# Equivalence Principle

### Acceleration is equivalent to gravity.

1400 years ago people used the mass balance, however nobody knew that it doesn't work in zero-g.

The equivalence principle expresses a property which is at the basis of Einstein's general relativity: the equivalence between acceleration and a gravitational field. More precisely, observations made in a system in acceleration (e.g. a rocket) are indistinguishable from those made in a gravitational field (e.g. on Earth).

The equivalence principle tells us that gravitational mass and inertial mass are identical. This is why it is at first so difficult to disentangle the notion of weight (related with the gravitational mass) from the notion of inertia (related with the notion of inertial mass).
Gravity, Why test the equivalence principle?, 2019

There is no difference between an experiment on Earth and an experiment on an accelerating spaceship. However this is not true for zero-g. The mass balance doesn't work in zero-g. If you put a mass on just one side then it won't tilt in zero-g. If the spaceship accelerates then this mass balance will work again as if it were still on Earth. This was known recently, however this was portrayed in the Quran 1400 years before it was discovered.

[Quran 55:7] And the sky, He raised; and He set up the balance.

If God raised the sky then this an accelerating frame in outer space. And in the same verse the mass balance is on Earth. Today we know why they are in the same verse, because the mass balance only works in these two conditions, gravity or acceleration.

How could an illiterate man who lived 1400 years ago have known that the mass balance only works in gravity or acceleration?

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