# Theory of relativity paradox? [duplicate]

I have seen the classical twin paradox before. It uses a twin stationary on Earth and the other traveling away and back. I have seen many contradictory solutions for it, some use general relativity, others use special relativity, either way, I have never been satisfied with what I've read. They always try to break the symmetry through the traveling twin's acceleration and deceleration, but never quite succeed.

So, let's do away with the classical twin paradox and let's explain a much simpler, perfectly symmetrical version of it where both twins are moving towards each other.

So imagine we have Twin A in a spaceship, and Twin B in another. They are both traveling at the same speed towards each other.

If I understand relativity properly:

• From Twin A's frame of reference, he's stationary and Twin B is moving at a constant speed towards him, therefore, Twin B is aging slower.
• From Twin B's frame of reference, he's stationary and Twin A is moving at a constant speed towards him, therefore, Twin A is aging slower.

When they both finally meet, they both think that the other is younger. Which one of them is right?

## marked as duplicate by ACuriousMind♦, user36790, John Rennie special-relativity StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Feb 15 '16 at 12:28

• Depends on how old they actually are. – user106422 Feb 14 '16 at 21:47
• This is the well-known twin paradox. Look at How is the classical twin paradox resolved? to see its resolution. – ACuriousMind Feb 14 '16 at 21:49
• The usual assumption is that they observe each others' ages accurately, so kid's comment is the whole story. If you believe their observations can be inaccurate (as your final question suggests) then of course this question has nothing to do with relativity and everything to do with whatever assumptions you care to make about the competence of the observers. – WillO Feb 14 '16 at 22:55
• @ACuriousMind I have seen that paradox before. It uses a twin stationary on Earth and the other traveling away and back. I have seen many contradictory explanations for it, some use general relativity, others use special relativity, either way, I have never been satisfied with what I've read. They always try to break the symmetry through the traveling twins acceleration and deceleration, but never quite succeed. Do away with the classical twin paradox and explain this much simpler, perfectly symmetrical version I just gave where they both move towards each other. Can someone give it a shot? – AxiomaticNexus Feb 16 '16 at 3:09