Without having properly researched this, I have two hypothetical possibilities in the concave Earth – both involve the Sun’s electromagnetism.
1. Positive electric currents from the Sun. The Sun is negatively charged because it is inducting. This negative charge is traveling towards Earth and terminates as cloud-to-ground negative lightning. Because of this, the Earth is constantly negatively charged, but is releasing this negative charge upwards again through the atmosphere and back into space (possibly back towards the Sun).
Measurements have shown that the negative charge on earth remains roughly constant with time. At first glance, this fact is difficult to understand since the charge on earth is continuously leaking off into the conducting atmosphere. In fact, calculations show that if the earth’s charge were not being continuously re-supplied, the charge on earth would disappear in less than an hour.
The standard description of direct electric current tell us that a current through a wire goes in two directions. The negative “electrons” slowly travel one way and the positive current flow travels at the speed of light in the opposite direction.
So although electrons would flow from negative to positive, by convention (agreement), physicists refer to conventional current as a flow from high potential/voltage (positive) to low potential/voltage (negative)… Electrons move from areas where there are excess of negative charges to areas where there are a deficiency (or positive charge). Electrons move from “-” to “+”, but conventional current is considered to move in the other direction. When you set up a circuit, conventional current is considered to move from the “+” to the “-” side.
So logically, as the negative current (“electrons”) is constantly moving upwards through the atmosphere back towards the center of the cavity, positive current is constantly traveling at the speed of light in the opposite direction towards the Earth from the center (probably the Sun). Circling, around the positive current is the magnetic field.
2. Ridiculously low-frequency electromagnetic radiation from the charged Sun. The once-every-6-months alternating electromagnetic field of the Sun produces an extremely weak ridiculously low frequency radio wave. If 1Hz is a super extreme low frequency radio wave and is one full cycle per second, then the Sun’s EM wave would be one full cycle per 1 year. Over 365 days of 24 hours this is 31,536,000 seconds which is 0.0000000317Hz (3.17 x 10-8Hz).
This kind of low frequency would easily be able to penetrate water and rock for considerable distances since even ELF (3 to 30Hz) radio waves can “penetrate seawater, which makes them useful in communication with submarines… ELF waves can also penetrate significant distances into earth or rock, and “through-the-earth” underground mine communication systems use frequencies of 300 to 3000 Hz.”
Gravity is also said to be an extremely low energy force compared to the normal frequencies of EM radiation, which fits this frequency – 1042 times weaker supposedly. The lower the frequency of light, the less energy it has. Light is said to travel in a spiral movement – circular polarization. Controversial gravity waves are also supposed to spiral towards the Earth according to south polar scientists – “Gravitational waves from inflation generate a faint but distinctive twisting pattern in the polarization of the cosmic microwave background, known as a “curl” or B-mode pattern.” 0.0000000317Hz would be extremely faint if not nearly undetectable. Interestingly, “gravitational waves are expected to have frequencies of 10-16 to 104Hz.”
Also, a supposed “government insider” has researched this topic and come to a similar conclusion that gravity is “magnetic and created through the energy of the Sun. The magnetic properties of the light coming from the Sun is actually what creates gravity as we know it”.
This theory is similar to the Le Sage particle gravity offshoots of the 19th and early 20th centuries. In 1748 Le Sage envisioned gravity as a stream of corpuscles emanating from mass.
According to this model, any two material bodies partially shield each other from the impinging corpuscles, resulting in a net imbalance in the pressure exerted by the impact of corpuscles on the bodies, tending to drive the bodies together.
This is in quasi-similar territory to the Bernoulli principle interpretation of the Cavendish experiment, except in a concave Earth there are no gravity particles or waves emanating from mass, but from the central Sun instead (if you can call such a low frequency a “wave”). After reading that wiki page, it looks like the Le Sage theory and its variations have continued to fail because of those two assumptions that gravity is an attractive force and that it originates from mass. Those assumptions are taken as fact purely because the heliocentric model was taken as fact also. (See how the well was poisoned from the start). If a house is built on faulty foundations, it will never continue to stand up.
The closest Le Sage interpretation to this concave Earth gravitational theory came from Thomas Tommasina and Charles F. Brush in the early 20th century:
Unlike Lorentz and Thomson, Thomas Tommasina between 1903 and 1928 suggested long wavelength radiation to explain gravity, and short wavelength radiation for explaining the cohesive forces of matter. Charles F. Brush in 1911 also proposed long wavelength radiation. But he later revised his view and changed to extremely short wavelengths.
Tommasina’s short wavelength radiation explanation for elemental/molecular bonding is a good one to explore also considering the new interpretation of the Gerlach experiment which has matter as consisting of very fast alternating magnetic fields (which of course produces shorter wave radiation).
Since the magnetic field of the Sun alternates every 6 months, we can hardly call this a wave (from our own perception living in the Earth) and more like gravity as a compression from the Sun’s magnetic field – super penetrative, extremely weak, obeys the square law, and originates from about the center of the cavity. It would be interesting to see if there are any very slight seasonal variations in gravity as the Sun moves very slightly away and to the center of the Earth cavity throughout the year. This applies to night and day where gravity should be very slightly stronger during the night because the Sun is very slightly nearer to the crust at night. Could this be measured?
This is probably my favourite candidate out of the two gravitational theories proposed so far.