A few days ago, an executive with Hyperloop Transportation Technologies — one of two major companies seeking to turn Hyperloop from science fiction to reality — said the major expense incurred during construction would be “the tube.” This is absolutely right if he’s thinking about building Hyperloop just as a proof of concept. If he wants to build something functional, that people use, he should prepare to spend the bulk of his company’s cash on carpeting. The problem with Elon Musk’s hyperfast hypertransport — the one people aren’t already talking about — is that riders are going to vomit everywhere.

That’s hyperbolic of course, but the fact remains that the way Hyperloop is being pitched to the public right now — a 750 mile-per-hour pneumatically-run speed tube designed to get Tesla owners from San Francisco to Los Angeles in 35 minutes — doesn’t touch on human physiology and creature comfort. When people are commuting to and from work, trekking across town to take care of errands, or traveling long distances, they want to do so without feeling profoundly uncomfortable. Speed is great, but there’s a reason people will forgo a crowded subway for ungodly traffic if it means they can at least enjoy some space in their car.

In a vacuum, humans can handle a constant speed, whether it’s at a snail’s pace or a bullet’s screech. But a real Hyperloop won’t just travel several hundred miles and then reverse course in the opposite direction. There will be stops, a lot of them if this kind of technology emerges as the successor to our aging railroad system. That means pods will be speeding up and slowing down more frequently than most of us would want. And the United States is not flat — it’s got a pretty gnarly terrain in some places. Hyperloop will be forced to go around these things, and above them in some instances.

Currently, high-speed trains (all the rage in Asian countries like China and Japan) top off at around 200 miles per hour or so. Moving at this high rate of speed consistently produces few physical symptoms, but acceleration, deceleration, and turns get problematic after a certain point. The human body can only handle about 0.2 Gs of acceleration vertically or laterally. Hyperloop would involve accelerations that would potentially push way past this limit (up to seven times greater than the limits imposed on Japan’s Shinkansen bullet train).

When you move, G-forces act on your body, in both positive ways (e.g. pushing you back to your seat) and negative ways (e.g. pulling you forward into your seatbelt). When you throw in the ability to move in three different planes, you’re contending with G-forces in three different directions. Your gastrointestinal tract can lurch in a different direction than your spinal column. That’s when nausea sets in — and if the ride doesn’t end soon or if you don’t find a way to adjust, prepare to say hello again to your latest meal.

Now that's a Hyperloop I can get behind.

The solution, naturally, is to go slower. The Hyperloop Pod Competition sponsored by Elon Musk and SpaceX requires teams to construct pods that go at least 240 miles per hour. Naturally, teams like MIT are building prototypes capable of moving considerably faster. As Hyperloop moves forward, however, the 240 mph range might actually be standard outside of the Great Plains. If too fast is too uncomfortable, the Hyperloop will slow down.

It’s important to emphasize that human physiology is relative — what causes one person to wretch is someone else’s cup of tea. Some people can handle a bumpy ride better than others. The problem with the Hyperloop — in its most ideal form — is that any turns it makes will make everyone uncomfortable. As a technology, the Hyperloop is one of the most intriguing projects under development right now. But we should approach it with a healthy sense of skepticism before we start to see how actual human passengers react to getting shipped around nine-tenths the speed of sound.

Photos via SpaceX