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What stops laser light?

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wurf

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I couldn't find it, but I recall an old thread here where someone gave a very good explanation of why light stops being visible at a certain distance. The explanation involved holding a piece of paper in front of a flashlight beam so that the light appeared on the paper as a circle, then increasing the distance so that eventually the circle was greater than the size of the paper, so the paper was receiving less and less of the light. What about a laser beam? Does a laser beam have a constant circumference at any distance? If so, with no dispersion, what would determine the distance at which it could not be seen?
 
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MeteorWayne

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The main benefit to a laser is that since all the light is at the same frequency, it can be focused much more tightly. So the beam doesn't expand as quickly; but it does still expand. The reflectors on the moon left by the Apollo astronauts are hit with lasers from earth, but despite the tighter focus a laser gives you, IIRC the beam is several miles across when it gets to the moon. I'll have to look up the exact numbers.<br /><br />So a laser beam expands slower, but it does still expand. <br />Over interstallar distances it makes some difference, but not much.<br /><br />You would still need an incredibly powerful laser to communicate.<br /><br />It may well be the best hope for such comminucation though. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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wurf

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I see, thanks, you answered a follow-up question I had about the lunar reflectors also.<br /><br />Theoretically, if you could somehow create a beam of light whose circumference didn't increase, what would stop the light from going "forever", even if it only had the brightness of a small candle? (Forever, or until it hit the first star in its path.)
 
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MeteorWayne

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If such a thing were theroretically possible, (and even though it expands, the same applies for for a non laser), the light goes on forever.<br /><br />The problem for detection is that eventually the signal falls below the strength of the background noise, so it can no longer be picked out from the background.<br /><br />The same applies to radio signals (which are the same as light, just in a different part of the electromagnetic spectrum.)<br /><br />Hope this helps. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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Mee_n_Mac

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<font color="yellow">Theoretically, if you could somehow create a beam of light whose circumference didn't increase, what would stop the light from going "forever", even if it only had the brightness of a small candle? (Forever, or until it hit the first star in its path.) </font><br /><br />Two things to think about. In the hypothetical case above, you are correct. The light would go on forever and you would receive the signal at the same strength no matter what the distance (lacking anything in it's path to scatter or absorb the beam). In practice MW is correct, there will always be some dispersion of the beam. <br /><br />Even with the flashlight beam as described earlier, the light goes on forever. It just keeps getting weaker and weaker and weaker. At some point it gets so weak we couldn't detect it with any sensor we have but theoretically it's still there. It get's weird to think about when you consider the wave/particle dual nature of light. We know light is quantized, that is that it comes in discrete levels of "strength" and the smallest level is 1 photon. So what happens when the signal strength predicted by the inverse square law gets to be less than that of 1 photon ? <br /><br />HINT: you now must think of signal strength in terms of how many photons per second there are. <img src="/images/icons/smile.gif" /> <br /><br /><br />EDIT : Here's the post you were refering to above. <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>
 
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Mee_n_Mac

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Just as a followup to my own answer (and MW's, which I hadn't seen as I typed my redundant reply above <img src="/images/icons/smile.gif" /> ), I want to go one step further down the path your question takes us. If we are properly interpreting the data we see and the Universe is metrically expanding, then as light goes on it's journey "forever" it'll be red-shifted. "OK" you say, "that's no surprise" but the energy of a a photon is inversely proportional to it's wavelength. A red photon has less energy than a blue one, a photon at microwave frequencies less than either of those. So as out light goes out on it's journey and get's red-shifted, at what point does it get so red-shifted that it has insufficent energy to be absorbed, and thus detected, by any theoretical detector ? I don't know the answer to this one so I'll have to hope a physics minded person will stumble on by and reply properly. My guess I could theoretically have a "test charge" that would react to the passing EM wave even as it was red-shifted down to the milli-Hz range but .... ??? <img src="/images/icons/crazy.gif" /><br /><br />So analagous to the old question about a tree falling in the forest; if a photon can no longer be detected, is it really there ? <img src="/images/icons/wink.gif" /><br /> <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>
 
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MeteorWayne

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Cute question! <img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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alokmohan

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I was thinking also.But quetion paper is out of syllabus.
 
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wurf

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Thank goodness you responded to yourself, lol. I got you on the first part of your first reply. But as to that first question, where you included a hint, I wasn't sure if you were actually addressing me for comment. I didn't want to seem rude by not replying, so I'll just reply that the question and the hint went over my head with such velocity that I had to go comb my hair.
 
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wurf

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My two cents: I think the tree-in-the-forest question could be answered like this: sound is like a pair of scissors, it requires two halves. Sound requires a transmission and a reception (the air molecules being moved by the tree hitting the ground provide the transmission, a set of ears provide the reception). The air molecules moving through air without a receptor do not produce what we know as sound, only air waves, since sound only exists when the human or animal brain converts the waves to sound. Sound waves are only potential sound. So I think the answer is no, a tree falling in the forest does not make a sound. And in a universe where there were no brains with the faculty of hearing, there would be no such thing as sound.<br /><br />So if sound waves were perfectly analogous to light waves, then in a universe where there were no brains with the faculty of vision (or at least a photo-sensitive spot), then there would be no such thing as light. But sound waves aren't analagous to light waves, as sound waves don't consist of any material feature such as a photon. So, if a photon is a bit of matter which exists regardless of a receptor for it, then I guess light would exist even if there were no receptor to receive it. Then again, as you said, light has two properties in one form, perhaps as a square is four lines in one form, or a cube is six squares in one, so... hard to say until we know more. Perhaps neither sound nor light exist apart from the biological capacity for reception of them.
 
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will25

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The anthromorphic Universe. Well , it has it's points.<br /><br />If a tree falls in the forest it produces sound waves. Preiod. If some creature hears them, fine, but if not the sound still exsists for that bit of time represented by the event. Being a person of the forest I can attest to this. <br /><br />So the Universe exsists only so that we can observe it. If we couldn't do that, in even the most mundane way, by being self aware for example, the universe would revert to it's other option, chaos.
 
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MeteorWayne

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All of which is unrelated to the light question.<br /><br />Sound only carries as far as the air pressure wave exceeds the local fluctuatiuons.<br /><br />Light does go on forever. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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wurf

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Well, I wasn't intending to suggest anthropomorphism, or any philosophical tenet. I thought I was saying something that any modern physicist would agree with; that movement of air molecules, and sound perceived by the ears and brain were different things.
 
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vogon13

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IIRC, the aperature (width) of the laser source and the wavelength are also facors in the dispersion of the beam.<br /><br />The greater the aperture and the shorter the wavelength, the less the dispersion.<br /><br />If you want to just illuminate the 3 moon of Panterax 7, your going to need a laser many kilometers across.<br /><br />Conversely, if you wanted to put just a 'dot' (say 1 km across) on alpha centauri, you are going to need a gamma ray laser.<br /><br />Please consult with the NSF for funding.<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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Mee_n_Mac

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<font color="yellow">But as to that first question, where you included a hint, I wasn't sure if you were actually addressing me for comment.</font><br /><br />It was something of an open question (improperly worded too) ... more a musing to lead to discussion. We, well I, usually think of light as a wavelike phenomon even though I know it's quantized. Let's say I have a square panel, 1 meter on a side, and so it's area is 1 sq meter. I could put this out in space someplace and beam some light at it. I measure the power of this light at some moment and I come up with a power density; that is how many watts/sq meter for that place at that moment. Knowing the total power of the light I was transmitting and the beam parameters and the distance for my source to my panel, I could have also calculated this power density. If I collected all the power hitting my panel for 1 second I'd could now speak of an energy density, how many joules / sq meter. But what happens when that energy density is less than the number I'd get for 1 photon of the wavelength of light I'm transmitting ? Well it means I may or may not collect a photon in that 1 second with my panel. It becomes a probablistic thing. The further away from the source I place my panel the less likely it becomes that I'll collect that photon in any 1 second. Usually I deal with power densities where this doesn't happen so it strikes me as "funny" to think about. Astronomers probably find it odd that I find it odd. <img src="/images/icons/smile.gif" /><br /><br />Just another strange thing that happens as a result of light having properties of both waves and particles. <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>
 
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yevaud

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As explanatory sources go, Wiki is generally not great. However, this is very well written and correct.<br /><br /><i>The Maxwell wave theory, however, does not account for all properties of light. The Maxwell theory predicts that the energy of a light wave depends only on its intensity, not on its frequency; nevertheless, several independent types of experiments show that the energy imparted by light to atoms depends only on the light's frequency, not on its intensity. For example, some chemical reactions are provoked only by light of frequency higher than a certain threshold; light of frequency lower than the threshold, no matter how intense, does not initiate the reaction.</i><br /><br />http://en.wikipedia.org/wiki/Photon<br /><br />In any event, if light of a sufficient energy does strike this it can:<br /><br />Reflect.<br />Interfere with itself, leading to virtual particles and then yet another (two actually) photon.<br />Impart energy to whatever it strikes, but in so doing generate the emission of another Photon.<br /><br />So regardless, something does happens, one way or another. <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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nexium

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Let's assume a better than usual laser illuminates one square kilometer on the moon at a distance of 400,000 kilometers. At 800,000 kilometers 4 square kilometers is illuminated. At 8 million kilometers;400 square kilometers is illuminated. At 8 trillon kilometers; 400 trillion kilometers is illuminated. The brightness per unit area is now reduced by 400 trillion times and we are a bit less than a light year from Earth = 0.8 lightyears. Neil
 
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mindmute

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So what happens when the signal strength predicted by the inverse square law gets to be less than that of 1 photon ?<br /><br />Mee_n_Mac, could you clarify for a dim-wit like me what an "inverse square law" is?
 
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MeteorWayne

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You can't detect it. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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pyoko

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I believe the main problem is that it is practically impossible to colimate a laser 100% perfectly. If it was, it woud keep going until it hit something (dust etc). <div class="Discussion_UserSignature"> <p> </p><p> </p><p><span style="color:#ff9900" class="Apple-style-span">-pyoko</span> <span style="color:#333333" class="Apple-style-span">the</span> <span style="color:#339966" class="Apple-style-span">duck </span></p><p><span style="color:#339966" class="Apple-style-span"><span style="color:#808080;font-style:italic" class="Apple-style-span">It is by will alone I set my mind in motion.</span></span></p> </div>
 
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Mee_n_Mac

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<font color="yellow">Mee_n_Mac, could you clarify for a dim-wit like me what an "inverse square law" is?</font><br /><br />It means that the power density is proportional to 1/(the square of the ratio of the distances). An example or 2 is easier to understand. Say we have X watts / sq meter at a distance D. So what would the power density be at twice the distance, 2D ? It would be 1/(2D/D)^2 = 1/2^2 = 1/4 so the power density is now 25% of what it was. At 3 times the distance, 3D, it would be 1/3^2 = 1/9 = 11.11% of what it is at D. At 4D it's only 1/16 = 6.25%. So some might think twice the distance gets you down to 1/2 the power but it's worse, it's 1/4th. <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>
 
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mindmute

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oh, like the monsters of Altair Seven. hmm... i see...(lol)
 
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mindmute

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hmmm... i'll have to go back for third cup of coffee.... <br />Thanks for the reply mee-n-mac <img src="/images/icons/smile.gif" />
 
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MeteorWayne

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BTW, it was Altair 4 <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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nyrath

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Ordinary <i>(i.e., non-coherent)</i> light fades out due to the Inverse Square law.<br /><br />Laser <i>(i.e., coherent)</i> light is not subject to the inverse square law. However it is subject to Diffraction which does much the same thing.
 
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