Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can light experience Gravity assist?

  1. Nov 15, 2014 #1
    Sorry this is a silly question but I'm an amateur and simple. Can someone explain Gravity-assist and the Penrose process to me? I'm obviously missing something basic but I'm confused. Technically, could a photon travelling at c experience a gravity assist (or something like the Penrose process) around a rotating black hole, and if so why couldn't the photon be accelerated faster than c? Thanks for reading this.
  2. jcsd
  3. Nov 15, 2014 #2
    The only silly question is one you do not learn from... but these things you mentioned and light are unrelated so I'll just try to break them down briefly.

    Gravity assist is simply using a large mass for its attractive force, in Newtonian physics, or "falling into" a gravity well in general relativity but this does not lead to an assist directly, since you have to "slingshot" around the mass and are pulled back by gravity slowing you back down or "climbing" back out of the well in GR. The gain in velocity is acquired while you are in "orbit" around the mass in which may be brief but effective in taking some of the orbital energy from a mass orbiting around another body (the opposite direction slows you down and gives the larger mass slightly more energy).

    The Penrose process is different because it doesn't involve the mass orbiting anything, it involves a smaller mass orbiting a quickly rotating larger mass and the smaller mass can leave the system, perhaps broken and part falls in with other other part being ejected with more energy, but it actually slows down the rotation of the larger mass a bit. If it all falls in it adds its energy to the rotation.

    As for photons at c? They are always at c! When we say c is the speed of light in a vacuum, that is its normal propagating velocity no matter who sees it at what speed they are moving in however much gravity. It only changes when it bounces off of something or it gets absorbed by an atom and a different photon may be emitted. What can change is its vector of travel, for example, gravitational lensing, or its frequency, which many factors can affect, but since you are interested in large masses I'll just touch on that. As a photon approaches a mass, it has to gain energy. Its velocity is stuck on max, but its energy can vary by wavelength, so to pass this energy to the photon the wavelength gets shorter... giving it a faster frequency. The opposite happens leaving a gravity well, causing it red shift to lower energy and slower frequency. I'm not sure if orbital or rotational energy affects it, but I'd bet it still goes c.
  4. Nov 15, 2014 #3
    jerromyjon that is EXACTLY the answer I was hoping for. Thank you!
  5. Nov 15, 2014 #4
    You're very welcome!
  6. Nov 15, 2014 #5


    Staff: Mentor

    Actually, the larger mass has to be a rotating black hole for the Penrose process to work; just an ordinary rotating body like a planet or star won't do it, because only a rotating black hole has an ergosphere.

    I haven't seen any analysis of these specific questions (which are different, as jerromyjon pointed out), but I don't see why a photon couldn't be affected by these processes just as a timelike object is. The only possible issue would be with the Penrose process, which requires the object that falls into the black hole's ergosphere to split in two; one piece goes down into the hole, while the other comes back up with more energy than it fell in with. A single photon can't split in two like that; but one could probably figure out some way of doing a similar trick with multiple photons.

    As jerromyjon said, photons always travel at c; when you "accelerate" a photon, you change its direction of travel and/or its frequency and wavelength, not its speed.
  7. Nov 15, 2014 #6
    Wouldn't that be like Hawking radiation, where a positron/electron pair are created from a particle at the EH of a black hole, one falls in and the other escapes taking energy from the system, theoretically "evaporating" the black hole if more energy is lost than incoming energy adds?
  8. Nov 15, 2014 #7
    Lack of sleep :confused: I meant entangled photons but similar physics...
  9. Nov 15, 2014 #8


    User Avatar
    Science Advisor

    Which would still allow extracting kinetic energy from the moving mass. It
    just doesn't go in the the photon's speed. as the OP thought, but its frequency.

  10. Nov 15, 2014 #9


    Staff: Mentor

    No. Hawking radiation involves virtual particles created in pairs; one of them becomes real as a result of the process (at least, that's one way of describing what happens, but not all physicists would describe it that way), but there are no real particles to start out.

    The Penrose process, applied to a photon the same way it is applied to a particle with nonzero rest mass, would have to involve a real photon falling into the ergosphere, then splitting into two real photons, one of which goes down the hole and the other of which comes back up out of the ergosphere with more energy than it had when it went in. But a real photon can't split into two real photons. So if the Penrose process can be applied to photons, it must be by some more complicated process than the one that works for particles with nonzero rest mass.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook