Here’s something that is, to me, a paradox.
❶ A radio signal leaves Earth and is received by the inhabitants of Proxima Centauri b in just under 4 years and three months.
❷ The inhabitants of Proxima Centauri b respond immediately and the reply is received on Earth in, again, just under 4 years and three months.
(Total time about 8 years and 6 months.)
❸ A space ship crewed by living human beings traveling at the speed of light and traveling neck-and-neck with the radio signal above leaves Earth at the same time.
❹ To the crew, they apparently arrive at Proxima Centauri b long after the radio signal; even if they turn around immediately they will return to Earth X number of years + ≈ 4 1/2 years after the Proxima Centauri b reply was recieved.
But they were traveling at the same speed as the radio signal !!!
There has to be something we humans are missing.