The me-sized universe

Some parts of the cosmos are right within our grasp

By Samuel Arbesman
September 19, 2010

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If you happen to think about the universe during the course of your day, you will likely be overwhelmed.

The universe seems vast, distant, and unknowable. It is, for example, unimaginably large and old: The number of stars in our galaxy alone exceeds 100 billion, and the Earth is 4.5 billion years old. In the eyes of the universe, we’re nothing. We humans are tiny and brief. And much of the physics that drives the universe occurs on the other end of the scale, almost inconceivably small and fast. Chemical changes can occur faster than the blink of an eye, and atoms make the head of a pin seem like a mountain (really more like three Mount Everests).

Clearly, our brains are not built to handle numbers on this astronomical scale. While we are certainly a part of the cosmos, we are unable to grasp its physical truths. To call a number astronomical is to say that it is extreme, but also, in some sense, unknowable. We may recognize our relative insignificance, but leave dwelling on it to those equipped with scientific notation.

However, there actually are properties of the cosmos that can be expressed at the scale of the everyday. We can hold the tail of this beast of a universe--even if only for a moment. Shall we try?

Let’s begin at the human scale of time: It turns out that there is one supernova, a cataclysmic explosion of a star that marks the end of its life, about every 50 years in the Milky Way. The frequency of these stellar explosions fully fits within the life span of a single person, and not even a particularly long-lived one. So throughout human history, each person has likely been around for one or two of these bursts that can briefly burn brighter than an entire galaxy.

On the other hand, while new stars are formed in our galaxy at a faster rate, it is still nice and manageable, with about seven new stars in the Milky Way each year. So, over the course of an average American lifetime, each of us will have gone about our business while nearly 550 new stars were born.

But stars are always incomprehensibly large, right? Well, not always. Sometimes, near the end of a star’s life, it doesn’t explode. Instead, it collapses in on itself. Some of these are massive enough to become black holes, where space and time become all loopy. But just short of that, some stars collapse and become massive objects known as neutron stars. While these stars have incredible gravitational fields and can be detected from distances very far away, they are actually not very large. They are often only about 12 miles in diameter, which is about the distance from MIT to Wellesley College. While its mass is 500,000 times the mass of the Earth, a neutron star is actually very easy to picture, at least in terms of size.

Moving to the other end of the size spectrum, hydrogen atoms are unbelievably small: You would need to line up over 10 billion of them in a row to reach the average adult arm span. However, the wavelength of the energy a neutral hydrogen atom releases is right in our comfort zone: about 21 centimeters (or 8 inches). This is only about one-eighth the average height of a human being. This fact was even encoded pictorially on the plaques on the Pioneer probes, in order to show human height to any extraterrestrials that might eventually find these probes now hurtling out of the solar system, and who might be interested in how big we are.

And let’s not forget energy, though it might seem hard to find energetic examples on the human scale. For example, the sun, a fairly unimpressive star, releases over 300 yottajoules of energy each second, where yotta- is the highest prefix created in the metric system and is a one followed by 24 zeroes. Nonetheless, there are energy quantities we can handle. The most energetic cosmic rays--highly energetic particles of mysterious origin that come from somewhere deep in space--have about the same amount of energy as a pitcher throwing a baseball at 60 miles per hour. This is the low end of the speed of a knuckleball, which is one of the slowest pitches in baseball. While the fact that a tiny subatomic particle has that much energy is truly astounding, it’s no Josh Beckett fastball.

While these examples might seem few and far between, there is good news: The universe is actually becoming less impersonal. Through science and technology, we are getting better at bringing cosmic quantities to the human scale. For example, the number of stars in our Milky Way galaxy is less than half the total number of bits that can be stored on a Blu-ray disc. The everyday is slowly but surely inching towards the cosmic.

Yes, the universe is big and we are small. But we must treasure the exceptions, and see a little bit of the human in the cosmic, even if only for a moment.

Samuel Arbesman is a postdoctoral fellow in the Department of Health Care Policy at Harvard Medical School and is affiliated with the Institute for Quantitative Social Science at Harvard University. He is a regular contributor to Ideas.