How can photons have momentum?

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thalion

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Given that:<br /><br />1.) Photons are massless.<br />2.) Photons have momentum, which is why they can propel a light sail.<br /><br />--how is it that photons can have momentum, even though they're massless? How can something massless propel something with mass?
 
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newtonian

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Thallion - Does energy have momentum?<br /><br />Can energy propel a sail?<br /><br />In fact, in the initial expansion of our universe, didn't the universe then have momentum even though it did not have matter yet?<br />I did not mean to make light of the matter!<br />
 
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datadog

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Found this at Wikipedia: "the energy gain seen by the solar sail from the radiation pressure is offset by energy lost by the reflected photons in the form of a frequency shift." Apparently, the molecules in the sails just gain a bit of kinetic energy from the photons, thus reflecting them at a slightly lower energy frequency. No momentum is actually involved.<br />
 
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qzzq

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colin098<br /><br />You mean they are just a ripple in a field? <br /><br />Electrons, like photons, don't exist as particles, but as ripples in a quantum field. Something like that?? <div class="Discussion_UserSignature"> <p> </p><p>***</p> </div>
 
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yevaud

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Yeah, but the problem is that you can't quantize gravity. It doesn't work. When you try to do what's called "perturbation" theory (countles small corrections used in Field Theory), you come up with infinites for Gravity. That's clearly wrong.<br /><br />Which is why String Theory was such a shock. Using it, there <b>is</b> a way to quantize gravity. Or so it seems... <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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qzzq

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<i>Did you get away clean - or with shrapnel wounds?</i><br /><br />I do have an itch... <img src="/images/icons/shocked.gif" /><br /><br /><i>Once again, an interesting image of QTF may be the 'graviton.' When it interacts with a helium atom in empty space, arguably, the graviton wave passes through the helium atom, continuing on its original vector line to outer space. And the interaction causes the helium atom in its quantum field to recoil 'in reverse' along the inward vector line of the graviton. THAT'S GRAVITY in QFT !!</i><br /><br />Could a graviton not just be a contracted area of space-time moving at c away from it's source? The atom would appear to be accelerated by the graviton, but it would have jumped a small distance, due to the contraction. <br /><br />Something like that? Instead of distance --- , it would travel distance -. Something like 'SpaceTime -- our flexible friend"? <div class="Discussion_UserSignature"> <p> </p><p>***</p> </div>
 
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heyscottie

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That is correct. Photons gain "inertial mass" (momentum), but do not gain "gravitational mass". This is true for all moving mass -- its inertia goes up, while its gravitational field does not.
 
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Saiph

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however, photons are energy, and so due curve spacetime. Just not much.<br /><br />I think.<br /><br /><br />Anyway, the "hand wavy, classical/physical, psuedo valid" explaination of photon momentum, w/o mass is:<br /><br />Every individual photon is "circularly" polarized, meaning it has a rotating linear polarization.<br /><br />When the photon strikes an object, the electric field causes the charged particles to move, and polarizes the neutral particles (shifts the negative to one side, positive to the other, even just a bit) which also causes some motion.<br /><br />With the particles in motion, the magnetic field can come into effect (in order for it to work, the object needs to be charged, and moving). This magnetic field exerts a force, via the right hand rule, that shoves the polarized particles (which revert to normal as soon as the photon is done messing with it) in just the direction you'd expect due to a particle collision.<br /><br />Kinda nifty if you ask me. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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heyscottie

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<font color="yellow"><br />however, photons are energy, and so due curve spacetime. Just not much. <br /><br />I think. <br /></font><br /><br />Really? I've always understood otherwise. I don't believe that energy as such curves spacetime, but only when it is actually converted to mass. If kinetic energy did curve spacetime, I think that would lead to all kind of wierd paradoxes.<br /><br />I'll have to research this one a little bit, though, as there may be something going on I don't understand here.<br /><br />Scott
 
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