No, you don’t need general relativity to ride a hoverboard.

Image credit: Technologistlaboratory.

This morning, someone sent me a link to a piece that appeared on WIRED

You Can’t Ride a Hoverboard Without Einstein’s Theory of General Relativity

The hoverboards in question here are the currently fashionable two-wheeled motorized boards that are driven by shifting your weight. I haven’t tried one, but it sure looks like fun.

I would have ignored this article as your average internet nonsense, but turns out the WIRED piece is written by someone by name Rhett Allain who, according to the website “is an Associate Professor of Physics at Southeastern Louisiana University.” Which makes me fear that some readers might actually believe what he wrote. Because he is something with professor, certainly he must know the physics.

Now, the claim of the article is correct in the sense that if you took the laws of physics and removed general relativity then there would be no galaxy formation, no planet Earth, no people, and certainly no hoverboards. I don’t think though that Allain had such a philosophical argument in mind. Besides, on this ground you could equally well argue that you can’t throw a pebble without general relativity because there wouldn’t be any pebbles.

What Allain argues instead is that you somehow need the effects of gravity to be the same as that of acceleration and that this sounds a little like general relativity, therefore you need general relativity.

You should find this claim immediately suspicious because if you know one thing about general relativity it’s that it’s hard to test. If you couldn’t “ride a hoverboard without Einstein’s theory of General Relativity,” then why bother with light deflection and gravitational lensing to prove that the theory is correct? Must be a giant conspiracy of scientists wasting taxpayers’ money I presume.

Image Credit: Jared Mecham

Another reason to be suspicious about the correctness of this argument is the author’s explanation that special relativity is special because “Well, before Einstein, everyone thought reference frames were relative.” I am hoping this was just a typographical error, but just to avoid any confusion: before Einstein time was absolute. It’s called special relativity because according to Einstein, time too is relative.

But to come back to the issue about gravity. What you need to drive a hoverboard is to balance the inertial force caused by the board’s acceleration with another force, for which you have pretty much only gravity available. If the board accelerates and pushes forward your feet (friction required), you better bend forward to shift your center of mass because otherwise you’ll fall flat on your back. Bend forward too much and you fall on your nose because gravity. Don’t bend enough, you’ll fall backwards because inertia. To keep standing, you need to balance these forces.

This is basic mechanics and has nothing to do with General Relativity. That one of the forces is gravity is irrelevant to the requirement that you have to balance them to not fall. And even if you take into account that it’s gravity, Newtonian gravity is entirely sufficient. And it doesn’t have anything to do with hoverboards either. You can also see people standing on a train bend forwards when the train accelerates because otherwise they’ll fall in dominoes. You don’t need to bend when sitting because the seat back balances the force for you.

What’s different about general relativity is that it explains gravity is not a force but a property of space-time. That is, it deviates from Newtonian gravity. These deviations are ridiculously small corrections though and you don’t need to take them into account for your average Joe on the Hoverboard, unless possibly Joe is a Neutron star.

The key ingredient to general relativity is the equivalence principle, a simplified version of which states that the gravitational mass is equal to the inertial mass. This is my best guess of what Allain was alluding to. But you don’t need the equivalence principle to balance forces. The equivalence principle just tells you exactly how the forces are balanced. In this case it would tell you the angle you have to aim at to not fall.

In summary: The correct statement would have been “You can’t ride a hoverboard without balancing forces.” If you lean too much forward and write about General Relativity without knowing how it works, you’ll fall flat on your nose.