# Thread: The gravity express or how to get to anywhere on this earth in 42 minutes or less

1. ## The gravity express or how to get to anywhere on this earth in 42 minutes or less

This approach to travel, assuming that one can build a tunnel through the center of the earth, would take relatively little energy (just enough to counteract friction). Of course, building the tunnel would be a challenge. In the comments that followed this article, some people have suggested that it would be impossible since parts of the earth move relative to each other. Others point out the the debris from the tunnel could be as much as the equivalent of 35,000 Mt. Everests. Other suggested that this may be a viable concept for smaller bodies, such as the moon.

On the other hand, it is an exercise in creativity and how to think outside of the box or, in the case, outside of "the hole".
• Why must the hole go through the core of the earth? Why not build a tunnel that is a chord (from one part of the circumference to the other)? While you may not get all the benefits of gravitational acceleration to the center of the earth, you would still be able to use some of the gravitional acceleration to save energy.
• Why do we need a big tunnel? If we are not transporting humans or big objects, it can be quite small (such as a fiber optic). By the way, apparently the Russians actually created a drilling device that drilled a nine-inch diameter hole 7.5 miles into the earth's crust, as described in The Deepest Hole. Suppose that we just set a drilling robot that could drag a cable that would provide it with power, lubricant (to wash out the contents of the hole, and a fiberoptic cable. We can pass light, x-rays, or even gamma rays through the cable.
• Do we really have to have a straight tunnel? A straight tunnel would perhaps produce the least amount of friction but, if what we are passing through the tunnel is energy, the energy could be bent around curves. Recently, Professor Eugene Polzik and his team at the Niels Bohr Institute in Copenhagen succeeded in quantum teleportation of a macroscopic object that contained several trillion atoms (Source). So, maybe someday, it will be "Beam me down, Scottie" instead of "Beam me up..."

http://www.damninteresting.com/?p=696

The Gravity Express
Posted by Alan Bellows on October 15th, 2006 at 11:22 pm

A forty-two minute gravity train route from New York City to HawaiiA forty-two minute gravity train route from New York City to HawaiiAbout four hundred years ago– sometime in the latter half of the 17th century– Isaac Newton received a letter from the brilliant British scientist and inventor Robert Hooke. In this letter, Hooke outlined the mathematics governing how objects might fall if dropped through hypothetical tunnels drilled through the Earth at varying angles. Though it seems that Hooke was mostly interested in the physics of the thought experiment, an improbable yet intriguing idea fell out of the data: a dizzyingly fast transportation system.

Hooke's calculations showed that if the technology could be developed to bore such holes through the Earth, a vehicle with sufficiently reduced friction could use such a tunnel to travel to another point anywhere on the on Earth within three quarters of an hour, regardless of distance. Even more amazingly, the vehicle would require negligible fuel. The concept is known as the Gravity Train, and though it seems inconceivably difficult to construct, it has received some serious scientific attention and research in the intervening centuries.

The basic concept behind the gravity train is straightforward: At each end of the tunnel, an observer looking into the hole would see a downhill slope. If a train at one end of the tunnel were to release its brakes, the force of gravity would immediately pull the train downhill and cause the train to accelerate much like a roller coaster. Steeper slopes would result in more speed, with the highest acceleration occurring in the straight-down tunnels which cross the Earth's center. The train would continue to accelerate until reaching the halfway point, at which time its inertia would be at odds with gravity and it would begin to decelerate. As Hooke's data indicates, if the train operated in a frictionless environment it would reach the surface on the opposite end of the tunnel at the exact moment that its speed reached zero. Naturally, a gravity train operating in a real-world environment would need to bring along enough horsepower to make up the friction loss.

One interesting property of the Gravity Express is that its transit time would always be very, very close to forty-two minutes regardless of the distance travelled. In fact, if the Earth were a perfect sphere, the trip time would always be exactly forty-two minutes and twelve seconds. Greater distances would be traversed in the same amount of time as short ones because the train's maximum speed would be increased enough to exactly make up the difference. Due to nature of gravity, this forty-two minute trip time would be consistent for any size of vehicle.

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2. That is interesting Dr,

It also raises an interesting question. Do you suppose "gravity holes" could be dug to act as a form of energy generation. The holes need not be but maybe a mile or two.

3. This really sounds very like the principles involved in many fair-ground rides. However if you are going to get anywhere in fourty two minutes, over the very long distances the top speed would have to be enormous and I can't help but feel that unless the train ran in a vacuum, overcoming the terminal velocity induced by air resistance would still require the use of sginificant amounts of fuel in order to meet the fourty two minute deadline. Is it realistic to build enormous tunnels that are vacuums?

One interesting property of the Gravity Express is that its transit time would always be very, very close to forty-two minutes
It makes you wonder if Douglas Adams knew more than he was letting on when he claimed the answer to life the universe and everything was fourty two.

4. Originally Posted by Rock
That is interesting Dr,

It also raises an interesting question. Do you suppose "gravity holes" could be dug to act as a form of energy generation. The holes need not be but maybe a mile or two.
Rock,

You have proposed an interesting idea but, as somebody much wiser than me once said, "there ain't no free lunch." To dig the hole, you would have to expend energy moving the contents of the hole out. Yes, if you then drop mass down the hole and use the impact energy at the bottom of the hole, you can generate the energy but the hole would simply fill up with the mass. Then you would have to expend the energy to remove that mass. It would be a wash.

Gravity holes are a potential approach of storing energy, however. For example, if one could harness the impact of an asteroid to make a hole and then regain the energy from filling up the hole, it may provide a source of energy but controlling that source is messy. There are probably easier and simpler ways to store energy. For example, that is what batteries do. We create energetic electron "holes" and the movement of electrons to those "holes" provide us with the energy that we use to run our cell phones and computers.

Wise.

This really sounds very like the principles involved in many fair-ground rides. However if you are going to get anywhere in fourty two minutes, over the very long distances the top speed would have to be enormous and I can't help but feel that unless the train ran in a vacuum, overcoming the terminal velocity induced by air resistance would still require the use of sginificant amounts of fuel in order to meet the fourty two minute deadline. Is it realistic to build enormous tunnels that are vacuums?

It makes you wonder if Douglas Adams knew more than he was letting on when he claimed the answer to life the universe and everything was fourty two.
Adrian, vacuums are only necessary if you are moving masses that can generate air resistance. If you are using a beam of light, for example, the photons do not hit enough air molecules to lose much of their energy.

Your question, however, raises an interesting question. One way of reducing air resistance is to generate air movement in the hole. Suppose, for example, you dig a hole through the earth and suck air through the hole when you want to send something through the hole from the other end. By the way, you can recapture some of the energy of the air if you blow the output of your vacuum pump onto a windmill.

I use to work in a Veterans Administration Hospital where they have these air tubes for sending messages from different parts of the hospital. You put your message in a plastic capsule/cart that you put into the air tube. The capsule/cart is sucked into the tube, presumably because of a vacuum that is generated somewhere in the system. The Mayo Clinic has a similar system that I think they are still using, for transferring patient files from central records to the examining rooms.

Wise.

6. I don't have time to comment in depth on this, so a couple of quickies:
If you get to any non-negligible (for these purposes) depth, heat and pressure become two big problems. If the tube were not evacuated, the air pressure at any reasonable depth would be considerable (likely causing enough diffusion to interfere with light transfer as well as causing friction), so vacuum pumps would need to be installed at the lowest depth.
I remember seeing the pneumatic tubes being used in a department store for sending money and receipts from place to place in the store. And of course some banks use them now to communicate with drive-up customers.
It is a neat idea, though.
Rock's idea is really the same as storing energy by pumping water into a reservoir (digging the hole) and later recovering it by running the water through a turbine (dropping the water back down).
Wise, that's a good analogy with the holes in a battery. That (especially with a capacitor) is much more efficient than moving mass around on a large scale.
- Richard

7. Yes, if you then drop mass down the hole and use the impact energy at the bottom of the hole, you can generate the energy but the hole would simply fill up with the mass. Then you would have to expend the energy to remove that mass.

I am not sure how much energy or how to even capture a falling mass generating electricity but the mass can be removed by a counter weight perhaps. I know it sounds like the Flintstones but the way current energy supplies are being used, then rolling stones may tune out to be one possibilty.

There would have to be many holes and only one mass per hole that is a smaller diameter so air can escape. Then, the mass is removed. Maybe even dropping an item thru a mountain top down thru the middle and exiting out the side would work too.

8. I have been contacted by CBS News Production/Discovery Channel on Future Trains concerning the subject of this discussion. They want to do an interview at the beginning of August. I told them that I am not an expert on this subject at all but they apparently want to something that reflects the discussion here. So, anyway, I thought that I might reactivate this thread to see if people have any more ideas.

Basically, a gravity train is a theoretical entity that uses gravity to accelerate a train moving in a tunnel that goes through the earth, as shown in the following figure. We had discussed a number of variants, including light.

http://en.wikipedia.org/wiki/Gravity_train
Wise.

9. Originally Posted by Wise Young
I have been contacted by CBS News Production/Discovery Channel on Future Trains concerning the subject of this discussion. They want to do an interview at the beginning of August. I told them that I am not an expert on this subject at all but they apparently want to something that reflects the discussion here. So, anyway, I thought that I might reactivate this thread to see if people have any more ideas.

Basically, a gravity train is a theoretical entity
Wise.
The concept is fine but the considerations needed to accomplish it - overcoming the tremendous heat and pressure at any appreciable depth, noted by rfbdorf, - are insurmountable with present technology and will remain so for the foreseeable future.

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