08-14-2008, 07:50 AM
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Religion: LDS
Title:meh...
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Join Date: Jul 2005
Location: Earth
Gender: 
Posts: 6,558
Frubals: 532516
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Originally Posted by Panda
Here we get to a problem. We are moving at this speed in reference to what? If a spaceship moves away from earth at half the speed of light the Earth is also moving away from the spaceship at half the speed of light, which one would experience the time dilation? This is a question no one seems to know the answer to.
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Here is a conversation I had on this topic on another forum:
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Originally Posted by soyleche
[maybe a bit of a tangent]
Here's something I've never quite been able to grasp. I'm not even sure if I can articulate my question very well, but I'll try.
With the whole time dilation (sp?) thing, we know that from my perspective someone who is moving relative to me is experiencing "slower" time. So, if Joe gets on a spaceship and travels at some high percentage of the speed of light for what I experience (staying here on Earth) a year, to him it will only be a month (actual numbers aren't important here - I'm just going for concept). When he gets back, I'll have aged a year, but he will only have aged a month. Am I right so far?
Now, lets look at this from Joe's perspective. Relative to Joe, I am the one that is moving at a high percentage of the speed of light, and so from his perspective I am experiencing "slower" time. So, what he experienced as a month would only be a week to me (again - the numbers aren't important). When he gets back, he will have aged a month, but I will only have aged a week.
So - which is it? What is wrong with what I have tried (and probably failed) to explain?
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Originally Posted by magillaG
This is called the twin paradox. The more standard argument for it in layman's terms is that one of you has to accelerate. Acceleration has a kind of "reality" to it that moving at a constant speed does not. For example, you can tell when you car is accelerating without looking outside, but you can't tell if it is moving (assuming the road is smooth enough.) This makes the two of you different. Clocks run more slowly for the one who has accelerated.
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Quote:
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Originally Posted by soyleche
Ok, then let's take acceleration out of it. Let's pretend that Joe was born on a spaceship that has been moving at a constant velocity (relative to the Earth) for his entire life. We live in a pretty small, spherical universe, so that Joe can pass by the Earth multiple times without needing to change velocity or direction (he never accelerates). Joe and I are pen-pals, so we send each other a message every time he is close enough. The rest remains pretty much the same. When Joe get's back next year (to me), has he only aged a month? If so - in that month (to Joe) have I only aged a week?
Something is still not clicking. (If we need to get the Earth's gravity out of the picture, put me on another ship going in a different direction - just as long as we can pass by each other multiple times to check each other's ages).
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Originally Posted by magillaG
Then it becomes a question of synchronization. Before you knew that you had both been born at the same time, and could synchronize your watches, since you are at the same place and not moving relative to each other. Now, you are moving relative to each other. There will be no way for you to agree on whether you watches are synchronized. If you think your watches are synchronized then he will disagree, and vice versa.
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Originally Posted by soyleche
Does synchronozation matter when we are only concerned with durration? I'm not wondering so much what our respective clocks will say. I'm wondering how much "time" has "elapsed" for each of us. For simplicity, imagine that we do actually have clocks that would tick away the exact same second if we were at rest relative to each other.
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Originally Posted by magillaG
It is the only way we can agree on how much time has passed between us. Let's say that I claim time is moving more slowly for you than for me, as relativity has to say since you are moving relative to me. Well, what I have done is take two readings on my watch, and two readings on your watch, and compared the differences. I have to take your first reading and my first reading simultaneously to me,and also the last readings. I take the first reading just as we pass by each other so that we agree that we take those at the same time. But you will disagree that I take the second pair of readings simultaneously. You will think that I took the reading on your watch much earlier than I took it on mine. You will claim this is why the time between the readings on your watch are closer together than the readings on mine. If only I had taken both of the original readings simultaneously, you will claim, then my watch would have said an earlier time than yours. (Now if you can accelerate, you change direction and come back to me, and then we can stand right next to each other and agree we are taking the final reading simultaneously.)
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There are other pretty good posts on the subject in that thread: Too fast for you.......... - Actuarial Outpost
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"In any compromise between good and evil, it is only evil that can profit." -- Ayn Rand
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