Ok so this one isn't about Space Travel, but it is heavy Sci-Fi.
It's from the new Total Recall.
This isn't really a spoiler, since it describes a technology that's shown in the opening moments of the film, so if yuo haven't seen it yet, no biggie.
It describes a form of mass transit, called The Fall. It's a large car, of sorts, that drops into a tunnel that goes from London to somewhere in Australia, and uses gravity and momentum to get it to its destination.
The characters board the vehicle, sitting in upright seats. Safety devices, similar to what you see on a roller coaster, drop down to hold them in place. The Fall is released, and they drop. As they pass the Earth's core, gravity suddenly goes to 0, and the entire compartment rotates 180 degrees so that when they come up in Australia, they will still be upright... A few moments after this rotation, gravity reasserts itself and they ride the rest of the way. The entire trip takes 17 minutes.
Now, hypothetical mass transit methods like this have been speculated about for some time, and aren't entirely unrealistic. There are some issues though, in how they are portrayed in this movie.
-Gravity. If you're sitting in this device and it's dropped down into the Earth, you will be in a state of 0g the entire way, not just in the middle. The reason for this is that you're in freefall... You drop at the same rate as the vehicle around you. Astronauts in the space station experience 0g for the same reason. Technically an orbit is just freefall where you keep missing the ground. As you pass the midpoint of the trip and the vehicle starts to slow down because it's now rising toward the surface instead of falling, gravity and momentum would still affect your body in the same way as the vehicle, so you'd experience 0g all the way to the end of the trip.
-Speed. I'm not sure whether 17 minutes is a realistic time for such a trip, but it MIGHT be. The rate of acceleration when the vehicle leaves, if it's heading straight down, would be 9.8 meters per second per second. That's pretty fast. The rate of acceleration would slow as the vehicle went deeper, because in a spherical shape like a planet, only the mass closer to the center of the sphere than you actually exerts gravitational force upon you. (I've seen the calculus that proves this in my high school AP Physics class, but I won't replicate it here.) As you drop further and further toward the core, less and less mass is pulling on you, so the rate of acceleration from gravity would approach 0. As you reach the core, it would be 0 so your acceleration would cease for that instant. As you start to leave the core, more and more mass is closer to the core than you, so the force of gravity would gradually increase. Ignoring friction and assuming the point of departure and the destination were both at the same elevation, the acceleration from gravity would be back to 9.8 meters per second per second, but pulling you down. Your momentum would thus bleed off the entire time you're rising and ideally, would reach 0 at the same moment you reach the station at the far end.
-Wind. In one scene in the movie, somebody opens a hatch that exposes the inside to the outside atmosphere. In the movie, it's just wind rushing by really, really fast. In reality... No. If that tunnel has air in it at all, it would be rushing by at thousands of miles per hour at the point of peak velocity. That's much, much faster than any aircraft man has ever built. The heat from the air ahead of it being compressed would be enough to vaporize the vehicle. That's why we have heat shields on spacecraft. (Contrary to popular belief, it isn't the friction of the atmosphere. The heat of re-entry is mainly from the compression of the air in front of the falling spacecraft.)
Not that it should be a problem, because...
-Vacuum. For a system like this to work, the tunnel would have to be completely depressurized and be in a vacuum. This would eliminate problems from wind resistance which would not only fix the aforementioned heating problem, but also eliminate friction from air which would deplete the vehicle's kinetic energy long before it reached the surface.
-Friction. Not only would wind resistance be a problem, but the vehicle would have to travel along some kind of frictionless rails. Maglift trains use a technology that could be of use in a system like this. The movie didn't address this either way, so all is well there.
-The Core. This is the biggest issue I had with the movie. The Earth's crust is only about 30 - 40 mi thick on average, so a system like this would go all the way through that and then be into the Mantle... Which is a hot, gooey magma mess that can't simply be tunneled through. Depending on the path of The Fall, it would also have to pass through the Outer Core which is even worse. The Movie suggested that The Fall took a course that curved around the Inner Core, but still... The film implied that only a short part of the trip was through a zone of really hot rock.
In theory, a system like this would work fine between any two points on the Earth, not just on opposite sides. A Fall could be constructed to connect, say, Denver to Baltimore. It wouldn't go as fast, but all the principles would be the same.
"He who takes offense when no offense is intended is a fool, and he who takes offense when offense is intended is a greater fool."
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