The History:
During World War II, American pilots were given hollow metal spheres
to be used when their planes went down over water. Metal, as you
probably know, tends to be quite heavy and quite useless as a flotation
device. So why were these pilots given chunks of metal? Well, they were
told that if they dropped the sphere into the ocean, their current
position could be worked out by allies in the region, and they would be
rescued. However, as foreign intelligence was soon to find out, the
spheres were just that – hollow metal spheres – no electronics, no
radio equipment, no imbued magical properties. So was the military just
playing with it’s pilots, or was there actually something to these
spheres?
Well, some of the details are still kinda top secret, but with a
little physics and an active imagination, it’s possible to work out how
they were used. Arrgh! Scary! I just said the P word. Don’t worry, I’ll
try to summarize things without delving to deep into the physics.
Simple Facts: The Ocean
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The deeper you go, the colder it gets
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The deeper you go, the higher the pressure
Simple Facts: Soundwaves
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Sound travels in waves
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These waves bend towards where the sound travels _slower_
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Sound travels faster in warm temperatures than cold temperatures
Combining the facts:
gradual as you go deeper; however, there is a zone (at around 500m)
where the temperature change is quite rapid. Therefore, any soundwaves
entering this area, will be bent down. Once you are past this zone, at
around 1km deep, the pressure becomes the dominant effect, and sound
will bend upwards.
these two areas of bending, it gets trapped, and ends up travelling
great distances. This is called the sound channel:
gets deflected somewhat, but doesn’t bend back enough to get trapped.
Note also, that the lines here show the path of the wave, not the wave
itself. View each line as a zoomed out version of the wave pictures
above.
Keep your eye on the sphere:
So, the spheres that the pilots were dropping into the ocean were of
a specific thickness that would be crushed by the ocean pressure at
about 1km deep. This would cause a “ping” that could be detected
by underwater microphones thousands of miles away. By triangulating the
sound (kinda like how GPS works) the Navy was able to work out where
the sphere was dropped and go rescue the pilot!
Implications – LOFAR:
I don’t believe it – I am about to recommend a Tom Clancey novel, The Hunt for the Red October. Tom goes into details as to how the US Navy took this technology and
used it to detect submarines that entered the sound channel. I’d
totally recommend buying it here for those of you who have enjoyed
reading this posting.
Implications – Nature:
It turns out that nature beat us to the punch. Whales have been using
the sound channel for years to communicate with each other over long
distance via Whale Song!
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