 The last, non-intuitive consequence of the speed of light being the same for all observers that we'll cover is that the idea that things can happen at the same time no longer has meaning across inertial frames of reference. In other words, there is no such thing as at the same time for different inertial frames. To see this, we'll go back to our train example. Here the observer on the train is in the middle of the car, and as he moves past the observer on the ground, lightning strikes the front and back of the train car. The light from both strikes travel the same distance at the same speed and reach the observer on the ground at the same time. He correctly concludes that lightning hit the front and back of the train car at the same time. The person on the ground also notes that the light from both strikes travels to the person on the train at the same speed, but because the person on the train is moving in the direction of the front lightning strike, it doesn't have as far to go and therefore it reaches him first. But how does this all look to the person on the train? In his reference frame, he is not moving forward. The person on the ground is moving backward. He knows that he is also the same distance from the lightning strikes, and he also knows that the speed of light is the same for both strikes. So for the front strike to reach him first, it must have happened first. If they had happened at the same time, they would have reached him at the same time. The person on the train sees the light from the first strike, start traveling to the person on the ground before the light from the second strike. But because he is moving in the direction of the second strike, they get to him at the same time. Who is right? They are both right in their own reference frame. They are simply measuring what they see. What is simultaneous at two locations in one reference frame may not be simultaneous in another reference frame. What happens in two places at the same time on earth will not be happening at the same time if we are viewing them from Venus. Simultaneity is relative, along with time, distance, speed, and momentum.