Home / Opinion / Columns /  How far is that oldest object, really?
Listen to this article

My last column in this space referred to the oldest object we humans have ever observed. That’s the galaxy GLASS-z13, which the James Webb Space Telescope focused on last month. Light from GLASS-z13 has taken 13.4 billion years to reach us, which means we are seeing it as it was that long ago. That makes it, indeed, the oldest object we’ve ever observed.

It also means the galaxy is 13.4 billion light years away. Or is it, really? That column pointed out that “astronomers estimate [GLASS-z13] is now actually about 33 billion ly from us."

How do they know this?

Try this thought experiment. I sit opposite you, blowing a sharp and quick note on a whistle every second. You’ll hear the note every second. No drama there. Now suppose I’m actually sitting in an aircraft. I continue to blow the whistle every second, but after my first whistle note from opposite you, the plane travels away from you at the speed of sound, which is about 1,250kmph, or 350 metres per second. How frequently will you hear the whistle? (Assume for the sake of this experiment that you can indeed hear it.)

Well, the second time I blow it is after one second, but by then, I’m 350 metres from you. That sound will take a second to reach you - so you hear my second note two seconds after the first. You might say the frequency of the notes has halved, from one every second to one every two seconds. Similarly, if the plane was flying towards you instead, the frequency would double.

This is a simplified way to think about the well-known Doppler Effect. Sound travels in waves. If the sound source is moving away from you, those waves lengthen, their frequency dips, and the sound becomes lower-pitched. The opposite, if it is moving towards you. The classic example familiar to us all is the horn of a passing train. Its pitch lowers as it speeds away from us. In fact, my whistles from the moving plane will also be lower in pitch than the first, which sounded before the plane started moving.

What does all this have to do with GLASS-z13 and how far from us it is? As I mentioned in my last column, there’s a clue in that “z13". That is a measure of what astronomers call “redshift", or a change in the frequency of light.

Like sound, light is also made up of waves. (It’s a little more complex than that, but let that be.) When the light comes from a source that is moving—like my whistle in the plane—its wavelength changes like sound’s does. If the source is moving away, the waves lengthen (and their frequency decreases). When that happens on the sound spectrum, you get a sound at a lower pitch. With light, you get light that’s nearer to the red end of the light spectrum: thus “redshift". The opposite, if the source is moving towards you: thus “blueshift".

You should wonder here: how do we detect this shift, whether red or blue? It’s not exactly that the light from the distant source suddenly looks redder or bluer. Instead, it has to do with substances the object is made up of—like iron or carbon, or magnesium.

When you heat such a substance, it emits light. A spectroscope (aka spectrometer and spectrograph) uses a prism to break up that light into a spectrum, in the same way that rainbows form from “white" light. Each such substance produces its unique pattern of lines in that spectrum, each line at a specific frequency.

So if you find the telltale lines of iron in a spectrum, you know there’s iron in whatever your spectroscope is pointing at. This fingerprint, if you like, is how we know the chemical composition of faraway celestial objects.

Here’s the fascinating thing. When astronomers first used spectroscopes on the light from distant stars and galaxies, they recognized fingerprints in the spectra, the characteristics of different substances. But to their surprise, in every case, these spectral lines were shifted along the spectrum. This led to a remarkable conclusion: these distant objects are moving.

Not just that. Since the degree of the shift speaks of how fast the object is moving, we know these objects are moving very fast indeed. (The pulsar I wrote about here a few weeks ago is travelling at two million kmph.) Not just that either; Hubble’s Law tells us that the farther an object is, the faster it is moving away from us, and thus the greater its light is redshifted. This is because the universe is steadily expanding.

(Note: there are some relatively nearby stars and galaxies whose light is blueshifted, meaning they are moving towards us. That’s because at those relatively close distances, the gravitational attraction between objects is greater than the expansion that drives them apart.)

Finally, the magnitude of the Doppler effect is measured by comparing the frequency of a shifted spectral line to its frequency at “rest". Specifically, if you divide the difference between these frequencies by the “rest" frequency, that ratio is the redshift, called “z". This measure of redshift tells us how fast the object is moving and how far from us it is.

And that will bring us back to GLASS-z13. In the name, “z13" stands for a redshift of 13 in the light from the galaxy. This means it is about 13.4 billion ly away; or, more correctly, its light has taken that long to reach us. But remember that the redshift also says the galaxy is moving away from us. Since we know its speed, we can calculate how far it has travelled in those 13.4 billion years. That number? Nearly 20 billion ly. So our best guess is that today, GLASS-z13 is actually about 33 billion years from us.

All that, from certain lines in the light from a tiny smudge in the sky. Astronomy invariably induces awe.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

Catch all the Business News, Market News, Breaking News Events and Latest News Updates on Live Mint. Download The Mint News App to get Daily Market Updates.
More Less
Subscribe to Mint Newsletters
* Enter a valid email
* Thank you for subscribing to our newsletter.

Recommended For You

Trending Stocks

Get alerts on WhatsApp
Set Preferences My ReadsWatchlistFeedbackRedeem a Gift CardLogout