Twin paradox

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[Jim]: When you saying clock rates change and the rate is measured how are the changes measured?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

The way any clock rate is measured -- by counting ticks.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I presume the traveling of the twins is just a story or mind game of some sort-I am only guessing but I don't know of any data that has been assembled on such a voyage.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

It has never been done with human twins, but is done every day with twin atomic clocks. Those in orbit tick at very different rates from those on the ground, by just the amount expected by relativity.

No one familiar with data doubts anymore that relativistic effects are real. Our discussions are about how best to explain them in simple physical models that make sense. -|Tom|-

Is the rate measured by a shift in the frequency of the signal that is used to communicate with the satelite? Sometime a shift toward blue and other time red.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[Jim]: Is the rate measured by a shift in the frequency of the signal that is used to communicate with the satelite? Sometime a shift toward blue and other time red.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

No. That's a doppler shift, and can be used to measure velocities.

If you are wondering how clock ticks are measured for moving clocks, that is easy for GPS satellites. In effect, an observer on the ground reads the satellite clock and his own clock at some moment. He then waits until the orbiting satellite next returns to the same spot and reads both clocks again. He can then compare their rates.

In practice, we don't have to wait to make a rate measurement because all satellites are continually under observation by ground stations all around the Earth. So any rate changes would be spotted as soon as they occurred. -|Tom|-

So the two clocks run at different rates only because the one on the satelite is moving and the one on Earth in not moving. I guess that if the clock in the satelite could be returned to Earth it would be telling a different time than the Earth bound clock. Is that right? How many seconds apart would the two be set at after 100 years or so?

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[Jim]: So the two clocks run at different rates only because the one on the satelite is moving and the one on Earth in not moving.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

It would be more to the point to say the two clocks have a relative motion.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I guess that if the clock in the satelite could be returned to Earth it would be telling a different time than the Earth bound clock. Is that right?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

No need to return it to Earth. We observe satellite time at ground stations continuously. Once the range from satellite to ground is known, we can easily correct for signal propagation delay and determine the true difference between satellite and ground clocks at any moment.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>How many seconds apart would the two be set at after 100 years or so?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Because of the speed difference alone, the difference would be about 0.26 seconds in 100 years. We can express the same answer in GPS terms: the difference would be 260,000,000 nanoseconds. It is fair to express the difference this way because the clocks keep time to better than 1 nanosecond. So the time difference between ground and satellite clocks is evident in less tha one minute, and just keeps getting larger day by day, year after year. -|Tom|-

The relative motion detail is not at all clear to me. How would this kind of motion effect two clocks going the same speed in the opposite direction?