Do Astronauts Age More Slowly?

From Prof. John D Norton of Pittsburgh University answers (more here):

Do astronauts age more slowly?

Image credit: NASA/Sandra Joseph and Kevin O'Connell Nov. 16, 2009.
Downloaded from http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts129/multimedia/gallery/09-11-16-4.html Dec 26 2011

YES

According to Einstein's special theory of relativity, all processes slow down when a system moves at high speed. The result applies to astronauts since they are moving rapidly. The amount of slowing is so slight as to be imperceptible for ordinary speeds. It becomes very significant when we get close to the speed of light:

An astronaut is really just a quick way of saying "someone who travels away from the earth at high speed and returns."

Car at 100 miles per hour	Rocket at earth's escape velocity (7 miles per second)	Rocket at 100,000 miles per second (53% speed of light)	Rocket at 185,800 miles per second (99% speed of light)
Lose 0.35 seconds in 1,000,000 years	Lose 0.022 seconds in 1 year (Astronaut is 0.022 seconds younger on returning after a one year trip.)	Astronaut metabolism slows to 84% of normal.	Astronaut metabolism slows to 4.5% of normal. (One year journey=aging 16 days)
Small effect...			...large effect

How can special relativity know that these effects will happen? They arise directly from the basic supposition of the theory: all uniformly moving observers must measure the same speed for light.--186,000 miles per second.

At first this seems impossible. Say I send out a light signal from earth. I measure its speed at 186,000 miles per second.

What about another observer that chases after the light signal at, say, half the speed of light. Shouldn't that observer see the light signal slowed to half its speed? All our common sense says yes. Special relativity says no.

How can that be? Something in our common sense assumptions must be wrong. There is not much room to look for the mistake. We find the speed of the light signal with just two instruments: a measuring rod to determine how far the light signal goes; and a clock to measure how long it takes to go that far. Classically we assume that neither is affected by rapid motion. At least one of these assumptions must be wrong if the speed of light is to remain constant. When we work through the details we find that both are: the rod shrinks in the direction of motion and the clock slows.

So rapidly moving clocks slow. How does that get to a rapidly moving astronaut aging more slowly. An astronaut's metabolism is a clock. You can use your pulse to time things if you like. So that metabolism clock must slow too. The legend is that Galileo used his pulse to time the period of a slowly swinging lamp while not attending to a cathedral mass and thereby arrived at the famous result of the isochrony of the pendulum, which just says that the period of a pendulum is fixed by its length. His pulse was the simple clock used to time the pendulum.