beejewel Posted July 20, 2015 Share Posted July 20, 2015 Relative velocity is absolute, the only cause of relative velocity is relative potential, therefore relative potential is absolute. -1 Link to comment Share on other sites More sharing options...
ACG52 Posted July 20, 2015 Share Posted July 20, 2015 False Link to comment Share on other sites More sharing options...
Mordred Posted July 20, 2015 Share Posted July 20, 2015 False Link to comment Share on other sites More sharing options...
Strange Posted July 20, 2015 Share Posted July 20, 2015 Relative velocity is absolute False the only cause of relative velocity is relative potential False therefore relative potential is absolute False (Note that using a larger font doesn't make it true.) Link to comment Share on other sites More sharing options...
ajb Posted July 20, 2015 Share Posted July 20, 2015 Your statements seem very confused. Unless you have some other meaning of 'absolute' then what you have said is a true oxymoron. Potentials are far from absolute, this is one of the key points of a potential, even when discussing electrostatics or Newtonian gravity. You really only have a torsor and can only measure energy differences; you have to pick a ground or zero energy. Link to comment Share on other sites More sharing options...
MigL Posted July 20, 2015 Share Posted July 20, 2015 Without jumping to conclusions... Notice he did say RELATIVE velocity and potential. In the case of the velocity or potential of X with respect to Y, there are only two frames to consider, that of X and that of Y. RELATIVE, by definition disregards all others, and so can be considered absolute, as it cannot vary. So, I wouldn't say it is false. However, I fail to see the sense or meaning of the statement. 1 Link to comment Share on other sites More sharing options...
beejewel Posted July 20, 2015 Author Share Posted July 20, 2015 All opinions appreciated, I merely want to understand if the statement is true or false, without resorting to any beliefs or assumptions. So let's break it up..Relative velocity is absolute, This means relative velocity between two bodies can have any value between zero, -c and +c, but no more or less. the only cause of relative velocity is relative potential, AFAIK the only way to set a body in motion relative to another body is via a potential difference, no magic or other spooky action will do it, even a transfer of velocity from one body to another must have it's roots in a potential difference of some kind somewhere. therefore relative potential is absolute. This just says, if the first and second statement is true, then you can't have an unlimited potential difference between two bodies, as this would imply that you could obtain an unlimited velocity, and we understand from special relativity that we can't. It is very difficult to prove a statement without resorting to beliefs (I like to call them fairies), which is why it would be helpful if everyone can make clear which part of the above statement is false and why. Link to comment Share on other sites More sharing options...
Strange Posted July 20, 2015 Share Posted July 20, 2015 All opinions appreciated, I merely want to understand if the statement is true or false, without resorting to any beliefs or assumptions. So let's break it up.. Relative velocity is absolute, This means relative velocity between two bodies can have any value between zero, -c and +c, but no more or less. Almost (should be any value less than c). But this is relative velocity. Not absolute. (Unless you are using the word "absolute" in a non-standard way?) therefore relative potential is absolute. This just says, if the first and second statement is true, then you can't have an unlimited potential difference between two bodies, as this would imply that you could obtain an unlimited velocity, and we understand from special relativity that we can't. This doesn't follow. However large the applied force (or potential) the speed is still limited to be less than c. But it seems you are using "absolute" to mean "maximum" or "limited". Is that what you mean? If so, one of my answers would be different. Link to comment Share on other sites More sharing options...
ajb Posted July 20, 2015 Share Posted July 20, 2015 Absolute would usually mean something more like 'frame independent'; this is not the meaning used in the opening post. What is true is that if one inertial observer measures the speed of another inertial observer relative to himself then that speed is <c and is 'fixed'. But this is not the usual meaning of 'absolute'. 1 Link to comment Share on other sites More sharing options...
beejewel Posted July 20, 2015 Author Share Posted July 20, 2015 Almost (should be any value less than c). But this is relative velocity. Not absolute. (Unless you are using the word "absolute" in a non-standard way?) Yes you are correct I was just pointing out that approaching and receding velocity is possible absolute would imply both as you point out. This doesn't follow. However large the applied force (or potential) the speed is still limited to be less than c. I see where you are coming from here, because we understand that excess energy is converted to mass, but can we honestly take this route? Can there exist such a potential difference between two bodies, such that the free fall of either body results in more mass than the original rest mass of the two bodies? But it seems you are using "absolute" to mean "maximum" or "limited". Is that what you mean? If so, one of my answers would be different. My definition of absolute in this context is an absolute value between zero and some upper limit. -1 Link to comment Share on other sites More sharing options...
ajb Posted July 20, 2015 Share Posted July 20, 2015 ...because we understand that excess energy is converted to mass... I don't follow what you are saying. We understand that mass has to be taken into account when considering total energy. Can there exist such a potential difference between two bodies, such that the free fall of either body results in more mass than the original rest mass of the two bodies? Something like binding energy? My definition of absolute in this context is an absolute value between zero and some upper limit. This is unclear. By absolute do you mean a numerical value of something that all (or at lest all inertial) observers will agree on? Link to comment Share on other sites More sharing options...
Strange Posted July 20, 2015 Share Posted July 20, 2015 (edited) I see where you are coming from here, because we understand that excess energy is converted to mass, but can we honestly take this route? Can there exist such a potential difference between two bodies, such that the free fall of either body results in more mass than the original rest mass of the two bodies? And that is why the idea of "relativistic mass" is discouraged by many people. It leads to erroneous arguments like that. My definition of absolute in this context is an absolute value between zero and some upper limit. But what do you mean by "an absolute value"? The velocity is still relative and is observer dependent. It is not absolute, even though there is an upper limit. Actually, I'm not sure it is clear that there is an upper limit. The value of c in this context is rather like infinity: you can get as close as you want; whatever speed you are doing less than c, it is possible to go faster. Edited July 20, 2015 by Strange Link to comment Share on other sites More sharing options...
beejewel Posted July 20, 2015 Author Share Posted July 20, 2015 I don't follow what you are saying. I was referring to how we can accelerate a proton in a ring giving it additional relativistic mass, this we can clearly do, but only at the expense of mass elsewhere, not sure if that qualifies as a single potential difference. Something like binding energy? I think I more or less answered my own question, Let's take the biggest potential we can think of, from the surface of a black hole to to the event horizon at the edge of time. and let a body fall through that potential, the total energy of the system will remain constant while potential is converted to relative velocity. This is unclear. By absolute do you mean a numerical value of something that all (or at lest all inertial) observers will agree on? I think it is best to stick with two bodies at this stage, so let's just say the relative velocity is definite and limited. A agrees and B agrees that their relative velocity is definite and limited by c. Strange: And that is why the idea of "relativistic mass" is discouraged by many people. It leads to erroneous arguments like that. No need for relativistic mass here, just a question of weather a potential difference can exist which would cause a body to exceed the speed of light. yes or no? But what do you mean by "an absolute value"? The velocity is still relative and is observer dependent. It is not absolute, even though there is an upper limit. Actually, I'm not sure it is clear that there is an upper limit. The value of c in this context is rather like infinity: you can get as close as you want; whatever speed you are doing less than c, it is possible to go faster. ajb, also questioned my use of absolute above, so I redefined it to the best of my current understanding. Regarding the speed of light, the potential through which a body must fall to reach the speed of light must follow an asymptotic function, so as we observe no bodies moving faster than the speed of light, it seems to imply some potential limit? -1 Link to comment Share on other sites More sharing options...
Strange Posted July 20, 2015 Share Posted July 20, 2015 I think I more or less answered my own question, Let's take the biggest potential we can think of, from the surface of a black hole to to the event horizon at the edge of time Not sure what the "event horizon at the edge of time" is. But the (gravitational) potential of a black hole depends on its mass. So there isn't any upper limit here. Unless, in practical terms, the entire mass of the universe is in a single black hole... I think it is best to stick with two bodies at this stage, so let's just say the relative velocity is definite and limited. A agrees and B agrees that their relative velocity is definite and limited by c. So not absolute at all. No need for relativistic mass here, just a question of weather a potential difference can exist which would cause a body to exceed the speed of light. yes or no? No. And it doesn't matter how large the potential difference is. Regarding the speed of light, the potential through which a body must fall to reach the speed of light must follow an asymptotic function, so as we observe no bodies moving faster than the speed of light, it seems to imply some potential limit? The very fact that it is an asymptotic limit implies that there isn't (or doesn't have to be) an upper limit to potential difference. However large you make the potential (tending towards infinity) the final velocity can increase (tending towards c). Link to comment Share on other sites More sharing options...
ajb Posted July 20, 2015 Share Posted July 20, 2015 I was referring to how we can accelerate a proton in a ring giving it additional relativistic mass, this we can clearly do, but only at the expense of mass elsewhere, not sure if that qualifies as a single potential difference. Okay, you are talking about relativistic mass, or as everyone today would call it (mod c^2) the total energy. I think I more or less answered my own question, Let's take the biggest potential we can think of, from the surface of a black hole to to the event horizon at the edge of time. and let a body fall through that potential, the total energy of the system will remain constant while potential is converted to relative velocity. The problem is the general relativity is formulated very differently to Newtonian gravity and one has to be careful with ideas like potential. You can use the Newtonian approximation or sometimes deal with effective potentials; you can calculate and effective potential for text particles on the Schwarzschild solution. The point is you cannot always just 'map' Newtonian ideas to ideas in general relativity. I think it is best to stick with two bodies at this stage, so let's just say the relative velocity is definite and limited. A agrees and B agrees that their relative velocity is definite and limited by c. Okay, but for sure this is not the standard meaning of absolute. Link to comment Share on other sites More sharing options...
beejewel Posted July 20, 2015 Author Share Posted July 20, 2015 Not sure what the "event horizon at the edge of time" is. But the (gravitational) potential of a black hole depends on its mass. So there isn't any upper limit here. Unless, in practical terms, the entire mass of the universe is in a single black hole... So not absolute at all. No. And it doesn't matter how large the potential difference is. The very fact that it is an asymptotic limit implies that there isn't (or doesn't have to be) an upper limit to potential difference. However large you make the potential (tending towards infinity) the final velocity can increase (tending towards c). Okay, my definition of absolute was not clear enough, so I shall think about the problem some more and see if it can be asked in a different way. For sure it does not seem reasonable to me that a potential can exist which accelerates matter beyond the speed of light, relativistic mass doesn't (shouldn't) come into play here because there is no force acting on a free falling body, a body in free fall is converting it's potential to velocity at a 1:1 ratio, so there ought to be a linear relationship between velocity and potential. As suggested further up the thread my original statement would have been better worded like this. Relative velocity is limited, the only cause of relative velocity is relative potential, it follows that relative potential is limited. Here is another argument in favour of limited potential; A body falling through a potential will accelerate with respect to a body at rest until such time as the relative speed is that of light by which time it is by definition a massless photon and as a consequence can no longer be accelerated by a potential. Link to comment Share on other sites More sharing options...
imatfaal Posted July 20, 2015 Share Posted July 20, 2015 ... A body falling through a potential will accelerate with respect to a body at rest until such time as the relative speed is that of light by which time it is by definition a massless photon and as a consequence can no longer be accelerated by a potential. Bodies do not become photons just because the approach the speed of light - they remain relativistic particles most of the time. And potentials can accelerate photons - but not change the speed of the photon; see gravitational lensing Link to comment Share on other sites More sharing options...
MigL Posted July 20, 2015 Share Posted July 20, 2015 Relative velocity can only be measured in two frames. If X and Y are moving relative to each other, you can only measure their relative motion in frame of X or frame of Y. And in those two frames the relative velocity is equivalent. That makes it absolute by definition. Link to comment Share on other sites More sharing options...
imatfaal Posted July 20, 2015 Share Posted July 20, 2015 Relative velocity can only be measured in two frames. If X and Y are moving relative to each other, you can only measure their relative motion in frame of X or frame of Y. And in those two frames the relative velocity is equivalent. That makes it absolute by definition. If you can only measure it in one or the other frame then it is not absolute it is relative. "And in those two frames the relative velocity is equivalent" - there is a minus sign; or you could say that they are equivalent when you are dealing with absolute values... oh damn Link to comment Share on other sites More sharing options...
Strange Posted July 20, 2015 Share Posted July 20, 2015 For sure it does not seem reasonable to me that a potential can exist which accelerates matter beyond the speed of light That is correct; it can't. relativistic mass doesn't (shouldn't) come into play here because there is no force acting on a free falling body, a body in free fall is converting it's potential to velocity at a 1:1 ratio, so there ought to be a linear relationship between velocity and potential. Off the top of my head, that looks wrong, even in a Newtonian world. Potential (gravitational) varies linearly with height, whereas free fall speed varies (I think) with the square root of height. Or, to look at it another way, when in free fall the object swaps potential energy for kinetic energy. Kinetic energy is related to velocity squared. And that isn't taking into account that in GR velocity addition and acceleration are not linear. Relative velocity is limited, Correct. the only cause of relative velocity is relative potential, We can allow that for the purposes of this discussion. it follows that relative potential is limited. No. See below. A body falling through a potential will accelerate with respect to a body at rest until such time as the relative speed is that of light No. It will accelerate continuously. Although not physically plausible, it could fall from infinity to an infinite mass and it would accelerate continuously and get closer and closer to (but never reach) the speed of light. To go back to your black hole example, an object in free fall towards a black hole from infinity will reach the speed of light as it crosses the event horizon (for any sized black hole). At which point, I don't think there is a meaningful definition of speed (compared to the outside world). Relative velocity can only be measured in two frames. If X and Y are moving relative to each other, you can only measure their relative motion in frame of X or frame of Y. And in those two frames the relative velocity is equivalent. That makes it absolute by definition. You could measure their velocities from a third frame (in which case the velocity between them is limited to be less than 2c) and get different values from either of them. Ditto a fourth, fifth, ... Link to comment Share on other sites More sharing options...
beejewel Posted July 20, 2015 Author Share Posted July 20, 2015 (edited) The confusion is becoming clearer, when I was referring to limited velocity and limited speed, I meant from the perspective of the observer. My understanding or belief if you like, is not that energy potential or velocity has any physical limit, but rather that there are restraints on what an observers domain. Rather like the "Trueman show", in which Trueman lived his life in a domain, it was limited in size and no matter how hard he tried, he was never able to make it out of the domain (except for at the very end). I like to say for fun: We are not acting out our lives on a stage called the Universe, we are staging the Universe in an act we call Life. So if we think of the Universe as a bubble surrounding the observer, the potential drop from the surface of that bubble to the surface of an imaginary black hole at the observers centre of gravity, it constitutes the absolute potential drop, and should be (must be) inversely proportional to speed. ref: https://en.wikipedia.org/wiki/The_Truman_Show Edited July 21, 2015 by beejewel Link to comment Share on other sites More sharing options...
MigL Posted July 20, 2015 Share Posted July 20, 2015 You can't measure RELATIVE velocity ( or speed ) from a third frame, strange. It is then no longer RELATIVE velocity, by definition. And as you correctly point out, you would get differing results. Agreed its probably semantics, but I believe I'm sticking to accepted definitions. And as this thread is now referencing movies ( and I couldn't figure out the original point, anyway ), I'll be bowing out. Link to comment Share on other sites More sharing options...
beejewel Posted July 21, 2015 Author Share Posted July 21, 2015 And as this thread is now referencing movies ( and I couldn't figure out the original point, anyway ), I'll be bowing out. I respect your decision to bow out of the thread, but I hope it was not my reference to the Trueman show that did it. Truth can to be found anywhere, if we limit our search for truth to peer reviewed journals, I can assure you that it will never be found, because it's not there. If on the other hand you believe that we already know all the answers, we should not waste any more time, bind it in leather, emboss it with gold letters and call it the Bible. Link to comment Share on other sites More sharing options...
Strange Posted July 21, 2015 Share Posted July 21, 2015 You can't measure RELATIVE velocity ( or speed ) from a third frame, strange. I don't see why not. And as you correctly point out, you would get differing results. That is what makes it relative, not absolute. So if we think of the Universe as a bubble surrounding the observer, the potential drop from the surface of that bubble to the surface of an imaginary black hole at the observers centre of gravity, it constitutes the absolute potential drop, and should be (must be) inversely proportional to speed. But, as noted, velocity is not inversely proportional to velocity, even if you ignore relativistic effects. I respect your decision to bow out of the thread, but I hope it was not my reference to the Trueman show that did it. Truth can to be found anywhere, if we limit our search for truth to peer reviewed journals, I can assure you that it will never be found, because it's not there. If on the other hand you believe that we already know all the answers, we should not waste any more time, bind it in leather, emboss it with gold letters and call it the Bible. What does "truth" have to do with it? I thought this was a discussion of science. But you are contradicting yourself: if "truth can be found anywhere" then that must include peer reviewed journals so it can't be true to say it's not there. Link to comment Share on other sites More sharing options...
StringJunky Posted July 21, 2015 Share Posted July 21, 2015 'Truth' iis what agrees with what we know now compared to what we knew before; it will always be plastic from our perspective. Link to comment Share on other sites More sharing options...
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