# Gravitational Slingshot

• October 25th, 2012, 06:46 PM
Quantime
Gravitational Slingshot
How would a photon behave if gravitationally slung shot by an incredibly dense black hole?

Can anyone explain to me how momentum is conserved? If it does not get a relative velocity boost then what does it get because it has to be conserved right?
• October 26th, 2012, 10:40 AM
Janus
A gravity slingshot is a lot like an elastic collision, so let's look at what would happen in such a collision.

You fire a light pulse at a mirror that is traveling towards you. Upon reflection, the mirror losses some momentum and the light pulse gains some. The light pulse however does not change speed with respect to you. What does change is its frequency (the momentum of a photon is related to its frequency. When it gets back to you, you will see that it is blue shifted. (this is the principle that doppler radars are based on.

If you do the same thing with an approaching black hole, (only in this case you shoot your pulse such that the BH's gravity whips it around and sends it back to you) you get the same result the light coming back has the same speed but a higher frequency.
• October 28th, 2012, 07:19 PM
forrest noble
Quote:

Originally Posted by Quantime
How would a photon behave if gravitationally slung shot by an incredibly dense black hole?

Can anyone explain to me how momentum is conserved? If it does not get a relative velocity boost then what does it get because it has to be conserved right?

I think gravitational sling-shot of a photon might be best characterized as a misnomer.

First the energy greatly increases of the photon as its frequency increases as it approaches a black hole. As this photon leaves the black hole its frequency is equally decreased and redshifted ending up at its starting frequency with no gain or lose of energy realized. Its momentum is its mass times its velocity. In this case its mass-equivalence is equal to its energy state which first increases then equally decreases, while its velocity remains unchanged (the speed of light), therefore its momentum remains unchanged afterwards and is conserved.
• October 29th, 2012, 08:44 AM
Kerling
Gravity slingshot? Well, Photon's as does everything in space. Move in straight lines over the space-time continuum called a geodesic. Since it follows a straight path it doesn't lose energy. So our planet revolving around the sun (assuming both point particles) do not lose energy due to their rotation.

However all objects curve space-time continuum to a certain degree. And a black hole does this a lot. Which make it look like as if it was a sort of lens. As for the light particle it follows a straight line. Gravitational lens - Wikipedia, the free encyclopedia
Is actually a very good explanation.

The slingshots used by sattelites use the rapidly curving space-time continuum around planets to more effectively use their engines. As a small displacement in a large curved field can more easily put you into a ' higher geodesic' which makes you gain more energy from the same rockets. Or more simply: Oberth effect - Wikipedia, the free encyclopedia

This is of course not appicable to massless particles or anything without propellent. Then it will just change direction.

On a more intersting note, The earth's moon (which originates from the earth itself) is pulling on the earth and vice versa. Normally this wouldn't make the moon and earth move closer or further away from eachother. But at birth it reached an initial value away from the earth, and due to the fact that it drags the earths oceans with it (which are braked by land) the earth loses rotation speed, and the moon has an increasing speed away from the earth. Yes, during dinosaurs the moon must have apperaed about 60 percent larger in the sky then it does now. :)