# Low gravity area water loop?

There is an area in the Indian Ocean that has low gravity. So much so, that the sea level is 100 meters lower than in the areas surrounding it.
Leaving aside the construction issues. If a pipe from the higher sea level areas was run out to the middle of the low gravity area. Would water flow from the higher sea level area to the lower one? The pipe outlet is a bit lower than 100 meters above the low gravity area for a bit of a slant to it.

Quick clarification:

The sea is lower where there is low gravity? Shouldn’t it be the opposite?

That is what I thought at first too. Is the surrounding higher gravity areas pulling the water out of that area? It does seem counter intuitive. But that is what I read.

No. There is already a path for water to flow. Gravity is going to pull the water to the level that it is currently at.

I am surprised that the difference in gravity is that dramatic that it causes a 100 meter dip. I wouldn’t have expected it to be that dramatic.

I imagined that might be the case. The water would be trying to go uphill in a way? But wasn’t sure.

I’ll flog this horse a bit more.
If the pipe ends were both submerged. The low gravity end being 99 meters above sea surface. Would the falling 99 meters of water keep a flow going? There is gravity there. Just weaker. The water in the vertical section will fall. Or would a vacuum occur and stop it?

“Uphill” is going to be difficult to define, because “up” from a gravity point of view isn’t going to be the same direction as “up” from a geometric point of view. The geometric “up” is going to be a line straight out from the center of the Earth, but the gravitational “up” is going to be skewed based on where the different parts of the Earth have a higher density.

From a gravity point of view, the Earth looks like this:

From Wikipedia’s article on the Earth’s gravity, here:

There’s an animated version available as well.

This bit confused me. Both ends are submerged but one end is above the surface?

If both ends are 1 meter below the surface at that location, then water will not flow, even though one end is 100 meters further away from the center of the Earth than the other end.

On the other hand, if both ends are at the same distance away from the center of the Earth, then the end where the density is higher will be submerged and the end where the density is lower will be 99 meters above the surface. The water level and the water density inside the pipe will be exactly the same as the water level and density outside the pipe, all along the pipe.

The pipe is 99 meters above the low gravity area, but has a 90 degree downward bend at the end. The pipe end goes down 100 meters, so it is submerged. Both ends of the pipe are underwater. But the low gravity end has a 99 meter section pointing down. The pipe is primed full of water.

The water is going to try to drain out of the pipe until the water level inside the pipe matches the water level outside of the pipe. But (assuming your pipe is air-tight) the water is going to be pulling against vacuum. So the water level on both ends of the pipe will be roughly 10 meters above the sea level on each side, because that is the maximum height that a vacuum can pull water. Different substances can be pulled to different heights, which is how old-time barometers used to work. The exact height you’ll get depends on whatever substance you are using (sea water in this case) and the air pressure on the sea outside of the pipe.

If you then put a hole in the pipe so that air can fill up the vacuum, the water level on both sides will drop to sea level in that location.

So if the pipe was arranged so there was only a 9 meter height difference. There might be a continuous flow?

No. Again, the water level inside the pipe is going to try to match the water level outside the pipe, but you are pulling against vacuum. As long as you don’t exceed 10 meters (-ish, varies depending on air pressure) then the pipe will stay full, but there’s no force to make the water actually flow. Even if you tilt the pipe, as long as both sides stay no more than 10 meters above the water, no water will flow.

If you then lift one end out of the water, all of the water in the pipe will rush out the other end as air rushes into the pipe from the “open” end. Once the pipe is drained though, the flow stops.

Thanks for the replies. Makes sense.

There was a thread many years ago where the poster saw a patent idea for a huge pipe from someplace like Las Vegas to NW Oregon. Since the average air pressure in Vegas was higher (I think) than at the other end air would flow from one end to the other. Put a turbine in the pipe and you could generate electricity.

Costs/benefits aside and all that, this had the same problem as the OP here. Why would air flow thru the pipe when it could more than just as easily flow next to the pipe?

If you wanted a low pressure/high pressure gradient over a much shorter (cheaper) distance pick the top/base of something like Steen’s Mountain in Oregon. But still it wouldn’t work.

In that case, the presumed virtue of the tunnel is that it would create a differential directly between those two points rather than going through all of the other intervening variation in air pressure and density. The flaw is that there would be sufficient losses over that distance that it would be practically impossible to extract useful work from the system due to wall drag and internal viscosity, which would doubtless heat the air (increasing pressure) until there was no useful gradient across a short enough span to operate a turbine.

As @engineer_comp_geek showed above, the actual shape of the gravitational field is quite ‘bumpy’, and is actually defined by the Earth Gravitational Model (EGM) of the World Geodetic System (WGS); the current edition is from the1984 high precision survey to support the Global Positioning System, so it is referred to as WGS84. WGS84 actually has a reference geoid—an ellipsoid that is the first approximation of the geodetic ‘shape’ of the Earth, and the EGM defines derivatives thereto for use in calculating gravitational potentials at any point about the globe. The reference geoid can actually be above or below Mean Sea Level (MSL) depending on the local gravitational field and other factors which impact MSL, which is a point that is often lost on non-geodicists. In terms of ‘up’ and ‘down’ in the gravitational field, it is this gravitational model, not local MSL or the reference geoid by itself that determines whether one point is has greater gravitational potential than another, and thus, whether work can be done by moving a mass between them.

Stranger

No, the flaw there is that there’s a reason for the difference in air pressure. If it were just the drag and viscosity, you could use the same principle on a smaller scale (with less drag) to make a perpetual motion machine. You can’t, because drag and viscosity aren’t the problem.

There are differences in air pressure at different points on Earth because of both altitude and meteorological phenomena. However, utilizing a pressure difference to extract useful work over a long distance is inherently limited by losses of the moving fluid, both due to viscosity (internal friction) and interaction at the boundary layer (skin friction). If these losses are greater than the power developed through the potential energy of the pressure difference, there is no net work that can be done and thus no possibility of extracting useful power.

Stranger

Which is completely irrelevant if the power developed through the potential energy of the pressure difference is zero.

Except the thesis of the poster was that there was a difference:

Please don’t turn this into yet another thread where you try to ‘logify’ why everyone else is wrong and only you are correct based upon some obtuse interpretation of the problem.

Stranger

According to the article, since the area is so large, no one notices or can see any 'dip".
It covers more than three million square kilometers and is centered about 1,200 km southwest of the southern tip of India. (Its enormity, as well as the fact that the ocean looks relatively flat at any given point, means the dip isn’t visible at the surface.)