How fast could you travel across the U.S. in the 1800s?
It's depressing how little travel time has changed in the past several decades. The Boeing 747 was introduced in 1970, and we're still traveling at around Mach 0.85. In the past four decades, we haven't made any significant improvements to travel time. How sad is that, when in the span of 3 decades in the 1830s, rail travel time doubled?I think (hope) that we're on a path to change this in the near future. The main reason for our lack of advancement in travel time is the cost of gasoline. The  Concorde demonstrated that supersonic transport was technically possible, but economically impossible. What will change the landscape is electric planes. This is not a fantasy - there are several companies with (somewhat) viable piston planes being engineered right now.The first company is Beyond Aviation (www.­beyond-­aviation.­com) which is producing an electric varient of the venerable Cessna 172 (most popular plane produced, the default trainer option for pilots). It's not a perfect plane -- only 2 people can ride in it, and it's designed for flight schools, not for cruising around, but it's a big step forward for aviation.The second company is Pipistrel, which is developing the Panthera (http://www.pipistrel.si/plane/panthera/technical-data). The Panthera Electro is a huge step in the right direction. Although it only seats 2, and cruises at 118 knots, it can go for 215NM. This is just shy of NYC -> DC. Imagine if you could travel between these cities for essentially free (solar power).On the more viable replace-business-jets route, Elon Musk has stated that he wants to build a VTVL electric jet. From what he's stated, it sounds like his main approach to overcoming the lack of energy density is flying higher. Current jets are limited to efficient operation at ~35k feet because the oxygen density any higher is too low for an ICE to operate. Current li-ion batteries, however, don't need oxygen to operate! So they can go to 50,60,70,80k feet for reduced drag and operate just as efficiently as if they were at 35k feet. Musk has stated that he thinks battery density needs to double before this jet can become a reality (and has shown that battery density doubles once every ~10 years historically).I'm extremely optimistic about the future of aviation. As soon as we can hook up cheap (nuclear, coal, wind, solar) and efficiently generated (~2x efficiency gains at large plants) power to planes, the cost of flying will take a huge drop.