THE LIFE STORY OF AN ATOM
Every one of those circa 7 × 1027 atoms has to have come from somewhere. They weren’t constructed from nothing when you were conceived, or born, or as you have grown. The atoms in your body are constantly being replaced at different rates – some remain only hours, others will have been in you for a few years, but over a ten-year period the majority of them will have been replaced. And there are only two obvious sources for atoms to join your body – the air that you breathe and the food and drink that you consume. We’ll cover the details of the consumption side in the next chapter, but the key consideration here is that the atoms that become incorporated in your body will previously have been in the air, plants, animals|| and minerals.
If we could follow an individual atom back through its history, it will have been incorporated many times into other animals and plants. There are so many atoms involved here that just as we can say that you are descended from royalty, we can also say with certainty that your body incorporates atoms that have previously been in the bodies of royalty or the historical celebrity of your choice. Bear in mind that your body alone contains around 100,000 times more atoms than the order-of-magnitude** estimate of 100 billion (1011) humans ever existing.
In fact, your atoms have been in pretty well every type of living thing, from trees to grass, insects to dogs. Go further back and we can say with certainty that the very same atoms will have been in dinosaurs, while throughout the presence of life on Earth many will also have been in bacteria. With the exception of a few atoms produced by radioactive decay, every atom in your body will have already existed when the Earth formed around 4.5 billion years ago.
So, if you are made up of atoms, and those atoms were already in existence way back when the Earth came into existence, where are you actually from? What is your atomic heritage? The universe is around 13.8 billion years old. What happens if we trace the atoms back before the Earth and our solar system came into being? The solar system formed from space-borne gas and dust which itself could only have had two sources. The earliest of these is, effectively, the big bang.
THE COSMOLOGIST’S TIME MACHINE
We can’t with 100 per cent certainty describe how the universe began. It would be unfair to be too hard on cosmologists in this respect. Compare the job of an archaeologist and a cosmologist. The archaeologist will typically be trying to deduce what happened a few thousand years ago, using artefacts that they can get their hands on and test using a whole range of tools and techniques. The cosmologist is looking back millions of times further into the past and can’t touch or directly analyse anything.
To be fair, astronomers and cosmologists do have one advantage. When I was young there was a TV show called The Time Tunnel which, as the name suggests, involved a psychedelic-looking tunnel-shaped device through which the characters could peer and see past events or even pass through to visit another time.†† If we limit the capabilities of a time tunnel to viewing, it is rather similar to the bonus that cosmologists have when looking into space over the archaeologist’s world. Although archaeologists can touch and examine relics, they can never see directly how they were used. By contrast, astronomers and cosmologists can’t directly interact with what they observe, but they can see into the past.
There was no meaningful explanation for the working of the time tunnel in the TV show, but our real, space-facing time tunnel functions due to a simple reality about light – it has a speed. Light moves quickly. Very quickly. A hummingbird’s wings flap 4,200 times in a minute, near invisible to the human eye. Yet in the duration of just ten flaps of those wings, a beam of light could have travelled the distance of the Earth’s circumference. Light is so fast that for a long time it wasn’t even clear whether it had a speed at all, or whether it got from one place to another instantly. The 17th-century French philosopher René Descartes favoured instantaneous travel, thinking that light acted rather like a snooker cue. Push one end of the cue and it appears that the other end moves instantly‡‡ – so imagine light starting at a distance as a push somewhere at the other end of some intervening substance and it arrives at your eye the same moment.
Not long after, the first attempts to measure the speed of light showed that it was indeed very fast, but not instantaneous. It travels at around 300,000 kilometres (186,000 miles) per second. In fact, unlike most other constants of nature, we can put an exact figure on its velocity: 299,792,458 metres per second in a vacuum (it goes slower when it travels through a substance). It’s possible to be this precise because a metre, originally defined as 1/10,000,000th of the distance from the North Pole to the equator through Paris, is now defined as 1/299,792,458th of the distance light travels in a second.
A JOURNEY INTO SPACE
There is, then, an inevitable time-shifting effect when looking up at the stars. The further an object is away, the further back in time you see it, as you have to wait for the light to get from the object to your eyes. The next clear night, see if you can spot the constellation Orion, recognisable by the distinctive belt made up of three stars.
Orion
Alnilam, the middle of the three stars in the belt is around 2,000 light years away. Despite regular attempts in sci-fi movies to use ‘light year’ as a measure of time, it’s actually a distance. As the name suggests, a light year is the distance light travels in one year, about 9.5 trillion kilometres. So when you look up at Alnilam, the light you see has been on its way towards you for the last 2,000 years. You are seeing Alnilam as it was then, not as it is now. Other, brighter objects give us a more penetrating look into the past.
The most distant object visible to the naked eye is probably the Andromeda galaxy, visible to those with good eyesight on a clear, dark night as a little fuzzy patch relatively near the W of Cassiopeia. Andromeda is the nearest big neighbour to our own Milky Way galaxy. But even so, it is 2.5 million light years away, so we see it as it was 2.5 million years in the past, long before humans existed.
Location of the Andromeda galaxy in the night sky.
With advanced modern telescopes and by using a wide range of the light spectrum from radio through X-rays and gamma rays, rather than just the limited light capacity of our eyes, astronomers can see billions of years into the past. But such views are limited, and a lot of what we think we know about the universe 13.8 billion years in the past involves a degree of speculation, dependent as it is on a series of very indirect measures.
HOW IT ALL BEGAN
The lack of direct evidence means that the big bang theory of the origin of the universe does have competitors as a description of how things began. However, for the moment, it’s the best interpretation of the data we have, and as such is the widely accepted theory.§§ According to the big bang theory (the actual theory, not the TV show), the universe began as an impossibly small dot of pure energy. Like the electron, it had no dimensions and, as such, gives us some theoretical issues. Shortly after its origin, though, it began to expand, and physics as we know it can describe what happened well.
Initially the stuff of the universe was so hot and energetic that there were no atoms. But physics tells us (and experiments, including the explosions of nuclear weapons, demonstrate) that energy and matter are interchangeable. When the young universe was a few minutes old, some matter, in the form of atomic nuclei, was able to come into existence. Initially all that we had was hydrogen. Lots and lots of the simplest atom of them all. Briefly, at this point in time, there was enough temperature and pressure for the whole baby universe to act as if it were a star.