How Einstein’s theory of relativity makes you smaller
Relativity is best known for giving us a new view of time. The faster you go, the slower time moves. There are other effects as well. For example, the faster you go, the more squished you get.
The most famous consequence of Einstein’s work with relativity is the idea that time slows down as an object speeds up. The less-famous fine print is that time seems to slow down only for the person who is watching the movement. If two twins synchronize their watches before one steps onto a train, the synchronized watches read the same right until the train starts moving. The twin that is on the train - let’s call her Maria - won’t see her watch slow down.