Is light blue-shifted by gravity?

  • 2 Replies
  • 2564 Views

0 Members and 1 Guest are viewing this topic.

*

Offline thedoc

  • Forum Admin
  • Administrator
  • Sr. Member
  • *****
  • 513
    • View Profile
Is light blue-shifted by gravity?
« on: 18/09/2012 09:30:01 »
Jeff Sykes  asked the Naked Scientists:
   
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?

What do you think?
« Last Edit: 18/09/2012 09:30:01 by _system »

*

Offline evan_au

  • Neilep Level Member
  • ******
  • 4246
    • View Profile
Re: Is light blue-shifted by gravity?
« Reply #1 on: 18/09/2012 11:48:08 »
Yes, gravitational blue-shift and red-shift has been demonstrated on Earth, and on stars with strong gravity.
http://en.wikipedia.org/wiki/Gravitational_redshift#Initial_verification

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).

http://en.wikipedia.org/wiki/Gravitational_redshift#Gravitational_redshift_versus_gravitational_time_dilation suggests that the gravitational redshift/blueshift is equal to the time dilation due to gravity.

If so, there should be no gravitational "slingshot effect". 

*

Offline imatfaal

  • Neilep Level Member
  • ******
  • 2787
  • rouge moderator
    • View Profile
Re: Is light blue-shifted by gravity?
« Reply #2 on: 18/09/2012 14:15:15 »
.../
If so, there should be no gravitational "slingshot effect". 

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
Thereís no sense in being precise when you donít even know what youíre talking about.  John Von Neumann

At the surface, we may appear as intellects, helpful people, friendly staff or protectors of the interwebs. Deep down inside, we're all trolls. CaptainPanic @ sf.n