Conducted sometime in the late 1940s or early 50s by the U.S. Army, the experiment shows radiation heat transfer occurs at the speed of light. When you cast a visible light shadow you also cast a thermal shadow, interrupting this process. This is seen in the video where the person is stopping the absorption of the sun’s heat on the wall. This along with a cooling breeze and a clear sky, allows the wall to cool. As he walks away his shadow remains. In an atomic blast where this is no natural cool

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