Our Mathematical Universe
Page 44
So how’s our Universe going to end, billions of years from now? I have five main suspects for our upcoming cosmic apocalypse or “cosmochalypse,” illustrated in Figure 13.2 and summarized in Table 13.1: the Big Chill, the Big Crunch, the Big Rip, the Big Snap and Death Bubbles.
Figure 13.2: We know that our Universe began with a hot Big Bang 14 billion years ago, expanded and cooled, and merged its particles into atoms, stars and galaxies. But we don’t know its ultimate fate. Proposed scenarios include a Big Chill (eternal expansion), a Big Crunch (recollapse), a Big Rip (an infinite expansion rate tearing everything apart), a Big Snap (the fabric of space revealing a lethal granular nature when stretched too much) and Death Bubbles (space “freezing” in lethal bubbles that expand at the speed of light).
Table 13.1: The future of space in five cosmic-doomsday scenarios
Click here to see a larger image.
As we saw in Chapter 3, our Universe has now been expanding for about 14 billion years. The Big Chill is when our Universe keeps expanding forever, diluting our cosmos into a cold, dark and ultimately dead place. I think of it as the T. S. Eliot option: “This is the way the world ends / Not with a bang but a whimper.” If you as Robert Frost prefer the world ending in fire rather than ice, then cross your fingers for the Big Crunch, where the cosmic expansion is eventually reversed and everything comes crashing back together in a cataclysmic collapse akin to a backwards Big Bang. Finally, the Big Rip is like the Big Chill for the impatient, where our galaxies, planets and even atoms get torn apart in a grand finale a finite time from now. Which of these three should you bet on? That depends on what the dark energy from Chapter 4, which makes up about 70% of the mass of our Universe, will do as space continues to expand. It can be any one of the Chill, Crunch or Rip depending on whether the dark energy sticks around unchanged, dilutes to negative density or anti-dilutes to higher density, respectively. Since we still have no clue what dark energy is, I’ll just tell you how I’d bet: 40% on the Big Chill, 9% on the Big Crunch and 1% on the Big Rip.
What about the other 50% of my money? I’m saving it for the “none of the above” option, because I think we humans need to be humble and acknowledge that there are basic things we still don’t understand. The nature of space, for example. The Chill, Crunch and Rip endings all assume that space itself is stable and infinitely stretchable.
We used to think that space was just the boring static stage upon which the cosmic drama unfolds. Then Einstein taught us that space is really one of the key actors: it can curve into black holes, it can ripple as gravitational waves, and it can stretch as an expanding universe. Perhaps it can even freeze into a different phase much like water can, as we explored in Chapter 6, with lethal fast-expanding bubbles of the new phase offering another wild-card cosmochalypse candidate. We also used to think that you can’t get more space without taking it away from someone else. However, as we saw in Chapter 3, Einstein’s gravity theory says the exact opposite: more volume can be created in a particular region between some galaxies without this new volume expanding into other regions—the new volume simply stays between those same galaxies. Moreover, Einstein’s theory says that space stretching can always continue, allowing our Universe to approach infinite volume as in the Big Chill and Big Rip scenarios. This sounds a bit too good to be true, which makes me wonder: is it?
A rubber band looks nice and continuous, just like space, but if you stretch it too much, it snaps. Why? Because it’s made of atoms, and with enough stretching, this granular atomic nature of the rubber becomes important. Could it be that space too has some sort of granularity on a scale that’s simply too small for us to have noticed? Mathematicians like to model space as an idealized continuum without any granularity, where it makes sense to talk about arbitrarily short distances. We use this continuous space model in most of the physics classes we teach at MIT, but do we really know that it’s correct? Certainly not! In fact, there’s mounting evidence against it, as we discussed in Chapter 11. In a simple continuous space, you’d need to write out infinitely many decimal places just to specify the exact distance between two random points, but physics titan John Wheeler showed that quantum effects probably make any digits after the thirty-fifth decimal place meaningless, because our whole classical notion of space breaks down on smaller scales, perhaps being replaced by a strange foamy structure. It’s a bit like when you keep zooming a photo on your screen and discover that what looked smooth and continuous is actually granular like a rubber band, in this case made up of pixels that can’t be further subdivided (see Figure 11.3).
Because that photo is pixelized, it contains only a finite amount of information and can be conveniently transmitted over the Internet. Similarly, there’s mounting evidence that our observable Universe contains only a finite amount of information, which would make it easier to understand how nature can compute what to do next. The holographic principle we mentioned in Chapter 6 suggests that our Universe contains at most ten to the power 124 bits of information, which averages to about 10 terabytes for each volume that can fit an atom.
Now here’s what bothers me. The Schrödinger equation of quantum mechanics that we encountered in Chapter 7 implies that information can’t be created or destroyed. Which means that the number of gigabytes per liter of space keeps dropping as our Universe expands. This expansion continues forever in the Big Chill scenario (the front-runner cosmochalypse candidate based on polling my astrophysics colleagues), so what happens when the information content gets diluted down to a megabyte per liter, which is less than a cell phone can store? To a byte per liter? We can’t say specifically what will happen until we have a detailed model to replace continuous space, but I think it’s a safe bet that it will be something bad that will gradually alter the laws of physics as we know them and make our form of life impossible—welcome to what I call the “Big Snap.”
Here’s what bothers me even more: a simple calculation suggests that this will happen within a few billion years, even before our Sun runs out of fuel and engulfs Earth. Our best theory for what put the bang into our Big Bang, the inflation theory from Chapter 5, says that there was an awful lot of rapid space-stretching going on in our early Universe, with some regions getting much more stretched than others. If space can get stretched only by a maximum amount before suffering a Big Snap, then most of the volume (and consequently most of the galaxies, stars, planets and observers) will be found in the regions that have stretched the most and are close to snapping.
What would an impending Big Snap be like? If the granularity of space gradually grows, then the smallest-scale structures would get messed up first. We might first notice the properties of nuclear physics starting to change, for example by previously stable atoms undergoing radioactive decay. Then atomic physics would start changing, messing up all of chemistry and biology. Fortunately, our Universe has provided gamma-ray bursts as a convenient early-warning system which, like a canary in a coal mine, might alert us long before a Big Snap could harm us. Gamma-ray bursts are cataclysmic cosmic explosions blasting out detectable short-wavelength gamma rays from halfway across our Universe. In continuous space, all wavelengths move at the same speed, the speed of light, but in the simplest kinds of granular space, shorter wavelengths move slightly slower. Yet we’ve recently observed gamma rays of quite different wavelengths race for billions of years through space from a distant explosion, arriving in a photo finish within a hundredth of a second of each other. Taken at face value, this rules out an impending Big Snap for billions and billions of years to come, flying in the face of what we predicted in the last paragraph.
In fact, the problem is even worse. Our space isn’t expanding uniformly: indeed, some regions, such as our Galaxy, aren’t expanding at all. One could therefore imagine galaxy-dwelling observers happily surviving long after intergalactic space has undergone a Big Snap, as long as deleterious effects from these faraway regions don’t propagate into the galaxies. But this scenario saves only the observers, not
the underlying theory! Indeed, the discrepancy between theory and observation merely gets worse: repeating the previous argument now predicts that we’re most likely to find ourselves alive and well in a galaxy after the Big Snap has taken place throughout most of space, so the lack of any strange gamma-ray time delays becomes even harder to explain.
So we’ve concocted a strange brew by blending together some of the most cherished ingredients of cosmology and quantum physics, adding some experimental data, and stirring it up. The result? The ingredients don’t mix well, suggesting that there’s something wrong with at least one of them. I love mysteries, and find paradoxes to be nature’s best gifts to us physicists, often providing clues to future breakthroughs. I think we’re due for a breakthrough on the nature of space, and that the Big Snap paradox is an interesting hint.
The Future of Life
Starting with the full physical reality of the Level IV multiverse, we’ve now zoomed in on our particular Universe and discussed its long-term fate. Let’s continue even closer to home and consider the future of life. Out of all the awe-inspiring properties that our Universe has, the one I find the most inspiring is that it’s come alive, containing self-aware entities such as us who can enjoy it and ponder its mysteries.
So what are the future prospects for life? Are we humans alone in our Universe, or are there other civilizations out there that might interact with us or destroy us? Will our human life spread throughout our Universe, perhaps in some evolved form? We’ll explore these fascinating questions below, but first let’s tackle some more pressing ones: what are some of the main threats to the future survival of life on our planet, and what can we do to mitigate them?
Existential Risk
When I was fifteen, I had a thought that really shocked me. I was well aware that we humans worried a lot. We worried about personal challenges such as health, relationships, money and career, and we also worried about threats to our family, our friends and our society. But what about the greatest threats of all, that could potentially destroy all human life—were we really worrying enough about this? No, we weren’t!
I realized that I’d lived my life lulled into a false sense of security, naively believing that everything that needed to be worried about was being taken care of by someone else. As a toddler, I never worried about dinner because I knew my parents had a plan for that. I didn’t worry about my safety because I knew that the fire and police departments had a plan for that. Gradually, I realized that the grown-ups around me weren’t as omniscient and omnipotent as I’d thought, and that there were many small problems that I had to solve myself. But the really big and most important problems facing humankind, they were given top priority by our political leaders. Surely?
I never questioned this until the frightening truth hit me like a brick when I was fifteen. My personal wake-up call was learning details about the nuclear-arms race. It really astonished me to realize that here we were together, billions of us, on this precious and beautiful blue planet, and even though essentially none of us wanted a full-scale nuclear war, there was significant risk we’d have one in my lifetime, most likely by accident. Perhaps the risk was 1% per year, perhaps 100 times less, perhaps 10 times more—in any case, the risk was absurdly high given the stakes. And yet it wasn’t even considered the number-one election issue in any country. Moreover, this is just one example among many of what Nick Bostrom has termed existential risk, something that could either annihilate Earth-originating intelligent life or permanently and drastically curtail its potential.1
The American futurist Buckminster Fuller has described this basic predicament much more poetically than I did in my teens, as our collective voyage on “Spaceship Earth.” As it blazes through cold and barren space, our spaceship both sustains and protects us. It’s stocked with major but limited supplies of water, food and fuel. Its atmosphere keeps us warm and (via its ozone layer) shielded from the Sun’s harmful ultraviolet rays, and its magnetic field shelters us from lethal cosmic rays. Surely any responsible spaceship captain would make it a top priority to safeguard its future existence by avoiding asteroid collisions, onboard explosions, overheating, ultraviolet-shield destruction and premature depletion of supplies? Well, our spaceship crew hasn’t made any of these issues a top priority, devoting (by my estimate) less than a millionth of its resources to them. In fact, our spaceship doesn’t even have a captain!
Later, we’ll explore why we humans are so bad at managing the greatest threats to our long-term survival, and what we can do about it. First, however, let’s briefly survey what some of these threats are. Figure 13.3 summarizes some of the existential risks I find most relevant. Let’s start on the right end of the timeline, in the distant future, and work our way back home toward the present.
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1For good introductions to existential risk, I recommend http://www.existential-risk.org and Martin Rees’s book Our Final Hour.
Our Dying Sun
Let’s begin with astronomical and geological threats, and then turn to human-created threats. Earlier, we discussed five “cosmochalypse” scenarios for the end of our Universe: the Big Chill, Big Crunch, Big Rip, Big Snap and Death Bubbles. Although we don’t know which of them, if any, will actually happen, my guess is that there’s no need to panic, and that our Universe will avoid wholesale destruction for tens of billions of years.
What we know with certainty, however, is that our 4.5-billion-year-old Sun will cause us problems much sooner. It keeps shining progressively more brightly, because of the complex dynamics of the fusion reactions in its core as the hydrogen fuel gradually gets depleted. Forecasts suggest that about a billion years from now, this solar brightening will start having a catastrophic effect on Earth’s biosphere, and that a runaway greenhouse effect will eventually boil off our oceans, much like what has already happened on Venus. Unless we do something about it, that is.
Figure 13.3: Examples of what could destroy life as we know it or permanently curtail its potential. Whereas our Universe itself will likely last for at least tens of billions of years, our Sun will scorch Earth in about a billion years and then swallow it unless we move it to a safe distance, and our Galaxy will collide with its neighbor in about 3.5 billion years. Although we don’t know exactly when, we can predict with near certainty that long before this, asteroids will pummel us and supervolcanoes will cause yearlong sunless winters. In the immediate term, we may face self-inflicted problems such as climate change, nuclear war, global pandemics and unfriendly superhuman artificial intelligence.
Click here to see a larger image.
Interestingly, there may be something that can be done. The astronomers Donald Korycansky, Greg Laughlin and Fred Adams have shown that, by clever use of asteroids, Earth can be kept at constant temperature by gradually moving it out to a larger orbit around the warming Sun. Their basic idea is to nudge a large asteroid to fly very close to Earth every 6,000 years or so and give us a gravitational tug in the right direction. Each such close encounter would be fine-tuned to send the asteroid passing near Jupiter and Saturn to get its energy and angular momentum reset to the required values for the next Earth encounter—we’ve successfully used such “gravitational assists” before, to send spacecraft such as NASA’s Voyager probes into the outer Solar System. If successful, this scheme could extend Earth’s habitability from about 1 billion to about 6 billion years. After that, our Sun will end its life as we know it, bloating into a red giant, and more radical measures may be required both to prevent it from engulfing Earth and to keep our atmosphere at a reasonable temperature.
Around the same time, a few billion years from now, our entire Milky Way Galaxy will collide and merge with its nearest big neighbor, the Andromeda galaxy. This isn’t quite as bad as it sounds, because their constituent stars are so far apart relative to their size that they’ll mostly miss each other: if our Sun were the size of an orange in Boston, then its nearest neighbor star, Proxima Centauri, would be in my native Stockholm. Instead
of colliding, most stars will intermingle to form a single new galaxy, “Milkomeda.” However, as we’ll see next, this may exacerbate problems with supernovae and asteroid impacts.
Asteroids, Supernovae and Supervolcanoes
Our fossil record reveals five major extinction events during the last 500 million years, each killing off more than 50% of all animal species. Although the details are actively debated, it’s widely believed that they were all triggered by various astronomical and geological events. The most recent of these “big five” extinctions appears to have been triggered by a Mount Everest–sized asteroid crashing into the Mexican coastline about 65 million years ago, whose most famous casualties were the non-avian dinosaurs. With an impact energy equivalent to many millions of hydrogen-bomb explosions, it blasted out a 180-kilometer crater and engulfed our planet in a dark dust cloud that blocked sunlight for years, causing widespread ecosystem collapse.
Earth regularly gets hit by objects from space of various sizes and compositions, so the question isn’t if we’ll suffer another similarly deadly collision, but when. The answer is largely up to us: a good network of robotic telescopes should be able to give us decades of advance warning of dangerous inbound asteroids, which is ample time to develop, launch and execute a mission to deflect them. If this is done sufficiently far in advance, only a gentle nudge is needed, which can be applied for example with a “gravity tractor” (a satellite whose gravitational pull nudges the asteroid toward it), a satellite-based laser (which ablates material from the asteroid’s surface and sends the asteroid recoiling in the opposite direction), or even by painting the asteroid so that the radiation pressure corresponding to solar heating will push it differently. If time is short, a riskier approach is required, such as a kinetic impactor (a satellite tackling the asteroid off course like a football player) or nuclear explosion.