That arguement is based on the unrealistic assumption that the time it would take them is the same that it would take humans with 20th century technology. Lets move into the 21st. We will soon be able to manipulate the inertial state of ships to change their natural state of motion to a state of acceleration. see-
http://home.aol.com/zcphysicsms/inertmanip.htm
Lets say we can acheive a = 1 ly/y2
(about earths surface acceleration). If you speed up half way to Alpha Centauri, slow down the other half and repeat the process to get back, then for those on the ship, the round trip would take
t' = 7 yrs 4 months.
while for those on earth it would take 11 yrs 11 months.
Moving further into the future of technology still, there are various methods of removing the limitation of the speed c as the top speed at which those on earth observe the ship to move. see-
http://home.aol.com/zcphysicsms/modernrelativity.htm

• Can you calculate stress required for 9.8 m/sec^2 acceleration? - alexander - April 4, 2001 - 21:34 UTC
  • Yes,but - fines137 - April 5, 2001 - 06:53 UTC
    • better yet, - fines137 - April 5, 2001 - 06:55 UTC
    • sign correction - fines137 - April 5, 2001 - 07:04 UTC
    • one more time - fines137 - April 5, 2001 - 07:13 UTC