I have only a very vague awareness of the idea of relativity but I’m aware that there’s a concept that people in orbit experience less time than those on the planet due to gravity, in some way.
Does this mean that the idea of “now”, as in a moment that is right now, is marginally different for people in other places? Are they experiencing a moment that is in my objective future/past, in a mathematical sense?
In: Physics
I’m going to ignore gravity because that stuff is difficult.
You’re moving at certain speed and direction. Everyone who is moving the same speed and direction as you is in the same “inertial reference frame” and you all experience time the same way. Your clocks all tick at the same rate. You age at the same rate. You all have the same concept of “now”.
If you and the other people watch someone in a different “inertial reference frame” you’ll all see that their clocks are moving slower and that they are aging more slowly.
Weirdly, those people in the other inertial reference frame will say the same about you. They’ll say it’s your clock that is moving slowly.
How to resolve this difference. How can you both be correct. It seems like there is a “twin paradox”. A pair of twins are born. One of them gets on a spaceship and travels very far very fast. Which twin is older? There isn’t an answer and there doesn’t have to be. The twins are far separated and nothing they do directly affects the other. The younger twin can’t use his smooth hand to feel the older twin’s wrinkled face. So long as that can’t happen, we don’t have to agree about which one is older. And even the twins don’t agree. They both think the other is younger. Both will disagree about who had their 40th birthday first. The order of events is different.
But what if one of them turns around and comes back? When they meet again don’t they need to agree which one of them has more wrinkles so the young hand can touch the old face? They do have to agree.
People in different inertial reference frames don’t have to agree about the order of events if the events don’t happen at the same location. They only have to agree when multiple events occur at a single location.
So how do we determine which twin is older? One of the twins changed his inertial reference frame. If they both stayed in the same inertial reference frame they could keep disagreeing about how much time had passed for each other. But in order to reunite, one of them changed inertial reference frames, and that moving between inertial reference frames changes the calculations.
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