The Earth's magnetic fiels is not stationary. It shifts and moves, as people who've witnessed an aurora know pretty well. And, due to the pressure of the solar wind, it changes drastically between Earth's dayside and nightside. So there would be motion through the magnetic field and induced currents. Big ones.

So, the material we need is infinitely resistant, extremely light and insulating, capable of withstanding extreme temperature and pressure variations, while keeping it's physical properties unchanged within the kind of strict parameters needed for frequent operation.

Unobtainium indeed.

If you want a tether that's moving through the magnetic field you can use an insulating material. The tethers that got melted by electric currents were made of some good conductor because the experimenters wanted to see if they could get useful power out of the effect & got a bit more than their system could handle.

Assi: As usual Wikipedia gives a good introduction to a subject.
http://en.wikipedia.org/wiki/Space_elevator

Ribbons made from carbon nanotubes are theoretically strong enough to be used in a practical space elevator. They're not there yet, but there is every incentive to get there — not just for the SE, but for thousands of uses for construction and others. Breakthroughs in strength, length, and manufacturing techniques are happening almost every month. This is a very fertile area of development and will be for the next decade or more.

WOW ! What news. I think everything has been answered on my Christmas list now .. Space X, Elevator and a few others I won't go into here. Thankyou Father Christmas !

There is a debate over whether oscillations can be controlled — the backers believe they can be — and if the cable is shaped into a curved ribbon, the chances of a meteorite severing it completely are greatly reduced. Further up, the cable can be maneuvered out of the way of trackable space junk in orbit.

In the end, if we're serious about exploiting orbital space, the Moon, Mars, and beyond, we're going to need a cheap, safe, and reliable way to get into orbit. So far, the Space Elevator is the most promising solution to the problem.

( knowledgeable reader can probably skip this)

Geostationary orbit is about 36,00 km up. A piece of string might just about hold up a kilometer of itself before snapping. So you dangle a kilometer of string, and tie the end to two kilometers of string, and tie them to four bits of string and so on. At the top end you would need less than 2^36000 pieces of string because you don't have any dead weight at geostationary orbit, but you would need an awful lot. Okay, so string is probably out. However, if you find some material that is light and tough so it can safely support a much longer length of itself – such as 4000 Km – then the top might be 512 times the cross-section of the bottom if we keep to our simple exponential model, which is not great but possible. A small improvement of the self-hanging length from 4000 KM to 6000 Km reduces the cross-section ratio to 64:1, so you save 7/8 of the mass.

If it snaps, you can't repair it. One half falls to earth, possibly wrapping itself entirely around the earth as it falls. The other end shoots off into a higher, elliptical orbit, which would probably dance in and out of the geostationary orbit and make a real nuisance of itself. We have to be really sure that the thing will not break, so it will have to be overdesigned somewhat.

So, what material has the best safe self-hanging length? Carbon has strong directional bonds to itself, and it is light. There isn't really anything below Carbon in the periodic table that shows much promise (though Boron and Nitrogen can make tough things). Unless you are going to make your elevator out of something other than conventional matter, some sort of unfaulted, regular Carbon-Carbon chain is likely to be about as strong and light as you will get.

It is clearly possible to make a buckytube to macroscopic lengths, and it is not a great leap from there to making almost infinite lengths. Such a material would be very useful for suspension bridges, cable cars, bulletproof jackets, and so forth. If it can be made, it will probably be made whether a space elevator uses it or no. We might well see it happen in the next decade. If and when we can get large lengths of Buckytube, then we will know how tough it is, and hence how practical a space elevator is.

Is a space elevator a practical solution? We won't know that without knowing the competition, and that is much harder to predict. My personal feeling is that an elevator is too easily wrecked, but we will see, won't we? Fun.

No spirit to achieve hard goals.

Saying no before you even try something is a pathetic excuse for human.

A little optimism is needed, and if you want a little madness, we, humans can build a space elevator.

There may be a show-stopper found along the way, but the space elevator is based on sound science not fantasy.

The concept seems sound. The energy for "horizontal" acceleration is supplied by the rotation of the Earth. The Earth will slow down a little with each launch… just like it does for "gravity assist" accelerations. That should be cool for the next few million or billion launches, or thereabouts.

Carbon Nanotubes ARE available. Ribbons have been made ("Bucky Paper") and are strong. Fabrics (Bucky Cloth?) are being made now and will be much stronger.

A few months ago "Colossal Carbon Tubes" were invented accidentally. Look up the properties, if you have the trime. Each molecule the size of an individual cotton fiber (visible to the naked eye, held together by Strong Interaction), 35 times stronger than Kevlar, as light as Aerogel.

With a bit of luck and determination, it'll be installed before NASA takes humans to Mars.

A confidence-inspiring bit of trivia: AC Clarke described geosynchronous comsats in detail long before they happened. He also described space elevators in great detail.

That energy comes from the decrease in horizontal speed of an elevator carriage of similar mass descending at the same time as another rises. Isn't that the point?

Yeah, I'm well aware of that. There have also been a great deal of supposed future breakthroughs and inventions dismissed as lunacy that turned out to be, well, lunacy.

Time will tell with the space elevator. In any case, it'll be quite some time of even if it is possible.

Regards,

Günther

Consider: "The moment that one definitely commits onself, then Providence moves as well. All sorts of things occur to help one that would never otherwise have occurred. A stream of events issues from the decision, raising in one's favor all manner of unforeseen accidents, meetings and material assistance that no one could have dreamed would come their way. Whatever you can do or dream that you can do, begin it. Boldness has genius, power and magic in it. Begin it now." — Goethe

There is also the Grimm Brothers' fable of Nail Soup, which pretty much comes down to the same thing: http://en.wikipedia.org/wiki/Stone_soup Looks like somebody's come up with the nail. Now for some leeks, veggies, potatoes, and maybe a slice of mutton . . .