One of the most subtle effects predicted by general relativity is a phenomenon known as rotational frame-dragging. This is caused by a massive spinning body, such as a planet, dragging space-time with it as it turns. That causes any small rotating particles in the vicinity to precess.

Needless to say, the effect is tiny and extremely hard to observe. The difference between Einstein’s predictions and Newton’s is in the region of one part in a few trillion.

Various attempts to measure this in orbit around Earth have had mixed success. The best was a $750 million spacecraft called Gravity Probe B that NASA launched in 2004.

The spacecraft consisted of four small, almost perfectly spherical gyroscopes, each coated with a superconducting layer in which the movement of electrons could be used to measure the rotation.

The idea was to monitor very tiny changes in the way these gyroscopes spun as the spacecraft orbited Earth. In theory, that should have allowed the measurement of frame-dragging with an accuracy of 1%. However, various problems with the spacecraft reduced its accuracy to about 20%.

Astrophysicists would dearly love to get a better measurement, but know that the chances of raising the cash required for another experiment of this type are as small as the effect itself.

But there is a much cheaper way of achieving the same goal, at least according to the Italian Space Agency ASI.

These guys put a “disco ball” in orbit around Earth and say that carefully measuring its orbit from the ground should produce a similar result.

This disco ball is an extraordinary object. It is entirely passive, with no thrusters or electronic components. Instead, it is a tungsten sphere about the size of a football, weighing 400 kg and covered with 92 reflectors that allow it to be tracked using lasers on Earth. These reflectors also make it look like a disco ball.

The ball’s small size but large mass make it the most perfect test particle ever placed in orbit, the first aerospace structure ever made from tungsten and the densest object orbiting anything anywhere in the solar system.

The ball is known as the Laser Relativity Satellite, or LARES. The Italians launched it in February of last year and have been carefully measuring its orbital characteristics ever since.

Today, Antonio Paolozzi at the University of Rome La Sapienza and Ignazio Ciufolini at the University of Salento described the results of this process.

To be sure, this experiment will be no easy ride. The idea is to measure the ball’s orbit by bouncing lasers off it and then to compare this with the theoretically predicted orbit that takes account of all the different forces that must act on the satellite.

The problem, of course, is that the many effects are often subtle and can swamp the signal they are looking for.

To cancel out the effects of the most subtle of these forces, the team need to compare data from LARES with other similar test particles in orbit. As luck would have it, the Italians have a couple of other disco balls already in orbit: LAGEOS 1 and 2.

Although these aren’t as perfect as LARES, they have been providing data for several years.

Paolozzi and Ciufolini are confident that the analysis will finally produce an accurate measurement of rotational frame dragging. “By adding the LARES orbital data, it will be possible to eliminate also the effects of [these perturbations], thus allowing the achievement of about 1% accuracy,” they said.

That will be impressive, not least because it will have been achieved at a tiny fraction of the cost of Gravity Probe B.

But it’s still too early to pop the champagne corks. As physicists who have attempted to measure this effect can testify, these highly sensitive experiments have a tendency to spring the odd surprise.

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The traditional method of imaging, which is at least 150 years old, relies on a lens to create an image and a device for recording photons such as an array of pixels, a light-sensitive film or even a retina.

But a dramatic revolution that is currently taking the world of imaging by storm means that this could soon change.

This revolution is based on a technique called compressive sensing, which is based on the idea that many common measurements have huge redundancy. That means it’s possible to acquire the same data with just a fraction of carefully chosen measurements.

The trick, of course, is knowing which measurements to take and how to reassemble them. Various teams have been excitedly experimenting with this idea. Back in January, for example, we looked at one group who have created 3D images using a single pixel in this way.

Today, this revolution gains pace because Gang Huang and his team from Bell Labs in New Jersey say they’ve used compressive sensing to build a camera that needs no lens and uses only a single sensing pixel to take photographs. What’s more, the images from this camera are never out of focus.

The new device is simple in nature. “The architecture consists of two components, an aperture assembly and a sensor. No lens is used,” says Huang. It consists of an LCD panel that acts as an array of apertures that each allow light to pass through and a single sensor capable of detecting light three colours.

Each aperture in the LCD array is individually addressable and so can be open to allow light to pass through or closed. An important aspect of this kind of imaging is that the array of open and closed apertures must be random.

The process of creating an image is straightforward. It begins with the sensor recording the light from the scene that has passed through a random array of apertures in the LCD panel. It then records the light from a different random array and then another and so on.

Although seemingly random, each of these snapshots is correlated because they record the same scene in a different way. And this is the key that the team use to reassemble an image. The process of compressive sensing analyses the data, looking for this correlation which it then uses to recreate the image.

Clearly, the more snapshots that are taken, the better the image will be. But it is possible to create a pretty good image using just a tiny fraction of the data that a conventional image would require.

For example, the Bell Labs team took this image of books using only a quarter of the data they could have recorded.

In fact, the less detail there is in the scene, the less data is required to reconstruct it.

This revolutionary lensless camera has a number of advantages over a conventional camera. First is the tiny amount of data required to create images. Without a lens, these images suffer none of the aberrations and focusing problems associated with lenses. The scene is entirely in focus and the resolution of the image depends on the size and number of the apertures and the point-like nature of the light sensor.

By using two sensors behind the same aperture array, it is possible to create two different images of the scene at the same time. Indeed, multiple sensors produce multiple images.

What’s more, the device is simple and cheap. Huang's team built their prototype using cheap off-the-shelf components that anybody would have access to.

Best of all, the same approach works for other wavelength of light such as infrared and millimetre waves. So it ought to be possible to create relatively cheap cameras for these wavelengths, too.

The disadvantage, at least for the moment, is that it takes time to acquire the data for each image, so the camera only creates images of still scenes.

But even that is useful for surveillance since it is possible to compare consecutive images of the same scene to determine things that have changed or to work out the speed of moving objects.

That’s an impressive piece of work that is likely to have far-reaching consequences for the way we record the world around us. Expect to hear a lot more about compressive imaging — or ghost imaging as it is sometimes called — in the near future.

It’s also interesting to see Bell Labs hitting its straps again. This is an organisation with a venerable history but a tumultuous recent past. With advances like this, there is always the chance it can recapture some of its former glory.

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Some of the most promising battery chemistries — which, in theory, could store several times more energy than today's lithium-ion batteries and cost much less — have a fatal flaw. They can't be recharged very often before they stop working, making them useless for applications such as electric vehicles. Now researchers at Stanford have created novel nanostructures that greatly increase the number of times one of these chemistries can be recharged, even to levels high enough for many commercial applications.

Whereas previous research reports have celebrated the ability to recharge a lithium-sulfur battery 150 times, the Stanford researchers recharged their battery 1,000 times and still retained substantial energy storage capacity. In some electric vehicle configurations, that would be enough to last several years. Commercial versions of the batteries could approximately double battery storage compared to lithium-ion batteries, says Yi Cui, a professor of materials science and engineering at Stanford. The batteries retained 81% of their capacity after 500 cycles and 67% after 1,000 cycles. Cui says that the nanomaterials can be made with simple methods that lend themselves to high-volume manufacturing. Read more…

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