3
$\begingroup$

Hawking's radiation emits particles from a black-hole.

Lets play with the idea of harnessing a mini black hole in the center of a star-ship. Let alone harnessing the gravity pull to the front and move the emitting particles out the back, (creating a Push-Pull effect).

Would the small black-hole be able to create enough gravity + emit enough particles to actually move the star-ship?

And would it be effective*?

(*Effective is define by actually reaching interesting speeds like at least 1/4 & 1/2 C)

$\endgroup$
8
  • $\begingroup$ The force of radiation would be negligible to the force of gravity. $\endgroup$
    – Jimmy360
    Commented Jun 4, 2015 at 12:01
  • $\begingroup$ Also, remember Newton's third law. $\endgroup$
    – Jimmy360
    Commented Jun 4, 2015 at 12:04
  • $\begingroup$ C is a unit for speed? What does it mean? $\endgroup$
    – Vincent
    Commented Jun 4, 2015 at 14:05
  • 2
    $\begingroup$ @Vincent, "c" means the speed of light in a vacuum. $\endgroup$ Commented Jun 4, 2015 at 14:52
  • $\begingroup$ Doesn't hawking radiation occur when the black hole absorbs new matter? In other words, you still need propellant, so it all depends on the efficience of that process (compared to doing more traditional things to that propellant). $\endgroup$ Commented Jun 5, 2015 at 7:18

1 Answer 1

5
$\begingroup$

Turns out there's a paper

http://arxiv.org/pdf/0908.1803.pdf

By these 2 guys

https://www.phys.ksu.edu/personal/westmore/

http://www.math.ksu.edu/people/personnel_detail?person_id=1330

I'm willing to trust someone who's phd dissertation was called "Optical black holes and solitons" on the matter as qualified to answer.

They outline the design for a ship and roughly outline the theoretical machines needed to produce small black holes.

tl;dr: Yes it is possible but you need to point particle beams at the black hole both to keep it's size stable and to control it's position.

Design requirements for a BH starship

  1. use the Hawking radiation to drive the vessel
  2. drive the BH at the same acceleration
  3. feed the BH to maintain its temperature

Item 3 is not absolutely necessary. We could manufacture a SB H, use it to drive a ship one way, and release the remnant at the destina tion. However this would limit us greatly as to performance, and be very dis appointing in the powerplant application discussed below.

We shall discuss these three problems in outline only here; a t the level of engineering they will each require an extended discussion. It is not hard to see how we might satisfy requirement 1. We sim ply position the SBH at the focus of a parabolic reflector attached to the bo dy of the ship. Since the SBH will radiate gamma rays and a mix of particles an d antiparticles, this is not simple. The proposal has been made in the context o f antimatter rockets, to make a gamma ray reflector out of an electron gas [11].

It is not clear if this is feasible (e.g., [2]).

Alternatively, we could allow the gamma rays to escape and di rect only the charged particle part of the Hawking radiation (cf. [2]), al though this produces a less capable ship. To improve the performance, we could add a thick layer of matter which would absorb the gamma rays, reradiate in opt ical frequencies, and focus the resulting light rays. An absorber which stops o nly gamma rays heading towards the front of the ship and allows the rest to es cape out the back causes gamma rays to radiate from the ship asymmetrically. I n this way, even the escaping non-absorbed gamma rays contribute some thrus t (cf. [12] or [13]). Modulo safety concerns, one would not want the absorber to be too massive. An extremely massive absorber could burden the mass of vehic le so much that the extra thrust it helps to deliver does not lead to an improv ed acceleration.

Yet another idea for the utilization of gamma ray energy is to exploit pair production phenomena. By interacting with the electric fiel d of atomic nuclei, high energy gamma rays can be converted into charged particl e-antiparticle pairs such as electrons and positrons. These particles can b e directed by elec- tromagnetic fields. It is not likely that even half of the gamm a ray energy can be utilized in this manner however (see Vulpetti [14], [15]).

It might be advantageous to use the Hawking radiation to ener gize a sec- ondary working substance which can then be ejected as exhaus t (as is done in thermal and ion rockets). However, the working substance mu st be ejected at 10 relativistic speeds so that the specific impulse will be high enough for interstellar travel.

The most optimistic approach is to solve requirements 2 and 3 together by attaching particle beams to the body of the ship behind the BH and beaming in matter. This would both accelerate the SBH, since BHs “move w hen you push them”(see [3] p270), and add mass to the SBH, extending the li fetime.

The delicate thing here is the absorption cross section for a particle going into a BH. We intend to investigate this question in the futur e. If simply aiming the beam at the SBH doesn’t work, we can try forming an accreti on disk near the SBH and rely on particles to tunnel into it. Alternativel y, we could use a small cluster of SBHs instead of just one to create a larger e ffective target, charge the SBH etc. It is also possible that because of quantu m effects SBHs have larger than classical radii, due to the analog of zero po int energy. This point must remain as a challenge for the future.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .