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

kelvinzero · Dec 6, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>...More interestingly, relativity prevents the existence of rigid bodies.  If there were such a thing as a rigid rod then you could use to sent a signal from one point to another instantaneously. ... Posted by DrRocket</DIV>This probably doesnt contradict what you said, but just to clarify: It would not be the compressibility of matter that prevents the paradox. Im sure it all works out somehow through relativistic contraction and all that.Likewise, just at the rotating beam from a lighthouse would actually form a spiral, like water from a hose, I expect a massless infinitely rigid rod spun over your head would also form a spiral. Not because it bent, but because it is doing its best impression of straight in a relativistic universe.EDIT: sorry just noticed your last post where you mention the same effect. The difference is just that where you say rigid bodies cannot exist, im saying that it is not a contradiction for ridgd bodies to behave in this way.For example, length contraction. If we see a craft zooming by so fast that it is flattened to a disk we don't say 'well, nothing can be perfectly rigid' because we know it is not flattened in its own frame of reference, and we are flattened in a million different directions right now from the point of view of a million different frames of reference that happen to be moving wrt to our own, eg the frame of the craft.

Bflowing · Dec 6, 2008

let's go back to Origin's thought experiment and tighten it up a bit.  2 reflective surfaces A and B 1 ly from us but 5 ly apart.  On the arc from A to B, a long reflective strip.  We shine a laser on point A and swing it in 1 second to point B.2 years later, we see a line of light go from A to B in 1 second.  This is an optical illusion. No light or information traveled from A to B in 1 second.Plus I doubt it would be a solid line.  I don't know how many photons the laser will emit in 1 second, but by the time it "paints" a line from A to B, they have to be scattered somewhat.  Hmmm, good math and physics question. <div class="Discussion_UserSignature"> </div>

vandivx · Dec 6, 2008

Replying to: <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.  And yes I'd agree that relativity places a max limit on the angular rate (and acceleration) for a given size wheel. Posted by Mee_n_Mac</DIV>non-rigid wheel just means delay in how parts of the wheel move as you say. Given how unreal otherwise such discussion is I didn't bother to speak about rigidity issue as I thought it besides the point here. I took it that the point of the OP was if the wheel couldn't turn at all past certain diameter in size and the answer is there is no such cutoff size where the wheel could no longer move - it can always move, just at glacial pace and then it depends if he was asking about the principle of the matter or if he was looking at it from the more practical angle (because the wheel whose center that we could examine as far out we were able to and which didn't make measurable motion in few generations might be deemed standing still for all practical purposes). I assumed his was a question of principle. <div class="Discussion_UserSignature"> </div>

Mee_n_Mac · Dec 6, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>non-rigid wheel just means delay in how parts of the wheel move as you say. Given how unreal otherwise such discussion is I didn't bother to speak about rigidity issue as I thought it besides the point here. I took it that the point of the OP was if the wheel couldn't turn at all past certain diameter in size and the answer is there is no such cutoff size where the wheel could no longer move - it can always move, just at glacial pace and then it depends if he was asking about the principle of the matter or if he was looking at it from the more practical angle (because the wheel whose center that we could examine as far out we were able to and which didn't make measurable motion in few generations might be deemed standing still for all practical purposes). I assumed his was a question of principle. Posted by vandivx</DIV> I believe we are all in agreement. <div class="Discussion_UserSignature"> -----------------------------------------------------Ask not what your Forum Software can do do on you,Ask it to, please for the love of all that's Holy, STOP ! </div>

vandivx · Dec 6, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Sorry Vandivx, but DrRocket was correct. http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3Think about how fast a shadow can move.  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. ... Posted by SpeedFreek</DIV> thx I see what you saying (I also looked at that link) but I still have difficulty with understanding very precisely or at all what 'falls' on that distant surface when you swing your light source and distance on the order of light years are involved. Perhaps that could be translated into difficulty of dealing with the superluminal phenomenon and if you can still speak of light moving (shadow is just inverse phenomenon) in terms of continuity (its like when do you stop talking about the heat when you move on bit by bit to microscale - here when it looses meaning talking about spot of light moving in terms of photon falling here and there). But as to the main thing I believe I was then wrong. That said I still don't know if there is not some change in difference between the states when the spot of light moves at below light speed and when it moves past that barrier. Because normally we talk about the shadow or light moving not at FTL speeds. Will do some thinking but now will be very busy for a few days at least with other things in life.when I was in shower now, it came to me that what is misleading here is that nothing really moves in the direction in which the shadow or spot of light moves - I mean in the sense science uses the term when it says something 'moves'. It is easier to see what goes on here when considering large distances, like on the order of light years. What happens is that a photon here and there arrives in approximately perpendicular direction at a given spot and then another one from the same source arrives some distance in one direction and so it repeats, say at points A, B, C. .... What we are talking here about 'moving FTL' is a phenomenon (more precisely a series of events) which has no direct causal happening between the points A, B, C, etc. (such as when we talk about something moving) but we have independently happening events that just happen to have the same cause in the origin of the light beam source. That is why it is at least to me so misleading that the talk is about something moving FTL, its almost like when a magician does something and claims what happened is 'magic', a sort of that deceit. In the same deceiving spirit, one could say that the bowshock wave from an exploded supernova as it expands into the far regions of cosmos in a spherical shape is causing things happening instantaneously (not just FTL) if one considers the bowshock hitting inumerable stars at a given spherical distance... In terms of the shadow moving FTL, here we could talk not about FTL but infinite speed of 'propagation' of the events between the points A, B, C, etc...  because all stars that found themselves at certain distance from the nova on some imaginary sphere would be hit at the same time (where the shadow moved from from point A to point B at FTL speed, here it would happen simultaneously but the principle is the same - because it happens simultaneously, nobody looks at it that way but we are really talking about same thing in principle). But that is non-sensical look on physics and I'd say this talk about FTL travel of the shadow or the beam of light is in the same category. It really strikes me as a fraud on those who are not familiar with what the trick consists in (it is sort of like play on words), same as one gets fooled by a magician even when one knows that real magic doesn't exist. As far as I noticed, I didn't see it explained in these terms on that link you supplied.I wrote this in quite a haste, so excuse some typos and long sentences. Also it seems my intuition was right and I wasn't  wrong as in wrong LOL rather I was taken in by a devious talk - I think it is false or at least not advisable idea to speak of 'motion of a shadow' in physics. Such talk should be left for everyday life talk but not science. Its almost like if one talked about a 'motion' when a deer jumped in Nebraska and Idaho and Oregon and in other states and somebody correlated the events and talked about FTL motion or more precisely about event travel if the jumps happened close enough in time succession being caused by meteorite exploding overhead say.  <div class="Discussion_UserSignature"> </div>

kelvinzero · Dec 7, 2008

It is necessary to consider this phenomona in order to explain why certain effects astronomers observe travel faster than light without violating relativity.Certainly no scientist who accepts Relativity (a very large majority) would wish to fool people into thinking that relativity has be violated.I have heard people missrepresent this evidence, but not scientists.

vandivx · Dec 7, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>It is necessary to consider this phenomona in order to explain why certain effects astronomers observe travel faster than light without violating relativity.Certainly no scientist who accepts Relativity (a very large majority) would wish to fool people into thinking that relativity has be violated.I have heard people missrepresent this evidence, but not scientists. Posted by kelvinzero</DIV> well, they almost fooled me, but almost. I suppose as long as they explicitly hastened to point out that nothing really moves in the claimed direction in the physical sense in this type of phenomenon, it might be ok but I still think this is not right to employ concepts this way in physics. You may have too ideal view of scientists I'd say. But then again, it might be widely known such explanation for all I know, I'd be curious to know. I didn't notice anybody explaing it on that linked webpage for example and there it should have been pointed out if anywhere as it supplies answers to begining students or people interested in the field I think. If I was teaching this stuff, I'd certainly make it a point that everybody understood what really 'moves' in the claimed direction here. Or rather that nothing moves except correlated series of events is happening due to them having cause in the same (rotating) source.I'd be curious if YOU knew what I said about this phenomenon and if not, shouldn't the doctor here told you (us) if he knew that instead of just talking about FTL motion and that it is harmless since no information can be transfered by its means etc. I suspect because no harm comes from not knowing my interpretation, nobody may bother to enlighten others. Also to find out that no FTL transfer of real stuff or information can happen in this setup, you don't really have to know what I said about it. I figured as much before I figured what really goes on in the physical sense. <div class="Discussion_UserSignature"> </div>

kelvinzero · Dec 7, 2008

I guess you are referring to Speedfreek's link.Physics is like that.When you are learning physics the lecturer or tutor can explain the same thing ten times and you just dont get it. You go away, struggle through the exercises, get more help, try again, suddenly it clicks. Everything they said was entirely true but somehow avoided the concept you were personally stuck on.Of course they are not trying to confuse you.  Replying to: <DIV CLASS='Discussion_PostQuote'>I'd be curious if YOU knew what I said about this phenomenon and if not..</DIV>If I understand the particular point then yes. I had wondered about how a spot of light on a wall could move faster than light, heard about the strange phenomona of observations of distant objects travelling faster than light, etc. The person who first told me presented it as a paradox, but they were not a scientist. I dug up the explanation  myself online and discoverd it was just this 'spot on a wall' concept, either that or the illusion of the thing moving towards you very rapidly.That particular idea is of interest to me because it means that if a colonising lifeform were to expand in a sphere at near lightspeed (which I have arguments to support for a billion year old replicator-like species) then it would appear to be expanding towards you at much greater than lightspeed. Our first experience of a billion year old species may infact be a disklike hole in the stars and galaxies that grows perceptably, eating thousands of lightyears every second as you watch. This is how it could appear.

adrenalynn · Dec 8, 2008

>> . If you move it relatively slowly, then you see it make a water line mark across the wall as you 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 delay untill finally it gets to the spot on the other end. Just as a note - I've seen this concept expressed by a couple people a couple times in the thread.  If I'm understanding what you and the other poster are alluding to - this is "persistence of vision" and is superfelous to the topic in practicality. <div class="Discussion_UserSignature"> .bipartisan (bī-pär'tĭ-zən, -sən) [Adj.] Maintaining the ability to blame republications when your stimulus plan proves to be a devastating failure.IMPEACH OBAMA! </div>

kg · Dec 8, 2008

Replying to: <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? 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? Posted by solidsnake</DIV> Would this object end up resembling an accretion disk of some kind?  Would jets of radiation erupt from it's axis of rotation?

adrenalynn · Dec 8, 2008

I'd guess "none of the above".  That infinite energy required to rotate what would then become infinite mass is going to rip-up space-time something fierce.  Not to mention that the infinite mass will then occupy all of space instantaneously... <div class="Discussion_UserSignature"> .bipartisan (bī-pär'tĭ-zən, -sən) [Adj.] Maintaining the ability to blame republications when your stimulus plan proves to be a devastating failure.IMPEACH OBAMA! </div>

vandivx · Dec 11, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I guess you are referring to Speedfreek's link.Physics is like that. When you are learning physics the lecturer or tutor can explain the same thing ten times and you just dont get it. You go away, struggle through the exercises, get more help, try again, suddenly it clicks. Everything they said was entirely true but somehow avoided the concept you were personally stuck on.Of course they are not trying to confuse you. If I understand the particular point then yes. I had wondered about how a spot of light on a wall could move faster than light, heard about the strange phenomona of observations of distant objects travelling faster than light, etc. The person who first told me presented it as a paradox, but they were not a scientist. I dug up the explanation  myself online and discoverd it was just this 'spot on a wall' concept, either that or the illusion of the thing moving towards you very rapidly ... Posted by kelvinzero</DIV>Yes, I referred to that link. Perhaps I didn't make it clear enough, I don't want to say anybody is deliberately after confusing anybody, rather that happens because people in the field feel they have to be able to explain everything and so they supply 'explanation' even in cases where they may or may not sense they do not have right answers, still they supply them and sometimes they or others can be quite adamant about those answers being right and anybody who doesn't accept them is made out as not understanding physics. Such people confuse pupils because they, the teachers I mean, don't know better, its not like they know the truth but hide it. Replying to: <DIV CLASS='Discussion_PostQuote'> <a name="3" title="3"></a>http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3Think ... These are all examples of things that can go faster than light, but which are not physical objects. </DIV> As I said, shadow or a spot of light moving on a surface is not a 'thing' that can go FTL, as I pointed out, nothing, i.e., no-thing, is actually moving across the surface, that is nothing is moving accross it in the sense of anything at all moving there in the local sense. The one who wrote that certainly doesn't have any deeper understanding of the phenomenon as I worked it out above, else he would have at least briefly outlined what really goes on in the phenomenon under discussion instead of ending with such a mystifying pronouncement as he does. It has nothing to do with them explaining ten times with me not getting it. They clearly are not capable of explaing the phenomenon at hand, no two ways about it. As another example of forced explanation just because whatever they teach they got to have answers for everything and can't be wrong... look at the same link, subject #14 Replying to: <DIV CLASS='Discussion_PostQuote'> <a name="14" title="14"></a>http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3Think 14. The Moon revolves round my head faster than light!<a name="14" title="14"></a>Stand up in a clear space and spin round.  ... Suppose the Moon is on the horizon.  How fast is it spinning round your head?  ... it is more than ... the speed of light!  ... according to general relativity all co-ordinate systems are equally valid, including rotating ones.  So isn't the Moon going faster than the speed of light?  This is quite difficult to account for. <a name="14" title="14"></a>What it comes down to is the fact that velocities in different places cannot be compared directly in general relativity. ... </DIV> Supposedly one can only compare velocities in the local sense. Talk about deliberate limiting of one's intellectual vision to save the theory... This is a cop out if I have seen one. Perfect example how to skate out of any difficulties. One makes up something to get out of the tight spot and physics be damned. It can't get in our way can it. Our theory can't be wrong or be inconsistent, can it. But I don't think the phd who wrote those lines above is after deliberately confusing anybody. Rather he in his limitless arrogance shared by the whole field (BTW of which he is the product of) he feels he has to save the theory at all costs, even at the cost of talking out of his behind (because he knows he can't come up with anything better). Here, that one can't compare velocities in different places directly. What more can I say. I certainly do not admire or share in your going out of your way the extra mile and then some to believe they know what they (scientists) are talking about. <div class="Discussion_UserSignature"> </div>

kelvinzero · Dec 11, 2008

Rotational frames are quite different from inertial ones. This is quite clear even without relativity. If you were in space and you threw and object, you would be able to tell you were rotating when the object appeard to drift off to one side and then spiral outwards instead of moving in a single line.This talk of copouts and arogance has lost me. Im leaving this conversation.

DrRocket · Dec 11, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Yes, I referred to that link. Perhaps I didn't make it clear enough, I don't want to say anybody is deliberately after confusing anybody, rather that happens because people in the field feel they have to be able to explain everything and so they supply 'explanation' even in cases where they may or may not sense they do not have right answers,...lanybody who doesn't accept them is made out as not understanding physics. Such people confuse pupils because they, the teachers I mean, don't know better, its not like they know the truth but hide it. .... The one who wrote that certainly doesn't have any deeper understanding of the phenomenon as I worked it out above, else he would have at least briefly outlined what really goes on in the phenomenon under discussion instead of ending with such a mystifying pronouncement as he does. It has nothing to do with them explaining ten times with me not getting it. They clearly are not capable of explaing the phenomenon at hand, no two ways about it. ..Posted by vandivx</DIV>Did it ever occur to you that the theory is subtle but valid, and the problem is that you simply don't understand it ?  Now that is a simple explanation.Sometimes the problem is not with the transmitter but with the receiver.  <div class="Discussion_UserSignature"> </div>

vandivx · Dec 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Rotational frames are quite different from inertial ones. This is quite clear even without relativity. If you were in space and you threw and object, you would be able to tell you were rotating when the object appeard to drift off to one side and then spiral outwards instead of moving in a single line. Posted by kelvinzero</DIV> You lost me with this talk about difference btw the two frames, what that has to do with the matter at hand? Isn't the context general relativistic?  The guy (John Baez - mathematical physicist at the university on whose servers the Q&A is published) himself explicitly says we are taking the issue up within GR context. I should quote the section here in its entirety for convenience and record keeping. (Markup was added by me)Replying to: <DIV CLASS='Discussion_PostQuote'><h3><a name="14" title="14"></a>14. The Moon revolves round my head faster than light!</h3> <a name="14" title="14"></a>Stand up in a clear space and spin round.  It is not too difficult to turn at one revolution each two seconds.  Suppose the Moon is on the horizon.  How fast is it spinning round your head?  It is about 385,000 km away, so the answer is 1.21 million km/s, which is more than four times the speed of light!  It might sound ridiculous to say that the Moon is going round your head when really it is you who is turning, but according to general relativity all co-ordinate systems are equally valid, including rotating ones.  So isn't the Moon going faster than the speed of light?  This is quite difficult to account for. <a name="14" title="14"></a>What it comes down to is the fact that velocities in different places cannot be compared directly in general relativity.  Notice that the Moon is not overtaking any light in its own locality.  The speed of the Moon can only be compared to the speeds of other objects in its own local inertial frame.  Indeed, the concept of speed is not a very useful one in general relativity, and this makes it difficult to define what "faster than light" means.  Even the statement that "the speed of light is constant" is open to interpretation in general relativity.  Einstein himself, on page 76 of his book "Relativity: the Special and the General Theory", wrote that the statement cannot claim unlimited validity.  When there is no absolute definition of time and distance it is not so clear how speeds should be determined. <a name="14" title="14"></a>Nevertheless, the modern interpretation is that the speed of light is constant in general relativity and this statement is a tautology given that standard units of distance and time are tied together using the speed of light.  The Moon is given to be moving slower than light because it remains within the "future light cone" propagating from its position at any instant. </DIV> Talk about putting on horse blinkers to limit your vision only to areas where you want to see. <div class="Discussion_UserSignature"> </div>

vandivx · Dec 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Did it ever occur to you that the theory is subtle but valid Posted by DrRocket</DIV>How so?Would you care to explain instead of taking easy cope out of personal attack? I am sure others are also curious how that theory jibes? Might it be that you sense the bugaboo of the absolute space in this issue that the other poster here in his innocence in effect hinted at and that it is best to avoid discussion and hide behind personal attacks. I only wish that people who propagate that one reference frame is as good as any other and that it is all relative in relativity would also eat their cake... but consistency is too much to ask for is it?  BTW I also noted that you fell silent on the subject of FTL motion of shadow/light spots... not even some closing personal attack of some sort. You know that saying about when going gets tough... and here when things get really discussed and it doesn't do anymore to regurgitate textbooks from memory... <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>How so?Would you care to explain instead of taking easy cope out of personal attack? I am sure others are also curious how that theory jibes?</DIV>Ok I will try to explain the theory so that even you might understand it.  There has been no effort to hoodwink anyone.  I was simply pointing out that your attack on those whose explanation you do not understand is more likely to lie with your ability to comprehend than in their ability to formulate a correct explanation.Suppose that you have a screen that is a circular arc section at a distance R.  You have a laser pointer which you activate, point towards the screen and swing in an arc at an angular velocity of Omega.  Photons leave that laser pointer and travel to the screen hitting the screen at the point with circular coordinates (R, Omega x t) at time t.  This produces a spot that travels along the screen at angular velocity Omega, the same as the initial angular velocity, but with a linear speed of R x Omega.  By making R arbitrarily large the linear speed is also made arbitrarily large, and can exceed c, the speed of light.  The effect of the finite speed of light is found in the travel time of the photons involved and results in a delay between the time when the photons create the moving spot on the screen and the time when that image is actually seen by the observer at the position of the laser pointer.  Yes it might take a year for the first photon to hit the screen, but the other photons were released during the short during which the laser was swung through the arc and have also been traveling for nearly a year  then selves. The apparent speed of that spot is not affected and it is faster than c if R is large enough.  There is no limitation on this speed imposed by relativity because the spot is not a physical thing and it is not a means for the transmission of information.This is all quite elementary stuff in special relativity and you can find it in any textbook on the subjec Replying to: <DIV CLASS='Discussion_PostQuote'>Might it be that you sense the bugaboo of the absolute space in this issue that the other poster here in his innocence in effect hinted at and that it is best to avoid discussion and hide behind personal attacks. I only wish that people who propagate that one reference frame is as good as any other and that it is all relative in relativity would also eat their cake... but consistency is too much to ask for is it? </DIV>No the bugaboo that I sense is your continual insistence on this nonsense of absolute space.  That idea was totally set aside over 100 years ago and the theory that showed that notion to be incorrect has been supported by a massive amount of experimental data ever since.  The theory is called relativity.  You really ought to learn about it and keep at it until you understand the basics.Special and genereal relativity have been shown to be completely consistent with observations and experiments.  There is nothing whatever inconsistent about those theories.  Replying to: <DIV CLASS='Discussion_PostQuote'> BTW I also noted that you fell silent on the subject of FTL motion of shadow/light spots... not even some closing personal attack of some sort. You know that saying about when going gets tough... and here when things get really discussed and it doesn't do anymore to regurgitate textbooks from memory... Posted by vandivx</DIV>I didn't see any point in pursuing that topic once you had admitted that such a spot could in fact move at an arbitrarily high speed.I am afraid that I don't have a good enough memory to regurgitate from textbooks.  I have to rely on actually understanding the material and formulating a response in my own words, using Wiki to supply longer explanations with symbology that I cannot replicate using the tools available to the forumYou might do well to read and attempt to understand some of those testbooks yourself.  This one might suit your needs.  <img id="prodImage" src="http://ecx.images-amazon.com/images/I/51ZBE8EF4JL._BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA240_SH20_OU01_.jpg" border="0" alt="The Complete Idiot's Guide to Understanding Einstein, Second Edition" width="240" height="240" /> <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 13, 2008

It is important that newcomers to science understand why the accepted principles of physics are accepted, and why attacks such as those mounted by people such as Vandivx are bogus and detrimental to those who would aspire to truly understand science.  It is important because challenging beliefs is the heart of research science, but challenging valid ideas in the wrong context is not only not profitable, it is just plain stupid.  To state, for instance, that Einstein's theory of relativity is completely invalid is to demonstrate profound ignorance.Physical principles are formulated by highly intelligent and extremely hard working people.  That work requires a great deal of creativity and the ability to challenge accepted beliefs.  But it more importantly requires a discipline to know when accepted principles are known to be valid and to make the challenge in an intelligent and constructive way so as to advance science and not merely play the fool.Any physical theory is accepted for only two reasons.  First, it must provide the power to predict the behavior of natural phenomena accurately from a set of established and measurable data.  Second, and more importantly it must agree with ALL valid experimental and observational data, or else be viewed as an approximation that is valid only in known and proscribed circumstances.  In fact, we know that none of the existing theories are perfect and they are all only approximations with known ranges of validity.  But we do know that within those ranges of validity the theories are extremely accurate.`The major physical theories of physics are general relativity and the quantum field theories that make up the Standard Model of particle physics -- the theory of the electroweak force and the theory of the strong force.  We know that general relativity and quantum field theory are not compatible because general relativity is deterministic (it predicts definite outcomes) while quantum theories are stochastic (they predict only probabilities).  General relativity applies in circumstances of macroscopic phenomena and is our best theory of gravity.  Quantum field theories apply at small scales and in the absence of significant gravitational effects, it predicts atomic an sub-atomic phenomena as observed in nature and in experiments in particle colliders to exquisite accuracy.  In situations where both gravity and quantum phenomena are important, such as near the predicted singularity in a black hole or in the earliest times following the Big Bang our theories fail, because both gravity and quantum effects are important.But in most circumstances quantum theories and general relativity have been subjected to many many sophisticated experiments and have been shown to be extremely accurate.  It is that massive amount of empirical data that supports these theories and it is that massive amount of data that shows conclusively that they are extremely accurate under almost all circumstances.  Claims that they are badly mistaken are simply foolish.  To state that Einstein was completely wrong or that quantum theory is useless are the mark of the uneducated and unintelligent.  Claims that there is perhaps a better theory, one that subsumes both relativity and quantum theory and that agrees with each of these pillars of physics in circumstances where they are supported by experimental data are the hallmark of a good researcher and a solid scientific mind.Of course relativity or quantum theory and likely both are ultimately wrong, since they are, after all, incompatible.  But whatever is correct must also agree with them in most circumstances.  That is the correspondence principle.  And it is valid.  To believe otherwise is to believe that nature is not understandable at all, and the success of the human mind in discovering the laws of physics is proof of that.  Einstein, Feynman, Schwinger, Tomogawa, Gell-Mann, and Weinberg were pretty smart guys and they formulated brilliant and valid theories.  Not perfect, but valid and consistent with a huge body of experimental data. It is also important to recognize that with general relativity and quantum field theory we still cannot predict everything, or even most things, aven under "ordinary" circumstances.  That is simply because, although, these theories provide the most fundamental rules for nature, the behavior of nature at a more complex level with many particles interacting is too complex for us to be able to solve the describing equations.  So even if a "theory of everything" that unites general relativity and quantum theory and is mathematically consistent is discovered, we have a long way to go to be able to predict the behavior of complex systems, and most of nature comprises complex systems.The road to understanding is not to reject accepted physics but rather to understand why it is accepted, and to see the ways in which our comprehension of nature can be expanded and the laws of physics expanded, generalized, and formulated so as to provide increased predictive power.  The road to ignorance is to reject what is accepted, not take advantage of the work of the geniuses that have preceded us, and attempt to replace them, not with refinements, but with nonsense.Physics is a field for enthusiastic and creative minds.  Crackpots need not apply. <div class="Discussion_UserSignature"> </div>

vandivx · Dec 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> I didn't see any point in pursuing that topic once you had admitted that such a spot could in fact move at an arbitrarily high speed.I am afraid that I don't have a good enough memory to regurgitate from textbooks.  I have to rely on actually understanding the material and formulating a response in my own words, using Wiki to supply longer explanations with symbology that I cannot replicate using the tools available to the forumYou might do well to read and attempt to understand some of those testbooks yourself.  This one might suit your needs.  Posted by DrRocket</DIV> Sorry, I took your reply as a comment on that other issue 'Moon is moving faster than light', my fault. That's why I noted the 'shadow moving FTL' issue at the end of that post as an afterthought.And I didn't mean to tease out the stock textbook explanation that you are so good at I must say (it may not be your memory at work but it sure comes across like that). I wanted some discussion of that issue that nothing (as in no physical object) moves FTL accross the surface and that in fact the concepts like 'shadow' or 'light spot'  can be misleading - the professor in that Q&A talks about 'thing moving' but that it is ok because no information can be transmited that way... And when I protested originally that nothing moves FTL, I was told I am wrong. It is really a matter of how one uses concepts. Shadow or light spot are not 'things' or anything physical in their own right (I'd categorize such concepts as virtual phenomenons or something like that) and if such concepts are to be used in physics, they better be shown for what they are. And then no question of FTL 'motion' would even arise. I thought you might at least acknowledge that my conclusions are correct to auswage the doubts of people who may not be able to decide for themselves given they are starting out in physics. Mere silence on your part is not enough given you attempt to discredit whatever I post if there is slightest possibility to do that.   -----As to the recommended reading, I must dissapoint you. I don't have such books on my shelves, same as I don't have Reader's Digest books there, nor would I recommend them to anybody who is serious about science. But everybody to his tastes I say. <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> ...I wanted some discussion of that issue that nothing (as in no physical object) moves FTL accross the surface and that in fact the concepts like 'shadow' or 'light spot'  can be misleading - the professor in that Q&A talks about 'thing moving' but that it is ok because no information can be transmited that way... And when I protested originally that nothing moves FTL, I was told I am wrong. ...Posted by vandivx</DIV>I think that most of the problem here lies with the limitations of language.  That is why it is usually best to discuss physics in the proper language, which is mathematics.  But it is often too hard to do that in a forum such as this, both because the participants have disparate backgrounds in mathematics and because the symbology necessary to express the mathematics is not available.It is true that nothing can move faster than light.  But this opens the question as to what is meant by a "thing".  For the purposes of special relativity a "thing" is either a particle or an information-carrying signal.  The particle can be massive or can have zero rest mass, but it must have energy.  An information carrying signal, going from point A to point B should allow the recipient to distinguish between two or more possible alternatives at B, so the receipt of one information carrying signal inherently implies the existence of an alternative signal.An illuminated spot or a shadow, even the point of impact of a garden hose, are not "things" in the above sense.  They have no physical realization and they have no energy -- the photons or the water molecules certainly carry energy, but the spot itself does not.  Similarly there is no alternate signal that carries information from one location of the spot to another location.This is why I prefaced the example that I gave with the light spot as saying that by taking advantage of an angular motion one could sort of create faster than light motion.  It was a means of introducing the topic in a way that some find interesting but also implying that there was no violation of special relativity.  The purpose of the example is to illuminate what special relativity says and what it does not say.Concepts like shadow and light are not really misleading.  It is simply important to recognize what they are and what they are not.  As way noted by a another poster, speedfreak if I recall correctly, such instances of apparently faster than light motion, are known in astronomy where a light source casts a spot on gas clouds which can be observed with telescopes and appear to be moving superluminally.  In that case the example given is important, and provides and explanation the observation which does not require a major revision of physics.  <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> ...I thought you might at least acknowledge that my conclusions are correct to auswage the doubts of people who may not be able to decide for themselves given they are starting out in physics. Mere silence on your part is not enough given you attempt to discredit whatever I post if there is slightest possibility to do that. ...Posted by vandivx</DIV>If you would like a recommendation to a book that is deeper and requires more mathematical background, then An Introduction to Special Relativityby Wolfgang Rindler is one of the best.<img id="prodImage" src="http://ecx.images-amazon.com/images/I/413T6S0WD5L._BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA240_SH20_OU01_.jpg" border="0" alt="Introduction to Special Relativity" width="240" height="240" />I discredit only your notions that involve contradictions to well-established science, in particular the notion of absolute space.  There are reasons for that.  First and foremost that notion has been demonstrated to be without meaning in both general and special relativity. Second, it is important that younger people reading these posts understand what is and is not real science.  Acceptance of the idea of absolute space would dramatically impede the learning of relativity, and do serious damage to progress of someone trying to learn true science.There is a real need for creative thinking in science,  but that creativity must be tightly constrained by the facts.  Special relativity, general relativity, quantum electrodynamics, quantum chromodynamics, electroweak theory, classical Newtonian mechanics, Maxwellian electrodynamics are well-established and their limitations are known.  These theories do have weak spots.  The process of renormalizability in quantum field theories, for instance, is not on firm mathematical ground -- but does result in exquisitely accurate predictions though the reason is mysterious.  General relativity predicts singularities in situations in which one might expect quantum phenomena to be important, and we have no theory that can explain gravity and quantum theory simultaneously and consistently. Nevertheless challenging such theories, backed by the enormous experimental data that supports them and demonstrates their amazing accuracy and defines their limitations, is not science, it is just plain silly, except with regard to those aspects that are known to be weak.  Every physics or mathematics department sees crackpot ideas on a regular basis.  Some nut always thinks he can square the circle, show that Einstein was a fraud, or some such other ludicrous notion.   On the other hand, the speculative theories, such a string theory, inflation, loop quantum gravity, etc. are clearly open to challenge, but not in a way that also challenges the established theories within their known domains of validity.Dark matter and dark energy are not really open to challenge either, but not because they are established physical theories.  They are not open to challenge simply because they are nothing more than code words that indicate a question.  You might challenge the data that leads to the question and whether there is or is not something of substance to te answered, but you cannot challenge the question itself.  You can also challenge an answer to a question, but neither dark matter nor dark energy are answers, but rather are just place holders for some future explanation.  You can most certainly challenge any purported statement as to the precise nature of dark matter or dark energy, and such challenges are issued daily in mainstream science.  In fact, it is probably a waste of time to make such a challenge in a forum like this, because almost certainly any recent explanation will have already received many challenges from professionals -- that is just the nature of on-gong research in science.A measure of understanding and depth of knowledge in the sciences is recognizing what one can profitably challenge and what is so firmly supported by hard evidence that no challenge is meaningful.  It is the difference between creative professional research scientists and crackpots.  <div class="Discussion_UserSignature"> </div>

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