Science Questions

Is light blue-shifted by gravity?

Mon, 24th Sep 2012

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Jeff Sykes asked:

Dear Naked Astronomers,


When we send probes into space, we use gravity assists to speed it up. If a photon of light were to pass by a large body, such as Jupiter, instead of going faster (since the speed of light is constant), could the beam get blue shifted?




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Yes, gravitational blue-shift and red-shift has been demonstrated on Earth, and on stars with strong gravity.

However, a photon generated on one side of Jupiter, and observed on the other side of Jupiter would show no blue-shift, because both the source and observer are at the same gravitational potential (eg in the Sun's gravitational well). suggests that the gravitational redshift/blueshift is equal to the time dilation due to gravity.

If so, there should be no gravitational "slingshot effect".  evan_au, Tue, 18th Sep 2012

I think gravitational sling shot relies on the planet moving.  ie whilst, as you have explained regarding light above, you have no net gain moving towards and away from the planet (viewed from the rest frame of the planet) when viewed from a solar frame in which the planet is moving (at a fair speed) the craft is able to be dragged to a higher speed by hitching a ride with the planet.  It is not gravity that is speeding up the craft - it is merely that gravity allows the craft to "attach" itself to the planet; the planets momentum is so vast that the craft makes next to no difference on the planet and gets a free boost imatfaal, Tue, 18th Sep 2012

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