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Are the Androids Dreaming Yet?

At the bottom end of the spectrum is infrared light. Pit vipers have evolved special organs on the sides of their heads to 'see' in this spectrum and they use this sense to hunt prey in the dark. I use the word see with some caution. We have no idea what their sensation of 'heat-sight' involves, but their organs are very precise, able to detect things only 0.2 degrees warmer than the background.

Infrared cues help several species of snakes, bats and insects locate things in the dark, but the animal that excels at the task, albeit using technology, is mankind. Special cameras allow us to use infrared to see in the dark or detect where our houses lose heat.

X-rays are much higher in frequency – about one hundred times that of the ultraviolet light that affects our T-shirts. The high frequency corresponds to a small wavelength that allows the rays to pass through our bodies. Later on in the book we will understand that frequency is not a proper explanation for light, as it is not a wave but rather a particle that obeys the laws of a wave. But for now we will ignore this detail.

The first use of X-ray images was to see broken bones. Bones block the rays as they are dense, but the soft parts of our bodies are almost completely transparent to X-rays. We can see the soft tissues if we turn the contrast up, but there are problems when using X-rays to view the brain. Our skull completely encases the brain and however much we turn the contrast up, all we see is bone. The solution to this problem is to perform sophisticated mathematical tricks using a computer to enhance the contrast ratio and make image 'slices' through the living head.

The slicing technique was invented independently in the 1970s by Sir Godfrey Hounsfield, working for EMI in England, and Allan Cormack, of Tufts University in America, and they shared the 1979 Nobel Prize for Medicine for their work. Legend has it that EMI was making so much money from The Beatles they could fund the enormous development cost of the CAT scanner from the profits; true or not, it’s a great invention.

The best way to understand the mathematics is to picture yourself in an episode of 'CSI', the American television crime drama. An intruder has attacked someone with a knife and there are blood spatters all over the walls of the room. Enter the brilliant pathologist who reconstructs the scene of the crime from the pattern of blood on the wall. She can map the trajectory of the blood spatters and back-calculate that the attacker must have been 5' 4", left-handed and wielding a 6" blade. In a CAT scan, our head is hit with billions of rays that bounce and scatter over the walls of the machine. Sensors detect the rays and a mathematical algorithm calculates an image of the body that would produce such a pattern. To

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