maddening way of his, because we can’t observe it without introducing some new element into the situation, a molecule of water vapour for it to hit, or a piece of light—things which havean energy of their own, and which therefore have an effect on what they hit. A small one, admittedly, in the case of Bohr …
Bohr Yes, if you know where I am with the kind of accuracy we’re talking about when we’re dealing with particles, you can still measure my velocity to within—what …?
Heisenberg Something like a billionth of a billionth of a kilometre per second. The theoretical point remains, though, that you have no absolutely determinate situation in the world, which among other things lays waste to the idea of causality, the whole foundation of science—because if you don’t know how things are today you certainly can’t know how they’re going to be tomorrow. I shatter the objective universe around you—and all you can say is that there’s an error in the formulation!
Bohr There is!
Margrethe Tea, anyone? Cake?
Heisenberg Listen, in my paper what we’re trying to locate is not a free electron off on its travels through a cloud chamber, but an electron when it’s at home, moving around inside an atom …
Bohr And the uncertainty arises not, as you claim, through its indeterminate recoil when it’s hit by an incoming photon …
Heisenberg Plain language, plain language!
Bohr This
is
plain language.
Heisenberg Listen …
Bohr The language of classical mechanics.
Heisenberg Listen! Copenhagen is an atom. Margrethe is its nucleus. About right, the scale? Ten thousand to one?
Bohr Yes, yes.
Heisenberg Now, Bohr’s an electron. He’s wanderingabout the city somewhere in the darkness, no one knows where. He’s here, he’s there, he’s everywhere and nowhere. Up in Faelled Park, down at Carlsberg. Passing City Hall, out by the harbour. I’m a photon. A quantum of light. I’m despatched into the darkness to find Bohr. And I succeed, because I manage to collide with him .… But what’s happened? Look—he’s been slowed down, he’s been deflected! He’s no longer doing exactly what he was so maddeningly doing when I walked into him!
Bohr But, Heisenberg, Heisenberg! You also have been deflected! If people can see what’s happened to you, to their piece of light, then they can work out what must have happened to me! The trouble is knowing what’s happened to you! Because to understand how people see you we have to treat you not just as a particle, but as a wave. I have to use not only your particle mechanics, I have to use the Schrödinger wave function.
Heisenberg I know—I put it in a postscript to my paper.
Bohr Everyone remembers the paper—no one remembers the postscript. But the question is fundamental. Particles are things, complete in themselves. Waves are disturbances in something else.
Heisenberg I know. Complementarity. It’s in the postscript.
Bohr They’re either one thing or the other. They can’t be both. We have to choose one way of seeing them or the other. But as soon as we do we can’t know everything about them.
Heisenberg And off he goes into orbit again. Incidentally exemplifying another application of complementarity. Exactly where you go as you ramble around is of course completely determined by your genes and the various physical forces acting on you. But it’s also completely determined by your own entirely inscrutable whims from one moment to the next. So we can’tcompletely understand your behaviour without seeing it both ways at once, and that’s impossible. Which means that your extraordinary peregrinations are not fully objective aspects of the universe. They exist only partially, through the efforts of me or Margrethe, as our minds shift endlessly back and forth between the two approaches.
Bohr You’ve never absolutely and totally accepted complementarity, have
Bohr Yes, if you know where I am with the kind of accuracy we’re talking about when we’re dealing with particles, you can still measure my velocity to within—what …?
Heisenberg Something like a billionth of a billionth of a kilometre per second. The theoretical point remains, though, that you have no absolutely determinate situation in the world, which among other things lays waste to the idea of causality, the whole foundation of science—because if you don’t know how things are today you certainly can’t know how they’re going to be tomorrow. I shatter the objective universe around you—and all you can say is that there’s an error in the formulation!
Bohr There is!
Margrethe Tea, anyone? Cake?
Heisenberg Listen, in my paper what we’re trying to locate is not a free electron off on its travels through a cloud chamber, but an electron when it’s at home, moving around inside an atom …
Bohr And the uncertainty arises not, as you claim, through its indeterminate recoil when it’s hit by an incoming photon …
Heisenberg Plain language, plain language!
Bohr This
is
plain language.
Heisenberg Listen …
Bohr The language of classical mechanics.
Heisenberg Listen! Copenhagen is an atom. Margrethe is its nucleus. About right, the scale? Ten thousand to one?
Bohr Yes, yes.
Heisenberg Now, Bohr’s an electron. He’s wanderingabout the city somewhere in the darkness, no one knows where. He’s here, he’s there, he’s everywhere and nowhere. Up in Faelled Park, down at Carlsberg. Passing City Hall, out by the harbour. I’m a photon. A quantum of light. I’m despatched into the darkness to find Bohr. And I succeed, because I manage to collide with him .… But what’s happened? Look—he’s been slowed down, he’s been deflected! He’s no longer doing exactly what he was so maddeningly doing when I walked into him!
Bohr But, Heisenberg, Heisenberg! You also have been deflected! If people can see what’s happened to you, to their piece of light, then they can work out what must have happened to me! The trouble is knowing what’s happened to you! Because to understand how people see you we have to treat you not just as a particle, but as a wave. I have to use not only your particle mechanics, I have to use the Schrödinger wave function.
Heisenberg I know—I put it in a postscript to my paper.
Bohr Everyone remembers the paper—no one remembers the postscript. But the question is fundamental. Particles are things, complete in themselves. Waves are disturbances in something else.
Heisenberg I know. Complementarity. It’s in the postscript.
Bohr They’re either one thing or the other. They can’t be both. We have to choose one way of seeing them or the other. But as soon as we do we can’t know everything about them.
Heisenberg And off he goes into orbit again. Incidentally exemplifying another application of complementarity. Exactly where you go as you ramble around is of course completely determined by your genes and the various physical forces acting on you. But it’s also completely determined by your own entirely inscrutable whims from one moment to the next. So we can’tcompletely understand your behaviour without seeing it both ways at once, and that’s impossible. Which means that your extraordinary peregrinations are not fully objective aspects of the universe. They exist only partially, through the efforts of me or Margrethe, as our minds shift endlessly back and forth between the two approaches.
Bohr You’ve never absolutely and totally accepted complementarity, have
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