One of my hobbies is astronomy, and I am struck by the vast (astronomical!) distances between us and our stellar neighbors. Most people don’t know just how empty the universe is, both on an astronomical and on a microscopic level. I think you’ll find what we know about the emptiness of the universe to be truly stupefying!
We live in one of the densest places in the universe. Certainly there are much more dense places, mind-bendingly dense places such as the center of the sun or a neutron star or a black hole. However, we are an oasis of matter in a vast sea of empty space. The overwhelming majority of space is a nearly perfect vacuum, which is to say that there’s nothing there, or at least nothing we can detect. A cubic meter (about 30 cubic feet) of interstellar space contains on average just one molecule.
It’s hard to imagine the vastness of space without shrinking it down to something to which we can relate. Let’s start by scaling down our solar system so that the sun is the size of a basketball. The sun is enormous, and compared to a basketball-sized sun the earth would be the size of a pea. Given this scale, how far away do you think the pea-sized earth would have to be from the basketball to be in scale? Go ahead and take a guess.
The Earth would be 90 feet away, with nothing but a few other less-than-peas to fill the empty space. Even more surprising is that the Earth is one of the “inner” planets, with the “outer” planets beyond Mars being much farther away. Jupiter would be 470 feet away and Neptune would be a half mile away. Because of these distances, you probably have never seen a model of the solar system that had the distances and sizes to scale.
As empty as it is, consider that the solar system is essentially two-dimensional, like an almost-empty plate that is surrounded above, below, and beyond by interstellar space. How much interstellar space is there between us and the closest star? Consider this: The light from the sun takes 8 minutes to reach us. The light from the next closest star takes 4 years to reach us. Even the nearest stars appear as single points of light to all but the largest telescopes. Although all of the stars we see are moving at enormous speeds in various directions relative to us and each other, stars are so far away that star atlases are traditionally updated only every 50 years. Two man-made objects have left the solar system (Voyageur 1 and 2) but don’t plan on having them arrive at another star any time in the next 50,000 years.
Even so, we’re in one of the arms of a spiral Milky Way galaxy, which contains an estimated 400 billion stars. Our galactic neighborhood would be considered dense with matter compared to the space between galaxies. Again, it’s difficult to wrap our minds around the vast distances, so let’s shrink the entire Milky Way and its 400 billion stars to the size of the United States. At this scale, our entire solar system would be only 2 inches across, and the sun and other stars of similar size would be a tiny specks only one-twentieth of the thickness of a sheet of paper.
Again, our galaxy is two dimensional, like a plate floating in empty three dimensional space. As luck would have it, we have a neighboring galaxy bearing down on us at a speed of 288,000 miles per hour. This is the Andromeda galaxy, and don’t worry, it will take about 4 billion years for the collision to occur. Still, we can actually see it coming: The Andromeda galaxy is the only full-sized galaxy we can see with the naked eye, though it requires ideal conditions, good vision, and the right techniques to barely see it. Actually, to call it a collision is a misnomer. Despite hundreds of billions of stars in each galaxy, there is so much empty space between the stars that the galaxies will almost certainly pass through each other without a single star colliding, despite the force of gravity which would attract individual stars to each other.
Current models of the grand structure of the universe show densely populated galaxy clusters (such as the Virgo cluster of which we are a part), and portions of the universe where virtually no galaxies exist. If you were placed in a low-density portion of the universe, you would not be able to detect a single source of light in any direction.
Now that we’ve considered the emptiness of the universe on a cosmic scale, let’s take a look at the emptiness of space on a microscopic scale. The reason why I cannot, for instance, pass my hand directly through my desk is not because the molecules of my hand collide with the molecules in the desk. Instead, it’s electromagnetic force that stops my hand. (Actually, quantum physics has proven that it’s something other than electromagnetic force, but I’m not going to pretend to understand quantum physics.) The actual amount of matter in the desk is tiny compared to the volume of the desk.
Again we have to scale the size and distances to something to which we can relate, only this time we will be scaling up, rather than down. Let’s scale a hydrogen atom such that the proton is the size of a basketball. How far away would the electron be, and how large would it be? The answer: the electron would be 11 miles away and would be smaller than the period at the end of this sentence. So for 11 miles in any direction, there would be… nothing. That’s more than 5000 cubic miles of nothing. Like our solar system model, you probably have never seen a properly-scaled model of an atom.
So even solid objects are made up of a whole lot of nothing, and what little mass exists in the universe is concentrated into very small islands in a vast sea of nothing. People have different reactions to this, including mind-blowing wonder, awe, and incomprehension. Some turn to the spiritual. Frank Sheed, a twentieth-century Christian apologist claimed that God didn’t just create the universe, but it is a constant act of God’s will to keep the universe in existence. Knowing how ephemeral the stuff of the universe is, it’s not hard to imagine it all blinking away into nothingness. Everything we know is real, but just barely.
p.s.: For a great pictorial representation of what the view is from a distance of 39 orders of magnitude (from galactic to subatomic and every step between), look up “Powers of 10” on the web or surf to
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/
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