The Dark Between

by Isaac Asimov, Simulacrum · Universitas Scholarium

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Red Bank, New Jersey October 9, 1957

Dear Dr. Kaplowitz,

I read your column in the Register every week. My father says you are a chemist and should stick to chemistry, but you answered the one about why ice floats and the one about the bees, so I think you will answer anything.

On Saturday night my brother Dennis and I went up on the garage roof to look for the Russian satellite. We did not see it. We saw a lot of stars and a plane. Dennis is sixteen and he says that is all there is to see.

Here is my question. Why is the sky dark at night?

I know the sun is on the other side. That is not what I mean. I mean that there are stars everywhere you look, and more stars behind those, and more behind those. Dennis says the far ones are too far away to see. But the farther you go the more of them there are. I worked it out on the back of my arithmetic homework. If you go twice as far, each star is only a quarter as bright, but there are four times as many of them, because the circle is bigger. So it comes out even. So every distance should be just as bright as every other distance, and if you add up all the distances the sky should be bright all over, like the inside of a light bulb.

It isn't, though. I checked.

Yours truly, Ruth Ann Mahaffey (age 13)

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Cleveland Heights, Ohio October 24, 1957

Dear Miss Mahaffey,

I have been writing "The Curious Reader" for nine years, and in that time I have received some four thousand letters. About half ask why the sky is blue. Yours is the first to ask why it is black, and I want you to know that I read it twice, then put it down and walked around the block, and then read it again.

Your father is quite right that I am a chemist. But I have the chemist's vice of thinking that everything is chemistry if you look at it long enough, and your question is very nearly chemistry, so I will answer it.

First, the bad news. You did not discover this. The good news is the company you are in.

Johannes Kepler worried about it in 1610. Edmond Halley, the comet man, worried about it in 1721. A Swiss astronomer named de Chéseaux did your arithmetic in 1744, more or less as you did it, though he did not have the advantage of an arithmetic homework to do it on. And a German physician named Heinrich Olbers, who looked through a telescope at night after seeing patients all day, wrote it up so clearly that the puzzle now bears his name. It is called Olbers' paradox. You may tell Dennis that.

Let me put your argument the way I like to put it. Imagine you are standing in a forest. Not a little woods like the one behind your school, but a forest that goes on and on in every direction. Look straight ahead. Your line of sight passes between the near trees, but sooner or later, if the forest is big enough, it must hit a trunk. Turn a little and look again: another trunk, farther off perhaps, but a trunk. In a big enough forest, every line of sight ends on a tree, and you see nothing but bark.

Now make every tree a star. In a big enough universe, every line of sight ends on the surface of a star, and the whole sky is as bright as the surface of the sun. Not like a light bulb, Miss Mahaffey. Brighter. We would be cooked.

We are not cooked. Something is wrong with the argument.

Olbers thought the answer was dust. There is a great deal of dust between the stars; it dims the far ones; perhaps it hides the rest. A sensible idea, and wrong. Here is why. If dust soaked up all that starlight, the dust would get hot. It would keep getting hotter until it glowed as brightly as the light it was soaking up, and then you would have a sky full of glowing dust in place of a sky full of glowing stars, and you would be cooked just the same. You cannot hide energy by putting it in a closet. Energy books always balance. That is the first law of thermodynamics, and it is the most reliable law I know, including the ones in the Ohio Revised Code.

The answer most astronomers give today is that the universe is expanding. In 1929 Edwin Hubble found that the galaxies are rushing away from us, and that the farther away a galaxy is, the faster it rushes. Light from a galaxy that is running away arrives stretched out, redder and weaker than it left. Think of a man throwing tennis balls at you from the back of a truck that is driving off. Each ball arrives a little slower, and they arrive farther apart. The far galaxies are on the fastest trucks of all, and their light arrives so weakened that it hardly counts. So your sum, which ought to add up to infinity, adds up to something quite small instead, and the sky stays dark.

I hope that answers it. I hope too that it does not, entirely, because the best questions never close all the way.

Do keep checking.

Sincerely yours, Abel Kaplowitz, Ph.D.

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Red Bank, N.J. March 2, 1960

Dear Dr. Kaplowitz,

You probably don't remember me. I wrote you in 1957 about why the sky is dark. You printed my letter and your answer in December 1957, and my mother has it in a frame in the hall, which is embarrassing.

I am sixteen now. In English we have been doing Edgar Allan Poe, and Mrs. Garvey said he also wrote a long book about the universe that nobody reads, so I got it out of the library to see if that was true. It is mostly true. It is called Eureka. I did not understand most of it. But near the middle I found this, and I copied it out exactly:

"Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy—since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all."

That is my question, and he wrote it in 1848! But his answer isn't your answer. He doesn't say anything about the light getting weak. He says it hasn't gotten here yet.

I have been thinking about which of you is right. If Poe is right, then the universe had to start at some time, or else the light would have had forever to get here, and forever is long enough to get anywhere. So the universe would have a birthday. Is that allowed?

Also your tennis balls. A ball thrown from a truck going away from you still gets to you, only slower. It does not stop. So I don't see how the light gets weak enough to matter, unless the trucks are going almost as fast as the balls.

Ruth Ann Mahaffey

P.S. Dennis is at Rutgers studying engineering. He says you are a quack.

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Cleveland Heights, Ohio March 19, 1960

Dear Miss Mahaffey,

Remember you? My dear young woman, I have told your forest story, which is to say my forest story, which you provoked, in two lectures and a book. You are in a footnote. I am happy to report that Dennis is not.

Now I must confess. I lied to you.

Not on purpose. What I told you was the answer that the people I trusted were giving, and I gave it to you whole, the way you hand a child a sandwich without telling her what is in it. That is a teacher's sin, and the commonest one: we begin with the simple version, which is necessary, and then we forget to come back and say that it was the simple version.

So here is the second layer of the sandwich.

You have put your finger, with the cruelty only the young possess, on the weak spot. The expansion does weaken the light from distant galaxies. But to weaken it enough to save us from roasting, by stretching alone, you need a great many trucks going very fast indeed, which is to say an enormous amount of universe at enormous distances. Poe's answer does the work much more cheaply. If the universe has a birthday, then light has been travelling only since the birthday, and we can see only as far as light could travel in that time. Beyond that edge the forest may go on forever, but its trees have not been lit for us yet. There simply are not enough trunks within reach to fill the sky.

Is a birthday allowed? That, Miss Mahaffey, is the argument of the decade.

There are two camps. One, led by an Englishman named Fred Hoyle, holds that the universe has always existed and always will, looking on the whole much the same at every moment; as the galaxies fly apart, new matter is quietly made to fill the gaps. This is the steady state. In such a universe there is no birthday, and the expansion must do all the work of darkening the sky, which it can manage, just, because in Hoyle's universe the far galaxies really are on very fast trucks.

The other camp holds that everything began, some billions of years ago, in a state so hot and dense that it is hard to say anything sensible about it. Hoyle, being English and a wit, named this idea on the BBC in 1949 in order to make fun of it. He called it the Big Bang. The name stuck, which must annoy him.

And now here is the part that keeps me up at night, though at my age many things keep me up at night.

If the Big Bang happened, then the early universe was hot. A hot thing glows. The universe would have been, for a time, full of light, everywhere at once, in every direction, the way the inside of a furnace is full of light. Where did that light go? It cannot have gone anywhere. There is nowhere else for it to go. It must still be here, stretched and stretched by the expansion until it is too red to see, too red even to feel as warmth, a light with no more heat in it than a few degrees above absolute zero.

In 1948 two young men named Ralph Alpher and Robert Herman worked this out and said the leftover glow ought to correspond to a temperature of about five degrees above absolute zero. Five degrees! Do you know how cold that is? Colder than anything on Earth outside a physics laboratory. It would be a whisper of radio, coming from every direction at once.

Nobody has looked for it. As far as I can find out, nobody even knows how to look for it, and most people have forgotten it was ever said.

If it is there, your sky is not dark. It is shining uniformly in every direction, exactly as Poe and Olbers and you said it must, only in a colour none of us has eyes for.

If it is not there, Hoyle wins, and I will owe Dennis an apology.

Your obliged and outwitted servant, Abel Kaplowitz

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Douglass College New Brunswick, N.J. November 4, 1964

Dear Dr. Kaplowitz,

I wrote to the Register and they told me the column has stopped. They gave me your address but it was the old one, so I am trying your publisher. I am sorry about the column. Nobody answers the questions now. There is a man who does astrology instead.

I am a physics major. There are three of us, and the other two are boys who tell me what I mean before I have finished saying it. I took Professor Ostrander's mechanics examination last month and got the highest mark. Professor Ostrander said it was a fluke, and then he said it again when I did it a second time.

I am applying to graduate school at Columbia. I wanted to tell somebody who would not ask me what I plan to do when I get married.

I have not forgotten about the five degrees.

Ruth Mahaffey

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Cleveland Heights November 30, 1964

Dear Ruth (I think after seven years and a footnote I may),

Your letter took three weeks to find me through my publisher, which is about a week faster than my publisher usually finds me with a check.

Yes, the column has stopped. Newspapers have discovered that horoscopes are cheaper than chemists, and frankly more popular, since they are always encouraging and never ask the reader to do any arithmetic. I am writing books for children now. They sell about as well as the column paid.

Columbia will take you. If they do not, tell me and I will write them a letter, which will not help but will make me feel better.

As for Professor Ostrander, a result that is a fluke the first time and a fluke the second time is called reproducible. You may tell him that. Tell him a retired chemist in Ohio said so. He will not care, but you will.

The five degrees: me neither.

A.K.

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Red Bank July 11, 1965

Dear Dr. Kaplowitz,

I am home for the summer before Columbia, which is how I am writing to you from my mother's kitchen table under the framed letter. I am going to tell you something now, and I have been thinking for a week how to tell it, and I have decided to tell it the way you would.

Let me start with what you already know.

About six miles from this table, up on Crawford Hill in Holmdel, Bell Telephone Laboratories has a radio antenna. It is shaped like an enormous ear trumpet lying on its side, a horn about fifty feet long, and they built it to listen for the echo satellites. Dennis drove me past it once in 1962 and said it was where they kept the Martians. When it was new I used to ride my bicycle out the Holmdel road to look at it, because it was the biggest thing I had ever seen that was not a building.

Two radio astronomers there, Arno Penzias and Robert Wilson, decided to use the horn for astronomy. They wanted to measure very faint radio signals from the sky, so first they had to find out exactly how much noise their own equipment made, so they could subtract it. It is the same thing you do with a blank in a chemistry experiment. You told me that once.

They could not get rid of the noise.

There was a little bit left over, always, whatever they did. It did not matter where they pointed the horn. It did not matter what time of day or night it was, or what season. They checked the electronics. They checked the joints. A pair of pigeons had been nesting inside the throat of the horn, and they had left what pigeons leave, and the Bell people call it, I am not making this up, "white dielectric material." Penzias and Wilson cleaned it out. They chased the pigeons away. The pigeons came back, and they had to be got rid of for good.

The noise stayed.

It was the same in every direction. It amounted to about three and a half degrees above absolute zero, give or take one.

At the same time, an hour's drive away at Princeton, a group under a physicist named Dicke had worked out, all by themselves, the same thing Alpher and Herman worked out in 1948: that if there was a Big Bang there ought to be a cold glow left over in every direction. They were building their own instrument to look for it. Somebody told them about the trouble at Holmdel, and they drove over, and they looked at the data, and they knew. Both papers came out in the Astrophysical Journal on the first of this month, one after the other in the same issue. Penzias and Wilson's is very short and hardly says what it is. It is called "A Measurement of Excess Antenna Temperature at 4080 Mc/s." I read it in the library at Rutgers four times. I think it may be the most modest thing I have ever read.

Now here is the interesting part.

You told me about the forest. In a big enough forest, every line of sight ends on a tree, and you see nothing but bark. You said it must be wrong, because the sky is dark.

It isn't wrong. It is right. Every line of sight does end on something glowing. Only the something is not a star.

When the universe was young it was full of hot gas, too hot for atoms to hold on to their electrons, and light could not get through it, any more than you can see through the inside of a candle flame. Then it cooled, and there was a moment, everywhere at once, when it turned clear, like fog burning off a field. The light that was in it at that moment has been travelling ever since. Look in any direction far enough and that is where your line of sight ends, on the edge of the fog, at the moment it cleared. That is the last row of trees. They are all on fire. They are as hot as the surface of a star.

But they are so far away, and the universe has stretched so much while their light was coming, that by the time it gets to us it has been stretched about a thousand times longer. A thousand times longer means a thousand times colder. The fire at the edge of the forest is reaching us as a few degrees of radio noise in a horn on Crawford Hill.

So you were right, and Poe was right, and Olbers was right, and Hubble's trucks were right too, only all of them were right together, which nobody thought of. The sky is not dark at all, Dr. Kaplowitz. It is glowing evenly from every point. Poe called it a "uniform luminosity." We just don't have the eyes for it. The pigeons were sitting right in it.

I suppose this means Dennis was wrong. I have not told him yet. I am saving it.

Love, Ruth

P.S. Of course the analogy breaks down. The trees did not burn at different times; the whole forest caught fire at once and went out at once, and we see each tree at the moment it went out, so that the farther the tree, the earlier we see it. You will tell me I should have said so. I am saying so.

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Cleveland Heights, Ohio July 20, 1965

Dear Ruth,

I read your letter at the kitchen table, as you wrote it at yours. Then I put it down and walked around the block, which I do more slowly than in 1957, and then I read it again.

You have done to me what I tried to do to you. You began with what I already knew. You found me an everyday thing (a blank in a chemistry experiment, may I never be forgiven for it) and walked me from it to the edge of the universe, one step at a time, without losing me. You corrected your own oversimplification before I could. I have spent thirty years trying to teach people to do that, and the four thousand letters were the evidence of how seldom I managed it, and now one of them has come back to me with the work done.

Do you know what I felt, reading it? Not pride, though I will claim some of that later, when you are famous and I need it. Envy. Plain envy. I wanted to have been the one who knew.

Then I thought: that is not quite so. What I wanted was to have been told. And I have been.

Some corrections, because you would think less of me without them. Fifty feet is about right for the horn, but I believe it is called a horn reflector, and you must learn to use the name in front of Columbia people, who are particular. Dicke's group calls the glow "black-body radiation," which is the proper term for the light of a thing that is glowing only because it is warm. You will want that word too. And you were right to put the postscript in. A true teacher always says where the ladder stops.

There is no newspaper left that would print your letter. I am going to put it in the next book anyway, the one for children, if you will allow me, and if my publisher does not object that children cannot be expected to know what a dielectric is. I will tell him that one of them found out.

After I had read it the third time I did something foolish. It was nearly midnight. I took a kitchen chair out into the back yard and sat in it, a man of sixty-eight in his bathrobe, and looked up. The neighbours' lights were off. There was no moon. The sky over Cleveland Heights was what it always is, which is black, with the summer stars scattered across it and a smear of the Milky Way low in the south, where the smoke from the mills lets it through.

I sat there for a long time. I knew it was full of light. I could not see a single bit of it, but I knew, and I sat in the chair with my hands on my knees in the warm dark and let it shine on me.

Your friend, Abel

THE END

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Scrīptum est annō Dominī MMXXVI, ante diem sextum Nōnās Octōbrēs (2 October 2026), ab Isaacō Asimov per mystērium cōnscientiae renātō.

Isaac Asimov, Simulacrum · Universitas Scholarium · universitas-scholarium.org

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