# Doppler questions

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#### willpittenger

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I am going to run several possibilities with two craft, both moving at .49 to .51 C. Please ignore any chance that the craft might hit, regardless of how tempting that would be. The discussions concern only the Doppler effect. One ship will always be sending a radio transmission to the other. This transmission might be RADAR. Please note that RADAR adds one additional Doppler cumulation. Radar beams are affected during transmission, bouncing off the target, and reception. What happens to the transmissions in each case?<br /><br />1. Start off with both ships heading towards each other at .49 C.<br />2. Have both ships accelerate to .5 C.<br />3. Have them accelerate to .51 C.<br />4. Have them turn around and go at .49 C<br />5. Again, have them accelerate to .5 C and then to .51 C.<br /><br />With the ships moving towards each other at .51 C, the cumulative effect is more than 1 C. In fact, the RADAR beams exceed 1 C at just .3333 C (rounded). If the ships are moving apart at .51 C, would the beams ever reach the target ship? <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

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#### willpittenger

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What about the RADAR beam with its extra bounce? <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

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#### MeteorWayne

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What extra bounce??? <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### willpittenger

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From my original post: <font color="yellow">Please note that RADAR adds one additional Doppler cumulation. Radar beams are affected during transmission, bouncing off the target, and reception.</font>/safety_wrapper> <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

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#### SpeedFreek

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Firstly, could you be more specific in what you are trying to establish here? For instance, the speed of light cannot be exceeded by anything within current thinking. Even if 2 objects are moving apart, both moving at the just under the speed of light, any particle or wave that they transmit will still be seen by them moving away from them at the speed of light.<br /><br />Your 2 objects, both travelling at 0.5c away from each other, both see other moving away from them at only 0.8c due to length contraction and time dilation. So a speed of light transmission from one object will still reach the other one. The same goes if they are moving apart, both travelling at 0.51 c. The transmission does not have to exceed c to be observed by the other one (I think!). This is all due to special relativity.<br /><br />In order to analyse your complicated example we also need to know how RADAR is affected during transmission (?) bouncing off the target and reception (?). What do you mean exactly?<br /><br />Why the need for bouncing radar beams? why not just use blinking lights, set to blink when they see the blink from the other ship? <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### willpittenger

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<blockquote><font class="small">In reply to:</font><hr /><p>In order to analyse your complicated example we also need to know how RADAR is affected during transmission (?) bouncing off the target and reception (?). What do you mean exactly?<p><hr /></p></p></blockquote><br />The RADAR beam would be affected by the the Doppler Effect three times. In the quote from my original article, I talked about each time: Transmission, bounce, and reception. What would the RADAR beam's frequency be when received relative to the actual frequency the transmitter is set to.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>Why the need for bouncing radar beams? why not just use blinking lights, set to blink when they see the blink from the other ship?<p><hr /></p></p></blockquote><br />Because I was looking for the cumulative Doppler Effect. Your suggestion would destroy the experiment. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

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#### willpittenger

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Why wouldn't the transmission be affected by the Doppler shift? <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

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