The Twin Paradox
One of Special Relativity's Most Important Thought Experiments
In Special Relativity, will a moving person age slower than a stationary one? This is a question which has been tossed around for years, with very interesting results.
One of the fundamental results of Albert Einstein's special theory of relativity is a phenomenon known as time dilation. Simply put, this result states that time moves more slowly for a moving object than it does for an object at rest.
As Einstein put it in his original paper: If two clocks are synchronized while in close proximity to each other, then one of them is taken away for some time, perhaps on a journey, then they are brought together, they will no longer be in tune with each other. The clock which has been in motion will have recorded time more slowly than the clock at rest.
The Paradox
This basic principle of special relativity seems perfectly suited to a very traditional, very simple thought experiment:
The characters are two twin brothers, one of whom is adventurous, one not so much. Upon growing up, the adventurous twin becomes an astronaut while the other one desides to spend his adult life firmly planted on Earth. Off the astronaut twin goes on his mission in an extremely fast rocket.
The first twin spends a long while in space, traveling very quickly to the far reaches of space, then turns around and flys back to Earth just as quickly, only to find that upon arriving, time dilation has occurred and his brother has aged considerably while he himself has only noticed a few years passing by.
On the surface, this seems like a perfectly natural (albeit counterintuitive) result from special relativity. Looking a bit deeper, however, one can easily see why this is such a paradox.
Special relativity claims that all inertial frames are equally valid - that there can be no "preferred" frame of reference. In the story of the twins, though, this doesn't seem to hold up. The twin in the spaceship is said to be "moving" while the other is "stationary." From the perspective of the spaceship, however, doesn't it also seem plausible that it could be the Earth itself which is moving while the ship remains motionless?
Thus, from the perspective of the Earth, it might appear that the clock on the ship is actually running faster - exactly the opposite of the result predicted by Einstein and others.
So Which is it?
Does this really present a valid paradox for special relativity, though? Does this somehow represent a chink in the theoretical armour? To some, perhaps, but there are certainly explanations to how this story will turn out which seem perfectly reasonable.
The most important explanation for the twin paradox came from French physicist Paul Langevin in 1911 (who, consequently, was the first to develop this particular form of the thought experiment).
This explanation revolves around the idea of acceleration. The fact that the twin in space has undergone acceleration (which, scientifically, consists both of speeding up and of slowing down) causes it to lose its status as an "inertial frame," and thus no longer be considered equal with the inertial frame on Earth.
As a result of the acceleration, therefore, the twin in space does indeed possess motion in a relative sense to the non-accelerating twin on Earth, and thus is effected by the time dilation of special relativity. It is the idea of acceleration which causes the "symmetry breaking" between a relativistic reference frame and a non-relativistic inertial frame.
In the end, then, if Langevin's view is accepted (as it generally is), this story is not so much a paradox within special relativity, but in some ways a confirmation, as it helps to exemplify some of the fundamental features of the theory using a very accessible thought experiment.
References:
Moring, G. F. (2004). The Complete Idiot's Guide to Understanding Einstein. New York, NY: Alpha Books.
Hawking, S. (1988). A Brief History of Time. New York, NY: Bantam Books.
