50 Shifts of Red ( and Blue)

When we talk about waves, there are two important concepts you should remember: blueshift and redshift. But before we really get into these topics, we need to understand what the color spectrum is.

The color spectrum is basically a measurement of which wavelength goes with each color. Remember, light is an electromagnetic wave, reflected by the objects. Depending on how large the wavelength of these waves are, certain colors will be seen by the human eyes.Look at this image:

Right on the visible light section, you can see that different pigments have different wavelengths, that vary between 400 and 700 nanometers ( which is ... pretty small).

And that is when the “shifts” come in. Suppose you are standing up on a street, looking at the cars passing by. As they move away from you, they reflect light, as any other object. However, when the vehicle is moving, the light it reflects is stretched as it gets to your eyes. This causes the wavelength to increase, what makes the car be slightly redder ( even though you can’t see it). Since red has the largest wavelength, objects moving away from an observer will, indeed, look reddish.

This is also true for object moving close to an observer, but, instead of the car, for example, getting red, it gets a bit blue ( but, again, the car won’t magically turn blue). Since the vehicle is now moving towards you, the light reflected get squished off, reducing its wavelength, and making it “blue”.

Don’t say you have never experienced this, because you did. The same phenomenon happens, but with soundwaves. When an ambulance is approaching you, doesn’t it sound like the siren has gotten louder, and then all of a sudden the noise is reduced? Those are blueshift and redshift, too. When sound waves move towards you, their wavelengths diminish, and the noise gets louder. But, as the source of sound moves away and waves get spread out, the volume in lessened.

Redshift and blueshift were used by the astronomer Edwin Hubble to discover that the universe is expanding. By analyzing light from distant galaxies, Hubble noticed that it was presenting redshift, therefore, the stars could only be moving away --- and, for everybody’s surprise, at an ever increasing rate. This was a big argument against the static cosmos -- this is, not expanding -- model proposed by many scientists.