# Time dilation, reference frames

1. Jun 4, 2013

Hi,
Basic question.
I'm confused by a time dilation example (37.3 in Young and Freedman 11th ed.). Mavis is moving at .600c relative to earth-bound Stanley, and at the instant she passes, both start timers. Part b asks "At the instant when Mavis reads .400 s on her timer, what does Stanley read on his?" The answer they get is .320 s.

My question is, doesn't this depend on what reference frame you're in? I think for the .320 s answer you'd need to assume we're in Mavis' frame. (Somehow the textbook's reasoning is not transparent to me.)

2. Jun 4, 2013

### ghwellsjr

You don't sound confused to me. You got everything correct.

3. Jun 4, 2013

### ghwellsjr

Here's a couple spacetime diagrams depicting the scenario from the two Inertial Reference Frames (IRF's) under consideration. First is Stanley's earth frame. Stanley is shown in blue with dots every tenth of a second of his Proper Time and Mavis is shown in red with similar dots:

You can see that when Mavis's red clock is at 0.4 seconds, Stanley's blue clock would be at 0.5 seconds (but I didn't drawn that in).

Now for Mavis's rest frame:

Now you can see that when Mavis's red clock is at 0.4 seconds, Stanley's blue clock is at 0.32 seconds.

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4. Jun 4, 2013

Thanks a bunch, ghwellsjr, much appreciated. I wasn't sure if I was going to be reassured or if I was somehow wrong. Definitely reassured.

5. Jun 4, 2013

### ghwellsjr

You're welcome.

6. Jun 4, 2013

### bobc2

I thought the wording implied that the author was talking about Mavis's instant of time (Mavis's simultaneous space), which means at Mavis's instant of time and in her instantaneous 3-D world, Stanley was seeing 0.320 s on his clock. I don't see how you could interpret this as Stanley's instant.

7. Jun 5, 2013