Maybe you know something I don't. At an altitude of 8000 miles gravity is reduced by 4 times, at 12,000 miles, 9 times, which is still not negligible.
At an altitude of 60 miles at 6 miles per second, air resistance is sufficient to heat, hundreds of square feet of shuttle surface white hot. We really cannot go 6 kilometers per second at much lower altitude than 60 miles, even with the help of streamlining and plasma to blow the air away. Even at 60 miles altitude heating very thin air and tiles takes lots of energy. So it is better to accelerate to about one kilometer per second at 60 miles altitude, and do most of our accelerating at 100 miles or higher. Structures more than a few kilometers long, need huge amounts of material to avoid collapsing under their own weight. This is even true of mountains. Earth likely never had a mountain more than 20% taller than Mt. Everest. The granite near the base would be crushed by the enormous weight.
An exception is something 20 times stronger than Kevar by weight. In theory CNT = carbon nano tubes can do that, however we have not made large scale CNT that is even twice as strong as Kevar so far, partly because twice as strong does not help much unless it is cost competitive with Kevar. 20 times stronger than Kevar by weight would be worth a million dollars per ton for some applications such as the SE, or a coil gun 60 kilometers long.
Diamond is superior for compressive loads, but even if we could make flawless diamond for $1000 per ton; a 10 kilometer tower with generous safety factors would likely cost a million times a million dollars, with the first one even more costly.
Balloons are fairly efficient at pulling up large loads to about 10 miles altitude. To achieve the 45 miles suggested by Space Island Group requires balloons that hold more than a cubic kilometer of hydrogen for even a tiny pay load and CNT or equivalent 20 times stronger than Kevar. Maybe some manufacturer is about to announce the large scale availability of such a material, but don't hold your breath. We have been waiting about 15 years with rather puny progress with CNT.  Neil

Extreme taper is needed if steel is used = much too heavy. For the cable to be stationary with respect to Earth's equator it needs to be more than 36,000 kilometers long. A 29,000 kilometer (or shorter) cable would burn up when it's end reached into Earth's atmosphere. CNT = carbon nano tubes with great specs is needed, perhaps 20 times stronger than steel by weight.   Neil

A lunar space elevator is great for travel from Moon to L1 or Titan to Titan L1, but not practical for most other applications. This is because the tether is moving close to the same speed as Earth's moon or Titan, so you gain little delta v.
An Earth or Mars space elevator moves several kilometers per second faster than the planet out near the counter weight. This allows you to go anywhere in the solar system with much less fuel and/or faster than lifting off from the planet.
Some moons (asteroids etc) are not tide locked to their planet and rotate briskly making a space elevator at least slightly useful in spite of the shallow gravity well.  Neil

As far as I know all of the 2nd bburwood post is also correct. We could perhaps anchor a Venus space elevator to a lighter than air habitat in the cloud tops of Venus. The upper atmosphere of Venus rotates much faster than the surface so perhaps 200,000 kilometers would be long enough, and the sky station would be above the sulphuric acid layer of the atmosphere. One report found 55 degrees f near the cloud tops where the pressure is close to Earth sea level and the sun shines about 55% of the time, providing solar energy. The heat and pressure below would not be a problem, unless the station lost buoyancy. The occupants might be able to escape on the space elevator in case of loss of buoyancy.
With present technology we would need a three stage heat pump to pump heat from 25 degrees c to 450 degrees c and it would use massive amounts of energy.
At Mercury, a bolo attached at the top of a tall tower over The Mercury polar colony, would work for leaving Mercury. Landing on the outer tip of a bolo spinning 10 kmps = kilometers per second, might not be safe even with future technology. Would 10 kmps plus the high speed of Mercury orbiting the Sun get us to Earth eventually, without much more delta v? It would always be about 300 degrees c on the bolo as the sun never sets slightly above the surface of Mercury at the poles.
I'm enthused about Earth grazing asteroids, as flying time could be less than to the moon, and the tiny asteroid habitat tours the inner solar system up to a billion kilometers from Earth. I think a permanent asteroid habitat/colony is possible for about what we will spend on the 6th flag and foot prints mission to the Moon. Safety is not good for the first few asteroid habitats/colonies as many surprises are likely, but I think there would be lots of volunteers anyway.   Neil

As far as I know bburwood has that exactly correct, except Venus and Mercury may be more difficult due to their very slow rotation rates and very hostile surface conditions. 9 times more solar flux near Mercury would likely cause problems for the entire million? kilometer length of a Mercury space elevator. In some respects the poles of Mercury may be the easiest place to build a human colony in our solar system. A long (300 kilometer) space elevator at Earth can get us to Mercury in about one month. The bottom of the polar craters of Mercury are always at -133 c according to one report, bur the waste heat from a colony would warm the crater to a comfortable temperature. Gravity is same as Mars, but essentially no atmosphere. The orbit of Mercury is tilted 9 degrees with respect to the plane of Earth's orbit, but the axis of rotation is not tilted with respect to the planet Mercury's orbit, so the sun never shines on the bottom of polar craters.   Neil

Perhaps about the same cost, but not cheaper. Even the lowest energy route to Mars takes slightly more energy than landing on the moon, besides taking years to get there. There has to be some cost for the long travel time, if only the equipment is obsolete when it arrives. Most things deteriorate a bit after years in space exposed to hard vacuum and uneven temperatures. A more costly space elevator can throw goods to Mars in a few weeks, but arriving at a million miles per day = 42,000 miles per hour, means some goods will be damaged by even a very sophisticated areo breaking system.   Neil

The ribbon is fastened to the anchor ship with a winch so the tension can be optimised, If the ship is 200 kilometers North or South of the equator, the ribbon sags = leans slightly toward the equator. East and West movements of the ship produce somewhat smaller  movements of the ribbon at LEO = low earth orbit altitude, 20 minutes later. Delay depends on the speed of the transient on the ribbon. Somewhat larger North or South movement is necessary to dodge at LEO as the sag is increased or decreased partly offsetting the horizontal movement. The ship can move perhaps ten kilometers in 20 minutes, but rarely needs to move that much, as the path of most space junk can be predicted + or - a meter, about a day in advance.
Generally we cannot predict a micro meteor, so the ribbon will be damaged, perhaps daily. The damage should produce a change in the many transient moving on the ribbon, so we should know about the damage in minutes to hours, so a thread laying climber can be diverted to make a repair.
I suppose we could build a crawler, somewhat like the machine that moves even the biggest rockets from the VAB = vehicle assembly building to the launch pad, at Cape Canaveral. There are however other advantages to at sea, such as extremely rare lightning and low peak wind speed. It also solves the NIMBY = not in my back yard problem. A no fly zone with deadly force is far more practical at sea, than at most land locations.
Advantages of launching from high altitude are puny compared to the 97,000 kilometer total length of the ribbon, or the 36,000 kilometers to GEO = geostationary Earth orbit.   Neil

Hi A_M_Swallow: that should be about the correct conclusion for 250 km, so the total mass is reduced by an even smaller percentage for 25 km. It seems to me if you optimize the tapper and other properties of the ribbon without the sky station then reduce the effective weight of the bottom 25 km by 500 kg then you can increase the payload by 500 kg if you start the climber from the sky station instead of the anchor ship. Perhaps more than 500 kg is possible as there are negligible wind gusts to deal with at the sky station. We do still need to deal with atomic oxygen and micro meteors. 500 kg divided by 35786 = 13.97 grams per km = 13.97 milligrams per meter of ribbon, so the 500 kg seems reasonable, if not low, considering the slight possibility of a bad storm with lightning for the bottom 25 km.   Neil

The laser beam can possibly be pulsed several billion times per second, but we still need the photovoltaic panels plus some device to convert the micro wave energy to a form which can power the motors. We have made the system more complicated, heavier, with more loss, but no better as far as I can tell.   Neil

If the platform is 23 miles above Earth's surface, that is 0.1% of the distance to GEO altitude. Tapering may reduce the strength required by several tenths of one percent, but that does not help much, and developing the sky station will be costly and could take decades even with lots of money. The weight will be reduced by much less than 1/3. Four ribbons from ground to sky station may be best as electric power can be sent up the ribbons as back up for photovoltac and perhaps  www.sky windpower.com  tethered to the skystation. Multiple tethers means the station and ribbon survive failure of one or two of the ribbons and climbers can be returned to Earth for reuse. People below will be unhappy if we just drop the surplus climbers and other trash. The crew, if any, may not want to use a parachute to return to Earth for vacation.
Transients traveling on the ribbon (both above and below)will shake the sky station, eventually causing stress fractures and annoying the crew, if any. Developing a reliable unmanned cargo transfering system controlled from Earth may prove quite difficult. Present balloons lose their helium or hydrogen in about one month, so resupply of the balloons gas will be a major cost. Some 5 ply balloon faberic is being developed that will hopefully reduce gas loss greatly.
A manned station at GEO also has some advantages, but cost and development time are severe disadvantages.   Neil

There is light pressure from laser and microwave beams, but at distances of thousands of miles, it is less than one gram per square meter, so it can't significantly push the climber up the ribbon. It is however thought practical to shine the laser beam on photo voltaic panels and get enough electricity to power electric motors which climb the ribbon, mechanically. Laser beams and microwaves have reached the moon and reflected detectable energy back to Earth, so 62,000 miles is possible, but the illuminated spot for micro waves is much larger than the climber even at 1000 miles, so microwave likely is not practical to propel the climber.
Much more powerful lasers can likely push by light pressure, except it would melt the climber and the ribbon with average beam intensities of gigawatts per square meter.   Neil

I think it was Marshal Savage who wrote Millenium Project. He described a vacuum tube (perhaps 10 feet in diameter) which ran up the side of a tall steep mountain. The space vehicle was accellerated inside the tube to perhaps 5 kilometers per second (with electromagnets) when it poped the door at the top and coasted upward to space. Do you have this in mind except for using balloons instead of a mountain to support the tube vertically?
Either method is costly, and requires a 2nd stage, for going to the moon or most any other purpose. The poping of the door allows considerable air to rush in, even if the door is opened only one second. The high speed blast of air stresses the tube requiring it to be sturdy and thus heavy and costly. The partially filled tube is heavier than the vaccuum tube, so it starts to decend. It may take several days and 100 megawatt-hours of electricity (costs about $10,000) to pump the tube to high vacuum again.
Besides the small payload, the helium for the balloons will cost perhaps a million dollars and needs to be replaced about monthly. Helium is a non-renewable resource so the world price would rise, as reserves were used up. Eventually the helium cost would be much higher than hydrogen, which has slightly better lifting power, and a negligible fire hazard at 80,000 feet.
An alternate approach is to use compressed air to accelerate the space craft up the tube. This can likely acheve the 5 kilometers per second with less research and development than the magnetic propulsion, and operating cost might be lower, but it would likely take about an extra day to recover the high vacuum. The craft will be white hot from air friction (shortly after leaving the tube) so more than 5 or 10 kilometers per second is likely impractical, unless it can be made taller than 80,000 feet.   Neil

Cable elevators rarely are more than 20 to 50 stories. Laser powered  building elevators = lifts could be faster than is safe with cable, and hundreds of stories are theororetically possible. Fast means fewer elevators are needed in a very tall building, so in the far future the conveniece, ecconomics and safety may favor laser propulsion inside buildings.   Neil

I can't do the math, but A_M_Swallow can, so we can be confident that the advantages are puny. I'm aware of several disadvantages: 1 to take one ton to 120,000 feet, requires 3 huge balloons. Starting from sea level the balloons are almost empty so they flap in the breeze. 2 If the balloon faberic touches the ribbon, a leak is all but sure, so we need a tube between the three balloons, perhaps five hundred feet long with the ribbon inside. The tube will fail if the tube touches the ribbon frequently, so some sort of high tech is needed to prevent touching. 3 At 120,000 feet or the max height achived, we need to get rid of the tube and 3 balloons. In theory the helium or hydrogen can be dumped and the tube will slide back down the ribbon with the limp balloons for reuse. The decent could take all day, so no new climbers can be launched that day 4 I suggested hydrogen as 300 launches per year would double or more the world price of helium. Helium is a non-renewable resource. 5 As the balloons are detached, the climber needs to grip the ribbon tightly or the climber will start falling. This probably is a minor problem, but perhaps could complicate the mission. We can not use the laser propulsion until the balloons are clear as the laser would damage the balloons.  Neil

Hi Jesse: The weight of the column of air is the same as another column of air without the tube, so there is no net force except sometimes 3% due to differences in barometer readings. A siphon works becase the fluid flows to a lower level. If your method worked we could get rid of the excess air that Venus has.  Neil