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Red Shifts and Blue Shifts: What does baseball have to do with it?
We know red shifts and blue shifts help explain how things move in the universe, but what does baseball have to do with it?
First, let’s clear up what red shifts and blue shifts are in the universe. A red shift in the spectra of light means that the emitter of the light and the receiver of the light are moving farther apart. If they are moving apart, it will take the light longer to reach the receiver than if they were both still. This causes the crests of the light waves to reach the receiver at a lower frequency. Light waves at lower frequencies appear on the red side of the spectrum. A blue shift occurs when the emitter and receiver are moving closer together. This causes the waves of light to reach the receiver at a faster/higher frequency than if they both were still. Higher frequencies appear on the blue side of the spectrum.
Now to the good stuff, baseball. Red and blue shifts are much like a pitching machine on wheels. If the pitching machine is approaching the batter as it shoots out the balls, the balls will reach the batter in more rapid succession mirroring a blue shift in frequency. But if the pitching machine is moving away from the batter, the balls reach the batter more slowly, mirroring a red shift in frequency. If the batter is moving toward the pitching machine, the balls will reach the batter at a higher frequency. But if both are moving, the frequency depends on whether they are generally moving closer or farther apart.
Note that it doesn’t matter whether the batter is moving towards the machine, the machine is moving towards the batter or they are both moving. The only thing that matters is whether the distance between them is increasing or decreasing as they both are moving and the resulting increase or decrease in the frequency with which the balls reach the batter.
Assume an astronomer on Earth is looking at the spectra of light from a distant galaxy. If the distance between the Milky Way and the galaxy is getting smaller, the astronomer will see a blue shift in the light from the galaxy. If the distance between the Milky Way and the galaxy is increasing, the astronomer will see a red shift in the light from the galaxy. The same would be true from any other star or planet in the universe. So all the red shift that Hubble discovered tells us is that the Earth and most distant galaxies are moving farther apart, but whether everything is moving outward or the universe is not expanding is another question.