What is the twin paradox, and how is it resolved?

The twin paradox is a thought experiment in special relativity. One twin makes a space voyage at relativistic speeds and returns to find that the twin who stayed on Earth has aged more. It seems puzzling because each twin sees the other as moving, so a naive use of time dilation suggests each should find the other aged less. Physicists say it is not a true paradox, because the situation is not symmetrical and there is no logical contradiction. [1]

A standard textbook example uses a speed at which the factor gamma is 30. A trip lasting 2 years for the astronaut lasts 60 years in the Earth frame. An astronaut who leaves at age 40 returns at 42, while the Earth twin would be 100. From the astronaut's window, Earth's clocks seem to run slow by the same factor, so the two accounts appear to conflict. [2]

The paradox arises because at any moment the traveler's clock runs slow in the Earth twin's inertial frame, while the relativity principle seems to allow the reverse argument as well. [1]

The Earth twin stays at rest in a single inertial frame throughout. The traveler must switch from a frame moving away from Earth to one moving toward it. Only the traveler feels acceleration, which an accelerometer could measure. This breaks the symmetry, so the two twins must be analyzed with different methods even though both perspectives predict the same age difference. [1] [3]

A Scientific American worked example shows how the accounts agree. Each twin sees the other's clock run at half speed on the outbound leg. On the return leg each sees the other's clock run at double speed. Both agree at the end that the traveler's clock reads 16 years and the homebody's reads 20. The asymmetry is that the traveler leaves Earth's reference frame and comes back, while the homebody never leaves. [4]

Explanations have varied since Paul Langevin's in 1911. They fall into two broad groups: those that stress differing standards of simultaneity in different frames, and those that name the traveler's acceleration as the main reason. Max von Laue argued in 1913 that the switch between two inertial frames is what matters. Albert Einstein and Max Born used gravitational time dilation to treat the ageing as an effect of acceleration. [1]

Against the acceleration view, the effect also appears in a version with no turnaround acceleration. An outbound traveler and an inbound traveler pass each other and transfer a clock reading, and the result is the same. In that version, the sources say, what matters is the length of each path through spacetime, measured as proper time, rather than how bent it is. Some textbooks still give acceleration as the sufficient reason for the asymmetry but note that this is incomplete, since the paradox can be posed without it. Neither general relativity nor acceleration is needed for the effect. [1] [3]

The OpenStax textbook presents acceleration as the key difference and adds that general relativity also predicts the astronaut ages less. The search results do not say which view most specialists hold, though the relay version is presented as showing that acceleration is not essential. [2]