# Aging and time dilation

1. Oct 10, 2011

### jbar18

After a fairly long time of thinking I had a decent grip on the concept of time dilation, it has suddenly occured to me that I don't. My issue is with the concept of the aging twins thought experiment (or whatever it's called) where if one shoots off at some comparable speed to the speed of light they will come back and be relatively younger than the twin that didn't move.

I will quote from wikipedia: "Time dilation would make it possible for passengers in a fast-moving vehicle to travel further into the future while aging very little, in that their great speed slows down the rate of passage of on-board time."

My issue with this is that relative to each twin, they themselves do not move at all. I.e. Each will observe the other to move slower through time than themselves. So then, if one twin shoots off at, say, 0.9c, there will be significant time dilation, and they will observe their twin staying young while they themselves age. My question is, how is it possible for both twins to witness each other age slower than themselves? In other words, who would be older when the twin came back?

I've read through many threads like this in the past and nodded my head thinking I understood, but now it seems apparent that I don't.

2. Oct 10, 2011

### jbar18

After a quick bit of research, I have found an analogous problem for length contraction called the "ladder paradox" thought experiment. I suppose this is kind of the same as that, but for time dilation. I still don't get it though. Can someone explain it so that this simpleton can understand??

3. Oct 10, 2011

### Staff: Mentor

4. Oct 10, 2011

### thenewmans

jbar18, Maybe I can save you some time from reading that other thread. I think it takes them a while to get to the point. Your question is exactly why Einstein was asked. And if I recall correctly, it took him a couple months to answer. To me, the importance of this question doesn't happen until the twins get back together. In that case, the travelling twin has to turn around. That's what makes them different. If you try keeping tract of simultaneity from the travelling twin's point of view, then simultaneity jumps forward quite a bit along the earth bound twin's timeline. So in a way, you could say that the earth bound twin does a lot of aging during the traveler's turnaround. Does that help?