Special Relativity is that time is relative. Relative to your position in space. And therefore, time and space are inextricably linked. They are the same thing. First published by Einstein in 1905. [Special Relativity Part 1: From Galileo to Einstein](https://www.youtube.com/watch?v=rBmYUEnafok&list=PLybg94GvOJ9FAFBqQGf5-4YbfKpWbJtGn&index=9) > Postulate 1: > The laws of physics are the same in every inertial reference frame. > Postulate 2: > The speed of light (c) is the same in every inertial reference frame. > Conclusion 1: > Time flows at different rates for different observers. # Time Dilation [Special Relativity Part 2: Time Dilation and the Twin Paradox](https://www.youtube.com/watch?v=iIEeSiT3SI4&list=PLybg94GvOJ9FAFBqQGf5-4YbfKpWbJtGn&index=9) > Time Dilation > The faster you move, the slower time passes. The faster you move relative to the speed of light. Time = Distance / Speed This is an explanation for how we know time is relative and affected by space. Because if it weren't true, GPS wouldn't work! We have to account for the difference in time that a GPS satellite experiences because it has to adjust its clocks to match the time on earth. # Length Contraction [Special Relativity Part 3: Length Contraction](https://www.youtube.com/watch?v=FPzGAksFCbs&list=PLybg94GvOJ9FAFBqQGf5-4YbfKpWbJtGn&index=10) Length Contraction basically means that since time and space are measured together, if your measure of time changes, then so does the measure of space. The faster you go the shorter the distance it appears you're going. # Mass-Energy Equivalence (E = mc²) [Special Relativity Part 4: Mass-Energy Equivalence or E = mc²](https://www.youtube.com/watch?v=FPzGAksFCbs&list=PLybg94GvOJ9FAFBqQGf5-4YbfKpWbJtGn&index=10) Okay this is a perfect series. Like you have to understand those four parts in order to understand what special relativity is, it's implications for the laws of physics, and therefore the shape of spacetime itself in [[General Relativity]]. E = mc², the most famous equation on earth. He just said something really interesting. [[Time does not exist to a photon]]. Because it is moving at the speed of light. A photon also experiences no space, because it has no mass. Ooh, I need a page on [[Energy]] too. It's impossible for any object with mass to reach light speed because that would require INFINITE ENERGY. That's what makes the speed of light a universal speed limit.