Caltech / Cornell SXS Collaboration

This diagram shows two spiral-shaped vortexes (yellow) of whirling space sticking out of a black hole, plus the vortex lines (red curves) that form the vortexes.

No one's ever seen a black hole up close, but physicists can nevertheless visualize how two colliding black holes send ripples through space-time like waves on the ocean. They've even invented a new word — "tendex" — to describe the lines of force that stretch the objects caught in a space-time warp.

A research paper published online this week in Physical Review Letters delves into the effects of black hole collisions in unprecedented detail. "We've found ways to visualize warped space-time like never before," Caltech theoretical physicist Kip Thorne said in a news release.

Thorne and his colleagues at Caltech, Cornell University and the National Institute for Theoretical Physics in South Africa combined theory and computer simulations to describe the beautiful patterns of gravitation force lines emanating from black holes. Such lines are analogous to the invisible field lines created by electromagnetic forces.

In some scenarios, warping space-time creates swirls of force lines that twist around each other in a region of space called a vortex. "Anything that falls into a vortex gets spun around and around," said Cornell physicist Robert Owen, the paper's lead author. An astronaut falling through a gravitational vortex would be wrung out like a wet towel.

Caltech/Cornell SXS Collaboration

In this simulation, two doughnut-shaped vortexes are ejected by a pulsating black hole. Also shown at the center are two red and two blue vortex lines attached to the hole, which will be ejected as a third doughnut-shaped vortex in the next pulsation.

Tendex lines describe the stretching effect of a strong gravitational field. "Tendex lines sticking out of the moon raise the tides on the earth's oceans," said David Nichols, the Caltech graduate student who coined the term. When many such lines are bunched together, as in the surroundings of a black hole, that creates a super-stretching region called a tendex. An astronaut passing through a tidal tendex would be pulled apart like taffy — an effect sometimes known as "spaghettification."

The researchers contend that the vortex and tendex concepts can lead to a clearer understanding of black hole collision modeling. If two black holes smash into each other head-on, that creates doughnut-shaped vortexes and tendexes that emanate from the merged black hole like smoke rings. But if the black holes spiral in toward each other before merging, the field lines swirl outward like sprays of water from a rotating sprinkler.

Whether they're more like smoke rings or sprinkler jets, the force lines create gravitational waves — the kinds of waves that the Laser Interferometer Gravitational-Wave Observatory, or LIGO, has been built to detect. "With these tendexes and vortexes, we may be able to much more easily predict the waveforms of the gravitational waves that LIGO is searching for," said Caltech physicist Yanbei Chen.

The researchers suggest that the vortex-tendex model could explain a theoretical phenomenon that other physicists noticed three years ago: Using computer models, the Rochester Institute of Technology's Manuela Campanelli and her colleagues found that a black hole collision could result in a gravitational kick so powerful that the merged black hole is thrown out of its galaxy. The newly published paper proposes that gravitational waves from spiraling vortexes and tendexes are added together on one side of the black hole, but cancel out each other on the other side. The result would be a burst of waves in one direction, creating the kick.

"Though we've developed these tools for black hole collisions, they can be applied wherever space-time is warped," said Cornell's Geoffrey Lovelace. "For instance, I expect that people will apply vortex and tendex lines to cosmology, to black holes ripping apart, and to the singularities that live inside black holes. They'll become standard tools throughout general relativity."

More about space-time warps:

• Wrinkles in space-time may help identify black holes
• Famed black hole sheds light on magnetic fields
• Galactic collisions create monster black holes
• Space-time distortions help find hidden galaxies
• Black hole puts a dent in space-time
• How a black hole will kill you

For more about "Simulating Extreme Spacetimes" project, including video visualizations of black hole collisions, check out Black-Holes.org, the Caltech-Cornell collaboration's website. In addition to Owen, Thorne, Chen, Lovelace and Nichols, the co-authors of the paper in Physical Review Letters, titled "Frame-Dragging Vortexes and Tidal Tendexes Attached to Colliding Black Holes: Visualizing the Curvature of Spacetime," include Jeandrew Brink, Jeffrey D. Kaplan, Keith D. Matthews, Mark A. Scheel, Fan Zhang and Aaron Zimmerman.

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