The Philadelphia Experiment is an alleged military experiment that is said to have been carried out by the U.S. Navy at the Philadelphia Naval Shipyard in Philadelphia, Pennsylvania some time around October 28, 1943. The U.S. Navy destroyer escort USS Eldridge (DE-173) was claimed to have been rendered invisible (or "cloaked") to enemy devices.
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The experiment was allegedly based on an aspect of the unified field theory, a term coined by Albert Einstein; the theory aims to describe — mathematically and physically — the interrelated nature of the forces that comprise electromagnetic radiation and gravity, in other words, uniting the fields of electromagnetism and gravity into a single field.
According to some accounts, unspecified "researchers" thought that some version of this field would enable using large electrical generators to bend light around an object via refraction, so that the object became completely invisible. The Navy regarded this of military value and, by the same accounts, it sponsored the experiment.
Another unattributed version of the story proposes that researchers were preparing magnetic and gravitational measurements of the seafloor to detect anomalies, supposedly based on Einstein's attempts to understand gravity. In this version, there were also related secret experiments in Nazi Germany to find anti-gravity, allegedly led by SS-Obergruppenführer Hans Kammler.
There are no reliable, attributable accounts, but in most accounts of the experiment, the USS Eldridge was fitted with the required equipment at the Philadelphia Naval Shipyard. Testing allegedly began in the summer of 1943, and it was supposedly successful to a limited degree. One test allegedly resulted in the Eldridge being rendered nearly invisible, with some witnesses reporting a "greenish fog" appearing in its place. Crew members supposedly complained of severe nausea afterwards. Also, it is said that when the ship reappeared, some sailors were embedded in the metal structures of the ship, including one sailor who ended up on a deck level below that where he began and had his hand embedded in the steel hull of the ship as well as some sailors who went "completely bananas." At that point, it is said that the experiment was altered at the request of the Navy, with the new objective being solely to render the USS Eldridge invisible to radar. None of these allegations have been independently substantiated.