Welcome back!

I hope you are all safe and doing well.

We are already midway through the third month of the new year. Time continues to fly at speeds that seem to exceed even that of light. As the Earth continues its cyclic track through interplanetary space, the changes that we wish for may be on their way; it seems that cases continue to decline as vaccination rates increase.

As we hurtle into the potentiality of a future in which some semblance of normalcy may return to life, it is always worth taking a step back and looking at things from a different perspective, worth revisiting the foundations that make so many things up.

So in today’s blog post, I thought we might visit a basic physics phenomenon that forms the root of several essential tenets of astronomy. This phenomenon has the added advantage of being essential for several aspects of life as well!

But what is this? you ask.

All in good time. For now, sit back, relax, and enjoy the ride!

The phenomenon that we are going to explore today forms the core of waves. Whether it be a ship moving through an ocean, a photon speeding at the speed of light, or a fire truck screaming towards a building aflame, this phenomenon is shared among all. This phenomenon even formed an integral part of an iconic episode of a popular sitcom.

This phenomenon is known as the Doppler effect.

Named after Austrian physicist Christian Doppler, the Doppler effect is defined most succinctly as the perceived change in frequency of a wave based on its speed relative to an observer.

Now, this does not make a lot of sense. Perceived change in frequency? Speed relative to an observer? But never fear— I’m here to break it down.

Let’s first look at a diagram:

Credit: Flipping Physics

When a wave-producing object (such as an ambulance) moves forwards, the waves that it is producing in front of it are being “pushed” by the object. As a result, for an observer in front of the ambulance, the waves that are being registered are more compressed.

On the contrary, when a wave-producing object moves forwards, the waves it is producing behind it are “pulled” by the object. As a result, for an observer behind the object, the waves that are being registered are more stretched. 

When the math for the Doppler effect is worked out, the frequency of the waves that the observer is receiving depends on the velocity of the observer and the velocity of the source. We’ve discussed this qualitatively, but for calculations its simply converted to a quantitative form.

We’ve discussed this in terms of sound, mostly, but the Doppler effect applies for all waves, even electromagnetic. Let’s look at another diagram:

 Blueshift - WikipediaCredit: Pob Schools

When an object producing light moves, the waves in front of it are compressed and the waves behind it are stretched out, just like the ambulance example that we had. Now, as we are looking at light, we can describe this stretch and compression in terms of color. Light with longer wavelength is considered red, and, therefore, the stretching of waves is considered to be redshift. Light with shorter wavelength is considered blue, and, therefore, the compression of waves is considered to be blueshift.

This concept of redshift is, in fact, how astronomer Edwin Hubble discovered that the universe was expanding. When he observed several distant galaxies, he noticed their spectra were stretched out, or redshifted, meaning that they were all moving away from the Earth. And if all the distant galaxies were moving away, the insinuation can be made that the universe is expanding!

The Doppler effect is a core physics principle that has several applications throughout astronomy—and, as a result, throughout the world. The next time that you hear an ambulance scream by you, feel free to talk about the Doppler effect to whoever you’re with. You might just blow their mind.

Clear skies!