The Long-Delayed Radio Echoes
In 1927, a Norwegian engineer heard his radio signal come back to him three seconds after it was sent. Nearly a century later, no one is sure why.
What was reported
Radio signals bounced around the Earth by the upper atmosphere normally return as an echo about a seventh of a second later. In 1927, Norwegian engineer Jørgen Hals noticed something stranger: a second, fainter echo arriving about three seconds after the signal. He wrote to the physicist Carl Størmer, who published the finding in the journal Nature.
Experiments in 1928 recorded echoes delayed by up to 30 seconds. They are now called long-delayed echoes, and amateur radio operators still occasionally report them.
Explanations offered
About 15 explanations have been proposed, including signals trapped and slowed in layers of charged particles high above the Earth, or reflected far out in space. A writer in 1973 even suggested they came from an alien probe. None has been confirmed, and the echoes remain a puzzle for radio physicists.
- When
- First reported in 1927
- Where
- Oslo, Norway
- Witnesses
- Radio engineers and amateur operators
- Status
- Unexplained
Famous cases are real, widely reported events. Each file sets out what witnesses said and the explanations that have been offered, without taking a side.