What is Redshift?

Question posed by Emma.

Imagine standing on the pavement by the side of a main road. An fire engine is approaching from the distance, lights blaring, siren wailing. As it approaches you it gets louder and louder until it whooshes past you at speed. What happens?

If you've ever experienced this in reality, you'll know that as the fire engine rushes past you, the sound of its siren siren appears to drop in pitch: as the fire engine recedes into the distance, its siren sounds noticably lower than it did as it was approaching you.

This perceived drop in pitch is a result of something called the Doppler effect which was proposed in 1842 to by Austrian physicist Christian Doppler, and is observed (or heard, of course) when an emitter (e.g. the fire engine's siren) and a receiver (your ears) are moving relative to each other*.

Sound travels in waves, and it is this fact that allows the Doppler effect to exist. The sound waves that are emitted by the fire engine's siren always travel at the same speed, but the wave-fronts are squished closer together by the motion of the fire engine as it approaches you, and the frequency of the sound is increased as a result. The rather groovy image to the right that I've borrowed from another website (click on the image to go there) illustrates this quite nicely, I think. Although I couldn't find one with a fire engine, the principle's exactly the same.

As the fire engine (or police car)goes past you, for a fraction of a second you hear the siren at the actual pitch that is being emitted, and then you're behind the generic emergency vehicle. This scenario is also included in the diagram, and you can see that as the policebulance engine recedes away from you, the wave-fronts are now spread out, the frequency of the sound decreases, and the pitch is perceived to be lower.

I asked about redshift. Why are you talking about emergency vehicles?
I stated before that it was the fact that sound travels in waves that allows the Doppler effect to happen. Electromagnetic radiation, which includes visible light, also travels in waves, so the Doppler effect happens with light too: as an object travels towards you at speed, the light waves being emitted (or reflected) by it are squished in the same way (and for the same reasons) as with sound waves, and the colour is changed. Specifically, when light waves are compressed in this way, the colours that are perceived by an observer move (or 'shift') towards the blue end of the visible light spectrum. If an object is moving away from the observer**, the waves of light are stretched out and the perceived colour is 'shifted' towards the red end of the spectrum, relative to the emitted colour.

But police cars don't turn red when they're driving away from me!
No. Because the wavelength of light is so much smaller than that of sound, any differences in speed that you're likely to experience on Earth don't make much of a difference to the spread of light (the 'spectrum') that you observe from a moving object.

But if you look into the night sky with the right kind of telescope, and take the right kind of photographs, you'll see what astronomers around a hundred years ago first discovered: most of the galaxies outside our own are a bit more red than they should be: they are 'redshifted'. This means that they are moving away from us at high speeds.

So, in short...
Redshift is an effect that is observed when an emitter of light and an observer are moving away from each other***, which causes the received light to look redder than that emitted. The opposite of redshift is 'blueshift', which is observed when an emitter and receiver are moving towards each other, causing the received light to look bluer than the light that was actually emitted.

Interesting bits

  • Redshift (and blueshift) can actually be used to determine the speed at which an object (such as a different galaxy or cluster) is moving away from us. Studies of the redshifts of different galaxies has indicated that galaxies that are further away from us are moving faster than closer galaxies. This implies that space itself is expanding.
  • Alternating red and blueshifts of an object in orbit around another body within our galaxy have enabled scientists to calculate orbital velocities, allowing an estimate for the mass of the orbited body to be calculated.
  • The same effect is observed at wavelengths other than those of visible light****, but an increase in wavelength is always referred to as 'redshift' and a decrease as 'blueshift'. This may be counter-intuitive, depending on how you think of such things.

Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

* Yes, I do mean relative to each other. The emitter doesn't have to be the one that's moving- in the example we're using here, exactly the same effect would be heard if you were to travel past a stationary fire engine with  its siren going, at the same relative speed.
** Or, of course, the observer is moving away from the object.
*** Relatively speaking.
**** Ultraviolet or infrared light, for example, or x-rays, gamma rays or microwaves.


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