One twin sets off on a long space voyage traveling at 0.9 c, while the other stays on Earth. When she returns 60 years later, the traveling sister has hardly aged at all, while her twin has aged 60 years. What aspect of relativity is demonstrated in this situation?
a. mass-energy equivalence
b. time dilation
c. length contraction
d. mass increase

b. time dilation

Explanation:

Time dilation is the change in time which is observed when an object is moving near the speed of light. Time flows slower for people which are moving. Time dilation is given by the equation.

where,

= Moving observer's period of the clock

= Time passed in frame of the clock itself.

v = Speed at which the person is moving

is always greater than .

So, here when one twin returns the other twin is 60 years old is due to time dilation.