# Special Relativity Question

I have two problems with the following paragraph from your article on special relativity.

"Imagine a 15 year-old traveling at 99.5 percent the speed of light for 5 years (from the astronauts perspective). When the 15 year-old gets back to earth, according to NASA, he would be only 20 years old. His classmates, however, would be 65 years old".

Problem 1: To an observer on earth, the astronaut is in motion. To the astronaut, the earth is in motion. That implies that both the observer and the astronaut would be aging more rapidly in the other person's system. That would make the astronaut the same age as his classmates.

Problem 2: If the astronaut is traveling for 5 years and ends up on earth, this implies a round trip. On a round trip the effects encountered going away should be cancelled by the effects encountered on the return trip. That would make the astronaut the same age as his classmates.

Where has my thinking gone wrong?

#### marty369

I have two problems with the following paragraph from your article on special relativity.

"Imagine a 15 year-old traveling at 99.5 percent the speed of light for 5 years (from the astronauts perspective). When the 15 year-old gets back to earth, according to NASA, he would be only 20 years old. His classmates, however, would be 65 years old".

Problem 1: To an observer on earth, the astronaut is in motion. To the astronaut, the earth is in motion. That implies that both the observer and the astronaut would be aging more rapidly in the other person's system. That would make the astronaut the same age as his classmates.

Problem 2: If the astronaut is traveling for 5 years and ends up on earth, this implies a round trip. On a round trip the effects encountered going away should be cancelled by the effects encountered on the return trip. That would make the astronaut the same age as his classmates.

Where has my thinking gone wrong?
if i have this correctly understood then here goes Einstein predicted that the closer you get to sol the slower time gets so since the astronaut is traveling so near to the sol time has slowed down so substantially for him only 5 years have past but from the perspective of his classmates time has passed 10 fold and I'm sure there's a formula to back this up and when you observe light coming from distant systems you get to a certain point when light times the expansion of the cosmos dictates that time is compounded and gets to the point where the observable universe will never be seen in its entirety by any means

#### Helio

That would make the astronaut the same age as his classmates.
Yes, but that means they both will be ignoring what they are observing and recognizing, mentally, namely that they both would have to understand what is happening that relativity applies to both observed motions.

Problem 2: If the astronaut is traveling for 5 years and ends up on earth, this implies a round trip. On a round trip the effects encountered going away should be cancelled by the effects encountered on the return trip. That would make the astronaut the same age as his classmates.
Right, but in order to travel from Earth and back requires something the Earth doesn't encounter... acceleration. The acceleration experience makes the difference, from what I am told, but don't understand, admittedly. If travel is uniform to a destination, then comes deceleration, then acceleration to get back. This has the same effect as if the travelers made one big circle since circular motion also requires acceleration (angular).

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