Existence: Why is the universe just right for us?
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IT HAS been called the Goldilocks paradox. If the strong nuclear force which glues atomic nuclei together were only a few per cent stronger than it is, stars like the sun would exhaust their hydrogen fuel in less than a second. Our sun would have exploded long ago and there would be no life on Earth. If the weak nuclear force were a few per cent weaker, the heavy elements that make up most of our world wouldn't be here, and neither would you.
If gravity were a little weaker than it is, it would never have been able to crush the core of the sun sufficiently to ignite the nuclear reactions that create sunlight; a little stronger and, again, the sun would have burned all of its fuel billions of years ago. Once again, we could never have arisen.
Such instances of the fine-tuning of the laws of physics seem to abound. Many of the essential parameters of nature - the strengths of fundamental forces and the masses of fundamental particles - seem fixed at values that are "just right" for life to emerge. A whisker either way and we would not be here. It is as if the universe was made for us.
What are we to make of this? One possibility is that the universe was fine-tuned by a supreme being - God. Although many people like this explanation, scientists see no evidence that a supernatural entity is orchestrating the cosmos. The known laws of physics can explain the existence of the universe that we observe. To paraphrase astronomer Pierre-Simon Laplace when asked by Napoleon why his book Mécanique Céleste did not mention the creator: we have no need of that hypothesis.
Another possibility is that it simply couldn't be any other way. We find ourselves in a universe ruled by laws compatible with life because, well, how could we not?
This could seem to imply that our existence is an incredible slice of luck - of all the universes that could have existed, we got one capable of supporting intelligent life. But most physicists don't see it that way.
The most likely explanation for fine-tuning is possibly even more mind-expanding: that our universe is merely one of a vast ensemble of universes, each with different laws of physics. We find ourselves in one with laws suitable for life because, again, how could it be any other way?
The multiverse idea is not without theoretical backing. String theory, our best attempt yet at a theory of everything, predicts at least 10500 universes, each with different laws of physics. To put that number into perspective, there are an estimated 1025 grains of sand in the Sahara desert.
Fine-tuned fallacy
Another possibility is that there is nothing to explain. Some argue that the whole idea of fine-tuning is wrong. One vocal critic is Victor Stenger of the University of Colorado in Boulder, author of The Fallacy of Fine-tuning. His exhibit A concerns one of the pre-eminent examples of fine-tuning, the unlikeliness of the existence of anything other than hydrogen, helium and lithium.
All the heavy elements in your body, including carbon, nitrogen, oxygen and iron, were forged inside distant stars. In 1952, cosmologist Fred Hoyle argued that the existence of these elements depends on a huge cosmic coincidence. One of the key steps to their formation is the "triple alpha" process in which three helium nuclei fuse together to form a carbon-12 nucleus. For this reaction to occur, Hoyle proposed that the energy of the carbon-12 nucleus must be precisely equal to the combined energy of three helium nuclei at the typical temperature inside a red giant star. And so it is.
However, Stenger points out that in 1989 a team at the Technion-Israel Institute of Technology in Haifa showed that, actually, the carbon-12 energy level could have been significantly different and still resulted in the heavy elements required for life.
There are other problems with the fine-tuning argument. One is the fact that examples of fine-tuning are found by taking a single parameter - a force of nature, say, or a subatomic particle mass - and varying it while keeping everything else constant. This seems very unrealistic. The theory of everything, which alas we do not yet possess, is likely to show intimate connections between physical parameters. The effect of varying one may very well be compensated for by variations in another.
Then there is the fact that we only have one example of life to go on, so how can we be so sure that different laws could not give rise to some other living system capable of pondering its own existence?
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Existence: Am I a hologram?
TAKE a look around you. The walls, the chair you're sitting in, your own body - they all seem real and solid. Yet there is a possibility that everything we see in the universe - including you and me - may be nothing more than a hologram.
It sounds preposterous, yet there is already some evidence that it may be true, and we could know for sure within a couple of years. If it does turn out to be the case, it would turn our common-sense conception of reality inside out.
The idea has a long history, stemming from an apparent paradox posed by Stephen Hawking's work in the 1970s. He discovered that black holes slowly radiate their mass away. This Hawking radiation appears to carry no information, however, raising the question of what happens to the information that described the original star once the black hole evaporates. It is a cornerstone of physics that information cannot be destroyed.
In 1972 Jacob Bekenstein at the Hebrew University of Jerusalem, Israel, showed that the information content of a black hole is proportional to the two-dimensional surface area of its event horizon - the point-of-no-return for in-falling light or matter. Later, string theorists managed to show how the original star's information could be encoded in tiny lumps and bumps on the event horizon, which would then imprint it on the Hawking radiation departing the black hole.
This solved the paradox, but theoretical physicists Leonard Susskind and Gerard 't Hooft decided to take the idea a step further: if a three-dimensional star could be encoded on a black hole's 2D event horizon, maybe the same could be true of the whole universe. The universe does, after all, have a horizon 42 billion light years away, beyond which point light would not have had time to reach us since the big bang. Susskind and 't Hooft suggested that this 2D "surface" may encode the entire 3D universe that we experience - much like the 3D hologram that is projected from your credit card.
It sounds crazy, but we have already seen a sign that it may be true. Theoretical physicists have long suspected that space-time is pixelated, or grainy. Since a 2D surface cannot store sufficient information to render a 3D object perfectly, these pixels would be bigger in a hologram. "Being in the [holographic] universe is like being in a 3D movie," says Craig Hogan of Fermilab in Batavia, Illinois. "On a large scale, it looks smooth and three-dimensional, but if you get close to the screen, you can tell that it is flat and pixelated."
Quantum fluctuation
Hogan recently looked at readings from an exquisitely sensitive motion-detector in Hanover, Germany, which was built to detect gravitational waves - ripples in the fabric of space-time. The GEO600 experiment has yet to find one, but in 2008 an unexpected jitter left the team scratching their heads, until Hogan suggested that it might arise from "quantum fluctuations" due to the graininess of space-time. By rights, these should be far too small to detect, so the fact that they are big enough to show up on GEO600's readings is tentative supporting evidence that the universe really is a hologram, he says.
Bekenstein is cautious: "The holographic idea is only a hypothesis, supported by some special cases." Better evidence may come from a dedicated instrument being built at Fermilab, which Hogan expects to be up and running within a couple of years.
A positive result would challenge every assumption we have about the world we live in. It would show that everything is a projection of something occurring on a flat surface billions of light years away from where we perceive ourselves to be. As yet we have no idea what that "something" might be, or how it could manifest itself as a world in which we can do the school run or catch a movie at the cinema. Maybe it would make no difference to the way we live our lives, but somehow I doubt it.
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Earth stalker found in eternal twilight
AN ASTEROID 300 metres in diameter is stalking the Earth. Hiding in the pre-dawn twilight, it has marched in lockstep with our planet for years, all but invisible to our telescopes.
The rock is Earth's first confirmed Trojan, which can orbit the sun in either of two gravitational wells along the same orbital path as our planet. From the sun's point of view, these wells lie 60 degrees ahead of and behind the Earth, at Lagrange points where gravitational forces between the sun and the Earth balance out.
Trojans are common - Jupiter alone boasts about 5000, and Neptune andMars each have their own smaller collections. But finding Earth's has proven difficult, because the Lagrange points lie towards the sun in the sky. Astronomers must look for the objects just before the sun rises or after it sets, and until now the glare of this sunlight has obscured the feeble light reflected from any rocks that might be hiding there.
Now Martin Connors of Athabasca University in Alberta, Canada, and colleagues have used a heat sensor to see past the gloaming. Using data from NASA's Wide-field Infrared Survey Explorer (WISE) satellite, they identified a 300-metre-wide Trojan now dubbed 2010 TK7.
The rock is leading the Earth, and based on the team's calculations it is expected to be stable in an elliptical orbit around its Lagrange point for at least the next 10,000 years, drifting at between 20 million and 300 million kilometres from us (Nature, DOI: 10.138/nature/10233).
Like most Trojans, says Connors, the story of where 2010 TK7 came from, and what it is made of, is an utter mystery.
It could be an errant, captured asteroid, or perhaps a "genesis rock" - a long-sought relic from the birth of the solar system about 4.5 billion years ago. If so, it may be identical to the rocks that came together to form the Earth, which means that studying its composition would tell us what the chemistry of our planet was like in the earliest stages of its existence.
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Next Mars rover will climb a mountain
It's official: The next Mars rover will explore a 5-kilometre-high mountain of sediment inside a crater called Gale, NASA announced today.
The Curiosity rover is scheduled to launch between 25 November and 18 December. It will examine Martian rocks and soil to learn about the history of the planet's climate and to look for chemical traces of life.
There has been vigorous debate
about where to send the rover. At a meeting in May, competing camps made their final arguments for each of the four sites then on the shortlist.
about where to send the rover. At a meeting in May, competing camps made their final arguments for each of the four sites then on the shortlist."These are all like different flavours of ice cream – all fantastic but slightly different," said John Grant of the National Air and Space Museum in Washington DC, one of the organisers of the May meeting, at a NASA press conference today.
In recent weeks, NASA said it had narrowed that list to two craters: Gale and Eberswalde.
Runner-up
Eberswalde contains a beautifully preserved fan of sediment from an ancient river delta. On Earth, organic material gets concentrated in such sediment, so this would be a good place to look for signs of past life on Mars.
But on Friday, NASA announced it will send Curiosity, also known as the Mars Science Laboratory (MSL), to Gale crater instead.
There, a giant mound of layered sediment rises 5 kilometres from the crater floor. It is not known exactly how it got there, but the sediment contains clays, a sure sign that it was exposed to liquid water at some point.
"If you start at the bottom of the pile of layers and you go to the top, it's like reading a novel," said John Grotzinger, the rover's project scientist at the California Institute of Technology in Pasadena. "And we think Gale crater is going to be a great novel about the early environmental evolution of Mars."
Multiple sites
It is too dangerous to land on the mountain itself, so Curiosity will touch down on a flatter part of the crater floor, then drive up the mountain.
But first, it will take a look at sediment deposited at the landing site by a river that once flowed into the crater. That sediment and the clays in the mountain are two of the places in Gale that may have been habitable and left organics behind.
But organics could also be preserved in several other places where life may once have existed. One is an ancient river canyon that cuts into the mountain. There are also layers in the mountain containing sulphate minerals, which require water to form. And there are also cracks in the mountain that appear to have been waterlogged in the past.
"There was a real preference for Gale in that it's not a one-trick pony," said Michael Meyer of NASA Headquarters in Washington DC. "There's several different environmental settings that can be explored [there], any one of which might have some possibility of preserving organics."
Organic hunt
Grotzinger agrees: "It has exceptionally high diversity for different kinds of habitable environments and it is possible that some of those might preserve organic carbon."
"Organic carbon" means any complex carbon-based molecules, whether they come from living things or some other source. For example, Mars has probably received organic carbon from meteorite impacts.
Grotzinger noted that finding organic carbon will be "a very, very difficult challenge" based on experience with Earth geology. Even though life is abundant on our home planet, ancient rocks very rarely preserve organic material, he says.
"We hope to be able to look for organic carbon," he said. "What we can promise to deliver with MSL is an understanding of the environmental history of Mars."
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Snowstorms on Mars may dwarf those on Earth
SNOWSTORMS more violent than any on Earth may have hit Mars - and could occasionally strike again, despite its extremely dry climate.
No rain or snowstorms have ever been observed on Mars, which has been mostly cold and dry for about 3.5 billion years. But mineral evidence suggests short-lived lakes have formed intermittently on the planet, sometimes inside craters. Lakes may form when meteorite impacts heat ice in the crust or when underground reservoirs of water kept liquid by geothermal heat leak onto the surface.
Edwin Kite of the University of California, Berkeley, and colleagues wondered how the planet's weather might change in the presence of such lakes. So they plugged Mars's present-day conditions - which may be much the same as they have been for the last 3 billion years or so - into weather-prediction software, and modelled what would happen if the planet had a 65-kilometre-wide lake near the equator.
The results were dramatic. Heat from the lake, although minimal, stirred the thin Martian atmosphere vigorously. In the simulation, this caused a plume of warm, moist air to rise from the lake at a top speed of 194 kilometres per hour, similar to the updraft speed in thunderstorms on Earth.
As the rising water condensed to ice within the plume, it formed a storm cloud that stretched to 35 kilometres in altitude. Earth's storm clouds are stopped about 20 kilometres up by a layer of warm air heated by ozone, which absorbs the sun's ultraviolet rays. Mars has no ozone layer to heat its upper atmosphere, so clouds can rise higher there. The simulated storm cloud dropped about 10 centimetres of snow per hour, comparable to extreme snowstorms on Earth (Journal of Geophysical Research, DOI: 10.1029/2010je003783).
"Imagine being in the most severe thunderstorm you've been in, where it's really dark and ominous-looking," says team member Scot Rafkin of the Southwest Research Institute in Boulder, Colorado. "Then make it darker and more ominous, with snow coming down at an unbelievable blizzard-like rate."
Periodic changes in Mars's tilt - thought to occur due to gravitational tugs by Jupiter - could later melt the snow. That may explain the presence of water-formed minerals downwind of suspected former lakes, including one in a canyon called Juventae Chasma, the team say.
The study shows Mars's climate can vary dramatically depending on local conditions, says Anthony Colaprete of NASA's Ames Research Center in Moffett Field, California.
