Editing Twin paradox (section)

==What it looks like: the relativistic Doppler shift==

In view of the frame-dependence of simultaneity for events at different locations in space, some treatments prefer a more phenomenological approach, describing what the twins would observe if each sent out a series of regular radio pulses, equally spaced in time according to the emitter's clock.<ref name='Kogut'/> This is equivalent to asking, if each twin sent a video feed of themselves to each other, what do they see in their screens? Or, if each twin always carried a clock indicating his age, what time would each see in the image of their distant twin and his clock?

Shortly after departure, the traveling twin sees the stay-at-home twin with no time delay. At arrival, the image in the ship screen shows the staying twin as he was 1&nbsp;year after launch, because radio emitted from Earth 1&nbsp;year after launch gets to the other star 4&nbsp;years afterwards and meets the ship there. During this leg of the trip, the traveling twin sees his own clock advance 3&nbsp;years and the clock in the screen advance 1&nbsp;year, so it seems to advance at {{frac|3}} the normal rate, just 20&nbsp;image seconds per ship minute. This combines the effects of time dilation due to motion (by factor {{nowrap|''α'' {{=}} 0.6}}, five years on Earth are 3&nbsp;years on ship) and the effect of increasing light-time-delay (which grows from 0 to 4&nbsp;years).

Of course, the observed frequency of the transmission is also {{frac|3}} the frequency of the transmitter (a reduction in frequency; "red-shifted"). This is called the [[relativistic Doppler effect]]. The frequency of clock-ticks (or of wavefronts) which one sees from a source with rest frequency ''f''<sub>rest</sub> is
:<math>f_\mathrm{obs} = f_\mathrm{rest}\sqrt{\left({1 - v/c}\right)/\left({1 + v/c}\right)}</math>
when the source is moving directly away. This is ''f''<sub>obs</sub> = {{frac|3}}''f''<sub>rest</sub> for ''v''/''c'' = 0.8.

As for the stay-at-home twin, he gets a slowed signal from the ship for 9&nbsp;years, at a frequency {{frac|3}} the transmitter frequency. During these 9&nbsp;years, the clock of the traveling twin in the screen seems to advance 3&nbsp;years, so both twins see the image of their sibling aging at a rate only {{frac|3}} their own rate. Expressed in other way, they would both see the other's clock run at {{frac|3}} their own clock speed. If they factor out of the calculation the fact that the light-time delay of the transmission is increasing at a rate of 0.8&nbsp;seconds per second, ''both'' can work out that the other twin is aging slower, at 60% rate.

Then the ship turns back toward home. The clock of the staying twin shows "1&nbsp;year after launch" in the screen of the ship, and during the 3&nbsp;years of the trip back it increases up to "10&nbsp;years after launch", so the clock in the screen seems to be advancing 3&nbsp;times faster than usual.

When the source is moving towards the observer, the observed frequency is higher ("blue-shifted") and given by
:<math>f_\mathrm{obs} = f_\mathrm{rest}\sqrt{\left({1 + v/c}\right)/\left({1 - v/c}\right)}</math>
This is ''f''<sub>obs</sub> = 3''f''<sub>rest</sub> for ''v''/''c'' = 0.8.

As for the screen on Earth, it shows that trip back beginning 9&nbsp;years after launch, and the traveling clock in the screen shows that 3&nbsp;years have passed on the ship. One year later, the ship is back home and the clock shows 6&nbsp;years. So, during the trip back, ''both'' twins see their sibling's clock going 3&nbsp;times faster than their own. Factoring out the fact that the light-time-delay is decreasing by 0.8&nbsp;seconds every second, each twin calculates that the other twin is aging at 60% his own aging speed.

[[Image:rstd4.gif|thumb|Light paths for images exchanged during trip<br>Left: Earth to ship. Right: Ship to Earth.<br>
Red lines indicate low frequency images are received, blue lines indicate high frequency images are received]]
The ''x''–''t'' (space–time) diagrams at right show the paths of light signals traveling between Earth and ship (1st diagram) and between ship and Earth (2nd diagram). These signals carry the images of each twin and his age-clock to the other twin. The vertical black line is the Earth's path through spacetime and the other two sides of the triangle show the ship's path through spacetime (as in the Minkowski diagram above). As far as the sender is concerned, he transmits these at equal intervals (say, once an hour) according to his own clock; but according to the clock of the twin receiving these signals, they are not being received at equal intervals.

After the ship has reached its cruising speed of 0.8''c'', each twin would see 1&nbsp;second pass in the received image of the other twin for every 3&nbsp;seconds of his own time. That is, each would see the image of the other's clock going slow, not just slow by the {{nowrap|''ε''}} factor 0.6, but even slower because light-time-delay is increasing 0.8&nbsp;seconds per second. This is shown in the figures by red light paths. At some point, the images received by each twin change so that each would see 3&nbsp;seconds pass in the image for every second of his own time. That is, the received signal has been increased in frequency by the Doppler shift. These high frequency images are shown in the figures by blue light paths.

===The asymmetry in the Doppler shifted images===
The asymmetry between the Earth and the space ship is manifested in this diagram by the fact that more blue-shifted (fast aging) images are received by the ship. Put another way, the space ship sees the image change from a red-shift (slower aging of the image) to a blue-shift (faster aging of the image) at the midpoint of its trip (at the turnaround, 3&nbsp;years after departure); the Earth sees the image of the ship change from red-shift to blue shift after 9&nbsp;years (almost at the end of the period that the ship is absent). In the next section, one will see another asymmetry in the images: the Earth twin sees the ship twin age by the same amount in the red and blue shifted images; the ship twin sees the Earth twin age by different amounts in the red and blue shifted images.