Go outside and you will be showered, every day, with proof of the theory of special relativity. This proof comes in the form of muons, so you won't actually see it, but we'll tell you why the presence of these particles proves Einstein right.
The theory of special relativity isn't controversial anymore. It's no surprise that many found it hard to believe. The idea that time can be stretched and compressed for people going at different speeds is hard to credit. When scientists started thinking about it, though, they realized that the theory had a lot of easily-observable proof.
As scientists observed they universe, they turned up more and more proof. One of the biggest proofs comes from one of the smallest objects in the universe. The muon isn't as small as an electron or a neutrino, but it's still smaller than any other particle of matter. Its small size is a relief, as the Earth is pelted with a lot of muons every day. Cosmic rays hit the atmosphere and create muons flying towards the ground. The muon don't overstay their welcome. The muon lifespan is about 2.2 microseconds. After that, it decays into an electron and a neutrino.
The muons created in the upper atmosphere are moving at about 99 percent of the speed of light. That's not enough. A muon traveling at 99 percent of the speed of light for 2.2 microseconds will decay long before it hits Earth's surface. Only a few very, very lucky and long-lived muons will strike the Earth.
Despite this, we are constantly being inundated in muons. Nearly 10 percent of the muons that get created make it down to us. The secret to that otherwise-unaccountable wealth of particles is special relativity. The muons are going so fast, relative to us, that their 2.2 microseconds of life lasts, from our perspective, 15.6 microseconds. If we were the muons, we'd still be experiencing only 2.2 microseconds, but all of us here on Earth see the particles living 15.6 microseconds, and coming down to bop us on our heads.
We are literally struck with evidence of special relativity every day.