Let me try again, if you were to wave good bye to your twin with an atomic clock on 1.1.01 and travel at c for 5 years and return (total ten years) with an atomic clock returning on 1.1.11. If you were 40 when you left, would you be 50 on your return, then how old would your twin be. And what would the clocks say ?

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Thank God for magnetism.

You'd be 50, your twin and his clock would be interstellar dust

Assuming your speed of (almost) c was relative to your twin

Wow. still fail to understand how c can have such an effect on time relative to the observer than the traveller.

Does this mean that any thing we are looking at in the universe over 5 light years away or less is inter galactic dust ?????

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Thank God for magnetism.

No. It's not moving away from us at the speed of light is it?

OTOH, what does "over 5 light years away or less" really mean? Aren't there things on your desk that fit that criteria?

Perhaps it might help you, Sean Clayden, to imagine that each twin carries both a watch to measure time and an odometer to measure mileage. The twin on the trip at near c has a watch that ends up recording almost no time, and an odometer that registers a huge distance. The twin who stays home has a watch that registers a large time, and an odometer that measures very little distance. Why should one of these results be more surprising than the other? The lesson of relativity is that both time elapsed and distance covered depends on the path taken to get there. The only reason we think that two cars can end up with different odometer readings but the same clock reading when later compared side by side is that cars never move at speeds anywhere close to c.

Thank you.

If you were the twin on the trip, would the watch appear to be working correctly or slow to him/her.

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Thank God for magnetism.

Correctly, to him/her

Anybody looking at their own watch would see it working normally - time is affected for everything on the trip. This includes the hands of your watch, your eyeballs, the chemistry of your brain, and the aging of your body.

However, if the twin on the trip used a telescope to look at the Earth twin's watch (or if the Earth twin used a telescope similarly), they would notice the movement of the hands being affected.

And note that's true even after correcting for time of flight effects-- the time dilation is not an illusion.