by Alok Jha
Vacuum Decay
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You would be forgiven for thinking that a vacuum is empty. Indeed, a vacuum is the very definition of empty space. There is nothing there. It is a total absence of stuff. And if there is no stuff, then you might think that there is nothing that could harm us or our world. End of chapter. But you would be wrong.
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Among the many strange things that quantum mechanics has revealed about the world is the curious idea that our empty vacuum is not, in actual fact, empty. This conclusion comes from the one bit of quantum mechanics that almost everyone is probably familiar with: Werner Heisenberg’s uncertainty principle. This says that it is impossible to know both the exact position and the velocity of a quantum particle, such as a photon or electron, at the same time. The more accurately you know one of these values, the less accurately you can know the other.
Another way to express the uncertainty principle is in terms of the energy and time of the particle, so it is possible that, for extremely short periods of time, a quantum system’s energy can be thought of as highly uncertain. In fact, the system can sometimes “borrow” enough energy from the vacuum to create entirely new particles, as long as those particles do not end up exisiting for very long. These “virtual particles” appear in pairs (an electron and its antimatter pair, the positron, for example) for a short period and then annihilate each other.
A vacuum, according to quantum mechanics, is not empty at all, but seething with pairs of virtual particles popping into existence and then vanishing. It is a soup of energy, and therein lies danger.
Inflation and false vacuums
To understand why the vacuum is more dangerous than you might at first think, we need to go right back to the start of the universe. Very soon after the Big Bang, after our baby universe had been expanding steadily for a few moments, it started to balloon at an incredible rate. During this period of “inflation,” it more than doubled in size every 10-35 seconds; by the time inflation switched off at 10-32 seconds after the moment of the Big Bang, this had happened a hundred times. To put that into context, imagine the universe had started off at 1 cm. After 10-32 seconds, one “tick” of inflation, it would be 2.7 cm wide. After two ticks, it would be 7.4 cm. Three ticks later, we’re at 20 cm. By 20 ticks, the universe is 4,850 km wide, and after 50 ticks, 5,480 light years. All that in less than 10-34 seconds.
By the time inflation had finished, after 100 ticks, the universe would have grown by a factor of 1043. And that is a conservative version of the theory—in some accounts, inflation was even more extreme, with the factor of expansion more like 10 multiplied by itself a trillion times.
According to Alan Guth, the physicist at the Massachusetts Institute of Technology who came up with the idea of inflation, this rapid expansion was caused by the release of energy from a form of matter he calls “false vacuum.” As this decayed into “true vacuum,” it exerted a strange type of repulsive gravity on the space around it, as opposed to the more familiar attractive kind that keeps us stuck to the Earth and the Earth moving around the Sun.
After inflation, things stabilized, and today the universe continues to expand at a much slower, steadier rate. And that is the last we hear of the false vacuum. Well, not quite. “The false vacuum is unstable, but in most versions of the theory it decays like a radioactive substance, such as radium,” says Guth. This means that the decay is described by a half-life, a time after which half of the false vacuum still remains. After two half-lifes, a quarter of the original vacuum will be left, and so on. That means that, today there is still false vacuum out there somewhere.
As it decays, the false vacuum will expand and the expansion will be faster than the decay. Although only half of the false vacuum will remain after one half-life, it will still be larger than the initial region. “The false vacuum would never disappear, but instead would continue increasing in volume indefinitely,” says Guth. “Pieces of the false vacuum region would randomly decay, producing new ‘bubble’ universes at an ever-increasing rate. Our universe would be just one of the universes on this infinite tree of bubbles.”
By sprouting these “bubble universes,” the false vacuum moves into a lower, more stable energy state. Physicists might assume that the same laws of nature would apply in all these individual bubble universes, but Guth is not so sure. “Other kinds of space might not be three-dimensional, and they might alter the masses of elementary particles, or the forces that govern their behavior. If there are many kinds of space, the infinite tree of bubble universes would sample all the possibilities.”
Regions of false vacuum could decay to create new “bubble” universes with their own laws of nature and different (perhaps no) particles and forces.
How can a vacuum be so dangerous?
So, we have established that a vacuum is more complex than we might have imagined: it can cascade from a higher-energy (false vacuum) to a lower-energy (true vacuum) state by creating bubble universes that expand at the speed of light, with each universe potentially having its own laws of physics.
What if the universe we live in today is in a region of space where the vacuum is stuck in an unstable, high-energy state? In other words, what if there is an even more stable form of vacuum than the one we exist in? It would be as if we are perched at the top of a hill—all it takes is for a nudge to send us tumbling into the lower-energy position at the foot of the hill. The question is, is our vacuum at the top or the bottom of that hill?
The Earth, our solar system, our Sun, our entire galaxy might be in such a false vacuum state right now. At any point, it could collapse into a lower-energy vacuum by creating a new bubble universe. This collapse would grow at the speed of light and rewrite the physics that we know. Under these new laws, the fundamental forces would have different strengths, and our atoms would not hold together in the ensuing wave of intense energy. We, and everything around us, would be torn apart and turned into energy. All that energy might recondense at some point into something else, new forms of matter governed by new laws of nature. But we would not be here to see any of it.
In 1980, the Harvard physicist Sidney Coleman calculated that vacuum decay would be the end for all life as we know it. “The possibility that we are living in a false vacuum has never been a cheering one to contemplate,” he wrote. “Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.”
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After vacuum decay, not only is life as we know it impossible, so is chemistry as we know it.
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How likely is it?
The theory that underpins vacuum decay is scientifically robust. Whether it would happen in real life is, to all intents and purposes, unknown and unpredictable. The fact that the universe we can see has existed for as long as it has suggests that a bubble nucleation of some vacuum decay has not happened, but that is no guarantee of safety for the future.
If that conclusion leaves you depressed or worried, there is one crumb of comfort you can take. Again, it comes from the same branch of physics that gave us vacuum decay: quantum mechanics.
This rule says that predicting the behavior of any quantum system is impossible; instead, its equations just provide a range of possible scenarios for each system and assign it a probability of happening. In the “many worlds” interpretation of quantum mechanics, each of these possibilities actually corresponds to a different universe.
For example, if you throw a six-sided die, each of the six possible ways it could land is represented by a different universe. When it lands on, say, a four, you and the entire arena of action move into one of those six universes. In the other five possible univer
ses, which do exist somewhere, the die landed on one of the other numbers.
The same rules of quantum mechanics that might one day lead to catastrophic vacuum decay also lead to the inevitable conclusion that, at the time of decay, new universes will be created that will be spared the destruction. On the one hand, quantum mechanics will take away our lives in an instant. On the other, everything will carry on just as normal.
Solar Collision
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Suppose our parent star was about to suffer a terrible attack. That a cosmic missile was zooming toward our solar system and making for the heart of the Sun. If that missile was a white dwarf—a dense star in its dying days containing the mass of the Sun in a volume one hundredth the size—it would mean the end of the solar system.
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As it approached, the white dwarf would alter the orbits of the planets, perhaps pulling one of them (maybe the Earth) into an eccentric path around the Sun or even knocking it out of the solar system altogether. Closer still, it would force a change in the shape of the Sun, elongating it as the missile’s intense gravitational field pulled solar gas toward it.
The mutual gravitational attraction would accelerate both stars, and the white dwarf would smack into the Sun at almost 650 km/s (400 miles per second). Anyone watching from Earth would be treated to quite a fireworks display while they managed to cling on to life.
The collision would create a shock wave in the Sun that would compress and heat the entire star above the temperature needed to fuse the hydrogen there. Until that moment, only the center of the Sun would have been hot enough to fuse anything. In the next hour, the superheated star would release as much energy from fusing atoms as it would have done in 100 million years of normal burning. Releasing energy this fast would cause the gas to expand more quickly than escape velocity—the Sun would have blown itself apart. On the Earth, all our oceans and atmosphere would be obliterated by the rising amount of radiation and the searing gas clouds from the Sun.
An hour after it entered the Sun, the white dwarf would emerge from the other side of our star, almost unchanged and on its way into deep space after destroying all life on Earth.
In space, collisions are common
The idea that a white dwarf is about to smack into our Sun without any warning sounds fanciful. And you can rest easy in that it is highly unlikely: certain parts of the galaxy might be a hotbed of collisions, but estimates of the risk of anything hitting our Sun are around once in 10 trillion trillion years.
For much of the 20th century, though, the whole idea that stellar collisions might even be possible or worthy of study seemed ludicrous to astronomers. According to Michael Shara, curator and chair of the department of astrophysics at the American Museum of Natural History in New York City, “The distances between stars in the neighborhood of the sun are just too vast for them to bump into one another. Other calamities will befall the sun (and Earth) in the distant future, but a collision with a nearby star is not likely to be one of them. In fact, simple calculations carried out early in the 20th century by British astrophysicist James Jeans suggested that not a single one of the 100 billion stars in the disk of our galaxy has ever run into another star.”
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Not a single one of the 100 billion stars in the disk of our galaxy has ever run into another star.
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The initial clues that stellar collisions were indeed happening in deep space came with the observation in the 1950s of strange blue stars sitting in the middle of certain globular clusters, which are regions of space dense with stars and dust. Where we are in the galaxy, there is around one star in every ten cubic light years of space; in a globular cluster, the same amount of space might hold hundreds of stars.
Blue stars are among the hottest of all stellar objects, and they burn through their hydrogen fuel much faster than smaller, yellower stars. The globular clusters in which they were situated, however, were known to have exhausted their clouds of gas, which normally give rise to new stars, billions of years earlier. The blue stars that astronomers were seeing were simply too young to have been formed where they were.
Measurements from the Hubble Space Telescope showed that these “blue straggler” stars were not, as some had thought, normal stars that had somehow conserved their fuel and managed to live longer. Instead, they were two much older stars that had collided and coalesced, creating a brand new star that simply looked a whole lot younger.
When stars collide ...
What happens when two stars collide depends on a number of things, not least the speed of the objects, what they are made of and how full-on their collision is. “Some incidents are fender benders, some are total wrecks and some fall in between,” says Shara. “Higher-velocity and head-on collisions are the best at converting kinetic energy into heat and pressure, making for a total wreck.”
Shara has modeled stellar collisions to work out their effects on the stars themselves and also on the objects around them. He first studied the possible results of a head-on collision between a Sun-like star and a vastly more dense star, such as a white dwarf, several decades ago. “Whereas the sunlike star is annihilated, the white dwarf, being 10 million times as dense, gets away with only a mild warming of its outermost layers,” he wrote in a 2004 essay for Scientific American. “Except for an anomalously high surface abundance of nitrogen, the white dwarf should appear unchanged.”
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Some incidents are fender benders, some are total wrecks and some fall in between.
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If the two stars happened to be of a similar size and density, something different happens. “As the initially spherical stars increasingly overlap, they compress and distort each other into half-moon shapes. Temperatures and densities never climb high enough to ignite disruptive thermonuclear burning. As a small percent of the total mass squirts out perpendicular to the direction of stellar motion, the rest mixes together. Within an hour, the two stars have fused into one.”
Globular clusters have more collisions than normal regions of space because the stars they contain move relatively slowly. While the Sun is traveling at around 76,000 km/h (47,000 mph) with respect to its cosmic neighbors, the stars in a globular cluster move at around half that speed past each other. This gives gravity some time to work between the objects, deflecting their paths and increasing the chances of a collision. “The stars are transformed from ballistic missiles with a preset flight path into guided missiles that home in on their target. A collision becomes up to 10,000 times more likely. In fact, half the stars in the central regions of some globular clusters have probably undergone one or more collisions over the past 13 billion years,” says Shara.
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RISK OF SOLAR COLLISION
1 in 10 trillion trillion years
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Is it likely that a collision will destroy us?
Astronomers are almost certain that collisions between stars must occur, and have uncovered the remnants of those collisions in faraway globular clusters. What they have never done, though, is manage to see one in action. “To be sure, all this evidence is circumstantial,” says Shara. “Definitive proof is harder to come by.”
Detecting these faraway collisions will be tough, and might have to be done by looking for gravitational rather than light waves coming from them. If two supermassive objects collide, the event would create a disturbance in space–time. Albert Einstein’s theory of general relativity says that any such collision would cause gravitational waves to propagate through space. These waves stretch and move space itself, making the distance between two points expand and contract as the energy passes by.
“The average time between collisions in the 150 globular clusters of the Milky Way is about 10,000 years; in the rest of our galaxy it is billions of years,” says Shara. “Only if we are extraordinarily lucky will a direct collision occur close enough—say, within a few million light-years—to permit today’s astronomers to witness it with present t
echnology.”
Detecting a real-life collision would be exciting for astronomers, giving them an unprecedented insight into the mechanics of stars. But it would also be a stark reminder of just how dangerous such collisions can be.
Scientists Create a Black Hole
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As scientists gathered in Geneva to switch on the Large Hadron Collider (LHC) at Cern in 2009, not everyone was celebrating. A handful of people had been sounding an alarm that this gigantic machine might rip apart space itself, creating black holes under the mountain in Geneva and swallowing all of us (and the Earth) into nothingness.
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The LHC is the most intricate and sophisticated machine built by humans, its intended use to bring scientists closer than ever before to understanding the Big Bang. It stands as a testament to human ingenuity, a physical embodiment of two decades of design and building work. There is little doubt that it will bring us a better understanding of the most fundamental physics humans have ever known.
But the detractors are not placated by such promises. They were instead focused on one of the scary-sounding scenarios that the particle physicists themselves had predicted: the creation of black holes on Earth.
Black holes of different types
Think “black hole” and you probably conjure up an image of something far away in the darkness of space, using its immense gravity to suck in and destroy anything that has the misfortune to wander nearby—stars, planets or spaceships.