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The Downfall of Simultaneity
At the heart of Einstein's Theory of Relativity is the concept that all observers in all reference frames are equal. Imagine, hypothetically, that you are floating in space with nothing else around you. Then you see another person zip by you. Was he moving, or were you? To you, the other person moved past you while you were motionless. And to the other person, it seemed that you were the one moving instead. There is no such thing as an absolute reference frame. Our observations of the universe around us are relative.
Couple this with the fact that nothing can move as fast as the speed of light. And no matter what, an observer will always measure the speed of light to be the same. To illustrate how this is important, we’ll construct another hypothetical scenario. Say that you can throw a baseball at 60 mph. Now, if you are in a car that’s moving 60 mph, and then you throw the baseball, you’d measure it moving at 120 mph. But now consider the speed of light. If you're standing still, you would measure that speed at roughly 300,000 km/s. Now if you’re in a spaceship traveling at 299,000 km/s, instead of a baseball, you throw out some photons by turning on a flashlight. If you measure the speed of light, you'd still measure it at 300,000 km/s. In other words, it would look the same as if you weren't moving at all.
All of this leads to some pretty shocking conclusions to us humans, who aren't accustomed to seeing the universe operate at this level, including time dilation, length contraction, and the downfall of simultaneity. Because two observers can be moving at different speeds, but two flashes of light must each be seen by them moving at the same speed, one observer could observe the events as occurring at the same time, and the other would see the events occurring at different times. And since the main premise of relativity is that each observer's reference frame is equally valid, that means there is no absolute way to say that the events happened simultaneously. Simultaneity is relative. Time is a concept now tied to your motion through space.

physics: The Downfall of Simultaneity At the heart of Einstein's Theory of Relativity is the concept that all observers in all reference frames are equal. Imagine, hypothetically, that you are floating in space with nothing else around you. Then you see another person zip by you. Was he moving, or were you? To you, the other person moved past you while you were motionless. And to the other person, it seemed that you were the one moving instead. There is no such thing as an absolute reference frame. Our observations of the universe around us are relative. Couple this with the fact that nothing can move as fast as the speed of light. And no matter what, an observer will always measure the speed of light to be the same. To illustrate how this is important, we’ll construct another hypothetical scenario. Say that you can throw a baseball at 60 mph. Now, if you are in a car that’s moving 60 mph, and then you throw the baseball, you’d measure it moving at 120 mph. But now consider the speed of light. If you're standing still, you would measure that speed at roughly 300,000 km/s. Now if you’re in a spaceship traveling at 299,000 km/s, instead of a baseball, you throw out some photons by turning on a flashlight. If you measure the speed of light, you'd still measure it at 300,000 km/s. In other words, it would look the same as if you weren't moving at all. All of this leads to some pretty shocking conclusions to us humans, who aren't accustomed to seeing the universe operate at this level, including time dilation, length contraction, and the downfall of simultaneity. Because two observers can be moving at different speeds, but two flashes of light must each be seen by them moving at the same speed, one observer could observe the events as occurring at the same time, and the other would see the events occurring at different times. And since the main premise of relativity is that each observer's reference frame is equally valid, that means there is no absolute way to say that the events happened simultaneously. Simultaneity is relative. Time is a concept now tied to your motion through space.