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Light energy

  1. Mar 20, 2007 #1
    Can anyone tell me how an object can emit light/radiation when it has no energy source?
  2. jcsd
  3. Mar 20, 2007 #2
    It must have some source of energy to emit light/radiation. Can you give an example of something that emits light/radiation without any source of energy?.
  4. Mar 20, 2007 #3
    I can't really give an example, but I was reading an article that explained the theory that an object moving at great speed emits radiation/light at the same speed as a stationery object. My thought was that if the stationary object has no energy source, how can emit anything?
  5. Mar 20, 2007 #4
    It's still unclear to me
  6. Mar 20, 2007 #5


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    Staff: Mentor

    All objects have an internal energy based on their temperature. As they emit light, they cool down.
  7. Mar 20, 2007 #6


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    Science Advisor

    Is there anything in that article that implies no energy source? The point of the article, or at least what you say of it, is that if an object moving at high speed relative to you emits a light radiation (from whatever energy source) then you will see that light coming to you at the same speed as light coming from a stationary object (from whatever energy source). Where did you get the idea that the stationary object "has no energy source". This computer screen that I am looking at that is spitting large numbers of photons at me (and God know what other energy) is stationary relative to me and certainly does have an energy source- I just plugged it in!

    The point of what you are reading is not "energy source", it is relative velocity. If you are standing on the side of a road and another person standing on the road (so stationary relative to you) throws a ball to you with speed (relative to him) 30 m/s, then it comes to you at (relative to you) 30 m/s. At the same time a truck comes toward you at 70 km/hr= 20 m/s and a person in the truck throws a ball at you at (relative to him) 30 m/s, then it comes at you at (relative to you) 20+ 30= 50 m/s.

    That's the "Newtonian physics" calculation. "Einsteinian physics" would give a slightly different answer but, because the velocities involved are much less than the speed of light, you couldn't measure the distance.

    Now, suppose that instead of throwing a ball, both people shine a light at you at (relative to each of them) c, the speed of light. Newtonian physics say that the light from the truck would be coming at you (relative to you) at c+ 20 m/s. Einsteinian physics say the light from both would be coming at you at c. It doesn't matter how fast the source of the light is moving, the light comes toward you at speed c.

    That has nothing to do with a "source of energy". I think you are misunderstanding if you think it is talking about light without a source of energy.
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