Sciences Participating with Arts & Culture in Education

"Why does it get dark at night?" You might think that this is a rather trivial question. After all, even a child knows that night falls when the Sun sets below the horizon, and that since there is nothing else in the night sky anywhere near as bright as the Sun, we have to make do with the feeble reflected light from the moon and the even more feeble light emanating from the distant planets and stars. And yet, it turns out this question is far more significant than it first appears. Indeed, astronomers puzzled over it for hundreds of years before they found the correct answer. It is known today as Olber's Paradox.

*

My uncle had to decide whether or not he was ready to stop having kids. His doctor told him that he should still consider himself lucky, because the cancer hadn't yet grown out of control. All they needed to do was a relatively simple operation, but afterwards, my uncle wouldn't be able to have a second child. Or a third. Or a fourth.

I never found out what he had originally planned for himself in terms of a family. I don't know if one kid was all he had ever wanted in the first place. But my uncle decided that he had to live for the golden-haired daughter he had waiting for him at home. Along with his wife, they were the only world he lived in, the only air he breathed. The operation went smoothly and miraculously, it seemed at the time, my uncle was declared 99.9 % cancer free. He wouldn't even have to go through chemo, his doctor told him. 99.9 %.  

What is the relevance of those numbers anyway?

*

Getting into the "bullet" position was hard enough. I had already been trying for 30 minutes; so long, it seemed to me, that the smile on Pops' face should have worn out by now. The wide skis on my feet didn't allow any easy maneuvers in the water. Plus, even though they were beginner's skis, they felt too big for me, and my feet were practically swimming in the rubber sockets. I flipped myself around a few times, tangled and untangled myself from the rope, and finally managed to curl up into an awkward version of the bullet. I gave my dad the thumbs up that meant "Ready!"

I saw his hand move towards the stick shift.

"One..." he shouted, followed by a deliberate dramatic pause.

"Two..." I started thinking of the triangular mouth racing towards me again.

"THREE!"

Our small 50 horsepower boat roared as it went from 0 to full blast, but, as usual, it blasted off without me.

*

Here then, is the problem. We have good reason to believe that even if the Universe is not infinite in size (and it may well be), it is so enormous that to all intents and purposes it goes on forever. Thus, in every direction we care to look out into space, we should see a star, and the sky should be even brighter than it normally becomes during the day; in fact, it should be so bright, all the time, that it should not even matter whether it is day or night according to our Sun. Imagine that our universe goes on forever with an infinity of stars evenly distributed throughout it. No matter where we look in the sky, we should always see a star in our line of sight. So there would not be any gaps where we do not see a star, and the whole sky should be as bright as the surface of the Sun, all the time.

*

My uncle was lying on his back, holding his daughter up as she hopped in slow-motion on his stomach, like an uncertain astronaut on the moon. As her feet came down to prepare for a new bounce, a sharp pain burst in his stomach. This time, he suspected the worst. This time, the doctor didn't use words like "lucky." He just told my uncle that somehow, the cancer had spread. It had mutated, into something he had never encountered before. Something he had never expected. As if it were some alien life form travelling through my uncle's veins. He went from 99.9% cancer free to an unfortunate patient filled with a rare form of sarcoma. Usually, what came attached to sarcoma was a survival expectancy of five years. At that point, feelings like worry mutated too, into feelings like fear. Into feelings that don't have names. Feelings that make you picture things you can't describe, probably because they were never meant to be felt. That's when things got scary. I started hearing terms like "chemo cocktails" for the first time. And when I'd visit my uncle, he'd be much thinner, and his hair would all be gone. Not his smile, though. A smile like his couldn't get swallowed up by alien life forms.

*

Every time I almost had it, I went flying forward, falling face-first on the water's black mirror. My legs were tired. My arms were numb. Once the boat finally managed to pull me to my feet, I either leaned forward too much and belly-flopped or pulled back and my skis would pop off from under me. How come I couldn't do it? All my brothers managed so naturally. Seb was even doing slalom on one ski now. And I couldn't even get up on two. I just let my skis float away from me in the water, and lay on my back, looking up at the grey sky. I hoped to catch a glimpse of blue, to at least make it feel like a real summer day on the lake. But all the blue was gone. I could feel the water in the air, and the goosebumps on my skin. The wind picked up, and small waves lapped against my face. I felt like I could float that way forever. I could just close my eyes and float, wherever the lake would take me.

*

When you first consider this dilemma, you might ask: wouldn't very distant stars simply be too faint for us to see? As it turns out, this doesn't actually matter. While it is true that more distant stars appear dimmer than closer ones, the corresponding patch of sky they are in would encompass a much larger volume since it is farther away, and so would contain more stars. A relatively simple bit of geometry is needed to show that these two effects balance out exactly. For any given patch of sky, the fewer, nearer stars should have a total brightness equal to the more numerous, more distant stars. Surely, the night sky should be as bright as an average galaxy — not quite as bright as the surface of the sun, but still blinding.

Well, no.

And, as we will see, the reason why not turns out to be one of the most profound truths about our universe that we have ever discovered.

*

I remember the phone call. Mom sitting on the floor in the kitchen, the telephone wire just a pile of black spirals beside her feet. I could tell by the look in her eyes that whoever was on the other end of the line was crying. Because the tears were reflected in her eyes too, even though she was holding them back. My uncle had been rushed to the emergency because his lungs were failing him. Tumors had sprouted in his lungs, and they were filling up with water. Lying in his hospital bed, he was practically drowning. To think that you could drown in a room filled with air, try solving that riddle. There were tubes to pump the water out and pull the air in. Tubes were keeping him alive. For how long, no one could really tell. But water in the lungs was never a sign you would hope for. 99.9 % cancer free to drowning in a room filled with air in only four years. Before then, I would have ruled that probability as being absurdly impossible.

*

"Even if you don't make it today, we'll try again tomorrow," Pops said. "We always have tomorrow." I opened my eyes to find his face in the grey sky. He looked down at me from behind his tortoiseshell sunglasses.

The afternoon was ticking away, and I just stayed floating on my back looking up at him. Sure we always had tomorrow, but I didn't want to feel like a failure going home that night with nothing accomplished. I knew they'd all be waiting to hear how it felt. How the first day on skis was. And I'd walk in with nothing but a tight throat and puffy eyes.

Pops leaned over the boat and gave me his hand: "Ready to come in?"

Yes, I was. But that didn't matter. I was too stubborn to let that matter.

"One more try," I puffed.

His smile was filled with approval. "Try standing up," he said. "Don't lean forward or pull back, just stand up."

I swam over to my skis, slipped my feet into the sockets and curled up into the bullet.

Thumbs up.

"One... Two... THREE!"

*

The reason the night sky is dark is not that the Universe is finite in size; for all we know, it may go on forever. It is not that the distant stars are too faint; the farther out we look, the more star-filled galaxies there should be, contributing  their cumulative light to brightening up the gaps we see between stars as we look out into space. No, the real reason for the darkness of space is more simple and profound than this. The night sky is dark because the Universe had a beginning. We can see out into space only as far as the age of the Universe allows us to. What we can see in the sky is just a tiny fraction of the whole cosmos. We call this the "visible universe" and we cannot, even with the most powerful telescopes, see beyond this horizon in space. And this is because it is also a horizon in time. The farther out we look, the farther back in time we are looking, because what we are seeing is the light that left its origins billions of years ago; and so we see it for what is was, not what it is. The edge of the visible universe is to us, therefore, also the earliest moment in time.

*

My uncle passed away in September. At seven p.m. in his hospital bed. It was the night of the harvest moon, when it's so full and yellow, and seems to be closer to Earth than it should be. By the end of it all, my uncle didn't drown in a room filled with air. His lungs hadn't filled up with water. He just slipped away in his sleep, which had become almost constant, and I only hoped it felt something like floating down a lake beneath a grey sky forever. My uncle passed before my mom made it to the hospital, but she said that when she got there, he looked so peaceful. Like an old king, his hands on his chest, his hair gone white and thin from all the chemo. He went from 99.9% cancer free to having a  5 year deadline stamped to his life. And within his years, he watched his daughter grow up. He watched as the golden curls grew, and grew messy, and grew until they reached her shoulders. He watched as she spoke her first words, and watched until she spoke full sentences, in a language far too developed for her age. He watched her leave for her first day of school. He watched it all in silence. A deep silence I could only ever observe from afar.

*

Running on water. A feeling I had never felt before. I was moving faster than I ever did. The water was splashing against my skis, creating a trail of white bubbles behind me. The black depth and triangular mouth didn't frighten me anymore. Whatever was lurking there, way below, couldn't reach me now. Not at this speed. Sure, my position probably looked horribly embarrassing, my legs wide apart, my butt sticking out, but I was doing it. My father was looking back at me from the boat, his face but a flash of a white smile and tortoiseshell sunglasses. It took a while before I realized that I was laughing.

*

A true paradox is a statement that leads to a circular and self contradictory argument, or describes a situation that is logically impossible. But the word "paradox" does tend to be used more broadly to include what I prefer to call "perceived paradoxes". For such puzzles there is a way out. It may be that the paradox has hidden within it a trick or sleight of hand that deliberately misleads the listener or reader. Once the trick is uncovered, the contradiction or logical absurdity disappears. Another type of perceived paradox is one which the statement and the conclusion, while initially sounding absurd or at the very least counterintuitive, turn out not to be so, even if the result remains somewhat surprising.

You just need to know where to look, where to find the Achilles’ heel, that can be exploited with careful prodding and a better understanding of the science, until the paradox is a paradox no more. 

 

Excerpts (in italics) taken from Paradox, The Nine Greatest Enigmas in Physics, by Jim Al-Khalili. Broadway Paperbacks: New York. 2012.