Hi, this is Kate from MinuteEarth.
In 1960, scientists blew up hundreds of pounds of explosives off the coast of Australia;
nearly four hours later, sounds from the blasts reached underwater microphones near Bermuda
- over 19,000 km away. In the air, these sounds would have traveled a few dozen kilometers
at most. And sure, sound generally travels a b it farther underwater because sound waves
don’t lose as much energy moving through water as they do through air...but not hundreds
of times farther. These sounds traveled halfway around the world thanks to an underwater sound
superhighway. That superhighway exists because sound travels
at different speeds in different layers of the ocean. The temperature of water - and
to a lesser extent, its pressure - affect how densely packed its molecules are and how
rigidly they are connected to each other - the two factors which determine the speed of sound.
At the surface, where the water is generally warm - sound moves pretty quickly, but with
increasing depth, the temperature plummets, drastically slowing the speed of sound. Around
a thousand meters or so down - depending where on the planet you are - the ocean’s temperature
bottoms out, and the effect of pressure takes over, causing the speed of sound to increase
again. But here’s where it gets weird, because
that layer of the ocean where the speed of sound is slowest is where sounds can travel
the farthest. If you’ve ever skied, you may get why; say you are speeding along on
a packed trail, then you start drifting into a much slower stretch of powder. The ski in
the powder slows down, turning you and pulling you farther into the fluffy stuff, toward
the slower direction of travel. And if you reach the other side of the powder and hit
another packed trail, your outside ski speeds up, turning you back into the pokey powder.
Sound waves are a lot like skiers; if they enter a layer of water where sound travels
particularly slowly, at just the right angle - or if they start out there in the first
place - they can get stuck in that slow layer, bending up and down and up and down. So, rather
than spreading out and scattering off the surface or getting absorbed by the ocean floor,
sounds in this layer get funneled along, and can go and go and go.
This layer - appropriately called the “SOFAR channel” - may be a weird quirk of physics,
but it’s also really useful. Based on whales’ behavior and calls, scientists think some
whales use the SOFAR channel as a reeeally long-distance telephone. And actually so do
we; monitoring systems in the SOFAR channel can detect sounds all over the ocean, from
the breaking up of ice shelves to supposedly secret nuclear tests. What’s more, using
this infrastructure, we can calculate how fast sounds move through this layer of water;
changes in the speed of sound in the SOFAR channel can help us track changes in the temperature
of the ocean - a critical measure of our planet’s health. And that is SO FAR out.
The Comprehensive Nuclear-Test-Ban Treaty Organization, or CTBTO, uses a network of
11 hydrophone triplets – anchored from the seabed - into the SOFAR channel to listen
in on our oceans. It’s part of their effort to collect and study sounds across the planet
to figure out which sounds are nuclear tests, and which are, for instance, simply the long-distance
calls of lonely whales. This work has been mandated by more than 180 member states, and
the data that is collected worldwide is available to researchers who want to further expand
our knowledge about our planet. If you’re interested, please contact ctbto.org. Thanks
to CTBTO for sponsoring this video, and for helping keep the world safe from nuclear weapons.