# Measuring motion

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1. Aug 1, 2015

### Stephanus

Dear PF Forum,
I'm sorry if I ask the basic question here again. Just need confirmation.

V = 0.6; Gamma = 1.25
TRAVEL travels at 0.6c. STAY stays.
Pic 02 is Pic 01 boosted -V.
1. All STAY knows about TRAVEL is:
Proper Time
Speed
Is this true? And mutually for TRAVEL
2. At B (and C) all STAY knows about TRAVEL proper distance is the distance at B1 (and C1 distance for C).
3 . At B, STAY never knows about C. And C never knows about B. Is this true?
4. It is not relevant for STAY to calculate TRAVEL clock/calendar
For example:
At B, supposedly 1st January 2015; 10:00 AM, there's no need for STAY to calculate when is B1, because there is no certainity/assurance that at start STAY and TRAVEL clocks were synchronized.
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In short what STAY knows about TRAVEL?
A. Proper Time
B. Speed
C. Distance (late by the time that light takes to travel from TRAVEL to STAY, for example B can only calculate proper distance from B1 to STAY. STAY's X coordinate always zero, right)
D. Red/Blue shifted. But it's already known right. How can we know proper time and speed if we don't know about Red/Blue shifted.
E. Anything else?

2. Aug 1, 2015

### Staff: Mentor

It can also know the crew size of the spacecraft, and many other things. In other words: do you expect knowledge to be limited somehow?

All they can observe up to that point: right.
What do you mean with "never"? They can observe that at some point in the future.
If you don't know the timekeeping system of someone else, you cannot say which time someone else has in this timekeeping system, but that is trivial: I don't know which year the Chinese calendar is in, even without relative motion.

3. Aug 1, 2015

### Stephanus

Come on, I like to make sense the basic SR here .
Thanks. I suspected that much. But I need confirmation to know that I'm on the right track. Now I know I am. Thanks.
In the future, yes. And that implies that but at B STAY knows nothing after B1. Thanks.
So proper time only. Yes.

It's 2566. And it always advanced by 551 years from Julian calendar. But Islamic calendar is faster 1/33 to Julian calendar. I think somewhere between the year of 15.000 the Islamic calendar will catch up Julian calendar.