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Volume 602 Issue 7897, 17 February 2022

Seeing red

Einstein’s general theory of relativity predicts that Earth’s gravity will sufficiently distort space-time so that clocks at different distances from the planet will tick at different rates — an effect called gravitational redshift. In this week’s issue, Tobias Bothwell, Jun Ye and their colleagues demonstrate this effect at a sub-millimetre scale. The researchers use a cloud of ultracold strontium atoms, effectively creating a series of atomic clocks. As illustrated on the cover, the atoms are trapped in pancake-like optical traps and then interrogated by a laser. The team was able to observe a linear change in frequency — the redshift — from one side of the cloud to the other, showing that each atomic clock was ticking at a slightly different rate.

Cover image: Ella Marushenko & Kate Zvorykina (Ella Maru Studio, Inc.)/Ye labs, JILA

This Week

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    • The torus of dust surrounding a quasar — a very luminous supermassive black hole that accretes matter from its surroundings — has now been captured with high-resolution infrared imaging.

      • Robert Antonucci
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    • Tests of relativity once required accurate clocks separated by thousands of kilometres. Optical techniques have now made such tests possible in an atomic cluster measuring no more than one millimetre in size.

      • Ksenia Khabarova
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  • Perspective

    • The concept of 'Embodied Energy'—in which the components of a robot or device both store energy and provide a mechanical or structural function—is put forward, along with specific robot-design principles.

      • Cameron A. Aubin
      • Benjamin Gorissen
      • Robert F. Shepherd
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