 Another consequence of the speed of light being a constant for all observers is that the size of an object shrinks along the line of motion when viewed by an observer in a different inertial frame. To see how this must also be true, we put a second identical clock on the moving frame, but this time we align it with the direction of motion instead of perpendicular to it. They tick at the same rate because they are identical clocks at rest together. They will also tick at the same rate for an observer on the ground. Only they will tick slower as we have already established. But the motion of the horizontal clock's mirror makes the distance the light has to travel different from the distance the light travels in the perpendicular clock. Here we see the perpendicular clock ticking one before the light in the other clock has returned. The only way to bring the clocks back into sync is for the length of the distance traveled horizontally to shrink. This is called space contraction. Here's a look at the new Orion spacecraft fully decked out for the planned long journeys to the moon, asteroids, and Mars. If we were traveling at the same speed as GPS satellites, it would look like this. If it were traveling at 0.9 times the speed of light, it would look like this. And if it were traveling at 0.99 times the speed of light, it would look like this. Unfortunately, we don't have the technology to take a measuring rod and accelerate it to almost the speed of light in order to measure how much it shrinks. But we do have the interesting example of muons created by cosmic rays in the upper atmosphere 15 kilometers above the ground. You'll recall from our segment on elementary particles in the How Small Is It? video book that we discovered muons with cloud chambers on mountaintops and in high flying balloons. We found that their half-life is only 2.2 microseconds. Traveling at near the speed of light, half the muons would decay with each 660 meters traveled. That would leave few, if any, reaching the Earth's surface 15 kilometers away. Yet we measure about 1 per square centimeter per minute. That's around 90% of the estimated muons produced in the upper atmosphere, which is way too high a percentage given the short half-life. This high count rate is explained using relativistic time dilation. The measured velocity of the created muons is 0.999986 times the speed of light. So by time dilation, the half-life would be 189 times longer. This gives the muons time to travel up to 125 kilometers before half of them have decayed. And that's plenty of time for most of them to reach the surface in line with the numbers observed. What does this look like in the muon's own reference frame where it is standing still and the Earth is approaching at 0.999986 times the speed of light? It will only last 2.2 microseconds. How will it make it to the surface? This is where space contraction comes in. The 15 kilometers of atmosphere is shrunk. For the muon, the distance to the surface is only 79.4 meters. So then we'll reach it in 2.2 microseconds.