Here is my recently completed “Cosmic Cable” poster. You can also download a PDF version. Its first appearance is at the GR22 conference in Valencia, Spain, this week.

This work investigates the mechanics of a rigid 1-dimensional object in an arbitrary static, spherically symmetric spacetime. Others have applied such ropes / cables / strings to thermodynamics of black holes (for example scooping up Hawking radiation in a box), or to harvest energy in an expanding universe. It is an interesting exercise in topics which don’t receive a lot of attention, such as extended rigid objects in relativity.

I review the case of static cables, which show a fascinating “redshift” of force effect. I then generalise to a simple case of moving cables, solve for the kinematics, tension, and also the power than can be generated (loosely, this is from a loss of gravitational potential). In case this stuff sounds simple, it is not at all obvious, indeed many papers fail already at the kinematics step. The frame dependence of the quantities is a conceptual challenge.

If you’d like more details I have a proceedings (forthcoming) from the Marcel Grossmann conference of 2018. I will expand this into a longer paper. I would also like to write a pedagogical paper explaining Gibbons (1972) — his 1-page paper is beautifully concise, yet is hard to understand, has an error (as others have pointed out), and lots of typos.