What are people made of? All life on earth is carbon-based. Carbon is the only atom that you can string together into an infinite variety of flexible chains, loops, and branches, decorated with a sprinkling of other atoms. In order for these large molecules to move, wiggle, change shape and interact with their neighbors, they need to be able to swim around in water. Thus, we are mostly made of water. In fact, Captain Jean-Luc Picard of the Starship Enterprise was once insulted by being called an ugly bag of mostly water. Since there is no other system of chemicals that allows such variety, scientists think that life in other parts of the universe also depends on water, and is based on carbon. So when you want to look for life on Mars, or anywhere else, the first thing you have to look for is water. The picture came from here <<<

I looked around the web a bit and found a few sites that listed what other elements make up the body. I decided to plot these, since the quantities ranged from kilos to micrograms, and numbers like that are difficult to read in a table. Here is a plot based on numbers from a book by John Emsley. As you can see, the Hydrogen and Oxygen that make up the water are prominent, and Carbon of course also. All the othe red dots show that other elements, all the way up to Uranium at 92, occur in minuscule amounts. There are better ways to plot the data, and I also looked up data for the composition of the Earth's crust, the solar system and more. Since all that is a bit much for this web page, have a look here for much more on this subject. There are some amazing tidbits to be seen in these raw distributions of elements.

• More on elements in the body, the earth and the universe
  
  
• Re: What is the mineral/chemical composition of the Human Body in percents
• Re: what is the elemental composition of the human body ?
• Elements in the Human Body
• Human body composition@Everything2.com
• Chemical Composition of Human Biology
• Elements in the Human Body (25)
• Igneous Geochemistry
• Elements and Atoms
• Dr John Emsley
• periodic table
• From NASA: what is your cosmic connection to the elements?
• Periodic Table & The Chemical Elements, with pictures of what the elements look like

I - Are there any?
As far as scientists know, all you need is a hospitable place and enough time, and life will emerge by itself. What is hospitable? Well, first you need a planet, with liquid water on it. If you've got water, you also have an atmosphere, and a sun shining down on you. How many earth-like planets could there be? In recent years, astronomers have found lots of planets circling around other stars. The count in March 2007 is 182., and that is only by looking at stars close by. These planets are more giants like Jupiter, not much like our little earth, but that is because we can't yet see smaller planets. However, since planets around stars seem to be common, we conclude that in the whole universe, there are probably countless planets pretty much like the earth, and given enough time, there will be life on those planets. Likely some of of them have evolved intelligence and technology, some more, some less than us. So yes, somewhere out there, there are aliens. Also check out this link.

II - Where are they all?
First, if aliens have to come to us from the stars, one problem is that space is incredibly, enormously, tremendously, stupendously big, and that there is this universal speeed limit of 300000 km/second (the speed of light). Then there is the matter of energy: if you want to accelerate a spaceship to a speed close to the speed of light, this requires tremendous amounts of energy. No amount of technological advance can circumvent the laws of nature. Suppose you want to visit someone at the other side of our own galaxy, going the speed of the fastest spacecraft that currently exists (62.100 km/h), it would take 2 billion years for a one-way trip, and then a phonecall home, which would travel at the speed of light, would still take 100.000 years to get back to the earth. And that is only inside our own galaxy. Aliens have to deal with the same laws of nature.

Next there is the matter of time: Mammal species on earth last for about a million years or so, and although humans may last longer than dolphins, we may not last forever, especially if we manage to wipe ourselves out in some man-made catastrophe. The earth itself is only 4.5 billion years old, and will be gone in a few billion more. Even if the earth got a dozen visits by aliens in its lifetime, there is only a 1/1000 chance that they would arrive during the existence of the human race.
The enormous size of the universe, in both space and time, works against anybody running into anybody else.

II - Listening and looking
People have been looking and listening - with radio telescopes - for aliens fora long time. See the SETI Institute and this excellent article at Wiki. This is ongoing work, if you want you can help!. All you need is a home computer. The way these searches work is that they listen for radio signals that 'sound unusual'. There have been a few occasions when strange signals were heard briefly, but they went away. What you really want is a signal that you can tune in to for a while.

III - Speaking
Of course if we can listen for their radio and TV broadcasts, they can listen for ours too. Humans have been broadcasting for decades, and the oldest radio signals have now swept out into space to a distance of 70 lightyears, reaching many stars. Who knows, we may have already been noticed. Question is, is that a good idea? If you walk into an unknown jungle, would it be a good idea to shout 'HERE I AM!'. What if a tiger hears you?. Plenty of messages have gone out from the earth, including pictures and sounds that are carried aboard spacecraft: Pioneer, and a record on Voyager containing sounds and pictures from all over the world. Go have a look/listen.

• Planets outside the Solar system
• Where are they? , at Wikipedia
• The SETI Institute
• Seti at home
• Just this week, the French government put al their UFO files online. Read the story here
• A direct link to the French UFO archives
• Follow the links on this page at APOD (Astronomy Picture of the Day), one of my personal daily favorites.

What are quarks? Until recently, I always started this story with the Greek philosopher Democritus (5th century BC), but I learned on Wikipedia about even older Indian atomic philosophies, as well as how Arabic scholars developed their atomic theories in the 11th century. Anyway, bring an apple and a big. sharp knife to show that the 'essence of apple-ness' remains present as you cut smaller and smaller pieces. Modern concepts of atoms flowed out of chemical experiments, and was formulated by John Dalton, about 200 years ago. Early in the 20th century, atoms were found to to be not hard spheres, but mostly empty, with a small nucleus in the center, and lightweight electrons buzzing around it (the Rutherford scattering experiment). This is like trying to infer the shape of a peach by tossing peas at it. By observing how they bounce off, or pass by without hitting, you could eventually infer the size of the peach, about 3-4 inches. However, if you try to do the same thing by shooting beebees at the peach, and see how they scatter, you would get a very different answer. Beebees would pass straight through the soft peach, and only scatter when they hit the pit, which is less than 1 inch. So what you measure depends on how energetic the probe is that is used to do the measurements. Producing high-energy particles to do this leads to bigger and bigger machines, and I put up a poster showing an aerial photograph of CERN, where I used to work. The biggest accelerator ring is 30 km around.

At the next level down, the nucleus is made up of protons and neutrons, and by doing scattering experiments at even higher energies, the protons and neutrons are found to be made up of quarks, called 'up' and 'down'. Although everything we can see and touch is made up of up quarks, down quarks and electrons, that is not the whole story. These particlea each have 2 heavier cousins, which have only a fleeting existence, and can only be seen in high-energy experiments. (to be continued)

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