# Homework Help: Relativity: Message Intervals

1. Jun 21, 2011

### zerakith

1. The problem statement, all variables and given/known data
During a space flight an astronaut communicates with earth by sending radio signals
at regular intervals. If he travels out and back at a speed of 0.9c. What is the ratio
of the frequencies of receipt by a receipt by a terrestrial observer on the outward
and return journeys?

2. Relevant equations

3. The attempt at a solution
Ok so in the frame of the traveller the time between intervals is the proper time and thus we can use the equation which will give the time between signals being transmitted by the traveller. This will be:

$$\Delta t=\gamma \Delta t'$$

To find out the difference in receipt times the difference in distance between subsequent emissions needs to be found. This is where I struggle, I get confused as to whose frame this should be done in, and how to get this into a ratio of frequencies.

Zerakith

2. Jun 21, 2011

### AJKing

Hey Zerakith, I'm new here (and to physics in general) but does relativity really fall under "Introductory" material?

You'll probably get responses here, I'm just saying that you might get more elsewhere.

3. Jun 21, 2011

### vela

Staff Emeritus
Look into the relativistic Doppler effect.

AJKing, yes, this is an introductory physics problem. The advanced physics forum is for material from upper-division physics courses.

4. Jun 21, 2011

### zerakith

Thank you for the response, my understanding of the situation was that these were burst signals sent at set intervals apart. Now that i think about it this is most likely equivilant to considering points of constant phase, as in the Relativistic Doppler stuff.

As for the category, i'm glad I got it right I am always a little unsure as to where introductory crosses over to Advanced, i would have thought University level would be counted as advanced but I don't think that's generally the case, i think (for the UK) it is probably about 3 year UG and above.

Thanks a lot.
Zerakith