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Comment by mikewarot | Hacker News Reader

Remember how there was a thought experiment where you couldn't tell the difference between accelerating at 1G in an elevator, or being in a 1g gravity field?

If the elevator is more than a few CM taller than the clocks, now you can. Bolt one to the floor, the other to the ceiling.

If they run at the same rate, it's 1g acceleration. If there is the predicted difference, you're in a 1g field.

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You can't tell the difference between 1g acceleration and a uniform gravitational field. The earths gravitational field is not uniform but radial.

ELI5 time - where would a truly uniform gravitational field exist?

Nowhere. Gravity is just inherently radial. (But if you have a very large spread it can look fairly uniform, of course. Especially if your measure of “flat” is actually curved to the same radius.)

Nowhere in the real world, but that’s okay since we are dealing with thought experiments. An infinite plane of mass is roughly what you’d need so that the contributions of each element add up to a uniform field.

Nowhere, i.e. if you are in the falling elevator the tennis balls will fall downwards if it's uniform but any real gravitational field (apart from something of infinite radius) will make them converge gradually towards eachother as they fall towards the CoG

In the pasture where I keep my spherical cows.

Why would that be? What if the acceleration happens from the top of the elevator?

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