# Redshift / blueshift of light in a gravity well

by Lino
Tags: blueshift, gravity, light, redshift
 P: 297 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.) Regards, Noel.
 Mentor P: 12,005 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
 P: 297 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)? Regards, *Noel.
 Mentor P: 12,005 Redshift / blueshift of light in a gravity well If it is moving relative to something else you can.
 P: 297 Thanks Drakkith. Regards, Noel.
P: 483
 Quote by Lino 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.) Regards, Noel.
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".
PF Gold
P: 9,488
 Quote by jartsa 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".
er, that is nonsense.
P: 4,837
 Quote by Lino 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.) Regards, Noel.
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.
 P: 297 Thanks Chalnoth. That (Sachs-Wolfe effect) is actually on my list of things to get a better understanding of! Regards, Noel.
P: 483
 Quote by Chalnoth 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.
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.
P: 297
 Quote by jartsa 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.
Thanks Jartsa. I assume that the "atmosphere" proportion is as a result of a different process. Is that correct?

Regards,

Noel.
P: 483
 Quote by Drakkith 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

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.
P: 483
 Quote by Lino Thanks Jartsa. I assume that the "atmosphere" proportion is as a result of a different process. Is that correct? Regards, Noel.
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.
P: 297
 Quote by jartsa ... A: Light experiences an elastic collision with the athmosphere of a planet ...
Ah! Just like craft / body bouncing off the upper atmosphere?

Regards,

Noel.
P: 483
 Quote by Lino Ah! Just like craft / body bouncing off the upper atmosphere? Regards, Noel.
Yes.
 P: 297 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? Regards, Noel.
P: 483
 Quote by Lino 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? Regards, Noel.

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.
 P: 297 Understood. Thanks Jartsa. Regards, Noel.

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