Monthly Archives: February 2010

Doppler Effect and Red Shift

Today I thought I would write about the Doppler Effect and Red Shift. What are they? Why do they occur? Why do we refer to Red Shift in the field of cosmology and astrology?

Red Shift is the name given to the perceived ‘shifting’ of waves through the spectrum from our perception. It gives us a measure of speed, or expansion, as is often the case. It can also called Blue Shift, and this is essentially the opposite of Red Shift. However, for the purposes of this post I shall refer to both of them as Red Shift unless otherwise stated.

Have you ever noticed the change in pitch when an object passes you at high-speed? Let’s take an ambulance or a Formula 1 car as an example. As it approaches we hear a higher than normal pitch, but as soon as the sound source starts to move away from us we hear a sudden drop in the pitch of the sound emanating from the source. Most people are aware of this phenomenon, but not so many fully understand it. It is useful for us to understand this, as it is almost the same as Red Shift, or at least it shares enough similarities with the Doppler Effect to be very useful towards our understanding of it. The effect I have just described to you has a special name, the Doppler Shift or the Doppler Effect, named after Christian Doppler who first proposed the effect in 1842.

A visual example of the Doppler Effect

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Why is the night sky dark?

It seems obvious at first, the night sky is dark because there is an absence of light. There is an absence of light because the Sun has moved behind the Earth from our perspective as part of the daily rotation of the Earth about its axis. Simple. So why am I asking this question? Well, as you may have guessed, the answer is not as simple as it may at first seem.

We have different understandings or thoughts on the universe, we call these models. Albert Einstein was among those who believed in a static model, one that states that we occupy a static universe, that is to say that it is neither expanding nor contracting, rather that it is infinite and eternal.

If we are to assume this to be true, and also assume that stars are evenly distributed around the universe, then we are still left with our question: “Why is the night sky dark?”. This is known as ‘Olbers’ Paradox’. Why are we left with this question? Read on! Read more »