The red shift measures the speed at which a celestial object is moving away from its observer. It is defined as a ratio of the speed of light (300000 km/s). The opposite of a red shift is a blue shift (the object is moving towards the observer). The term red shift relates to two well known physical phenomenons: spectroscopy and the Doppler effect.
When visible light goes through a prism, it is decomposed into what is called a spectrum. All the colours composing the visible light appear separately, creating some kind of rainbow.
A Bavarian physicist, Joseph von Fraunhofer, made significant progress regarding the spectrum, using diffraction gratings. He discovered that when it is magnified, some mysterious dark rays can be observed: these are called absorption lines. These dark lines seemed to occur at particular wavelengths (i.e. colour), and depend on the type of light used as a source. It is only decades later that scientists made a giant leap forward: each dark ray corresponds to an element (e.g. hydrogen, iron, oxygen…), and the absorption lines actually show the chemical composition of the light source.
Using spectroscopy, it became possible to determine the composition of any light source: stars (including our Sun), galaxies, nebulae… By analysing where the absorption lines are placed in the spectrum, and matching these lines with known elements, scientists know what’s in a star without leaving Earth. Surprisingly, we discovered that these celestial bodies were made of basic elements that we could also find on Earth!
Analysing distant stars
However, when we apply this principle to more distant stars and celestial bodies, like galaxies or nebulae, the spectrum looks very different. Absorption lines are present, but they don’t match any known element.
We finally understood that the lines were simply shifted, either towards the red, or towards the blue end of the spectrum:
The reason of this shift is actually simple: the object is in movement relatively to the Earth! And due to this movement, the wave appear to us at a higher (or a lower) frequency than they truly are. This phenomenon, called the Doppler effect, applies to all type of waves, and is often associated with sound.
The Doppler effect
Let’s take an example: when an ambulance is moving towards you, the sound produced by its siren seems higher pitched; on the contrary, when it is moving away from you, the sound seems lower pitched. In physics, a high pitch tone translates into a high frequency, and a low pitch tone, into a low frequency.
But this is just an illusion, because the sound emitted by the ambulance doesn’t change its frequency: when you’re in the ambulance, the sound is always the same. However, the fact that it is in movement creates a distortion, only perceived by the observer, and the strength of that distortion depends on the distance between the object and the observer.
Now, since the frequency is the inverse of the wavelength, we can deduce that when the wavelength appears lower (i.e. higher frequency), the object is moving towards the observer; and when the wavelength appears higher (i.e. lower frequency), it is moving away from the observer.
If we apply this principle to a celestial body that is emitting visible light, we obtain the same results as the ambulance. A star that is moving away from Earth, has its spectrum moved towards the red end: it’s a red shift. Another star that is moving towards us, has a spectrum moved towards the blue end: it’s a blue shift.
An accelerating universe
Moreover, the American astronomer Edwin Hubble discovered a direct relation between the shift and the distance to the object: the further it is, and the more shifted its spectrum appears. By measuring the “size” of the shift, we are now able to determine the speed of the object. This is how scientists today can estimate the composition, distance, speed and direction of stars, galaxies and nebulae… without leaving Earth!
And that’s not all! Since we knew how to measure the speed of observable galaxies, we discovered that most of them are actually moving away from us. And they are also accelerating. Since the universe doesn’t have any centre, it suggests that everything is moving away from everything.
Finally, all of this constitute two strong evidences that:
- The Big Bang happened, tossing galaxies away;
- The universe is expanding in an increasing rate.
I have a telescope. Can I analyse stars and galaxies too?
Yes! It is possible to mount a diffraction gratings as a filter on your telescope. These will allow you to view the spectrum of a celestial body, and analyse its chemical composition. Also, check our this interesting topic.
And in real life?
These phenomenon can be observed in real life as well:
- As mentioned, the Doppler effect can be witnessed every day in the street. But it also has a far more practical usage. The police uses the Doppler effect to measure the speed of a vehicle: the radar sends a micro wave on the vehicle and measures the distortions induced by its speed. Since the speed and the amount of distortion is proportional, it is possible to determine the speed of the vehicle.
- The Doppler effect is also used in medical imagery.
- The spectrum of the visible light can also be seen quite often. When it rains, water drops can act as thousands of little prisms. When the Sun is shining through the clouds, at certain angles, the water drops decompose the visible light and create a rainbow.