17.4.1 - Gravity Catapults

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

It is possible to conceive of "machines" that are capable of pushing a body, using gravitational force, up to fairly impressive suboptic velocities. These devices violate no known basic laws of physics, but it difficult to see how to do the required engineering.

The most "conventional" of these, first discussed by Freeman Dyson, has been called the Contact Binary Catapult.¹⁰²³ A starship approaches a binary star system whose members are both white dwarfs orbiting each other at close range. By swinging in very near to one star on the proper trajectory, the starcraft benefits from gravitational slingshot or Newtonian "gravity whip" effect. The vessel withdraws orbital energy from the stellar pair.

Dyson estimates that a system involving two 1 Msun white dwarfs could accelerate delicate and fragile objects to a velocity of 0.7%c -- at about 10,000 gees. Since there are no engines or propellants, there are no physical stresses on the payload -- gravity acts on all parts of a material body equally. (Tidal forces, on the order of D/80 gees where D is starship diameter in meters, should not prove troublesome.)

Dyson also considered the possibility of using a pair of orbiting neutron stars as a gravity catapult. Unfortunately, they cannot exist! A neutron star binary would radiate away all of its orbital energy as gravitational radiation in less than 2 seconds. The two objects would coalesce almost immediately with a spectacular "gravity flash" at a frequency of about 200 Hz.

A more exotic technique is the Black Hole Catapult. This scheme requires that and find a rotating black hole (stellar mass) somewhere in space, then travel around it in the direction of spin very near to the equator. Besides losing a lot of time because of General Relativistic time dilation effects, some of the BH’s rotational energy would be converted to linear kinetic energy of the star ship. There would be a substantial increase in velocity. Tidal forces during transit will be rather extreme -- about 500 million gees per meter -- but apparently there are tricks with dense masses in a space vehicle that can be used to cancel the tides.²⁰¹⁴ A Neutron Star Catapult is also possible, with similar effects.¹⁰⁹⁹ The most speculative gravity machine of all is the Smoke Ring Catapult, which consists of a rotating torus of dense matter (such as neutronium) turning inside out like a smoke ring.²⁷³⁹ Relativity theory predicts a force in the direction of the rotation, so a starship fired through the center of the massive hoop could be kicked up to very high velocities -- depending upon the rotational energy of the torus.