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Doing The Wave

There has been a lot of digital ink spilled over the recent paper on the reactionless thrust device known as the EMDrive. While it’s clear that a working EM Drive would violate well established scientific theories, what isn’t clear is how such a violation might be resolved. Some have argued that the thrust could be an effect of Unruh radiation, but the authors of the new paper argue instead for a variation on quantum theory known as the pilot wave model.

One of the central features of quantum theory is its counter-intuitive behavior often called particle-wave duality. Depending on the situation, quantum objects can have characteristics of a wave or characteristics of a particle. This is due to the inherent limitations on what we can know about quanta. In the usual Copenhagen interpretation of quantum theory, an object is defined by its wavefunction. The wavefunction describes the probability of finding a particle in a particular location. The object is in an indefinite, probabilistic state described by the wavefunction until it is observed. When it is observed, the wavefunction collapses, and the object becomes a definite particle with a definite location.

While the Copenhagen interpretation is not the best way to visualize quantum objects it captures the basic idea that quanta are local, but can be in an indefinite state. This differs from the classical objects (such as Newtonian theory) where things are both local and definite. We can know, for example, where a baseball is and what it is doing at any given time.

The pilot wave model handles quantum indeterminacy a different way. Rather than a single wavefunction, quanta consist of a particle that is guided by a corresponding wave (the pilot wave). Since the position of the particle is determined by the pilot wave, it can exhibit the wavelike behavior we see experimentally. In pilot wave theory, objects are definite, but nonlocal. Since the pilot wave model gives the same predictions as the Copenhagen approach, you might think it’s just a matter of personal preference. Either maintain locality at the cost of definiteness, or keep things definite by allowing nonlocality. But there’s a catch.

Although the two approaches seem the same, they have very different assumptions about the nature of reality. Traditional quantum mechanics argues that the limits of quantum theory are physical limits. That is, quantum theory tells us everything that can be known about a quantum system. Pilot wave theory argues that quantum theory doesn’t tell us everything. Thus, there are “hidden variables” within the system that quantum experiments can’t reveal. In the early days of quantum theory this was a matter of some debate, however both theoretical arguments and experiments such as the EPR experiment seemed to show that hidden variables couldn’t exist. So, except for a few proponents like David Bohm, the pilot wave model faded from popularity. But in recent years it’s been demonstrated that the arguments against hidden variables aren’t as strong as we once thought. This, combined with research showing that small droplets of silicone oil can exhibit pilot wave behavior, has brought pilot waves back into play.

How does this connect to the latest EM Drive research? In a desperate attempt to demonstrate that the EM Drive doesn’t violate physics after all, the authors spend a considerable amount of time arguing that the effect could be explained by pilot waves. Basically they argue that not only is pilot wave theory valid for quantum theory, but that pilot waves are the result of background quantum fluctuations known as zero point energy. Through pilot waves the drive can tap into the vacuum energy of the Universe, thus saving physics! To my mind it’s a rather convoluted at weak argument. The pilot wave model of quantum theory is interesting and worth exploring, but using it as a way to get around basic physics is weak tea. Trying to cobble a theoretical way in which it could work has no value without the experimental data to back it up.

At the very core of the EM Drive debate is whether it works or not, so the researchers would be best served by demonstrating clearly that the effect is real. While they have made some interesting first steps, they still have a long way to go.

Paper: Harris, D.M., et al. Visualization of hydrodynamic pilot-wave phenomena, J. Vis. (2016) DOI 10.1007/s12650-016-0383-5

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Here's a new favorite photo of a small village near Mount Fuji in Japan. I've just finished working on this one over the past few days while holed up here in Denver! :)

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Laboratory (Berkeley Lab) have discovered a possible secret to dramatically boosting the efficiency of perovskite solar cells hidden in the nanoscale peaks and valleys of the crystalline material.
Solar cells made from compounds that have the crystal structure of the mineral perovskite have captured scientists’ imaginations. They’re inexpensive and easy to fabricate, like organic solar cells. Even more intriguing, the efficiency at which perovskite solar cells convert photons to electricity has increased more rapidly than any other material to date, starting at three percent in 2009, when researchers first began exploring the material’s photovoltaic capabilities, to 22 percent today. This is in the ballpark of the efficiency of silicon solar cells.
Now, as reported online July 4 in the journal Nature Energy, a team of scientists from the Molecular Foundry and the Joint Center for Artificial Photosynthesis, both at Berkeley Lab, found a surprising characteristic of a perovskite solar cell that could be exploited for even higher efficiencies, possibly up to 31 percent.
This atomic force microscopy image of the grainy surface of a perovskite solar cell reveals a new path to much greater efficiency. Individual grains are outlined in black, low-performing facets are red, and high-performing facets are green. A big jump in efficiency could possibly be obtained if the material can be grown so that more high-performing facets develop.
Using photoconductive atomic force microscopy, the scientists mapped two properties on the active layer of the solar cell that relate to its photovoltaic efficiency. The maps revealed a bumpy surface composed of grains about 200 nanometers in length, and each grain has multi-angled facets like the faces of a gemstone.
Unexpectedly, the scientists discovered a huge difference in energy conversion efficiency between facets on individual grains. They found poorly performing facets adjacent to highly efficient facets, with some facets approaching the material’s theoretical energy conversion limit of 31 percent.
The scientists say these top-performing facets could hold the secret to highly efficient solar cells, although more research is needed.
If the material can be synthesized so that only very efficient facets develop, then we could see a big jump in the efficiency of perovskite solar cells, possibly approaching 31 percent says Sibel Leblebici a postdoctoral researcher at the Molecular Foundry.
Leblebici works in the lab of Alexander Weber-Bargioni, who is a corresponding author of the paper that describes this research. Ian Sharp, also a corresponding author, is a Berkeley Lab scientist at the Joint Center for Artificial Photosynthesis. Other Berkeley Lab scientists who contributed include Linn Leppert, Francesca Toma, and Jeff Neaton the director of the Molecular Foundry.

A team effort
The research started when Leblebici was searching for a new project. I thought perovskites are the most exciting thing in solar right now, and I really wanted to see how they work at the nanoscale, which has not been widely studied she says.
She didn’t have to go far to find the material. For the past two years, scientists at the nearby Joint Center for Artificial Photosynthesis have been making thin films of perovskite-based compounds, and studying their ability to convert sunlight and CO2 into useful chemicals such as fuel. Switching gears, they created pervoskite solar cells composed of methylammonium lead iodide. They also analyzed the cells’ performance at the macroscale.
The scientists also made a second set of half cells that didn’t have an electrode layer. They packed eight of these cells on a thin film measuring one square centimeter. These films were analyzed at the Molecular Foundry, where researchers mapped the cells’ surface topography at a resolution of ten nanometers. They also mapped two properties that relate to the cells’ photovoltaic efficiency: photocurrent generation and open circuit voltage.
This was performed using a state-of-the-art atomic force microscopy technique, developed in collaboration with Park Systems, which utilizes a conductive tip to scan the material’s surface. The method also eliminates friction between the tip and the sample. This is important because the material is so rough and soft that friction can damage the tip and sample, and cause artifacts in the photocurrent.

Surprise discovery could lead to better solar cells
The resulting maps revealed an order of magnitude difference in photocurrent generation, and a 0.6-volt difference in open circuit voltage, between facets on the same grain. In addition, facets with high photocurrent generation had high open circuit voltage, and facets with low photocurrent generation had low open circuit voltage.
This was a big surprise. It shows, for the first time, that perovskite solar cells exhibit facet-dependent photovoltaic efficiency says Weber-Bargioni.
Adds Toma These results open the door to exploring new ways to control the development of the material’s facets to dramatically increase efficiency.
In practice, the facets behave like billions of tiny solar cells, all connected in parallel. As the scientists discovered, some cells operate extremely well and others very poorly. In this scenario, the current flows towards the bad cells, lowering the overall performance of the material. But if the material can be optimized so that only highly efficient facets interface with the electrode, the losses incurred by the poor facets would be eliminated.
This means, at the macroscale, the material could possibly approach its theoretical energy conversion limit of 31 percent says Sharp.
A theoretical model that describes the experimental results predicts these facets should also impact the emission of light when used as an LED. Linn Leppert, Sebastian Reyes-Lillo, and Jeff Neaton performed this particular work.

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California startup HyperSolar and University of Iowa researchers have teamed up to make renewable energy in a way that draws inspiration from plants. Using water and sunlight, they are able to make renewable hydrogen energy. At the end of May, HyperSolar announced a “breakthrough” in efficiency, and the University of Iowa just renewed a year-long research agreement with the startup.
The two have been laboring on technology to create clean hydrogen energy by using a process similar to photosynthesis. Their technology is important because the sustainable method of creating hydrogen power by splitting water molecules is very expensive. HyperSolar’s way of producing hydrogen power could ultimately be far more cost effective.

To achieve this goal, they’ve created an electrochemical device that’s solar-powered. The device is placed in any type of water, including wastewater or seawater. When sunlight hits the device, it converts the water to hydrogen, and that can be “stored like a battery.” When that hydrogen is converted back to water, the researchers can harvest power.
HyperSolar Lead Scientist Syed Mubeen said they’re planning to scale up, and to do so they’ll find ways to cut more costs and strengthen their process. Ultimately, the energy they produce could be used in hydrogen-powered cars or as a source of clean electricity.
Mubeen said in a press release Developing clean energy systems is a goal worldwide. Currently, we understand how clean energy systems such as solar cells, wind turbines, et cetera, work at a high level of sophistication. The real challenge going forward is to develop inexpensive clean energy systems that can be cost competitive to fossil fuel systems and be adopted globally and not just in the developed countries… If one could develop these systems at costs competitive to fossil fuel systems, then it would be a home run.

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A few days ago in Queenstown there were some wicked clouds out over the Remarks...
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