Brian Clegg - The Universe Inside You: The Extreme Science of the Human Body From Quantum Theory to the Mysteries of the Brain

Stand in front of a mirror, preferably full length, and take a good look at yourself. Not the usual glance really take in what you see. You may become a little coy at this point. Its easy to start looking for imperfections, noticing those extra centimetres on the waistline, perhaps. But thats not the point. I want you to really look at a human being.

In this book you are going to use the human body, your body, to explore the most extreme aspects of science. Its all there. Everything from the chemistry of indigestion to the Big Bang and the most intractable mysteries of the universe is reflected in that single, compact structure. Your body will be your laboratory and your observatory.

You can look at the whole body, treating it as a single remarkable object. A living creature. But you can also plunge into the detail, exploring the ways your body interacts with the world around it, or how it makes use of the energy in food to get you moving. Zoom in further and you will find somewhere between ten and 100 trillion cells. Each cell is a sophisticated package of life, yet taken alone a single cell is certainly not you. Go further still and you will find complex chemistry abounding you have a copy of the largest known molecule in most of your bodys cells: the DNA in chromosome 1.

Continue to look in even greater detail and eventually you will reach the atoms that make up all matter. Here traditional numbers become clumsy; a typical adult is made up of around 7,000,000,000,000,000,000,000,000,000 atoms. Its much easier to say 7 10, simply meaning 7 with 27 zeroes after it. Thats more than a billion atoms for every second the universe is thought to have existed.

Theres a whole lot going on inside that apparently simple form that you see standing in front of you in the mirror.

In a moment well plunge in to explore the miniature universe that is you, but lets briefly stay on the outside, looking at your image in the mirror. Heres a chance to explore a mystery that puzzled people for centuries.

Stand in front of a mirror. Raise your right hand. Which hand does your reflection raise?

As youd expect from experience, your reflection raises its left hand.

Heres the puzzle. The mirror swaps everything left and right something we take for granted. Your left hand becomes your reflections right hand. If you close your right eye, your reflection closes its left. If your hair is parted on the left, your reflections hair is parted on the right. Yet the top of your head is reflected at the top of the mirror and your feet (if its a full-length mirror) are down at the bottom. Why does the mirror switch around left and right, but leave top and bottom the same? Why does it treat the two directions differently?

Heres a chance to think scientifically. There are three things influencing how the mirror produces your image. The way light travels between you and the mirror, the way that you detect that light (with your eyes) and, finally, the way that your brain interprets the signals it receives. We will explore all of these aspects of your body in more detail later in the book, but one significant point may leap out immediately as you think about the process of seeing your reflection. Your eyes are arranged horizontally. You have a left and a right eye, not top and bottom eyes. Could this be why the switch only happens left and right?

Sadly, no. Its a pretty good hypothesis, but in this case its wrong. Thats not a bad thing; much of our understanding of science comes from discovering why ideas are wrong. Lets try a little experiment that will help clarify what is really happening.

Experiment On reflection

Hold up a book (or magazine) in front of you, closed, with the front cover towards you. Look at the book in the mirror. What do you see? Be as precise as possible. List everything that you can say about the reflected book. Does this help explain why the mirror works the way it does?

Do try this yourself first, but heres what I see:

• The book in the mirror is printed in mirror writing, swapped left to right.
• The reflected book is as far behind the mirror as my book is in front of it.
• The books colours are the same in the mirror as they are on my side.
• The front cover of the book in the mirror is the back cover of my book.