# Light Travel

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#### vijaywantstoknow

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What is the maximum distance light can travel in space? What are the factors(external/internal disturbances) that could affect this propagation?

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#### search

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Basically there is no limit on the distance other than the physical limit of the universe size if there is any. <br /><br />imagine that the line is the size of the universe and the dots the "outside":<br /><br />_________________________________-----------------------------------<br /><br /><br />Light can travel the distance equivalent to the line but could not travel outside otherwise this would mean that energy would not be conserved in the universe. Energy would be lost. <br /><br />In vacuum light travels at speed c=299,792,458 metres per second (1,079,252,848.8 km/h), 186,282.397 miles per second or 670,616,629.384 miles per hour.<br /><br />It is the speed of all electromagnetic radiation in a vacuum, not just visible light.<br /><br />When light is subject to physical conditions other than a vacuum, the effective velocity of the electromagnetic waves that make up the light can change.<br /><br />In reality the speed of the photon did not change but depending on the refraction index the photon takes a path that is a longer distance than the metric displacement along its trajectory appearing to slow down the speed of light since the photons are traveling the speed of light along a longer distance.<br /><br />Same distance different medium. Speed of photon is the same but distance needed to travel is longer:<br /><br />Vacuum _______________________ photon arrives<br /><br />Other __-_-_-_-_-_-_-_-_-_-_-_-_- photon arrives later <br /><br />It is a bit like driving a car on a free highway at 100Km/h and then driving same highway with lots of traffic and same speed but having to change lanes all the time. The second case will arrive later although it was travelling at same speed because the distance travelled was longer.

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#### vijaywantstoknow

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If light is able to travel to the end of the universe, what stops it from travelling an extra inch? Could there be a shield at the end of the universe, which stops/reflects the light?

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#### nexium

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Photons continue to travel until they come to a barrier. Several percent of the photons produced 13 billion years ago are likely still enroute. The photon density decreases as the square of the distance, so light from 13 billion years ago is very weak. Neil

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#### search

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<font color="yellow">If light is able to travel to the end of the universe, what stops it from travelling an extra inch? Could there be a shield at the end of the universe, which stops/reflects the light?</font><br /><br />The present laws of physics.<br /><br />Do you know aliens that saw light from outside the universe? <img src="/images/icons/wink.gif" />

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#### search

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That is because photons when interacting with matter can be absorved and the left over is radiant energy or electromagnetic radiation. CMBR is a relic radiant energy. The oldest known (detected) form of electromagnetic radiation.

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#### heyscottie

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Tough question. Nobody knows what is beyond the "edge" of the universe, or even whether there IS an edge, or even whether it makes sense to talk about an edge. Hypothesizing about what happens to light at this uncertain spot is not likely to be fruitful.<br /><br />It is even possible that there is in fact a defined edge, but that light will never get to it, because it is receding from all points at a velocity greater than c.

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#### search

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Remember also that we do not know in fact the curvature of the universe.<br />If ? = 1, the curvature is zero;<br />If ? /> 1, there is positive curvature; <br />If ? < 1, there is negative curvature.<br />Light would follow this curvature.

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#### centurion001

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<p>Question.&nbsp; </p><p>We know that light travels @ 186k miles per sec..&nbsp; If light can travel @ that speed, why can't something else travel at that speed??&nbsp; </p><p>Could it be possible to surround ourself's with light, or&nbsp;to travel at the same speed?&nbsp;A light wave consists of energy in the form of electric and magnetic fields...</p><p>Is that what warp speed is??</p><p>&nbsp;Alan&nbsp;</p>

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#### why06

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Question.&nbsp; We know that light travels @ 186k miles per sec..&nbsp; If light can travel @ that speed, why can't something else travel at that speed?? </DIV></p><p>&nbsp;</p><p>Light has no mass. An object with mass could never accelerate to the speed of light because it would require an infinite amount of energy. It takes a HUGE particle accelerator and an INCREDIBLE amount of power just to get protons, sub-atomic particles. anywhere near the speed of light. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Could it be possible to surround ourself's with light, or&nbsp;to travel at the same speed?&nbsp;A light wave consists of energy in the form of electric and magnetic fields...Is that what warp speed is??&nbsp;Alan&nbsp; </DIV></p><p>I'm not sure what you mean by "surround ourselves with light" and I believe warp speed is a fictional term which deals with going faster than the speed of light or bypassing the effect of relativity in one way or another. </p> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

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#### centurion001

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;Light has no mass. An object with mass could never accelerate to the speed of light because it would require an infinite amount of energy. It takes a HUGE particle accelerator and an INCREDIBLE amount of power just to get protons, sub-atomic particles. anywhere near the speed of light. I'm not sure what you mean by "surround ourselves with light" and I believe warp speed is a fictional term which deals with going faster than the speed of light or bypassing the effect of relativity in one way or another. <br />Posted by why06</DIV><br /><br />It's hard to imagine something not having mass, but if it could be possible to in effect, creat an envelop around an object with mass made up of&nbsp; the actual makeup of light, could it be possible to reach sppead light?&nbsp;&nbsp; Light Photons do not have mass?

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Question.&nbsp; We know that light travels @ 186k miles per sec..&nbsp; If light can travel @ that speed, why can't something else travel at that speed??&nbsp; Could it be possible to surround ourself's with light, or&nbsp;to travel at the same speed?&nbsp;A light wave consists of energy in the form of electric and magnetic fields...Is that what warp speed is??&nbsp;Alan&nbsp; <br />Posted by centurion001</DIV></p><p>Anything with a rest mass of zero can, must and does travel at the speed of light.</p><p>Anything with a non-zero rest mass experiences a mass increase, quantified by the Lorentz transformation, that would result in infinite mass, hence infinite energy at the speed of light.&nbsp; Since infinite energy is not available, nothing with positive rest mass can travel at the speed of light.</p> <div class="Discussion_UserSignature"> </div>

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#### why06

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>It's hard to imagine something not having mass, but if it could be possible to in effect, creat an envelop around an object with mass made up of&nbsp; the actual makeup of light, could it be possible to reach sppead light?&nbsp;&nbsp; Light Photons do not have mass? <br /> Posted by centurion001</DIV></p><p>Wrapping something in light might just create a really shiny spaceship. There is only one way for mass to go the speed of light and that is for it to be purely converted to energy. However, though blowing your self up into pure energy may reduce your travel time, I don't think the end destination will be as worthwhile if you remain in that state. There may be a way to put yourself back together again, but it would be incredibly complex; like building a living breathing creature with your color printer.</p><p>So if your interested in ways to circumvent relativity I suugest you learn about wormholes.&nbsp; http://en.wikipedia.org/wiki/Wormhole</p> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

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#### ZenGalacticore

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<p>Do not photons have a minimal mass? Maybe not.</p><p>Warp drive, however, is not light speed. Warp refers to the warping of the fabric of spacetime. The Enterprise, in warp drive, did not travel from point A to point B across space. Rather, the power of the warp engines folds the intervening space to where point A meets point B, in a sense bringing pt B to A. It creates it's own artificial wormhole. This is why Captain Kirk and Co. didn't have to worry about time dilation. How convenient!!</p><p><font color="#ff9900">Kirk- Scotty, what's our speed?</font></p><p><font color="#ff0000">Scotty- We're going around in circles at warp 10 sir; and at that speed we're goin' nowhere mighty fast.</font></p><p><font color="#000000">BTW, I like your handle name, in Spaceghost voice: CENTURION 001!!!!!</font></p><p>I&nbsp;just noticed that this is your first&nbsp;post. Welcome Centurion001! It's a lot of fun here, and very informative.</p><p>I would like to add that the warping of space is a legitimate topic of physics. After all, celestial bodies warp space. Even a single person warps space, however infintesimally. Unfortunately, it would take the energy equivalent of an entire typical spiral galaxy to warp space and zip around(a mere)&nbsp; 3,000 light-year radius like&nbsp;Kirk, Spock, Scotty&nbsp;and the boys.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p>ZenGalacticore</p> </div>

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#### ZenGalacticore

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Anything with a rest mass of zero can, must and does travel at the speed of light.Anything with a non-zero rest mass experiences a mass increase, quantified by the Lorentz transformation, that would result in infinite mass, hence infinite energy at the speed of light.&nbsp; Since infinite energy is not available, nothing with positive rest mass can travel at the speed of light. <br />Posted by DrRocket</DIV><br /><br />But there's nothing to stop one from <em>approaching </em>the speed of light, from what I've read on the subject. If we could achieve 10%c, that would be good enough for me!<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-cool.gif" border="0" alt="Cool" title="Cool" /> <div class="Discussion_UserSignature"> <p>ZenGalacticore</p> </div>

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#### centurion001

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>But there's nothing to stop one from approaching the speed of light, from what I've read on the subject. If we could achieve 10%c, that would be good enough for me! <br />Posted by ZenGalacticore</DIV></p><p>&nbsp;</p><p>To all, thank you.&nbsp; Very interesting and appreciative.&nbsp; </p><p>So light has no mass @ all????? Photn, nothing??<br /></p>

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#### amaterasu

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Anything with a rest mass of zero can, must and does travel at the speed of light.Anything with a non-zero rest mass experiences a mass increase, quantified by the Lorentz transformation, that would result in infinite mass, hence infinite energy at the speed of light.&nbsp; Since infinite energy is not available, nothing with positive rest mass can travel at the speed of light. <br />Posted by DrRocket</DIV><br /><br />are you implying that 'relativistic' mass implies special relativity here? <div class="Discussion_UserSignature"> </div>

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#### centurion001

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>are you implying that 'relativistic' mass implies special relativity here? <br />Posted by amaterasu</DIV></p><p>Let me try this.&nbsp; Is there any one part or element of light, or something that has mass in the makeup of light?&nbsp; If it has mass, than something else with mass can traqvel that speed, no?</p>

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#### amaterasu

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Let me try this.&nbsp; Is there any one part or element of light, or something that has mass in the makeup of light?&nbsp; If it has mass, than something else with mass can traqvel that speed, no? <br />Posted by centurion001</DIV><br /><br />oh, DrRocket says, <em>'anything' with a non-zero rest mass experiences a mass increase, quantified by the Lorentz transformation</em>' but i thought the concept of the mass increasing with speed was obsolete in modern physics literature,&nbsp;or so i heard.<br /><br />here's what Einstein himself had to say on this:<br /><br />"It is not good to introduce the concept of the mass of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the 'rest mass' m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion."<br />http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/tdil.html#c5<br /><br />you just can't calculate such effects using variable mass.</p> <div class="Discussion_UserSignature"> </div>

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#### why06

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>oh, DrRocket says, 'anything' with a non-zero rest mass experiences a mass increase, quantified by the Lorentz transformation' but i thought the concept of the mass increasing with speed was obsolete in modern physics literature,&nbsp;or so i heard.here's what Einstein himself had to say on this:"It is not good to introduce the concept of the mass of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the 'rest mass' m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion."http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/tdil.html#c5you just can't calculate such effects using variable mass. <br /> Posted by amaterasu</DIV></p><p>WTH is going on? Eienstien wouldn't bash his own theory like that. I think maybe your misintepreting what he said or this was taken out of context...</p> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

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#### amaterasu

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>WTH is going on? Eienstien wouldn't bash his own theory like that. I think maybe your misintepreting what he said or this was taken out of context... <br />Posted by why06</DIV><br /><br />&nbsp; oh no worries.&nbsp; Einstein didn't bash anything here.&nbsp; he was most certainly not saying the concept of relativistic mass is wrong per se, either.&nbsp; it's just that&nbsp;the concept seems&nbsp;not to be terribly practical when it comes to calculating&nbsp;inertia and gravity effects correctly.&nbsp; it also tends to be&nbsp;misleading for a lot of amateurs like myself.&nbsp;<br />it'd be more reasonable to call inertial and gravitational mass just 'energy' rather than 'relativistic mass', although physicists might say inertial mass and gravitational mass of an object are not the same thing.<br /><br />http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html<br /><br />http://www.mathpages.com/rr/s2-03/2-03.htm</p> <div class="Discussion_UserSignature"> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>oh, DrRocket says, 'anything' with a non-zero rest mass experiences a mass increase, quantified by the Lorentz transformation' but i thought the concept of the mass increasing with speed was obsolete in modern physics literature,&nbsp;or so i heard.here's what Einstein himself had to say on this:"It is not good to introduce the concept of the mass of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the 'rest mass' m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion."http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/tdil.html#c5you just can't calculate such effects using variable mass. <br />Posted by amaterasu</DIV></p><p>Within the context of special relativity, and that is the context in which this thread started, the concepts of mass and rest mass are well-defined.&nbsp; A photon has zero rest mass.&nbsp; As such it always travels at the speed of light.&nbsp; It also has energy, and energy and mass are the same thing, so one can talk about the mass of an electron in terms of its energy.</p><p>A body of positive rest mass is seen to have a larger mass when in motion.&nbsp; And that mass increases without bound as the speed approaches the speed of light.&nbsp; So does the energy.&nbsp; It is impossible for a body of positive rest mass to travel at the speed of light since that would result in infinite mass and require an infinite amount of energy.&nbsp; It is possible in principle for such a body to travel at any speed less than light, though there are obvious practical limitations imposed by the energy required to do so.</p><p>Within the context of special relativity one can, carefully, address the acceleration of a particle.&nbsp; One must be careful in doing so, as the familiar equation of Newton, F=ma, only applies for systems of constant mass.&nbsp; In relativity, mass is not a constant.&nbsp; Attempts to work with F=ma run into serious problems although such attempts were made, and the mass used in those attempts was rest mass.&nbsp; However, Newton originally stated his law as F = dp/dt where p is momentum (mass times velocity).&nbsp; That equation works perfectly well in special relativity if the mass is taken to be the relativistic mass as defined via the Lorentz transformation.&nbsp; But the fundamental quantity is momentum rather than mass, which is reflected in the quote attributed to Einstein.&nbsp; In looking at the web site from which the quote was taken it also appears that the author is promoting the idea of working with the 4-vector that represent energy and momentum as a single entity rather than dealing with variable mass.&nbsp; The&nbsp; 4-vector approach has much to recommend it, and is probably to be preferred.&nbsp; But you can get along with variable mass, and it is not incorrect to do so.</p><p>Within the context of general relativity, which applies when gravity is a consideration or when the bodies being considered undergo any form of acceleration, the concept of mass is not well-defined.&nbsp; Energy is also not clearly defined.&nbsp; There are various attempts to define mass in that context, but there is no universal concept.&nbsp; http://en.wikipedia.org/wiki/Mass_in_general_relativity<br /></p> <div class="Discussion_UserSignature"> </div>

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#### amaterasu

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Within the context of special relativity, and that is the context in which this thread started, the concepts of mass and rest mass are well-defined.&nbsp; A photon has zero rest mass.&nbsp; As such it always travels at the speed of light.&nbsp; It also has energy, and energy and mass are the same thing, so one can talk about the mass of an electron in terms of its energy.A body of positive rest mass is seen to have a larger mass when in motion.&nbsp; And that mass increases without bound as the speed approaches the speed of light.&nbsp; So does the energy.&nbsp; It is impossible for a body of positive rest mass to travel at the speed of light since that would result in infinite mass and require an infinite amount of energy.&nbsp; It is possible in principle for such a body to travel at any speed less than light, though there are obvious practical limitations imposed by the energy required to do so.</DIV><br />yes, but the problem with the phrase 'a photon has zero rest mass' is that it doesn't make sense to lay people.&nbsp; in reality, would the photon ever be at rest?&nbsp; i don't think so.&nbsp;&nbsp;then why not simply say 'photons are&nbsp;massless' instead?<br /><br />&nbsp; <br />Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Within the context of special relativity one can, carefully, address the acceleration of a particle.&nbsp; One must be careful in doing so, as the familiar equation of Newton, F=ma, only applies for systems of constant mass.&nbsp; In relativity, mass is not a constant.&nbsp; Attempts to work with F=ma run into serious problems although such attempts were made, and the mass used in those attempts was rest mass.&nbsp; However, Newton originally stated his law as F = dp/dt where p is momentum (mass times velocity).&nbsp; That equation works perfectly well in special relativity if the mass is taken to be the relativistic mass as defined via the Lorentz transformation.</DIV><br />agreed.<br /><br /><br />Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>But the fundamental quantity is momentum rather than mass, which is reflected in the quote attributed to Einstein.&nbsp; In looking at the web site from which the quote was taken it also appears that the author is promoting the idea of working with the 4-vector that represent energy and momentum as a single entity rather than dealing with variable mass.&nbsp; The&nbsp; 4-vector approach has much to recommend it, and is probably to be preferred.&nbsp; But you can get along with variable mass, and it is not incorrect to do so.</DIV><br />yes, the 4-vector approach seems to me so much more practical indeed.<br />although i am not too sure if i want to get along with a bunch of electrons myself.&nbsp; hasn't each and every individual got its own mass?<br /><br /><br />Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Within the context of general relativity, which applies when gravity is a consideration or when the bodies being considered undergo any form of acceleration, the concept of mass is not well-defined.&nbsp; Energy is also not clearly defined.&nbsp; There are various attempts to define mass in that context, but there is no universal concept.&nbsp; http://en.wikipedia.org/wiki/Mass_in_general_relativity <br />Posted by DrRocket</DIV><br />that is way beyond my comprehension, however, considering&nbsp;the increase of kinetic energy (or mass) with velocity does not originate in the body in the real world (but rather in space-time itself), i'd still reckon&nbsp;relativistic mass is confusing and&nbsp;unnecessary.<br /><br />anyway, thanks a million for your clear explanation as always.&nbsp; <div class="Discussion_UserSignature"> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>yes, but the problem with the phrase 'a photon has zero rest mass' is that it doesn't make sense to lay people.&nbsp; in reality, would the photon ever be at rest?&nbsp; i don't think so.&nbsp;&nbsp;then why not simply say 'photons are&nbsp;massless' instead?&nbsp; </DIV></p><p>A particle with zero rest mass is never at rest.&nbsp; In fact it is always traveling at the speed of light,&nbsp; c, in all inertial reference frames.</p><p>But because it has energy (E = h*n, where E is energy, n is frequency and h is Planck's constant) and because energy and mass are the same thing, you cannot just say that a photon is massless, at least you cannot say that and be correct.&nbsp; That energy/mass can create gravity, so the effect of the mass can be real (you don't see it in ordinary phenomena but it is a factor when cosmologists try to account for all of the mass/energy in the universe).&nbsp; John Archibald Wheeler once&nbsp;did some theoretical studies of large gravitational fields that might be created by photons alone -- he called them geons. <br /></p> <div class="Discussion_UserSignature"> </div>

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