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?'s about light

  1. Aug 28, 2004 #1
    I had some questions about light. Mb some1 could explain 2 me. I know about red shift and blue shift. If the object is moving towards you it's color shifts to blue and vice versa. Is this because if the object is moving towards you it compresses the light waves and if moving away it stretches the light waves? What would an object moving near light speed look like if viewed from the side passing by you? Would the light have like a rainbow effect blue to red shift in colors? When light near a Black Hole enters past the event horizon is the reason you don't see light because the light waves are pulled to like a flate line? If you are moving towards or away from a stationary object are the shifts in light the same? If that is true then would it mean that light waves don't get stretched out compressed but by moving toward or away you are changing the percieved time between the modulations in the lights frequency. Like shortening or lengthing the percieved time between the the peaks in a light waves frequency?

    Thanks Ed
  2. jcsd
  3. Aug 28, 2004 #2
    Yes, yes it is.

    If it was moving to the side of you then for you to see it light would have to be emitted from it in your direction. In this case it would be blueshifted as the object approached, redshifted as it moves away from you, and it would not be altered at all as it passed directly perpendicular to you.

    Kind of, though I'm not certain what you mean.

    In theory, no. According to the current ideas, you do not see the light because the velocity of the light, c, is not larger than the escape velocity of the event horizon. Basically, the light cannot reach your eyes because it cannot escape from the event horizon.

    I don't believe that moving towards or away from an object will shift its wavelength at all. But then again, I'm not sure.

    I'm not too sure of my last statement, so I better leave that to someone else.

    :: Ben ::
  4. Aug 28, 2004 #3
    The resaon light is blue and red shifter is because of the expansion of spce itself. When light is emitted from a star, space is expanding all the time and this expnsion of space ellongates the wavelength of light, amking it red shifted.
  5. Aug 28, 2004 #4
    I think that is only true when the direction of emission is opposite of the direction of star movement. When light is emitted from a stationary object, it appears to be normal. Only then is the wavelength inversly proportional to it's frequency (Maxwell). If you light compared to a moving system, Maxwell's definition of light being emitted from a stationary source does not apply.
  6. Aug 28, 2004 #5
    :uhh: :uhh: :uhh: :uhh: Im just telling him why light from other stars is red shifted. :uhh: :uhh: :uhh: :uhh:
  7. Aug 28, 2004 #6


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    Just for clarification, red shift due to expansion only occurs between galaxies, not within them. Any red shift observed in light from stars within our galaxy is due to proper motion. Also, any light passing by in front of you is not visible. Only light that 'hits' you can be seen.
  8. Aug 28, 2004 #7
    Moving toward or away from a light source does blue-shift red-shift the light. That's the way police radar works :smile:
  9. Aug 28, 2004 #8
    Are you sure about this ? I've never seen it stated that way, and can't imagine an equation that would expand the universe but leave out lumps where galaxies reside :smile: But I could be wrong.

    Last edited: Aug 28, 2004
  10. Aug 28, 2004 #9


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    This is mentioned in the sci.physics.faq

    Basically, things that are bound together don't expand as the universe expands. Atoms are bound together by the electromagnetic force, the galaxy is bound together by gravitation.
  11. Aug 28, 2004 #10
    Wrong. Look up 'transverse doppler shift'. You would be correct if there such a thing as absolute time (classical doppler effect).
  12. Aug 28, 2004 #11
    Could the Compton Effect be interpreted as a Doppler effect on the wave properties of X-rays and the shift to longer wavelength that depends only on angle?
  13. Aug 28, 2004 #12
    This must be only in theory; I can't imagine reality operating that way :smile: I thought that there was gravitational attraction between galaxies also ??

  14. Aug 28, 2004 #13
    In the Compton Effect energy is transferred so I don't think the doppler effect could be to blame :smile:

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