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    JimGunther's Avatar
    JimGunther Posts: 436, Reputation: 38
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    #1

    Jun 28, 2008, 09:09 AM
    Where's My Kinetic Energy?
    My question is very brief but the explanation may be a bit more complex. My question is this: Wherz my kinetic energy??

    OK, here is what I am asking about: Letís say there are two parallel railroad tracks in the desert. I am sitting on one track in a stationary railroad car with a rifle and there is a target on the track in front of me 100 yards away. On the other track is a train coming up behind me at 100 miles per hour. You guessed it, there is a person on this train with a rifle just like mine and a target on the track right next to mine.

    At the instant that we are right next to each other, both of us shoot at the target. Which bullet will hit first? The answer is, of course, obvious. The other bullet will hit the target first because it has been imparted with 100 mph of kinetic energy that my bullet doesnít have.

    Now letís change the location of this scenario to outer space and use light beams instead of
    Bullets. I am sitting 10,000 miles from Earth in a spaceship that is stationary relative to Earth. Coming up behind me and to one side is another spaceship traveling 1000 miles per hour. At the instant that we are equidistant from Earth, we each fire a strong beam of light at the Earth. Which beam of light will hit Earth first?

    I took a lot of astronomy in college (I majored in G & P) and was taught that the speed of light is constant. According to this notion, light is apparently not effected by kinetic energy. But I also learned that kinetic energy doesnít simply vanish, it must dissipate or be absorbed in a process such as the action of friction. How could 1000 mph of kinetic energy be imparted to a mechanism that produces a beam of light, but not to the light itself? In short, wherz my kinetic energy??
    rsain2004's Avatar
    rsain2004 Posts: 207, Reputation: 6
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    #2

    Jun 28, 2008, 09:35 AM
    Just a SWAG,. The vacuum of space contains solar winds, planetary radiation belts, micro meteorites, gravity fields and other "stuff". Hence it is not an "empty" vacuum. Perhaps these physical and energy obstructions absorb the lights' kinetic energy.
    JimGunther's Avatar
    JimGunther Posts: 436, Reputation: 38
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    #3

    Jun 28, 2008, 09:57 AM
    If that were true it would absorb the kinetic energy equally from both beams of light.
    Capuchin's Avatar
    Capuchin Posts: 5,255, Reputation: 656
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    #4

    Jun 28, 2008, 10:07 AM
    As far as I can tell, your light beams will have different frequencies, as E = hf
    JimGunther's Avatar
    JimGunther Posts: 436, Reputation: 38
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    #5

    Jun 28, 2008, 10:21 AM
    So you're saying that the frequency of the light waves would be different, but that their speed toward Earth would be the same, correct? I assume you are referring to the Doppler affect and red/blue shift. What if the second spaceship were travelling away from Earth at 99.999 percent of the speed of light? Do you still think that both beams would strike Earth at the same time? I am a relative layman in these matters, but that is hard for me to believe.
    Capuchin's Avatar
    Capuchin Posts: 5,255, Reputation: 656
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    #6

    Jun 28, 2008, 11:06 AM
    Quote Originally Posted by JimGunther
    So you're saying that the frequency of the light waves would be different, but that their speed toward Earth would be the same, correct? I assume you are referring to the Doppler affect and red/blue shift. What if the second spaceship were travelling away from Earth at 99.999 percent of the speed of light? Do you still think that both beams would strike Earth at the same time? I am a relative layman in these matters, but that is hard for me to believe.
    Yes, the speed of light is invariant under transformation. The extra kinetic energy will change the wavelength, just like in doppler shift. It is hard for anyone to believe, you're right, but that doesn't mean it's not true :)
    Capuchin's Avatar
    Capuchin Posts: 5,255, Reputation: 656
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    #7

    Jun 28, 2008, 11:09 AM
    Quote Originally Posted by JimGunther
    So you're saying that the frequency of the light waves would be different, but that their speed toward Earth would be the same, correct? I assume you are referring to the Doppler affect and red/blue shift. What if the second spaceship were travelling away from Earth at 99.999 percent of the speed of light? Do you still think that both beams would strike Earth at the same time? I am a relative layman in these matters, but that is hard for me to believe.
    The even weirder thing, if the man on the spaceship moving away from you were to measure the speed of the light he sent towards you, he would measure the speed of light, so he would assume that under newtonian physics, the light would be nearly stationary in a stationary reference frame - however this is not the case, in a stationary reference frame the speed would also be measure as the speed of light.

    This is the weirdness of special relativity.
    JimGunther's Avatar
    JimGunther Posts: 436, Reputation: 38
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    #8

    Jun 28, 2008, 11:30 AM
    And under that line of thinking, I assume it would make no difference as between the light source and the target of the light source, as to which one is actually doing the moving. The red/blue shift would occur no matter which one is moving. That would suggest to me that the kinetic energy I am trying to impart to the light beam won't affect it at all, since only the relative speed matters for the shift to occur. I also know that it is possible to impart kinetic energy to anything that has mass, even subatomic particles. That tells me something about what light really is. Not bad for a G& P major!
    Capuchin's Avatar
    Capuchin Posts: 5,255, Reputation: 656
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    #9

    Jun 28, 2008, 12:05 PM
    Quote Originally Posted by JimGunther
    And under that line of thinking, I assume it would make no difference as between the light source and the target of the light source, as to which one is actually doing the moving. The red/blue shift would occur no matter which one is moving. That would suggest to me that the kinetic energy I am trying to impart to the light beam won't affect it at all, since only the relative speed matters for the shift to occur. I also know that it is possible to impart kinetic energy to anything that has mass, even subatomic particles. That tells me something about what light really is. Not bad for a G& P major!
    You can impart kinetic energy to massless objects too, you just have to define it differently. For instance light still has momentum (classically p = mv), but since it's massless you have to define it differently.
    JimGunther's Avatar
    JimGunther Posts: 436, Reputation: 38
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    #10

    Jun 28, 2008, 01:01 PM
    That is where I suppose I'm getting in over my head a bit. I know that light is composed of photons, said to be a "massless particle." To a layman like me, that sounds like a contradiction in terms, but no matter. I learned in college that even though photons have no mass, they do carry kinetic energy and I believe it can be said that photons must have kinetic energy to exist. That is one reason I thought I could add to that energy by speeding up the light-generating source in a fast-moving spaceship. That is how a layman like me defines kinetic energy-the energy of motion, so the faster you go, the more you have.

    You are right, it is weird and it will be great when when get around to figuring all this stuff out. Meanwhile I will be tooling around in my make-believe spaceship. I will get off the computer, pick out a good book, and set the autopilot to accelerate faster and faster through space. I will lay down, turn on a lamp, and start to read the book. The lamp, me and everything else will go faster and faster. But the light from the lamp will always be going at the same speed. Amazing!

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