What you're about to read is the Holy Grail of physics lectures. Well... it's the introduction to the Holy Grail of physics lectures. It's the lecture every professor wants to give and the one every student wants to hear. In the next couple of paragraphs, I'm going to lay down the foundation of Special Relativity and explore some of the paradoxes that arise from it. This is some seriously cool stuff.
Within this post, I'm going to include links to several helpful videos. I'll try to write in such a way that you don't need the visual aids, but I highly recommend that you click the links as they come up-- they will prove very helpful in wrapping your head around this new way of thinking (I'm speaking from personal experience).
First thing's first. If we want to understand special relativity, we need to understand "frame of reference," Simply put (as everything should be), this is just the idea that things appear to move differently from different perspectives. For example, if you're driving your car around your neighborhood, the bobble head on your dashboard seems to sit still (aside from the occasional nod it gives you when you slow down or speed up), while the mailboxes, houses, and and old men sitting on their porches are zooming past you. From the perspective of one of those old men, however, it's you who appear to be traveling very quickly, while his glass of lemonade sits motionless beside him. Video.
This idea probably isn't new to you! This thought first crossed my mind sometime around third grade, when I was tossing a tennis ball up and down in the back seat of our truck on the way to grandma's house. How is it that our vehicle can be traveling so fast, yet the ball doesn't fly backward when I let go of it? As you could easily reason, it's because I'm effectively throwing the ball at 60 miles per hour (or however fast the vehicle is moving) every time I let go of it. This is why cricket players get a running start when they bowl-- the velocity of the ball when they release it is equal to the combination of the speed at which they're running and the speed at which their arm is moving when they let go of the ball.
Now you're ready for the first (and most) mind-bending idea I'm going to hit you with.
Imagine that I have a flashlight. If I aim it straight in front of me and turn it on, the light will travel at 300,000,000 meters per second (300 million m/s). Great. Now, let me get a running start. Let's say I get going at like... 7 meters per second, then turn on the flashlight. From my perspective-- my "frame of reference," how fast does the light travel?
I know, I know, you're smart enough not to fall for the trap I set. I just explained reference frames, and you know the from my perspective, the light is still moving at 300,000,000 meters per second. But how fast does my mom, who's standing next to me, wondering why I'm running around the house with a flashlight, see the light moving? Well, if I'm moving at 7 meters per second, and light travels at 300 million meters per second, then my mom should see the light moving forward with the sum of these two velocities: 300,000,007 m/s!
But she doesn't.
What my mom sees light traveling at 300,000,000 m/s-- the same speed it was traveling before I took off running.
Wait... what? Video.
Light travels at a constant velocity, regardless of from what reference from you observe it-- inertial (moving), or otherwise.
This fact is the foundation of Special Relativity, and it is not easy to swallow, yet we know it to be true.
The fundamental implication here is the first that comes to mind-- one that you might have already considered: nothing can travel faster than the speed of light, or we're going to encounter some serious problems. For example, If I ran at twice the speed of light and flicked on my flashlight, then the light, which doesn't change its velocity with mine, would be behind me, while I'm running forward, flashlight in-hand. Or maybe the light just wouldn't escape at all? Once I reach that threshold of 300 million meters per second, the light just vanishes? Who knows. It's spooky stuff, and it can't happen.
So with the axiom that the speed of light is constant follows the rule that nothing can travel faster than the speed of light. Nothing
However... as things move faster and faster, approaching 300,000,000 m/s, some interesting "paradoxes" arise. I say "paradoxes" because these aren't paradoxes at all! They are truths that follow from that first axiom: the speed of light is constant. We call them paradoxes because they counter the intuition that we've developed from a lifetime traveling at ordinary, "non-relativistic" (AKA not very fast) speeds.
Now, when I set out to write this post, I planned on explaining some of these paradoxes in my own words, but then I realized that if you just watch the videos that I provide links to, you'll get a better, clearer, more complete explanation than I would be able to give you in a then redundant paragraph.
So what I'm going to do instead is leave you with videos explaining the two most famous paradoxes of relativity along with brief explanations, just so you can get a taste of what they're about and decide whether you want to invest the five or ten minutes watching the coolest things you'll ever see. Then, you can ask me any questions you have about things that don't make sense or sit well with you!
So I leave you with these links and the feeling that the rug has just been ripped out from under you because that's what learning new things about physics does each and every time.
The Twin Paradox:
This is the idea that if you start out with two people at the same age (twins), then send one of them, person A, off in a spaceship at really, really high speeds, person A will return to earth younger than person B.
[this is because of something called time dilation]
The Pole-Barn/Ladder Paradox:
In this paradox, a 6 foot pole can fit completely into a 4 foot barn.
[this is because of something called length contraction]