A gravity train is a train that falls to its destination without needing fuel. A common example is one that goes straight through the Earth, where you fall until you reach the same height on the other end.
However, a gravity train doesn't have to go straight down. It could also be fairly flat. As long as you connect two points at the same distance from the center of the Earth (or any other planet, if built somewhere else), you can build a gravity train that travels between those two points. If the tunnel is a straight line, it will always take 42 minutes to reach your destination.
Even though a train going through the center of the Earth is currently unfeasible due to the high temperatures and pressure, a train that never leaves the Earth's crust should be feasible. A quick calculation shows that the tunnel can be a few hundred kilometers long while staying within Earths's crust, where the temperatures and pressure are moderate. If the tunnel has to be longer, it would need to follow Earth's tangent, and the travel time would exceed 42 minutes.
Since the train would be fairly fast and require little to no fuel (depending on how low the friction is), shouldn't we consider building this as another form of transportation? Have there been any serious proposals to build a gravity train that stays within Earth's crust?
4 Comments
The earth is really fking huge!
Mount Everest itself is less higher relatively than the skin to an apple (I might be wrong but im pretty sure that’s close). Wonder if someone can math out the amount of dirt we’d excavate assuming the average metro tunnel size in unit of Mount Everests.
First because we don’t have the technology to dig a tunnel like you suggest. The deepest we have ever dug was 12km along a single small borehole that went straight down. It only gets harder making them big enough for a train and lateral. The temperatures at such depths are immense so good luck keeping anything down there at a safe temperature. The pressures would crush anything but the most resilient pressure vessels that we can build good luck making a train out of one. Second the amount of force you would get from gravity at such angles would be far too small to be useful at all. Friction would consume pretty much all of it at the steepest parts and that ignores that you still have to deal with the friction at the parts that are tangential to earths gravity. Third the cost of such a project would dwarf the price of fuel of any surface train by MANY orders of magnitude. It is already hilariously expensive just to dig a tunnel through a bit of mountain… Gravity trains will never be more effective than surface trains. And if you want to go faster just use an airplane…
A 100 km tunnel would have to be 200 m deep with the avg. train speed of 140 km/h.
A 500 km tunnel would have to be 5 km deep, with the avg. train speed of 700 km/h.
The deepest mine in the world is about 4 km deep.
The deepest (under a mountain) and longest tunnel is Gotthard Base Tunnel – 50 km long, 2.5 km deep, with natural temperature reaching 46 C. It cost 11 bn. USD. It took 23 years from start to commissioning.
A 100 km tunnel would then cost at least about 20 bn USD, which is equal to a cost of a nuclear power plant.
> However, a gravity train doesn’t have to go straight down. It could also be fairly flat. As long as you connect two points at the same distance from the center of the Earth
What’s the lowest limit to this? Most straight tunnels probably roughly fulfill the “same distance from the center” criterium but obviously aren’t gravity train tunnels.
So presumably there’s a minimum angle required.