max.yevs Posted March 29, 2009 Share Posted March 29, 2009 i realized recently- light travels at different speeds through a vaccum, air, water, glass, etc. which is why optical lenses work... if you were to shine light through a very dense, yet clear compound, could you actually see the light progressing from side to side? and would it be like after you turn off the light it would still keep shining for some time? Link to comment Share on other sites More sharing options...
NowThatWeKnow Posted March 29, 2009 Share Posted March 29, 2009 Most of the experts around here will argue that light can not be slowed down but the average speed can be slowed down as it passes through different substances. The light is absorbed and then re-emitted with a delay coming while it is absorbed. That being said, here is how to stop or slow down light. http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html Link to comment Share on other sites More sharing options...
timo Posted March 29, 2009 Share Posted March 29, 2009 I would rather assume that most experts would argue that light traveling through some medium with a high refractive index will not suddenly emit light that is not slowed down. Hence, you do not see a "ball of bright" moving through the high-refraction medium. Seeing light from a source that no longer emits is an old hat - it is commonly believed that some stars that we see on the night sky do no longer exist. Putting in obstacles with a high refractive index does not qualitatively change anything. It might quantitatively change something; namely put time and distance in a different relation. Link to comment Share on other sites More sharing options...
max.yevs Posted March 29, 2009 Author Share Posted March 29, 2009 ah ok... yeah that is what i basically what I had in mind, a "ball of bright" well since this question has been explained, how about an unrelated one: if you shine a lot of light onto a reflective surface, would the reflective surface be pushed back by the light? From the photons hitting and bouncing off it? Link to comment Share on other sites More sharing options...
lakmilis Posted March 29, 2009 Share Posted March 29, 2009 (edited) Most of the experts around here will argue that light can not be slowed down but the average speed can be slowed down as it passes through different substances. The light is absorbed and then re-emitted with a delay coming while it is absorbed. That being said, here is how to stop or slow down light. http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html I got a bit shocked, cos my whole knowledge of everything would just collapse like a probability wave collapsing had one stopped light entirely.. that would be the same efeect as if someone told me they had reached 0 Kelvin. Then I even saw this article was from 2001 or so.. then i really went nuts. Until I run the video, where the dane says ye so it ende dup being 1 m/s... REMOVED... have they really stopped light? Ok.. that's it.. im never ever gonna think again.. just gonna sing duppeyyydoooppooopppppp to myself and accept anything and everything... hell why not.. slowing it down to anything but 0 i could accept... I just can not accept yet.. that light can be stopped to 0 m/s. ddoooopppeeeeyyddddoooppppddddiiidddooopppppppppplllaallaaiillaaaaaaaaaaaaaa.breakdown.end of line Edited March 29, 2009 by lakmilis Link to comment Share on other sites More sharing options...
NowThatWeKnow Posted March 29, 2009 Share Posted March 29, 2009 if you shine a lot of light onto a reflective surface, would the reflective surface be pushed back by the light? From the photons hitting and bouncing off it? Solar sails come to mind so I think yes. http://en.wikipedia.org/wiki/Solar_sail Link to comment Share on other sites More sharing options...
cameron marical Posted March 29, 2009 Share Posted March 29, 2009 i dont think its stopped to o m/s. i think that really is impossible. its just having to go through so many things its absorbed and re-emitted at a speed we cant see. its still moving though, at the constant c. Link to comment Share on other sites More sharing options...
NowThatWeKnow Posted March 29, 2009 Share Posted March 29, 2009 ... have they really stopped light? Ok.. that's it.. im never ever gonna think again.. just gonna sing duppeyyydoooppooopppppp to myself and accept anything and everything... hell why not.. slowing it down to anything but 0 i could accept... I just can not accept yet.. that light can be stopped to 0 m/s. ddoooopppeeeeyyddddoooppppddddiiidddooopppppppppplllaallaaiillaaaaaaaaaaaaaa.breakdown.end of line Don't quit thinking yet. There are several in these forums that say the light is really not slowed down or stopped. That is why I started my last post with "Most of the experts around here will argue that light can not be slowed down but the average speed can be slowed down as it passes through different substances. The light is absorbed and then re-emitted with a delay coming while it is absorbed." There is a thread about all this somewhere around here not far away. Link to comment Share on other sites More sharing options...
max.yevs Posted March 29, 2009 Author Share Posted March 29, 2009 recently i was thinking, if you were going at the speed of light, and you shine a flashlight forwards, would it be going at double the speed of light? of course i found out that no it can't. but this leads to an interesting modification. If you're going forward at the speed of light, and you shine a flashlight behind you, how fast would the light be moving? at 0 mph or still at the speed of light? Link to comment Share on other sites More sharing options...
NowThatWeKnow Posted March 29, 2009 Share Posted March 29, 2009 but this leads to an interesting modification. If you're going forward at the speed of light, and you shine a flashlight behind you, how fast would the light be moving? at 0 mph or still at the speed of light? Light speed is independent of the motion of the source. Relative to you, your light will travel c in any direction. You can think of it as you are standing still and everything around you is moving. Link to comment Share on other sites More sharing options...
max.yevs Posted March 29, 2009 Author Share Posted March 29, 2009 Light speed is independent of the motion of the source. Relative to you, your light will travel c in any direction. You can think of it as you are standing still and everything around you is moving. wait a sec.... say there's a beam of light... traveling at c, speed of light ther's two guys, one is standing still, and one is traveling at 0.5c know we now light always goes at speed 1c. so relative to the second guy, shouldn't it be 0.5c? or if its 1c relative to him, shouldn't it be 1.5c relative to the other one? I think that speed of anything, even light, should be taken in relation to earth, because even though its not completely stationary, its speed is insignificant compared to c. Link to comment Share on other sites More sharing options...
swansont Posted March 29, 2009 Share Posted March 29, 2009 Light can be slowed down. The individual photons that comprise it can't. The so-called stopped light is light that has been absorbed in such a way that it can be recreated. You could envision a sample of material that gives rise to slow light, that scatters some off to the side, so that you could track the progress if the sample were very thin and of sufficient length. Photon reflection or scattering does indeed exert a force. As has been mentioned, this is the idea behind the solar sail. It's also the concept behind the 1997 physics Nobel prize of laser cooling, which uses radiation pressure to slow down atoms and ions via resonant scattering. Link to comment Share on other sites More sharing options...
lakmilis Posted March 29, 2009 Share Posted March 29, 2009 recently i was thinking, if you were going at the speed of light, and you shine a flashlight forwards, would it be going at double the speed of light? of course i found out that no it can't. but this leads to an interesting modification. If you're going forward at the speed of light, and you shine a flashlight behind you, how fast would the light be moving? at 0 mph or still at the speed of light? It would be moving at c away from you... thats the great mindboggling part of relativity AND if there was an external reference point, they would also measure both to be travelling at c. The thing with those thought experiments is that an inertial reference point can't be travelling at c.. which kind of makes that thought experiment invalid... of course we can say well a photon which then emits another photon.. soo all fair. Whatever you do.. in reltivity , light will always from all valid reference frames be travelling at c, no matter how you are trying to bend this rule, it won't work. Merged post follows: Consecutive posts mergedLight can be slowed down. The individual photons that comprise it can't. The so-called stopped light is light that has been absorbed in such a way that it can be recreated. You could envision a sample of material that gives rise to slow light, that scatters some off to the side, so that you could track the progress if the sample were very thin and of sufficient length. Photon reflection or scattering does indeed exert a force. As has been mentioned, this is the idea behind the solar sail. It's also the concept behind the 1997 physics Nobel prize of laser cooling, which uses radiation pressure to slow down atoms and ions via resonant scattering. Thanks for saying this swansont, cos that is how my reality works.. and had you nto said this, I would seriously be going bleep bleep BSOD.... I would of course counter this stopping thing with something hypothetical like : If you hit a mirror with a light beam in space normal to the surface, and c is maintained.. how then can the beam change its direction 180 degrees without slowing down? (When I was young, this I posed to myself to look into refraction etc, if in fact absorption , emission etc, always creates another photon which has information passed on from the incoming photon to fire off at the appropriate direction and velocity.. eg. that mirror or light from space hitting another medium lets say, refracts and 'slows' down, for me was perhaps an indication oof a photon pair interaction... Anyway.. these were just things I was thinking about as a youngster.. standard physicist teachers just said w00t? Link to comment Share on other sites More sharing options...
NowThatWeKnow Posted March 29, 2009 Share Posted March 29, 2009 (edited) wait a sec.... say there's a beam of light... traveling at c, speed of light ther's two guys, one is standing still, and one is traveling at 0.5c know we now light always goes at speed 1c. so relative to the second guy, shouldn't it be 0.5c? or if its 1c relative to him, shouldn't it be 1.5c relative to the other one? I think that speed of anything, even light, should be taken in relation to earth, because even though its not completely stationary, its speed is insignificant compared to c. Relative to Earth? OK. Accelerate away from Earth to .99c and shut off your rocket engines. While traveling away from Earth at .99c, send a second rocket from your rocket pointed away from Earth and have it accelerate to .99c. You will see yourself moving away from Earth at .99c and the second rocket moving away from your at .99c for a combined speed of almost twice the speed of light. However, from Earth the second rocket will be moving away from Earth at just under the speed of light. Your example is frame mixing and that does not work. Each frame has its own time dilatation and length contraction conditions and should not be mixed. Relative to you, light will move at c. The only exception is using the expansion of space to separate faster then the speed of light. Edited March 29, 2009 by NowThatWeKnow Link to comment Share on other sites More sharing options...
John Cuthber Posted March 29, 2009 Share Posted March 29, 2009 Does anyoone know where I can get hold of roughly a hundred million metres of (very low loss) optical fibre? Then I could do the experiment. I think it would work- optical delay lines certainly do and I don't see any difference in principle. Link to comment Share on other sites More sharing options...
max.yevs Posted March 29, 2009 Author Share Posted March 29, 2009 (edited) Photon reflection or scattering does indeed exert a force. As has been mentioned, this is the idea behind the solar sail. It's also the concept behind the 1997 physics Nobel prize of laser cooling, which uses radiation pressure to slow down atoms and ions via resonant scattering. You know i believe i saw like a toy once, in a vaccuum, where there were like leaves painted black on one side and white on the other- and it would spin slowly in the light like a solar sail... actually yeah i've found a picture (bottom left picture). But wait a sec... if the photons bounce off, without losing any speed or weight or anything, couldn't one theoretically design a motor where like a laser beam hits the sail, bounces off and onto a mirror, bounces back onto the sail, back onto the mirror, forever... if efficiency of the mirrors and design is high enough, you could actually make more energy then you put into the lasers... there must be some flaw to this? I drew a (very stupid) sketch of such a thing...on the type of solar sail that reflects light 90 degrees in all directions. I'm not sure if a prism will work there, but I know there's things that can replace it. The mirrors don't have to be perfect. Just efficient enough so that the amoung of energy created is higher than the amount of energy put into the laser... Edited March 29, 2009 by max.yevs Link to comment Share on other sites More sharing options...
max.yevs Posted March 29, 2009 Author Share Posted March 29, 2009 (edited) where's the flaw? I'm so tired of hearing the word 'free energy', but from what i've heard, it should be possible to create more energy with the generator then you put in with the laser. assuming 100% mirror efficiency, the lasers never get wasted. So, just one pulse of light with the laser will keep bouncing around forever... And when talking about forever, the pulse of laser light is insignificant. But the tiny amount of force on the sail is very significant. i think there's some catch in this statement: "photons bouncing of a surface exert a force on it without loosing speed or energy" and i know for sure it works, just look at the solar sail toy, but how? photons have a mass of 0! Edited March 29, 2009 by max.yevs Link to comment Share on other sites More sharing options...
insane_alien Posted March 29, 2009 Share Posted March 29, 2009 you are assuming that if you have a 100mW laser, split the beam into 3 and you'll get 3 100mW beams you will not. you will get 3 33.33mW beams also, solar sails are pretty crap at extracting power from a light beam. Link to comment Share on other sites More sharing options...
max.yevs Posted March 29, 2009 Author Share Posted March 29, 2009 you are assuming that if you have a 100mW laser, split the beam into 3 and you'll get 3 100mW beams you will not. you will get 3 33.33mW beams no, i am assuming that the million or so beams of light scattered by the solar sail can somehow be recollected into the original beam of light...i know you can't see it by the sketch and the solar sail i drew there can't do that since it has a limited number of petals... but something like a spiral worm type (see picture below) solar sail probably would. also, solar sails are pretty crap at extracting power from a light beam. yeah, but even a little amount of force would eventually add up if the beam can be recollected... (this will need theoretical 99.9% efficient mirrors, but everything is theoretical in physics!) Link to comment Share on other sites More sharing options...
Cap'n Refsmmat Posted March 29, 2009 Share Posted March 29, 2009 http://blag.xkcd.com/2008/02/15/the-laser-elevator/ Link to comment Share on other sites More sharing options...
max.yevs Posted March 29, 2009 Author Share Posted March 29, 2009 thanks cap'n, very useful link. so the momentum of a photon could really be described by energy * speed of light; since it has no mass; and the energy is higher frequency (i.e. like gamma rays are more energetic than radio waves)... so i guess if a blue light is used, it will eventually turn red after bouncing back and forth a couple hundred times... and i like the idea of the dyson sphere around the sun = a super powerful death ray laser... Link to comment Share on other sites More sharing options...
swansont Posted March 30, 2009 Share Posted March 30, 2009 You know i believe i saw like a toy once, in a vaccuum, where there were like leaves painted black on one side and white on the other- and it would spin slowly in the light like a solar sail... actually yeah i've found a picture (bottom left picture). A Crookes radiometer actually spins the wrong way for the effect to be radiation pressure. Merged post follows: Consecutive posts mergedthanks cap'n, very useful link. so the momentum of a photon could really be described by energy * speed of light; since it has no mass; and the energy is higher frequency (i.e. like gamma rays are more energetic than radio waves)... so i guess if a blue light is used, it will eventually turn red after bouncing back and forth a couple hundred times... p= E/c (not E*c) Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now