How can I tell the difference between acceleration and up/down?

Einstein once proposed a thought experiment involving an elevator, basically, you can’t tell the difference between acceleration and gravity. Since I spend a lot of time on trains, metros, and buses, I’ve been curious about how my body can tell the difference between acceleration and going up or down a hill.

Ultimately, I can tell the difference because of sound. When a train/metro/bus is accelerating or decelerating, it makes a distinct sound or vibration (in the case of sound-canceling headphones). That wasn’t enough for me, I always felt like there was something else.

Several weeks ago, I remembered how sensitive phone barometers are. And after getting in a fistfight with a balloon in a car, remembered that ‘denser’ air will move to the back of a vehicle. So, I was curious if my barometer on my phone could detect this and provide any interesting insights.

Sure enough, acceleration and deceleration made a noticeable difference in the volume of air in the vehicle. Usually less than half an hpa for moderate acceleration. My ears were not able to discern a difference, even when I concentrated on it. However, I was able to discern a difference in my ears when the change was >1.25 hpa in a relatively short span (very slight tingling in the ears). So, it’s conceivable I could subconsciously be aware of very small pressure changes.

However, when going up or down hills, changes in air pressure are much more drastic. Much more drastic than acceleration or deceleration of the same amount (>0.5 hpa change, or more than a 4 meter/13 foot hill). So there you have it. I finally solved a very old curiosity I’ve had, assuming that I can subconsciously tell the difference between very small air pressure changes.

Side note: this doesn’t work in environments that have a very controlled internal pressure — like airplanes. Cars, trains, etc., are usually not controlled in any sort of way. In fact, pressure drops pretty significantly once at speed (see Bernoulli’s Principle and how fast moving air across all the small holes on the outside of the vehicle would effect the pressure inside the vehicle.)

This is by no means scientific and may even already be well-known to some people, but I found my empirical observations enlightening. If anyone out there does so happen to scientifically study this, drop me an email, I’d love to read a proper paper on this.

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