Time travel can work like this according to the discovery channel:
The speed of light is impossible to break, so what happens if you are on a train and are going 99.99% the speed of light and run on the train? 99.99%(speed of train) + 1% From running on the train would break it wouldn't it(ex: if you run down a school bus aisle while it is moving, you are going faster than the bus)? Well to protect the barrier time slows down for you so your 1% running isn't 1%, thus protecting the barrier.
The hadron collider or w/e it is called proved this by finding subatomic particles acted different near the speed of light.
Like someone said before it would not feel like 600 years to those traveling, but it would be to those not traveling.
Hopefully this offers a simple explanation, I recommend watching Stephen Hawkings into the universe if you want to learn more about this stuff.