The Invisible Universe

Why There's More to Reality than Meets the Eye

Matthew Bothwell

• Publisher: Oneworld Publications
• Year: 2021
• eBook ISBN: 9780861541263
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

From the discovery of entirely new kinds of galaxies to a window into cosmic ‘prehistory’, Bothwell shows us the Universe as we’ve never seen it before – literally.

'A compelling read.' Lord Martin Rees

Since the dawn of our species, people all over the world have gazed in awe at the night sky. But for all the beauty and wonder of the stars, when we look with just our eyes we are seeing and appreciating only a tiny fraction of the Universe. What does the cosmos have in store for us beyond the phenomena we can see, from black holes to supernovas? How different does the invisible Universe look from the home we thought we knew? Dr Matt Bothwell takes us on a journey through the full spectrum of light and beyond, revealing what we have learned about the mysteries of the Universe.

This book is a guide to the ninety-nine per cent of cosmic reality we can’t see – the Universe that is hidden, right in front of our eyes. It is also the endpoint of a scientific detective story thousands of years in the telling. It is a tour through our Invisible Universe.

1

What is light?

Thinking is difficult at high altitude. Here in the control room of the Very Large Telescope,1 in the Atacama Desert nearly three kilometres above sea level, breathing the thin air provides you with what feels like a persistent hangover. There’s no way around it: the human body simply didn’t evolve to function on top of mountains. This is where I found myself in the spring of 2012, fighting the mental fog and the pounding headache, doing my best to carry out a carefully planned sequence of observations. Luckily for me I had made the schedule earlier, back in the blissfully oxygen-rich atmosphere at the foot of the mountain.

The headache was worth it, for one simple reason. Appearing on the computer screen in front of me was something no human being had ever seen before: a relic of the primeval Universe, hanging there in the ancient darkness. Just by looking at this image I was reaching across a vast ocean of cosmic time, peering back through billions of years into an alien cosmos that existed long before planet Earth formed. If this feels surreal, know that you wouldn’t have to go back very far in human history before this paragraph would start sounding more like magic than science. To be totally honest, it feels more than halfway to being magic to me, even now. How is this trick – time travel, essentially – made possible?

The answer, of course, lies in the properties of light. Light, which zips around the Universe at an incomprehensible speed, brings messages from the past and is our tool through which we understand our cosmos. Almost everything we know about our place in the Universe is built on a foundation of light.

Given that this book promises to be a guide to the ‘invisible Universe’, it might seem strange to start by extolling the importance of light, which by definition, you would think, reveals a thoroughly visible Universe. But we shouldn’t be fooled into thinking that the light we see is the end of the story. T. S. Eliot said that light was ‘the visible reminder of invisible Light’ – a line which beautifully describes the perspective of modern astronomy. The ancient galaxy I was observing above, in my altitude-addled state, was being captured in ‘infrared’ light – a snapshot of the Universe’s deep past that would have been completely invisible to my eyes without the aid of modern technology. As we shall see, we are surrounded by a universe of invisible light which reveals to astronomers a rich storehouse of cosmic wonders that would have been completely unimaginable to our ancestors.

In this introductory chapter, I want to talk about light. Light is undoubtedly one of the wonders of the Universe – a wonder which we are so familiar with in our everyday lives we can easily overlook how deeply strange it really is. I also want to introduce a handful of ideas about how light behaves – these ideas will make up a ‘toolbox’ of concepts that we can take with us on our journey through an invisible cosmos.

How does light work?

The basic idea behind our modern understanding of light is fairly simple. Light sources – like bulbs, fires and stars – produce waves of energy which then enter our eyes, allowing us to see things. Sometimes these waves enter our eyes directly, in which case we see the light source itself, and sometimes these waves reflect off other objects. This incredibly basic idea is so fundamentally embedded in our worldview it’s hard to imagine anyone describing it differently. But it’s worth remembering that the picture of light we take for granted was reached only after centuries of debate; many brilliant scientists and philosophers throughout history believed things about light which now seem downright ludicrous. But if we want to understand how our modern model of light came about, it’s worth looking at the road we took to get here.

The ancient Greek ‘pre-Socratic’ philosophers were, in many ways, the first scientists. They were the first to grapple with questions that we would now call ‘scientific’: asking where things come from, what things are made of, and how reality actually works on a deep-down, fundamental level. And one of the things worth explaining was, of course, light.

The philosopher Democritus (460–370 bce) was amazingly prescient when it came to anticipating modern science. Amongst other things, he was the first to suggest that all matter is composed of tiny ‘atoms’. At the same time, though, he proposed a theory of light and vision which sounds extraordinarily bizarre to modern readers. He proposed that all objects are constantly expelling ghostly versions of themselves called ‘eidola’ – images – which fly through the air, shrinking as they go, until they eventually enter our eyes. If you look at a cow, you are able to see it because a thin layer of cow peeled off the original and floated into your eye. The idea that objects are constantly losing thin layers of themselves rather neatly explains erosion, of course. If this seems crazy, you might regain some sympathy by trying to come up with a thought experiment that disproves this idea – without resorting to scientific evidence that would have been unavailable at the time. It’s not as easy as you might imagine.2

Competing against Democritus’ theory of light were a range of philosophical heavyweights including Pythagoras, Euclid and Plato. This other school of thought believed something equally strange to modern ears: that light was projected outwards from our eyes. These light beams, they supposed, interact with the world and bring information back to us, rather like a bat using echolocation. Again, this idea seemed to have plenty of supporting evidence: cats’ eyes seem to illuminate at night (allowing them to see in the dark), and if you poke your eyeball hard enough it seems to produce flashes of light.3

There were some dissenting voices. The Roman poet Lucretius casually spoke about light and heat originating from the Sun in his poem On the Nature of Things:

As light and heat of sun, are seen to glide

And spread themselves through all the space of heaven

Upon one instant of the day, and fly,

O’er sea and lands and flood the heaven . . .

. . . which is pretty spot on. These views, however, were not to be accepted for many hundreds of years.

The reason I mention these arguments isn’t to ridicule these people. The only reason their ideas seem absurd to us is that our modern scientific worldview is ‘in the water supply’, so to speak. The fact that lots of very smart people over hundreds of years didn’t come to the right answer should tell us that arriving at our seemingly ‘obvious’ picture was a hard-won battle. If we really want to get a sense of what it feels like to be on the cutting edge of science, exploring the world and pushing back the boundaries of human knowledge, it helps to take ourselves out of our comfort zone and imagine ourselves at a time when even our ‘obvious’ ideas, now barely worth a second thought, were still deeply and profoundly mysterious.

While Lucretius was certainly on the right track, it was the Arabic astronomer Hasan Ibn al-Haytham (known as ‘Alhazen’ in the West), who was the first to put forward a theory of light that we would agree with today. In his magnum opus, the Book of Optics (written between 1011 and 1021 ce), he carefully lays out arguments against the older Greek and Roman theories – for example, the fact that looking at a bright light can be painful suggests that light is an external ‘thing’, which is having an effect on our eyes. And while it’s difficult, now, to imagine thinking about light in any other way, the fact that it took humanity well over a thousand years to reach this point suggests that this idea – the right answer – is anything but obvious.

Ibn al-Haytham did more than dismantle the faulty ideas of the past. He also experimented with lenses and mirrors, eventually putting forward a recognisably modern theory of optics. For the first time, we had a valid working model which explained how we see the Universe, in which light is emitted by ‘light sources’ and travels in straight lines, bouncing from surfaces and being picked up by our eyes.

In his 1962 book The Structure of Scientific Revolutions, Thomas Kuhn talks about ‘paradigms’ of science, arguing that all science is done within a particular overarching worldview (a ‘paradigm’), which both colours our observations and sets limits on what can be known. Al-Haytham’s new ideas about light represent a wholly new ‘paradigm’; a revolutionary idea, the effects of which are still resonating with us today. If, as the Greeks held, light is basically part of ourselves, then it will be of limited use for telling us about the distant Universe. But once we accept that light comes from elsewhere, we can begin to see it as a messenger, bringing information about the cosmos. Without this new and important way of seeing the world, the Scientific Revolution centuries later would not have been possible. Ibn al-Haytham’s ideas, passed down from a thousand years ago, were the critical first steps on an intellectual journey that allowed us to take the measure of the stars.

Speed

Measuring the speed of light is no easy feat. It travels so much faster than anything in our normal experience that it took humanity many thousand years to realise that ‘travelling’ was a thing it did at all. In the ancient model – where light left our eyes, scouted the Universe, and returned bearing news – light would presumably have to be infinitely fast (after all, you can open your eyes and see the stars instantly). But once we understood that light is a messenger which leaves distant objects and then enters our eyes, we needed to find out how fast it travels.

Early attempts to measure the speed of light were well intentioned, but doomed to failure. Galileo famously tried to get a handle on it by getting two volunteers to go out at night with shuttered lanterns. The idea was that the first person would uncover their lantern, and as soon as the second person saw the light they would then, in turn, uncover their own lantern. Any delay above and beyond the normal reaction time would be caused by the time taken for light to travel – which, combined with some basic maths, will give you the speed. After some close-range practice (to get the reaction times down), the volunteers traipsed to the top of two hills a few miles apart to run the experiment for real. And the result was . . . anticlimactic. The time delay was indistinguishable from the one they measured during close-range practice. Galileo concluded that light was, at the very least, very fast indeed.

The main problem with this idea isn’t the method. Everything about this experiment is perfectly sensible. The only problem is that light is so absurdly fast – by human standards – that our comparatively glacial reaction times have no hope of keeping up over these short distances. If Galileo and his friend could have stood a million kilometres apart there would have been a very easily measurable time delay, about six seconds, before the first volunteer saw the light from the second. Making measurements over these distances isn’t possible on Earth, of course (not to mention the fact that holding a lantern visible a million kilometres away would probably be hazardous to both your health and the landscape in front of you). It’s no surprise, then, that the first good estimate of the speed of light came from astronomy, where distances of millions of kilometres are commonplace.

Galileo’s efforts to measure the speed of light ended up being in vain, but he did end up playing a small (and unexpected) part in the eventual victory. Even though the first good estimate of the speed of light didn’t come until decades after Galileo’s death, getting the answer would not have been possible without one of his most important discoveries: the moons of Jupiter. In the geocentric culture of the early seventeenth century, it was taken for granted that the Universe was a revolving clockwork machine centred on Earth. Everything orbited around us: the Moon, the planets, the Sun and even the distant stars. But when Galileo pointed his new telescope at Jupiter, he saw what we now call the ‘Galilean moons’ (Io, Europa, Ganymede and Callisto), clearly orbiting around their parent planet – and not the Earth.4 This came as something of a shock, being the first time humanity had clear proof that we were not actually the centre of everything after all.

It was Io, the innermost moon of Jupiter, that eventually held the key to measuring the speed of light. Io is flung around at very high speeds by Jupiter’s immense gravity, taking just forty-two hours to complete one orbit of the giant planet. With a small telescope and some patience you can watch Io’s orbit, seeing it first passing in front of its parent planet, then swinging behind Jupiter into the shadow: an eclipse of the little moon. If you want to time how long Io takes to orbit Jupiter, the start of this eclipse is actually rather useful, making a nice clear ‘marker’ point to start your clock. The Danish astronomer Ole Rømer was doing this exact experiment in the 1670s, when he noticed something odd. The time Io took to orbit around Jupiter seemed to be varying, often being off by several minutes. Given that gravity is normally very well behaved, this was a clear sign that there was something odd going on.

Rømer realised that Io’s orbit around Jupiter was changing in a predictable way: the timing seemed to change at different times of year. Whenever Earth was travelling towards Jupiter, Io seemed to speed up. Six months later, when Earth had swung ...

Table of contents

• Cover
• Title Page
• Dedication
• Contents
• Introduction
• 1 What is light?
• 2 The hidden infrared cosmos
• 3 Microwaves and the start of the Universe
• 4 Monsters in the dark: the quest to find the Universe’s hidden galaxies
• 5 Black holes: agents of destruction, agents of creation
• 6 Astronomy at the longest wavelengths
• 7 Dark matter: a cosmic ghost story
• 8 Ripples in space and time
• 9 Dark energy, and the future of our Universe
• Conclusion: The complete picture: visible and invisible
• Picture Section
• Acknowledgements
• Picture credits
• Suggestions for further reading
• Imprint Page