The Conversation – Akshat Rathi

‘Supernova in a bottle’ will help create matter from light

May 20, 2014May 20, 2014 Akshat RathiLeave a comment

In 1934, two physicists came up with a theory that described how to create matter from pure light. But they dismissed the idea of ever observing such a phenomenon in the laboratory because of the difficulties involved setting up such an experiment.

Now, Oliver Pike of Imperial College London and his colleagues have found a way to achieve this dream, 80 years after US physicists Gregory Breit and John Wheeler explained the theory. This group hopes to use high-energy lasers aimed at a specially designed gold vessel to convert photons into matter-antimatter particle pairs, recreating what happens in some exceptional stellar explosions.

Pike, who led the research published in the journal Nature Photonics, said, “The idea is that light goes in and matter comes out.” To be sure, the matter created won’t be every day-objects; instead the process will produce sub-atomic particles.

“To start with, the matter will consist of electrons and its antimatter equivalent positrons,” Pike said. “But with higher energy input in the lasers, we should be able to create heavier particles.”

Pike concedes this won’t be the first time light has been converted into matter. In 1997, US researchers at the Stanford Linear Accelerator Centre (SLAC) were able to do so, albeit in a different way.

The SLAC experiment used electrons to first create high-energy light particles, which then underwent multiple collisions to produce electrons and positrons, all within same chamber. This is called the multi-photon Breit-Wheeler process, named after the two physicists who came up with the theory in 1934.

“The key difference in the SLAC experiment and the one we propose is that our process will be more straightforward,” Pike said. In the new proposal, the laser beam will still be generated using free electrons, but it will be separated from the electrons.

Why create light using matter and then convert it back? Apart from showing that the Breit-Wheeler process can happen without the multiple photons the SLAC experiment needed, Pike thinks their process provides a clean way of doing particle physics experiments.

Current particle-physics experiments involve smashing sub-atomic particles at great speeds and sorting through the mess of new particles that are created in the explosion. This is how the Higgs boson was found in the Large Hadron Collider.

The new experimental design will be similar. Rather than involving a complicated mix of particles and photons, the laser beam will be sent into a small gold hohlraum (German for “empty room”). There, individual photons can interact with the radiation field that’s generated when the hohlraum is excited by a laser, creating the electron-positron pairs.

“While physicists have excellent methods to sift through such data, our process has the advantage that it will be easier to analyse,” Pike said. “Light will go in from one end of the hohlraum and particles created will come out from the other end.”

Pike and colleagues are now working to secure time on high-energy laser beams to carry out the experiment. The two likely candidates are Aldermaston, Berkshire in the UK or Rochester, New York in the US.

Andrei Seryi at the University of Oxford found the work interesting, but warned it is still too far away from being used in particle-physics experiments. “Theoretically, however, it would be great if we are able to create particles from only light.”

“With such high energy lasers, we may not need to build big particle colliders, such as the Large Hadron Collider, which is a 22km underground tunnel,” Seryi said.

Even if we do manage to create a photon collider, we would only be catching up with the natural world, where a specific type of supernova, called “pair instability,” involves the creation of proton-antiproton pairs. If Pike is able to achieve this phenomenon, he will essentially be creating a supernova in a bottle.

First published on The Conversation.

Jellyfish are the most energy efficient swimmers, new metric confirms

May 17, 2014 Akshat Rathi1 Comment

Even though a blue whale is much heavier than a tuna, the mammal consumes less energy per unit weight than the fish when they travel the same distance. For years, these sort of comparisons have dominated our understanding of the energy efficiency of animal movement, which is important for designing vehicles inspired by nature, such as underwater drones.

But Neelesh Patankar, professor of mechanical engineering at Northwestern University, believes that this measure has only limited benefit. Instead, with his colleagues, he has come up with a new measure that allows comparison of animals as small as bees or zebrafish with animals as large as albatrosses or blue whales.

The new measure has two implications. First, among those that have typical swimming and flying actions, which includes most fish and all birds, each animal is as energy efficient as it can be. This means that, given their size and shape, each animal is able to spend the least amount of energy to move the most distance. Second, this measure confirms a previous finding that jellyfish are unusually energy efficient, beating all the thousands of fish and birds Patankar studied.

“Put another way, a whale and a tuna are equally energy efficient,” Patankar said. “Except jellyfish, which have an unusual action that makes them more efficient.”

A new measure

To understand why jellyfish are special, we need to first answer the question why we need a new measure for energy efficiency. Patankar offers an analogy: if there are two cars that are of equal weight, would you expect them to have the same mileage? Just as in cars, animals’ motion will vary based on factors other than their weight.

John Dabiri, professor of aeronautics and bioengineering at California Institute of Technology, said, “It is not immediately obvious how to compare the swimming efficiency of a bacterium and a blue whale, for example, but Patankar and colleagues have developed one.”

To make the comparison, Patankar borrowed from a well-known concept in physics called the Reynolds number, which explains the relationship between two forces that act on any body that is moving through a fluid. The first is viscous force, which is, crudely put, the push you feel when you put your hand out of a moving vehicle. The second is inertial forces, which is the tendency of a moving object to keep moving (or that of a stationary object to remain stationary).

Depending on the size of a body and the speed at which it travels, the body faces either a low Reynolds number, where the forces acting on a body are mostly viscous forces, or a high Reynolds number, where inertial forces dominate. This creates a natural difference in how much energy is spent countering these forces.

Reynolds number was developed to look at the aerodynamics of stiff bodies, such as aeroplanes and ships. But Patankar reckoned he could use it to help compare animals of different sizes. He gathered data from thousands of birds and fish to come up with a metric called the energy-consumption coefficient, which he has described in the Proceedings of the National Academy of Sciences. Using it, he found that all the animals he looked at (except jellyfish) are as energy-efficient as they can be.

Note that Y-axis is for energy-consumption coefficient, not for energy efficiency.
Rahul Bale

“The idea that animals are tuned for energy-efficient locomotion is not surprising, but the authors have devised a fresh approach to the issue of how to compare the efficiencies of different animals,” Dabiri said.

Patankar finds, as he had hoped, that small animals find themselves in low Reynolds number situations, and large animals find themselves in high Reynolds number situations. This means they expend energies differently, which is what Patankar’s coefficient represents. Using the coefficient, one can compare the energy efficiency of bodies weighing few grams to many tonnes.

The coefficient also indicates that animals that fly are less energy-efficient than those that swim. This, Patankar thinks, must be because those in flight have to expend more energy to counteract gravity than those in water.

Jelly’s secrets

While working on the energy-consumption coefficient, he came across recent work done by Dabiri and his colleagues which showed that the unique contract-and-relax action of jellyfish allowed it to recapture some of the energy it spends on motion. This means a jellyfish can travel a lot more distance for the same amount of energy spent by other animals adjusted for its weight and size.

When Patankar used Dabiri’s data and plotted it on his energy-consumption coefficient chart, he found that the only animals that were more energy efficient than he had predicted were jellyfish.

“We found that each swimming or flying animal can spend all the energy it has at its disposal. However, our coefficient is a fair way to conclusively show that indeed jellyfish are more efficient,” Patankar said.

Dabiri is already working on exploiting jellyfish propulsion. However, he thinks that, apart from providing a new metric to compare different types of animals on the energy-efficiency scale, Patankar’s measure could be a used for evaluating the performance of aerial and underwater drones that are being developed, especially those with designs that are inspired by flying and swimming animals.

First published on The Conversation.

Search for alien life could remain fruitless

April 30, 2014April 30, 2014 Akshat RathiLeave a comment

Given that we are unlikely to be visiting an exoplanet any time soon, astronomers have been contemplating whether it might be possible to detect indications of simple life – a biosignature – from a distance. Many think that the strongest case for extraterrestrial life would be the discovery of oxygen and methane on the same body. They also think that the likelihood of finding such a biosignature is greatest on an Earth-like planet that is orbiting a sun-like star.

Astronomers who hope to search for these biosignatures in expolanets, however, may be in for a disappointment. New research finds that there is no way we can confirm that such a signature is actually the result of extraterrestrial life. The problem, it turns out, is that an exomoon’s atmosphere will be indistinguishable from the one of the planet it orbits.

Finding E.T.

Searching for extraterrestrial life is no easy feat. Astronomers have to first search for a star that has planets. Then they have to ensure that there is at least one planet that orbits this star in the habitable zone, which is a region around the star in which we might expect liquid water. Finally, they have to record the faint light that originated from the bright star and was reflected off the exoplanet after having passed through its atmosphere.

This faint light, even if only a handful of photons, when compared with light from the parent star is enough to give some indication of the chemicals in the atmosphere of this planet. Life as we know it creates two gases that wouldn’t naturally be present in an atmosphere at the same time – oxygen from photosynthesis and methane from microbes.

Both oxygen and methane can be created independently by non-living processes, so their individual presence is of little interest. What scientists are looking for is both of them in the atmosphere of a single body. If these reactive gases are not constantly replenished by living things, they will react with each other, creating carbon dioxide and water. As a result, we should not observe them in the same atmosphere without a large, living source.

False hopes

In the new study, published in the Proceedings of the National Academy of Sciences, Hanno Rein at the University of Toronto and his colleagues wanted to know whether anything else could mimic this biosignature. While working through potential false positives, which are signals that would show signs of life but in reality there isn’t life, he found a big one: exomoons. Rein found that observers on Earth will not be able to tell whether the signs of methane and oxygen originate from a single celestial body, or come from two nearby worlds.

This could happen because, just as Earth has a moon, there is a chance that exoplanets will have exomoons. While we have yet to find an exomoon, looking at the various moons of our solar system’s planets suggests that exomoons ought to be plentiful. However, even if they are plentiful, chances are that exomoons will be difficult to spot.

If both these celestial bodies have an atmosphere and in their atmospheres the exoplanet has oxygen and the exomoon has methane (or vice-versa), then an observer on Earth will record an oxygen-methane biosignature. This might seems like evidence for life, whereas in reality both these gases are being produced by non-living processes on two separate celestial bodies. Since they can’t react with each other, they will be able to build up to high levels.

Futile technology

“Even if we somehow developed ways of finding exomoons, we won’t be able to tease out the difference between their atmospheres given the limited amount of light that reaches us,” Rein said. This fundamental limit on the light that reaches us is called photo noise.

Rein limited his analysis to biosignatures coming from Earth-like planets orbiting a sun-like star, which is the combination that astronomers are betting has the greatest chance of hosting life. The American space agency NASA recently announced that they had found such an Earth-sized planet less than 500 light years away, although the star it orbits isn’t sun-like.

While their analysis might seem quite restrictive and involves a number of assumptions, it does not really matter: interpretation of biosignatures needs to be flawless. According to David Cullen at the University of Cranfield, “This study seems to highlight a real issue that will needed to be considered when interpreting biosignatures.”

Rein himself was surprised to find such a limitation. However, he sees the results of his work in positive light. “Finding such a limitation tells us what we should focus on in the future. Rather than a restricted search for Earth-like planets orbiting sun-like stars, we should broaden our search,” he said.

What this research shows is a need to move away from a highly focused search for extraterrestrial life that is currently in place. Rein points out that the chances of eliminating such false positive biosignatures increases as the star becomes dimmer or larger planets are considered. Perhaps alien life is not just unlike that on Earth, but it is also resides in a place that is unlike Earth.

First published on The Conversation. Image credit: bflv.