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Thread: speed of light

  1. #1 speed of light 
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    a jet engine needs air and fuel to thrust it forward but what does light use to thrust it forward to the incredible speed of 186000 miles per second?


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    Bullshit Intolerant PhDemon's Avatar
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    There is no thrust. Light always travels at c.


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by openmind View Post
    a jet engine needs air and fuel to thrust it forward but what does light use to thrust it forward to the incredible speed of 186000 miles per second?
    A jet has mass and so you need a force to accelerate it. Photons have no mass and so always travel at the speed of light.
    ei incumbit probatio qui dicit, non qui negat
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    as you say the speed of light is constant but where does the energy come from to power it? Sorry but I'm a bit thick when it comes to physics.
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    Genius Duck Dywyddyr's Avatar
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    Quote Originally Posted by openmind View Post
    as you say the speed of light is constant but where does the energy come from to power it?
    They're not required to be "powered".
    Photons are massless, they can't travel at anything less (or more) than c.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by openmind View Post
    as you say the speed of light is constant but where does the energy come from to power it? Sorry but I'm a bit thick when it comes to physics.
    Energy is only need to accelerate something with mass. Remember, force = mass * acceleration.

    Something with no mass (light) does not accelerate and so does not need any energy to "power it".
    ei incumbit probatio qui dicit, non qui negat
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    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by openmind View Post
    as you say the speed of light is constant but where does the energy come from to power it? Sorry but I'm a bit thick when it comes to physics.
    Even a normal massive object doesn't need something to power it to keep it moving as long as there is nothing to resist its motion. Objects in motion tend to stay in motion unless acted on by a force. The reason a jet must be under continuous power to keep moving is that its motion is being resisted by the air it is pushing through and a part of that interaction produces lift to keep it in the air. If we were to remove the air and gravity, once moving, the jet would continue to move at that speed without having to use its engines.

    Light is a little bit special in that it never goes from a stop to moving, but is created already moving. But like anything else it will keep moving in a straight line unless something interferes with it, without having to be powered.

    Now even our jet with no air and no gravity has to have some energy added to it it get it started moving. Light has energy also, so that energy does have to come from somewhere. That somewhere is what ever generated the light. You turn on a flashlight, the battery produce a current in the filament of the bulb, which excites the atoms in the filament causing them to vibrate very rapidly and give off light. In the creation of this light the atoms give up some of their energy and slow down a bit. This energy is replaced by the current supplied by the battery.

    So ultimately, the energy the light comes from the battery. If you turn off the switch, the current stops, the atoms in the filament give up the little bit of energy they have left and slow back down. The light that was already generated continues on. It does not need any more energy to be pumped into it to keep moving, and will just keep going until something stops it.

    The one other difference between light and massive objects is that with a massive object, the more energy you give it, the faster it moves. Light, no matter how much energy was used to create it, always travels at the same speed. So where does all the extra energy go?. It is reflected in the frequency of the light. If I double the energy of a photon, I double its frequency. Blue light is about twice as energetic as Red light, but they both travel at c.
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