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Laser-controlled clocks can already use thorium-229 nuclei inside crystals. Can their environmental shifts be measured and controlled well enough to establish a complete fractional-frequency uncertainty near 10^-18?
The first flash of a long gamma-ray burst reveals an ultra-relativistic jet releasing energy. Which radiation processes produce the photons, and how do their contributions change during the burst?
Why did the dominant rhythm of ice ages change about a million years ago?
What drove Greenland’s repeated rapid warmings during the last ice age?
What controlled the rise and fluctuations of atmospheric oxygen about 2.4 billion years ago?
Which carbon sources triggered and amplified the rapid warming about 56 million years ago?
Which erosion histories produced the huge gaps beneath many younger sedimentary rocks?
What produces unusual low-frequency seismic signals deep beneath volcanoes?
How quickly do water and iron-bearing mantle rocks generate hydrogen underground?
How much water remained in the Mediterranean during the crisis’s final freshwater-influenced stage?