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At 04:43 PM 9/25/01 -0700, Hal wrote:

*>The idea is simply not of practical use today.
*

Quite so. Sorry, I should have made it clear that my own interest in

raising the topic is that I want to plug such a gadget into a post-Spike

future sf novel, which allows me a measure of near-magic physics to help

with drilling and sustaining the tunnel (exotic matter, maybe, how would I

know). But I don't wish to shrug away *all* known physics with, say, an

anti-inertial drive or teleport system.

I find it amusing that some here think the trip will be weightless, others

that the difference in weight will hardly be noticed. So much for physical

intuition. :)

Alex Eremenko, whose site I ref'd, points out to me that

*>Indeed, the one-way travel time of a gravity train is
*

*>exactly 1/2 of the time of orbiting the Earth on a low orbit.
*

*>This is easy to figure out without any calculation once the
*

*>fact that the travel time is independent on the destination
*

*>is established. Indeed, conssider the travel to the antipodal point
*

*>(through the center of the Earth). This is the limiting case of
*

*>a Kepler orbit, it can be thought as a very long and thin ellipse.
*

*>According to the Third Kepler Law, the time depends only of the long
*

*>axis of the ellipse. But this long axis is the radius of the Earth,
*

*>the same as for a low circular orbit.
*

He also commented:

*>Speaking of the passengers sensation during the trip, they will depend on
*

*>the distance traveled. For example, on a train from Chicago to
*

*>Detroit they will feel almost nothing special.
*

*>As the distance increases they will feel some decrease of their weight,
*

*>and in the extreme case, a travel through the center of the Earth
*

*>(the longest trip possible, to an antipodal point) they will be
*

*>completely weightless (like in a space orbital trip)
*

*>ALL THE TIME OF THE TRIP.
*

*>There will be never any weight increase in any of these trips, only
*

*>some loss of weight, longer the trip, larger the loss.
*

*>The weight loss will not be constant during the trip[...]
*

*>One more remark: the gravity train running on a chord is not the fastest
*

*>possible one. It only has an amazing property that the time of the travel
*

*>does not depend on the distance. The fastest possible gravity train
*

*>will describe a more complicated curve inside the Earth called
*

*>the brachistochrone, and in this case the time of the travel will
*

*>depend on the distance. Actually the chape of this brachistochrone
*

*>was computed by Newton in his "Principia", but of course he did not mention
*

*>trains.
*

Damien Broderick

**Next message:**CurtAdams@aol.com: "Re: Is there still a chance for us?"**Previous message:**Spike Jones: "Re: ballistic subterranean trains"**In reply to:**hal@finney.org: "Re: ballistic subterranean trains"**Next in thread:**Robert J. Bradbury: "Re: ballistic subterranean trains"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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