Category: physics

There is something romantic about a scientific paper whose references count ones from the 1950s. I imagine a large industrial empire growing in the United States, hinging largely on its government funding basic research, maintaining military and defense research, and racing to the moon. The Eisenhower era marked a point in human history where Russia and the United States vyed for the top spot as the world’s technological leader. Competition possibly brought out the best in physics research of the decade. Certainly, questions brought up then have underscored much of the research going on today.

A Nature Communications paper from this past April cited a couple academic articles from the ’50s:

Morrison, P. Approximate nature of physical symmetries. Am. J. Phys. 26, 358–368 (1958).

and 

Schiff, L. I. Sign of the gravitational mass of a positron. Phys. Rev. Lett. 1, 254–255 (1958).

The Morrison paper tried to prove that antimatter and matter would have the same mass, using a thermodynamics proof to codify the argument. The Schiff paper argued the same thing. Both papers were highly theoretical and provided no direct experimental proof that neither gravitational nor inertial mass differed between matter and antimatter. All of the arguments were theoretical but carry a lot of weight today. Following a logical argument with mathematics is as important to scientific research as seeing the evidence of the proof with our own eyes. The theoretical arguments of these 1958 papers have stood the test of time.

The recent experiment described in the paper was the first of its kind – ever- to directly observe the behavior of antimatter under the force of gravity. As is the case with many Nature Communications papers, no solid conclusion on the underlying question was made, but the method to experimentally poke around the subject was worth noting. The jury is still out on whether the masses of the two kinds of physical entities are not different, but we’ll get there as the Berkeley-CERN team perfect their set-up.

Rebecca Boyle of PopSci covered the experiment pretty well on May 1st, the day after it was published, but not without a little help from the press release. 

Amole C., Ashkezari M.D., Baquero-Ruiz M., Bertsche W., Butler E., Capra A., Cesar C.L., Charlton M., Eriksson S. & Fajans J. & (2013). Description and first application of a new technique to measure the gravitational mass of antihydrogen, Nature Communications, 4 1785. DOI: 10.1038/ncomms2787

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It’s amazing that we’re still discovering things about our own planet at a time when we have the technology to explore planets other than our own. A study that was published in Nature Geoscience last week found that iron is weaker than previously thought. What’s interesting about this is that we didn’t know before this experiment how iron acted at high pressures, like those found in Earth’s core.

What we know about the composition of the center of the Earth is just made from inference. The Earth’s core is thought to be made of an iron-nickel alloy, but there is no direct physical evidence of this. Along with what’s in there, the intense pressure that would theoretically exist deep down below the Earth’s crust, where we live, is based on theoretical calculations. The pressure is thought to be 364 GPa (and probably as hot as 5500 Kelvin or 9440 °F). As far as our society has come technologically, we haven’t yet been able to drill down to our planet’s inner core to actually measure the pressure down there.

The authors, Ariana Gleason and Wendy Mao, from Stanford University and SLAC, used spectroscopy and diffraction to study the loading response of a sample of iron to large pressures that reached higher than 200 GPa. Attaining this pressure in the lab was an accomplishment since other lab experiments have only been able to produce 10 GPa, at the most.

What Gleason and Mao found was that iron started deforming at 1GPa, but irregularly. Basically, they learned that at such high pressures, grains of iron material start to line up to a configuration that would better withstand the pressure pushing in on them in the Earth’s core. This phenomenon, called creep, would happen up until 1 GPa. At this point, layers of iron atoms would be misaligned with one another, forming a dislocation in the material. The iron then deforms, or changes shape. This suggests that our iron core constantly changes its structure, a process called graining.

It’s really important to understand those changes in structure because our theoretical calculations of what’s going on at the Earth’s center need to be modified. And they will be because of this study.

This discovery is really a story of how lab researchers keep theoreticians in check. Lovers of equations and mathematical models can theorize about everything in the world as long as there is a mathematical proof for it. However, when lab scientists come along with physical evidence of a limitation to those models, theoreticians need to revise their models. For example, we know that that water freezes at 0 °C and boils at 100 °C because we’ve tested it in our laboratories (as well as our kitchens), so now, physicists can predict how liquid water will act between these temperatures. Outside of those temperatures, the research problem would be about either ice (water at or below 0 °C) or vapor (water at or above 100 °C). Based on physical evidence, mathematicians would then sophisticate their idealistic models with realistic constraints.

Do we need a model for everything? No. But for things that we can barely observe with our eyes or ears (or telescopes and radar), those models give us something to grip onto. The Stanford researchers’ findings shine a light on something that’s right under our feet. It makes me wonder whether we’ll finish exploring our planet before we’ve finished with another one.

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Gleason A.E. & Mao W.L. (2013). Strength of iron at core pressures and evidence for a weak Earth’s inner core, Nature Geoscience, DOI: 10.1038/ngeo1808

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