In this chapter we analyze another of the chapters of The Big Bang Theory series to explain what exactly the Doppler effect is. Even if the name does not even sound familiar, you should know that it is much more present in your life than you can imagine …
Have you ever noticed that when an ambulance or a racing car passes by, as the sound approaches it is louder and as the sound recedes, the sound is more serious? This happens because the ambulance, when advancing towards us, causes the sound waves it emits to crowd in the direction of movement, causing them to be heard higher than they really are. And it happens just the other way around when you walk away. And, thanks to this, the police are able to know how fast we are going.
Why does this effect occur?
This is just one of many daily examples of the so-called Doppler effect, named for the Austrian physicist Christian Andreas Doppler. Basically it is a change of frequency (apparently) in the displacement of the waves . Furthermore, it is not only applicable to sound waves, but also, for example, to light waves.
A light source when it approaches us, it looks more bluish, while when it moves away we see it more red . Of course, it is important to bear in mind that the speed of the source must be comparable to the speed of the wave. The difference between sound waves and light waves is the difference in speed between the two. In the case of light, it is 300,000 kilometers per second, much greater than the speed of sound (about 1,230 kilometers per hour in the Earth’s atmosphere), hence the Doppler effect is especially noticeable with examples such as a car at full throttle. velocity.
The Doppler effect in the universe
However, there is a place where we can observe the Doppler effect applied to light: the universe. The American astronomer Edwin Hubble discovered in the late 1920s that the light coming from the galaxies was shifted towards the red, which made him suppose that all the galaxies were moving away from ours and from each other. In addition, he found that the further away a galaxy was, the faster it was moving away from us. What Hubble had discovered is that the universe is expanding. We might think that if everyone is moving away from us, it is because the Milky Way is at the center of the universe, but this is not the case: in reality, all galaxies are moving away from everyone. But the question is more subtle, because the galaxies do not move away because they go faster and faster, but because the space between them grows larger and larger.
How can we imagine such an extraordinary event? Think of a balloon inflating with galaxies painted on its surface. If we assume that the surface of the balloon is our universe, when we inflate it, the space between them grows, as the universe becomes bigger and bigger, and it will seem that the galaxies are moving away from each other. However, this view of the universe is also misleading, as it gives the idea that it expands within something, while the universe expands within nothing. This is the problem with analogies, which are used to explain concepts that are difficult to understand, but always bear in mind that they do not represent reality.
