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Redshift / blueshift of light in a gravity well

  1. Dec 12, 2012 #1
    I appreciate that as light approaches (say) a star, the light is blueshifted by gravity, and that as it leaves the area of the star, the light is redshifted as it climbs out of the gravity well. However, given that spacecraft execute gravity assist manoeuvres to increase / reduce speed, does light also get a positive or negative “jolt” such that the blue and red shift does not cancel exactly?

    (I appreciate that, if it happens, the magnitude the jolt would depend on the specific trajectory.)


  2. jcsd
  3. Dec 12, 2012 #2


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    A gravity assist happens because the gravitational source, usually a planet, is moving. The spacecraft steals a tiny bit of the planets orbital momentum. Stars move also, but I am not sure if light can take momentum away from the planet or star. I wonder, would reflection off a moving object be similar to a gravity assist? I only bring that up because of the ball bouncing off the train picture they use in the wiki article below.

    See here: http://en.wikipedia.org/wiki/Gravity_assist
  4. Dec 12, 2012 #3
    Thanks Drakkith. (In relation to the gravity assist, I appreciate that the numbers involved are very small, but) Am I right in saying that the spacecraft changes velocity while the planet moves to a different orbit?

    Do you know if the same would apply to objects with peculiar velocities (i.e. assuming that the various vectors matched-up, could I use a slingshot around a rogue planet / star, that is not orbiting … anything, to obtain the boost / brake – I assume that it does and that there is a negligible impact on the trajectory of the rogue)?


  5. Dec 12, 2012 #4


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    If it is moving relative to something else you can.
  6. Dec 13, 2012 #5
    Thanks Drakkith.


  7. Dec 13, 2012 #6
    If light aproaches a matter-antimatter bomb, which explodes when the light has blueshifted, then the light keeps half of the blueshift.

    If light aproaches an object, which is pulled away at nearly speed of light, when the light has blueshifted, then the light does half of the work of separating the light and the object, and loses half of the blueshift.

    If we ask a large object, that does not do any sudden movements, about the speed change of a passing small object, the large object says: "the small object approched me at some speed, and left at that same speed".
  8. Dec 13, 2012 #7


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    er, that is nonsense.
  9. Dec 13, 2012 #8


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    If the gravitational potential well changes as the light ray passes through it, then yes, the light can pick up a total redshift or blueshift. This actually happens at very large scales, as dark energy makes it so that the gravitational potential wells for very large galaxy clusters get shallower over time, so that the light rays going into them don't redshift quite as much on the way out as they blueshifted on the way in. Underdense regions of the universe have a similar but opposite effect.

    This is known as the Integrated Sachs-Wolfe Effect.
  10. Dec 13, 2012 #9
    Thanks Chalnoth. That (Sachs-Wolfe effect) is actually on my list of things to get a better understanding of!


    Last edited: Dec 13, 2012
  11. Dec 14, 2012 #10
    Also the plain old slingshot effect works with light. The deflection is small though, unless we use the athmosphere of the planet to deflect the light.
  12. Dec 14, 2012 #11
    Thanks Jartsa. I assume that the "atmosphere" proportion is as a result of a different process. Is that correct?


  13. Dec 14, 2012 #12

    A "photon rocket" using "powered slingshot effect":

    The rocket approaches a planet, at the nearest point it fires its engines. Because the speed of the rocket is high, the increase of kinetic energy is large, this is matched by a large redshift of the propellant (photons).

    Let's say the spacecraft carries photons as a cargo. These photons become blueshifted. If the planet lost kinetic energy in this process, then part of that energy went into the cargo photons.

    EDIT: Oh yes, I could avoid the silly "cargo photons" by saying that the "photon rocket" uses the "powered slingshot effect" in order to lose a lot of kinetic energy. The propellant (photons) gains all this energy, assuming the planet's energy did not change.
    Last edited: Dec 14, 2012
  14. Dec 14, 2012 #13
    Well, looks like same process to me:

    A: Light experiences an elastic collision with the athmosphere of a planet.
    B: Light experiences an elastic collision with the gravity field of a planet.

    Although there is a slight difference in the details of A and B.
    Last edited: Dec 14, 2012
  15. Dec 14, 2012 #14
    Ah! Just like craft / body bouncing off the upper atmosphere?


  16. Dec 15, 2012 #15
  17. Dec 16, 2012 #16
    I was thinking about this and I can see how the "bounce" would produce a change in velocity for objects travelling at less than c, but would it produce a red / blue shift in light as well?


  18. Dec 17, 2012 #17

    Sure it produces. All people say that policeman's radar measures the Doppler shift of microwaves that bounce back from an approaching car.

    Most people say that a scientist's Doppler radar measures the Doppler shift of microwaves that bounce back from an approaching black hole. (black holes scatter some microwaves back)

    I say that the microwaves experience compression when slowing down while entering the gravity field, and expansion when speeding up while leaving the gravity field, and compression and expansion are unequal when the gravity field is moving.
  19. Dec 17, 2012 #18
    Understood. Thanks Jartsa.


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