What I’ve learned from reading Why Does E=mc2? (and why should we care?) by Brian Cox and Jeff Forshaw:
I am learning!
Okay, so suppose your car’s clock is 100% accurate and always keeps perfect time. And suppose your best friend’s watch is also 100% accurate and always keeps perfect time. And both of these time pieces start exactly in synch. You get in your car and drive somewhere, so from your friend’s perspective, you are moving and she is standing still. With me so far? I’m still with me, at least (I understand this in my head, but this is my first attempt to explain it back).
Okay, so according to your friend you are moving (there is always the possibility that from your perspective you are sitting still in one spot and the rest of the world is moving around you, but we’re not going to look at that perspective right now). You are moving, and in your car, your clock continues to keep its perfect time, and outside, not moving, your friend’s watch continues to keep its perfect time.
But after you’ve been driving around for a really long time (there are some equations where you can figure this out exactly, but I’m still at baby-basic level here), you return to your friend and compare clocks. The clock in your car will be slow compared to your friend’s watch.
Your car’s clock didn’t break. Okay, here’s where I try to explain what I’ve learned:
Time is the 4th dimension of space. You can’t really picture this like you can picture the 3D world around you, but it’s there and all intertwined with speed. I like this explanation: Time and Speed have a set quota to reach that will always be the same. If there is no Speed (as in your best friend who is just standing around not moving) then Time gets to fill up the quota itself–so Time will move fastest if there is no Speed. When you start putting Speed in the mix, there is less Time, so Time will start moving slower. So if you end up going the speed of light Time will pretty much stand still because Speed has filled up the quota.
So even though your car doesn’t go anywhere near the speed of light, over time your car’s clock will still run slow.
And that is what I’ve learned.