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WonderScience fact

Strong gravity makes time pass more slowly, an effect called gravitational time dilation. Near a black hole it becomes extreme: a clock close to the edge would tick far more slowly than one far away, and to a distant watcher, a clock falling in would seem almost to stop.

Physics

Science

The story behind the card

Albert Einstein first predicted the effect in 1907, and it is part of his general theory of relativity of 1915. It happens on Earth too, only on a tiny scale. In 1971 physicists flew atomic clocks around the world on airliners and measured the small differences from clocks on the ground that relativity predicted, and laboratory clocks can now detect the effect over a height difference of less than a metre. GPS satellites depend on it: mostly because gravity is weaker at their height, their clocks gain about 38 millionths of a second a day, and without correction the positions they give would drift by around 11 kilometres a day. A black hole's edge, the event horizon, is the boundary beyond which nothing, not even light, can get out; seen from far away, the slowing of time becomes total there.

Türkçe oku
Güçlü kütleçekimi zamanın daha yavaş akmasına yol açar; buna kütleçekimsel zaman genişlemesi denir. Kara deliğin yakınında etki aşırı boyutlara ulaşır: Sınırına yakın bir saat, uzaktaki bir saate göre çok daha yavaş işler; uzaktan bakan biri, içeri düşen bir saatin neredeyse durduğunu görür.

Einstein bu etkiyi ilk kez 1907'de öngördü; etki, 1915'te ortaya koyduğu genel görelilik kuramının da bir parçasıdır. Dünya'da da görülür, yalnızca çok küçüktür. 1971'de fizikçiler atom saatlerini yolcu uçaklarıyla dünyanın çevresinde dolaştırdı ve yerdeki saatlerle aralarında göreliliğin öngördüğü küçük farkları ölçtü. Bugün laboratuvar saatleri, bir metreden az yükseklik farkında bile bu etkiyi yakalayabiliyor. GPS uyduları bu etkiyi hesaba katmak zorundadır: Uydulardaki saatler, çoğunlukla o yükseklikte kütleçekimi daha zayıf olduğu için, yerdekilere göre günde yaklaşık 38 mikrosaniye ileri gider. Bu düzeltilmeseydi verdikleri konum günde 11 kilometre kadar kayardı. Kara deliğin sınırı olan olay ufku, ışık dâhil hiçbir şeyin dışarı çıkamadığı sınırdır; uzaktan bakıldığında zamanın yavaşlaması orada tamamlanır.

Sources and further reading

Background reading: Black hole
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