You Don’t Accelerate When Free Falling

Speed is relative.

Acceleration is absolute.

If two objects in space are moving closer to, or further away from, each other at a constant speed, there’s no way to know which once is moving because speed is relative.

But, if two objects in space are moving closer to, or further away from, each other at a variable speed, we can figure out which one (or both) are changing speed because acceleration is absolute. This is how accelerometers work… they measure the absolute change in acceleration. 

Keep in mind that acceleration isn’t just a change in speed, acceleration can also be a change in direction. An object can move at the same speed, but still experience acceleration if it changes direction. 

We say that an object dropped from a building accelerates at 9.8 m/s², but that’s actually not the case. If we drop an accelerometer from a building, it will register 0g’s of acceleration on its way down and this is completely accurate. What’s really happening is that, since spacetime is warped, the ground is moving toward the dropped object at an accelerated rate. 

To picture this, think of a satellite in orbit, such as the ISS. It, too, seems to be accelerating at 9.8 m/s² as it falls toward earth, but its speed doesn’t change. Instead, it’s literally following an unaccelerated straight line through spacetime. Whether falling from a building or orbiting earth, both are weightless conditions and, therefore, there’s no acceleration.

Another way to think of this is imagine traveling in the passenger seat of a car that’s moving at a constant speed of 25 mph. You can easily throw a tennis ball up and catch it without any problems. This is because there’s no change in forward or lateral speed; hence, no acceleration. Now, continue to do this while the car is making a turn at a constant speed of 25 mph through an intersection. The ball will appear to move to the side window during the turn; and the further the ball is from the side of the car, the faster it will move relative to the window. This will give the ball the appearance that it’s accelerating but it's actually the car that's accelerating around the ball.

The next big question to answer is how and why does matter curve spacetime?