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Grab the popcorn ladies and gents, I'm gonna talk about a movie real quick.

In the movie Total Recall (2012), the main character and his friend step into a huge earth elevator/public transportation system that zooms them through the Earth's interior. We can assume that the elevator went straight through the core and out the other side. During the ride, they seemed perfectly comfortable and really relaxed, which shows that the environment they were in was controlled (and there wasn't much turbulence/"shaky-shaky").

Now the question:

What are the consequences of doing this? I know that the removal of the core is a pretty gigantic task by itself, but also combatting the temperatures itself would be enough to make most engineers walk away.

1) Can we emulate/imitate the presence of a core (to preserve magnetic fields, etc...)

2) What systems could we use the combat the temperatures?

3) How advanced of a society would we have to be to be able to make this?

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All sane engineers would walk away unless they are paid by the hour without penalty for failure and they have ethics of Wally because it is far beyond engineering and science that we know.

1) You mention removing the core -- this is even way more insane than drilling through the core.

2) How would you combat the temperature? No material can survive the temperatures required. The wall of the tunnel will melt or vaporize. You can't even cool the walls because part of the tunnel would necessarily be in contact with the core (and under extreme pressures - up to 3.6 million atmospheres) so even running liquid helium through the tunnels walls would not prevent its failure.

BTW, the tiniest leak would result in explosively filling the tunnel with magma, molten iron, etc. Casualty insurance premiums are going to be very pricey indeed.

If the tunnel were in a vacuum so that no heat was conducted into the train, it would still be nearly impossible to keep the train cool because of the high rate of heat transfer from infrared and visible light radiation as the interior temperature is expected to reach almost the same as the surface of the sun.

3) How advanced would you need to be to make this work? You would have to have force fields far in advance of star trek. We don't even have a working theory as to any physics that would allow for force fields. You need brand new physics. I.e., you need tech that seems like magic to us.

Assuming no friction, for the entire 38 minute duration of your trip, you are in free fall. You accelerate down until you reach the center at max. velocity of 28,440 kph and you start decelerating until reach the surface at zero velocity. At the exact center there is no acceleration and you are in a micro-gravity environment, i.e., your head would experience a small tidal force (a few milli-Newtons) pulling it down and your feet would experience a pull in the opposite direction but your net acceleration would be zero. Of course, no matter where you are in the vicinity of the Earth you experience similar tidal forces.

A final complication, antipodal locations are relatively rare that have land on each antipode. Only 4 percent of the earth surface terminates at land on each end. For example, Africa has no land mass at the antipodal locations except for some Pacific islands. Europe, basically nothing but a small fraction of France that has New Zealand as an antipode.

For a minimal comparison, consider the complexity of actually drilling the train tunnel under the Swiss Alps - recently completed and expected to open for service in 2016. Watch the video. This is nearly state of art drilling without the complications of the temperature and pressure you will encounter in a gravity train tunnel. Sorry about the annoying dramatics on the video, could not find a straight up quality documentary. I am also reminded that the Alaskan Way Viaduct has been very troubled (broken down digging machine). Digging tunnels is hard, even close to the surface.

Other than the fact that it is totally impossible for the foreseeable future, it would be a very interesting and efficient ride.

This article has nice explanation of the actual fall including the Coriolis effect that I neglected when describing the trip as free fall. @lorenpetch noted the omission.

The full truth is that I am not 100% certain of the real effect. Nearly all articles that refer to gravity trains simply refer to it as free fall, but I am pretty sure that there should be the Coriolis issue. So, I think Wikipedia, etc. is wrong, but did not want to invest the time to prove this to myself. Any takers? If you can do this rigorously it would be real nice of you to update Wikipedia too. It seemed beyond the necessary scope of this answer so I omitted it initially. The reason I am so nearly sure is because of angular momentum; I can't see any way to explain away this problem. So, I use the dodge of lateral acceleration coming from the train track (that does not affect the vehicle speed, just its path, keeping it in a straight line).

In a tunnel from pole to pole the Coriolis effect would not be an issue.

Continental drift and internal flows within the Earth would certainly complicate the tunnel, making expensive maintenance on ongoing problem.

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  • $\begingroup$ 4) The moment you reach the center, you are weightless because gravity pulls you equally in all directions. Obviously not a total showstopper if you have the ability to solve everything else anyway, but adds further significant complications. $\endgroup$
    – user
    Nov 25, 2015 at 10:31
  • $\begingroup$ @MichaelKjörling Yes, you are weightless, but assuming the tube is in near-vacuum (probably wise!), you still be traveling at $11.2 km/s$ near the center, which is Earth's escape velocity. $\endgroup$
    – type_outcast
    Nov 25, 2015 at 15:27
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    $\begingroup$ And let's not forget the pressure. The strongest known materials have compressive strengths in the tens of gigapascals, and those are wacky things we don't even know how to mass-produce yet like folded graphene sheets. The pressure in the Earth's core is hundreds of gigapascals. $\endgroup$
    – Saidoro
    Nov 25, 2015 at 17:33
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    $\begingroup$ @type_outcast Of course. :) The math is interesting, I just wanted to make sure that the result was unambiguous. $\endgroup$
    – Draco18s no longer trusts SE
    Nov 25, 2015 at 21:35
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    $\begingroup$ You're not in free-fall while you go through this unless the tunnel is pole to pole. Suppose you're at the equator--you're moving 1042mph. The exit is moving 1042 mph in the opposite direction. That's .91mph per second of acceleration--1.34f/s^2 in more reasonable units. .04g, not free fall. (Note that this force is sideways, not on your axis of travel.) $\endgroup$
    – Loren Pechtel
    Nov 25, 2015 at 22:04
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As Gary Walker said, there are not that many places that have land on a straight line tunnel that goes straight through the core.

The good news is that you could go around the core by taking a hypocycloid path between two non-antipode points. You still have to go deep into the mantle, which is still way beyond our technology level for the foreseeable future, but it's slightly less crazy.
All hypocycloid routes would take exactly the same travel time.
A tunnel from France to England would have to be 55km deep, which is into the mantle.

Alternative
That being said, a much more feasible way to accomplish the same thing would be a space plane. The Space Liner concept had flight times of 90 minutes from Australia to Europe, and 60 minutes from North America to Europe. This is slower than a gravity train, but it's also possible to achieve in our lifetimes with present/near future technology.

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    $\begingroup$ Small quibble, all such tunnels have the same travel time only if the Earth has a uniform density, but since this is not true the fastest tunnel is straight thru the core (about 38 minutes), tunnels much closer to the surface would have travel times a little over 42 minutes. And still insanely difficult to make. $\endgroup$
    – Gary Walker
    Nov 25, 2015 at 17:57
  • $\begingroup$ @GaryWalker that's kind of why I left times out of my write up. Density differences probably wouldn't amount to that much time wise except in rare cases. $\endgroup$
    – AndyD273
    Nov 26, 2015 at 2:15
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This is preposterous :)
I could see Three conditions that might, in a sci-fi realm, justify an underground "elevator" (do you even call it elevator?).
1. We are not talking about earth but about a rock in space. The rock has no atmosphere and no protection from meteors. Travelling on it or it its "skies" would be dangerous. The rock will also not have a core and will be small enough for a tunnel to be bored in a generation.
2. You are not going through the CORE of the earth but through a "chord". That chord has been naturally created by water or gas flow in hot lava and since the tunnel is already there, it might make sense to line it up and use for transport.
3. Like 2, a pre-existing chord. This time, it was not a natural phenomenon but a result of mining. We found a vain of gold, mined it, lined up rails for our mine gear, mined it, lined up rails... For a couple of decades until it was mined out - but we still had the rails in there so maybe worth to use for transport. Especially viable if there's a little left to mine here and there along the vain to justify keeping it open.

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  • $\begingroup$ This doesn't seem to answer the question, because while it gives three quite interesting methods for constructing a space elevator elsewhere, it doesn't address the scenario given in the question. $\endgroup$
    – HDE 226868 ♦
    Nov 28, 2015 at 23:35
  • $\begingroup$ Have you guys looked at the other answers? My answer is the only one that addresses the question directly instead of: "you can't do that. period. do something else." $\endgroup$
    – DraxDomax
    Nov 30, 2015 at 10:41

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