"At the same time, the twin in the spaceship considers himself to be the stationary twin, and therefore as he looks back towards Earth he sees his brother ageing more slowly than himself...Ah, but in order to return to Earth, the spaceship must slow down, stop moving, turn around and go back the other way. During those periods of deceleration and deceleration, it is not an inertial frame and therefore the normal rules of special relativity don't apply. When the twin in the spaceship turns around to make his journey home, the shift in his frame of reference causes his perception of his brother's age to change rapidly: he sees his brother GETTING SUDDENLY OLDER. This means that when the twins are finally reunited, the stay-at-home twin is the older of the two." http://topquark.hubpages.com/hub/Twin-Paradox
David Morin, Introduction to Classical Mechanics, Chapter 11, p. 14: "Twin A stays on the earth, while twin B flies quickly to a distant star and back...For the entire outward and return parts of the trip, B does observe A's clock running slow, but ENOUGH STRANGENESS occurs during the turning-around period to make A end up older." https://scholar.harvard.edu/files/david-morin/files/cmchap11.pdf
The "enough strangeness" occurring during the short turning-around period and "suddenly" changing the age of all people on the earth sounds preposterous, and yet this is just a euphemism. The original, Einstein's 1918 homogenous gravitational field, is immeasurably more preposterous:
Albert Einstein 1918: "A homogenous gravitational field appears, that is directed towards the positive x-axis. Clock U1 is accelerated in the direction of the positive x-axis until it has reached the velocity v, then the gravitational field disappears again." http://sciliterature.50webs.com/Dialog.htm
That is, as the traveling twin turns around (but considers himself to be at rest), he finds that a homogeneous gravitational field appears and this homogeneous gravitational field causes the earth, with his brother and all other stationary people on it, accelerate towards him until it reaches speed v. Then the field disappears.
Why is the gravitational field "homogeneous"? Because otherwise it would not go up to the earth and would not cause its acceleration. Also, if the field is homogeneous, calculations are much easier.
Einstein's 1918 homogenous gravitational field is perhaps the most preposterous concept in the history of science. Einsteinians feel some shame and never teach it explicitly.
David Morin, Introduction to Classical Mechanics, Chapter 11, p. 14: "Twin A stays on the earth, while twin B flies quickly to a distant star and back...For the entire outward and return parts of the trip, B does observe A's clock running slow, but ENOUGH STRANGENESS occurs during the turning-around period to make A end up older." https://scholar.harvard.edu/files/david-morin/files/cmchap11.pdf
The "enough strangeness" occurring during the short turning-around period and "suddenly" changing the age of all people on the earth sounds preposterous, and yet this is just a euphemism. The original, Einstein's 1918 homogenous gravitational field, is immeasurably more preposterous:
Albert Einstein 1918: "A homogenous gravitational field appears, that is directed towards the positive x-axis. Clock U1 is accelerated in the direction of the positive x-axis until it has reached the velocity v, then the gravitational field disappears again." http://sciliterature.50webs.com/Dialog.htm
That is, as the traveling twin turns around (but considers himself to be at rest), he finds that a homogeneous gravitational field appears and this homogeneous gravitational field causes the earth, with his brother and all other stationary people on it, accelerate towards him until it reaches speed v. Then the field disappears.
Why is the gravitational field "homogeneous"? Because otherwise it would not go up to the earth and would not cause its acceleration. Also, if the field is homogeneous, calculations are much easier.
Einstein's 1918 homogenous gravitational field is perhaps the most preposterous concept in the history of science. Einsteinians feel some shame and never teach it explicitly.