by Alok Jha
This structure is the result of a collision between an elongated galaxy (left) and a ring-shaped galaxy (right). This image, taken by the Hubble Space Telescope, is of galaxies 500 million light years away from Earth, in the constellation of Ursa Major.
“Tides between galaxies are much more disruptive than terrestrial ocean tides because galaxies pass much closer to each other, relative to their size, than do the earth and moon,” say Barnes, Hernquist and Schweizer. “If the moon orbited the earth at half its present distance, the gravitational force it exerts would increase by a factor of four, because gravity is inversely proportional to the square of the distance. But the difference between the forces on the near and far sides of the earth, which is what determines the height of the tides, would increase by a factor of eight. In other words, tidal forces are inversely proportional to the cube of the distance. In close collisions the tidal forces between a pair of galaxies can be strong enough to rip both apart.”
The collision between Andromeda and the Milky Way is likely to be a multipart process. Before the full merger in around 5 billion years, the galaxies will pass and hit each other with glancing blows. This will happen first in around 2 billion years, and there is a slight chance that our solar system will be thrown out of the Milky Way in the process, part of a tidal stream of material dragged out of our galaxy by Andromeda.
It will only be a few billion years before Andromeda is back to repeat the process. It will take a few swipes and several more billion years before the two galaxies settle down into a merged mass of stars that orbit a common center of gravity. The combined galaxy itself would be a huge elliptical object, without the sweeping arms we know of our Milky Way.
The colossal energies released every time the galaxies meet would cause shock waves to pass through the vast clouds of dust and gas that float between the stars. Each collision would compress the hydrogen in the clouds sufficiently to start the process of fusion, giving birth to a flurry of new stars every time our galaxies get close.
The timescales over which the collisions will occur are unimaginably vast. By the time the situation has settled down, in more than 7 billion years’ time, the Earth will be having problems of its own as the Sun swells up and swallows its closest planets. Assuming we are somehow still alive as a species to watch the formation of the new mega-galaxy, however, what will be the fate of our solar system?
A computer simulation by scientists at the Harvard-Smithsonian Center for Astrophysics reckons that our Sun will keep its remaining planets, but there is a high probability that the ensemble will be pushed away from the center of the newly merged galaxy—perhaps up to 100,000 light years from the twin black holes (one coming from the center of each original galaxy) compared to our distance of 26,000 light years from the center of the Milky Way at present.
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The slower an encounter between two galaxies, the more time there is for gravity to produce huge, disruptive tides and the greater the resulting damage.
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What can we do? How likely is the collision?
It seems vaguely ridiculous to think about whether we could do anything about an event of this magnitude that is predicted to happen so far in the future. But for anyone harboring hopes, it is worth pointing out that astronomers are not yet certain that the collisions will definitely happen.
Andromeda and the Milky Way are moving toward each other, of that there is no doubt. They will also come very close to each other and their mutual gravitational attraction will have tide effects on the objects within the galaxies as they pass. But no one is quite sure how the two galaxies will come together.
The uncertainty arises from the fact that astronomers know how fast Andromeda is spinning, but cannot measure its transverse velocity. This means we do not know if Andromeda will hit us square on, or whether it will be a glancing blow where only the outskirts of the galaxies (or just the haloes of dark matter that surround them) make contact.
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There is a chance that our solar system will be thrown out of the Milky Way in the process.
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Either way, let us hope that our descendants are watching the whole thing from a vantage point very far away.
The End of Time
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Good old time. Sitting in the backdrop of our daily lives, giving them order, providing the arena upon which all of us can get things done. It will always be there, right? Wrong. Some theories suggest that, at some point in the future, there is no “after.”
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It might have suffered at the hands of Albert Einstein, who was convinced that time was part of a bigger, more profound arena called space–time, but to the rest of us it stays the same. Ticking clocks, shifting seasons, babies growing up. We and our world might finish up as dust, but the universe will carry on; nothing really stops.
But that is not enough for physicists. In the past century, scientists have pondered on the basics of what time is and how it really fits into the scientific picture of the universe. Their ideas are startling and dangerous: perhaps time does not really exist, say some; perhaps it is draining away from the universe, argue others. Unfortunately for the survival of our universe, the death of time is frighteningly possible.
What is time?
Trying to define time feels something like asking what air is: it is a property of the world that is just there; it exists and without it our lives would, well, stop. Physicists and philosophers, though, always want to go further. Is time simply a way of labeling a sequence of events so that we know what has happened, in what order and how far apart? Or, like space, are we actually measuring some sort of “stuff” when we measure seconds and hours? We might not know what it is, but this “stuff” ticks away in the background.
Our experience of time is that it flows from one moment to the next. We are in a present that keeps moving, leaving a set of events in the past in our memories. “We have a deep intuition that the future is open until it becomes present and that the past is fixed. As time flows, this structure of fixed past, immediate present and open future gets carried forward in time. This structure is built into our language, thought and behavior. How we live our lives hangs on it,” says Craig Callender, a philosopher at the University of California, San Diego.
But this natural way of thinking is not reflected in science. “The equations of physics do not tell us which events are occurring right now—they are like a map without the ‘you are here’ symbol,” says Callender. “The present moment does not exist in them, and therefore neither does the flow of time. Additionally, Albert Einstein’s theories of relativity suggest not only that there is no single special present but also that all moments are equally real. Fundamentally, the future is no more open than the past.”
In Isaac Newton’s day, it seemed as though the universe came with a clock that split our experience of the world into segments that we call seconds, minutes or hours. During these moments, things would happen. Since the middle of the 19th century, however, scientists have known that scientific laws do not require these segments of time to proceed in any particular direction. In other words, there is nothing inherent in physics that says that time must move “forward” rather than “backward.”
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You cannot generally think of the world as unfolding, tick by tick, according to a single time parameter. In extreme situations, the world might not be carvable into instants of time at all.
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The laws of physics work perfectly whether time is moving forward or backward, and the 19th-century Austrian physicist Ludwig Boltzmann went so far as to suggest that the difference between the past and the future was something not intrinsic to time itself but simply a result of differences in how matter in the universe was arranged.
Albert Einstein’s theories of special and general relativity in the early 20th century put more nails into the coffin of the Newtonian “universal clock” idea. Special relativity made time part of the coordinate syst
em with space, melding them into a four-dimensional space–time where different people moving at different speeds would feel seconds ticking at different rates. In general relativity, gravity itself distorts the passage of time, so a second next to a massive star “ticks” at a different rate to a second in deep space, where there is less gravity.
These developments have left us with a conundrum. “You cannot generally think of the world as unfolding, tick by tick, according to a single time parameter,” says Callender. “In extreme situations, the world might not be carvable into instants of time at all. It then becomes impossible to say that an event happened before or after another.”
Which begs the question that many physicists have been pondering for decades—does time exist anyway? “The idea of a timeless reality is initially so startling that it is hard to see how it could be coherent,” says Callender. “Everything we do, we do in time. The world is a series of events strung together by time. Anyone can see that my hair is graying, that objects move, and so on. We see change, and change is the variation of properties with respect to time. Without time, the world would be completely still. A timeless theory faces the challenge of explaining how we see change if the world is not really changing.”
Where time ends
General relativity, Einstein’s theory of how gravity works, predicts the existence of something else disturbing to anyone still stuck in the Newtonian universe of time: singularities. These are points of infinite density in space where matter is squashed, physics essentially breaks down and time stops. They are the objects at the center of black holes, the result of supermassive stars that have collapsed into points smaller than the following full stop. The intense gravity sucks in anything that gets too close (within a boundary called the event horizon), and at the singularity itself, time does not move at all. Get into one of these, and there is no such thing as “after.”
In M-theory, particles and forces are made of strings of energy that vibrate at different frequencies. The strings (shown as loops and lines) are contained on membranes, called “branes.” It is possible that we live on a brane that is embedded in a higher-dimensional space called the “bulk” (shown as the shaded box).
There are other places where time seems to disappear. Much of the universe is known to be missing—the matter we are made of accounts for only four percent of the mass in the universe, with the rest split 20–76 between dark matter and dark energy. This latter substance (no one really knows what it is) seems to be pushing the galaxies apart, keeping the universe expanding despite the gravitational attraction between all the familiar and dark matter.
In 2007, José Senovilla, Marc Mars and Raül Vera of the University of the Basque Country, Bilbao, and the University of Salamanca, Spain, proposed an alternative explanation for the effects of dark energy on the universe. In a paper for Physical Review D, they suggested that, rather then there being an anti-gravity force at work in the universe, the effects we observe are due to time slowing down as it leaks away. We do not notice it at an everyday level, but it should become apparent in the motion of galaxies as they move over the span of billions of years.
Our observed expansion of the universe, therefore, is an illusion. In fact, time is slowing down.
The idea is based around a concept in string theory, which is an attempt to explain the universe at a more fundamental level than current scientific ideas. In string theory, all particles are made of multi-dimensional strands of energy, each vibrating at a different frequency. One of the resulting ideas is that our universe exists on a membrane (known as a “brane”), part of a “bulk” of other branes and universes. “Ordinarily, we are free to roam around our 4-D prison,” says writer George Musser, explaining Senovilla’s idea in an article for Scientific American. “But if the brane is blown around fiercely enough, all we can do is hold on for dear life; we can no longer move. Specifically, we would have to go faster than the speed of light to make any headway moving along the brane, and we cannot do that. All processes involve some type of movement, so they all grind to a halt.”
Anyone on the brane would be unaware that this is happening to them because their clocks would stop too—they would have no way of telling that time was turning into space. “All we would see is that objects such as galaxies seemed to be speeding up,” says Musser. “Eerily, that is exactly what astronomers really do see and usually attribute to some unknown kind of ‘dark energy.’ Could the acceleration instead be the swan song of time?”
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Our observed expansion of the universe, therefore, is an illusion. In fact, time is slowing down.
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Is it likely?
Whatever time is, we know that it was created 13.7 billion years ago with the Big Bang, and if the universe were to end, whatever time is would go with it. When pondering the fate of the universe, cosmologists think the most likely result is that everything will continue expanding forever, eventually whimpering to an end when energy is so sparsely spread that, though time technically never ends, it has become meaningless. In this “heat death” scenario, everything is in equilibrium and any process or interaction is quickly undone by a process that goes in the opposite direction.
“Physicists argue it both ways,” says Musser. “Some think time does end. The trouble with this option is that the known laws of physics operate within time and describe how things move and evolve. Time’s end points are off the reservation; they would have to be governed not just by a new law of physics but by a new type of law of physics, one that eschews temporal concepts such as motion and change in favor of timeless ones such as geometric elegance.”
Even if the universe continued on forever, without time, we would not live.
Strangelets
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The subatomic world has plenty of surprises, though typically none of them are cataclysmic. There is a secret, though, in the laws of physics. A particle so deadly that its mere presence would spell the end for our planet.
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Perhaps predictably, the weirdest stuff comes from one of the most successful, but also most bizarre, physical theories ever devised: quantum mechanics. This science of the very small is filled with peculiar ideas, such as the inability to predict exactly where subatomic particles are or what they are doing. Some interpretations even suggest the existence of multiple worlds, each new one branching off every time we make a decision. But none of these ideas, however odd, is necessarily cataclysmic.
Yet the equations harbor a terrifying secret. Among the descriptions of fundamental particles and the ways they can bind together to form the everyday matter we are familiar with, something ghastly is hiding—a theoretical particle so stable that it can transform any other particle of matter into a copy of itself.
There would be no energetic coercion going on. The laws of physics state baldly that if this particle, the strangelet, came into contact with a particle of normal matter (made of protons and neutrons), the latter would somehow recognize that it was in a hopelessly inefficient energy state and immediately reorganize itself into a strangelet too. These copies would then go on to convert other particles into still more strangelets.
It is the ultimate doomsday weapon: in just a few short hours, a small chunk of these particles could turn an entire planet into a uniform, featureless mass. Everything that planet was made of, everything that was on it, would be no more.
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In just a few short hours, a small chunk of these particles could turn an entire planet into a uniform, featureless mass.
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What is a strangelet?
To understand what a strangelet is, we need to step back into the basics of what makes up the stuff of the universe.
The standard model of particle physics gives a precise quantum mechanical description of all the subatomic particles that are known to exist. It says that all matter particles consist of a combination of six quarks (some of which make up protons and neutrons, others that are so heavy they only s
urvive for fractions of seconds before decaying into lighter particles) and six leptons (including the electron and neutrinos). There are also particles (called bosons) that carry the fundamental forces, which include the particle of light, the photon, and gluons that stick quarks together in the nucleus of an atom.
The strangelet contains one of the lesser-seen quarks. There are three “generations” of quarks, each one featuring particles that are more massive than those in the set before it: up and down, strange and charm, top and bottom. Only two of these quarks —up and down—affect us in our daily lives. A proton consists of two up quarks and a down quark; a neutron is made of two down quarks and a single up quark.
A strangelet is a hypothetical particle consisting of equal numbers of up, down and strange quarks. Because strange quarks are so heavy, this composite would be the same size as a small atomic nucleus that might otherwise contain scores of other up and down quarks. In normal life, a strange quark is unstable and decays very soon after it is formed into lighter quarks.
However, it has been hypothesized that if lots of up, down and strange quarks got together, the resulting mass would be less prone to decay. According to the strange-matter hypothesis, thought up by, among others, Ed Witten at the Institute of Advanced Study in Princeton, a strangelet with lots of quarks would be even more stable than a normal atomic nucleus.