# Relativity struggle [duplicate]

This question is an exact duplicate of:

Ok so there is something that I don't really understand , suppose we have the proper time A :8 years and gamma factor 2. Now I understand that the time slow downs for the relative time (t') so the relative time (dilated time) is 16 years. However guy who is moving in the t' frame is actually 8 years old and the guy in proper frame is 16 years which is opposite from what is said above so why do we say that t' is 8 years old if his dilated time is 16 ? How can we transform between the age and time ? Maybe my misunderstanding is the fact that I don't interpret t and t' in the good way ?

What I understand from this is that A: 16(t')=2x8(t) so in john frame after 16 years Mary age by 8 years ? Isn't t' the dilated time or is it the age what is it specifically and what do they mean in the book exactly

## marked as duplicate by AccidentalFourierTransform, ZeroTheHero, sammy gerbil, Qmechanic♦Jun 11 '18 at 2:03

This question was marked as an exact duplicate of an existing question.

• Hi Miguel, you need to say what you don't follow about the answers to your earlier (duplicate) question: physics.stackexchange.com/q/410993. Also, including pictures of book text that, on my phone and tablet are constantly at right angles to your text no matter how I spin the tablet, will unfortunately not help you get an answer. – user197951 Jun 10 '18 at 18:37
• I see I just think I didn't explain well on my last post so I am correcting the last one – Miguel Guevara Valencia Jun 10 '18 at 18:50
• Possible duplicate of Struggling with relativity – sammy gerbil Jun 11 '18 at 1:27
• Possible duplicate of first comment??? – Miguel Guevara Valencia Jun 11 '18 at 2:12

I think what you don't understand is symmetry of relativity.

If John's time (t) ticks 2 times faster then Mary's (t2), then John will age 2 times faster then Mary.

So 16 years in John's frame will mean 8 years in Mary's frame.

So if John got older by 16 years then Mary got older by 8 years.

Let's say they are twins. So if John becomes 16, Mary will only be 8 years old.

The problem with this is that it only works in this way in General relativity.

In special relativity, this relation would be symmetric, so if John would say Mary is getting older, Mary could say John is getting older and both would be wrong, they would age the same.

In general relativity, in this case Mary should be in a stronger gravitational field, and so under acceleration, and that is absolute. That is why Mary would age less, because a stronger gravitational field will make Mary move in the time dimension slower.

• Oh alright is just confusing because if t'=2x16 it equals 32 but it only means her time is dilated so for her to be 16 she travels 32 years , I think its only the math that is confusing because there seem to be no way of seeing mathematically that if earth frame guy is 16 years old then the spaceship frame is 8 years you just have to assume by what you said earlier – Miguel Guevara Valencia Jun 11 '18 at 4:49
• Or maybe yes there is you just have to look at the other frame.. – Miguel Guevara Valencia Jun 11 '18 at 4:50