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What if alien civilizations were deliberately staying silent?

Liu Cixin's science-fiction novel The Dark Forest imagines civilizations hiding because contact could be dangerous. The idea offers a speculative response to cosmic silence, not an established explanation. Explore which assumptions make secrecy seem rational and whether observations could distinguish the story from competing possibilities.

3–5 min

Questions you could explore

  • Which assumptions would make staying silent seem sensible?
  • What observation could challenge a proposed explanation for cosmic silence?

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Space

A crescent Moon is not a partly switched-off Moon

Sunlight illuminates half the Moon, while its changing position around Earth changes how much of that lit half we can see. A phase is different from an eclipse, where one body blocks light from reaching another.

3–5 min

A crescent Moon is not a partly switched-off Moon

Space3–5 minSunlight illuminates half the Moon, while its changing position around Earth changes how much of that lit half we can see. A phase is different from an eclipse, where one body blocks light from reaching another.

Space

Two eclipses, two different shadows

A solar eclipse and a lunar eclipse both involve the Sun, Earth, and Moon, but the shadow falls in a different place. In one, the Moon’s shadow reaches Earth; in the other, the Moon moves into Earth’s shadow. The Moon’s tilted orbit helps explain why these alignments do not happen every month.

5–7 min

Two eclipses, two different shadows

Space5–7 minA solar eclipse and a lunar eclipse both involve the Sun, Earth, and Moon, but the shadow falls in a different place. In one, the Moon’s shadow reaches Earth; in the other, the Moon moves into Earth’s shadow. The Moon’s tilted orbit helps explain why these alignments do not happen every month.

Space

What can century-old photographs reveal about stars today?

A star photographed on a glass plate a century ago can become a new measurement when compared with later observations. Harvard's DASCH project digitizes an astronomical plate archive, alongside dates and logbooks. Explore how calibration makes those old images useful for investigating changes across decades.

8–10 min

What can century-old photographs reveal about stars today?

Space8–10 minA star photographed on a glass plate a century ago can become a new measurement when compared with later observations. Harvard's DASCH project digitizes an astronomical plate archive, alongside dates and logbooks. Explore how calibration makes those old images useful for investigating changes across decades.

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