imagine two twins let’s call them space to an and earth to n space to entrap away from earth at very high speed and returns some years later meanwhile earth twin stays in order to not miss the next and seasons of Game of Thrones under Newtonian law when space twin returns from his trip he and his brother should be the same age makes sense right it turns out that’s wrong when applying Einsteins laws of special relativity space twin will be younger than Earth twin upon his return what quick recap by 1905 Einstein had published a theory which we now refer to a special relativity it comprises three naturally occurring phenomena first time dilation or moving clocks run slower second length contraction or moving bodies shrink third relative mass gain or moving bodies are heavier these three effects become relevant for any two objects moving at more than 10% of light speed relative to each other that’s when

Newton’s laws produce error margins of more than 1% if that’s news to you and you don’t know anything about special relativity you should really watch part 1 first where I explain it from scratch back to the twins when earth tune observes fast-moving space twin he will see space twins time runs slower than his own because of time dilation therefore space twin should be younger upon his return right well it’s not that simple from the perspective of space twin he is himself at rest and earth is moving at high speed relative to him thus when he observes earth according to special relativity

he will see their time running slower so upon space twins return shouldn’t earth to and have aged less and Bianca what’s going on here this apparent paradox was used by Einsteins professional critics to argue against his theory today the paradox has multiple accepted resolutions and should be called the twin problem but well the easiest to understand resolution lies and separating the reference frames of the twins and examining how much time each twin measures on their own clock during the journey of space plan let’s start with earth twin say space twins turning point is Alpha Centauri which is four light-years away spaceman travels there at around 80 percent of Lightspeed and back assuming instant acceleration for now how much time will earth women measure while spaceman is on his journey space twin will take five years each way thus in total earth one will have measured ten years of time on his clock when his brother returns spaceman on the other hand experiences the following because of length contraction the entire distance he travels shortens to only two point four light-years this results in three years of travel each way or six years for the entire trip on space twins clock only six years of time will have passed upon his return to earth boom solved but at home you say didn’t you argue that they that they see each other aging slower than themselves because of time dilation what if what is faced when an earth man observe each other during the entire trip how does this fit into the separate frames resolution let’s say space twin at earth to an actually observe each other which could be imagined as them sending each other a live stream of themselves how it is each twin seen sibling during the trip there are several effects at play here first time dilation will work its magic and make them see each other age more slowly than themselves second there’s the shear time it takes light on this case information to travel as the distance between the twins increases the images they send each other need increasingly long time to arrive this also makes them see each other age more slowly so that when space twin arrives at the turning point he will be looking for years into Earth’s past as he sees the light that earth sent off four years ago since space twin has traveled for three years at this point measured from his reference frame he will see his brother aged by only one year after turning point this effect reverses to make up for the shortening time light takes to travel between them they will see each other age more quickly this effect is called the relative the stick doppler shift and it stacks with the effects of time dilation together these two effects determine the image each twin will see of his sibling the result can be summarized in a spacetime diagram it shows two different paths through space-time of Earth twin space twin and the images they sent each other space twin sees earth to an aged by one year at the turning point and he will see him aged by another nine years after the turning point so that in total space to envelop seen his brother age by ten years earth turn on the other hand will seen his brother reached a turning point after nine earth years during which time he will have seen an age by three years during the last earth year of space – and strip earth twin will see him age by another three years adding up to a total of six years on screen for two twin sisters it would look something like this that’s how the twin paradox solved Einstein was right