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Effect of gravitational tides on the moon

  1. Sep 11, 2012 #1
    Ocean tides are considered to be mostly caused by the force of gravity from the Moon. Tides are presumably caused by the difference of the force applied on the "front" and on the "back"of the Earth. Meaning, gravity from the Moon cause Earth's shape to become elliptical in the plane of the moon, and these tides are more apparent on the oceans than on land simply because land is rigid while water ain't.

    Now. The moon is in tidal lock wrt Earth, so the forces from the Earth should create a very big tide on the Moon: there should be a big bulge (or a small one, doesn't matter) both on the dark side and on the bright side of the Moon, and the poles should be flattened, especially because the tides are locked in basically the same places, so there's no way for the forces from the Earth to be distributed over the surface (which would only minimize the tidal effects – there still should be flattening at the poles).

    So how come we actually see a negative tide on the moon?

    Look at this image to what I mean by negative tide:
    http://ase.tufts.edu/cosmos/pictures/Explore_figs_8/Chapter5/Fig 5_22copy.jpg

    Instead of a bulge on the bright side, we see a sort of obliteration of the crust, revealing the so called "mare". Ths is clear from the words "thin crust" and "thick crust" front and back.

    Isn't this evidence against tidal theory? Or is there a satisfactory explanation?
  2. jcsd
  3. Sep 11, 2012 #2


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    Maybe it's due to the core of the moon being denser than the mantle.
  4. Sep 11, 2012 #3
    You do know that for most places on earth, the high tide comes twice a day. There are high tides pretty much on opposite sides of the earth from the moon. Topography biases the tide because the land sticks out of the ocean. However, the surface topography of the moon is much simpler than the surface topography of the earth. I would expect the rock tides on the moon to be nearly symmetric.
    Rock tides could stretch the moon a small amount bit in a symmetric manner,, but they probably wouldn't cause asymmetry. That picture in your link doesn't state what causes the asymmetry. It merely shows that currently the crust is thinner on the near side of the moon. So I would not call it a "negative tide".
    Tides do not explain the asymmetry of the moon too well. So your picture of a "negative tide" still doesn't explain the initial cause for the asymmetry. You have to explain why the tide is different on opposite hemispheres of the moon.
    You are probably asking about what the symmetry breaking mechanism is that led to the hemispherical asymmetry of the moon. Tidal forces are not the only processes that could break the hemispherical symmetry of the moon. Tidal force is only one possible mechanism for breaking the symmetry.
    Most of the theories hypothesize that whatever mechanism broke the symmetry caused the near side of the moon to have a thinner crust. The visible asymmetry was directly caused by basaltic flows, which were more frequent on the near side.
    The theories that explain the asymmetry are generally divided between impact-based and geology-based theories. Impact based theories claim that the asymmetry is caused by an asymmetry in the orbit of the moon which biased the collisions. Geology based theories are based on the internal dynamics of the moon.
    There are two impact theories that I have been able to learn about. One theory is that the symmetry was broken by the spiraling motion of the moon away from the earth. The tides play an indirect role by causing the spiraling motion. As the moon moves away from the earth, there is a slight bias for the far side colliding with bolides. A second impact theory is that when the moon first formed, there was one or more bolides in an orbit that went in the same general direction of the moons orbit. Objects closer to the earth moved faster in their orbits. Thus, the near side of the moon would be hit more often. Tides do not play a role in the second theory.
    One geology based model is that when the moon first formed, the earth was still very hot. So the blackbody radiation (earth shine) emitted by the earth heated the near side of the moon. That is why the moon has a thinner crust on the near side. Tides are not significant in this theory.
    Another theory is that the tidal forces somehow caused the near side of the moon to have a thinner crust on the near side of the earth. However, this is not as simple as it first appears. To first approximation, the tidal force of the earth on the moon is a dipole interaction which would be nearly symmetric through the center of the moon.

    “The theories for the formation of the lunar maria can be divided into impact-based and geology-based explanations.

    The hypothesis that the lunar maria are the result of impacts by small Earth satellites proposed by Gilbert (1893) has been refined to postulate that collisions between the Moon and small external Earth satellites traveling in the same direction and in roughly the same orbital plane as the Moon would primarily take place on the far side of the Moon as the Moon spiraled away from the Earth, and that if the present Earth side of the Moon were actually the far side of the Moon at the time during which most satellite impacts were occurring, the asymmetric distribution of maria would be explained (Metcalfe and Barricelli, 1970).

    Geology-based theories about the differences between the near and far side of the Moon include variations of crustal thickness that modulate the amount of magma that reaches the surface (Thurber and Solomon, 1978), tidal force mechanisms (Smith J. V., 1970), tilted convection in the lunar magma ocean (Loper and Werner, 2002), rapid crystallization of the magma ocean on the far-side that created an asymmetric core (Wasson and Warren, 1980), and gravitational instability during the fractionation of an anorthositic crust (Parmentier, et al., 2002).”

    “Morota and Furumoto (2003) proposed that the currently observed nearside-farside asymmetry,
    such as density of the basin-scale craters and crustal thickness, is a consequence of different impact energy or the impact velocity between the leading side and the trailing side of the Moon in its orbital motion. They suggested that the current nearside could be the leading side, and the current farside be the trailing side.”
  5. Sep 11, 2012 #4


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    You may be making an order of magnitude error here. If the earth was entirely covered with water, the tidal range would be less than 1 meter. Even with the amplification caused by shallow water around the coastline and other "accidental" features of geography, the maximum amplitude everwhere on earth is less than 20m.

    Earth tides on land are measurable, but much smaller - IIRC a few mm.

    All of the above are much smaller than the "random" changes in surface geometry extending over large areas, caused by events like large meteorite impacts, etc.
  6. Sep 15, 2012 #5
    Thanks for the responses.
    I say negative tide because it is a negative bulge, oposite to the bulge predicted by tidal theory.

    None of the theories explain all the features. For instance, the near side could have been heavily impacted to explain the negative bulge, but then you'd have to explain why there's no flattening at the poles, or no tide on the far side. Flattening at the poles and tides on the near and far side are a direct prediction of tidal theory, and a body in tidal lock should have these features increased.

    Current tidal theory is based on Newton, with gravity as a pulling force at a distance. In GR, gravity is the shape of the field (space-time), and there are no forces (Einstein said so explicitly). Without forces, there's no mechanism for tidal theory to rely on, so we stick with Newton. So is gravity an attractive force or the geometry of the field? We can't have it both ways. Does this bother anyone else or is it just me?

    But back to the moon. Darwin123 said that "Tidal force is only one possible mechanism for breaking the symmetry", but that's backwards, tidal theory predicts a tidal symmetry front and back, not a breaking symmetry. It's not that "Tides do not explain the asymmetry of the moon too well". It's that they explicitly contradict the evidence.

    The proposition that objects in lower orbits wrt Earth would be moving faster and therefore cause more damage on the near side is a bit ridiculous and I can't take it seriously. By that reasoning, the moon should have an even larger lateral asymmetry, since tangential impacts would be less energetic than direct impacts on the trailing and leading sides of the moon. That's why it was proposed that the moon was differently oriented in the past, but that's such a wild guess that I could just as easily propose that it was upside down, or retrogade, or pink. This is particularly obvious considering the other theory that proposed a different orientation, where the far and near side would be reversed, and then proposing that impacts from the outer regions would cause the negative bulge we presently see. What caused all this flipping? Again, you can propose anything you like, so this is all as far from satisfactory as you can get.

    Geological explanations are even wilder. Say the moon has an uneven core, it isn't necessarily concluded that the surface would follow it's shape. If you can propose a complex mechanism to convince that the core is asymmetric, you can just as easily propose other mechanisms to explain any shape you like, any relations between core, mantle and surface, and with enough postulated mechanisms you can have a triangular core with a oblong surface or what have you. And even if the core happens to match the shape of the surface, the causes are still just assumptions. So we have assumptions over assumptions over assumptions where with impact-based theories you had just assumptions over assumptions.

    I don't know the answer to all this, of course, and I would be speculating just like everyone if I made any suggestions, but let me point out that gravity is not fully understood, and maybe not understood enough to fully describe tides. The history of the perihelion of mercury problem should teach us a lesson. Instead of believing Newton's gravity was complete and start looking for hidden planets to explain data on mercury, we could only find a partial solution when Einstein reviewed Newton, and precisely because that same theory explained other phenomena. Shouldn't we stop looking for hidden bodies impacting a differently oriented moon and base tidal theory on Newton? If GR is an improved description of gravity and if we really have ditched force at a distance, we should use GR to explain tides, and if that seems impossible, either GR is not a complete theory of gravity or gravity is not the cause of tides. There's no third choice, is there?

    After all, is there any evidence supporting tidal theory on other bodies in our solar system? Are there any moons that show the predicted tides, even if not in tidal lock? I couldn't find any. Is tidal theory supported only here on Earth?
  7. Sep 15, 2012 #6


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    If I understand you correctly, you expect to see some great bulges caused by the tidal effects, and since you don't see them, you conclude that there's something wrong with... I'm not sure what exactly. The whole of mechanics I guess?

    As AlphaZero has just told you, the crust deformation, as can be seen e.g. on Earth, is just too small to cause noticeable flattening. If you could do as accurate measurements on the Moon as on Earth, then you would indeed find out that the poles are flatter by some dozens of milimeters or something in this order of magnitude.

    This in no way can be compared to the size of irregularities we see, so other causes must have been in effect.
  8. Sep 15, 2012 #7


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    Tidal effects from the Earth on the Moon are very small. The average difference in acceleration towards the Earth that the near and far sides experience is around 1.1% according to my calculations. This contrasts with 6.8% for the difference between the near and far sides of the Earth. Whatever the reason for the 2km difference in the Moons crust between the near and far sides I doubt that tidal forces played a major role.

    And by the way, gravity is very well understood. Any claim that we may not understand it enough to describe tidal effects only shows the ignorance of the person making that claim. We use newtonian gravity to explain tidal effects because it still works extremely well for the scale and distance we are using it at. We even use it to send space probes out to other planets. The math for GR is horribly complex and is only necessary for long term predictions, such as the error in the perihelion of Mercury, very long distances such as multi-mega parsecs between galaxy clusters, and very high gravitational force areas such as around black holes.

    There is a mountain of evidence. Every single body in the universe experiences tidal forces. We've seen stars being ripped apart thanks to tidal forces. The opening paragraph on wiki's article on Io, a moon of Jupiter, explains that it experiences tidal heating thanks to the interaction of Jupiter and the other moons. The wiki article on tidal force has several examples: http://en.wikipedia.org/wiki/Tidal_force
    Last edited: Sep 15, 2012
  9. Sep 15, 2012 #8
    Although I've just been called ignorant, I understand the irritability of people since life's hard and all. Still, I thought I was being clear, but let me be more specific. The mechanics of gravity are inexistent. GR is a geometrical theory, not a mechanical one, so it begs the question as to HOW the bodies cause the curvatures on space-time. Please, I am not claiming GR is wrong, nothing like that, all I am saying is that still a lot to learn about gravity. As another example, GR can't explain the impetus to motion. This is admitted by Einstein himself, and he was with me regarding the incompleteness of GR and our understanding of gravity, so please, let's not claim omniscience.

    If we used Newton in such situations because it was simpler and easier to do calculations, I would have no issue with it, but it is not the case. GR cannot explain tides at all. Again, this is admitted by Einstein himself, and is one of the reasons we started tinkering with gravitons. If GR could explain gravity fully, we would never talk about gravitons. Bit there are NO FORCES in GR, and we can't or don't know how to apply GR to tides. Explain tides without forces and I will give you a cookie.

    With that cleared up, of course every body in the universe experience tidal forces (although forces contradicts GR). If there are tides here, there must be tides everywhere. You calculated that the difference front and back due to tides is just 1%, and that is precisely my point, the theory predicts symmetry on the near and far sides. That the shape of the moon does not follow from the calculations is the problem and the reason we have many theories. Tidal forces can't have played a major role assuming the theory is complete and there's no room for corrections.

    Also, the only extra examples on wiki (which i checked before asking here, thank you) are spaghetification and Io's tidal heating. Neither are hemispherical bulges on a single body as predicted by tidal theory. I asked for links to examples of hemispherical tidal bulges as the ones seen on earth and predicted by theory.

    I will give you that it would be hard to measure pole flattening at the moon from the earth, but not with satellites, but nobody's looking anyway, so we won't know.

    And I am saying that there's something INCOMPLETE with current theory, but of course, there could be something wrong too, even though that's not what I am saying.

    What I'm looking for is ANY example (other than earth, of course) of the bulges predicted by tidal theory on a single body as should be evident on the moon due to tidal lock, where the effects of the forces should accumulate over time.
  10. Sep 15, 2012 #9
    I agree with you. Tides do not explain the asymmetry. I only mentioned tides as "one possible reason" because one of the links that I posted mentioned it.
    There are unlikely ways that tides could cause symmetry breaking. The usual expression of the tidal forces are characterized by the dipole contribution of the gravitational potential. However, there are some ways the octopole component could get involved.
    I can visualize some of these. However, I don't have references. I suspect there are no references because the actual calculations show it can't happen.
    For instance, the earth equator is tilted with respect to the moons orbit. The tides would no longer center on the equator. The spin of the earth could create some asymmetry.
    I basically agree with you. The hemispherical asymmetry seen on the moon are far too big to be caused by tides. For completeness, I wanted to include this highly unlikely scenario which is also the most "obvious" to people who don't understand tides.

    You are misrepresenting the theory. It isn't just that the nearside collisions do more damage.
    Before the moon cooled to the "freezing point", more objects impacted on the nearside than the farside. The number concentration of objects will be higher on the nearside. The velocity of the objects will be greater on the nearside. So the flux of impacting objects will be greater on the nearside.
    The extra nearside collisions thinned the crust. They didn't immediately cause the visible features seen on the nearside. The maria seen on the nearside aren't craters in the usual sense. They are lava flows. Meteors punched holes in the thinner crust.
    The asymmetric crust formed while the moon was still hot. Nearside collisions caused the nearside crust to remain hotter longer, forming a thin crust. After the moon cooled off a little, the crust was thinner on the nearside than the far side. After it cooled of a bit, meteors came an punched holes in the crust.
    The problem is the thinner crust on the nearside than the far side. The lava flows are asymmetric because of the asymmetric crust. It appears to me that whatever initially caused the asymmetry must have been a slow process. Just not tides.

    The collisions that caused the asymmetry in this theory occurred before the crust completely cooled. The crust was still "liquid". It took a long time for the crust to cool, so there was time for magma to flow. So damaged caused by individual meteor strikes was "washed out". The flow eliminated the small scale "damage". All that was left was the thinner crust.

    This is nonsense. Gravity is the best know of all the forces that govern the dynamics of the solar system. Newton's Laws seem to govern geological processes sufficiently well that few geologists have to even consider General relativity. General relativity causes very small deviations from Newon's Laws that are easily calculated, even when they are insignificant.
    Deviations from General Relativity may occur on distance scales comparable to the diameter of the visible universe. There are strange observations concerning the dynamics of galaxies and galactic clusters that some scientists think may be caused by a deviation from General Relativity. However, nothing in the solar system has a distance scale comparable to these distances.
    There is no evidence that gravity behaves any differently on the distance scale of the solar system then it behaves on earth. Every experiment with "simple" systems shows that General Relativity is sufficiently accurate to predict orbital behavior within this solar system. Newton's Law of gravity is a sufficiently good approximation for geochemistry purposes.
    Playing around with Newton's Laws of gravity just adds more unknowns to problem. Even without a deviation from Newton's Law, there are more unknowns that variables. So your suggestion probably won't be helpful.
    There are a lot of other unknowns. As you point out, there are so many more variables than there are unknowns that multiple models seem to work. However, the laws of gravity are not one of the unknowns.

    Tidal theory? You mean gravitational theory! There is no "tidal theory". If a theory of gravity is validated, then it confirms tidal theory. That is why you didn't find anything in your search. "Gravity theory" works fine in this solar system.
    The law of gravity has been validated all over the solar system by our space probes.
    There is a questionable deviation of one of our space probes that left the solar system. However, the moon orbit is not nearly as long as the distance this space probe went.
  11. Sep 15, 2012 #10


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    Please provide a reference, as what you say borders on nonsense to me. GR should reproduce the effects of newtonian gravity at the scales where newtonian gravity is accurate. This includes tidal forces.

    The issue here isn't that GR is wrong or incomplete, it most certainly is incomplete, as our math leads to nonsense at certain scales. The issue is that you are claiming that GR doesn't explain effects that newtonian gravity does. To my knowledge this is absolutely wrong.

    Why do you believe tidal forces are even relevant to the discrepancy of the thickness of the Moons crust? Do you realize that the Earth's crust differs in thickness too? I'd be willing to bet that both Mercury and Venus do not have a perfectly even crust as well. (And we are talking about a difference of 2 km between the two sides, something around a 4% difference, as the Moon's crust is on average around 50 km thick) It is possible that tidal forces have had negligible effect and that there is some other effect of combination of effects that have changed the Moon. I can guarantee you that tidal forces are NOT the only thing responsible for the Moon's shape.

    No, you most certainly did not ask for "examples of hemispherical bulges", you asked for exactly what I gave you. Evidence for tidal forces. I quoted you exactly and replied to it in my earlier post.

    Let's get things straight. You are, and have been saying that tidal forces cannot be explained by current gravitational theories. This is wrong. Your claim that gravity must either be a force or be curvature is wrong. Both effects fit our observations very well within the scale that they are accurate. That is why we use them!!! They work!!
  12. Sep 15, 2012 #11
    Why exclude earth? Earth has typographical features that can't be explained by gravitational tides. Earth has continents, most of which at the current time seem to be on the Northern hemisphere. This asymmetry is described as a historical state caused by continental drift. Supposedly, it did not exist before the Mesozoic era. No one has tried to explain this asymmetry with tides.
    Mars has craters, and so does Mercury. In fact, I have never seen any mainstream scientist explain any typographical feature on any planet using tides. Your argument sounds like a straw man.
    You did not initially ask for references concerning asymmetries in other planets in the solar system. You initially asked for references concerning “tidal theory” in other parts of the solar system. Since there is no such thing as tidal theory, no one has been able to find you references on it. However, there are plenty of references on asymmetries on planets.
    No mainstream scientist tries to explain all asymmetries in the solar system by tides. You are the first person I have ever communicated with who has ever tried to explain topographical features with tides.
    Typographic features on planets are by definition asymmetric. There are all sorts of explanations with regard to the origin of typographic features: bolide collision, volcanism, etc. However, tides are very seldom invoked for explaining asymmetries.
    I can imagine unlikely scenarios where a planet, while molten, could be stretched out by tidal gravity. If the planet was tidally locked so it keeps one face at the planet doing the stretching, the planet would be egg shaped. This happened in the imaginary world of Jinxx, described by science fiction writer Larry Niven.
    The tidal forces on the moon by our planet are too small to stretch it into such an “egg shape”. Furthermore, the shape of our moon isn’t egg shaped. So tides don’t nearly explain how the asymmetry occurred. One day, I hope we discover an egg shaped planet. Then I will believe that tidal gravity caused it to have an egg shape. However, there have been no reports of egg shaped planets.
    Maybe next Easter!
    Nothing is available about general relativity not being able to explain tides. I can’t find a quote from Newton or any other scientist that says that. However, I found a few references that says that tides are consistent with both Newton’s Laws and GR. Here is one.
    “In Newton's theory, tidal gravity is the fact that parts of extended objects are at different distances from a source of gravity. The gravitational force vectors are therefore different on different parts of the extended object. This can stretch and/or squeeze an object.
    In relativity theory, the same effect is explained by means of curved space-time. The curvature is different for different regions of extended objects. This causes the space-time geodesics to be different for each region, hence the stretch and/or squeeze that is experienced.
    Like always, the differences between Newton's and relativistic effects are slight at large distances from objects. It is only in the very near-regions that there is a marked difference.”

    I couldn’t find a reference which said that general relativity (GR) can’t explain impetus. However, I found a Wikipedia article which says that the GR concept of inertia is different from the Newtonian idea of inertia. You may have misinterpreted this article or one like it.
    “As a result of this redefinition, Einstein also redefined the concept of "inertia" in terms of geodesic deviation instead, with some subtle but significant additional implications. The result of this is that according to General Relativity, when dealing with very large scales, the traditional Newtonian idea of "inertia" does not actually apply, and cannot necessarily be relied upon. Luckily, for sufficiently small regions of spacetime, the Special Theory can still be used, in which inertia still means the same (and works the same) as in the classical model.[dubious – discuss]”
    This is probably marked “dubious” only since the mass varies with velocity in special relativity (SR). I think that what the author meant to say is that there the force equation derived with Newton’s Laws (F=ma) can be generalized with four-vectors in SR.

    I found a rather strange article in Conservapedia where is said that GR doesn’t explain anything. I sincerely hope that you are not using Conservapedia as a source. Conservapedia is not on this Forums list of approved references. I hope it never is.
  13. Sep 15, 2012 #12


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  14. Sep 15, 2012 #13
    This is probably the best example of tidal force in the solar system!
    The real reason that none of us can come up with a planetary example is that other planets are not covered with visible liquid.
    The tide on the earth is so noticeable for three reasons. First, the earth is nearly covered with liquid water. Second, the moon (selene) is close enough to the earth to generate tides in the water. Third, humans live on the surface of a solid land mass so they can measure the height of the tide accurately.
    Of the inner planets, only earth is both covered with fluid and has a nearby moon that is almost as massive as it is. The conditions on the outer planets aren't too good for tides, either. I suppose that the atmospheres of the gas planets would show a proper bulge if any of the moons were nearly as massive as the planet itself. However, the moons of the gas giants too small to distort the shape of the atmospheres properly.
    The moons of the planets are not covered with oceans. Europa has an ocean covered with solid ice.
    I wonder if anyone has seen a tidal bulge on the atmospheres of any of the moons of the gas giants? It seems to me that the atmospheres are fluid enough to be distorted by gravity. However, measuring a small tidal bulge would be difficult with an atmosphere. Gases are transparent.
    Your picture of Shoe Maker was great! A large snow ball stretched and broken by Jupiter's tide! Just as one would expect from a tide!
  15. Sep 17, 2012 #14
    Ok, but it doesn't matter, all I am saying is that these are all speculations and wild guesses, and I don't that's terrible, after all, there's no other way to solve problems other than theorizing and testing, but these are hardly verifiable and it's impossible to decide between them.

    Why is it nonsense? I'm not saying that the math of gravity doesn't work, that GR doesn't work, anything like that, I'm saying that we are short on mechanics on gravity. We had force at a distance, which was a problem, now we have a body curving an abstract field (abstract in the sense that it is an outcome of the math, not of observation), and no mechanism as to how a body curves the field, we just suppose that it does since the math works and go with it, but there's no explanation as to how the body curves the field. Also, we had to propose gravitons because the geometry of space-time can't cause all the effects that we see, and also, we can't unite gravity with EM at the quantum level, and there's nothing absurd about what I am saying, every theory has unexplained postulates, or assumptions, and further theorizing has the job to keep looking for explanations, that's just standard. For instance, we don't know why or how light travels at c and not some other velocity, we just know it does, there's always more to learn.

    No, I mean tidal theory, resonances, spring and neap tides, the relation between coastal shape and the variation of water levels. These use gravity, but have specific calculations, models and so forth. And I totally agree with you that gravity has been validated! I'm just asking for a better explanation to tides and specifically what we see on the moon, and you provided a pretty good answer with various theories that try to explain the shape of the moon precisely because gravity and tides do not explain it, that's all. Everything else was just me discussing my thoughts and opinions, finding out what other people think about all of this. You perfectly addressed my questions and were the most thorough and respectful of all posters.

    One more thing. Here in PF someone else already asked how GR can explain tides with no forces and only geodesics:
    Do you find that answer satisfactory? How can no forces stretch atomic bonds? How does GR calculate this interaction? If particles do not follow geodesics (which is strange, since the body composed by the particles is following a geodesic, i take this to mean that they don't follow different geodesics individually, so space-time geometry can't explain tides), how does atomic bonds get stretched by GR? Are there forces involved? How come if there are no forces in GR?
    Also, wikipedia (http://en.wikipedia.org/wiki/Introduction_to_general_relativity) talks of forces in GR all over the place, directly contradicting Einstein. That's why I consider the usage of Newton sometimes and GR other times a problem. One explain motions with forces, the other with geometry, they can't coexist theoretically. You can use them mathematically to solve problems since they get the results we seek, but they can't coexist theoretically since they are in contradiction as to the cause of gravity and of motions, so even if we use Newton to calculate stuff because it's easier to do so, we still have to think about what's physically going on beneath the equations. Do you agree?
  16. Sep 17, 2012 #15
    I looked for a reference where Einstein admitted GR can't explain tides and couldn't find any, I remember reading it in a forum discussion, but it seems it was a misquote since I couldn't find it anywhere else, so I apologize. What Einstein admitted was that there are no forces in GR and no inertia. He actually called inertia a phantom! He said that the equations of GR "avoid the inertial system (the phantom which affects everything but is
    not itself affected)" - http://archive.org/stream/TheBornEinsteinLetters/Born-TheBornEinsteinLetters_djvu.txt
    But without forces it seems impossible to put a body in motion from rest. GR has field differentials that describe the geodesic a body in motion would follow, but no forces to cause motions. Can you explain how abstract mathematical differentials can "push"an object at rest relative to the field into motion, without forces? If you can, please give me a thorough explanation (with equations if possible) for this really bugs me.

    I don't believe it, I just take it that a body in tidal lock should be egg-shaped as predicted by theory. If the moon has some mysterious past that flattened the near side, it must have flattened it further since tides from the earth would act against this shape. Also, with tidal lock, tides are always on the same places, and if you take gravity as a force field, the effect should be cumulative over time, but we have no evidence of this on the moon.

    Well, that was what I intended, so forgive me for not being clear enough. I'm looking for another body that has hemispherical bulges (I don't care about the magnitude), since we have two contradicting evidence on earth and moon, and we try to explain the discrepancy with wild guesses as to what could have happened to the moon in a distant past, and I don't find any of the explanations appealing. Maybe tides are not only gravitational, maybe EM has some part on it, solar wind, I don't know and am not in place to propose anything, we just know tides exist and have a very very complex mathematical model with lots of variables to calculate them on earth, so there's plenty of room for corrections, and maybe the shape of the moon is caused by tidal forces after all, and can help us understand tidal mechanisms better. Maybe not. That's why I'm curious if there are any egg-shaped bodies or, even more interesting, if there are other bodies that follow the shape of the moon.

    No. I'm stating that tidal forces cannot explain the shape of the moon, and I'm saying that tides can't be explained with GR, but can with Newton. And precisely how can gravity be two things at the same time? If I make you lose your step with my hand, I could be either pulling you or could be twisting the carpet you stand on, but I can't do both things at the same time with the same hand. Gravity is either a curving of space-time with no forces or it is a pulling force, it can't be a force and not be a force at the same time, that's totally negligent. And I'm not saying we can't CALCULATE effects with both theories, there are various gravitational theories that work on specific problems, Newton and GR are the most general, but they fundamentally contradict each other. You can use the set of equations you like to get an accurate result, but you have to decide what's physically happening, even if you like to use both equations. One thing is to calculate a result, and there are always many ways to do that. Another is to understand what's physically happening.
  17. Sep 17, 2012 #16

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    Several hundred millimeters, not a few. The Earth's h2 Love number is about 0.61. The h2 Love number is a unitless parameter where a value of zero means a completely rigid body, a value of one a constant density incompressible fluid. In other words the Earth's solid body tides are about 61% the size of the ocean tides (which as you mentioned would be a bit less than a meter in height if only those pesky continents weren't around to make the equivalent of a resonant cavity).

    The Moon is a much more rigid body than the Earth, so the lunar Love numbers are considerably smaller than those for the Earth. The lunar h2 Love number is about 0.048.

    You did the math wrong. The tidal forces exerted by the Earth on the Moon are about 22 times those exerted by the Moon on the Earth: [itex]\frac{M_{\text{earth}}r_{\text{moon}}} {M_{\text{moon}}r_{\text{earth}}} \approx 22.18[/itex].

    Since the Moon is tidally locked with the Earth, most of the lunar solid body tides take the form of a permanent tide. However, because the Moon's orbit isn't circular, nutation and variations in orbital distance do result in time-varying solid body tides on the Moon. The Sun also creates time-varying solid body tides on the Moon. These variations are measurable thanks to the retroreflectors the Apollo astronauts left on the surface of the Moon.
  18. Sep 17, 2012 #17
    It's not that kind of asymmetry I intend to discuss, but specifically the tidal variations due to gravity, which have actually already been measured here on earth.

    You are mistaken. It's not topographical features that must be explained with tides. The variations due to tides as mostly symmetric front and back, with flattening at the poles, which is what we see on earth. Theory predicts this for all bodies, of course. The moon doesn't follow this description, so other theories try to explain this. I'm aware I wasn't clear in my first request, but it was my intention. I'm looking for other bodies that show either the egged shape predicted by theory or a similar shape as the moon. I'm trying to verify if the negative tide (call it whatever you want) we see on the near side of the moon is a common feature or if it is just the moon that exhibits it. The best candidates should be other bodies in near tidal lock, but I wasn't able to find any such analysis on other bodies, and that's what I'm looking for, since the provided theories to explain what we see on the moon are not satisfactory.

    GR and tides are a common subject of debate. If you consider different parts of the body to follow different geodesics, in a considerable amount of time the body would be ripped apart. You find explanations that state that the particles of a body do not travel in different geodesics, and that gravity acts on atomic bonds (I just replied someone else with a link to this), which i find inconsistent, since there are no forces in GR to stretch molecular bonds. Stretching implies force, which is non existent in GR.

    Inertia is not different in GR from Newton. Inertia is avoided in GR. There's no inertia and no forces in GR. This Einstein said directly, I just posted a link to this in another reply, he says so in the Born-Einstein letters, and it is clear from his books regardless. It is marked dubious because it is a wrong interpretation of GR, not intended by Einstein himself. He got rid of inertia in GR.
  19. Sep 17, 2012 #18

    You got me thinking that the tides from the sun should be very strong on the moon also, maybe even greater then earth's, i'll try to calculate it asap as a proportion to the force from the earth, but if have the numbers could you post it, pls?

    What are your thoughts concerning gravity as a force and gravity as field geometry? Is GR able to describe tides? Does it bug you that sometimes we take gravity as force and sometimes not? Do you think GR has proven that gravity is not a force? How would you explain tides with no forces?
  20. Sep 17, 2012 #19


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    You are searching for a discrepancy where none exists. It doesn't matter how the mechanism is described: the math works the same either way.
  21. Sep 17, 2012 #20


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    But what is a force? It is something we have DEFINED! It is a word that describes an interaction that causes a change in a system. The shape of spacetime that causes gravity in GR has the exact same effect! Whether we call it a force or not is irrelevant, the effect is the same. (And as DH explains below, forces apparently do exist in GR)

    From here: http://lro.gsfc.nasa.gov/moonfacts.html [Broken]
    + The moon is not round, but egg shaped with the large end pointed towards earth.
    (Although I did see several contradictory statements by different sites online as to which end was pointed at the Earth, the big or little end)
    According to NASA it is egg shaped, so I feel your entire argument is based on faulty information.

    You don't find the explanation appealing? So what? It is incredibly difficult to determine what happened to something 4+ billion years in the past. Of course its going to be difficult. And please, stop suggesting that we don't know what causes tides. We know exactly what causes them. If you keep suggesting this I will report your for being unwilling to learn real science, which is what PF is here for. EM forces are NOT responsible for tides, nor is the solar wind.

    Well, as has been said repeatedly, this is wrong. GR can and does explain tidal effects.
    Last edited by a moderator: May 6, 2017
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