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Question on Photons and perception

  1. Oct 17, 2012 #1
    Its my understanding that a photon will travel and exist in space forever. My question is if light, whether it be from a distant galaxy or star, can be perceived after it has passed by our location in space. To me it seems a confusing subject to think about but i assume you can see a beam of light from both directions, traveling towards your direction and past you? For example, If the light from a galaxy that was billions of years old passed through our location in space before humans were here to see it, it would continue on its path through space. Once humans are here and able to detect light wouldn't we see not only the light from that galaxy as it is currently from its source but also the light that has passed us going in the opposite direction ? This implies that we could see the same object in two different spots in the sky from two different times. I also understand redshift says that the light from a moving object is stretched in wave length to the red end of the spectrum, but is it possible that as light passes your location towards the other direction that it also is stretched to longer wavelengths as it travels away?

    I understand i must be missing a fundamental concept of the way light works, so i'm just looking for a answer that will explain why this wont work. Thank You
     
  2. jcsd
  3. Oct 17, 2012 #2

    Doc Al

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    If the light has passed you, how can you see it?
     
  4. Oct 17, 2012 #3
    I guess that was my question, once light passes your location is it possible to detect it going away from you? If not why? I realize this sounds like a stupid question but I'm curious.
     
  5. Oct 17, 2012 #4

    Doc Al

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    In order to detect light, it must hit you. If it's already passed by you and moving away, you missed your chance.
     
  6. Oct 17, 2012 #5
    I understand that you have to detect the light, but what is it about the light wave that makes it only detectible from head on as opposed to going away from your direction. For example, and i know light waves are not sound waves, but you hear sound coming towards you and also away. What makes light waves different in that aspect?
     
  7. Oct 17, 2012 #6

    Doc Al

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    No you don't. You might hear the sound emitted by something moving away from you, but the sound itself is moving towards you. Else you won't hear it. Same for light.
     
  8. Oct 17, 2012 #7
    Understood but what's the scientific reasoning of why light cannot be seen unless directly looked at. What are the properties of a light wave that make it detectable from only straight on and not from behind or the sides or any other angle.
     
  9. Oct 17, 2012 #8
    No, not understood I'm afraid. Any wave - even in photon description - is detectable from all angles as long as it comes towards you. Try again to analyse better the example of sound: you can hear a sound wave coming at you from all angles, but of course you cannot hear a sound wave going away from you. And also the same for water waves. And even the same for bullets: a bullet that is going away from you can't hit you!
     
  10. Oct 17, 2012 #9

    Drakkith

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    I think you have a misunderstanding of what light is. An EM wave, or light wave, IS light. You cannot see the wave because you are using it to see in the first place! You cannot "look at the light", you can only look at an object that the light has bounced off of or an emitter that is sending out light.

    If the wave isn't able to enter your eye, which is transparent to the wave, then it is absorbed by your skin or whatever is blocking your view.

    Imagine you are in a boat on the water. A large wave comes by and raises you in the air before setting you back down. This is analogous to an EM wave passing by. Unless you have something past you to reflect the wave back, neither the water wave nor the EM wave will come back and interact with you again. The wavefront has already passed!
     
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