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I see I did incorrectly use the tectonic plate terminology here. Sorry for the confusion. Yeah, i wondered about that but could not find any info. Thanks. I am not worried at all. Just questioning. The relative radius of the earth is irrelevant: e.g. a 50 meter center of mass shift would cause a huge variation in local sea-levels. My reasoning is as follows: When isostacy is the only mechanism creating deviations from an idealized, smooth Earth, than the earth gravitation should be the same everywhere (except close to large density variations). If you look at the picture, mountainous areas seem to have higher gravitation. As i see it, this contradicts isostacy being the only factor. Of course, this could be an artefact of the way it is measured, it could be a local fenomena only, etc. Anyways, I found a reference that the lithosphere has a different chemical composition than the asthenosphere (i learned this terminology here ). My assumption was that those were more or less the same. So this different composition allows for a difference in density and make isostacy on a global scale a valid mechanism. Thanks for the info. I learned a lot.

Hmm, you are suggesting this forum must not be used by ignorant people to seek knowledge but only for asking questions for which one already knows the scientific acceptable answer? That is quite a certain way to stop scientific progress... Please enlighten me, i do not not know what my flawed assumptions are. of course not. I did never say that. so, what does that have to do with the earth gravitational field? I am not doubting tectonic plate movement or the idea that the current land distribution comes from breaking up the the latest 'supercontent'. I am just doubting the assumed relative water distribution with it, based on, for me, quite solid reasons. Considering i am not an expert on this i might very well be wrong. That is the whole reason for posting topic: finding out where i went wrong in my reasoning! So what are the incorrect assumptions?

Thanks for the info. I didn't know the floating-ice cube principle was valid for tectonic plates. Sounds plausible when the earth crust indeed has a lower density than the magma directly below and does not have a significant 'stiffness' (is that the correct terminology). However, the isostacy principle for tectonic plate mechanics seem to have the state of a hypophesys, not that of a scientific theory. So, I am not completely convinced, considering: 1. looking up the density of magma (2.18 and 2.80) and the average density of the earth crust (2.83). 2. solidification of a fluid (magma) tends to increase the density (i know there are some exceptions such as water or iron below certain pressures). This seems to confirm (1). 3. at a glance, measured variations in in the earth gravitational field do not seem to support the isostacy hypophesys (https://en.wikipedia.org/wiki/Gravity_of_Earth ) Is there any reason that solidification of magma would a decrease in density or does the earth crust somehow have a different composition than the magma below?

I have be wondering a long time about the idea of having 1 super continent (Pangea) and 1 huge ocean about 300 million years in the past. It always sounded a bit unreal to me. Let me explain why. Let me first make clear: i am not putting the tectonic plate movement up to discussion! The theory about that is sound: i assume the geologic science is correct and that the tectonic plates forming the continents as we know now, originate from 1 tectonic plate about 300 million years ago (as detailed in https://en.wikipedia.org/wiki/Pangaea). The part of the theory i doubt is the distribution of water around the planet being the same as in the present, giving rise to 1 main landmass and the remainder of the world being 1 big ocean. The reason i doubt this, is because the geologists seem to forget that having 1 landmass on 1 side of the globe means that the center of gravity is no longer the same as in the present: it shift to that landmass, causing the ocean to flow to the landmass, causing the center of gravity to shift even more to the landmass, etc, until a balance is new balance found. The redistributed water will partially drown the Pangea landmass, and the sea-level on the other side of the world will drop, giving rise to other (big) landmasses. I could not find any reference to a different center of gravity 300 mln years ago versus the current distribution. Any ideas about this?

Indeed, no discussion needed: that is just specified in piezo specs: from http://www.piezo.ws/piezoelectric_actuator_tutorial/Piezo_Design_part2.php "The maximum allowable field strength in piezo actuators is between 1 and 2 kV/mm in the polarization direction. In the reverse direction (semi-bipolar operation), at most 300 V/mm is allowable (see Fig. 10). The maximum voltage depends on the ceramic and insulation materials. Exceeding the maximum voltage may cause dielectric breakdown and irreversible damage to the piezo actuator." So there is no use to increase the voltage above this. I think it is better to optimize for the frequency and the waveform.

You could drive the piezo at resonance and use an inductor to boost the voltage. This is explained a bit more here: https://www.powerelectronics.com/technologies/power-management/article/21864451/use-resonance-with-a-highvoltage-piezo-driver Keeping the electric resonance tuned to the mechanical might be challenging. The downside is that you loose control over the exact waveform driving the piezo. Separating the resonance driver piezo and the piezo that should do the actual work could be a solution.

As a none native speaker i had to look up 'parsimonious'. Not very flattering... But ok, in this case it more or less fits me: I did not know there was a distinction between closed and isolated, so thank you for the free education. With the thought experiment i proposed i think closed-system is still appropriate though: energy created from 'nothing', will actually be supplied by the negative energy of the expanding gravitational field originating from the created energy. Here you have the chicken and egg problem... 😁 This seem to be the thing: my simple thought experiment is not so simple. So we did find a new physics law preventing creating energy from nothing: an egg shall be made by a chicken and a chicken shall come from an egg. 🤣

I don't know what "time translation symmetry" means. I'll look it up. Thanks. That said, i thought that conservation laws applied to closed systems in the widest meaning possible. But note that a closed system is a pure hypothetical construct: e.g. anything accelerating within a closed system will cause gravitational waves leaking out of the 'closed' system. Perhaps it's the same for me not understanding your comment? Bit confused here. What has a single point mass in an empty universe to do with, eh, with what? Looks like kind of a philosophical question to me: in that case the point-mass is the universe. 🤔

I just mean (for as far as i know) that all energy gravitates: It doesn't matter whether it is stored as mass (e=mc^2) or not.

Some years ago i reasoned that the gravitational field must have a negative energy content. Apparently that was not a very original though. I was also speculating whether the total energy (gravitational energy + the mass_energy it originates from) would be negative, equal to zero, or positive. My guess was that it must zero but i did not know how to make a logic argument for that, let alone make calculation with that. The best answer that i could find was that is was arbitrary, but lately i stumbled upon a lecture of Lawrence Krauss in which he also reasons that the total energy must be zero (although interesting, that is not what i wanted to discuss here. He also wrote a book: "a universe from nothing", explaining this). Then another though came up: when this is correct (mass_energy + gravitational energy = 0), then using the first law of thermodynamics as argument against creating energy from nothing no longer holds: creating energy would immediately also create the negative gravitational energy resulting in a total of zero energy added to the universe (to be clear: i do not know how to do this ) . Or would there be other fundamental principles or nature laws that would forbid creating energy from nothing?

I know. Those are known factors but are not the whole story. Different species can also evolve in the same habitat. E.g. i can imagine that adaption A gives an advantage and adaption B give a different advantage, but the mix gives a disadvantage relative to A or B separate. Genetic incompatibility is than also an advantage. Perhaps not for the individual but for the new species it is.

I think encryption would complicate making sense of things, but it would not make detection impossible. The mentioned broadband spread-spectrum techniques would complicate detection and making sense of things. The nail on the coffin is that i think that each generation of wireless communication is lower-power and relying on a finer grid. Also, the frequency is increasing by which absorbption in the atmosphere is increasing. The spillover radiation would simply be too weak to be distinguish from background noise. All we can detect is a very strong directional signal send by a civilization that wants to be seen... I think seeing megastructures in space is a better candidate for detecting alien civilizations level 2 and 3 (Kardashev scale).

I wonder if this can be explained by the species not sharing a habitat: then there is no evolutionary pressure for genetic incompatibility as a species advantage. The generic compatibility is then only very unlikely.

I am impressed by the effort you put into this. Especially the way you drive the piezo stack can make a huge difference to the simplistic way Woodward is doing this. However, having followed Woodward's efforts for about 25 years (yes really!), i am also a bit skeptical about the results: Woodward manages to only show about 5 to 20 micro newton, and even that is highly debatable because of measurement problems. Vibrations, temperature differences, electric and magnetic fields, etc can very easy give false signals. So i would advice to keep on the good work but also to be very skeptical to any 'thrust' signal and try everything to debunk your test-setup.

Very cool. It is now more than three years later. Do you have new results that confirm or contradict this?

Indeed, it is completely irrelevant. I only mentioned this to show that it is possible to nullify the difference in start of acceleration for both observers by positioning the rocket motor between both observers instead of one at the level of the motor and the other one a some distance.

I don't think that is relevant, because that can be compensated for by a mechanic construction (e.g. with the rocker motor is positioned in the middle of the rocket). But for the sake of the argument lets assume the rocket stays in the back, what would that entail? Let's see: The difference in start and stop is given by a constant time: For a complete rigid rocket this only depending the height: delta_t = h/c where: h: height difference between observers c : speed of light. The top observer will start later, but also stop later. The result is that it is 'pushed' as long as the bottom observer (in the reference frame of the bottom observer). Now the difference in traveltime, can be calculated as follows. The timedilation is given by (for relative small acceleration and 'height' difference): v = a * h/c T = t1/t0 = 1/sqrt( 1 - v^2/c^2 ) where: a: acceleration of rocket as observed by observer in the bottom. The result T is slightly lower than in reality, but the difference will only be become significant for very large a or h. So, the total traveltime disagreement is: dt = T * tb where: tb: total traveltime for observer in bottom of of rocket. Clearly, since delta_t is constant (and canceled out in the end, and can be compensated for) while dt depends on the total traveltime of observer in the bottom, the observers will disagree on the total traveltime at the end of the trip.

Sorry to be absent for some time. They don't have to climb: the rocket just has to stop accelerating. That is the whole thing. for instance, when the rocket stops accelerating half-way the trip, then turns around and starts accelerating again. Then stops accelerating when the velocity is zero relative to the starting point. The observers will disagree on the trip-time! I think that is just weird.

I realized this last night myself as well: the proper local acceleration depends on the 'height' within the rocket because the observed differences in speed of time. It even does explain the contradiction i started with: the travelers will experience different travel-time although traveling the same. I think i just have to accept relativity produces weird effects. 🤪

i know it is accurate, but in this case it is not relevant. I you want to go into technical details: the deformation can also be compensated for with actuators so the rod will act as completely rigid up to measurement precision.

Correct. I can be wrong, but i think not this one. If so, please show me how.

I though this thought experiment would be clear. Appearently it is not. The time-dilation will be ridiculously small for any practical rocket unless you are using a start-trek like impulse drive for a very long lime, and we all know that is not available. So the though experiment is just that, and handles with an idealized 'rocket' or call it a space-hook if you like. We assume the rocket is idealized rigid. Clock synchronization is done while the rocket is not accelerating! Edit: i inadvertently posted to early.

Correct, this is not a contradiction. When the travelers synchronize clocks before acceleration, then accelerate with 1 in to top and one in the bottom of the rocket, then after acceleration they compare clocks again: they will not have experienced the same time! Or do they and is my reasoning incorrect?

I know this. I did not want to complicate things by mentioning this.because it is not relevant here.

The gravitational time-dilation is not caused by the difference in gravitation gradient (caused by the distance from a gravitational attractor), but only by the difference in gravitational potential (i hope i use the correct terminology). Hence the equivalence principle. If that is not convincing: tt is also possible to derive the time-dilation within the accelerating rocket simply using SR.