Thursday, June 16, 2016

Gravitational Waves Explained: Feynman's "Sticky Bead"

With yesterday's report from the LIGO collaboration indicating that they had observed a second black hole merger event, the 'net is once again abuzz with talk of gravitational waves, but some of you might still be struggling to understand precisely what a gravitational wave is, and what's so significant about it. Fortunately, way before anyone even dreamed of building a project as ambitious as LIGO or LISA, famous physicist Richard Feynman came up with an excellent way to explain gravitational waves.
Feynman called his thought experiment the "Sticky Bead" argument, and it goes something like this:

Imagine you have two sticks, fixed in place, oriented parallel to one another, and separated by a great distance. On one stick, there is a small bead which can slide freely without friction. On the other stick, there is a large bead made of extremely dense matter—let's say neutron star, to give it a nice heft.

You take the neutron star bead and slide it to one end of the stick. In the absence of other forces, your small bead on its parallel stick will feel the attraction from the large bead, and slide to the same end of its own stick, trying to get as close as possible to the neutron star bead. If you slide the neutron star bead back to the opposite end of the stick, the small bead will eventually make its way to the other side as well. The fact that one object can influence another in this way is solid evidence that the force of gravity carries information with it—in this case, it's information about the location of the neutron star bead.


This information, like everything in our universe, travels at a finite speed—specifically, the speed of light. The fact that transmission of this information is not instantaneous offers another clue that there's some kind of carrier-wave phenomenon occurring here.

Feynman, shown here, submitted the "Sticky Bead" argument
anonymously, in part to ensure it received unbiased consideration.
Image Source: Richard-Feynman.net
The most important part, though—the part that drove debate among brilliant minds for years—is that this wave necessarily carries energy, and this is where things get "sticky". Imagine your small bead has some friction to it—that it offers some resistance to sliding, but not enough to prevent it from moving entirely. Now, we move the neutron star bead from one end of the stick to the other. As the small bead begins to follow, it must overcome the frictional force against the rod, and—as everyone knows—where there's friction and motion, there's heat!

With this simple thought experiment, Feynman put to rest the question of whether gravitational waves could carry energy, effectively ending the debate. A discussion that had previously centered around tensors and pseudotensors—complex mathematics impenetrable to anyone without training in advanced math and physics—suddenly became a discussion about beads and string which anyone could grasp, one with an obvious resolution: if the motion of the massive bead can heat up the smaller bead's rod from a great distance, even a tiny amount, there's energy being transferred via gravity.

This kind of thinking is part of why, despite the controversy surrounding his personal life, Feynman continues to be regarded as one of the greatest physicists of the modern day, and it's a testament to the power of unconventional thinking.

1 comment:

  1. It's worth noting that this "stickiness" would no doubt be interpreted by other physicists as both heat and wave radiaton loss (just like dipoles in electrostatics).

    Also, I liked your more complicated take on the Feynman industry. I had the luck to meet him and take a class from him. He still came to house dinners into the 80s... so I can personally say he was womanizing until he succumbed to cancer.

    From personal experience Gell-man can also be a self-important jerk... and, along with his second wife, has always had a bit of a chip on his shoulder relative to Feynman.

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