How vast is the Universe? Let’s look at it on a human scale
Updated: Mar 17, 2021
“Space is big. You just won’t believe how vastly, hugely, mind-boggling big it is. I mean, you may think it’s a long way down the road to the chemists, but that’s just peanuts to space” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy, 1979.
This short parody is one of Adams’ best-known quotes, replete with his usual inventive humor. The point about the vastness of space, and its sheer size compared with wandering down the street to the shops, is very well taken. But how well?
We usually talk about different things on different scales; subatomic scale, atomic and cellular scale, human scale, and astronomical scale. The reason we have these scales is, bluntly, not so much because science has such a need to measure, but because we need them in order to comprehend things relative to us – where we fit in the world.
At the subatomic level there is not anything in our daily environment that we can relate it to. The same goes for the astronomical. There is very little by way of signposts. The smallness of nanoparticles or the vastness of the Universe on a human scale is far beyond the normal pencil-to-skyscraper type comparisons. So how can we go about appreciating vastness?
To start, just for us to even reach the tip of space, what is known as the Karman line, 60 miles (100 km) above sea level, would take 59,520 humans with an average height of 5’ 6” stacked one on top of the other. By the Karman line, the beginning of space, stacked humans viewed from Earth are already microscopic
Having reached the outer limits of Earth by metaphorically piling people one on top of the other, let’s keep going on our outward journey, into the solar system. The Sun is a safe 93 million miles away from us. That’s a good, secure distance, by the way: if it was much further away we would have a “snowball” Earth, and much closer and we would fry. Indeed, we really can’t be too much closer. In Greek mythology, Icarus found this out. He did not make it very far towards the Sun before things got overly hot for him, and the wax keeping his feathers on melted.
That’s but a metaphor. There’s also the matter of comparative size. The Earth’s radius is 7,926 miles in diameter. The diameter of the sun is 100 times this—although it doesn’t look that big from our vantage point, looking out at it from Earth.
What of the other planets in our solar system? There is a very useful model created by Guy Ottewell in the 1980s which instructs people curious about relativities on how to use everyday objects, such as a chestnut, peppercorn or pinhead, to model the size and distance of the planets, and this aspect even of our local space is, to use Douglas Adams’ words, quite mind-boggling.
Just as an example, Jupiter is the largest of the planets. Its diameter is 11.2 times that of Earth’s. Mercury is the smallest, in both mass and volume. It is two and half times smaller than Earth. In Ottewell’s model if the Sun was a ten-pin bowling ball, Jupiter would be a large chestnut, Earth a peppercorn and Mercury a pinhead.
But it isn’t the size of the planets that’s overwhelming. It’s the distance between them. Using Ottewell’s model, the Sun represented by a bowling ball and Pluto (still a planet at the time he developed his model, before it was demoted to a “dwarf” planet or “Kuiper belt object”, the smallest and the furthest from the Sun) would have to be represented by something smaller than a pinhead. At that size, you would have to place these over 1,000 paces apart to equate to the distance.
The spaces in-between the rest of the planets are pretty mind blowing too. From the Sun-bowling ball, pace 10 steps to Mercury’s pinhead, 19 to Venus’s peppercorn, and 26 to Earth’s peppercorn. Another 14 paces (we’re now 40 paces away) gets you to Mars, another pinhead.
This completes the inner planets. You might already be amazed that the gravity of the Sun, supposedly a weak force according to physicists’ models, holds these planets in orbit around it. That’s why it’s called the solar system.
Now it gets really interesting, because bodies much further away than these are also of course under the Sun’s gravitational power. Keep striding away from the Sun, and count to 136 steps. You have arrived at the chestnut we call Jupiter. Now pace to step 247 and you are at Saturn, a hazelnut-sized body, with those famous rings. Go a further 249 paces (total now: 596) and you encounter Uranus, a coffee-bean sized object. You are by now a little over half way to the outermost “planet,” Pluto.
Now walk another 281 paces (sum of paces: 877) to another coffee-bean sized planet, Neptune. This is your last port of call. Now walk another 242 paces away from the Sun and you reach the smallest body which many people have as their favorite, Pluto, not even a pinhead in Ottewell’s model, at 1,019 paces.
That’s the wondrous scale of the solar system. If we go further now, we are moving outside of our solar system, across our galaxy, the Milky Way. Now imagine our entire solar system as a small coin, say an American quarter. At this scale the Milky Way is the size of the United States (http://www.cfa.harvard.edu/seuforum/howfar/across.html).
The galaxy of ours is home to somewhere between 200 and 400 billion stars. Many, we believe, have their own planetary systems – exoplanets. But not all stars are created equal. For a long time it was thought that for physical reasons a star could not be more than 150 times as large as our Sun (which would make for a very large body). But this was proved wrong with the discovery of R136a1, a star 165,000 light years from earth, which is twice this scale, and has a mass 265 times that of our Sun.
Regardless of this, let’s be very clear: the Milky Way is, for lack of a better word, immense. At the speed of light—299,792,458 (close enough to 300,000) kilometres per second—it would take 100,000 years to travel across it.
You can see how we’ve already reached a point at which earthly comparisons don’t offer much help in understanding the scale of the Universe. Most people would struggle with apprehending such speeds and distances. Sadly, it doesn’t get much easier from here.
The image to the right, known as the Hubble Deep Field, was taken over 10 consecutive days in 1995. It gave us the first ever glimpses of the billions of galaxies that exist outside of our own. Each light looks like a star, but it’s not. It’s an entire galaxy with up to a billion stars – some, even more.
You might be looking at this and thinking: Wow! I’m just a speck in all of that. But actually you are far smaller than just a speck. At this scale specks are whole galaxies. Over 3,000 of them have been counted in this one picture alone. But that’s not all.
The space this Figure covers is an area only about one 24-millionth of the whole sky. To visualize this, imagine Roger Federer hits a tennis ball over 100 yards above you in the sky. That’s how small an area this image covers compared to the observable Universe.
Since this landmark image there have been a number of similar images, taken with improved and refined technology, which have cemented the belief that the Universe looks mostly the same all around us. And even more sobering, cosmologists reckon that we do not occupy a particularly special region in that Universe. It looks similar to everywhere else.
Astronomers have also revealed many more galaxies. This much sharper image is from the Hubble Ultra-Deep Field (HUDF), taken in 2004, and it shows over 10,000 galaxies.
From these images cosmologists estimate that in the observable Universe there are perhaps 200 billion galaxies. But these galaxies are so far away that they are also way back in time. What we see is just their left over light. We see them as they were billions of years ago, as the Universe cooled after the Big Bang. As far as we can calculate with our limited tools at this point, our Universe began 13.75 billion light years ago, and if it is not infinite, by our best guess, it is probably about 97 billion light years across.
Now we’re talking vast. But then most of us aren’t physicists, used to thinking in such measures, so it’s hard for us to fathom this scale. We ran out of human-sized comparisons a few paragraphs ago.
All this and we are just talking about the known Universe. We cannot say that ours is the only one; we can’t even say that it is not infinite. In short, the vastness of our Universe is virtually incomprehensible. It extends much further than our imaginations ever allowed us to go, in all prior human history. And it may be only one of zillions!
Thanks to the internet there is a very useful, interactive online system you can visit called The Scale of the Universe. It will help you visualize these enormous size differences: http://htwins.net/scale2/
Go to that site and play with the scale slider if you feel in need of a lift in your spirits, because it’s very interesting and engaging. But be prepared to be … well, dumbfounded is probably not too strong a word. Go especially if you are struggling with appreciating my assessment here. It’s a mind-bending opportunity to experience the Universe’s scope in all its majesty.
Adams, Douglas (1979). The Hitchhiker's Guide to the Galaxy. London: Pan.
Crowther, Paul A, Schnurr, Olivier, Hirschi, Raphael, Yusof, Norhasliza, Parker, Richard J, Goodwin, Simon P, Kassim, Hasan Abu (2010). The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit. Monthly Notices of the Royal Astronomical Society 408 (2): 731-751.
Ottewell, Guy (1989). The Thousand Yard Model or the Earth as a Peppercorn. Raynham, MA: Universal Workshop