Debhoro

OK! Maybe (just maybe) I got it. So you’re saying that the satellite is actually following the geodesics like any object would do, it’s just that the geodesics is circular and not straight because of the gravity of Earth. Correct?

ummm….let’s see….the satellite orbit around Earth and doesn’t get lost in space because geodesics lines keep it there, isn’t it? So the geodesics are changing the direction of the satellite, thus are applying a force, and that means it’s accelerating. Also, doesn’t circular motion require a constant acceleration (as stated by Delta1212 above)?? I’m getting confused…

Wait, if I understood it right, on Earth we are accelerating even if we don’t feel it. What’s different with GPS satellites? They are orbiting the Earth so they are accelerating Am I wrong?

Exactly!! But what I wanted to know was (without considering gravity): why should we think GPS satellite are delayed? If we see it from the point of view of the GPS, they are stable and the Earth is moving, so the clock of the GPs is actually moving faster than clocks on Earth. I guess the answer to this is:

Thanks for your reply. Then I’ll try to imagine a different example, without acceleration or deceleration: Mr.Green is stable somewhere in the universe and Mr.Red is moving in circle around Mr.Green. As you can imagine the speed of the space ship of Mr.Red is almost the speed of light, there is no acceleration or deceleration, they are in that situation since always (pretty dumb scenario, I know…). Which one will live longer? Mr.Red is the person travelling at high speed, but from his point of view he’s stable and Mr.Green is moving around him. Would it be correct to say that both of them could state that they are going to live longer than the other person (even if they will never find it out)?

Hi, I’m not a physicist but I’m very curious about relativity, I read and studied a little and I have one big issue I can’t understand. Let’s make an example, it would be easier for me to explain my point as English is not my first language: There are 2 old friends Mr.Green and Mr.Red who are in their own space-ships in the middle of the universe, at this moment the 2 spaceships are connected through a door and Mr.Green and Mr.Red can meet each other. From the windows they can’t see anything, it’s all dark and they don’t have any reference point. Moreover the 2 space ships are moving forward at a low constant speed (let’s say 50km/h) in a straight line. The 2 friends decide to synchronize their watches and then each one return to his space ship, they close the connection door and separate the 2 space ships. Now Mr.Green is at the right side of Mr.Red and they are seeing each other through a little window. While the space ship of Mr.Green keep the same speed and direction, the space ship of Mr.Red start to accelerate and turn to the left. However, both the acceleration and the change of direction occurs very slowly and it’s impossible for Mr.Red to perceive them. The space ship of Mr.Red accelerate up to about the speed of light, executing a very big round of 360º to the left and returning to the initial point. Once there, it starts again to move in a straight line trying to reach Mr.Green and decelerating little by little. Again it’s impossible for Mr.Red to perceive the change of speed. After a while the 2 space ships are side by side and connected through the door, the 2 friends open the door and they immediately check their watches: the clock of Mr.Red should be late compared to the clock of Mr.Green, as he travelled close to the speed of light (even if he doesn’t know it as it was impossible for him to perceive it), but accordingly to the theory of relativity Mr.Red has the right to say that he was stable and it was Mr.Green who moved. The question is: the watch of which one is late compared to the other? Thanks for your help