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May 20, 2019 5 tweets 2 min read Read on X
It's #WorldMetrologyDay, and this is an important one: Today a revamped International System of Units (SI) goes into effect. It's the most radical set of changes to the SI since it was established in 1960 doi.org/10.1063/PT.6.2…
A lot of attention has focused on the fact that a lump of metal in Paris will no longer be used to define the kilogram. That's a big deal, because it means that there are no longer any SI units that depend on a physical artifact.
But that's just part of what makes this change so significant: Beginning today, every SI unit—not only the kilogram but also the volt, the newton, the weber, and more—will be derived by some combination of seven invariants of nature. Image
Those seven fundamental constants, including the speed of light and Planck's constant, have been set at exact values based on dozens of experiments that measured them with remarkable precision.
Want to learn more? @usnistgov physicist David Newell previewed the SI changes in Physics Today's July 2014 issue doi.org/10.1063/PT.3.2… Image

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More from @PhysicsToday

May 19, 2022
A historian’s dream, the Farm Hall transcripts capture the secretly recorded conversations of Werner Heisenberg and 9 other German physicists discussing the atomic bomb in 1945. So why do scholars disagree on what they tell us? 🧵#histSTM #twitterstorians physicstoday.scitation.org/do/10.1063/PT.…
The Farm Hall transcripts are among the most famous primary sources in 20th-century physics. They document conversations between 10 German physicists suspected of working on an atomic project for Nazi Germany, including Heisenberg, Otto Hahn, and Carl Friedrich von Weizsäcker.
The physicists were rounded up at the end of World War II by Allied intelligence and imprisoned in a luxurious English country mansion called Farm Hall, where their conversations were secretly recorded.

📷 AIP ESVA/NARA Farm Hall, in 1945 or 1946.
Read 7 tweets
Aug 26, 2021
Albert #Einstein’s neat cursive has achieved pop culture status: A letter he wrote featuring the E = mc² equation recently sold for nearly $1.25 million at auction. But Einstein’s handwriting is also a microcosm of his turbulent life, argues @_rdahn [1/5] physicstoday.scitation.org/do/10.1063/PT.…
Einstein actually employed a different type of German handwriting early in his life. Called Kurrent script, it looks quite different than the Latin cursive still taught in some English-language schools today. [2/5]

Credit: Der Damen-Briefsteller, 1866/Public Domain Kurrent script.
Einstein switched from Kurrent script to Latin cursive in 1905—the same year he published his annus mirabilis papers. Part of the reasoning behind his decision was surely practical: Foreign scientists could read German, but they often struggled to read Kurrent. [3/5]
Read 5 tweets
Dec 10, 2020
This year's #NobelPrize laureates have received their medals. If the October prize announcements feel like two years ago rather than two months, check out this thread on some of the prizewinning work:
Roger Penrose received half of the #NobelPrize in Physics for his theoretical work on black holes doi.org/10.1063/PT.3.4…
Andrea Ghez and Reinhard Genzel shared half the #NobelPrize in Physics “for their discovery of a supermassive compact object at the center of our galaxy” doi.org/10.1063/PT.3.4…
Read 4 tweets
Oct 1, 2020
This month in Physics Today: our second annual careers issue, with a focus on early careers. Read about the initial employment of physicists who just earned their PhDs, advice on landing a tenure-track professorship, and more (thread) physicstoday.scitation.org/toc/pto/73/10 Physics Today's October 202...
Securing a faculty position can take months or even years. From applications to negotiations, Omar Magana-Loaiza, a new professor at @LSUphysastro, advises candidates on what to expect and how to prepare physicstoday.scitation.org/doi/full/10.10…
For the first time since 2001, more newly minted physics PhDs hold potentially permanent positions than hold postdocs physicstoday.scitation.org/doi/full/10.10…
Read 5 tweets
Sep 1, 2020
New in Physics Today's September issue: Graphene kirigami, Europe's particle physics strategy,@NASAJuno at Jupiter, and are we at the dawn of the topological age? physicstoday.scitation.org/toc/pto/73/9 Physics Today September 202...
Graphene gets bent: Two-dimensional nanomaterials are bending the rules of the papercraft known as kirigami doi.org/10.1063/PT.3.4…
Particle physicists hash out long-term strategy for Europe: Among the goals are to pick a Higgs factory, carry out R&D on accelerators and detectors, conduct feasibility studies, and improve environmental sustainability doi.org/10.1063/PT.3.4…
Read 5 tweets
Aug 3, 2020
New in the August issue of Physics Today: Sarah Frances Whiting's x-ray photography, bridging the gap between physics and biology, the warmth of wind power, and more physicstoday.scitation.org/toc/pto/73/8 Image
Sarah Frances Whiting and the “photography of the invisible”: A team of women working in the physics laboratory at Wellesley College carried out some of the first successful x-ray experiments in the US doi.org/10.1063/PT.3.4…
Does new physics lurk inside living matter? The link between information and physics has been implicit since James Clerk Maxwell introduced his famous demon. Information is now emerging as a key concept to bridge physics and biology doi.org/10.1063/PT.3.4…
Read 4 tweets

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