A Wave-Tracking Experiment | Hacker News Reader

A Wave-Tracking Experiment

npr.org

103 pointssrikar20 comments

Anyone familiar with standard wave mechanics will expect that crossing waves will pass right through each other. The part that confuses me is why the waves don't spread out laterally. For instance, take the waves in the article that went from the Indian ocean to the Mexican coast. Wouldn't they fan out across the entirety of the Pacific ocean after passing between Australia and Antarctica? Or maybe they do, but they're so big to begin with that even after they do so, they're still big enough to crash on the entire west coast of the Americas? Would the same storm have caused gigantic waves to crash on the much closer southern Australian shore?

It may not answer your question directly, but the following is a nice write-up: http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

"Water waves are an example of waves that involve a combination of both longitudinal and transverse motions. As a wave travels through the waver, the particles travel in clockwise circles. The radius of the circles decreases as the depth into the water increases."

The animation is mesmerizing.

Here's another visualization, which let's you play with the wave parameters. http://web.mit.edu/~njwilson/www/waves.html

Good visualization of what any aware surfer/bodyboarder/bodysurfer/etc. has likely noticed.

That's a cool animation, and I've seen it before, but I'm talking about the waves spreading out in the 3rd dimension, horizontally along the length of the wave (i.e. perpendicular to the screen in that animation).

You mean like ripples in a pond? Yes, that does happen for both deep and shallow water waves. And yes, the wave power would be expected to spread laterally, causing the amplitudes observed at a point to be smaller further from the source.

Here's a model visualization of the tsunami set off by the 2004 Sumatran earthquake. http://youtu.be/46ovp1rZeL4

Flying you can see when strong winds blow from land. The sea is flat at the beach. But just 3 or 5 miles inland waves can be 1 meter high or more. It's amazing how fast they go from milimeters to meters.

It's also possible to see how the wind blows differently, when guided by hills and valleys. Just like pointing a huge hair dryer to a bathtub.

Edit: typos

I'm having trouble visualizing what you're talking about here. What do you mean by inland waves?

I think he mean't offshore waves

So, what's the bandwidth and latency?

Who is doing the work tracking waves today? This reminds me of the butterfly flapping his wings of fractal fame. In this case though, they are tracking the whole path.

NOAA and a lot of secondaries using their data: surfline, magicseaweed, swell3d, etc.

This is so broken on mobile that I cannot scroll down to read the article on latest Chrome on iOS.

Works fine in mobile IE

> "The astonishing thing is, you'd think it would bump into a million other waves that are coming at it from every direction; that it would pass through other storms, spreading, bumping, traveling, that all this travel would sap its momentum. But, as Walter Munk would discover, that's not what happens."

Kinda weird that the article spends so long explaining high-school physics, and in such astonished terms. Zomg! Waves pass through each other! The Superposition principle! If they think that's cool, they should see some of the stuff you can do with light and sound.

Many Americans don't take physics in high school. I didn't and I'm a computer science phd student (so it's not that I wasn't intelligent enough to take it). I was surprised to learn that waves coming from different directions just pass through each other. I did take physics in college, from a guy who could barely speak English.

If this is true (and I have no evidence one way or the other), then surely this is the bigger surprise (coming from an environment where science is mandatory to middle-school equivalent, and at least one science subject almost un avoidable before university level.

>coming from an environment where science is mandatory to middle-school equivalent, and at least one science subject almost un avoidable before university level.

School districts are ran locally and the curriculum is usually set at the state level, so there are differences, but 3 years of science is mandatory in middle school in every state I've ever heard of. In addition, almost all American high schools require 3 or 4 years of science for university bound students (maybe a bit less for non college bound students but usually not less than 3 years).

At my high school, in a not very big town in Georgia, everyone took physical science, chemistry, and biology. However, you had choices for senior year--physics, AP physics, AP chemistry, ecology or anatomy (you could take more than one if you wanted).

If you consider them travelling self-sustaining standing wave packets is it still totally confusing? There are many examples of self-contained resonating bodies travelling through medium without interfering with each other in physics. Usually only in electromagnetic radiation though. Harmonics, induction, and current aren't excluded from sound and water waves. Not sure why we shouldn't see other analogies to draw from micro and quantum principles.

> Kinda weird that the article spends so long explaining high-school physics

Physics curricula on waves work like this: in high school or early college you learn about noninteracting waves because they're simple. Then, once you can handle that, you're introduced to waves that do interact with their surroundings. Electrons scatter off of imperfections in conductors, emit/absorb photons, etc. Sound, light, and current/voltage waves scatter off of impedance changes and nonlinearities of certain types. The general intuition is that disruptions in the wave medium lead to nontrivial behavior. Depth, temperature, wind, and currents all cause behavior that isn't modeled by high-school physics.

The fact that these higher-order terms ultimately don't matter much in this specific scenario is surprising in light of the fact that the medium through which ocean waves propagate has an abundance of disruptions that could plausibly make the behavior more complex. The fact that someone unaware of the higher-order terms could conceivably blunder accidentally into the correct explanation isn't impressive or enlightening. The analysis which determines that the simple model actually works well in this scenario is both.

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