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01292008, 12:57 PM  #17  
 
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Light always travels at the speed of light. Now if you change the speed of light, like with a piece of glass or water or a plasma then the light will go that speed, but it is still the speed of light it is just slower than c. If you look at the relativistic equations they contain the letter c, so everything is setup so that you cant get to it or make stuff go faster than it. http://en.wikipedia.org/wiki/Special_relativity  
01292008, 01:19 PM  #18  
 
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The way I understood the slower speeds of light was that within substances, a photon flies with the original c, but hits an atom, gets absorbed, and some time later (fs? ns?) reemitted. That little time delay is what causes photons to "appear" slower, although the lower speeds in matter are just the average speed of the photon going stopandgo.
 
01292008, 01:22 PM  #19  
 
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01292008, 01:31 PM  #20  
 
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so than if someone could make light that has mass you could have a variable amount of force being emitted from light. would it be completely impossible to do this? maybe some headlights that can blow a hummer off the road...anyone
 
01292008, 01:56 PM  #21  
 
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Yes, there is no rest mass. But yu can say that energy has mass, too (so a full battery would actually be heavier than an empty one), so you can say a photon has mass. Better speak of a photon's imulse, though (although that is NOT a photon's mass). Quote:
So the 100tons figure is just arbitrarily picked. See, there is this formula, m = m[sub]rest[/sub]/sqrt(1(v/c)²). So moving at, say .86c (which is coincidentally sqrt(3)/2), your mass would double, compared to your mass at rest. Well, where does the extra weight come from? It's the energy you put up to accelerate. See, if you (70kg) accelerate to 1m/s, your kinetic energy is 1/2*m*v²= 35j. Now you can just use the e=mc² formula to calculate the equivalent mass, 3.9e16 kg. If you were to burn up that mass in a nuclear reactor, you'd get just that same energy. Now, as you approach the vicinity of c, you have already gained a lot of kinetic energy (=additional mass). But so far, it was that extremely tiny amount per m/s. Yet now, you start to notice that extra mass. At 86% c, your weight (erm, mass) is now 140kg. So now, to increase your speed, you can no longer say "I need 35J per m/s"  now you need to accelerate 140kg! Keep on accelerating. At 99% c, your mass is now almost 500kg! That is 70kg of "massatrest" to accelerate, as well as the 430kg that is actually energy you already put up to even get so fast! So (as mindboggling as this whole concept may sound), the faster you get, the harder you make it for yourself to get faster. That is why you can't reach c, because even if you were to hit .999c, your mass were now around 1500kg, and as you inch towards c, your mass explodes into infinite heights. Now, we are of course talking about a vacuum. .99c in an atmosphere? Haha, let's just say you lose 50% of your speed (down to 50% c) within the first second (arbitrary guess). Then you'd release 415 kg of energy. That is 3.73e19 joules. Comparison  the tsar bomb, the world's larges nuclear bomb, recorded at 50Megaton TNT equivalent  a mere 2e17 joules. So your slowing down is about hundred times more violent and "exotherm" that the worlds largest nuke ever exploded. Now try and calculate your chance of survival of that  
01292008, 02:41 PM  #22  
 
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Although why it is exactly c, or why c is at that place, or why in the world it is e = mc squared and not cubed or linear, no one knows. I would say, that goes off into metaphysics. Quote:
So, a photon that has 1 joule of energy (oh, that would be deep into gamma rays) would have, e=mc², a mass of 11 femtograms (ridiculously nothing). You then feed that...wait, nvm. Now: Impulse of a photon is h/lambda (h being planck's constant). So your average 532nm photon has an impulse 1.24e27 Ns. As it has an energy of 3.73e19 joules, your average 100mW greenie emits 2.7e20 of these buggers per second. That is around 30.000 quadrillion . Multiply that with your impulse, and you get 3.37e7 N What does this mean? Well, if you hit your friend with your 100mW laser, you excert that force on him. Now you were talking about headlights. Assuming they are 60W each, and 100% efficient, you'd excert a force 1200 times as strong. *But we're still counting at .3mN. Okay, let's do this way bigger. Let's mod our headlights with two 2500W xenon burners, and assume 100% efficiency again. Then we're at 16mN, or the equivalent of 1.6g  almost noticeable! Neat thing here is that as it is independend of your wavelenght, you can say any light source emitting 100mW will excert that force. Moving towards the hummer: You want to move a hummer off the road, using photons? Okay, lets look into this. Now I am lazy and will only consider static friction. The weight of an H2 (including a fat driver or soccer mom + kid) is 3000kg (2900+100). Coefficient for friciton rubber on asphalt is .9, so the force needed to push that car off the road is m*g*.9 is 26500 Newtons. Multiply that with your 100mW force, and you'd see you need 8 Gigawatts. Now, there is a trick to help us there. Assuming the hummer is fully chromed, so that every photon is reflected 100% back, that means the force delivered doubles (f = delta p, delta p is 2p). So, no we're down to "merely" 4 GW, or assuming 2 headlights, 2 GW per headlight. There you have it, and you don't even need a laser for it! Good luck explaining why you need those light to the next police officer checking your car... little side note here: If the light is reflected, which has the advantage that the hummer won't be evaporated before it hits the side of the road, then it all comes straight back at you. Now, due to Newtons second law (was it second?), the porce of the photons expelled exquals the force excerted on you, your tires have to push against 13250N. But the photons coming back at you are now hitting you again! Your best chance would be to let them shoot "through" you, then you dont't have to hold them, too. But that means, somebody behind you will get the full shot of 4GW photons. But what I mean to say with this is, get a heavier car than a Hummer. This won't work with a portable since thath force excerted on you would send you flying through the city, but not move the hummer. whew. Easy, see?  
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