2 Silence
If we are interested in the ultimate nature of reality, our Great Predictor is the one to talk to. After all, he obviously knows so much. Before Bell’s Theorem came along, I would have said that I would dearly love him to tell me what he knows.
The problem would have been, though, that the Great Predictor doesn’t talk very much. The Predictor is reticent. There are questions he never answers. If I ask him what will come to pass, he will reply with the utmost specificity. But if I ask for more he falls silent. Quantum theory makes predictions all right—but it does no more.
As an analogy, suppose that the Predictor tells me that tomorrow I will be in two places at once. And lo and behold, when tomorrow evening rolls around, I realize with a start that I do indeed have a vivid memory of having lunched with a friend at noon—and also a memory of having participated in a noontime pick-up basketball game. Of course my memory might well be mistaken, a delusion. But no! I ask my friend and he confirms having lunched with me, and all my basketball buddies vividly remember our game.
I’d better figure this out. So I approach the Predictor and ask him, “How is it possible to be in two places at once?”
The Predictor makes no reply. He refuses to answer my question.
Here’s another analogy. There is a tree. It’s autumn, the time that leaves turn color and fall to the ground. The Predictor says, “Next week half the tree’s leaves will fall, while the other half will remain on the tree. The week after that, half the remaining leaves will fall. And so on.”
I wait and watch. I find that indeed he had been correct. But now I find myself wondering, for many weeks into the autumn I notice that there are still a few isolated leaves clinging to the branches of the tree. They are resisting the buffeting of the winds. But why? What is the difference between the leaves?
Are those still on the tree more hardy, and those that fell more fragile? Do some have thick stems and others thinner? I climb the tree. I rummage around in the pile of leaves at its base. I find no differences. The leaves are all the same.
I go to the Predictor for enlightenment. I ask him, “Why did some leaves hang on for longer than others?”
Again the Predictor makes no reply. Again he refuses to respond to my question. He just sits there.
I ask again. I point to a particular leaf. “When will this one fall?” No response.
These are analogies, of course. The first is a translation into everyday terms of famous quantum-mechanical experiments that demonstrate that electrons can be in two places at once, as demonstrated by a phenomenon known as interference. Interference is a property of waves—but electrons are not waves, they are particles, and quantum theory refuses to explain how a particle can do such a thing. And the second analogy is of the radioactive decay of a nucleus, in which quantum theory correctly predicts the rate of decay, but refuses to explain why one nucleus decays sooner than another.
Don’t focus on what quantum mechanics does. Focus on what it avoids doing. The theory steadfastly refuses to speak of many things. An electron can be emitted here and detected there, but the theory cannot describe the path the electron took. It tells us that an atom can have many different energies at the same time, but it does not tell us how this may be possible. It says that a particle can spin—indeed, that it must spin—but in no particular direction until it is observed. It tells us that events in the atomic realm occur randomly, but it fails to describe their causes. The theory deals only in probabilities, and it never gives explicit descriptions of events—first this happened, and then that. It never explains why an event occurred.
This refusal of the theory to respond to certain questions, this inability to give explanations for its predictions, to describe what happened, and to express certain things, deeply puzzled the theory’s creators. And it has deeply puzzled many physicists ever since.
Return to our examples of everyday predictors. The investor, if she is in a talkative mood, might be willing to tell us what she knows about financial reality. A scientist in the weather bureau would be willing to bend your ear for hours on what he knows of atmospheric conditions. You can learn a lot about the mood of the public by talking to a media executive.
But you will never learn anything by asking the Great Predictor about the real physical situation that we do not perceive, but of which he seems to know so much.
There is a problem with the analogy I just used of the tree in autumn. The problem is that in my analogy the leaves were all the same. But real leaves are not all the same. If one stays up on its branch for longer than the others, there is a reason. Its stem might have been thicker, or maybe it was protected from the wind. But in the world of quantum mechanics there is no such reason, for radioactive nuclei are all alike—absolutely alike. And yet they behave differently.
So my analogy was flawed. And why did I not use a better one? Because there isn’t any. In the world of our experience identical objects behave identically. And if two objects behave differently, it is because they only seem to be identical—were we to look more closely, we would eventually spot the difference. But for nuclei there is no difference.
There is a lesson in all this. It is that nothing in the normal world of daily experience prepares us for quantum mechanics. The microworld is alien—absolutely alien. If there is anything I have learned about the world of the quantum, it is that normal thinking simply does not apply.
So a brief warning. Throughout this book, I will often be using analogies. I will be doing this because I want to place the strange, unfamiliar world of quantum mechanics into a comfortable context. But some of these analogies are going to be misleading. I will try to warn you when they are, but it’s going to be an awkward situation.
So be it.
And while I am at it, I should warn you of something else. Even now, so many years after the creation of quantum mechanics, physicists keep on arguing about it. There is still a profound disagreement among researchers about how to understand what it is telling us about the world. There have even been alternative theories proposed, designed to replace standard quantum theory with something else. Some of these can be thought of as re-interpretations of the theory, and some are outright modifications.a
I want to warn you that everything I am going to say in this book refers to the standard version of the theory. Which is to say, I am completely ignoring these alternative approaches. Why do I do this? Because that is what most physicists do. None of these alternatives has attracted the amount of attention that the standard theory has attracted.
It is actually an unfortunate situation, for there is much to recommend each one of these other approaches. They are all worthy of more attention than they have received. As I am sure you will appreciate as this book goes along, standard quantum theory is utterly strange, and in the long run one of these alternative approaches may well prove to be a better way of dealing with the mysteries of the microworld. If so it will be that one that comes to attract the lion’s share of physicists’ attention.
But as of now, they lie on the outskirts. So in this book I will confine myself to the standard approach.
Here is something that I wish I could tell you: that back when I was a student, I simply could not reconcile myself to the Great Predictor’s silence. That in those days I wanted to grab him by the lapels and shake him to and fro. That I wished I could yell at him to “speak! Say something! Explain yourself!”
But to be honest, I cannot really tell you this. Yes, I was sometimes irritated at the Great Predictor’s silence. But as I have already mentioned, mostly I was irritated at myself. Irritated and even perhaps ashamed. Ashamed that I was so dense. Ashamed that I was Just Not Getting It. Ashamed that I was not understanding what my professor was trying to teach me. That professor certainly seemed pretty sure of himself, striding so confidently back and forth in front of the blackboard as he filled it with all that weird stuff. And, glancing sideways at my fellow students, I could not help but feel that they also seemed pretty sure of themselves. Was I the only one so confused? It was true in most of my courses.
Of course, I would never reveal such weakness in public. So I put on a brave face and soldiered on, writing in my notebook with a bored and superior mien. Who knows? Perhaps I even managed to fool myself.
For truth to tell, it is hard being a student. There is so much you have to learn. Everything is unfamiliar, and often it is foreign to your customary way of thinking. (I will illustrate this a little in chapter 6.) Before the school year began you had felt pretty sure of things—but now you are out of your depth, in new and uncharted territory.
It is true of every form of learning. Some time ago I decided that I needed to improve my tennis game. So I took a few lessons. The serve was something I found particularly hard. To this day I remember vividly all the contortions I was putting my body through in my efforts. I found myself twisting this way and that, bending into the most bizarre poses. Everything I was trying to do felt unnatural and awkward. Meanwhile my instructor was utterly graceful and at home as he demonstrated the proper form.
I now believe that the same is true of all learning. Tennis, quantum theory—it is all the same. To the newcomer it is alien and uncomfortable, and one’s self-confidence can be undermined. In the long run, as the new material sinks in and is internalized, the sense of solidity and confidence returns. But in the first stages the whole situation can be very bruising to the ego.
And so it was with me. I found myself floundering during my initial exposure to quantum mechanics. And while this was going on, I simply had no energy for anything more than learning the material. It would be pleasing to me to be able to tell you now that, even as a student, I had questioned the very foundations of what I was struggling to learn. But that is not really true. The questions, I would tell myself, could wait.
But not always.
Every so often, I would approach my professor after class and try to speak about these mysteries. I wanted to ask him how an object could be in two places at once, or how something could happen without a cause. And although he was being polite, I could tell that my questions did not really interest the professor.
And more than that: I had a faint but unmistakable feeling that he regarded my questions as juvenile. “Kids,” I would imagine him thinking. “You’ve got to love them—aren’t they great? But in the long run this guy Greenstein will have to grow up.”
Even then, at the very beginning, I had encountered a stigma—the faint but all-pervasive sense that I’d better not spend too much time asking such questions. I had encountered a second kind of silence. It was a silence that had paralyzed the field for decades.
a. The most prominent of these are the “pilot wave” theory developed by Bohm, the “spontaneous collapse theory” of Ghirardi, Rimini, and Weber, and the “many worlds” theory of Everett.