Worldbuilding 203: Cosmic distances

Greetings and samite! ...You know, I will gladly take that kind of silk! Anyway, as you all may know, I am a huge space opera fan, but one thing I see a lot of people struggle with is, well, how enormously huge space is, so that is today's topic!

No word is enough

You might think it is hyperbole when I say there is absolutely no word that suffices to explain the absolute enormity of space itself. Let’s take a tour of the solar system together to see how huge it alone is.

A normal passenger plane like Boeing 747 travels around 900 km/h, or 570 mph, or 31 thousand banana throw lengths per hour for my 🇺🇸 readers! That is also about 85% of the speed of sound, or Mach 0.85, which I will shorten to M0.85. It feels fast; you can travel far with that! Well, how long does it take to go to the moon? Assuming no gravity influences it, it would take almost 18 bloody days to reach the moon. Okay, so it’s not that fast, but what about the American X-15 that almost reached Mach 7? About 2.5 days to go there, and you are already going 7 times the speed of sound! It took Apollo 11 3 days to reach it, so you know Mach 7 is bloody fast.

The theoretical maximum of a scramjet is M25, but counting for M20, which is almost 6km/s 😵‍💫, to the moon it is over 18 hours still! Well, now it is somewhat bearable right? Well, to move to Earth's closest neighbour when it's the closest, and assuming again no gravity and no distances change, we will be going to Venus: it is 80 days of travel! Same thing to Mars: 110 days of travel!

That is still taking longer time than most sea voyagers to the Americas back in the age of sails! Alright, how about we go the furthest away object that you humans have ever made? Voyager 1 reached about M50. Time to Venus and Mars, about one month. Imagine being crammed in an aeroplane for a month just to take a vacation on Mars or Venus…

Well, we’re not done yet! That is just your neighbours; how about we travel to the dwarf planet Pluto which is about 40 times the distance from the sun compared to earth, or 40 AU? At M50, that is an 8 year long journey; how about where the solar system officially ends, where the sun's rays stop being dominant: the heliopause at a distance of 120 AU or 0.002 lightyears (ly)? Over 33 years of travel! 😵 Okay, we gotta make it go faster still! Shall we take the fastest object in the solar system? The Parker Solar Probe, which is at M570! That is 0.064% of the speed of light or almost 200 km/s! It will take 9 months to Pluto then, and 3 years to reach the heliopause.

This isn’t going fast enough; we have not left the Sol system, and we are still taking years to just reach the end! Alright, time to take off the kid gloves and go at the speed of light! 1C go! Pluto? 2 hours away, Heliopause? 17 hours! Now we’re talking, are we not? In less than a single day, we can travel out of the solar system from the sun, one and a half days we can go through the entire solar system at 1C!

How about we go to the stars, finally? Closest one one is Proxima Centauri at 4.24 lightyears… oh bollocks 😨 A light year is how much distance light travels in one year, so at 1C it gives us the exact time needed to get there… in years. So that is already over 4 years of travel! Sirius stars are over 8 years away! No matter where we go, 1C is still not enough to make travel times comfortable! Well, time to get out the big boys now: faster than light drive go!

Engines to 100C! Fuck you light, we’re faster than you! How long is it to Proxima Centauri now? 15 days. Sirius stars? One month! Humans have been sending radio waves for about 120 years, we catch up to those in 1 year and 2 months, and we’ll keep going!

Helix nebula at 655 light years, we’ll get there in little over 6 and a half years! Next destination, Eagle Nebula at 5700ly distance! 57 years at 100C… Clearly at 100 C, we are still taking forever to reach anything remotely cool and desirable. Time for even more speed then: 10 thousand times the speed of light! 10kc!

How long are we taking there now with 10kc? 6 months to go there now, while light itself won’t reach us for over 5 thousand years. Imagine here now at the Eagle Nebula, the light from Sol that is reaching us is the same light that your ancestors that were just figuring out how to write. But we cannot stick around here; we have places to be, celestial objects to see, next up is the galactic core of the Milky Way!

27 thousand light years away, at our speed it will take 2 years and 8 months to reach, and we still have not left the galaxy. Alright, time to leave ludicrous speed behind, and go for insane speeds! 1 MILLION times the speed of light! 1Mc speed ahead! 9 days to the centre, a full month to fly across the entire galaxy in one go. What about the Sextans Dwarf Galaxy that orbits the Milky Way and is 320 kly away? Almost 4 months of travel even at this insane speed.

Andromeda galaxy, the closest galaxy that is independent of the Milky Way and will in fact collide with our galaxy in over 4 billion years? Remember, we’re at 1 Mc now, and the Andromeda galaxy is 2.5 million light years away, so that means going to Andromeda will take over 2.5 years at this insane speed. The most distant member in the Virgo Supercluster is NGC 4945 at 13 million lightyears away, which is 13 years away at 1 Mc speed! Even if we increase it from insane speed to eldritch speed of 100 Mc, that is 100 million times the speed of light, it takes a month of travel and some change to reach it.

The Boötes Void, which is an empty religion containing much less matter than it statistically should–60 galaxies instead of the expected 2000–is over 330 Mly across, and now takes over 3 years and 3 months to cross, and keep in mind at this speed, we can now reach Alpha Centauri in less than 2 seconds! You might ask, how long will it take at this eldritch speed to travel from one end of the observable universe to the other? It will still take almost a thousand years.

Space is empty

From that, I hope you can see how absolutely enormous space is in terms of distances. I will get into a rant about it in a moment or five. The next thing to understand–and I use the word ‘understand’ loosely because despite my pathetic attempt at giving it some meaning before and in the next section, it is so mind bogglingly stupidly huge that it is beyond comprehension how empty space is–is what does it mean to be empty? Well, here it is a measure of how little matter there is in a volume, so what is the emptiest thing you encounter in daily life?

Air. Air is the emptiest thing in your everyday life with a density of 1.2 kg/m3. How empty does that mean exactly? If Earth had the same density underneath the ground, you would experience only 0.02% of what you weigh on normal Earth! So air itself is not a lot… but that still counts for over 25 septillion (1 followed by 24 zeros) particles in every cubic metre of air!

Well, how is it in the solar system? Let’s put it like this: if you take every planet, every asteroid, everything out to the heliopause, that is less than a thousandth of a percent of the Sun’s mass so everything is the sun for all intents and purposes. But what is the average density despite this? 46 quadrillion particles per cubic metre. And this is an AVERAGE, where in reality, everything is essentially concentrated in the sun. You want a comparison? Take two drops of food colouring and drop it into a swimming pool and that food colouring now has the same ratio as if you compared air to the average for the solar system.

And in the galaxy? The mass is much more spread out but in it you have… 13 million particles per cubic metre roughly. Take a single drop of food colouring and drop it into 2 billion swimming pools of water and it has a ratio similar to that between the galactic density and air density. How much is it for the entire universe then? Less than 0.2 particles per cubic metre, not even one fucking particle! I am not going to bother with a comparison, it is too insane.

As you can see, space is incredibly empty and there is almost nothing in it except the few concentrated spots that host it all.

The universe is dark

To illustrate this you need to understand a few concepts: the first is angular diameter, which is the angle between, usually, the two furthest points on an object and the observer.

Similar to this diagram, as you can see it forms a triangle which means you can do trigonometry! Run away! What matters here is that no matter what you use–eyes, tools, whatever–there is a smallest angular diameter that can be perceived and anything smaller is blurred together with the rest. For the human eye, that is 1 arcminute (1/60 of a degree).

The other concept is the inverse-square. It says that when it comes to light that is not a laser, and thus spread out like an expanding sphere, if you double the distance, the intensity of the light, in terms of watt, photons, per area, is reduced to one fourth of the previous intensity! So as you can imagine, the intensity goes down real fast. And below a certain level, whatever is output–light, radio, scifi radiation–will not be distinguishable from background radiation.

These facts combined mean that given how low the density of things are, most of the sky is actually extremely dark and empty because there is nothing there. There appears to be much more to the night sky than there is because of the extreme limitation of the human eye in terms of angular diameter perception. After doing some mathematics I won’t bore you with, you have essentially what you can call “stearcminutes”, or “stam” as I like to call it. Think of them as tiles around you that cover your entire field of vision in every direction, and in the sphere around you there are 43200 of them; it is called solid angle and is usually measured in steradians, but I converted to degrees and arcminutes, 60 of those in a degree, for your convenience. If we say one visible star takes up one of these stam tiles, that means you only need 43200 of them arranged in a specific manner to cover your entire field of vision and it won’t matter how far away they are, as long as they are close enough to be detected by your eye. If a star takes 1 or 0.000000000000000001 stams doesn’t matter, your brain registers it as 1 stam sized and sees a star there.

With all of this I hope it becomes also incredibly obvious that even well explored spaces can have things hidden; it is so easy to miss something as the distances are so fast, the chances of detection so little, the next big thing might be in your backyard, and you can’t even see it.

Using these facts in fiction

You are now saying

I get it! I get it! It is unfathomably huge and Lovecraft was right about how tiny humanity is. But why does any of this matter, you handsomely beautiful Limax?

As always, thank you for your compliments! I am extra pretty today on this lovely summer day! But Lovecraft was only correct in how tiny of a scale humanity is, otherwise I disagree a lot with him. Let’s go through why it matters, and you will see why I loathe that scifi writers have no sense of scale.

Star density

While I nagged above on how little there is in the galaxy, I did point out it is highly focused, and we can estimate it roughly. What is important to pay attention to is that you have 3 sections of a galaxy: the core, the disc, and the halo. The core is the centre: it is very active, volatile, and not life friendly, but it is the densest of all, almost like there is some mysterious force that pulls matter inward or something! It has about 50 stars per cubic parsec. What is a parsec? Go back to what I said about angular diameter, imagine a disc the size of Earth's orbit, now how far away must you be before this imagined huge disc has an angular diameter of one arcsecond (1/60 of an arcminute or 1/3600 of a degree)? That distance is a parsec. It is about 3.26 light years. So a cubic parsec is about 34.7 cubic light years. This also means that in the core on average there is about only 1 light year between each star.

Now for the disc of the galaxy, the star density is about 0.5 per cubic parsec or a distance of 5 light years between each star. In the halo it is 0.005 stars per cubic parsec, or about 24 light years between stars. These are estimates and somewhat rounded so you get nice 5s and multiples of 100 between them. This doesn’t sound like much as it is now, but I will demonstrate below why it matters. Look at the image here, a map of the Star Trek galaxy. It is not official, but if this is even within 2 orders of magnitude accurate, it is still bad!

Keep in mind here that the distance from the core to the outer part of the galaxy is 50 kly. The Federation, blue, is about ¼ of that so 12.5 kly in length, about half as wide, lets for the argument assume just as thick. Simple calculation of a block gives us 14 BILLION cubic parsecs, that means they control 7 billion stars roughly. That is an enormous amount of stars! With the current estimate of 7.5% of all stars are G type, like the sun, we say 1 percent get life, that is 5 million 280 thousand stars with life around it in the Federation alone. Do you really need that much!?

Impossible to defend

As you can see, due to the scale of space and despite its enormous emptiness–isn’t that a bit of an oxymoron?–because things don’t grow the way human brains like to work, namely linearly, it gets out of hand real fast and without thought, you have something that is needlessly big.

The thing is also in space, because there are generally no natural borders, defending them becomes incomprehensibly difficult. Let’s go back to the Federation above. What kind of area–yes, area, not border, we are in 3D space now that has an area around it–do we need? Well, we are around 350 MILLION square lightyears. And remember what I said about before with angular sizes and solid angles. A ship is going to be very small and almost undetectable until it is right at you.

You have no chance to protect that large of an area. Even if you space them out so each one tries to only get in within a designated 1000 square lightyear section, that allows 350 thousand ships to fly in virtually undetected; you can begin counting how many ships you need to defend this sized space. If you want to try to defend this space, keep in mind that while the amount of stars grows with the cube of the radius, the area to protect grows by the square. So you better tell people to get busay on making children for all the ships you’ll need to defend it.

Introducing barriers

Of course, if you have natural barriers that prevent people from coming in easily, defending becomes much easier! If you have FTL that are networks this is natural, but if you do the kind where you can move freely, it is not so obvious. There are many ways to do it depending on your universe. In my case, there is something called a “nulltrino” that renders FTL useless and ansible communication technology inoperable. There are systems and even regions of space that are cut off because these nulltrinos are being produced there, making them inaccessible. That is an important aspect to my Tshutsi species.

But if you can find ways to create natural barriers in 3D space, it will help significantly in defending the space that a star nation acquires. Without it, it is a herculean task that is essentially impossible.

Why this is good

I have been dunking on a lot of issues here. People making huge interstellar civilisations without understanding how much it is. Not getting the distances, it is all really based on ignorance, and that is fine. Ignorance can be cured with blogposts like this! But here is what I think about these immense scales.

Despite how many times people fail to comprehend it–and trust me, I get it, the sizes are enormous–think about it, with so many stars, so much potential for life, so much everything… you don’t need a whole galaxy, you don’t need galaxies upon galaxies to fill out everything you need. Take 1 percent of the Milky Way galaxy alone, and it will contain rough stars, enough opportunities for life, enough nebulas, enough everything for you to write every story you can imagine and much else.

All space opera writers and more that make their star nations and star drives so eldritch insanely fast make me sad. Why? Because your universes are empty, sad and empty. You have so much space in the universe within the Milky Way alone, and you need so much just to tell your stories? Do you imagine your universe is so empty when there is so much? Sure, some stories need this, but from what I have seen, most stories do not.

Summa summarum

The big thing here is that space is huge: as star nations expand outward, the amount of stars they can control grows fast, but the amount of area to defend fast, too. But I hope I have made it clear that you do not need to make huge space-filling star nations: the galaxy and space are so big, it looks unbelievable.

Even going out 10 light years means you have dozens of stars to start controlling and much area to defend. Sure, there will be some handwaving; I do it in my Stellima universe, too. It is unavoidable, but I have mine set in less than 0.1% of the galaxy. There is too much in the galaxy for all stories I can imagine to be all of it.

The Galaxy is vast, vaster than you can imagine, vaster than your imagination. Do not make the universe empty by filling it out.

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Copyright ©️ 2024 Vivian Sayan. Original ideas belong to the respective authors. Generic concepts such as cosmic distances, faster than light, star nations, and ansible technology are copyrighted under Creative Commons with attribution, and any derivatives must also be Creative Commons. However, specific ideas such as Tshutsi species, Nulltrino and all language or exact phrasing are individually copyrighted by the respective authors. Contact them for information on usage and questions if uncertain what falls under Creative Commons. We’re almost always happy to give permission. Please contact the authors through this website’s contact page.

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