The equivalence principle simply puts says that you cannot tell the difference between gravity and acceleration.
keep in mind that acceleration is a constant change in speed, not a constant speed.
Given that, why is gravity so hard to understand or believe its real.
Also why is it so hard to believe that gravity can hold the atmosphere in place. Even though space is a vacuum, gravity can hold the air in place.
There is a pressure gradient, as evidenced by the thinner atmosphere in the mountains. Why is it so hard to believe that the air gets thinner and thinner the higher you go and the line between 'space' and our atmosphere is tenuous.
something else to think about, you have an unopened soda bottle. you shake it up and open it, obviously it will spew out everywhere.
Now consider this, if the soda bottle is accelerating, is there an acceleration that will prevent that bottle from spewing everywhere. Can you accelerate that bottle so fast even if you shake it up and open it, all the liquid and gas just stays in the bottle.
a typical car goes zero to 60 in 10-12 seconds, supercars can do it in 4 seconds or less.
now lets pretend we can accelerate that bottle super fast, say zero to 500 mph in 10 seconds. Do you think the liquid might stay in the bottle?
now lets think about even faster, say zero to 10,000 mph in 10 seconds (for the sake of the thought experiment, dont worry about HOW such acceleration would be achieved, just imagine the pressures that such acceleration would cause on the bottle). In such circumstances its not too difficult to imagine that even if you shake a soda bottle and open it, the bottle will remain full, all the gas and liquid will remain inside the bottle, even if the bottle is in a total vacuum.
Also note that once you reached your end speed (whether 500 mph or 10,000 mph), then the acceleration stops and now the liquid and gas would be free to leave the bottle. This also helps to demonstrate the difference between a constant speed and a constant acceleration.