What happens if you rotate a HUGE merry-go-round?

[solidsnake]

<p>Even if you rotate it really slow, if the diameter of the merry-go-round was really huge, wouldn't the outer rim exceed the speed of light?</p><p>&nbsp;</p><p>I realize that you can't exceed the speed of light, so would the laws of physics prevent a huge merry-go-round from being rotated at all? </p>

 

[MeteorWayne]

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Even if you rotate it really slow, if the diameter of the merry-go-round was really huge, wouldn't the outer rim exceed the speed of light?&nbsp;I realize that you can't exceed the speed of light, so would the laws of physics prevent a huge merry-go-round from being rotated at all? <br />Posted by solidsnake</DIV><br /><br />The point is you would never be able to supply enough energy to make the outer edge move at the speed of light. <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>

 

[SpeedFreek]

<p>For instance, if your roundabout had a diameter of 60,000 miles, you would have to having it spinning once every second in order for the outer edge to be moving at the speed of light. The Earth has a diameter of less than 8000 miles. </p><p>Can you imagine building a roundabout with a diameter more than seven times the size of the Earth? What would you make it out of that would be structurally capable of spinning once a second? How much energy would you require to rotate something with that much mass, once a second? </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[solidsnake]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>For instance, if your roundabout had a diameter of 60,000 miles, you would have to having it spinning once every second in order for the outer edge to be moving at the speed of light. The Earth has a diameter of less than 8000 miles. Can you imagine building a roundabout with a diameter more than seven times the size of the Earth? What would you make it out of that would be structurally capable of spinning once a second? How much energy would you require to rotate something with that much mass, once a second? <br /> Posted by SpeedFreek</DIV></p><p>Doesn't have to be a merry-go-round.</p><p>&nbsp;</p><p>How about a really long fishing line with a weight at the end? </p>

 

[MeteorWayne]

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Doesn't have to be a merry-go-round.&nbsp;How about a really long fishing line with a weight at the end? <br />Posted by solidsnake</DIV><br /><br />It still doesn't change the fact that it would require <strong>infinite</strong> energy to accelerate any object with mass to the speed of light. <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>

 

[why06]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> What would you make it out of that would be structurally capable of spinning once a second? <br /> Posted by SpeedFreek</DIV></p><p>Lets not even get nto structuaral inegrity or this thread will go no where. I think we have to assume that what ever material this merry go round or fishing line is made out of it is impossible to break.</p><p>So now that we have eliminated the other variables we can focus purely on the energy required to spin the wieght. In which case that energy would still be an infinite amount. So your merry-go-round will not break the speed of light.</p><p>Anyway a moving in an arc would be incredibly ineffecient way to reach light speed since your velocity keeps changing. </p> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Even if you rotate it really slow, if the diameter of the merry-go-round was really huge, wouldn't the outer rim exceed the speed of light?&nbsp;I realize that you can't exceed the speed of light, so would the laws of physics prevent a huge merry-go-round from being rotated at all? <br />Posted by solidsnake</DIV></p><p>They would prevent a merry-go-round of <em><strong>infinite diameter</strong></em> from rotating as a <strong><em>rigid body</em></strong>.</p><p>However, there is shortage of merry-go-rounds of infinite diameter, even if the universe is spatially finite which it may or may not be.</p><p>More interestingly, relativity prevents the existence of rigid bodies.&nbsp; If there were such a thing as a rigid rod then you could use to sent a signal from one point to another instantaneously.&nbsp; Special relativity (and general relativity) prevent the transmission of information superluminal speeds as well as limiting the speed of massive objects to the speed of light.</p><p>An extraordinarily large merry-go-round would be very difficult to get started.&nbsp; Classically the moment of inertia would be huge, and relativistic effects would only compound the difficulty.&nbsp; A structure that large would not, even classically, be amenable to treatment as a rigid body, but rather would have to be modeled using the theory of elasticity.&nbsp; So in practice what would happen is the center would start to rotate, but the outer edge would lag quite a bit as an elastic wave propagated throught the structure.&nbsp; </p><p><br /><br />&nbsp;</p> <div class="Discussion_UserSignature"> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Lets not even get nto structuaral inegrity or this thread will go no where. I think we have to assume that what ever material this merry go round or fishing line is made out of it is impossible to break.So now that we have eliminated the other variables we can focus purely on the energy required to spin the wieght. In which case that energy would still be an infinite amount. So your merry-go-round will not break the speed of light.Anyway a moving in an arc would be incredibly ineffecient way to reach light speed since your velocity keeps changing. <br />Posted by why06</DIV></p><p>Actually one way to, sort of, get something to travel faster than light is with an arc.</p><p>Suppose you have a searchlight (or just a good flashlight) and shine it on a wall a long distance away.&nbsp; The move the light through and arc and watch the illuminated spot on the wall.&nbsp; You can in fact cause that dot of light to move at speeds faster than light.&nbsp; You can make it move as fast as you please, with no limit whatever (assuming that the wall can be made to be arbitrarily far away).</p><p>So why does this not violate special relativity ?&nbsp; The answer is that there really is nothing that is moving.&nbsp; That spot of light is not a "thing".&nbsp; The photons are still traveling at c.&nbsp; No information is transmitted by the motion of the "spot". <br /></p> <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Actually one way to, sort of, get something to travel faster than light is with an arc.Suppose you have a searchlight (or just a good flashlight) and shine it on a wall a long distance away.&nbsp; The move the light through and arc and watch the illuminated spot on the wall.&nbsp; You can in fact cause that dot of light to move at speeds faster than light.&nbsp; You can make it move as fast as you please, with no limit whatever (assuming that the wall can be made to be arbitrarily far away).So why does this not violate special relativity ?&nbsp; The answer is that there really is nothing that is moving.&nbsp; That spot of light is not a "thing".&nbsp; The photons are still traveling at c.&nbsp; No information is transmitted by the motion of the "spot". <br />Posted by DrRocket</DIV><br /><br />Indeed, this is in fact observed with some objects in space. <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>

 

[why06]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Actually one way to, sort of, get something to travel faster than light is with an arc.Suppose you have a searchlight (or just a good flashlight) and shine it on a wall a long distance away.&nbsp; The move the light through and arc and watch the illuminated spot on the wall.&nbsp; You can in fact cause that dot of light to move at speeds faster than light.&nbsp; You can make it move as fast as you please, with no limit whatever (assuming that the wall can be made to be arbitrarily far away).So why does this not violate special relativity ?&nbsp; The answer is that there really is nothing that is moving.&nbsp; That spot of light is not a "thing".&nbsp; The photons are still traveling at c.&nbsp; No information is transmitted by the motion of the "spot". <br /> Posted by DrRocket</DIV></p><p>Thats interesting. Heres a question. Say I was in a huge room with a circular wall all around me say 100,000 kilometers in diameter. So I spent a laser pointer in the center of the room at such a speed that the spot generated on the wall was moving faster than the speed of light. Now would the brightness of the spot decrease when the phenomenon was traveling at speeds faster than light. How would it do this? </p> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Thats interesting. Heres a question. Say I was in a huge room with a circular wall all around me say 100,000 kilometers in diameter. So I spent a laser pointer in the center of the room at such a speed that the spot generated on the wall was moving faster than the speed of light. Now would the brightness of the spot decrease when the phenomenon was traveling at speeds faster than light. How would it do this? <br />Posted by why06</DIV></p><p>Why would you expect the brightness to have anything to do with the speed at which the illuminated area was moving ? You are getting x number of photons per second from your laser pointer and if we assume perfect reflectivity just to make the math simple, you are receiving x photons per second back to your eye.&nbsp; <br /></p> <div class="Discussion_UserSignature"> </div>

 

[aphh]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Doesn't have to be a merry-go-round.&nbsp;How about a really long fishing line with a weight at the end? <br /> Posted by solidsnake</DIV></p><p>There are 3 motion vectors in this scenario, one is always tangential to the motion of the weight at the end of the line, one points to the center and the 3rd is the impulse moment vector and points upwards from the center.&nbsp;</p><p>Star/planet systems retain the impulse moment that was created at the formation of the system. Hence the closest to your lightspeed merry-go-round system are the giant exoplanets that race around their parent star in about a day.</p><p>Even this is still far from the energy required, if the rotational period had to be one second instead of one day, like for the massive exoplanets.&nbsp;</p><p>This should give an idea of the amount of energy required; if the impulse moment was enough to make the exoplanet circle it's parent star in a day, perhaps a smaller mass could complete the revolution in a second, if similar impulse moment was transferred to a object with a lot lighter mass than the giant exoplanet?</p><p>Your near lightspeed merry-go-round requires a hot jupiter scale impulse moment. We can not manipulate orbits of planets at this point.<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /> </p>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>There are 3 motion vectors in this scenario, one is always tangential to the motion of the weight at the end of the line, one points to the center and the 3rd is the impulse moment vector and points upwards from the center.&nbsp;Star/planet systems retain the impulse moment that was created at the formation of the system. Hence the closest to your lightspeed merry-go-round system are the giant exoplanets that race around their parent star in about a day.Even this is still far from the energy required, if the rotational period had to be one second instead of one day, like for the massive exoplanets.&nbsp;This should give an idea of the amount of energy required; if the impulse moment was enough to make the exoplanet circle it's parent star in a day, perhaps a smaller mass could complete the revolution in a second, if similar impulse moment was transferred to a object with a lot lighter mass than the giant exoplanet?Your near lightspeed merry-go-round requires a hot jupiter scale impulse moment. We can not manipulate orbits of planets at this point. <br />Posted by aphh</DIV></p><p>Interesting.&nbsp; What you are calling "iimpulse moment"&nbsp; is what I would&nbsp;call "angular momentum".&nbsp; I have never seen the term "impulse moment" before.&nbsp; Is that standard terminology in your circles ?<br /></p> <div class="Discussion_UserSignature"> </div>

 

[aphh]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Interesting.&nbsp; What you are calling "iimpulse moment"&nbsp; is what I would&nbsp;call "angular momentum".&nbsp; I have never seen the term "impulse moment" before.&nbsp; Is that standard terminology in your circles ? <br /> Posted by DrRocket</DIV></p><p>Yes, retaining impulse moment is the equivalent of conservation of angular momentum here. I didn't realize they are not the same everywhere, but I found something that speaks about both of them: http://emweb.unl.edu/Negahban/em373/note13/note.htm </p>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Yes, retaining impulse moment is the equivalent of conservation of angular momentum here. I didn't realize they are not the same everywhere, but I found something that speaks about both of them: http://emweb.unl.edu/Negahban/em373/note13/note.htm <br />Posted by aphh</DIV></p><p>I'm not sure what you mean bby "...they are not the same everywhere..."&nbsp; What I see at the&nbsp;link that you posted is a discussion of "angular momentum" and the "impulse <strong>of a </strong>moment" and the statement tht angular momentum is conserved&nbsp;"only if the impulse of the resultant moment is zero".</p><p>This strikes me as 1) rather standard and 2) an incredibly contorted way to state a standard result.&nbsp; Let me explain what I mean.</p><p>If you consider the analogous issue with ordinary linear momentum, then momentum change is either mass times velocity change&nbsp;or the integral of force with respect to time.&nbsp; And the statement that momentum is conserved(constant) is seen to be trivially equivalent to the statement that the time integral of force is zero.&nbsp; Impulse is just another name for the integral of force with respect to time. </p><p>All that is said in that link is exactly the same thing with torque in place of force and angular momentum in place of linear momentum.&nbsp; The author has managed to take a very simple idea and make it sound complicated, which is, unfortunately, the opposite of good pedagogy.&nbsp; Impulse of a moment is nothing more than the time integral of the torque (aka moment).&nbsp; </p> <div class="Discussion_UserSignature"> </div>

 

[why06]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Why would you expect the brightness to have anything to do with the speed at which the illuminated area was moving ? You are getting x number of photons per second from your laser pointer and if we assume perfect reflectivity just to make the math simple, you are receiving x photons per second back to your eye.&nbsp; <br /> Posted by DrRocket</DIV></p><p>exactly</p><p>I figure if the light is spent fast enough, maybe if the wall even rotated as well (in the opposite direction) Then the photon would scatter apart until perhaps only one photon hit the wall say for every inch the laser traveled. So yeah the brightness would decrease because there would be less light in a certain spot per set amount of time. </p><p>Hmmm.. I thought I was getting at something here, something like doing something with my single photon I pulled out of the laser, but I now it doesn't seem very worth while any more.</p><p>Wait! I do expect we will begin to see lines on the wall when the laser gets up to higher speeds. Lines that would model the Alternating Current of the power sourcebeing switched to DC power via a Convertor. Other than that I don't see much practical application for doing such a thing....</p> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

 

[origin]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Actually one way to, sort of, get something to travel faster than light is with an arc.Suppose you have a searchlight (or just a good flashlight) and shine it on a wall a long distance away.&nbsp; The move the light through and arc and watch the illuminated spot on the wall.&nbsp; You can in fact cause that dot of light to move at speeds faster than light.&nbsp; You can make it move as fast as you please, with no limit whatever (assuming that the wall can be made to be arbitrarily far away).So why does this not violate special relativity ?&nbsp; The answer is that there really is nothing that is moving.&nbsp; That spot of light is not a "thing".&nbsp; The photons are still traveling at c.&nbsp; No information is transmitted by the motion of the "spot". <br />Posted by DrRocket</DIV><br /><br />I always assumed that this was not possible because light can only travel at c (in a vacuum).&nbsp; Since it came from you, Dr.,&nbsp;I had to reevaluate my thinking and I was way off -big surprise.&nbsp; So to help me visualize it, I assumed a wall (a very big wall) that was 1 ly away and was 2 ly wide.&nbsp; If I swept the light across an arc corresponding to the wall in 1 sec then it would take one year to for the beam to reach the 'starting' end of the wall and 1 second later the beam would have traveled 2 light years to the other end of the wall - pretty quick I would say!</p><p>Thanks for the though provoking post it answered alot of little nagging questions in the back of my mind.<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"> </div>

 

[aphh]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Impulse of a moment is nothing more than the time integral of the torque (aka moment).&nbsp; <br /> Posted by DrRocket</DIV></p><p>I tried to think about this, and I think it's an issue of translation. <br /><br />'Impulssimomentti' is the word here, but in this occasion 'momentti' is more like momentum, so it would in fact be impulse momentum, not impulse moment, which is closer to angular momentum i.e. energy retained within the system.</p><p>'Impulssimomentti', which I perhaps wrongly translated as impulse moment, means the energy within the system, aka angular momentum.&nbsp;</p><p>I am not an expert in physics, but I try to learn. Some of the books are in Finnish and some are in English. I have Randall D. Knight's book 'Physics for scientists and engineers', so I will try to look up the correct definitions in both Finnish and English. </p>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I tried to think about this, and I think it's an issue of translation. 'Impulssimomentti' is the word here, but in this occasion 'momentti' is more like momentum, so it would in fact be impulse momentum, not impulse moment, which is closer to angular momentum i.e. energy retained within the system.'Impulssimomentti', which I perhaps wrongly translated as impulse moment, means the energy within the system, aka angular momentum.&nbsp;I am not an expert in physics, but I try to learn. Some of the books are in Finnish and some are in English. I have Randall D. Knight's book 'Physics for scientists and engineers', so I will try to look up the correct definitions in both Finnish and English. <br />Posted by aphh</DIV></p><p>I cannot begin to help with a translation between Finnish and English.&nbsp; But I can help with the physics, in English only.</p><p>:Momentum" is force x time or mass x velocity.&nbsp; "Impulse" is usually used to denote the integral of force over time, i.e. force x time.&nbsp; So impulse is the momentum added by the applicationof force over time.&nbsp; A "moment" about a point or axis is the torque exerted by a force x the distance from the point or axis to the application point of the force.&nbsp; An "impulse moment" would be the integral of a force x the distance from a point or axis with respect to time, which would yield "angular momentum".&nbsp; That seems to be consistent with the useage in the link that you provided earlier.&nbsp; "Impulse momentum" seems to me to be improper terminology since both impulse and momentum involve the application of force over time and the units for "impulse momentum" would be force^2 x time^2 which doesn't make sense.</p><p>Fortunately the mathematics is independent of the language.</p><p>It is quite clear to me that your English is a lot better than my Finnish.&nbsp; <br /></p> <div class="Discussion_UserSignature"> </div>

 

[vandivx]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Actually one way to, sort of, get something to travel faster than light is with an arc.Suppose you have a searchlight (or just a good flashlight) and shine it on a wall a long distance away.&nbsp; The move the light through and arc and watch the illuminated spot on the wall.&nbsp; You can in fact cause that dot of light to move at speeds faster than light.&nbsp; You can make it move as fast as you please, with no limit whatever (assuming that the wall can be made to be arbitrarily far away).So why does this not violate special relativity ?&nbsp; The answer is that there really is nothing that is moving.&nbsp; That spot of light is not a "thing".&nbsp; The photons are still traveling at c.&nbsp; No information is transmitted by the motion of the "spot". <br />Posted by DrRocket</DIV></p><p>Actually that is a hogwash if I have&nbsp;seen one spoken. A beam of light&nbsp;doesn't move like a stick of wood&nbsp;that you make the beam of&nbsp;light to be like. Rather it moves sort of like a stream of water from a garden hose. That is, you turn on water and it gushes out at some speed untill it reaches a wall some distance away. Suppose the wall has some width and you move the hose end sideways to aim it at the opposite end of the wall. If you move it relatively slowly, then you see it make a water&nbsp;line mark across the wall as you&nbsp;progress sideways. If you move it fast, then the stream is falling on the wall as-it-is-able-to-reach-it and there is a noticable&nbsp;delay&nbsp;untill finally it gets to the spot on the other end. What I am trying to say is once the stream hits the wall, you are not at liberty to aim the hose around&nbsp;at will and expect the stream hitting the wall as fast as you can swing the hose around and aim it for a spot on the wall.</p><p>Light is like that even if it moves faster. Moving the light beam around means that for photons to reach the wall at the new amied for spot, they have to again fly there the same way they had to fly to that first spot. In that example Origin gave, it took one year for the light to hit the wall and if you spun your light gun sideways to aim it at&nbsp;the other end of that wall, it would take another light year (assuming it is one light year wide)&nbsp;for the light to arive there and make a spot. During that light year wait, the spot of light on that wall would move across it&nbsp;untill it would reach the opposite end of the wall. </p><p>If things were as you say, we would have superluminal communication now, make no mistake about it.&nbsp;Really this is high school stuff. Sadly your forum authority buildup in easy areas of science (when it comes to basic physics) blinds people so much so&nbsp;that you are believed even when you say plain&nbsp;nonsense or when&nbsp;it comes to issues at the&nbsp;forefront of&nbsp;physics where few are able to&nbsp;judge for themselves as it is. There you stounchly&nbsp;stomp on anything other than the&nbsp;officially accepted&nbsp;views and employ personal slighting remarks to boot. I for one find such approach stifling to discussion and thoroughly&nbsp;odious.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>

 

[vandivx]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Even if you rotate it really slow, if the diameter of the merry-go-round was really huge, wouldn't the outer rim exceed the speed of light?&nbsp;I realize that you can't exceed the speed of light, so would the laws of physics prevent a huge merry-go-round from being rotated at all? <br />Posted by solidsnake</DIV><br /><br />Taking it up as a&nbsp;pure 'what if' gedanken experiment, then I would say that&nbsp;the wheel could be rotated but increasingly slowly as it grew in size but it could always be turned, no matter what&nbsp;the size of the wheel was. But that depends on what you call 'slow turning' -&nbsp;what we would normally call static conditions, such as the position of&nbsp;stars or galaxies are not static at all from the 'point of view of universe' and even if we could not make&nbsp;such huge wheel to budge at all during our lives or the history of men, still it would move no matter its size&nbsp;if millions of years&nbsp;were allowed&nbsp;to&nbsp;pass or whatever time would be needed for the turning&nbsp;motion at the pivot to be noticable.</p><p>&nbsp;Thus, in the physical sense, the wheel could always be turned, albeit extremely&nbsp;slowly, at the glacial pace and then some. In the mathematical, that is in&nbsp;abstract&nbsp;sense, you could say that in the limit as the wheel size approached infinity, the turning motion at the pivot would come to a halt (approach zero value). But that is abstract theoretical conclusion which is unphysical, meaning that such conditions can't exist even if we allow for the fantasy setup&nbsp;of the experiment in the first place.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>

 

[SpeedFreek]

<p>Sorry Vandivx, but DrRocket was correct.&nbsp;</p><p>http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3</p><p><a name="3" title="3"></a>Think about how fast a shadow can move.&nbsp; If you project the shadow of your finger using a nearby lamp onto a distant wall and then wag your finger, the shadow will move much faster than your finger.&nbsp; If your finger moves parallel to the wall, the shadow's speed will be multiplied by a factor <em>D/d</em> where <em>d</em> is the distance from the lamp to your finger, and <em>D</em> is the distance from the lamp to the wall.&nbsp; The speed can even be much faster than this if the wall is at an angle to your finger's motion.&nbsp; If the wall is very far away, the movement of the shadow will be delayed because of the time it takes light to get there, but the shadow's speed is still increased by the same ratio.&nbsp; The speed of a shadow is therefore not restricted to be less than the speed of light.</p> <p><a name="3" title="3"></a>Others things that can go FTL include the spot of a laser that has been aimed at the surface of the Moon.&nbsp; Given that the distance to the Moon is 385,000 km, try working out the speed of the spot if you wave the laser at a gentle speed.&nbsp; You might also like to think about a water wave arriving obliquely at a long straight beach.&nbsp; How fast can the point at which the wave is breaking travel along the beach?</p> <p><a name="3" title="3"></a>This sort of thing can turn up in Nature; for example, the beam of light from a pulsar can sweep across a dust cloud.&nbsp; A bright explosion emits an expanding spherical shell of light or other radiation.&nbsp; When this shell intersects a surface, it creates a circle of light which expands faster than light.&nbsp; A natural example of this has been observed when an electromagnetic pulse from a lightning flash hits an upper layer of the atmosphere.</p> <p><a name="3" title="3"></a>These are all examples of things that can go faster than light, but which are not physical objects.&nbsp; It is not possible to send information faster than light on a shadow or light spot, so FTL communication is not possible in this way.&nbsp; This is not what we mean by faster than light travel, although it shows how difficult it is to define what we really do mean by faster than light travel.&nbsp; See also the FAQ The Superluminal Scissors.</p><p>&nbsp;</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Actually that is a hogwash if I have&nbsp;seen one spoken. A beam of light&nbsp;doesn't move like a stick of wood&nbsp;that you make the beam of&nbsp;light to be like. Rather it moves sort of like a stream of water from a garden hose. That is, you turn on water and it gushes out at some speed untill it reaches a wall some distance away. Suppose the wall has some width and you move the hose end sideways to aim it at the opposite end of the wall. If you move it relatively slowly, then you see it make a water&nbsp;line mark across the wall as you&nbsp;progress sideways. If you move it fast, then the stream is falling on the wall as-it-is-able-to-reach-it and there is a noticable&nbsp;delay&nbsp;untill finally it gets to the spot on the other end. What I am trying to say is once the stream hits the wall, you are not at liberty to aim the hose around&nbsp;at will and expect the stream hitting the wall as fast as you can swing the hose around and aim it for a spot on the wall.Light is like that even if it moves faster. Moving the light beam around means that for photons to reach the wall at the new amied for spot, they have to again fly there the same way they had to fly to that first spot. In that example Origin gave, it took one year for the light to hit the wall and if you spun your light gun sideways to aim it at&nbsp;the other end of that wall, it would take another light year (assuming it is one light year wide)&nbsp;for the light to arive there and make a spot. During that light year wait, the spot of light on that wall would move across it&nbsp;untill it would reach the opposite end of the wall. If things were as you say, we would have superluminal communication now, make no mistake about it.&nbsp;Really this is high school stuff. Sadly your forum authority buildup in easy areas of science (when it comes to basic physics) blinds people so much so&nbsp;that you are believed even when you say plain&nbsp;nonsense or when&nbsp;it comes to issues at the&nbsp;forefront of&nbsp;physics where few are able to&nbsp;judge for themselves as it is. There you stounchly&nbsp;stomp on anything other than the&nbsp;officially accepted&nbsp;views and employ personal slighting remarks to boot. I for one find such approach stifling to discussion and thoroughly&nbsp;odious.&nbsp; <br />Posted by vandivx</DIV><br />&nbsp;</p><p>Wrong again.&nbsp; Your record remains umblemished by success.</p><p>If this is high school stuff, then you had better go back to high school.</p> <div class="Discussion_UserSignature"> </div>

 

[Mee_n_Mac]

Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Taking it up as a&nbsp;pure 'what if' gedanken experiment, then I would say that&nbsp;the wheel could be rotated but increasingly slowly as it grew in size but it could always be turned, no matter what&nbsp;the size of the wheel was. But that depends on what you call 'slow turning' -&nbsp;what we would normally call static conditions, such as the position of&nbsp;stars or galaxies are not static at all from the 'point of view of universe' and even if we could not make&nbsp;such huge wheel to budge at all during our lives or the history of men, still it would move no matter its size&nbsp;if millions of years&nbsp;were allowed&nbsp;to&nbsp;pass or whatever time would be needed for the turning&nbsp;motion at the pivot to be noticable.&nbsp;Thus, in the physical sense, the wheel could always be turned, albeit extremely&nbsp;slowly, at the glacial pace and then some. In the mathematical, that is in&nbsp;abstract&nbsp;sense, you could say that in the limit as the wheel size approached infinity, the turning motion at the pivot would come to a halt (approach zero value). But that is abstract theoretical conclusion which is unphysical, meaning that such conditions can't exist even if we allow for the fantasy setup&nbsp;of the experiment in the first place.&nbsp; <br />Posted by <strong>vandivx</strong></DIV><br /><br />If you allow that the wheel is non-rigid then you can get it to rotate though not all the pieces at the same rate and same time.&nbsp; And yes I'd agree that relativity places a max limit on the angular rate (and acceleration) for a given size wheel. <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>

 

[DrRocket]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If you allow that the wheel is non-rigid then you can get it to rotate though not all the pieces at the same rate and same time.&nbsp; And yes I'd agree that relativity places a max limit on the angular rate (and acceleration) for a given size wheel. <br />Posted by Mee_n_Mac</DIV></p><p>Absolutely correct, and this observations echos the point made earlier that special relativity does not admit the existence of rigid bodies.&nbsp; Treatment of a sufficiently large wheel within special relativity would require modeling the wheel using the theory of elasticity rather than as a rigid body, and the center of the wheel would lead the outer edge in angular velocity.&nbsp; The&nbsp;angular velocity would be limited so that the linear velocity of the outer rim never exceeded the speed of light.</p><p>That observation applies to any physical wheel and any mechnism capable of conveying information.&nbsp; It does not apply to things that are not physical and cannot convey information, such as the rate at which an illuminated area seems to move as a beam of light is swept across a surface.&nbsp; In that case there is nothing physical that is actually moving faster than light&nbsp;and no information is conveyed by the movement of the beam of light.&nbsp; </p> <div class="Discussion_UserSignature"> </div>