Long delayed echos
Radio waves travel at the speed of light. In fact they are, in effect, very low frequency light. Or to put it
another way - visible light is just very high
frequency radio waves. Our eyes are receivers that work on very high frequency radio waves.
Light travels 299,792,458.8 metres per second or 300,000 km per second (186,000 miles per second) in a vacuum.
This means a radio wave can reach the other side of the world in a fourteenth of a second (0.071), and go completely around the world in a seventh of a second (0.142).
If you had enough power and the conditions were right, when you transmitted, you would hear the "echo" of your
signal a seventh of a second later as it travelled around the world and came back to you. If it had enough
power, it might go round a few times, and you might even hear it twice or three times.
I've never heard my own echo. It's hard enough to make the signal go halfway around the world!
The moon is between 356,500 and 406,700 km away. For a radio wave to travel from the earth, to the moon, and
back again that works out to about 2.7 seconds. You can see this in this video of Fay Lovsky singing to the moon.
It's actually quite hard to do this. You need a lot of power, a big dish, and sensitive receivers. A lot of the signal is lost on the way to the moon, and as the moon is not a very reflective surface as far as radio waves go, very little is reflected, and again, most of that small amount of reflected signal is lost on the way back to the earth.
Even longer delays
What's odd though, is that since the start of amateur radio, operators have reported echos longer than 2.7
seconds (roughly the length of time for moon-bounce). The longest have been 40 seconds. Radio signals would
travel 12,000,000 km in that time.
This has puzzled amateur radio operators and scientists. Where were the radio signals going? Were they bouncing back from something or being "stored" somewhere?
There are currently 5 hypotheses for long delayed echos:
- Ducting in the Earth's magnetosphere and ionosphere at low HF frequencies (1–4 MHz)
- Signals travelling many times around the world.
- Mode conversion: Signals couple to plasma waves in the upper ionosphere.
- Reflection from distant plasma clouds coming originally from the sun.
- Non-linearity in addition to mode conversion. Two transmitted signals combine to generate a difference frequency, which travels with a plasma wave, then is converted back.
Whatever the reason, it's some pretty freaky occurrence.
I look forward to the day though when I have a good enough station to hear my own echo.