Are there more actually more stars in the sky, than there are
grains of sand on all the world's beaches?

Sitting on Limantour Beach at Point Reyes awhile back, watching the waves slosh in and out, listening to gulls and feeling very lazy, I found myself looking about me at all that sand, and wondering how it could possibly be true. Reaching out, scooping up a mere handful of grains and letting–what?–a few hundred thousand of the would-be star proxies fall through my fingers, the notion seemed even more absurd.

Raising my eyes from the bit of the cosmos cupped in my hand and taking in the comparatively vast reaches of sand about me–a hundred or so feet between me and the waves, at least a mile or two of beach visible to the north, another stretch to the south, and who knows how many feet of depth beneath the surface? I simply couldn’t believe it. So, I pulled out my journal and started to write down some figures, working out the problem rationally.

So, is it true? Well, here's what I came up with:

Stars: Astronomers have estimated that there are about 200 billion stars in the Milky Way Galaxy. Galaxies come in many sizes, both much larger and considerably smaller than our home galaxy. I don't know what the average number of stars in each galaxy is, but for the sake of this calculation I chose a conservative 10 billion stars per galaxy. Astronomers have also estimated that there are between 50 billion and 100 billion galaxies in the Universe, based on observations made by the Hubble Space Telescope. Again being conservative, I chose the lower figure of 50 billion. So, with those numbers, I calculate a number of stars in the Universe at 10 billion times 50 billion, or 500 billion billion—or in exponential notation, 5 X 10²⁰.

So how does the number of sand grains in the entire world's beaches stack up against that?

To get to that number, I had to do some estimation. First, pulling some numbers out of the air, I decided that an average sandy beach is 30 meters wide (about 100 feet), and 10 meters deep (about 33 feet). Some beaches are wider, some much less so. I don't imagine that the sand on the average beach is as deep as 10 meters—but I've never taken a shovel and found out, either.

Next, I assume that the average sand grain is a millimeter across, giving it a volume of about a cubic millimeter. With that number, I figure the sand grain density to be 1000³, or one billion, sand grains per cubic meter of beach.

The final piece of the equation–after density, width, and depth–is length: the total length of beach shorelines in the entire world. Here's where I made some serious assumptions. Starting with the total length of shorelines of all continents and islands in the world, I got a figure of 356,000 kilometers from the CIA World Factbook. That's 356 million meters.

Now here's where my estimate becomes truly conservative. In my final calculation, I assumed that all 356 million meters of world coastline consisted of sandy beaches– which is not the case, of course; there are plenty of coastlines that are rocky, pebbly, gravely, ice-covered, or sheer cliffs, all without much, if any, sand.

So what were my results? Well, doing the math, 1 billion grains per cubic meter times a 30 meter beach width times a 10 meter beach depth times a 356 million meter beach length and assuming 100% of the coastlines consist of my hypothetical average beach, I get:

1 billion x 30 x 10 x 356 million x 100% = 1.068 x 10²⁰ grains of sand

Compared to the estimate of stars in the Universe, that's about 5 times as many stars in the Universe as grains of sand in all the beaches in the world! I guess the old adage was not only right, but somewhat of an understatement…

But it's all a thing of scale. I also calculated that there are about 3000 times as many water molecules in a glass of water than there are stars in the Universe…

The Allen Telescope Array.

Well, it turns out our chances are much better than I thought. Grote Reber began conducting sky surveys in the radio frequencies with his newly invented radio telescope in 1937, and detected the first signals from outer space in 1938. In the seven decades since then, we've seen a multitude of radio telescope designs pop up all over the world, but we still haven't gotten signals from any little green men. What I didn't understand, until I spoke to Jill Tarter and Seth Shostak at the SETI Institute, is that in all that time, we've hardly looked at any space at all.

Since SETI's first experiment in 1960 by Dr. Frank Drake, and until very recently, they've only looked at a thousand stars out of about 400 billion stars in our galaxy, and there are 100 billion other galaxies to look at! There are two reasons for this: 1) The radio telescopes they've been using can only look at narrow swaths of the sky, and 2) they've had to RENT time on other people's telescopes, which constrains their search and budget. Now, the new Allen Telescope Array is being built just for them, and with it they'll be able to capture millions of frequencies from multiple star systems simultaneously. It will be the biggest and fastest tool in the world for seeking signs of ET!

To learn why scientists use radio frequencies in the hunt for intelligent life, and to learn more about the history & future of the search, watch our story SETI: The Search for ET. You can also watch our extended interview with Astronomer Jill Tarter. And hey folks, the SETI Institute is a non-profit organization, so if you'd like to help them out with the search, consider adopting a scientist like Jill Tarter or Seth Shostak. Go to Adopt-a-Scientist, or join Jill's team and become a TeamSETI member at Join TeamSETI.
Also, check out U.C. Berkeley’s SETI@home page and turn your home computer into a tool that downloads and analyzes radio telescope data.

Watch SETI: The New Search for ET story online, as well as find additional links and resources.
Joan Johnson is an Associate Producer for QUEST on KQED Television.