<p> </p><p>When astronomers peer into space and see the light from distant galaxies and stars they see the light as it comes to us from a long way away. When the astronomers look in the exact opposite direction, why can they not see the light that has already passed them? why can we see light that travels towards us, but not see light that travels away from us? ie: shouldn't we be able to see the universe as though we were standing in front of a mirror?</p> <div class="Discussion_UserSignature"> <p><font size="4" color="#ff0000"><font size="2">Assumption is the mother of all stuff ups</font> </font></p><p><font size="4" color="#ff0000">Gimme some Schmack Schmack!</font></p> </div>
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<p>______________</p><p>shouldn't we be able to see the universe as though we were standing in front of a mirror?</p><p>______________</p><p> </p><p>We can! And it is exactly like a mirror. IF the light that passes us reflects off of something, and comes back, then we could get a second chance to see it. However, if it does not reflect off of anything, then we won't get a second chance.</p><p>The photons that make up light are just like any other object. We can only see them when they impinge upon our eyes, telescopes, cameras, etc. If they miss...well, then we don't see them.</p><p> </p><p>There are some nebulae out there called "Reflection" nebulae, because they reflect nearby starlight, but don't create any of their own. All the light we see has the basic spectra of nearby stars whose light is reflecting, with a few other differences introduced by the nebulae doing the reflecting (i.e. it isn't a perfect reflection). </p> <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>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> When astronomers peer into space and see the light from distant galaxies and stars they see the light as it comes to us from a long way away. <u>When the astronomers look in the exact opposite direction, why can they not see the light that has already passed them? </u>why can we see light that travels towards us, but not see light that travels away from us? ie: shouldn't we be able to see the universe as though we were standing in front of a mirror? <br />Posted by <strong>schmack</strong></DIV></p><p>We can't pretty much for the same reason we can't catch a baseball that has already passed by our glove. It's gone past. Now if it bounces off something and comes back to us or if we run really fast to get ahead of the baseball, we may get another chance to catch it otherwise we're out of luck. In the case of light, the latter anaolgy is rather hard to do. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /></p><p>Now I have the gut feeling that I haven't really understood your question ....</p><p> </p> <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>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> When astronomers peer into space and see the light from distant galaxies and stars they see the light as it comes to us from a long way away. When the astronomers look in the exact opposite direction, why can they not see the light that has already passed them? why can we see light that travels towards us, but not see light that travels away from us? ie: shouldn't we be able to see the universe as though we were standing in front of a mirror? <br />Posted by schmack</DIV><br /><br />The real question is this: If Einstein was right about the universe being unbounded, meaning you can circumnavigate the universe and end up back at your starting point, why is it that we haven't seen light that has itself circumnavigated the universe?</p><p>I guess I can think of four possibilities to answer my own question:</p><p>1: I don't know enough about physics to understand the mechanics behind this!</p><p>2: The traveling light may be warped, skewed, or lost within the expansion of the universe.</p><p>3: The universe simply isn't old enough for light to have circumnavigated the entire universe and made it back to Earth. </p><p>Or 4: Einstein was wrong altogether about the overall nature of the entire known universe.</p><p>Now someone come in here and tell me I don't know what I'm talking about! <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /> </p> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>We can't pretty much for the same reason we can't catch a baseball that has already passed by our glove. It's gone past. Now if it bounces off something and comes back to us or if we run really fast to get ahead of the baseball, we may get another chance to catch it otherwise we're out of luck. In the case of light, the latter anaolgy is rather hard to do. Now I have the gut feeling that I haven't really understood your question .... <br />Posted by Mee_n_Mac</DIV><br /><br />Hi Mee_n_Mac,</p><p>The baseball analogy is interesting. because i can understand not being able to catch a ball that has gone past me. But i can still see the ball once it has gone past me. </p><p>I think maybe the question should be more like - "why don't we see continuous light lines, like a torch or laser light through mist and smoke?" or ""why can't i just turn around and see the back side of the light that has come from the moon and stars after it has passed me just like i can turn around and see the back side of a baseball that has passed me? especially since the light is travelling in a continuous stream."</p><p> </p> <div class="Discussion_UserSignature"> <p><font size="4" color="#ff0000"><font size="2">Assumption is the mother of all stuff ups</font> </font></p><p><font size="4" color="#ff0000">Gimme some Schmack Schmack!</font></p> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'><font color="#3366ff">Hi Mee_n_Mac, The baseball analogy is interesting. because i can understand not being able to catch a ball that has gone past me. But i can still see the ball once it has gone past me.</font>Posted by schmack</DIV></p><p>The reason you can see the ball once it has passed you is that the Sun (or floodlights at night) is throwing photons at the moving baseball and those photons bounce back at you and hit you directly in the eye. You have to catch the photons with your eyeball. </p><p>You only see photons that enter your eye, hit your retina and send a message to your brain. If someone is standing behind you, looking over your shoulder, you cannot see the photons that bounce off them and come past you unless those photons are reflected back into your eye by a mirror at which point you see them looking at you from over your shoulder! </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Hi Mee_n_Mac,The baseball analogy is interesting. because i can understand not being able to catch a ball that has gone past me. But i can still see the ball once it has gone past me. I think maybe the question should be more like - "why don't we see continuous light lines, like a torch or laser light through mist and smoke?" or ""why can't i just turn around and see the back side of the light that has come from the moon and stars after it has passed me just like i can turn around and see the back side of a baseball that has passed me? especially since the light is travelling in a continuous stream." <br />Posted by <strong>schmack</strong></DIV><br /><br />Can we agree that you only get to see light that hits your eyeballs and that the light you then see is the light that is hitting your eyeballs at <strong>that</strong> moment. Same would go for any detector other than your eyes, it can only detect the EM wave (light) as it impinges on the detector. So now imagine a laser which is off, pulsed on for 1 second and then turned of again. In a true vacuum you only see the laser if it's aimed at your eyes, or detect it if our detector is in the path. If there's smoke or other small particles in the path, they may reflect a portion of the laser to your eyes or detector, but in their absence you don't receive any light. Even if your eyes or detector is in the path, the laser can only affect it when the "on" pulse is hitting them. After the pulse has passed, it can't affect the eyes/detector. In this case you can imagine the light to be a packet of particles. The particles can only cause you to see, or our detector to detect, when and if they are hitting something.</p><p>Another analogy would be a pond with a ping pong ball floating on it. It detects waves by moving up and down. The ball knows nothing about the universe other than by what it can detect by it's motion (heck, it's only a ping pong ball, what can you expect <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /> ). You throw a rock into the pond causing a series of ripples. They travel outwards towards the ball. Until they arrive, the ball detects nothing. It can not "see" the "light". Then the ripples arrive and move the ball up and down. The ball now "sees". The ripples pass by and the ball then remains motionless again. Light hitting your eyeballs, or a detector, acts in the same way.</p><p>Now <em>why</em> does the universe act this way .... I dunno ... it just does.</p><p>EDIT : It appears SF had a similar answer. I just gots to lurn to type faster ....</p> <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>
<p>The photon is not a source of light, it is a carrier of electromagnetic radiation. A single photon does not emit light in all directions. A photon, being a quanta of light, can not emit other photons in the opposite direction it is travelling. </p><p>Once that particle has passed you, you can not see it because there is nothing being emitted from it. Only through direct interaction with some form of receptor (our eyes being one) can that photon be detected and interpreted.</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
<p> </p><p>perfectly explained, and now understood. thanks heaps guys!</p> <div class="Discussion_UserSignature"> <p><font size="4" color="#ff0000"><font size="2">Assumption is the mother of all stuff ups</font> </font></p><p><font size="4" color="#ff0000">Gimme some Schmack Schmack!</font></p> </div>
Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> perfectly explained, and now understood. thanks heaps guys! <br />Posted by schmack</DIV><br /><br />You're welcome heaps! I have an Aussie friend who always said Heaps! Oh how I miss her! It's definitely not used in American lingo. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> When astronomers peer into space and see the light from distant galaxies and stars they see the light as it comes to us from a long way away. When the astronomers look in the exact opposite direction, why can they not see the light that has already passed them? why can we see light that travels towards us, but not see light that travels away from us? ie: shouldn't we be able to see the universe as though we were standing in front of a mirror? <br />Posted by schmack</DIV></p><p>I ever heard somepeople say that if we can travel extremely fast to one direction in the universe, we will end up where we started, so perhaps a very old light may have circle around the universe and back to us again? or perhaps it is too old and fade away? i am not a physicist, just want to share something. 
</p> <div class="Discussion_UserSignature"> </div>
</p> <div class="Discussion_UserSignature"> </div><p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I ever heard somepeople say that if we can travel extremely fast to one direction in the universe, we will end up where we started, so perhaps a very old light may have circle around the universe and back to us again? or perhaps it is too old and fade away? i am not a physicist, just want to share something. <br /> Posted by lukman</DIV></p><p>Current observations have the geometry of spacetime flat to with in 2%. IOW, if 2 parallel beams of lights are emitted they will never cross (excluding gravitational lensing). This is, of course, based only on the observable universe which could be only a small segment of the entire Universe. That is something we can't know with our current physics capabilities. It's like looking at a footbal field and trying to discern the curvature of the earth.</p><p>As for fading light... there is no known mechanism to cause a photon to fade. Beams of light may scatter and become diffuse, but the actual photon itself will never fade with age or distance. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
<br /> Posted by lukman</DIV></p><p>Current observations have the geometry of spacetime flat to with in 2%. IOW, if 2 parallel beams of lights are emitted they will never cross (excluding gravitational lensing). This is, of course, based only on the observable universe which could be only a small segment of the entire Universe. That is something we can't know with our current physics capabilities. It's like looking at a footbal field and trying to discern the curvature of the earth.</p><p>As for fading light... there is no known mechanism to cause a photon to fade. Beams of light may scatter and become diffuse, but the actual photon itself will never fade with age or distance. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div><p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Current observations have the geometry of spacetime flat to with in 2%. IOW, if 2 parallel beams of lights are emitted they will never cross (excluding gravitational lensing). This is, of course, based only on the observable universe which could be only a small segment of the entire Universe. That is something we can't know with our current physics capabilities. It's like looking at a footbal field and trying to discern the curvature of the earth.As for fading light... there is no known mechanism to cause a photon to fade. Beams of light may scatter and become diffuse, but the actual photon itself will never fade with age or distance. <br />Posted by derekmcd</DIV></p><p>True. But the notion flatness can be a bit non-intuitive. A typical right circular cylinder is also flat. You can locally map it onto a plane and preserve distances, and geodesics (just roll it as they do with a printing press). I do not believe that it is known whether the universe admits closed geodesics or not. Before the current fascination with the accelerated expansion, the big question was whether there was enough mass in the universe to close it. If it is closed then in principle light might return to the point of origin. I think there may be some problems involving the expansion of space-time, that would prevent this, but would need to do some book research to check that out. Maybe somebody else already this information at his fingertips.</p><p>I am a little bit curious as to what it might mean to be flat within 2%. Curvature is a tensor.</p><p>Even if there are closed geodesics and if light could complete one, I suspect that the scattering and diffusing that you mentioned would prevent formation of any real images from way, way back. One photon does not make an image, even if has not changed in energy since time immemorial.</p> <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>True. But the notion flatness can be a bit non-intuitive. A typical right circular cylinder is also flat. You can locally map it onto a plane and preserve distances, and geodesics (just roll it as they do with a printing press). I do not believe that it is known whether the universe admits closed geodesics or not. Before the current fascination with the accelerated expansion, the big question was whether there was enough mass in the universe to close it. If it is closed then in principle light might return to the point of origin. I think there may be some problems involving the expansion of space-time, that would prevent this, but would need to do some book research to check that out. Maybe somebody else already this information at his fingertips.I am a little bit curious as to what it might mean to be flat within 2%. Curvature is a tensor.Even if there are closed geodesics and if light could complete one, I suspect that the scattering and diffusing that you mentioned would prevent formation of any real images from way, way back. One photon does not make an image, even if has not changed in energy since time immemorial. <br /> Posted by DrRocket</DIV></p><p> </p><p>After rereading my post, i realize I didn't really answer his question correctly. I, apparently, confused the curvature of space with the possibilities of the topology of space. Topology is something i'm not all too familiar with. I understand some the concepts, but it's really beyond understanding. The 2% I referred to is the measure of critical density, not the actual flatness. The critical density has been measured to be 1.02 (+/- 2%), so it can be said the universe is flat or nearly flat with positive curvature. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
See if you were a photon of light during your trip either 1 meter or 1 million light years you would experience no time or distance. See a photon makes no sense if it does not have a place to be absorbed or reflected. So a photon experience zero time and distance and this can be explained very simply. The momentum of E.M. particles (photons) is the power divided by the speed of light squared (c^2). This only applies in vacuum, now for instance if you have a light source flash light, laser ect. and direct it toward the cosmological horizon or your own horizon (if your in constant acceleration)where the photon had no place to be absorbed. Than the light directed towards that direction of the horizon should lower the power of your source being used. <font color="#003399">derekmcd</font> you are also wrong in your earlier statement as a quote what you said "A single photon does not emit light in all directions." Did you ever read Q.E.D.? Richard P. Feynman stated that monochromatic light being emitted, a single photon at a time and squeezed through a small space will tend to spread out in various direction's. Not to insult you, but what is your educational level in physics? <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>See if you were a photon of light during your trip either 1 meter or 1 million light years you would experience no time or distance. See a photon makes no sense if it does not have a place to be absorbed or reflected. So a photon experience zero time and distance and this can be explained very simply. The momentum of E.M. particles (photons) is the power divided by the speed of light squared (c^2). This only applies in vacuum, now for instance if you have a light source flash light, laser ect. and direct it toward the cosmological horizon or your own horizon (if your in constant acceleration)where the photon had no place to be absorbed. Than the light directed towards that direction of the horizon should lower the power of your source being used. derekmcd you are also wrong in your earlier statement as a quote what you said "A single photon does not emit light in all directions." Did you ever read Q.E.D.? Richard P. Feynman stated that monochromatic light being emitted, a single photon at a time and squeezed through a small space will tend to spread out in various direction's. Not to insult you, but what is your educational level in physics? <br />Posted by dabiznuss</DIV></p><p>derekmcd does pretty well with physics. You are a bit off base and probably ought to brush up a little yourself. If one detects a photon, even after it has been put through a small space like a slit it will be detected as a point. You seem to have misunderstood what Feynman was saying. I have read QED among other things. Here is a series of pictures, from Wikipedia http://en.wikipedia.org/wiki/Double_slit_experiment showing the classic double slit interference patterns, using electrons (the results would be the same with photons), showing the interference patterns being created, one particle at at time. You will note that the electrons are not spread out. What spreads out is a probability distribution describing where the electron, which is a point, will be detected if one does detect it. In addition to QED you might consider reading <em>More than One Mystery </em>(there is a similar set of pictures in that book) which is at a slightly higher level but still accessible. Or if you fancy yourself a physicist you might try Feynmans <em>Quantum Electrodynamics </em>which requires a bit more mathematics.</p><p> </p><div class="thumbinner" style="width:202px"><img class="thumbimage" src="http://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Double-slit_experiment_results_Tanamura_2.jpg/200px-Double-slit_experiment_results_Tanamura_2.jpg" border="0" alt="Electron buildup over time" width="200" height="580" /> <div class="thumbcaption"><div class="magnify"><img src="http://null/skins-1.5/common/images/magnify-clip.png" alt="" width="15" height="11" /></div>Electron buildup over time</div></div><p><br /><br /> </p> <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>See if you were a photon of light during your trip either 1 meter or 1 million light years you would experience no time or distance. See a photon makes no sense if it does not have a place to be absorbed or reflected. So a photon experience zero time and distance and this can be explained very simply. The momentum of E.M. particles (photons) is the power divided by the speed of light squared (c^2). This only applies in vacuum, now for instance if you have a light source flash light, laser ect. and direct it toward the cosmological horizon or your own horizon (if your in constant acceleration)where the photon had no place to be absorbed. Than the light directed towards that direction of the horizon should lower the power of your source being used. derekmcd you are also wrong in your earlier statement as a quote what you said "A single photon does not emit light in all directions." Did you ever read Q.E.D.? Richard P. Feynman stated that monochromatic light being emitted, a single photon at a time and squeezed through a small space will tend to spread out in various direction's. Not to insult you, but what is your educational level in physics? <br /> Posted by dabiznuss</DIV></p><p>DrRocket correctly described it as a probability distribution. QED can not predict where that single photon will land. However, given enough photons, QED can, quite accurately, predicted a pattern. </p><p>For any particle to emit anything in a vacuum, it needs to spontaneously decay. Seeing as you understand that a photon does not experience time, it obviously can not have a half-life to decay and emit (radiate) anything. Considering a photon does not experience time and is considered to be massless in its rest state, I see no possible mechanism for a photon to emit other photons (radiation) in any direction. </p><p>I stand by my statement that a photon does not emit light in all directions. </p><p>Additionally, I'm not quite clear on your description of momentum. Classic momentum is described as mass * velocity. However, a photon is massless, so you must use its energy which is described using either it's frequency or wavelength.</p><p>The momentum of a photon is, essentially, E/c. </p><p>Not sure what you mean by power/c^2 or your follow-up statement of loweing the power of your source.</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>derekmcd does pretty well with physics. You are a bit off base and probably ought to brush up a little yourself. If one detects a photon, even after it has been put through a small space like a slit it will be detected as a point. You seem to have misunderstood what Feynman was saying. I have read QED among other things. Here is a series of pictures, from Wikipedia http://en.wikipedia.org/wiki/Double_slit_experimentshowing the classic double slit interference patterns, using electrons (the results would be the same with photons), showing the interference patterns being created, one particle at at time. You will note that the electrons are not spread out. What spreads out is a probability distribution describing where the electron, which is a point, will be detected if one does detect it. In addition to QED you might consider reading More than One Mystery (there is a similar set of pictures in that book) which is at a slightly higher level but still accessible. Or if you fancy yourself a physicist you might try Feynmans Quantum Electrodynamics which requires a bit more mathematics. Electron buildup over time <br />Posted by DrRocket</DIV></p><p>I am not referring to the two slit experiment, and in quantum mechanics we are concerened with probability distribution. b/c it seemingly gives classical result such as angle I = angle R. And the example i was giving was the more you squeeze light through say two bricks the more it tends to spread.</p> <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I am not referring to the two slit experiment, and in quantum mechanics we are concerened with probability distribution. b/c it seemingly gives classical result such as angle I = angle R. And the example i was giving was the more you squeeze light through say two bricks the more it tends to spread. <br /> Posted by dabiznuss</DIV></p><p>Your two brick analogy is precisely describing slit experiments. You also used that experiment to imply that the photon radiates in different directions by trying to counter my statement that photons do not radiate in different directions.</p><p>You are not being clear in what you are trying to describe here. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I am not referring to the two slit experiment, and in quantum mechanics we are concerened with probability distribution. b/c it seemingly gives classical result such as angle I = angle R. And the example i was giving was the more you squeeze light through say two bricks the more it tends to spread. <br />Posted by dabiznuss</DIV></p><p>How might one go about squeezing "light through two bricks"? I have no idea what that is supposed to mean.<br /></p> <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> When astronomers peer into space and see the light from distant galaxies and stars they see the light as it comes to us from a long way away. <font color="#ff9900">When the astronomers look in the exact opposite direction, why can they not see the light that has already passed them?</font> why can we see light that travels towards us, but not see light that travels away from us? ie: shouldn't we be able to see the universe as though we were standing in front of a mirror? <br /> Posted by schmack</DIV></p><p>that is good question, we 'see' by means of photons (particles of light) hitting our eyes and there is no other 'light' by means of which we could watch the 'photon light' except again by photons</p><p>point is we do not see photons as such with our eyes but we see other things by means of them, that is by means of photons, so when photons that carry some image of star have passed us by, we have no means to see that image 'from behind' as it is receding from us, it would only be possible if there were some secondary light other than photons and faster than photons (like some hypothetical tachyons, particles that travel faster than photons - that is faster than light) </p><p>now, photons have this special ability of not ever coliding with themselves, they just pass through themselves (as far as we can tell) when they are on a collision path (if you shine two flashlights so that the two light beams cross themselves, they just pass through each other without any interference) that phenomenon is called superposition in physics - the upshot of this phenomenon is that even if some photons could catch up other photons from behind which wouldn't happen because they all travel at the same speed but lets say they would intercept them from side and reflect back to you (your eyes) in a glancing collision... but wait that is precisely what they cannot do, they can't collide and so they can't reflect off other photons to carry back to you the 'picture' that has already passed you</p><p> </p><p>what you seem to be assuming is that picture of stars as we look out at night sky travels to us in the form of a bunch of photons arranged in the shape of them (of stars) and as it comes close to us we look at this picture by some separate means of vision (separate from photon vision) and that then implies we should see that picture from its rear side mirror reversed as it is receding from having passed us already but we have no such separate means of vision, as I said we do not see photons but we see by means of them as they get absorbed in our eyes, that said it is still like mirracle that we see as we do and grasping how we can see things falls into the same category like grasping dimensions of cosmos or microscopic world, both of which are hard or impossible to truly grasp and stunning when we try to comprehend them<br /> </p> <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>...now, photons have this special ability of not ever coliding with themselves, they just pass through themselves (as far as we can tell) when they are on a collision path (if you shine two flashlights so that the two light beams cross themselves, they just pass through each other without any interference) that phenomenon is called superposition in physics ...Posted by vandivx</DIV></p><p>Superposition most certainly does not mean that there is no interaction. With respect to waves superposition produces interference patterns, light interfering with itself. Look at the double slit experiment for an example. Interaction among light waves has been known and understood for many many years. </p><p>At the quantum level photons may collide with photons. Such collisions are a topic in particle physics. http://mrx.pppl.gov/</p><p>Basically you have seriously misunderstood the physics of light. <br /></p> <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Basically you have seriously misunderstood the physics of light. <br /> Posted by DrRocket</DIV> </p><p>I am right on the macrolevel I talk about which was the level on which the question was asked, your problem is that you didn't see or ignore the context of the question asked, such explanation as you might have offered would get the answer burried by unnecessary details of quantum level about which clearly he wasn't asking (in fact everybody here mised the point what his querry was about, that's why I replied at all and if you noticed I didn't feel the need to put anybody down for that and wouldn't mention it if you didn't come at me as you did), you would make poor teacher if I may say so, a little bit less haste in your criticism would be more becoming and friendly, as I said before this shouldn't be one upmanship forum </p><p>first or second year physics explains it as I did, same with your typical 'physics for laymen' books out there unless they are dealing with light on quantum level *, besides all that the quantum level interference wouldn't make one whit of a difference in what he was asking - you still can't see the light that has passed you, from behind it (or are you saying you can?) - so why pull it in unless one would want to exhibit one's knowledge, I believe one should focus rather on answering what was asked and only if the questioner then want's more detail explanation is the time to offer it</p><p>(*before you say it, yes there may be some interference on classical level where two flashlight beams cross (constructive/destructive depending on coherence of the beams) but the beams come out the same as they went in and that's what was relevant there) </p><p> </p><p>here's one maybe not too authoritative source (but at least it doesn't come from me who 'seriously misunderstands' the subject as you put it) that deals with this issue on the level the question was originally asked </p><p>http://answers.google.com/answers/threadview/id/700062.html</p> <div class="Discussion_UserSignature"> </div>
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> I am right on the macrolevel I talk about which was the level on which the question was asked, your problem is that you didn't see or ignore the context of the question asked, such explanation as you might have offered would get the answer burried by unnecessary details of quantum level about which clearly he wasn't asking (in fact everybody here mised the point what his querry was about, that's why I replied at all and if you noticed I didn't feel the need to put anybody down for that and wouldn't mention it if you didn't come at me as you did), you would make poor teacher if I may say so, a little bit less haste in your criticism would be more becoming and friendly, as I said before this shouldn't be one upmanship forum first or second year physics explains it as I did, same with your typical 'physics for laymen' books out there unless they are dealing with light on quantum level *, besides all that the quantum level interference wouldn't make one whit of a difference in what he was asking - you still can't see the light that has passed you, from behind it (or are you saying you can?) - so why pull it in unless one would want to exhibit one's knowledge, I believe one should focus rather on answering what was asked and only if the questioner then want's more detail explanation is the time to offer it(*before you say it, yes there may be some interference on classical level where two flashlight beams cross (constructive/destructive depending on coherence of the beams) but the beams come out the same as they went in and that's what was relevant there) here's one maybe not too authoritative source (but at least it doesn't come from me who 'seriously misunderstands' the subject as you put it) that deals with this issue on the level the question was originally asked http://answers.google.com/answers/threadview/id/700062.html <br />Posted by vandivx</DIV></p><p>First you overlooked the first two responses to the OP, which provided a correct and quite simple answer to the question. Basically you don't see a photon that has passed you for the same reason that you can't catch a basefall that has passed you -- it has passed you. Then you misstated key points regarding the physics of light, at both a macroscopic and quantum level. And you seem to think that quantum interference and macroscopic interference are somehow manifestations of different phenomena. The interference patterns are explained quite nicely by classical wave theory, until you start to deal with the formation of those patterns one photon at a time. But the phenomena are the same whether you are talking in the language of quantum theory or not.</p><p>Your example of flashlight beams was incorrect in terms of what you said about photons and interference and irrelevant to the original question. You succeeded in failing to address the question and in misleading people about the behavior of light at both a macroscopic and a quantum level.</p><p>The reason that you don't see interference patterns with flashlights is that the white light spectrum is quite broad. </p><p>So before you start trying to dress anyone down you might want to first understand the original question, see what answers have been provided, get your facts straight, and then post an answer that is both correct and addresses the question.</p><p>You missed on all counts. Then compounded the error in your next post.</p><p> </p> <div class="Discussion_UserSignature"> </div>
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