Entertainment | Science

The Miracle Of The Levitating Slinky

NPR | Sept. 11, 2012 3:57 a.m.

Contributed By:

Robert Krulwich

I should say right off, this is no miracle. The slinky I’m going to show you does what all slinkies do, even if it seems so astonishing, you figure, “Oh, come on. Somebody doctored this footage, this can’t be.”

It can be. It is. Nobody manipulated anything.

Here’s what’s going to happen. Derek Muller from the Australian science video website Veritasium is going to take a slinky and hold it from the top with his hand. He will then release the lower part. It will slink down to its full extension, elongating, and come to a dangling rest.

Then Derek is going to let the slinky go.

Now comes the miracle. If you keep your eye on the bottom of the slinky, on the last curl at the very end, you will notice that as the top of the slinky starts to fall, the bottom DOESN’T DROP. It just hangs in the air, levitating, as if it had its own magic carpet. It will stay there, hovering quietly, until a wave, or signal, passing through the slinky finally reaches it. Apparently, the bottom doesn’t know it’s supposed to fall, so it sits there, seeming to defy gravity, until the very end.

In this first video, Derek and a physics professor named “Rod” will drop a slinky. It drops very fast, but then they run the footage at 300 frames a second – that’s very slowly. Once again, this is not a trick. It’s just deeply mysterious.

Derek asked, what if we attach a tennis ball to the bottom of the slinky, then drop them both? Will the tennis ball hover, too? Or will it drag the slinky down? (Or up?) Here’s the answer:

So why does this happen? Why — going back to the slinky — why doesn’t it fall right away?

Psst, Pass It On

As the physics professor says, the bottom didn’t “know” it was supposed to fall until it got a signal from the top of the slinky. Only when the metal directly above the last coil transmits the signal, only then do you see the bottom coil start to twist and fall.

This idea, that information has to pass through an object for the whole thing to know what to do, applies not just to slinkies, but to ballpoint pens, logs, arrows, and yes, to all of us.

This week on our Radiolab podcast, we invited Cornell mathematician Steve Strogatz and Hayden Planetarium astrophysicist Neil deGrasse Tyson to dissect the miracle of the levitating slinky. Steve finds the physics beautiful, profound, even tragic. Neil is more the mechanic. Signals rule the world, he says. He explains that what happened to our slinky has become the essence of tank warfare. What? If you’re curious, check out the podcast (if you’re reading this on your computer, you can click on the Radiolab box just to the right).

Or, if you seek a zen moment, and wish to contemplate more slinkies not falling, here’s another video; this time the slinky is huge. Doesn’t seem to matter.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.

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