clocks to . When Carol passes Bob, she synchronises her clock to Bob's, . Finally, as Carol passes Alice, they compare their clocks against each other. In Newtonian physics, the expectation would be that, at the final comparison, Alice's and Carol's clock would agree, . It will be shown below that in relativity this is not true. This is a version of the well-known "twins paradox" in which identical twins separate and reunite, only to find that one is now older than the other. If Alice sends a flash of light at time towards Bob, then, by the definition of the k-factor, it will be received by Bob at time . The flash is timed so that it arrives at Bob just at the moment that Bob meets Carol, so Carol synchronises her clock to read . Also, when Bob and Carol meet, they both simultaneously send flashes to Alice, which are received simultaneously by Alice. Considering, first, Bob's flash, sent at time , it must be received by Alice at time , using the fact that the k-factor from Alice to Bob is the same as the k-factor from Bob to Alice. As Bob's outward journey had a duration of , by his clock, it follows by symmetry that Carol's return journey over the same distance at the same speed must also have a duration of , by her clock, and so when Carol meets Alice, Carol's clock reads . The k-factor for this leg of the journey must be the reciprocal (as discussed earlier), so, considering Carol's flash towards Alice, a transmission interval of corresponds to a reception interval of . This means that the final time on Alice's clock, when Carol and Alice meet, is . This is larger than Carol's clock time since provided and . Radar measurements and velocity In the k-calculus methodology, distances are measured using radar. An observer sends a radar pulse towards a target and receives an echo from it. The radar pulse (which travels at , the speed of light) travels a total distance, there and back, that is twice the distance to the target, and takes time , where and are times recorded by the observer's clock at transmission and reception of the radar pulse. This implies that the distance to the target is Furthermore, since the speed of light is the same in both directions, the time at which