A controversial approach to gravity that challenges Albert Einstein and suggests dark matter doesn’t exist has passed its first test.

The vast majority of physicists agree that gravity acts according to rules laid down in Isaac Newton’s law of gravitation and Einstein’s theory of general relativity. Yet observations of the universe show that the motion of the galaxies can’t be explained by the gravitational pull of all the ordinary matter out there – hence the belief in unseen, dark matter that exerts its own pull.

Now, a team of astronomers studying the distribution of matter in more than 30,000 galaxies say their observations can be explained by an alternative theory that does away with dark matter. If this “modified gravity” is correct, it would up-end hundreds of years of fundamental physics.

at Leiden University, the Netherlands, and her colleagues looked at the gravitational lensing of these galaxies – the way they bend the light of more distant galaxies as predicted by Einstein’s theory – to measure their dark matter content.

To their surprise, they discovered the observed lensing could just as readily be accounted for by a new model of gravity, without invoking dark matter.

, a theoretical physicist at the University of Amsterdam in the Netherlands, has been developing a competing model of gravity that borrows heavily from quantum mechanics, relativity, information theory and string theory. It also builds on controversial models of so-called modified gravity, such as the Modified Newtonian Dynamics (MOND) theory of Mordehai Milgrom.

Verlinde’s calculations fit the new study’s observations without resorting to free parameters – essentially values that can be tweaked at will to make theory and observation match. By contrast, says Brouwer, conventional dark matter models need four free parameters to be adjusted to explain the data.

“The dark matter model actually fits slightly better with the data than Verlinde’s prediction,” says Brouwer. “But then if you mathematically factor in the fact that Verlinde’s prediction doesn’t have any free parameters, whereas the dark matter prediction does, then you find Verlinde’s model is actually performing slightly better.”

Galactic lenses

Brouwer’s study takes advantage of catalogues of distant galaxies released in and and looks at regions close to the visible disc of each galaxy. These regions are where gravitational lensing should be bending light from distant galaxies beyond.

Using statistical algorithms that consider the shape and color of the background galaxies, the researchers inferred a lensing profile for the foreground galaxy. It’s a bit like projecting an image onto a warped and uneven sheet of glass and then, knowing what the original image looks like, figuring out the optical properties of the glass sheet from what we see on the far side.

From the gravitational distortions inferred for each foreground galaxy, the researchers devised a lensing profile based on Verlinde’s gravitational model, and another based on a conventional dark matter approach.

So if Verlinde’s is the better match, what’s the problem? Gravitational heresy. Verlinde’s gravity is stronger and dies off more slowly with distance compared with the models of Newton and Einstein.

To most physicists and astronomers today, that’s an issue, to put it mildly. Newton’s and Einstein’s theories of gravity have been so rigorously and comprehensively validated experimentally that it borders on sacrilege to suggest gravity could be something other than what they describe. String theorist Lubos Motl in a recent blog post: “I wouldn’t okay this wrong piece of work as an undergraduate term paper.”

Milgrom, however, supports the work. He also points out that according to his own 2013 , MOND produces similarly impressive results as Verlinde’s gravitational model does in Brouwer’s study.

“My equations work differently than Milgrom’s, and in the case of [galaxy] clusters this can be quite important,” Verlinde says. But in the case of Brouwer’s work, “They put in the formula I get,” he says, “and I have to admit it’s the same formula that Milgrom would have got, and… they just put it on the data. It looks like a fit.”

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THOUGHT your smartphone or tablet packed a big punch for its size? Pah, that’s nothing. The next generation of computers will be able to carry out complex calculations but will be little bigger than a snowflake.

Such tiny computers – nicknamed smart dust – would work much like their larger cousins, says at the University of Michigan in Ann Arbor. They will have tiny CPUs that run programs on a skeleton operating system and be able to access equally small banks of RAM and flash memory. The plan is for such sensor-packed machines to be embedded in buildings and objects in their hundreds or even thousands, providing constant updates on the world around us.

Dutta’s group is creating the first prototypes, which they have dubbed . These devices, a cubic millimetre in size, come equipped with sensors to monitor temperature or movement, say, and can send data via radio waves.

But how do you charge something so small? “The vision of blanketing the world with smart sensors is very compelling,” says Joshua Smith, head of the Sensor Systems Laboratory at the University of Washington in Seattle. “But a lot of sensor networks researchers found themselves surrounded by mountains of depleted batteries and dead sensor nodes.”

So, like microscopic Robinson Crusoes, the motes will live off the power they can scavenge from their surroundings. A mote near a light source might use a tiny solar panel, while a mote running somewhere with greater temperature extremes can be built to tap into that, by converting the heat energy that flows between hot and cold into electricity.

So what will be smart dust’s killer app? The Michigan team says Micro Motes could be used to monitor every tiny movement of large structures like bridges or skyscrapers. And motes in a smart house could report back on lighting, temperature, carbon monoxide levels and occupancy. With motes embedded in all of your belongings it might be possible to run a Google search in the physical world. For example, asking Google “where are my keys?” would give you the right answer if they have been fitted with a mote.

“It might be possible to do the equivalent of a Google search, say, to find your keys in the physical world”

Smart dust computers could make efficient medical implants too. The idea is that motes placed inside the body would monitor a patient’s vital signs. For example, in as-yet-unpublished research, the Michigan team has implanted a Micro Mote inside a mouse tumour so that it can report back on its growth.

, with his wireless identification and sensing platforms (WISPs). Further along in development than Micro Motes – albeit larger – WISPs communicate via radio frequency identification devices, using the same computer language that your next-generation credit card uses. Like Micro Motes, WISPs don’t need batteries and only consume what they can scavenge – stray signals from a nearby TV tower might do the trick, for instance.

But communication remains a key bottleneck for the next wave of computer miniaturisation, says Dutta. For the same chunk of energy a mote could perform 100,000 operations on its CPU but only transmit one bit of information to the outside world, he says.

This article appeared in print under the headline “A sprinkling of smart dust”