at which not even light can escape the effects of gravity of a black hole (gravity of course being an effect of bent spacetime). Now imagine we fall into the black hole along with the book. (Nevermind spaghettification, imagine we're just fine falling in with book in hand.) Now as inside observers we see the information of the book as being forever inside the black hole with us. But wait! An outside observer can get information about our book via Hawking radiation (the book is outside), yet we have the book in our hands on the inside. How can it be in both places?
The issue with this stems from the fact that we can't have information of the book both inside and outside (Hawking radiation) of the black hole at the same time. This would violate the no-cloning theorem, wherein information cannot be duplicated; something that is impossible in quantum mechanics. However, both can be true if the outside observer considers what's 'below' the event horizon as nonexistent. S/he can never know what lies beneath without falling in as well. Once inside, whatever exists outside is forever out of reach, and therefore can be considered to be nonexistent.
As Nomura has stated, the outside observer needs not describe the interior of a black hole as s/he can never access it anyway. Not even in principle can it be accessed. The "interior of spacetime [is] nonexistent" as Nomura puts it. Same would hold true for the inside observer, with regard to outside spacetime. However, as Nomura points out, this scenario must ignore Hawking radiation, which we just discussed as containing information from the interior. But ignoring it is allowed by the inside observer since s/he can never know of it.
It is nonexistent for all intents and purposes.
The event horizon, the boundary at which information within the black hole cannot escape, is similar in a sense to the cosmological horizon we discussed earlier in this really long (sorry!) blog. To an observer beyond (outside) the cosmological horizon, whatever is inside is nonexistent. Likewise for us "trapped" inside the cosmological horizon. We are forever prevented from knowing what lies beyond it. To us, what lies beyond is nonexistent. See the correlation?
Harking back to the no-cloning theorem, in which information cannot be duplicated, we now realize that considering both the inside and outside information of a book is impossible. Likewise for the universe along the cosmological horizon. Everything inside (and on) the horizon is known only to those of us inside, whereas it is unknown (nonexistent) to any observers outside. If we were to imagine information could be known on both sides, then as Nomura explains, we would be "overcounting the information"! Impossible in quantum mechanics!
So in this sense, to ask the question of what our universe is expanding into isn't even a valid question. Again, it's like apple is longer than smile. What? Exactly.
What this means for all those bubble universes we imagined "outside" the cosmological horizon, is that they exist in superposition just like that pencil; as probabilistic outcomes just like any other process in quantum mechanics. In Nomura's words, "[j]ust as a quantum measurement could spawn many different results distinguished by their probability of occurring, [eternal] inflation could produce many different universes, each with a different probability of coming into being. In other words, the quantum state representing eternally-inflating space is a superposition of worlds...representing different universes, with each of these [worlds] including only the region within its own [cosmological] horizon", [and as such, each is finite].
Solving the Predictability Problem with New Perspective
Earlier in this blog we discussed how an infinite number of universes made all possible outcomes possible, and this made predictability impossible; predictability, of course, being at the heart and soul of theory. Just as we cannot have information of the book exist both inside and outside the event horizon (no-cloning theorem), neither can we have a universe exist along side another simultaneously in real space.
However, since each world is finite into and of itself, we avoid the issue of non-predictability that plagued eternal inflation. As Nomura states, these other worlds do not exist simultaneously in a single real space, but "coexist only in probability space" (just like that quantum pencil), wherein observers can predict possible outcomes for their world.
Nomura's unification of the eternally-inflating multiverse and the many worlds interpretation of quantum mechanics, effectively solves the conundrum of predictability by pointing out the sense of 'sameness' between many worlds and the multiverse. Pure genius in my humble opinion. But like Jon Snow, I know nothing.
As we discussed earlier, this is all testable in that we can measure for negative curvature in spacetime, among other things mentioned above. And as I write this, Nomura and others continue to pursue these theories further, and have already begun to tackle the immense questions of what is time, and how does it emerge.
The answers to these profound questions have yet to be answered, and as Nomura has said in the past, in an eternal multiverse, the concept of time may be end up being little more than an illusion. And if so, then that is a perspective that fundamentally changes everything we know and think of our world.
To read Nomura's article, I highly recommend you pick up a copy of this month's issue of Scientific American. I also invite you to watch any, and all of my friend Phil and his wife's documentaries on the different theories on the early universe which include, but are not limited to; conformal cyclic cosmology, string theory cosmology, loop quantum cosmology, and the documentary he invited me to participate in on eternal inflation and the multiverse. His YouTube channel can be found here:
As always, thanks for reading.