I've got my FTL boots on (another silly black hole question)

  1. Good Afternoon, to those of you that are existing in the afternoon. For all else, good morning, good evening and so on.

    Before I begin, this is more of a question about the behavior of light in immense gravity than anything else. I'm well aware that under the best of circumstances, or the worst, I'm spaghetti'd and anyone outside observing me fall into a black hole will see me slow down and turn red. I mean, after all, it IS pretty embarassing to voluntarily point your self at a black hole and fall in.

    Alright, to the meat and potatoes, as they say.

    I've got my FTL boots on. I'm falling into a black hole. Nevermind an outside observer, I'm hoping there are none to record my silly actions from this point forward.

    Right as I pass the EH falling toward the singularity, I flip my FTL boots on and I hover, just inside the EH and take a gander back at the universe.

    Nevermind that in mere seconds I will realize that my logic is completely wrong, and my FTL boots will cease to exist and I shall be sucked into the singularity against all whims or wishes.

    My question is, what do I see as the light falls into the Black Hole and hits me, hovering just inside the EH?

    I've read Larry Niven's "Neutron Star" and in it, as I'm sure anyone who's read it knows, the light falling on the ship passing close to the star ends up giving the pilot/astronaut "star burns".

    Because I'm able to hover in my ill-logic for a couple seconds, and I'm not falling at the speed of light, is the light blue shifted?

    Thanks for your patient answers. :)
  2. jcsd
  3. Drakkith

    Staff: Mentor

  4. Hi Drakkith, thank you for that link. I'm actually pretty familiar with Andrew Hamilton's site, however, I can't seem to find an answer that tells me what the behavior of observed light is hovering just inside the EH. I'm pretty sure that as you fall into the singularity, if you were somehow able to observe the light falling in as you fell it would be red shifted, since it has to catch up with you as you fall at the speed of light.

    I'm curious to know that if you aren't falling, and yet stuck just inside the EH, is the light blue shifted? More intense/bright because it's trapped there etc?

    Edited to add: I'm also aware that the answer is probably in there somewhere, it's just not very apparent to a layman like myself.
  5. Drakkith

    Staff: Mentor

    Not sure honestly. Off the top of my head I would guess that the light is more blue shifted since you aren't moving away from it anymore, but thats just a guess.
  6. Chronos

    Chronos 10,349
    Science Advisor
    Gold Member

    Anything free falling into a black hole will perceive light from the outside is redshifted. You would have to be stationary to see it blueshifted, which is of course impossible for an object in free fall.
  7. Hovering inside the horizon of a black hole is impossible, so asking what you'll see makes no sense.
    Still a small region of spacetime inside a black hole will look no different to you than a region outside the black hole, and you can still get the light as blue-shifted as you want by accelerating hard enough.
  8. Actually that answered it perfectly for me, thanks Chronos. My FTL Boots would hold me right inside the EH, so I would not be falling at that point. (Until logic kicked in and with a terrible realization the whole "got my ftl boots on" thing becomes apparently impossible and I do get spaghettified immediately after). I would be stationary at the EH, so would the light not be blueshifted at that point, since I'm facing away from the singularity and the light would be trapped as it came towards me?

    Well. It could be said that following a beam of light is impossible too, and yet, that impossible thought experiment changed physics. (Einstein). Sometimes people, curious people especially, like to use an impossible situation to get a better understanding of the concept as a whole, so I'd tend to disagree. Also, you might want to address that with Andrew Hamilton, as he actually visualized the impossible, falling into a black hole AND rendering a visual of it for all to see. ;) (Impossible being that nothing can send back that kind of information from inside a black hole)
  9. I would say that your view would be no different from any point of the universe. I do not know why you should see something differet as far as i understand physics.
  10. Can you explain that a little further?

    Considering you get an Einstein ring around a black hole or neutron star just approaching it, your view of the universe is already drastically changed by the immense gravity's effects on the light around a black hole. As you're falling into a blackhole, light would be red shifted because you're moving at the speed of light already, (once you pass the EH and are doomed to hit the singularity), and that light has to catch up to you as you fall in. Both the redshift, and the Einstein ring are examples of a very different view from in and around a black hole.
  11. Drakkith

    Staff: Mentor

    We don't have to use FTL boots to keep us from falling into the black hole to see the effects on light. Simply accelerating against the gravity will allow us to see the light being less redshifted than when we were freefalling.
  12. HallsofIvy

    HallsofIvy 41,269
    Staff Emeritus
    Science Advisor

    Much like the cartoon character who runs off the cliff and hovers in air until he realizes there is no ground under him?
  13. Exactly! He has a few moments of false lucidity to consider, and then remembers Newton in a stomach lurching drop, resulting in a small poof somewhere down... there...

  14. Agreed. The only reason I used the "FTL boots" was to keep ahead of the gravitation pull, (gotta move faster then the speed of light to not be sucked in completely). I really just wanted to understand more about the behavior of light around the EH.

    This was all inspired to me by Larry Niven's "Neutron Star". I thought it was a brilliant detail to add in how badly blue shifted the light was in the vicinity of the Neutron Star, being that it has a harder time escaping there, than in pure vacuum.
  15. Drakkith

    Staff: Mentor

    Harder to escape the neutron stars gravity than pure vacuum? I don't follow you.
  16. Bah, bad wording there. I meant that the light has a harder time escaping/moving through the neutron star's gravity than moving through vacuum, hence the blue shift. Apologies for the confusion! :)
  17. Drakkith

    Staff: Mentor

    The light is blue shifted because it is heading into a gravity well and gains energy. Which is only noticeable if the observer is also inside the gravity well. If you were on the opposite side of the neutron star from a flashlight the light would appear the correct color, as it gained then lost energy equally as it made its way to you. It doesn't really have a "harder" time moving through this than open space nowhere close to a massive object.
  18. This part confused me a little bit, the rest of your post made alot of sense. I guess by "harder time" I meant that light gets altered around a massive object, hence the disturbance in it's path. It really ends up being rhetoric, (on my part, not yours, I just don't have the terminology to frame it correctly), but in the end I understand exactly what you're saying, and it actually adds alot of clarity to my concept of light around massive objects.
    Last edited: Feb 15, 2012
  19. Chronos

    Chronos 10,349
    Science Advisor
    Gold Member

    In free fall, by the time you reach the event horizon you are also travelling near light speed, so light from the outside is struggling to keep up.
  20. Has anyone here read "Neutron Star"?

    Would blue shifted light actually be harmful if you were exposed to it going close to c?

    Or stuck in a gravity well for that matter, around a Neutron star...

    I mean, nevermind tidal effects, mass increase, dust particles nipping you and causing thermonuclear reactions...
  21. Drakkith

    Staff: Mentor

    Absolutely. Get to a high enough speed and the microwave radiation background blue shifts into high energy gamma rays.
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