Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravitational Propulsion

  1. Jun 13, 2012 #1
    Theoretically is it possible to propel a vessel through space by creating a wave in space time using intense amounts of light to bend space time?
  2. jcsd
  3. Jun 13, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    High energy densities will bend space-time and generally drag stuff along with it ... eg. if you are in orbit about a planet you get dragged along with the planet. So, in principle, yes.

    But I suspect you have an example in mind ... say, that if you use light for this you should get dragged along at the speed of light?

    In order for a passing planet to drag you along, it has to be going slow enough to capture you by some mechanism. The same would need to apply to the light. In principle a very dense pulse of light passing you could give you a gravitational tug as it goes by but, considering it is very fast indeed, it won't be able to drag you along. (That's not really explaining properly though - be warned.)

    Besides, you'd do better to intercept the light and use the transfer of momentum to push you along.

    All this requires the light-source to be someplace else of course.
  4. Jun 13, 2012 #3
    So could you then emit a pulse of light from the back of a capsule at a strong enough energy level to propel you forward?
  5. Jun 14, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Of course not - if your ship projected the pulse it would recoil just as hard in the opposite direction.

    You could use such an engine in the manner of a rocket though.

    Also, did you read the bit about how the light moves too fast to drag you along anyway? The pulse always goes away from you at the speed of light.

    The gravitational field due to light is a bit special but the above should be indicative without actually doing the calculation.
  6. Jun 14, 2012 #5
    Nitron, if I understand your question, you are essentially asking "can we in principle bend spacetime ahead of a ship so as to propel it"

    So Simon's answer, 'in principle', seems right on:

    when an object 'falls to earth' it is moving along a curvature in spacetime in just the way I think you are proposing.

    I wonder if one could create a higher gravitational potential behind the ship, and a lower relative potential ahead of the ship', if we could also get the ship to 'move along'....fall into that gravitational well....seems again 'in principle' that works.
  7. Jun 14, 2012 #6
    Here is a variation on the OP theme:

    Had to look up how this was titled:

    Gravity assist
    From Wikipedia, the free encyclopedia


    This has ALWAYS seemed to me 'too good to believe' but I guess it works!!! It is [loosely] a version of gravity propulsion!
    Last edited: Jun 14, 2012
  8. Jun 14, 2012 #7
  9. Jun 17, 2012 #8
    Thank you for the responses. Yes that is precisely what I am talking about. It would seem that if we could create a pulse strong enough to bend space-time in the manner showed by Antiphon's link, it could create a wave in space-time that would propel a ship forward. It seems that it could be a very reliable source of propulsion in the vacuum of space where combustion engines become complicated employing the use of oxidizers and fuels.
  10. Jun 17, 2012 #9
    The problem is creating the expanding part of space-time. It is not possible to do that with ordinary mass-energy (not with light or anything else with positive mass-energy). It is known that to create such a bend in space-time requires mass-energy that violates the averaged null energy condition (Wikipedia- Energy condition). There is nothing that is known to violate this energy condition, but theoretically such things could exist (Arxiv paper on the theoretical violation of said energy condition).

    So while, in principle, it may be possible to construct an Alcubierre drive, it cannot be done with light alone.
  11. Jun 19, 2012 #10

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    ... even if you did get the exotic matter - you'd still need to get the pulse to propagate ... so it is still more like firing your pulse from something and hitching a ride.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook