mistermack Posted August 8, 2017 Share Posted August 8, 2017 3 minutes ago, studiot said: The two forces are the body force due to gravity downwards and the reaction between you and the surface upwards. That's fine, except that I keep reading that gravity isn't a force. That's the essence of what I was saying really. It's not a force, it's curvature of space time, needing a real force to counteract it and stop me from following it. But it's not acceleration either, because I'm not gaining kinetic energy. Link to comment Share on other sites More sharing options...
studiot Posted August 8, 2017 Share Posted August 8, 2017 6 minutes ago, mistermack said: That's fine, except that I keep reading that gravity isn't a force. That's the essence of what I was saying really. It's not a force, it's curvature of space time, needing a real force to counteract it and stop me from following it. But it's not acceleration either, because I'm not gaining kinetic energy. In Newtonian mechanics, which is all we need here, gravity is most definitely a force. The interpretation of gravity as some sort of warping of spacetime (not space) is non Newtonian. You should not mix Newtonian and non Newtonian physics. Link to comment Share on other sites More sharing options...
Tim88 Posted August 9, 2017 Share Posted August 9, 2017 14 hours ago, studiot said: In Newtonian mechanics, which is all we need here, gravity is most definitely a force. The interpretation of gravity as some sort of warping of spacetime (not space) is non Newtonian. You should not mix Newtonian and non Newtonian physics. Adding to that, it depends on one's definition of "force" if we call it a force or not. In fact, Einstein still called it a force in 1916 when he explained GR. Plays on words should not hinder our understanding! Link to comment Share on other sites More sharing options...
Tim88 Posted August 9, 2017 Share Posted August 9, 2017 (edited) 16 hours ago, mistermack said: That's fine, except that I keep reading that gravity isn't a force. [..] But it's not acceleration either, because I'm not gaining kinetic energy. How about this: Einstein speculated in 1916 that "The general theory of relativity renders it likely that the electrical masses of an electron are held together by gravitational forces." (in Relativity: The Special and General theory, English translation of 1920). - https://en.wikisource.org/wiki/Relativity:_The_Special_and_General_Theory/Part_I#cite_note-14 Once more (sticking to the topic), it's all a matter of interpretation and definitions, quite independent of the mathematical theory. Edited August 9, 2017 by Tim88 added link as courtesy Link to comment Share on other sites More sharing options...
studiot Posted August 9, 2017 Share Posted August 9, 2017 (edited) 1 hour ago, Tim88 said: Adding to that, it depends on one's definition of "force" if we call it a force or not. In fact, Einstein still called it a force in 1916 when he explained GR. Plays on words should not hinder our understanding! If more than one definition / interpretation is available, then it is incumbent upon a writer to make it plain which one is in use at any point in the writing. Otherwise it can easily lead to one definition being set against another, as I noted. Edited August 9, 2017 by studiot 1 Link to comment Share on other sites More sharing options...
Tim88 Posted August 9, 2017 Share Posted August 9, 2017 (edited) 38 minutes ago, studiot said: If more than one definition / interpretation is available, then it is incumbent upon a writer to make it plain which one is in use at any point in the writing. Otherwise it can easily lead to one definition being set against another, as I noted. What does that obvious fact have to do with the issues that PrimalMinister and mistermack have with interpretations of relativity by certain writers? As I understand their issues, they are struggling with a mix-up that happens at the source of information. The issue here, I think, is not that writers are unclear or confusing about their definitions. All too often -even in textbooks- metaphysics is sold as physics, sometimes even accompanied by misleading or false information. That causes the most confusion. Let's wait for their comments. Edited August 9, 2017 by Tim88 Link to comment Share on other sites More sharing options...
studiot Posted August 9, 2017 Share Posted August 9, 2017 (edited) 7 minutes ago, Tim88 said: What does that obvious fact have to do with the issues that PrimalMinister and mistermack have with interpretations of relativity by certain writers? As I understand their issues, they are struggling with a mix-up that happens at the source of information. The issue here, I think, is not that writers are unclear or confusing about their definitions. All too often -even in textbooks- metaphysics is sold as physics, sometimes even accompanied by misleading or false information. That causes the most confusion. Let's wait for their comments. Perhaps I used the wrong word, "writers" includes writers of posts here. In fact my comments here have been directed towards (not necessarily against) posts here. I don't think I commented directly in this thread on something Einstein etc wrote In particular bvr offered 'equilibrium', whereby a four force 'balanced' a three force, which is unacceptable. Edited August 9, 2017 by studiot Link to comment Share on other sites More sharing options...
swansont Posted August 9, 2017 Share Posted August 9, 2017 19 hours ago, mistermack said: That's fine, except that I keep reading that gravity isn't a force. That's the essence of what I was saying really. It's not a force, it's curvature of space time, needing a real force to counteract it and stop me from following it. But it's not acceleration either, because I'm not gaining kinetic energy. You would be accelerating in freefall, but in GR, freefall (in a uniform field) is an inertial frame. Link to comment Share on other sites More sharing options...
Janus Posted August 9, 2017 Share Posted August 9, 2017 21 hours ago, mistermack said: That's fine, except that I keep reading that gravity isn't a force. That's the essence of what I was saying really. It's not a force, it's curvature of space time, needing a real force to counteract it and stop me from following it. But it's not acceleration either, because I'm not gaining kinetic energy. For one, you can accelerate while maintaining a constant speed, just travel in a circle. Secondly, kinetic energy is a relative and not absolute measurement. When you say you are gaining kinetic energy, what frame are you gaining it relative to? Different frames will say that your kinetic energy at any given moment have different values. When Einstein says the gravity and acceleration are equivalent (not "indistinguishable") He means as measured from the rest frame of the system concerned. The elevator is not gaining kinetic energy relative to itself nor are you gaining kinetic energy relative to the elevator. It doesn't matter if you are or are not according to some other frame. 2 Link to comment Share on other sites More sharing options...
mistermack Posted August 9, 2017 Share Posted August 9, 2017 55 minutes ago, Janus said: For one, you can accelerate while maintaining a constant speed, just travel in a circle. Secondly, kinetic energy is a relative and not absolute measurement. When you say you are gaining kinetic energy, what frame are you gaining it relative to? Different frames will say that your kinetic energy at any given moment have different values. When Einstein says the gravity and acceleration are equivalent (not "indistinguishable") He means as measured from the rest frame of the system concerned. The elevator is not gaining kinetic energy relative to itself nor are you gaining kinetic energy relative to the elevator. It doesn't matter if you are or are not according to some other frame. Thanks, I get what you are saying there. One question that occurs to me though, is can an accelerating frame of reference be real? I can see how it works locally and temporarily, but as far as the Universe goes, it would mean that the total energy of the Universe is constantly rising. So I can picture an accelerating reference frame, no problem. But can one exist, other than as a local effect? If you take it to it's logical conclusion, an accelerating reference frame would have everything in the Universe moving faster than light eventually. Or, on the elevator, a constant acceleration of 1g would take you past the speed of light. Link to comment Share on other sites More sharing options...
studiot Posted August 9, 2017 Share Posted August 9, 2017 6 minutes ago, mistermack said: Thanks, I get what you are saying there. One question that occurs to me though, is can an accelerating frame of reference be real? I can see how it works locally and temporarily, but as far as the Universe goes, it would mean that the total energy of the Universe is constantly rising. So I can picture an accelerating reference frame, no problem. But can one exist, other than as a local effect? If you take it to it's logical conclusion, an accelerating reference frame would have everything in the Universe moving faster than light eventually. Or, on the elevator, a constant acceleration of 1g would take you past the speed of light. You could just curl up with this good book https://www.google.co.uk/search?site=&source=hp&q=anderson+tau+zero&oq=anderson+tau+zero&gs_l=psy-ab.3..0i22i30k1l4.1928.8716.0.9839.17.17.0.0.0.0.451.3638.0j6j10j0j1.17.0..3..0...1.1.64.psy-ab..0.17.3622...0j0i131k1._CEwwF8hsy0 Janus, you have a way of cutting through the crap to a simple explanation in Relativity (and perhaps other matters). I take my hat off to you. +1 Link to comment Share on other sites More sharing options...
koti Posted August 9, 2017 Share Posted August 9, 2017 2 hours ago, Janus said: For one, you can accelerate while maintaining a constant speed, just travel in a circle. Secondly, kinetic energy is a relative and not absolute measurement. When you say you are gaining kinetic energy, what frame are you gaining it relative to? Different frames will say that your kinetic energy at any given moment have different values. When Einstein says the gravity and acceleration are equivalent (not "indistinguishable") He means as measured from the rest frame of the system concerned. The elevator is not gaining kinetic energy relative to itself nor are you gaining kinetic energy relative to the elevator. It doesn't matter if you are or are not according to some other frame. Simple, scaringly coherent. You are wasting your talents on an internet forum Janus +1 Link to comment Share on other sites More sharing options...
mistermack Posted August 9, 2017 Share Posted August 9, 2017 16 minutes ago, koti said: Simple, scaringly coherent. You are wasting your talents on an internet forum Janus +1 Maybe Janus works on the theory that if you don't use it, you lose it. I should know, I lost it years ago. Link to comment Share on other sites More sharing options...
swansont Posted August 9, 2017 Share Posted August 9, 2017 1 hour ago, mistermack said: One question that occurs to me though, is can an accelerating frame of reference be real? I can see how it works locally and temporarily, but as far as the Universe goes, it would mean that the total energy of the Universe is constantly rising. Energy conservation only applies within an inertial frame. Energy is not invariant, which is the term that applies when switching frames. IOW, kinetic energy is relative. Link to comment Share on other sites More sharing options...
Tim88 Posted August 10, 2017 Share Posted August 10, 2017 19 hours ago, mistermack said: Thanks, I get what you are saying there. One question that occurs to me though, is can an accelerating frame of reference be real? I can see how it works locally and temporarily, but as far as the Universe goes, it would mean that the total energy of the Universe is constantly rising. So I can picture an accelerating reference frame, no problem. But can one exist, other than as a local effect? If you take it to it's logical conclusion, an accelerating reference frame would have everything in the Universe moving faster than light eventually. Or, on the elevator, a constant acceleration of 1g would take you past the speed of light. Indeed accelerating frames are for local use; that was already done in classical mechanics but GR extended it for all physical phenomena. There's another issue that none of us addressed but is coming back every time: there is no "moving faster than light eventually" involved. Accelerating objects as measured with a "stationary frame" (also called "coordinate acceleration") accelerate less and less fast as they reach high speed. That was even one of the first predictions of SR, about the fact that electrons cannot be accelerated to the speed of light. Note also the subtle difference with proper acceleration, which is the acceleration relative to instantly co-moving reference frames.The fact that nothing can break the speed of light is true for someone who has a constant proper acceleration of 9.8 m/s2. Link to comment Share on other sites More sharing options...
MigL Posted August 10, 2017 Share Posted August 10, 2017 Relativity cannot be incorrect for the conditions where it is applicable. Ever ! It has been tested for a century, and it IS correct. Same with Newtonian gravity. It IS correct where applicable, and has been tested for almost 4 centuries. Keep in mind that these are models. Analogues for the reality which tends to bring masses together. It has been modelled as a force, by Newton, and it works well enough to put people on the moon ! It has been modelled as space-time curvature, which greatly extends the conditions of applicability, approaching billionths of a second after the Big Bang and close to the center, deep inside a Black Hole. Could it be modelled as something completely different in the future, extending our understanding to t=0, and explaining away the infinities of singularities and quantized gravity ? Of course. But that doesn't make Newton and Einstein incorrect. Link to comment Share on other sites More sharing options...
mistermack Posted August 11, 2017 Share Posted August 11, 2017 I agree with all of that. One thing that I don't agree with though, is the often posted claim that science is just about modelling, and accurately describing and predicting the world. Newton and Einstein would have LOVED to discover what causes gravity, rather than just modelling and calculating what it does. (I don't mean "just" in a derogatory way) The actual mechanism was not detectable to them, as it's still not identifiable now. The similarity between acceleration and gravity I find fascinating, and I think is probably a strong clue to the actual mechanism. But the differences are also substantial, as in the problem with reaching the speed of light, if you were to have a constant acceleration, whereas you don't, on the surface of the Earth, even though it feels exactly the same. You can model it by imagining an accelerating reference frame, but that only makes sense for temporary and local events. If each bit of mass in my body had a spring attached to it, connected to the centre of gravity of the earth, you could model the effect as curved space time and accurately predict the motion if I jumped up and down. But if the spring was in plain sight, you would concentrate on that. Link to comment Share on other sites More sharing options...
swansont Posted August 11, 2017 Share Posted August 11, 2017 41 minutes ago, mistermack said: I agree with all of that. One thing that I don't agree with though, is the often posted claim that science is just about modelling, and accurately describing and predicting the world. Newton and Einstein would have LOVED to discover what causes gravity, rather than just modelling and calculating what it does. (I don't mean "just" in a derogatory way) The second observation in no way contradicts the first. Link to comment Share on other sites More sharing options...
swansont Posted August 11, 2017 Share Posted August 11, 2017 On 8/9/2017 at 11:11 AM, Janus said: For one, you can accelerate while maintaining a constant speed, just travel in a circle. Also, this effect has been experimentally confirmed, with the same sort of methodology used in the Pound-Rebka experiment: A Mössbauer effect transition moves out of resonance when you put the system in a centrifuge, with one part (emitter, IIRC) near the center and the absorber spinning at distance away. references are listed at the endhttp://blogs.scienceforums.net/swansont/archives/1426 Link to comment Share on other sites More sharing options...
mistermack Posted August 11, 2017 Share Posted August 11, 2017 Travelling in a circle is a different kind of acceleration though. It's not continuous, but constantly changing in direction. More of a series of infinitely small accelerations, all in slightly different directions. Unlike the effect of gravity which is constant in magnitude and direction. The first wouldn't result in encountering the speed of light barrier whereas a constant acceleration in a straight line would. Link to comment Share on other sites More sharing options...
swansont Posted August 11, 2017 Share Posted August 11, 2017 1 hour ago, mistermack said: Travelling in a circle is a different kind of acceleration though. It's not continuous, but constantly changing in direction. If I use a spherical or cylindrical coordinate system, which is appropriate for this sort of geometry, the direction is constant: radially inward. Quote More of a series of infinitely small accelerations, all in slightly different directions. Unlike the effect of gravity which is constant in magnitude and direction. The first wouldn't result in encountering the speed of light barrier whereas a constant acceleration in a straight line would. If the acceleration is not constant in my scenario, you most certainly can't say that gravity is. Gravity is directed radially inward as well. Link to comment Share on other sites More sharing options...
mistermack Posted August 11, 2017 Share Posted August 11, 2017 (edited) 5 hours ago, swansont said: If I use a spherical or cylindrical coordinate system, which is appropriate for this sort of geometry, the direction is constant: radially inward. If the acceleration is not constant in my scenario, you most certainly can't say that gravity is. Gravity is directed radially inward as well. Gravity doesn't have to be directed radially inwards though. On a non-rotating planet, it would be a constant force in a constant direction, as in the elevator in space. One big difference in rotational acceleration, compared to linear acceleration, appears to be how energy is involved. No external source of energy is needed for a constant rotational acceleration. It could continue for ever without any input. But to accelerate a mass in a straight line, you need to put in energy, in greater and greater quantities as the speed increases. In one case the forces are just re-directing the velocity. In the other, they are increasing it. Gravity seems to be a cross between the two. The feeling of acceleration without the need for energy input, or any change of direction. Edited August 11, 2017 by mistermack Link to comment Share on other sites More sharing options...
studiot Posted August 11, 2017 Share Posted August 11, 2017 (edited) 1 hour ago, mistermack said: Gravity doesn't have to be directed radially inwards though. On a non-rotating planet, it would be a constant force in a constant direction, as in the elevator in space. Oh dear Oh dear Oh dear, Surely someone has said this before. Gravity acts towards the centre of mass of the Earth. This is nothing to do with rotation. Edited August 11, 2017 by studiot Link to comment Share on other sites More sharing options...
mistermack Posted August 11, 2017 Share Posted August 11, 2017 1 hour ago, studiot said: Gravity acts towards the centre of mass of the Earth. This is nothing to do with rotation. Good lord ! I had no idea ! Link to comment Share on other sites More sharing options...
Endy0816 Posted August 11, 2017 Share Posted August 11, 2017 (edited) Why can't I keep going the direction my mass is already going? :'( The rotation actually decreases the effective force you feel slightly. Imagine being on a spinning ride with everything restraining you vanishing. Whee Edited August 11, 2017 by Endy0816 Link to comment Share on other sites More sharing options...
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