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Shooting a star (with torchlight)

  1. Apr 10, 2013 #1
    I don't know whether I should post this under cosmology, but since it's mainly about light rays so I'll put it here.

    I was reading Hawking's book and a thought suddenly struck me. Since light travels in a straight line, if I direct a torchlight into the night sky, if the light isn't reflected back by any particle on the way, and if it managed to travel through the atmosphere perpendicularly(or not exceeding refractive index of the atmosphere) would it escape from the earth?

    2. If it managed to escape successfully, and if the universe really has so many stars, would the light of the torchlight end up falling on a distant star many years later?
     
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  3. Apr 10, 2013 #2

    phyzguy

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    Yes.

    Probably not. Stars cover only a small part of the sky. And since the universe is expanding at an accelerating rate, the stars are getting further apart and covering less and less of the total sky. So probably the light will just keep going.
     
  4. Apr 10, 2013 #3

    cepheid

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    It wouldn't be too hard to aim it at a star in our galaxy (within which expansion is irrelevant, also). EDIT: all of the individual stars we can see in the night sky lie within our own galaxy, btw, so they are not getting farther apart.

    I'm pretty sure SETI has aimed radio waves at specific astronomical objects, which is the same thing.
     
  5. Apr 10, 2013 #4

    phyzguy

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    Right. I didn't mean to say that you couldn't hit a star. Clearly if you point the torch properly you can hit whatever you want. What I meant was that a direction chosen at random probably wouldn't hit a star.
     
  6. Apr 18, 2013 #5
    I was suddenly reminded about damping. Supposedly, light from the torchlight wouldn't be strong enough to reach any stars, right?
     
  7. Apr 18, 2013 #6

    HallsofIvy

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    If the light were strong enough to get through the atmosphere (which you included in the original post, "if the light isn't reflected back by any particle on the way, and if it managed to travel through the atmosphere") there would be relatively little dust to absorb it. That is, I presume, what you mean by "damping"?
     
  8. Apr 18, 2013 #7
    Damping does not occur in vacuum?
     
  9. Apr 18, 2013 #8

    phyzguy

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    No. Light propagates in a vacuum without loss of energy, except for the loss of energy which occurs due to the expansion of the universe, which only occurs over very long time and distance scales. Even then, the light still arrives, it is just red-shifted to longer wavelengths. We see light which has been propagating for billions of years and has traveled billions of light years.
     
  10. Apr 18, 2013 #9
    practical considerations would affect this strongly.

    First, atmospheric scattering would attenuate the light, and the refractive index gradients in the atmosphere would cause a lot of dispersion, beyond the normal dispersion of the torch itself. Once it enters deep space there won't be much absorption of the light, but the dispersion will strongly reduce the irradiance (power per unit area). I would guess that you'd also have a lot of trouble aiming the torch in the correct direction due to astronomical aberration .
     
  11. Apr 19, 2013 #10
    I'm no expert in relativity, but you would probably have to account for the fact that the star you're aiming at is not there anymore, and wherever it is, it won't be by the time the light gets to it.
     
  12. Apr 19, 2013 #11

    Danger

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    I'm thinking that the light "cone" is going to be ridiculously huge by the time it gets anywhere of significance, so no matter how weak they might be, the photons are likely to hit thousands of stars. Not like anybody would notice...
     
  13. Apr 19, 2013 #12
    I think there is no way a light beam from a torch would reach the far reaches of our solar system let alone other Galaxies. As kids alot of us pointed a torch out into the night sky imagining it would reach some distant star/planet some 10-50 years later. In reality I think the source would be to weak and the disipation would be to great to reach any meaningful distance, much like ocean swells disapate the longer they travel.

    /Tek
     
    Last edited: Apr 19, 2013
  14. Apr 19, 2013 #13

    phinds

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    Yes, the BEAM would spread extremely widely, but individual photons would hit stuff. Did you not read what Danger said? Did you not understand it?
     
  15. Apr 19, 2013 #14
    I left school in year 9, Im from a poor family and im not really educated. But I think common sense should not be under estimated in any field.

    To me the answer to 1) Is yes it could possible leave earth but no meaningful distance. Correct me if im wrong but light still travels in a wave length form. Wave lengths generally get greater the further they travel.

    Question 2) Re:Falling on a star. How could you define that in a measurable form? Certainly wouldnt be visible and in reality the photons you speak of would be unditectable.. So the answer is NO.
    /Tek
     
  16. Apr 19, 2013 #15

    phinds

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    Addressing only this statement and not your responses on light:

    No, there are fields where common sense not only SHOULD be ignored, it MUST be ignored since it is SO wrong. We humans evolved within a narrow spectrum of physical experience and that is where our "common sense" does in fact make sense. We don't HAVE any "common sense" when it comes to the incredibly tiny (quantum mechanics) and the incredibly large (cosmology).

    Read about the "double slit experiment" and you will begin see what I mean.
     
  17. Apr 19, 2013 #16

    phinds

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    We receive light from entities that are now about 50 BILLION light years away ... why do you think light FROM us would not travel as far?

    Undetectable is NOT the same as not present. This is NOT semantics. Undetectable could just mean we haven't figured out yet how to detect something. Not present (your answer) would mean that it would never be possible to detect it because it isn't there to be detected. Do you see the difference?
     
  18. Apr 19, 2013 #17

    NascentOxygen

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    Light would have to travel until it is absorbed by something. I'm thinking interstellar dust would be more likely to be encountered than a star. But either way, it must hit something otherwise it will keep on going until it does hit something that absorbs its energy.
     
  19. Apr 19, 2013 #18

    Danger

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    Ah, yes... I failed to account for dust, or even dark matter. There will certainly be a high level of attenuation if such falls between the source and the stars. Still, the infinitesimal dimensions of a photon pretty much guarantee that at least a couple of them can pass through a dust cloud unimpeded. It would be somewhat equivalent to firing a load of #9 birdshot through a tennis racquet; you'll probably annoy a few strings, but some pellets are going to make it. (Mathematical accuracy subject to change without notice...)

    There is also the fact that light is affected by gravity, so photons that might not be traveling directly toward a star might be diverted by one.
     
  20. Apr 19, 2013 #19

    phyzguy

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    The universe is largely transparent. That's why we can see objects that are billions of light-years away, and can see the CMB radiation that has been traveling since nearly the beginning of the universe.

    You say, "it must hit something otherwise it will keep on going until it does hit something that absorbs its energy".

    Why can't it just "keep on going"? Since the universe is expanding and getting continually less dense, a very likely possibility is that it will just "keep on going".
     
  21. Apr 19, 2013 #20

    Danger

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    Entropy precludes that, but so far in the future that it's essentially meaningless. On the other hand, we really don't know what the fate of the Universe might be. There's still a chance that it could stop expanding and then collapse back into a "Big Crunch". An extremely unlikely (pretty much impossible) alternative is that it will simply stop expanding and stay there. The effect upon photons, therefore, is still very much uncertain.
    (Personally, I want to just track them all down and whap them with a flyswatter, but that's a bit impractical.)
     
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