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We know sound travels about one thousand feet per second, so we can fire the weapon and see how long it takes before we hear the blast at the back. If the sound takes one second to get there, we are standing still in that direction. We can turn our ship and keep firing. When the sound gets from the front to the back in the shortest time, it means we've found the direction we're traveling. The air, running the length of the ship, carries the sound along. If the sound always travels one thousand feet per second, then we are definitely standing still.
3. We might get into a heap of trouble firing randomly like that in a fog, so we need a more gentle way. We stick a tuning fork, vibrating four hundred forty cycles per second, up in the front end and another with the same frequency in the back, and we'll put the ear phones right between them.
If we travel through the air towards the first tuning fork, the pitch will be higher coming from that one, perhaps four hundred forty-one cycles per second. The frequency will be lower from the one we put up in the back, vibrating at four hundred thirty-nine times per second. We hear this same effect when the pitch of a car horn lowers as the car passes us. If the difference in frequencies is too small to hear the two pitches clearly, we can use the same trick as the piano tuner and listen for a wavering sound. We can rotate our ship until the wavering frequency is highest, and then we'll know our direction and our speed (and we won't run out of bullets).
So far, we have some ways to test for absolute motion in air by using sound.
WILL THE ABOVE METHODS WORK WITH LIGHT IN SPACE
If light travels in waves, there must be a medium, but what does it mean if we think there's a medium? First of all, space can't be empty.
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