The Beginning of Infinity - By David Deutsch Page 0,23

only at a much lower range of temperatures.

The universe is pervaded with microwave radiation – the afterglow of the Big Bang. Its temperature is about 2.7 kelvin, which means 2.7 degrees above the coldest possible temperature, absolute zero, or about 270 degrees Celsius colder than the freezing point of water. Only very unusual circumstances can make anything colder than those microwaves. Nothing in the universe is known to be cooler than about one kelvin – except in certain physics laboratories on Earth. There, the record low temperature achieved is below one billionth of a kelvin. At those extraordinary temperatures, the glow of ordinary matter is effectively extinguished. The resulting ‘non-glowing ordinary matter’ on our planet is an exceedingly exotic substance in the universe at large. It may well be that the interiors of refrigerators constructed by physicists are by far the coldest and darkest places in the universe. Far from typical.

What is a typical place in the universe like? Let me assume that you are reading this on Earth. In your mind’s eye, travel straight upwards a few hundred kilometres. Now you are in the slightly more typical environment of space. But you are still being heated and illuminated by the sun, and half your field of view is still taken up by the solids, liquids and scums of the Earth. A typical location has none of those features. So, travel a few trillion kilometres further in the same direction. You are now so far away that the sun looks like other stars. You are at a much colder, darker and emptier place, with no scum in sight. But it is not yet typical: you are still inside the Milky Way galaxy, and most places in the universe are not in any galaxy. Continue until you are clear outside the galaxy – say, a hundred thousand light years from Earth. At this distance you could not glimpse the Earth even if you used the most powerful telescope that humans have yet built. But the Milky Way still fills much of your sky. To get to a typical place in the universe, you have to imagine yourself at least a thousand times as far out as that, deep in intergalactic space.

What is it like there? Imagine the whole of space notionally divided into cubes the size of our solar system. If you were observing from a typical one of them, the sky would be pitch black. The nearest star would be so far away that if it were to explode as a supernova, and you were staring directly at it when its light reached you, you would not see even a glimmer. That is how big and dark the universe is. And it is cold: it is at that background temperature of 2.7 kelvin, which is cold enough to freeze every known substance except helium. (Helium is believed to remain liquid right down to absolute zero, unless highly pressurized.)

And it is empty: the density of atoms out there is below one per cubic metre. That is a million times sparser than atoms in the space between the stars, and those atoms are themselves sparser than in the best vacuum that human technology has yet achieved. Almost all the atoms in intergalactic space are hydrogen or helium, so there is no chemistry. No life could have evolved there, nor any intelligence. Nothing changes there. Nothing happens. The same is true of the next cube and the next, and if you were to examine a million consecutive cubes in any direction the story would be the same.

Cold, dark and empty. That unimaginably desolate environment is typical of the universe – and is another measure of how untypical the Earth and its chemical scum are, in a straightforward physical sense. The issue of the cosmic significance of this type of scum will shortly take us back out into intergalactic space. But let me first return to Earth, and consider the Spaceship Earth metaphor, in its straightforward physical version.

This much is true: if, tomorrow, physical conditions on the Earth’s surface were to change even slightly by astrophysical standards, then no humans could live here unprotected, just as they could not survive on a spaceship whose life-support system had broken down. Yet I am writing this in Oxford, England, where winter nights are likewise often cold enough to kill any human unprotected by clothing and other technology. So, while intergalactic space would kill me in a matter of seconds, Oxfordshire in its primeval state might do

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