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Light and the distance it travels

Updated: Aug 19, 2021

The average Life expectancy for Homo-sapiens is around 73 year. Throughout this time, we spend almost twenty years studying or learning, then at least 40 more working and finally maybe twenty more relaxing and spending our probable last moments in this universe.

How about if we don’t do any of that?

Let’s walk, throughout our life, from the moment we are born to the moment we die. Let’s keep walking towards the sky, what do you think will happen?

Where would you be able to go?

Will you see the gigantic Sun up close?

Or will we able to see the entirety of the Milky Way in a gigantic manner?

The answer, unfortunately, is none. The total approximate distance you would be able to cover might be only 170,000 Kilometers, Which is not even enough to reach our moon.

Now, let’s talk about our dear light. Light travels at the speed of 299,792,458 m/s which is roughly around 300,000 Km per second, all those years of walking and all your whole life, spent in a split second by a mass-less particle.

These are the particles that help us calculate the distances in our universe; Light years, often confused as a unit of time, defines the distances the universe hold and even if you haven’t yet realized the majestic size of our universe, you will soon.

Talking about moon, with the modern technological advancements, the fastest mission till now that reach near the moon took 8 hours and 35 minutes to cover the distance. The rocket that achieved this feat was NASA’s Atlas V traveling at 16.26 Km/s and in comparison, the fastest vehicle on land can travel up to 20Km/min (Thrust SSC).

Animations, CGI and paintings have given us quite an insight on how the planets and stars faraway look like, which often creates an illusion in our minds that every single planet discovered, is plain into our sight. May it be a science fiction movie or a beautifully designed picture; they all show us the planets and stars in a gigantic manner. They indeed are huge, magnificent and colossal, but they don’t usually appear to us in that way.

We can always blame scientists for saying vague stuff like “there might be life on that planet”; I mean, if you know where the planet is, just look at it and figure it out!

But that’s not how it works, why?

Because we don’t have the luxury to look at the planets up close, we don’t even have the luxury to even look at most of the exoplanets (Planets that exist outside of our solar system).


One way for us to figure out the existence of an exoplanet is through Transit Method of exoplanet detection; this method relies on the detection of a dip in the brightness of a star which insists that a star might have passed between our line of sight. Through further rigorous calculation taking certain assumptions we can finally characterize planetary masses, radii, densities, atmospheric composition, orbital alignments and much more by simply taking the account of the depth of the dip, the size of the dip, distance of the star, the distortion by the dip, etcetera.

One of the pictures taken by Kepler Telescope.

Of course there are other methods like Radial Velocity Method, Micro-lensing, and Direct-Imaging.

Now then let’s come back to our dear light. Now that you know how really fast light is, let’s see why it holds so much importance.

Distance the light travels

You may have heard that speed of light is absolute and that we can never go near it, but why?

Einstein stated that no object with mass can go beyond the speed of light, why can light travel at the speed of light?

Because it simply has no mass, it can only propagate through its momentum. Now why can’t anything with mass go beyond?

Because it’s basically impossible to feed something so much energy; you still have to take all of this with a grain of salt as the technicalities are slightly different but to get a gist of how important the speed of light is, let’s just say that we can’t go beyond it.

In order to understand the vastness of the universe, we had to take measure of the fastest possible speed we can physically achieve but we still don’t compare it the way we usually compare lengths. As a lot of people, who are familiar with the term Light-year, know that a light year is a distance that even light needs a whole year to complete. Imagine, if one second is equivalent to one whole life’s worth of human walking, what about one whole year be?

Anyway, now boom-boom, let’s assume we just throw away what Einstein said out of our hypothetical window and make a spaceship that travel at the speed of light. Now, as I am a teenager and can’t travel into the unknown streets of outer space yet (My mum won’t allow it, you know), it’s all unto you to go and travel to the ends of the Universe. Bon Voyage!

Here you go! You start from earth, and you are already at moon in less than 3 seconds. Eh, we don’t want moon, let’s go beyond that. 3 minutes in and there you see it, our red faced shy neighbor or maybe 22 minutes depending on when you start your trip. In about 5 to 6 hours you will be able to see the Dwarf Planet Pluto but what about our solar system?

The scale is logarithmic; each indicated distance is ten times farther out than the previous distance For this calculation, it really depends on us what we consider the end of solar system, let take it the icy region out the heliosphere, The Oort cloud.

Considering the distance of Oort cloud from us to be around 7.4 Trillion Km, Your space ship will take around 285 days to get there.

Proxima Centauri, closest star?

About 4.23 light years, you will need to keep on going and going on that ship of yours for about 4.23 years to get to the closed star even that when traveling at the Einstein-forbidden speed, the light-speed. More-over, we are just getting started; you remember I told you about those 3000 exoplanets?

Imagine how unbelievably far they are from us!

Let’s take our previous analogy, one human life was for half a light-second, then that about 266 Million people! That’s 3.8% of our total population, just to reach to the nearest star. What about leaving the Galaxy?

That’s 500 light-years or about 31 Billion people worth of walking.

The Edge and Light

As we know, in order for our naked eyes to observe anything, light has to bounce back from that object into our eyes. Thus, for us electromagnetic waves play a very crucial role in order to understand the existence or maybe even know about the existence of something. One example of it is how we came to know about the total amount of normal matter present in this universe, how? Lets keep it for some another article.

For now, all we need to understand that light and the distances in our universe are more related to each other than we think, another example is the exact value the velocity of light holds the limit of causality!

What is causality? In Einstein’s General and Special Relativity, he states Causality means that an effect cannot occur from a cause that is not in the past light cone of that event and a cause cannot have an effect outside its future light cone, where a light cone refers to the path that a flash of light which is emitted from a single point in space and a single moment of time and is traveling in all possible direction would partake. Thus, Causality, in laymen terms, is the only reference frame of function in which the order or the sequence of events is consistent to all the observers. That’s cool but why is it so important? Read this paragraph again and you might get a small glimpse into what causality holds.

Moving forward, this means that light is unbelievably important in modern day physics and that’s why it holds so many secrets that a mere article of 1,500 words could never discuss all of that but what we can discuss is how we found out the length of the observable universe with the help of it.

As we are already riding a lot in the temporal dimension, lets ride once again and move back to 1842, when Christian Doppler discovered the idea of Doppler Effect. This phenomenon holds the key to find the size of the observable universe and its age. Well, I won’t be explaining it to you now as the subsequent article will hold a much deeper explanation to this idea than I can explain in the given word limit. Let’s just establish that Doppler Effect results in Red Shifts and Blue shifts, which tells us whether an object is accelerating away from us or towards us; if you want a deeper understanding of this effect and the causes I would recommend you to go and read the subsequent article first and then read ahead but if you only want to know how and why, let’s move forward.

So more or less, this acceleration is measured and then reverse calculated to estimate the age of this universe; Why? If it hasn’t clicked yet, then Volla! Our Universe has been expanding since its estimated birth which was about 13.8 Billion years ago. Now, as we have already established that in order to see anything that exist, we have to observe the light from it. Now, this is very important to understand the edge of this universe because as we know it is expanding, this means that the light from very distant objects is always reaching us as time passes by and we are always able to see more of this universe today than we were able to do in the future but to simple give it as value, we just reversed the reverse calculations we did to figure out that the total diameter of the observable universe is about 93 Billion Light years and that’s an astonishingly big number and as some of you might have thought, but that’s more time for light to travel than anything in our universe got since its existence!

Of course if anything which was 93 Billion Light years away would have emitted a ray of light, it would never reach us. Why? Simply because the age of the universe itself is 13.8 Billion years and light would just not have enough time to travel this much distance but this only arises when we establish that the speed of expansion of the universe in not limited to the limit of causality or as we say limit of light speed because it isn’t traveling in space, the space itself in expanding itself in between.

Coming over to our first topic, how is the light from something 93 Billion Light year away reach us for it to consider in the observable universe? The answer is it did not. The light was simply emitted at the time when that object was nearer to us than 13 Billion Light years but with the expansion of The Universe, it is now so far away from us that the only way for us to reach that object that I can think of is through a Wormhole.

Isn’t it fascinating how huge our universe can be when a common metaphor on a planet in this Planetary System of this one out of Billions of other galaxy is “The World is a Big Place.” And this is just the gist of our observable universe!

Thanks a lot for reading along this blog but if you are a little bit concerned about what might lie beyond of our reach then keep on read because the next blog will go on to discuss The Non- Observable Universe. Thanks a lot again!

By Christopher George


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