# Effect of gravitational tides on the moon

More nonsense. Forces are not nonexistent in general relativity. Forces are messy in general relativity, but then everything is a bit messy in general relativity.

I quoted Einstein directly. No forces. Considering forces in GR is the outcome of mixing Newton's forces with Einstein's space-time geometry, and that force was a force at a distance, which is presumably ditched.

Why would you think that? The Earth and Moon are more or less at the same distance from the Sun, and the Moon's radius is about 27% that of the Earth. Solar tides on the Moon are about 27% those on the Earth -- so a whole lot less than are tides caused by the Earth.

Agreed, faulty logic on my part, I take it back. Still, over 1/4 is very strong indeed.

Of course general relativity can describe tides.

How? Do you think the answer that tidal forces from GR act on molecular bonds is appealing? Do you know of another interpretation? Geodesics don't work as far as I know.

Not a bit. Gravity is a real force in Newtonian mechanics, but a rather strange one. It's the only force we can't measure with local experiments.

Gravity is viewed as a fictitious force in general relativity, but that's most a consequence of how general relativity and Newtonian mechanics differ with regard to inertial frames of reference. OTOH, that we can't measure gravity -- it's a fictitious force. You can't measure fictitious forces.

It is strange because it defies logic. Force at a distance is a logical impossibility, and it bugged everyone until GR came along and ditched it. It was either ditched or not, both cannot coexist logically, only operationally since the results are close enough for most calculations, but physically only one can be true. Are we to believe Einstein's own interpretation and goal of his own theory is wrong? Was he mistaken getting rid of forces?

Please. That's faulty logic. This site is not the place to put forth crackpot notions, which is where it looks like you are headed.

? Am asking for an explanation for how GR causes tides consistent with what we see on earth, that's all! I'm being very consistent, I am quoting Einstein, there are no forces and no inertia in GR, Einstein's usage of Riemann space have no way to describe forces, precisely because he wanted no forces, he wanted bodies to travel with constant velocities in a curved space-time, with no accelerations, forces and inertia, and I don't see (or know) of tides explained in these terms. Everywhere I look for we talk about forces, accelerations and so on. It seems that we always use Newton, with or without GR. If GR alone can describe tides, please inform me how, I am not a crackpot and I have no theory of my own, I'm just pointing to some inconsistencies that bug me, and I think it's too much of a coincidence that the same side that's facing the earth at all times is the same side that is obliterated, and I don't believe in coincidences. Maybe it is, maybe it isn't, I just find it strange, but this discussion has long diverted from that topic, and I don't mind, the answer I was looking for already came up and now I know a bit about the present theories that try to explain the shape of the moon, and I am entitled to remain unconvinced since nobody is convinced with any of this theories, that's not crackpotery! So again, if GR can describe tides, post me a link as to how. I have only found sites that explain it with forces, and I checked Einstein's original papers and he is very clear that there are no forces in GR, so I don't know what to think anymore... Is this crackpotery???

But it does "follow the description". The data from the Clementine spacecraft's LIDAR shows ~2km flattening on the poles.
I couldn't find a freely-accessible paper analysing the data, but even by reading this one's abstract:
http://www.agu.org/pubs/crossref/1997/96JE02940.shtml
you can see that bit of information there.

The problem with the Moon, is that other topographical deformations are much larger in magnitude(~8km), obscuring the tidal effect. But then again, it's the same on Earth, isn't it? That max 300mm deformations(http://en.wikipedia.org/wiki/Earth_tide#Tidal_constituents) aren't exactly strikingly visible.

It is clear from this abstract that it doesn't follow the shape predicted from gravitation – equatorial ellipticity (∼800 m) – and that it is a challenge to explain it's shape. Again, if it did follow prediction, no theories would be necessary to explain it's overall shape. If we have 2km flattening at the poles the problem is even worse, since pole flattening should be somewhat the same size (actually smaller if you consider the maximum deviations instead of mean volume deviations) than the equatorial bulges.

And the topographical features do not impede us from measuring tides on land here on earth, so there's no obscuring, if they are strikingly visible or not is not to the point, the point is if they follow prediction, that's all. But that was already answered, I am already convinced it doesn't, it says so everywhere, and there are many theories trying to explain why it doesn't. To me this issue is settled, and I don't understand why you insist that the shape of moon follows from tidal theory since it doesn't, something else must be the cause. This is widely admitted, so I don't know what you're saying.

D H
Staff Emeritus