1 Comment »Posted on Thursday 28 August 2008 at 1:18 pm by Jacob Aron
In Space & Astronomy

Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.

Douglas Adams, author of The Hitchhiker’s Guide to the Galaxy, makes a good point. The sheer giganticness of the universe is extremely difficult for us to comprehend. I find that even with adequate analogies, my brain still wants to dribble out my ears in protest at just how big it is. Even so I’ll give it a try. Let’s start locally, in our own solar system.

The Earth orbits the sun at an average distance of around 149,598,000 km – a distance known as one astronomical unit, or AU. In comparison, the average circumference of the Earth is just 40,041 km, or 0.0003 AU. Already, the mind boggles. Don’t Panic, as Douglas Adams’ Guide would tell us. We can use a trip to the chemist’s to help us understand.

Imagine we shrink the Earth to a circumference of 1 km. Forget about pesky things like changes in gravity or the question of where all 6.5 billion of us are going to stand – we’re just interested in making a new scale. On one side of our 1 km Earth is your house. On the exact opposite side is the chemist’s – at 500 meters away, it’s about a 5 minutes walk. The orbit of the Earth around the sun is now 3736 km – still pretty big, but we can visualise it. Roughly the same as the journey from New York to Los Angeles on the real Earth, it would take you a little over three and a half weeks to walk there from your house. That’s quite a few trips to the chemist’s.

Suppose the chemist’s decides to open a new branch, on the sun. Maybe head office decided they’d make a killing in the sun cream market, who knows. The trouble is they’ve closed your local branch on the 1 km Earth, and you really can’t be bothered to make the three and a half week trip to the new solar store. You call up head office to complain, and they agree to cut you a deal. They’re still going to open their new branch, but they’ve offered to shrink the solar system for you, so that the Earth is 1 km away from the sun. It’ll be twice the walk you had before, but it’s only ten minutes away so you can’t really grumble.

In other words, we’re now working at a scale of 1 km = 1 AU. The nearest star to the sun, Proxima Centuri, is about 4.2 light years away. A light year is exactly what it sounds like: the distance light will travel in a year. Light is pretty speedy at 300,000 km/s, and takes approximately eight minutes to get from the sun to us on Earth. Even so, Proxima Centuri is going to take you much longer to get to than the chemist’s. It works out that 4.2 light years is just over 265,600 AU, so in our shrunken solar system a walk to Proxima Centuri takes five years.

Time to kick things up a notch. The Andromeda Galaxy, the largest in our Local Group of galaxies, is 2,560,000 light years away. You’ve got some friends there, but you don’t see them very much because in a 1 km = 1 AU universe it takes 3 million years to walk and really, that’s hard to fit in to a day trip.

Thing is, they have some sort of magic communication device that can send messages across the vastness of space in an instance and they keep telling you about this great chemist’s, just round the corner from them. You borrow the magic communication device and call up the chemist’s to explain the situation. They’re very sympathetic, and shrink the universe for you down to 1km = 1,000,000 light years. We’re talking the better part of half an hour’s walk to get there, but it really is a very good chemist’s. Sure, your old chemist’s back on the other side of the Earth is now only a 0.000000000003 seconds away and you could walk there and back 15 million times in literally the blink of an eye, but let’s be honest – you never liked them anyway.

What does the universe look like on our new scale? Our current best estimate of the diameter of the observable universe is 93 billion light years. The observable universe is a subtly different concept to the universe proper. Since nothing can travel faster than light, we can have no knowledge of an object in the universe unless light emitted by it has reached us. The Big Bang, which created the universe, took placed 13.73 billion years ago.

“Hang on a moment,” you cry. “How can light have reached us from 93 billion light years away if there have only ever been 13.73 billion years for it to travel in?” The answer is expansion. The universe has been expanding ever since the Big Bang, and the space between galaxies and all other objects can expand faster than the speed of light.

It’s a difficult concept to grasp. The question that immediatly pops up is “what is the universe expanding into?”, but the truth is that there is nothing to expand into – it just expands. It’s a topic I plan to return to in the future, but for now let’s just stick to our figure of 93 billion light years.

After all this walking to and from various chemist’s, you’re probably pretty hungry. I’m going to steal another Hitchiker’s device, the Restaurant at the End of the Universe. In the book the restaurant is at the temporal end of the universe, the end of everything, but I’m going to place my restaurant at the edge of the observable universe. Oh, and it’s a chain of restaurants so there is another one on the other side, separated by 93 billion light years. Even on our absolutely massive 1km = 1,000,000 light years scale, it would still take nearly two years to walk from one restaurant to another. That’s certainly a little bit further than a trip to the chemist’s.


  1. One Comment

  2. Great Post! v. entertaining.Gives me something to thing about this morning as I go to buy some antihistamine, though I must admit I’m feeling less allergic to big concepts…

    By Pam Johnson on Friday 29 August, 2008 at 7:59 am

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