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I am looking to have a Tunnel drilled through a world and deal with the physics of What happens in the centre of a planet, what happens with the heat at the core and whether it is faster to travel through a planet. However I'm interested in a reason for doing this?

Obviously with any form of surface transport on any kind of reasonable sized world, it would be impractical to drill a hole through a planet to get to the other side, as surface transport would be easier.

What kind of technological, environmental or chemical environment might force a society to decide to go through rather than around a planet?

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  • $\begingroup$ As it stands this is is very broad. Can you try and give as much detail as possible about the problem you're trying to solve? meta.worldbuilding.stackexchange.com/questions/14/… $\endgroup$
    – Liath
    Sep 30, 2014 at 10:27
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    $\begingroup$ Yes, could you please add a little detail about the world? Technology, creatures, etc. I'm going to try and answer, but some of my answer may be invalid depending on what you say. $\endgroup$
    – DonyorM
    Sep 30, 2014 at 10:55
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    $\begingroup$ "surface transport would be easier" -- Well, in the Total Recall remake, they decided to have a tunnel from Australia to England, so there's at least one person who things a tunnel would be worthwhile on Earth. Or maybe just "cool" :) $\endgroup$
    – Brian S
    Sep 30, 2014 at 15:57
  • $\begingroup$ About the why: I remember a sci-fi novel, where in the distant future the human population lived completely underground, because the surface was uninhabitable. They used tunnels almost entirely through the planet (not through the inner core, but through the lowest parts of the mantle and maybe even the outer core), because the conditions on the surface were not much better, and they had the technology to drill and maintain safe tunnels even very deep down, as practically the whole civilization was based on tunneling. $\endgroup$
    – vsz
    Sep 30, 2014 at 19:05
  • $\begingroup$ As I'm sure you know, the inner part of a habitable world is highly dynamic. The inner core of Earth spins at a different rate to the rest of the planet. The mantle is turbulent and generates enough force to raise mountains and shift continents. A completely solid planet without an active core would likely lack a geomagnetic field and would have no protection from solar radiation. $\endgroup$
    – superluminary
    Oct 23, 2014 at 10:33

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How they would work

The classic design for the tunnels would be a tube with a near-vacuum inside. Objects are dropped into the tube and they fall through to emerge at the other side. The tunnel itself would need to curve so the object can fall matching the spin of the planet as it does so. You would also need something (perhaps magnetic systems built into the tunnel walls) to keep the speed of the object up as even in a near-vacuum it would lose some speed, the same systems could be used to keep the object away from the walls of the tunnel though as otherwise drift could bring it into contact.

The tunnel wouldn't need to go directly through the center of the planet and out at the same time, you can make the tunnel go to one side of the core and then bend around it to come out at any other point on the surface, like an orbit around the center of the planet although gravity gets weaker the deeper you go which makes the orbital math more interesting. For nearby destinations that would not be practicable but for anything a long way away it would be pretty fast and efficient once the tunnel is built.

The main problems are heat and pressure, particularly as you approach the core. Keeping out the high pressure high temperature magma would be a massive engineering challenge. Vessels passing through would actually not be too badly effected though as vacuum is a good insulator. This would be made much easier in a smaller world without a molten core.

Why they would be built

The main advantages of going through the planet are speed and security. For example a planet under siege or in a state of cold war might make such tunnels in order to transport resources rapidly to wherever they are needed.

The initial construction of the tunnels would be a huge effort but once they are built it would be very fast and efficient so ecologically sensitive civilizations may actually prefer it to air travel.

A common concept with warp drives is that they only function in a low enough gravitational gradient. There is no science that I'm aware of backing up that theory but it's a common literary device. Drives working that way normally means the ships need to be a long way out in space to jump. The center of the planet though also has no gravity gradient so the hypothetical warp drives would function there.

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  • $\begingroup$ You mention one of the challenges being magma; that assumes a planet with a fluid core (i.e. core of the moon is mostly solid). That said, at those pressures rock may still flow. $\endgroup$
    – CoderTao
    Oct 3, 2014 at 21:55
  • $\begingroup$ One other advantage is that, barring the needed corrections to keep within the tunnel, and assuming equal heights on both ends; the energy cost of sending materials to the other side of the planet is basically 0. $\endgroup$
    – CoderTao
    Oct 3, 2014 at 21:58
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I have to chime in here on a reason for the tunnel that no one has seemed to touch on. The most likely, and I think interesting reason to bore all the way through a planet is pure hubris. Why do we feel compelled to build the tallest building? Why do we care about record flights across bodies of water, or circumnavigating the world in a sail boat? The purest reason to make a tunnel through a planet is because you could.

This reason might be seen as a sort of cop-out, but the reality is fairly well documented and even better known in a metaphorical context. The story of icarus, the tower of babel, etc.

For world building this sort of idea could be really rich. Maybe an unknown ancient group made it, maybe it was made by "gods" or the like depending on your setting.

I think everyone has covered the how, but narrative-wise remember that it doesn't necessarily have to be stable, or well-engineered. If done well your tunnel could be believable without anyone even understanding its background or history, or the physics of the thing. It could be an imposing, mysterious character on its own.

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On a small enough planet, where such a tunnel is feasible, dropping an object through to the other side (or via many possible gravity-assisted routes) has the same delivery time as a near-surface ballistic orbit, with less work required to accelerate and decelerate because the planet's mass does that for you. The tunnel would be need to be held at a near vacuum for this to work effectively.

If there was a constant requirement to deliver material one side to another, this might offset the rather large cost of building such a tunnel.

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I'd like to propose a different cause for the tunnel. You mention being forced to go through it, but is there really a reason for them to be forced to make it before they're forced to use it?

A tunnel to the core, reaching the other side can be dug out for mining (impractical but it might be hitting two birds with one stone), exploitation of heat for energy generation, technological and scientific experimentation, cheap housing and storage (energy generation and heating are close by, transporting good is cheap).

It could also allow the easy manufacturing of various minerals and metals, by exploiting the extreme conditions through specialized scaffolding. Instead of creating artificial diamonds through explosions, you create pockets within the highly pressurized mantle and bake them there - just like how mom used to make them at home :P.

Magma taps could provide exotic materials or allow using the magma to alloy with common materials for specialized uses. The lower gravity, combined with controlled near-vacuum conditions (for the constructed route, as the rest of the others have proposed), provide tremendous opportunities for construction, experimentation and energy saving. You can build a huge construct, like a spacecraft or space station, out of heavy materials that are close by, then break it up modularly and transport it easily to the surface, or launch it through rails on the side of the tube (considering the depth, there would be enough time to slowly build up speed to avoid the massive acceleration otherwise required to catapult spacecraft into orbit).

And now, suppose everything goes wrong and the underground tunnel is mankind's only hope for survival. There it is, already furnished and ready for use, with housing available due to tourism and other market causes. Now they're forced to go underground and the tunnel is very efficient, necessary even - but they didn't build it out of necessity, which would make less sense no matter the overland threats and dangers. The planet is very, very, very deep - too deep to ever drill down that far just for cheap travel.

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  • $\begingroup$ The Part with the Magma taps remindet me on a documentytion where it is mentioned that beneath 100 km + there are dimonds about the sice of scyscrapers. That could be used FOR SCIENCE! $\endgroup$
    – Fulli
    Oct 1, 2014 at 7:07
  • $\begingroup$ @Fulli This could attract a lot of Dwarf Fortress players xD $\endgroup$
    – mechalynx
    Oct 1, 2014 at 9:56
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$\begingroup$ 
Would there be any reason to bore a tunnel entirely through a world?

I can think of one reason and it's the same as why the chicken crossed the road - to get to the other side. ;)

So, why then would someone want to use a hole instead of surface transport like you suggest?
1) It's cheaper
2) It's more reliable
3) It's more 'eco-friendly'
4) It's faster

I could go on.

Before going further we need to clarify some apparent assumptions.

You seem to be comparing "a world" with our world. The primary reason I suggest this is because you suggest there is heat at the core - not all planets have hot cores.

...surface transport would be easier.

What kind of technological, environmental or chemical environment might force
a society to decide to go through rather than around a planet?

What if "a world" had a mountain range around the equator which was too high to get over easily (even with an airplane)?

What if "a world" was nothing but a mucky swamp, or a sandy desert, or an accordion of mountains that never ended? Again, assuming no airplanes. One does not need as much technology to dig a hole if a hot core is not an issue.

And those are just a few geological hazards. We could also talk about wind patterns that made the equator a 'hot zone' from natural or man made radiation. What about a group of people, animals, insects, ... that make it nearly impossible (or at least extremely hard) to traverse from one side of the globe to the other.

There are many scenarios one can imagine that makes it hard to get from one side of a world to another.

Lastly, as other have noted, gravity is the main force providing the 'work' to get from one side of the planet to the other so once the hole was 'built' transportation costs could be minimal. If friction-less the object would emerge to the same height above the ground as it was dropped. Since a friction-less system of any kind is very difficult one could easily use electromagnets, tethers or other simple means to bring the payload whatever extra distance to the surface was required due to the losses from friction.

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There are always reasons.

  • If a civilization how a highly effective creature to dig tunnels, but no good system of ground transportation, (No extra fast creatures or flying creatures) it could actually be an effective method of transportation. And a strong tunneling creature would make the cost reasonable. Of course, depending on the planets size, supporting the tunnel in the center may not be possible.

  • Another option similar to the one above would be if the above ground world was unsafe. If going above ground for even a short time was highly dangerous, people might have to go through the earth to easily travel. If it's safe to go through the center of the planet, they might try to do so.

  • The tunnel could be an elaborate secret passage. If someone country's worse enemy's capital was on the opposite side of the world, the best way to secretly get there would be to go through the world. Other paths would give the enemy too much time to prepare.

  • Neil Slater's idea is also good. (Trying to be inclusive without copying)

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Another reason to make such a tunnel: They are sending probes to nearby stars. The probes are boosted at extreme accelerations via a linear motor (or perhaps some technology we don't know) and a tunnel through the planet is the longest possible motor and thus the highest ejection velocity.

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The tunnel could hold Ethernet cables to reduce latency for communication to the other side of the planet by pi/2.

That assumes the light speed couldn't be worked around despite massively advenced technology.

That shifts the problem to "why would low latency be so important"...

EDIT: @ Toby's quip about High-Frequency Trading actually points to a semi-plausible scenario: a rich Corporation building it to gain an edge in stock trading. See real-world analogy

Or there could be a permanent, highly automated war in which faster coordination can yield a big advantage.

Better yet: all inhabitants spend their days in Virtual Reality, some sort of MMORPG on steroids, and lags are inconvenient when interacting with someone half a world away. Resources are plentiful enough to make it worth it.

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  • $\begingroup$ HFT will even try to scam alien worlds! $\endgroup$
    – Toby Allen
    Oct 23, 2014 at 21:29
  • $\begingroup$ On an earth size planet, I would rather communicate with neutrino than make such a tunnel :D $\endgroup$
    – Madlozoz
    Feb 25, 2016 at 19:30
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A good example of the above ground world being unsafe (as suggested by DonyorM) would be to place it very close to a star. Lethal surface temperatures combined with extreme radiation and no atmosphere would make it necessary to have everything underground, at which point radial tunnels look quite attractive.

Another option would be a small planet with no atmosphere but a very high population, where the only remaining space for a new mass transit project is through the middle. There could be just a very high demand for sunlight and surface space (e.g. for farming)

The key bit to managing the core temperatures and pressures is not really the size of the planet, but the density.

It would also help if you were on a planet that rotated relatively slowly, since your radial transport vehicle will start the journey travelling at surface rotation speed and you will need a large part of your energy budget to slow that down as you descend and speed up on the other side (or risk hitting the wall of the tunnel).

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    $\begingroup$ You should be able to curve the tunnel to allow for the effects of radial velocity and still drop the cargo in free-fall for the entire trip without needing anything more than basic stabilization to keep it centered in the tunnel.. $\endgroup$
    – Tim B
    Sep 30, 2014 at 14:39
  • $\begingroup$ @TimB That might work but I don't know if it'll really be necessary. How curved would it really have to be, considering conservation of angular momentum? Also, why wouldn't rails work much better than curving? $\endgroup$
    – mechalynx
    Sep 30, 2014 at 21:07
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    $\begingroup$ With free-fall through vacuum you lose very little energy and can attain extremely high speed. If you place the capsule on rails then that will both slow down the maximum speed and require you to input more energy to keep it moving. $\endgroup$
    – Tim B
    Sep 30, 2014 at 22:47
  • $\begingroup$ @TimB - a subterranean orbital. Awesome! $\endgroup$
    – superluminary
    Oct 23, 2014 at 10:36
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Because, for whatever reason, this civilization is forbiden to reach orbit and going at the center is the only way to reach zero G

Well... parabolic flight is another way to get zero G but come on.... let's make a tunnel

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