Science News

21 Lutetia has puzzled astronomers since its discovery. Now they have made a daring set of predictions about what the Rosetta spacecraft will find when it flies past this mysterious asteroid in July

On 10 July, the European Space Agency's Rosetta spacecraft will fly within a few thousand kilometres of 21 Lutetia, a main belt asteroid that orbits the Sun between Mars and Jupiter.

Lutetia is an unusual object. It is classified as an M-type asteroid, which are thought to be made mainly of nickel and iron. However, Lutetia's spectrum does not seem to show any evidence of metals. In fact, exactly what Lutetia is made of puzzles astronomers. That's partly why it was chosen for the fly by.

So come July, astronomers should know the answer to this conundrum. But in the run up, they're indulging in a little fun. The game they've invented is to see how good a prediction they can make about what Rosetta will find.

Today, Irina Belskaya at the Observatoire de Paris and a few friends take a stab. They make several detailed predictions about Lutetia based partly on observations dating back to the 1960s but mostly on data taken since 2004, when interest picked up after the asteroid was chosen as a flyby target.

So what do they think Rosetta will find?

Belskaya and co say that Lutetia will be 132x101x76 km in size (that's technically known as potato-shaped). They say its texture and mineral content will vary across its surface. At least part of Lutetia's surface will be covered by a layer of loose dust having a mean grain size less than 20 micrometres across. And Lutetia's surface will be made of stuff that has more in common with the carbonaceous chondrite meteorites found on Earth than the iron-nickel ones.

But they're most interesting prediction is that Lutetia will be "non-convex" in shape. That means a large crater will be visible on its surface. In fact its shape will be dominated by this crater.

Great fun to see a daring set of forecasts like this. And only four months until we find out how well they've done.

Ref: arxiv.org/abs/1003.1845: Puzzling Asteroid 21 Lutetia: Our Knowledge Prior To The Rosetta Fly-By

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New Charging Method Could Slash Battery Recharge Times

Apply an oscillating electric field to the anode of a lithium battery and the recharge time drops dramatically, say chemists.

One of the biggest problems with batteries is the time it takes to recharge them. Run out of juice and it'll be several hours before you're mobile again, a particular showstopper for electric vehicles.

Today, Ibrahim Abou Hamad at Mississippi State University and few buddies reveal an entirely new technique for charging lithium ion batteries that could lead to exponential improvements in charging time.

The business end of a lithium battery, the anode, consists of a graphite electrode, in other words a stack of graphene sheets, bathed in an electrolyte of ethylene carbonate and propylene carbonate molecules through which lithium and hexafluorophosphate ions diffuse. During charging, an electric field pushes the lithium ions towards and into the graphene sheets, where they have to cross a potential barrier to become embedded and stored, a process called intercalation.

The Mississippi team have studied the movement of these ions and molecules by creating a computer model of the forces acting on them. Their model consists of 160 carbon atoms arranged in 4 graphene sheets, 69 propylene carbonate and 87 ethylene carbonate molecules forming a liquid electrolyte and finally, two hexafluorophosphate ions and10 lithium ions. They then apply an electric field across this system and watch what happens.

It turns out that while the electric field pushes the lithium ions towards the graphene, the rate limiting step is the process of intercalation--the rate at which the lithium ions can cross the potential barrier into the graphene .

What Hamad and co have found is a relatively simple way to overcome this barrier. The trick is to superimpose an oscillating electric field onto the charging field. This has the effect of helping the lithium ions to hop over the barrier.

But get this: the team says there is an exponential relationship between the intercalation time and the oscillating field amplitude. So a small increase in amplitude of the field leads to a massive speed up of the process of intercalation.

"These simulations suggest a new charging method that has the potential to deliver much shorter charging times, as well as the possibility of providing higher power densities," they say.

That's a neat piece of work which should be relatively straightforward to test in a real battery.

That doesn't mean that we'll see a ten minute charging time for electric vehicles any time soon.

Battery performance is a complicated balance between huge numbers of competing factors. If this oscillating field does improve charging time in real batteries, manufacturers will then have to check its effect on other performance metrics such as the number of these charging cycles a battery can withstand and how long it holds its charge, to name just two.

Nevertheless, these Mississippi guys have come up with an interesting new approach that will have more than peaked the interest of battery makers around the globe.

Ref: arxiv.org/abs/1003.1678: A New Battery-Charging Method Suggested By Molecular Dynamics Simulations

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How to Build a Superluminal Computer

Physicists have come up with a way to process information faster than the speed of light. But what could they do with such a hypercomputer?

The speed of light represents one of the fundamental limits of the laws of physics. Nothing can travel faster than the speed of light, right?

Well, yes and no, say Volkmar Putz and Karl Svozil at the Vienna University of Technology in Austria. They say there are several ways that signals can cross the superluminal line, although none of them allow the kind of time travel paradoxes beloved of science fiction writers. For example, the quantum phenomenon of entanglement occurs when two quantum particles are described by the same wave function. These particles can be separated by the diameter of the universe and yet a measurement on one will instantaneously influence the other.

So-called "nonlocal" phenomenon cannot be used to transmit information faster than the speed of light but Putz and Svozil today ask whether it can be used to process it, to carry out computational tasks at superluminal speeds. They say there is no reason why not, provided the processing does not lead to any time travel paradoxes.

How might such a machine work? Putz and Svozil point out that nonlocal phenomenon can lead to materials in which the index of refraction is less than one, thereby allowing superluminal speeds. For example, light travelling through a vacuum can be made to spontaneously form into an electron-positron pair--an entangled pair--which then recombine to form a photon again. This process happens instantaneously, allowing the photon to effectively "jump" across space.

A material in which this kind of pair formation and recombination was promoted would have a refractive index less than one, they say. Various physicists have proposed such materials made of things like metamaterials. Putz and Svozil themselves suggest that a vacuum filled with either electrons or positrons would do the trick.

Having created a medium in which the refractive index is less than one, Putz and Svozil's idea is simply to immerse a computer in it. That simple act (and presumably some clever design to create an optical computer in the first place) would allow superluminal computation to take place.

Assuming that this device could actually be built, what could you do with a superluminal computer? That's a good question that Putz and Svozil do not address directly. They say such a device would fall into a class of processing machine known as hypercomputers. These are hypothetical devices more powerful than Turing machines, that allow non-Turing computations. They were first discussed by Alan Turing in the 1930s.

In theory, hypercomputers can compute certain kinds of otherwise noncomputable functions. That sounds handy but even though there are uncountably many non-computable functions, it's actually quite hard to come up with an example of one that might seem useful. If you have any ideas, post them in the comments section.

Otherwise sit back and wait for a new era of superluminal hyprcomputers. But don't hold your breath.

Ref: arxiv.org/abs/1003.1238: On the physical limit of communication speed by light signals

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Theoretical Breakthrough For Quantum Cryptography

Quantum cryptography only works if Alice and Bob share their relative positions in advance. Now physicists have worked out how to do it without this information.

The world of cryptography is currently undergoing a quantum revolution. The weird laws of quantum mechanics allow cryptographers to create codes that guarantee perfect secrecy. Until recently, the best cryptographers could aim for was just pretty good secrecy with codes that were always compromised in some way or another. Quantum cryptography, on the other hand, is perfect: theoretically and practically secure.

A few companies have even sprung up to sell the gear that can send perfectly secure messages, mainly to banks and governments (although the gear itself creates some loopholes that eavesdroppers can attack).

But it's still early days for this technology and naturally it suffers from several drawbacks. For example, one well known limitation is that quantum cryptography can only be used over point-to-point connections and not through networks where data has to be routed. That's because the routing process destroys the quantum properties of the photons used to secure messages.

A lesser known limitation is that the sender and receiver of quantum encrypted messages--the famous Alice and Bob--must be perfectly aligned so that they can carry out well-defined polarisation measurements on the photons as they arrive. Physicists say that Alice and Bob must share the same reference frame.

That's not so hard to do when Alice and Bob are both based in labs on the ground. But it's much harder when one or the other is moving, in a satellite, for example, which would be both spinning and orbiting the Earth.

Today, Anthony Laing from the University of Bristol and a few pals show how to get round this. The trick is to use entangled triplets of photons, so-called qutrits, rather than entangled pairs.

This solves the problem by embedding it in an extra abstract dimension, which is independent of space. So as long as both Alice and Bob know the way in which all these abstract dimensions are related, the third provides a reference against which measurements of the other two can be made.

That allows Alice and Bob to make any measurements they need without having to agree ahead of time on a frame of reference. There is one proviso: Alice and Bob cannot move too quickly during the measurements since this changes their relative orientation and a new qutrit will be needed to establish a reference.

That'll be useful for quantum encryption over satellite links, the kind of thing that government agencies and the military might want to do. But there's another, more valuable application.

If quantum encryption is ever to be widely used, it'll need to work between one microchip and another without the need to share a frame of reference in advance. That's always been a problem because the chips inside computers are constantly on the move (relative the the wavelength of light) and because photon polarisations drift as they move through optical fibres, introducing another source of error.

That's why quantum cryptography that is reference frame independent is an enabling technology and so potentially hugely valuable. It means that Laing and co may have made one of the key breakthroughs that will bring quantum cryptography to the masses.

Ref: arxiv.org/abs/1003.1050: Reference Frame Independent Quantum Key Distribution