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Authors: Amanda Gefter
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host of possible states and yet we only ever measure one. What happens in the course of a measurement that collapses the wavefunctionâs probability distribution down to a single outcome? Given the many positions allowed by the photonâs probability distribution,how does it choose one? The choice appears to be truly randomâan effect with no cause. Was the universe at bottom truly random? Einstein didnât buy it, but the universe didnât seem to care.
Bohr argued that quantum phenomena, like particles, have real properties only after they are measured; it makes no sense to even ask about their pre-measurement state. Thereâs no mysterious collapse, he said, because thereâs nothing to collapse. Bohr didnât believe that observers magically influence the outcomes of experiments or create reality through their mindsâit was just that a measurement outcome was objectively relative to the frame of reference of a measuring device, be it a detector or a photographic plate or a human eye.
Thatâs not to say he didnât realize how seriously weird the whole thing was, requiring, as he wrote,âa radical revision of our attitude toward the problem of physical reality.â But in some sense the fact that properties were relative to observers wasnât that different from Einsteinâs relativity, a fact that Bohr happily pointed out after Einstein had insisted that quantum theory couldnât be a complete description of reality. âI like to think that the moon is there even if Iâm not looking at it,â Einstein had said. In response, Bohr wrote that quantum theoryâmay be paralleled with the fundamental modification of all ideas regarding the absolute character of physical phenomena brought about by the general theory of relativity.â In other words,
Sure
,
quantum theory fucks with reality
,
but you started it.
Then again, there was something distinctly weirder about quantum mechanics than relativity. At least in relativity there was some basic realityâthe unified four-dimensional spacetimeâthat simply
looked
different relative to different observers, and Einstein had kindly offered up tools such as Lorentz and diffeomorphism transformations to translate between different points of view. But what was the basic reality in quantum theory? It was as if there was no reality at all until someone made a measurement.
Of course, if that was true, you couldnât have an observer to make the measurement in the first place. The observerâs got to live in some kind of reality. That was the problem with Bohrâs view. If measurement is the arbiter of reality, then the measuring device has to sit outside realityâwhich, even within the bizarro universe of quantum mechanics,is downright impossible. Besides, any measuring device, human or otherwise, is ultimately made up of subatomic particles, so drawing some kind of ontological line between the two was just plain schizophrenic.
The assertion that a particle doesnât have any ârealâ attributes until someone measures them becomes particularly weird when you realize that certain attributes canât be measured at the same time. Which means that certain attributes canât
exist
at the same time. Take position and momentum. Thereâs no conceivable experiment that can measure both a particleâs position and momentum to perfect accuracy. If you want to accurately measure position, you need a rigidly fixed measuring device that wonât move when the particle hits it; otherwise its movement will smear out the position measurement. But if you want to accurately measure momentum, your device had better move easily when hit, so that its recoil can register the amount of momentum imparted by the particle.
No matter how you set it up, the two measurements are mutually exclusive. The more accurately you know position, the less accurately you know momentum. And itâs not merely a
Bohr argued that quantum phenomena, like particles, have real properties only after they are measured; it makes no sense to even ask about their pre-measurement state. Thereâs no mysterious collapse, he said, because thereâs nothing to collapse. Bohr didnât believe that observers magically influence the outcomes of experiments or create reality through their mindsâit was just that a measurement outcome was objectively relative to the frame of reference of a measuring device, be it a detector or a photographic plate or a human eye.
Thatâs not to say he didnât realize how seriously weird the whole thing was, requiring, as he wrote,âa radical revision of our attitude toward the problem of physical reality.â But in some sense the fact that properties were relative to observers wasnât that different from Einsteinâs relativity, a fact that Bohr happily pointed out after Einstein had insisted that quantum theory couldnât be a complete description of reality. âI like to think that the moon is there even if Iâm not looking at it,â Einstein had said. In response, Bohr wrote that quantum theoryâmay be paralleled with the fundamental modification of all ideas regarding the absolute character of physical phenomena brought about by the general theory of relativity.â In other words,
Sure
,
quantum theory fucks with reality
,
but you started it.
Then again, there was something distinctly weirder about quantum mechanics than relativity. At least in relativity there was some basic realityâthe unified four-dimensional spacetimeâthat simply
looked
different relative to different observers, and Einstein had kindly offered up tools such as Lorentz and diffeomorphism transformations to translate between different points of view. But what was the basic reality in quantum theory? It was as if there was no reality at all until someone made a measurement.
Of course, if that was true, you couldnât have an observer to make the measurement in the first place. The observerâs got to live in some kind of reality. That was the problem with Bohrâs view. If measurement is the arbiter of reality, then the measuring device has to sit outside realityâwhich, even within the bizarro universe of quantum mechanics,is downright impossible. Besides, any measuring device, human or otherwise, is ultimately made up of subatomic particles, so drawing some kind of ontological line between the two was just plain schizophrenic.
The assertion that a particle doesnât have any ârealâ attributes until someone measures them becomes particularly weird when you realize that certain attributes canât be measured at the same time. Which means that certain attributes canât
exist
at the same time. Take position and momentum. Thereâs no conceivable experiment that can measure both a particleâs position and momentum to perfect accuracy. If you want to accurately measure position, you need a rigidly fixed measuring device that wonât move when the particle hits it; otherwise its movement will smear out the position measurement. But if you want to accurately measure momentum, your device had better move easily when hit, so that its recoil can register the amount of momentum imparted by the particle.
No matter how you set it up, the two measurements are mutually exclusive. The more accurately you know position, the less accurately you know momentum. And itâs not merely a
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