According to Greene, "the gravitational force allows us to declare that all observers - regardless of their state of motion - are on absolute equal footing. Even those whom we would normally think of as accelerating may claim to be at rest, since they can attribute the force they feel to their being immersed in a gravitational field."

 

I can understand how we're all under the same force, so we're all in the same reference frame with respect to that force. But I don't see how that allows acceleration to be nulled. I thought that if I travel at 60 mph to work, while my buddy travels 40 mph on his way to work, that he has aged more than me - albeit in billionths of billionths of a second probably.

 

Anyone care to explain what I'm missing here?

I think you're mission the acceleration of both yourself and your friend to and from your speeds, but I'm very tired so probably missunderstood the situation