For more than 50 years I have been trying to understand the gravitational potential. I used it for orbit determinations when I worked at my first job at the CIA, then later worked on a NASA project to determine the geopotential itself using satellite orbits and measurements to solve for the shape and properties of the potential. And over the subsequent years I follow new developments.
But the thing that intrigued me the most was a concept I associate with Robert Lull Forward whose work I first read when I was at University of Maryland College Park, on the energy density of the gravitational field. I have said it so many times before, but it is important. The energy density of the gravitational field at the surface of the earth is equivalent to a magnetic field of about 379 Tesla. You hear about 3 Tesla magnets for MRI and those are supposed to be “soo powerful”. Yet we are immersed every moment of our lives in a field that is more than a hundred times as powerful – and the energy density goes up as the square of the field.
No one has much interest in that. But I keep reminding myself. If you hold something in your hand, you are overcoming the gravitational force from the earth. But it is not an instantaneous process where the whole earth is every moment reaching out to that mass. Rather the gravitational potential at the location where you hold that mass varies from place to place, and its gradient, its slope, is the acceleration. The field needs energy to do its work, and the work has to get to the mass in orderly fashion, even if we don’t see how that happens. The potential is already there.
I spent almost a year using a network of superconducting gravimeters to measure the speed of gravity, now almost 20 years ago. It changed the way I understood gravitational fields. Because the effect of the sun and moon (I used them for the measurement) at a station has already diffused from the sun and moon long ago, and is in the location of the detector. I say “the field, second by second, is in equilibrium. Few new changes come in to change the main shape and character of the field. The field exists at the location, and is only updated by distant mass movements – at the speed of light.
Yes, the changes to the gravitational field get from where mass moves (the sun, the moon, the changing density of a volume element (voxel) when there is a seismic wave, the changing density of a voxel above those huge waves where surfers play) at the speed of light and gravity. If you change the speed a tiny bit in the calculation for a signal at a gravimeter station, the error goes up.
Now, few will know what I am saying, but it is important. The gravitational potential field is “linear”. For slow measurements, the contributions for many masses add linearly. So fitting the model of the field at some location on earth, to the measurements at that location only requires a linear regression. Two numbers is all that is needed to fit a very complex signal for a month. It is really amazing and beautiful.
https://hackaday.io/project/164550-low-cost-time-of-flight-gravimeter-arrays – click on the graph on the left and look how close the signal and calculated values follow each other. The calculated value only uses the positions of the sun moon and earth, the position of the station, mass of sun moon and earth, and the rate of rotation of the earth, and the orientation of the sensors axes. Those are all readily available for anywhere on the surface of the earth.
But what I am working on new is “how does it work” at an atomic level? And I am making a tiny bit of progress. I have known for 40 years that the gravitational energy density can come from many different kinds of physical models. I have tried magnetic fields that do the same thing, electric fields that do the same thing, electromagnetic fields, acoustic fields. All of those “work”. You can build models using those methods and get a gravitational field that fits the real thing, arbitrarily close, depending on how much computer and field generators you can afford.
But in terms of “what is going on at an atomic level”? The energy density is roughly equivalent to an omnidirectional blackbody radiation field with the same energy density. That is a routine calculation in astrophysics and fusion research. It comes to millions of degrees Kelvin. The units that many use are “electron volts” and the gravitational field at the surface of the earth is equivalent to a blackbody field of about 1250 electron volts.
That is in the “soft x-ray” region of the electromagnetic spectrum. And a rather difficult and expensive region of spectroscopy. The hard xrays are easy, we did those for a hundred years. But generating and measuring “soft” x-rays is really only coming into its own in very recent decades and years. An interesting history, but what I want to convey is that the field is always there. It is very powerful.
When I wrote an essay 40 years ago for the Gravity Research Foundation about the gravitational energy density, I was asking whether it would be detectable and have a strong influence when fusion experiments and other experiments began getting to the magnetic field needed (379 Tesla).
Now there are “laser vacuum” experiments that are trying to generate particle antiparticle pairs from the vacuum (at the surface of the earth) using intense lasers (I did that energy density too, it is in the 10^17 Watts/meter^2) range. Readily accessible but no one yet has measured the fine variations over weeks and months to check the gravitational signals from the sun and moon. That would be fairly conclusive, that they are actually heating the gravitational potential itself. It can be modeled as a superfluid with very specific properties that can be measured in many ways. The most powerful is the sun moon tidal signal – since it is accessible (money, education and skill) to most every country in the world.
I am working out a model of surface array of field generators, powerful enough to lift an Elon Musk payload on his largest chemical rocket – to orbit. Keeping the cost of the lifting separate. This means the lift is not dependent on how much fuel you can carry. I am also working on “atomic fuels” where the energy is stored in bonds that are 10,000 times what are available in chemical fuels. The fuel needs for an Elon Musk rocket can fit in a container that is one hundredth or less of the size of the current fuel tanks.
Such problems can be calculated with no outstanding issues. It is just routine engineering. And for gravitational problems, orbit problems, solar system colonization problems, I call it “gravitational engineering”. Much of the technology is readily available now, but I have had to wait 50 years for most of it to get to where it can generate and control gravitational potential fields and their gradients.
Richard Collins, The Internet Foundation
That post was so long, I was worrying that Facebook would crash. It has an unknown limit on how much it can handle.
I have a project now on ResearchGate with much of this written out. Many of the updates I put there are different examples of how the gravitational potential field works at an atomic level. And how to use gravitational fields for imaging the interior of things – the earth, its oceans, its atmosphere. The sun’s interior, its “oceans” and atmosphere. The moon and planets.
Solar System Gravimetry and Gravitational Engineering