Take a bicycle outside and pedal it, and you’ll quickly discover just how much power output it takes to travel anywhere at any appreciable velocity. The tyranny of physics? Not exactly. When you try to pedal a bicycle, or really any human-powered mode of transportation, you’re having to work against the mass of the air around you that wants to stay in place — recall, per Newton, a body at rest wants to stay at rest — as well as friction, in the case of a bicycle coming from rolling resistance of rubber tires on the ground. Even pedaling up to 10 miles per hour can be exhausting…but notice that once you reach such a speed, it’s much easier to stay at that speed than it was to accelerate to that velocity in the first place — recall, per Newton, a body in motion wants to stay in motion. It’s just that the mass of air and the friction of the ground will act on your vehicle, slowing you down.
But what if those factors were just…removed? Strikingly, we know how to do that with current technology. Vehicles need not have physical contact with the ground at all; magnetic levitation allows a vehicle to hover over a surface against the force of gravity, removing friction entirely, and is already used today in “maglev” trains. With a maglev train, less of the vehicle’s energy output is lost to friction with the track, making acceleration easier, and hence greater speeds can be achieved. Maglev trains are also quieter, all other things being equal, because there are no wheels contacting the track to generate noise: there’s just a swoosh of air…which reveals the major limitation of magnetically levitated travel: even without friction with the ground, you are still being slowed down by drag, the resistance of the air you’re moving through. Helpfully, we also know how to create tubes that are devoid of air, i.e. vacuum tubes or evacuated tubes, even at large scale. Inside a specially constructed tube or tunnel, sealed from leakage from the surrounding atmosphere, there need not be any air. Combined with magnetic levitation, evacuated tubes (often dubbed “vactrains”) make acceleration ludicrously easy compared to what we’re accustomed to.
How easy? Without friction, and without drag, the simplicity of Newton’s formulas prevail. As many people who’ve paid attention at the gym are aware, a human pedaling a bicycle, untrained and at an easy recreational level, can sustain 100 watts of power output over an hour. One watt is 3600 joules, so we’re dealing with 360,000 joules of energy. How much it’ll accelerate an object depends on the weight, but let’s suppose that you’re dealing with one metric ton. That sounds heavy compared to a bicycle, but remember you’re carrying yourself, an enclosure (there’s no air to breathe from outside, so you’ll need something like a space capsule, even if featherweight), a linear motor or some such to maintain magnetic levitation, and perhaps some cargo as well. One ton, for reference, is roughly as much as the original Volkswagen Beetle weighed.
“Oh no!”, you might say at the number you’re probably expecting to see. Well…pushing even a Beetle down the road on your own power would be hopeless, but without friction, without drag, all your 360,000 joules is being converted to kinetic energy. Plug it into the well-known equation, where kinetic energy in joules equals one half of its mass in kilograms multiplied by the square of its velocity in meters per second, and solve for velocity, and the velocity you achieve after one hour of pedaling this one-ton vehicle is…26.8 meters per second, or almost exactly 60 miles per hour.
Now, 0 to 60 in an hour isn’t exactly an impressive number in a car magazine, but consider that it’s not that much slower than the world record speed for a production-line bicycle…and that was achieved while going downhill. And this, on the other hand ,is with an untrained person pedaling along no more vigorously than an average recreational biker…while going on a flat slope. Consider also that since this maglev vehicle we’re considering is in a vacuum, it does not slow down. If our man stops pedaling after an hour, he’ll just keep cruising along at 60 mph forever. There’s no air or friction to slow him down (there are still other energy losses so he would slow even without air or friction, but the effects are tiny on a human scale and can be safely ignored here). So after his hour of pedaling he could just sit back and relax as he cruises 60 miles every hour until he reaches his destination. Comparable in velocity to a car along the freeway…only on your own muscle power.
Keep on pedaling for three hours and our man has input 1.08 million joules of energy. Now his speed, at the hour three mark, is 46.5 meters per second, or 104 miles per hour. Notice that despite pedaling three times as long he’s not even quite up to twice the speed; Newton’s equations mean that it doesn’t scale up proportionally. A full day of bicycling, however, perhaps 2 million joules total, would get our vehicle up to 63 meters per second, or 140 miles per hour…comparable to a high-speed train.
It gets even fancier: when you’re in vacuum and frictionless, you won’t slow down or stop until you brake, and, like hybrid cars, maglev trains make use of regenerative braking, which converts the kinetic energy of speed back into a usable form, which may be tapped later for acceleration. Physics doesn’t allow you to recapture all of the energy you used, but efficiencies of 80-90% have been achieved experimentally even today in some contexts. Which is meaningful: once “charged up” in the maiden voyage, to reach the same speed again, only 10-20% as much energy input from our man would be required. So instead of having to contribute all 2 million joules by pedaling again to reach 140 mph, he would only need to put in the energy that was lost from the previous trip, 10-20%, or 200,000-400,000 joules. Which, at our easy pedaling rate, equals not six hours, but rather around one hour of pedaling on his second and subsequent journeys.
On this maglev freeway, our personal transit vehicle would be pedaled up, with power assist from regenerative braking, to 140 mph and then keep cruising on. Transcontinental journeys by pedal power become feasible; Los Angeles to New York at this speed would only take the better part of one day. You could circumnavigate the entire planet in a week. Depending, of course, on straight-line vacuum tubes with magnetic track (or rather pavement) being available for you to drive on. The eerie part is that no on-board fuel or other external means of propulsion would even be required: just the tube, a magnetic pavement, and your own muscle power with your own vehicle. And no noise would result from this infrastructure: cities and countryside would be completely silent and devoid of traffic noise. No matter how fast or how crowded it got, because vacuum has no medium to transmit sound through (hence the saying “there is no sound in space”).
It gets even fancier when you take it to more science-fictional extremes. Pedaling is not the only way to collect energy from human activity; just plain ordinary footfall imparts kinetic energy onto the ground beneath you, a property that is of some interest by scientists to use as a source of power in urban areas with heavy foot traffic. Floors could absorb and convert this power if they were properly engineered, the key property being studied being springy…and yes, they would be springy in the same fashion as dance floors. Plain ordinary footfall generates 200 joules per step, though only a small fraction can realistically be captured by a floor. 10 joules per step. Vigorous dancing can perhaps quadruple these figures. But let’s stick with pacing for now.
A typical person paces at a rate of perhaps 100 steps per minute, so just by wandering around the vehicle, that’s 10 joules captured multiplied by 100, or 1000 joules per minute; over the course of an hour one person pacing can generate 60,000 joules of energy. Not nearly as much as pedaling, but it is nevertheless enough, per Newton’s equation, to accelerate our vehicle from zero to 11 miles per hour after an hour. Just from pacing. Which doesn’t sound like much, but 11 miles per hour is a decent speed for a recreational bicycle, and our driver has gotten that much speed without even trying. Just from pacing around as he thinks, like the one-dimensional man would do in his corporate office as he worked on a problem.
If you were so hyped up you paced around all day, for at maximum 18 hours let’s say, you’ve imparted 1 million joules into your vehicle. Good for 0 to 44 mph. Notice also that the energy contribution from a full afternoon’s worth of pacing as you think about life, the universe, and everything is roughly equal to what you’d need to return to 140 mph starting from zero, after you used your brake on your previous journey. Not only could a human power such a vehicle, “exercise” wouldn’t even be needed. That’s how absurdly forgiving the physics of motion are.
But remember what I said about dancing? Well…consider that in a vigorous dance party, you’re expending perhaps quadruple as much energy. After an hour one person wouldn’t have generated 60,000 joules, but rather more like a quarter million joules. Good for 22 mph. Or, if we’ve already “banked” energy from regenerative braking from our 140 mph run, back to 140 mph (recall, this gives you a 5-10x power assist, in practical terms).
Of course a dance party of one person is nigh-oxymoronic, so let’s say our vehicle contains 10 people. 1 ton isn’t going to cut it, but like a space capsule our vehicle could be very lightweight (it only really has to structurally support the crew and cargo; it won’t crash into anything on such a track, after all, especially since it’s in a vacuum…). Let’s say 2 tons, since volume scales up more than surface area, and this is roughly in the range of a heavy car, once the weight of the passengers is added in. 10 people should generate 2.5 million joules in an hour of hard dancing, which for a two-ton vehicle equals a velocity of 50 meters per second. 112 miles per hour. From dancing.
Keep the party going for three hours instead of one hour and you’re up to 86 meters per second, 192 miles per hour. Six hours of hard dancing, and your vehicle is up to 272 miles per hour.
Which might sound extreme, but once you’ve done an initial charge with six hours of hard dancing, you once again only need 10-20% as much energy for subsequent trips. So instead of 15 million joules you’d need 2 million joules or so, which is within the range of what one person with a bicycle pedal could do. Or, more to the point, an hour or less of hard dancing instead of six hours, assuming you once again have 10 people.
Let’s take the figure implied by 10 people dancing hard for six hours: if they’re in a vehicle that’s already been charged up, then their effective power boost from regenerative braking is 5-10x, so those 15 million joules will have as much effective power as 75-150 million joules. 150 million joules, on a two-ton vehicle? That equals enough energy to accelerate it to 387 meters per second, or 865 miles per hour, faster than a commercial jetliner today.
The power of joy acquires a whole new meaning…
Now, as you can intuit here, this isn’t a free energy machine: you do run into limits. But since the charge-up effect can be cumulative, assuming you have enough on-board energy storage capacity, speeds that would strike people today as fantastical for a human-powered vehicle can be achieved.
A kind of “cheat code” to squeeze even more speed would be to take advantage of the effects of gravity: go downhill, you accelerate; then as you go uphill, you decelerate. In air and with friction, you’re not going to get nearly as far uphill as you went downhill, but in vacuum and without rolling resistance you could regain almost all of your original altitude. Without any additional energy input. This effect has been explored in the context of a “gravity train”: take the spherical surface of the Earth, travel through the planet along a straight line, and the effect is gravity will first accelerate you and then decelerate you, with your speed being zero at the beginning and end of the journey, and reaching maximum speed halfway.
Gravity trains have several freaky and fascinating effects (e.g. the travel time is exactly the same for any journey no matter the distance, and it depends only on the density of the planet; for Earth the time is 42 minutes, 38 once density differences in the interior are taken into account), but a showstopper is that a tunnel between two points of appreciable distance would involve a maximum depth in the mantle…or even the core. But the effects are pretty significant even with modest vertical distance. To wit, we could dig a tunnel, say, 20 kilometers under the surface. With a maglev operating in a vacuum, it’s pure Newtonian formulae all the way down: an object two tons in mass dropping 20 kilometers in our gravity well releases 392 million joules of energy into motion. Velocity? 626 meters per second, or 1400 miles per hour. Entirely passive. And twice as fast as today’s best jetliners. All our passengers would need to do with their energetic dancing with gravity’s help is make up for the losses from this system.
The ultimate limit in theory would be a gravity train that extends across the Earth’s core connecting antipodal points; such a train would begin falling straight down (i.e. in full free-fall), pass through the Earth’s core, and then decelerate straight up. This would unleash 125 trillion joules, enough for our two-ton vehicle to reach a maximum speed of 11.18 kilometers per second. Not coincidentally this is exactly the same figure as Earth’s escape velocity. Alas, in a gravity train, you’re reaching that speed going down, so it’s then all “used up” as you head back up, and your speed zeroes out at the surface, not in outer space.
But hey, we’re not only working with gravity: we’re capturing the energy from footfall here. Notice that starting from 11 kilometers per second, you only reach zero speed after traveling thousands of miles straight vertically. Because we have enormous amounts of energy here that gravity only acts on gradually, and there’s no air for our train to rub up against (a good thing too, since otherwise there would be a tremendous amount of heat generated, exactly like a re-entering spacecraft or a hypersonic aircraft). So “charge up” a vehicle enough along these maglev tracks, and in principle you can, starting from the Earth’s surface, reach orbit, or even escape velocity.
For a two-ton vehicle the energy required to reach escape velocity is 125 trillion joules, which sounds like a lot, but remember that our vigorous dance party of 10 people is generating 2.5 million joules per hour. That’s 50,000 hours of dancing, which at 6 hours per day equals…8300 days worth of partying. 23 years. Okay, that’s a lot. But along these tracks on the surface, if they could just keep going in a straight line, they could eventually store up enough energy to reach escape velocity.
The big problem at this point would be “how do you create a vacuum tube to outer space?”. Since if you’re dumped into the lower atmosphere at 11 kilometers per second speed you’ll burn to a nice crispy ceramic. Well…science already has an answer to that: it’s possible to build a structure that connects Earth’s surface and outer space, in the form of the famous “space elevator”. Advanced materials currently known to us, albeit only manufacturable in the laboratory, would have enough tensile strength to withstand the extreme forces that would act on it. Anchor one end of the cable at Earth’s equator, and attach a counterweight beyond geostationary orbit, and you now have a structure on which you could climb up to outer space. Reach the altitude of geostationary orbit, and you have enough velocity, courtesy of the cable’s rotation along with the Earth, to just gently push away. Now you’re in space, and in orbit around the Earth in stable fashion.
Typically the “cars” of this space elevator are envisaged as being supplied by an external power source a la an electric rail, but if a suitable evacuated tube connected our terrestrial network to the base of this elevator cable, our vehicle could be inserted into the vertical tube at 11 kilometers per second, and you’d come out with a decent amount of velocity into Earth orbit, from which a momentum exchange tether system could put you on course to the Moon or to another destination. ToughSF’s blog has an excellent overview of how these tethers could work to exchange energy and alter an object’s orbit, such as our friendly neighborhood dance-powered vehicle. Notably, momentum exchange tethers could gather kinetic energy and stay in the proper position by use of passive means such as solar sails, and I might add solar sails could in principle be used by vehicles like I’m discussing here, perhaps in mothership fashion — dock with a solar sail module when you exit the space elevator, and then jettison it in a suitable parking orbit when you reach your destination, for the next traveler to use.
Notably all these applications demand the use of some fairly advanced materials, but they are not remotely energy-intensive to operate. Some of these applications such as tunnels 20 miles underground connecting every home and shopping mall would demand cheap and easy tunneling en masse, which basically requires something like a nuclear subterrene. And if you have enough nuclear energy to just melt rock, why not just use it for propulsion and forget about all this dance-your-way-to-the-Moon nonsense? Well, consider that aside from being extremely energy-efficient, railways or even roadways that are in vacuum tubes are quiet, potentially enabling cities and a whole world where traffic noise is effectively zero, which is never going to happen if your vehicles are zipping along at 1000 miles per hour or better within the atmosphere. And if your infrastructure is underground anyway, why not just make the tunnels a vacuum? If nothing else it’s a much more forgiving environment on the materials you’re making the vehicles out of…
Consider also that a key advantage of this slow form of spaceflight with a space elevator is that you can, in principle, go to anywhere in outer space you choose on your own power, and with literally the same vehicle you drive on Earth. Tough to say that about any other method of travel. Like people who sail today, the autonomy might be attractive.
And there’s an even bigger advantage: in stark contrast to rocketships, a momentum exchange tether emits no light that pollutes the night sky. ToughSF speculates that the use of tethers for kinetic energy exchange might be favored by the criminal element such as pirates or the military, especially coupled with stealth features of the spacecraft themselves, since they are far harder to observe from afar than a rocket that’s belching out radiation, but I would point out that in a world where every other point of light you see up above is a rocket engine rather than a star or a planet, the idea of a mode of travel that just shuts all that visual noise off might really be appealing.
Space elevators as a way to pristine dark skies? Gravity trains as a way to quiet cities? It’s a possibility that’s often overlooked. Oh, I think it’s more likely that a civilization that possesses advanced materials science but hasn’t yet discovered the power of nuclear energy would use these methods — think steampunk, of a kind with the Victorian Internet, or a world without fossil fuels readily available — but for all we know, the ancient human yearning for a green world and a black sky may yet prevail…