I had a though last night that kept me awake trying to figure out what I was trying to understand. I have been out of school many years now so I have forgotten much.. Here is my question. Lets imagine there is a 30 meter diameter hole (complete with a vacuum) in my front yard that goes through the center of the earth all the way through to the other side. Basically, a hole through the center of the earth. This earth is not rotating around our sun, it is isolated in some space all by its self. Nor is it rotating etc. Lets imagine we have three persons.. and I am on of the three. Lets imagine there are two hollow 2 meter metal spheres in my front yard. One is sitting on the ground next to the hole, and the other is suspended over the hole at surface level by an electromagnet. Lets imagine that in each of the two spheres contains one of the people, and I am standing in the open air in my front yard watching these two spheres and such. We all have an accelerometer and a notepad and a perfect clock. One last detail, once inside the sphere, the person has no knowledge of what the situation is. It's like they woke up magically in a sphere not knowing if they were on a planet, in this universe, etc. They simply have no idea what is up except they are in a round spherical container, and they are going to try to figure out what is up. I think that sets up for my question So, I have a button to turn off the electromagnet which wakes the two people up in the spheres at the same moment, and the clock starts a timer. So I was imagining the situation for the two people in the spheres. Sphere A was over the hole and when the magnet was released he awakes to find him self in a gravity free sphere where he is thinking he is perfectly still floating somewhere in space. His accelerometer is reading zero on x,y and z. Sphere B was on the ground and thus wakes up to find him self in a sphere sitting on one side. He is a space man so he thinks he is in a ship taking off, and doesn't realize he is just sitting on the ground. So he does the math with his clock and the accelerometer and decides is traveling ever faster in the direction opposite where he is sitting. The interesting thing to me in this question is that I, the third guy, knows exactly what the situation is as I am watching it. When the falling sphere passes through the earth to the other side, it will just come back to where it started and I will catch it with the electromagnet and get the guys out of the spheres and then we will compare notes as to what we experienced. Lets imagine it took 90 minutes for the falling sphere to return to the electromagnet to be caught, the rough time it takes a satellite to orbit the earth per say, the exact number is not important I don't think. In our discussion after, over a beer, we relate our experiences. The guy in Sphere A just figured he was stationary in space while he was actually in free fall motion unpretrubed on his trip. So he just noted the 90 minutes of floating perfectly still. I noted that he traveled to the other side of the world and back. The guy in Sphere B just figured he was in a rocket and was accelerating in a straight line at 1g for the 90 minutes and figured he had traveled about 142,884km and should have been going about 190,512kph at about that time. He also calculated that since he weighed exactly 1_kg (skinny dude), that it took about 1.4 x 10^9 Joules of energy to get him going that fast. I noticed he didn't change location or speed, and no energy change occurred. So here is my question ..... I understand the guy in Sphere A could also have guessed he was in a sphere in free fall and that the guy in Sphere B could have guessed he was just sitting on a massive body with a gravity of 1g. This is the equivalency principle I believe. What I don't get is the energy change part of the Sphere B situation. Sphere A did have a kinetic energy change during his trip however he returned to the original energy and thus no net change occurred. Sphere B either 'might have' or 'might not have' had an energy change. I am not sure how to reconcile this idea. Additionally, I am thinking the Sphere A clock might have slowed down during the 90 minutes, as he was in motion. However, the clock in Sphere B could have slowed down also, or maybe not, as he may have been in motion or may not have been in motion. I guess, what I am puzzled with is that we have lived in an accelerated field all of our lives here on Earth, and if accelerated fields are the same whether gravitational or inertial, and if I lived my whole life in that Sphere B, could I be going close to the speed of light in the space inside the sphere as I have had the idea that I was accelerating all these years at 1g. As you can probably see, I am a bit confused, but interested :) Thanks for considering my question.