What I remember from physics class makes me think this isn't viable. If you have sound moving through the tube, you have energy pushing the sound. Either you h

What I remember from physics class makes me think this isn't viable. If you have sound moving through the tube, you have energy pushing the sound. Either you have to keep the whole tube 'humming' or you have to send people off in a blast wave. With the humming idea, you might be able to create vacuums that travel around the tube at the speed of sound (sound works by changing the density of air, oscillating from high pressure to low pressure), and then you can move vehicles around the tube at the speed of sound with very little drag. So then you need some way to keep the vehicles floating, and you need to get them to the speed of sound in the first place (rail gun?). I'm going to pretend that we have a cheap way to do both of those things. So, how expensive would it be to keep 100s of miles of hyperloop humming with a sound powerful enough to create vacuums large enough to fit transportation units in? Even if you are only using cars large enough to hold 1 person and a suitcase, you are going to need a lot of energy powering the sound waves throughout the tube. You might be able to do some acoustics-like engineering to keep the sound loud at little energy cost, but I don't think that solar power above the tunnel is going to be enough. Using this model, Musk would need a handful of technological advances up his sleeve. So I think the big 'secret' that makes a hyperloop viable is not this one. Then again, with the Tesla and his space adventures, he has had to deal with at least a handful of difficult obstacles. So maybe he is expecting to develop multiple technologies to make this viable.